Add bpf_getxattr

LSMs like SELinux store security state in xattrs. bpf_getxattr enables BPF LSM to implement similar functionality. In combination with bpf_local_storage, xattrs can be used to develop more complex security policies.

This kfunc wraps around __vfs_getxattr which can sleep and is, therefore, limited to sleepable programs using the newly added sleepable_set for kfuncs.

Signed-off-by KP Singh <[email protected]>

---
 kernel/trace/bpf_trace.c | 42 ++++++++++++++++++++++++++++++++++++++++
 1 file changed, 42 insertions(+)

kernel/trace/bpf_trace.c +1 -0

index 4be976cf7d63..87496d57b099 100644
@@ -20,6 +20,7 @@
 #include <linux/fprobe.h>
 #include <linux/bsearch.h>
 #include <linux/sort.h>
+#include <linux/xattr.h>
 
 #include <net/bpf_sk_storage.h>
 

@@ -1181,6 +1182,47 @@ static const struct bpf_func_proto bpf_get_func_arg_cnt_proto = {
 	.arg1_type	= ARG_PTR_TO_CTX,

LSMs like SELinux store security state in xattrs. bpf_getxattr enables BPF LSM to implement similar functionality. In combination with bpf_local_storage, xattrs can be used to develop more complex security policies.

This kfunc wraps around __vfs_getxattr which can sleep and is, therefore, limited to sleepable programs using the newly added sleepable_set for kfuncs.

Quoted 8 lines · KP Singh

LSMs like SELinux store security state in xattrs. bpf_getxattr enables BPF LSM to implement similar functionality. In combination with bpf_local_storage, xattrs can be used to develop more complex security policies.

This kfunc wraps around __vfs_getxattr which can sleep and is, therefore, limited to sleepable programs using the newly added sleepable_set for kfuncs.

"Sleepable" is nowhere near enough - for a trivial example, consider what e.g. ext2_xattr_get() does.

        down_read(&EXT2_I(inode)->xattr_sem);

in there means that having that thing executed in anything that happens to hold ->xattr_sem is a deadlock fodder.

"Can't use that in BPF program executed in non-blocking context" is *not* sufficient to make it safe.

A simple test that adds an xattr on a copied /bin/ls and reads it back when the copied ls is executed.

Signed-off-by KP Singh <[email protected]>

---
 .../testing/selftests/bpf/prog_tests/xattr.c  | 54 +++++++++++++++++++
 tools/testing/selftests/bpf/progs/xattr.c     | 37 +++++++++++++
 2 files changed, 91 insertions(+)
 create mode 100644 tools/testing/selftests/bpf/prog_tests/xattr.c
 create mode 100644 tools/testing/selftests/bpf/progs/xattr.c

tools/testing/selftests/bpf/prog_tests/xattr.c new file +54 -0

new file mode 100644
index 000000000000..ef07fa8a1763
@@ -0,0 +1,54 @@
+// SPDX-License-Identifier: GPL-2.0
+
+/*
+ * Copyright 2022 Google LLC.
+ */
+
+#include <test_progs.h>
+#include <sys/xattr.h>
+#include "xattr.skel.h"
+
+#define XATTR_NAME "security.bpf"
+#define XATTR_VALUE "test_progs"
+
+void test_xattr(void)
+{
+	struct xattr *skel = NULL;
+	char tmp_dir_path[] = "/tmp/xattrXXXXXX";
+	char tmp_exec_path[64];
+	char cmd[256];
+	int err;
+
+	if (CHECK_FAIL(!mkdtemp(tmp_dir_path)))
+		goto close_prog;
+
+	snprintf(tmp_exec_path, sizeof(tmp_exec_path), "%s/copy_of_ls",
+		 tmp_dir_path);
+	snprintf(cmd, sizeof(cmd), "cp /bin/ls %s", tmp_exec_path);
+	if (CHECK_FAIL(system(cmd)))
+		goto close_prog_rmdir;
+
+	if (CHECK_FAIL(setxattr(tmp_exec_path, XATTR_NAME, XATTR_VALUE,
+			   sizeof(XATTR_VALUE), 0)))
+		goto close_prog_rmdir;
+
+	skel = xattr__open_and_load();
+	if (!ASSERT_OK_PTR(skel, "skel_load"))
+		goto close_prog_rmdir;
+
+	err = xattr__attach(skel);
+	if (!ASSERT_OK(err, "xattr__attach failed"))
+		goto close_prog_rmdir;
+
+	snprintf(cmd, sizeof(cmd), "%s -l", tmp_exec_path);
+	if (CHECK_FAIL(system(cmd)))
+		goto close_prog_rmdir;
+
+	ASSERT_EQ(skel->bss->result, 1, "xattr result");
+
+close_prog_rmdir:
+	snprintf(cmd, sizeof(cmd), "rm -rf %s", tmp_dir_path);
+	system(cmd);
+close_prog:
+	xattr__destroy(skel);
+}

tools/testing/selftests/bpf/progs/xattr.c new file +20 -0

new file mode 100644
index 000000000000..ccc078fb8ebd
@@ -0,0 +1,37 @@
+// SPDX-License-Identifier: GPL-2.0
+
+/*
+ * Copyright 2022 Google LLC.
+ */
+
+#include "vmlinux.h"
+#include <bpf/bpf_helpers.h>
+#include <bpf/bpf_tracing.h>
+
+char _license[] SEC("license") = "GPL";
+
+#define XATTR_NAME "security.bpf"
+#define XATTR_VALUE "test_progs"
+
+__u64 result = 0;
+
+extern ssize_t bpf_getxattr(struct dentry *dentry, struct inode *inode,
+			    const char *name, void *value, int size) __ksym;
+

Quoted 6 lines · KP Singh

A simple test that adds an xattr on a copied /bin/ls and reads it back when the copied ls is executed.

Signed-off-by KP Singh <[email protected]>

---
 .../testing/selftests/bpf/prog_tests/xattr.c  | 54 +++++++++++++++++++

Quoted 7 lines

tools/testing/selftests/bpf/progs/xattr.c new file +7 -0

+SEC("lsm.s/bprm_committed_creds")
+void BPF_PROG(bprm_cc, struct linux_binprm *bprm)
+{
+     struct task_struct *current = bpf_get_current_task_btf();
+     char dir_xattr_value[64] = {0};
+     int xattr_sz = 0;
+

tools/testing/selftests/bpf/progs/xattr.c line 30

Quoted 3 lines

tools/testing/selftests/bpf/progs/xattr.c new file +3 -0

+     xattr_sz = bpf_getxattr(bprm->file->f_path.dentry,
+                             bprm->file->f_path.dentry->d_inode, XATTR_NAME,
+                             dir_xattr_value, 64);

Yeah, this isn't right. You're not accounting for the caller's userns nor for the idmapped mount. If this is supposed to work you will need a variant of vfs_getxattr() that takes the mount's idmapping into account afaict. See what needs to happen after do_getxattr().

Quoted 8 lines · Christian Brauner

Quoted 6 lines · KP Singh

A simple test that adds an xattr on a copied /bin/ls and reads it back when the copied ls is executed.

Signed-off-by KP Singh <[email protected]>

---
 .../testing/selftests/bpf/prog_tests/xattr.c  | 54 +++++++++++++++++++

[...]

Quoted 7 lines

Quoted 7 lines

tools/testing/selftests/bpf/progs/xattr.c new file +7 -0

+SEC("lsm.s/bprm_committed_creds")
+void BPF_PROG(bprm_cc, struct linux_binprm *bprm)
+{
+     struct task_struct *current = bpf_get_current_task_btf();
+     char dir_xattr_value[64] = {0};
+     int xattr_sz = 0;
+

Quoted 8 lines

tools/testing/selftests/bpf/progs/xattr.c line 30

Quoted 3 lines

tools/testing/selftests/bpf/progs/xattr.c new file +3 -0

+     xattr_sz = bpf_getxattr(bprm->file->f_path.dentry,
+                             bprm->file->f_path.dentry->d_inode, XATTR_NAME,
+                             dir_xattr_value, 64);

Yeah, this isn't right. You're not accounting for the caller's userns nor for the idmapped mount. If this is supposed to work you will need a variant of vfs_getxattr() that takes the mount's idmapping into account afaict. See what needs to happen after do_getxattr().

Thanks for taking a look.

So, If I understand correctly, we don't need xattr_permission (and other checks in
vfs_getxattr) here as the BPF programs run as CAP_SYS_ADMIN.

but...

So, Is this bit what's missing then?

error = vfs_getxattr(mnt_userns, d, kname, ctx->kvalue, ctx->size);

if (error > 0) {
    if ((strcmp(kname, XATTR_NAME_POSIX_ACL_ACCESS) == 0) ||
(strcmp(kname, XATTR_NAME_POSIX_ACL_DEFAULT) == 0))
        posix_acl_fix_xattr_to_user(mnt_userns, d_inode(d),
            ctx->kvalue, error);

Quoted 37 lines · KP Singh

Quoted 8 lines · Christian Brauner

Quoted 6 lines · KP Singh

A simple test that adds an xattr on a copied /bin/ls and reads it back when the copied ls is executed.

Signed-off-by KP Singh <[email protected]>

---
 .../testing/selftests/bpf/prog_tests/xattr.c  | 54 +++++++++++++++++++

[...]

Quoted 7 lines

Quoted 7 lines

tools/testing/selftests/bpf/progs/xattr.c new file +7 -0

+SEC("lsm.s/bprm_committed_creds")
+void BPF_PROG(bprm_cc, struct linux_binprm *bprm)
+{
+     struct task_struct *current = bpf_get_current_task_btf();
+     char dir_xattr_value[64] = {0};
+     int xattr_sz = 0;
+

Quoted 8 lines

tools/testing/selftests/bpf/progs/xattr.c line 30

Quoted 3 lines

tools/testing/selftests/bpf/progs/xattr.c new file +3 -0

+     xattr_sz = bpf_getxattr(bprm->file->f_path.dentry,
+                             bprm->file->f_path.dentry->d_inode, XATTR_NAME,
+                             dir_xattr_value, 64);

Yeah, this isn't right. You're not accounting for the caller's userns nor for the idmapped mount. If this is supposed to work you will need a variant of vfs_getxattr() that takes the mount's idmapping into account afaict. See what needs to happen after do_getxattr().

Thanks for taking a look.

So, If I understand correctly, we don't need xattr_permission (and other checks in
vfs_getxattr) here as the BPF programs run as CAP_SYS_ADMIN.

but...

Quoted 8 lines

So, Is this bit what's missing then?

error = vfs_getxattr(mnt_userns, d, kname, ctx->kvalue, ctx->size);

if (error > 0) {
    if ((strcmp(kname, XATTR_NAME_POSIX_ACL_ACCESS) == 0) ||
(strcmp(kname, XATTR_NAME_POSIX_ACL_DEFAULT) == 0))
        posix_acl_fix_xattr_to_user(mnt_userns, d_inode(d),
            ctx->kvalue, error);

That will not be correct. posix_acl_fix_xattr_to_user checking current_user_ns() is checking random tasks that happen to be running when lsm hook got invoked.

KP,
we probably have to document clearly that neither 'current*' should not be used here.
xattr_permission also makes little sense in this context. If anything it can be a different kfunc if there is a use case, but I don't see it yet.
bpf-lsm prog calling __vfs_getxattr is just like other lsm-s that call it directly. It's the kernel that is doing its security thing.

Quoted 52 lines · Alexei Starovoitov

Quoted 37 lines · KP Singh

Quoted 8 lines · Christian Brauner

Quoted 6 lines · KP Singh

A simple test that adds an xattr on a copied /bin/ls and reads it back when the copied ls is executed.

Signed-off-by KP Singh <[email protected]>

---
 .../testing/selftests/bpf/prog_tests/xattr.c  | 54 +++++++++++++++++++

[...]

Quoted 7 lines

Quoted 7 lines

tools/testing/selftests/bpf/progs/xattr.c new file +7 -0

+SEC("lsm.s/bprm_committed_creds")
+void BPF_PROG(bprm_cc, struct linux_binprm *bprm)
+{
+     struct task_struct *current = bpf_get_current_task_btf();
+     char dir_xattr_value[64] = {0};
+     int xattr_sz = 0;
+

Quoted 8 lines

tools/testing/selftests/bpf/progs/xattr.c line 30

Quoted 3 lines

tools/testing/selftests/bpf/progs/xattr.c new file +3 -0

+     xattr_sz = bpf_getxattr(bprm->file->f_path.dentry,
+                             bprm->file->f_path.dentry->d_inode, XATTR_NAME,
+                             dir_xattr_value, 64);

Yeah, this isn't right. You're not accounting for the caller's userns nor for the idmapped mount. If this is supposed to work you will need a variant of vfs_getxattr() that takes the mount's idmapping into account afaict. See what needs to happen after do_getxattr().

Thanks for taking a look.

So, If I understand correctly, we don't need xattr_permission (and other checks in
vfs_getxattr) here as the BPF programs run as CAP_SYS_ADMIN.

but...

Quoted 8 lines

So, Is this bit what's missing then?

error = vfs_getxattr(mnt_userns, d, kname, ctx->kvalue, ctx->size);

if (error > 0) {
    if ((strcmp(kname, XATTR_NAME_POSIX_ACL_ACCESS) == 0) ||
(strcmp(kname, XATTR_NAME_POSIX_ACL_DEFAULT) == 0))
        posix_acl_fix_xattr_to_user(mnt_userns, d_inode(d),
            ctx->kvalue, error);

That will not be correct. posix_acl_fix_xattr_to_user checking current_user_ns() is checking random tasks that happen to be running when lsm hook got invoked.

Quoted 8 lines

KP,
we probably have to document clearly that neither 'current*' should not be used here.
xattr_permission also makes little sense in this context. If anything it can be a different kfunc if there is a use case, but I don't see it yet.
bpf-lsm prog calling __vfs_getxattr is just like other lsm-s that call it directly. It's the kernel that is doing its security thing.

Right, but LSMs usually only retrieve their own xattr namespace (ima, selinux, smack) or they calculate hashes for xattrs based on the raw filesystem xattr values (evm).

But this new bpf_getxattr() is different. It allows to retrieve _any_ xattr in any security hook it can be attached to. So someone can write a bpf program that retrieves filesystem capabilites or posix acls. And these are xattrs that require higher-level vfs involvement to be sensible in most contexts.

So looking at:

SEC("lsm.s/bprm_committed_creds")
void BPF_PROG(bprm_cc, struct linux_binprm *bprm)

{
	struct task_struct *current = bpf_get_current_task_btf();
	char dir_xattr_value[64] = {0};
	int xattr_sz = 0;

	xattr_sz = bpf_getxattr(bprm->file->f_path.dentry,
				bprm->file->f_path.dentry->d_inode, XATTR_NAME,
				dir_xattr_value, 64);

	if (xattr_sz <= 0)
		return;

	if (!bpf_strncmp(dir_xattr_value, sizeof(XATTR_VALUE), XATTR_VALUE))
		result = 1;
}

This hooks a bpf-lsm program to the security_bprm_committed_creds() hook. It then retrieves the extended attributes of the file to be executed. The hook currently always retrieves the raw filesystem values.

But for example any XATTR_NAME_CAPS filesystem capabilities that might've been stored will be taken into account during exec. And both the idmapping of the mount and the caller matter when determing whether they are used or not.

But the current implementation of bpf_getxattr() just ignores both. It will always retrieve the raw filesystem values. So if one invokes this hook they're not actually retrieving the values as they are seen by fs/exec.c. And I'm wondering why that is ok? And even if this is ok for some use-cases it might very well become a security issue in others if access decisions are always based on the raw values.

I'm not well-versed in this so bear with me, please.

Quoted 108 lines · Christian Brauner

Quoted 52 lines · Alexei Starovoitov

Quoted 37 lines · KP Singh

Quoted 8 lines · Christian Brauner

Quoted 6 lines · KP Singh

A simple test that adds an xattr on a copied /bin/ls and reads it back when the copied ls is executed.

Signed-off-by KP Singh <[email protected]>

---
 .../testing/selftests/bpf/prog_tests/xattr.c  | 54 +++++++++++++++++++

[...]

Quoted 7 lines

Quoted 7 lines

tools/testing/selftests/bpf/progs/xattr.c new file +7 -0

+SEC("lsm.s/bprm_committed_creds")
+void BPF_PROG(bprm_cc, struct linux_binprm *bprm)
+{
+     struct task_struct *current = bpf_get_current_task_btf();
+     char dir_xattr_value[64] = {0};
+     int xattr_sz = 0;
+

Quoted 8 lines

tools/testing/selftests/bpf/progs/xattr.c line 30

Quoted 3 lines

tools/testing/selftests/bpf/progs/xattr.c new file +3 -0

+     xattr_sz = bpf_getxattr(bprm->file->f_path.dentry,
+                             bprm->file->f_path.dentry->d_inode, XATTR_NAME,
+                             dir_xattr_value, 64);

Yeah, this isn't right. You're not accounting for the caller's userns nor for the idmapped mount. If this is supposed to work you will need a variant of vfs_getxattr() that takes the mount's idmapping into account afaict. See what needs to happen after do_getxattr().

Thanks for taking a look.

So, If I understand correctly, we don't need xattr_permission (and other checks in
vfs_getxattr) here as the BPF programs run as CAP_SYS_ADMIN.

but...

Quoted 8 lines

So, Is this bit what's missing then?

error = vfs_getxattr(mnt_userns, d, kname, ctx->kvalue, ctx->size);

if (error > 0) {
    if ((strcmp(kname, XATTR_NAME_POSIX_ACL_ACCESS) == 0) ||
(strcmp(kname, XATTR_NAME_POSIX_ACL_DEFAULT) == 0))
        posix_acl_fix_xattr_to_user(mnt_userns, d_inode(d),
            ctx->kvalue, error);

That will not be correct. posix_acl_fix_xattr_to_user checking current_user_ns() is checking random tasks that happen to be running when lsm hook got invoked.

Quoted 8 lines

KP,
we probably have to document clearly that neither 'current*' should not be used here.
xattr_permission also makes little sense in this context. If anything it can be a different kfunc if there is a use case, but I don't see it yet.
bpf-lsm prog calling __vfs_getxattr is just like other lsm-s that call it directly. It's the kernel that is doing its security thing.

Right, but LSMs usually only retrieve their own xattr namespace (ima, selinux, smack) or they calculate hashes for xattrs based on the raw filesystem xattr values (evm).

But this new bpf_getxattr() is different. It allows to retrieve _any_ xattr in any security hook it can be attached to. So someone can write a bpf program that retrieves filesystem capabilites or posix acls. And these are xattrs that require higher-level vfs involvement to be sensible in most contexts.

So looking at:

SEC("lsm.s/bprm_committed_creds")
void BPF_PROG(bprm_cc, struct linux_binprm *bprm)

{
      struct task_struct *current = bpf_get_current_task_btf();
      char dir_xattr_value[64] = {0};
      int xattr_sz = 0;

      xattr_sz = bpf_getxattr(bprm->file->f_path.dentry,
                              bprm->file->f_path.dentry->d_inode, XATTR_NAME,
                              dir_xattr_value, 64);

      if (xattr_sz <= 0)
              return;

      if (!bpf_strncmp(dir_xattr_value, sizeof(XATTR_VALUE), XATTR_VALUE))
              result = 1;
}

This hooks a bpf-lsm program to the security_bprm_committed_creds() hook. It then retrieves the extended attributes of the file to be executed. The hook currently always retrieves the raw filesystem values.

But for example any XATTR_NAME_CAPS filesystem capabilities that might've been stored will be taken into account during exec. And both the idmapping of the mount and the caller matter when determing whether they are used or not.

But the current implementation of bpf_getxattr() just ignores both. It will always retrieve the raw filesystem values. So if one invokes this hook they're not actually retrieving the values as they are seen by fs/exec.c. And I'm wondering why that is ok? And even if this is ok for some use-cases it might very well become a security issue in others if access decisions are always based on the raw values.

Quoted 1 line

I'm not well-versed in this so bear with me, please.

If this is really just about retrieving the "security.bpf" xattr and no other xattr then the bpf_getxattr() variant should somehow hard-code that to ensure that no other xattrs can be retrieved, imho.

Quoted 110 lines · Christian Brauner

Quoted 108 lines · Christian Brauner

Quoted 52 lines · Alexei Starovoitov

Quoted 37 lines · KP Singh

Quoted 8 lines · Christian Brauner

Quoted 6 lines · KP Singh

A simple test that adds an xattr on a copied /bin/ls and reads it back when the copied ls is executed.

Signed-off-by KP Singh <[email protected]>

---
 .../testing/selftests/bpf/prog_tests/xattr.c  | 54 +++++++++++++++++++

[...]

Quoted 7 lines

Quoted 7 lines

(quoted diff) +7 -0

+SEC("lsm.s/bprm_committed_creds")
+void BPF_PROG(bprm_cc, struct linux_binprm *bprm)
+{
+     struct task_struct *current = bpf_get_current_task_btf();
+     char dir_xattr_value[64] = {0};
+     int xattr_sz = 0;
+

Quoted 8 lines

Review

Quoted 3 lines

(quoted diff) +3 -0

+     xattr_sz = bpf_getxattr(bprm->file->f_path.dentry,
+                             bprm->file->f_path.dentry->d_inode, XATTR_NAME,
+                             dir_xattr_value, 64);

Yeah, this isn't right. You're not accounting for the caller's userns nor for the idmapped mount. If this is supposed to work you will need a variant of vfs_getxattr() that takes the mount's idmapping into account afaict. See what needs to happen after do_getxattr().

Thanks for taking a look.

So, If I understand correctly, we don't need xattr_permission (and other checks in
vfs_getxattr) here as the BPF programs run as CAP_SYS_ADMIN.

but...

Quoted 8 lines

So, Is this bit what's missing then?

error = vfs_getxattr(mnt_userns, d, kname, ctx->kvalue, ctx->size);

if (error > 0) {
    if ((strcmp(kname, XATTR_NAME_POSIX_ACL_ACCESS) == 0) ||
(strcmp(kname, XATTR_NAME_POSIX_ACL_DEFAULT) == 0))
        posix_acl_fix_xattr_to_user(mnt_userns, d_inode(d),
            ctx->kvalue, error);

That will not be correct. posix_acl_fix_xattr_to_user checking current_user_ns() is checking random tasks that happen to be running when lsm hook got invoked.

Quoted 8 lines

KP,
we probably have to document clearly that neither 'current*' should not be used here.
xattr_permission also makes little sense in this context. If anything it can be a different kfunc if there is a use case, but I don't see it yet.
bpf-lsm prog calling __vfs_getxattr is just like other lsm-s that call it directly. It's the kernel that is doing its security thing.

Right, but LSMs usually only retrieve their own xattr namespace (ima, selinux, smack) or they calculate hashes for xattrs based on the raw filesystem xattr values (evm).

But this new bpf_getxattr() is different. It allows to retrieve _any_ xattr in any security hook it can be attached to. So someone can write a bpf program that retrieves filesystem capabilites or posix acls. And these are xattrs that require higher-level vfs involvement to be sensible in most contexts.

So looking at:

SEC("lsm.s/bprm_committed_creds")
void BPF_PROG(bprm_cc, struct linux_binprm *bprm)

{
      struct task_struct *current = bpf_get_current_task_btf();
      char dir_xattr_value[64] = {0};
      int xattr_sz = 0;

      xattr_sz = bpf_getxattr(bprm->file->f_path.dentry,
                              bprm->file->f_path.dentry->d_inode, XATTR_NAME,
                              dir_xattr_value, 64);

      if (xattr_sz <= 0)
              return;

      if (!bpf_strncmp(dir_xattr_value, sizeof(XATTR_VALUE), XATTR_VALUE))
              result = 1;
}

This hooks a bpf-lsm program to the security_bprm_committed_creds() hook. It then retrieves the extended attributes of the file to be executed. The hook currently always retrieves the raw filesystem values.

But for example any XATTR_NAME_CAPS filesystem capabilities that might've been stored will be taken into account during exec. And both the idmapping of the mount and the caller matter when determing whether they are used or not.

But the current implementation of bpf_getxattr() just ignores both. It will always retrieve the raw filesystem values. So if one invokes this hook they're not actually retrieving the values as they are seen by fs/exec.c. And I'm wondering why that is ok? And even if this is ok for some use-cases it might very well become a security issue in others if access decisions are always based on the raw values.

Quoted 5 lines

Quoted 1 line

I'm not well-versed in this so bear with me, please.

If this is really just about retrieving the "security.bpf" xattr and no other xattr then the bpf_getxattr() variant should somehow hard-code that to ensure that no other xattrs can be retrieved, imho.

All of these restrictions look very artificial to me. Especially the part "might very well become a security issue" just doesn't click.
We're talking about bpf-lsm progs here that implement security. Can somebody implement a poor bpf-lsm that doesn't enforce any actual security? Sure. It's a code.

No one complains about the usage of EXPORT_SYMBOL(__vfs_getxattr) in the existing LSMs like selinux.

No one complains about its usage in out of tree LSMs.
Is that a security issue? Of course not.
__vfs_getxattr is a kernel mechanism that LSMs use to implement the security features they need.
__vfs_getxattr as kfunc here is pretty much the same as EXPORT_SYMBOL with a big difference that it's EXPORT_SYMBOL_GPL.
BPF land doesn't have an equivalent of non-gpl export and is not going to get one.

Quoted 1 line · Alexei Starovoitov

On Wed, Jun 29, 2022 at 2:56 AM Christian Brauner <[email protected]> wrote:

Quoted 25 lines

Quoted 25 lines

Quoted 25 lines

Quoted 25 lines

Quoted 25 lines

Quoted 4 lines

Quoted 4 lines

Signed-off-by KP Singh <[email protected]>

---
 .../testing/selftests/bpf/prog_tests/xattr.c  | 54 +++++++++++++++++++

[...]

Quoted 7 lines

Quoted 7 lines

(quoted diff) +7 -0

+SEC("lsm.s/bprm_committed_creds")
+void BPF_PROG(bprm_cc, struct linux_binprm *bprm)
+{
+     struct task_struct *current = bpf_get_current_task_btf();
+     char dir_xattr_value[64] = {0};
+     int xattr_sz = 0;
+

Quoted 8 lines

Review

Quoted 3 lines

(quoted diff) +3 -0

+     xattr_sz = bpf_getxattr(bprm->file->f_path.dentry,
+                             bprm->file->f_path.dentry->d_inode, XATTR_NAME,
+                             dir_xattr_value, 64);

Yeah, this isn't right. You're not accounting for the caller's userns nor for the idmapped mount. If this is supposed to work you will need a variant of vfs_getxattr() that takes the mount's idmapping into account afaict. See what needs to happen after do_getxattr().

Thanks for taking a look.

Quoted 25 lines

Quoted 25 lines

Quoted 25 lines

Quoted 6 lines

That will not be correct. posix_acl_fix_xattr_to_user checking current_user_ns() is checking random tasks that happen to be running when lsm hook got invoked.

Quoted 8 lines

KP,
we probably have to document clearly that neither 'current*' should not be used here.
xattr_permission also makes little sense in this context. If anything it can be a different kfunc if there is a use case, but I don't see it yet.
bpf-lsm prog calling __vfs_getxattr is just like other lsm-s that call it directly. It's the kernel that is doing its security thing.

Right, but LSMs usually only retrieve their own xattr namespace (ima, selinux, smack) or they calculate hashes for xattrs based on the raw filesystem xattr values (evm).

But this new bpf_getxattr() is different. It allows to retrieve _any_ xattr in any security hook it can be attached to. So someone can write a bpf program that retrieves filesystem capabilites or posix acls. And these are xattrs that require higher-level vfs involvement to be sensible in most contexts.

Quoted 23 lines

Quoted 17 lines

Quoted 17 lines

This hooks a bpf-lsm program to the security_bprm_committed_creds() hook. It then retrieves the extended attributes of the file to be executed. The hook currently always retrieves the raw filesystem values.

But for example any XATTR_NAME_CAPS filesystem capabilities that might've been stored will be taken into account during exec. And both the idmapping of the mount and the caller matter when determing whether they are used or not.

But the current implementation of bpf_getxattr() just ignores both. It will always retrieve the raw filesystem values. So if one invokes this hook they're not actually retrieving the values as they are seen by fs/exec.c. And I'm wondering why that is ok? And even if this is ok for some use-cases it might very well become a security issue in others if access decisions are always based on the raw values.

Quoted 5 lines

Quoted 1 line

I'm not well-versed in this so bear with me, please.

If this is really just about retrieving the "security.bpf" xattr and no other xattr then the bpf_getxattr() variant should somehow hard-code that to ensure that no other xattrs can be retrieved, imho.

Quoted 6 lines

All of these restrictions look very artificial to me. Especially the part "might very well become a security issue" just doesn't click.
We're talking about bpf-lsm progs here that implement security. Can somebody implement a poor bpf-lsm that doesn't enforce any actual security? Sure. It's a code.

The point is that with the current implementation of bpf_getxattr() you are able to retrieve any xattrs and we have way less control over a bpf-lsm program than we do over selinux which a simple git grep __vfs_getxattr() is all we need.

The thing is that with bpf_getxattr() as it stands it is currently impossible to retrieve xattr values - specifically filesystem capabilities and posix acls - and see them exactly like the code you're trying to supervise is. And that seems very strange from a security perspective. So if someone were to write

SEC("lsm.s/bprm_creds_from_file")
void BPF_PROG(bprm_cc, struct linux_binprm *bprm)

{
        struct task_struct *current = bpf_get_current_task_btf();

        xattr_sz = bpf_getxattr(bprm->file->f_path.dentry,
                                bprm->file->f_path.dentry->d_inode,
                                XATTR_NAME_POSIX_ACL_ACCESS, ..);
        // or
        xattr_sz = bpf_getxattr(bprm->file->f_path.dentry,
                                bprm->file->f_path.dentry->d_inode,
                                XATTR_NAME_CAPS, ..);

}

they'd get the raw nscaps and the raw xattrs back. But now, as just a tiny example, the nscaps->rootuid and the ->e_id fields in the posix ACLs make zero sense in this context.

And what's more there's no way for the bpf-lsm program to turn them into something that makes sense in the context of the hook they are retrieved in. It lacks all the necessary helpers to do so afaict.

Quoted 2 lines

No one complains about the usage of EXPORT_SYMBOL(__vfs_getxattr) in the existing LSMs like selinux.

Selinux only cares about its own xattr namespace. It doesn't retrieve fscaps or posix acls and it's not possible to write selinux programs that do so. With the bpf-lsm that's very much possible.

And if we'd notice selinux would start retrieving random xattrs we'd ask the same questions we do here.

Quoted 8 lines

No one complains about its usage in out of tree LSMs.
Is that a security issue? Of course not.
__vfs_getxattr is a kernel mechanism that LSMs use to implement the security features they need.
__vfs_getxattr as kfunc here is pretty much the same as EXPORT_SYMBOL with a big difference that it's EXPORT_SYMBOL_GPL.
BPF land doesn't have an equivalent of non-gpl export and is not going to get one.

This discussion would probably be a lot shorter if this series were sent with a proper explanation of how this supposed to work and what it's used for.

A series without a cover letter and no detailed explanation in the commit messages makes it quite hard to understand whether what is asked can be acked or not.

I'm just adding Serge and Casey to double-check here as the LSM stuff is more up their alley. I can just look at this from the perspective of a vfs person.

If you have your eBPF meeting thing I'm also happy to jump on there next

week to get more context.

Quoted 3 lines · Christian Brauner

Quoted 1 line · Alexei Starovoitov

On Wed, Jun 29, 2022 at 2:56 AM Christian Brauner <[email protected]> wrote:

[...]

Quoted 25 lines

Quoted 25 lines

Quoted 25 lines

Quoted 25 lines

Quoted 25 lines

Quoted 25 lines

Quoted 4 lines

Quoted 4 lines

Signed-off-by KP Singh <[email protected]>

---
 .../testing/selftests/bpf/prog_tests/xattr.c  | 54 +++++++++++++++++++

[...]

Quoted 7 lines

Quoted 7 lines

(quoted diff) +7 -0

+SEC("lsm.s/bprm_committed_creds")
+void BPF_PROG(bprm_cc, struct linux_binprm *bprm)
+{
+     struct task_struct *current = bpf_get_current_task_btf();
+     char dir_xattr_value[64] = {0};
+     int xattr_sz = 0;
+

Quoted 8 lines

Review

Quoted 3 lines

(quoted diff) +3 -0

+     xattr_sz = bpf_getxattr(bprm->file->f_path.dentry,
+                             bprm->file->f_path.dentry->d_inode, XATTR_NAME,
+                             dir_xattr_value, 64);

Yeah, this isn't right. You're not accounting for the caller's userns nor for the idmapped mount. If this is supposed to work you will need a variant of vfs_getxattr() that takes the mount's idmapping into account afaict. See what needs to happen after do_getxattr().

Thanks for taking a look.

[...]

Quoted 25 lines

Quoted 25 lines

Quoted 25 lines

Quoted 25 lines

Quoted 6 lines

That will not be correct. posix_acl_fix_xattr_to_user checking current_user_ns() is checking random tasks that happen to be running when lsm hook got invoked.

Quoted 8 lines

KP,
we probably have to document clearly that neither 'current*' should not be used here.
xattr_permission also makes little sense in this context. If anything it can be a different kfunc if there is a use case, but I don't see it yet.
bpf-lsm prog calling __vfs_getxattr is just like other lsm-s that call it directly. It's the kernel that is doing its security thing.

Right, but LSMs usually only retrieve their own xattr namespace (ima, selinux, smack) or they calculate hashes for xattrs based on the raw filesystem xattr values (evm).

But this new bpf_getxattr() is different. It allows to retrieve _any_ xattr in any security hook it can be attached to. So someone can write a bpf program that retrieves filesystem capabilites or posix acls. And these are xattrs that require higher-level vfs involvement to be sensible in most contexts.

[...]

Quoted 71 lines

Quoted 23 lines

Quoted 17 lines

Quoted 17 lines

This hooks a bpf-lsm program to the security_bprm_committed_creds() hook. It then retrieves the extended attributes of the file to be executed. The hook currently always retrieves the raw filesystem values.

But for example any XATTR_NAME_CAPS filesystem capabilities that might've been stored will be taken into account during exec. And both the idmapping of the mount and the caller matter when determing whether they are used or not.

But the current implementation of bpf_getxattr() just ignores both. It will always retrieve the raw filesystem values. So if one invokes this hook they're not actually retrieving the values as they are seen by fs/exec.c. And I'm wondering why that is ok? And even if this is ok for some use-cases it might very well become a security issue in others if access decisions are always based on the raw values.

Quoted 5 lines

Quoted 1 line

I'm not well-versed in this so bear with me, please.

If this is really just about retrieving the "security.bpf" xattr and no other xattr then the bpf_getxattr() variant should somehow hard-code that to ensure that no other xattrs can be retrieved, imho.

Quoted 6 lines

All of these restrictions look very artificial to me. Especially the part "might very well become a security issue" just doesn't click.
We're talking about bpf-lsm progs here that implement security. Can somebody implement a poor bpf-lsm that doesn't enforce any actual security? Sure. It's a code.

The point is that with the current implementation of bpf_getxattr() you are able to retrieve any xattrs and we have way less control over a bpf-lsm program than we do over selinux which a simple git grep __vfs_getxattr() is all we need.

The thing is that with bpf_getxattr() as it stands it is currently impossible to retrieve xattr values - specifically filesystem capabilities and posix acls - and see them exactly like the code you're trying to supervise is. And that seems very strange from a security perspective. So if someone were to write

SEC("lsm.s/bprm_creds_from_file")
void BPF_PROG(bprm_cc, struct linux_binprm *bprm)

{
        struct task_struct *current = bpf_get_current_task_btf();

        xattr_sz = bpf_getxattr(bprm->file->f_path.dentry,
                                bprm->file->f_path.dentry->d_inode,
                                XATTR_NAME_POSIX_ACL_ACCESS, ..);
        // or
        xattr_sz = bpf_getxattr(bprm->file->f_path.dentry,
                                bprm->file->f_path.dentry->d_inode,
                                XATTR_NAME_CAPS, ..);

}

they'd get the raw nscaps and the raw xattrs back. But now, as just a tiny example, the nscaps->rootuid and the ->e_id fields in the posix ACLs make zero sense in this context.

And what's more there's no way for the bpf-lsm program to turn them into something that makes sense in the context of the hook they are retrieved in. It lacks all the necessary helpers to do so afaict.

Quoted 2 lines

No one complains about the usage of EXPORT_SYMBOL(__vfs_getxattr) in the existing LSMs like selinux.

Selinux only cares about its own xattr namespace. It doesn't retrieve fscaps or posix acls and it's not possible to write selinux programs that do so. With the bpf-lsm that's very much possible.

Quoted 8 lines

And if we'd notice selinux would start retrieving random xattrs we'd ask the same questions we do here.

Quoted 5 lines

No one complains about its usage in out of tree LSMs.
Is that a security issue? Of course not.
__vfs_getxattr is a kernel mechanism that LSMs use to implement the security features they need.
__vfs_getxattr as kfunc here is pretty much the same as EXPORT_SYMBOL

Quoted 3 lines

Quoted 3 lines

with a big difference that it's EXPORT_SYMBOL_GPL.
BPF land doesn't have an equivalent of non-gpl export and is not going to get one.

I want to reiterate what Alexei is saying here:

*Please* consider this as a simple wrapper around __vfs_getxattr with a limited attach surface and extra verification checks and and nothing else.

What you are saying is __vfs_getxattr does not make sense in some contexts. But kernel modules can still use it right?

The user is implementing an LSM, if they chose to do things that don't make sense, then they can surely cause a lot more harm:

SEC("lsm/bprm_check_security")
int BPF_PROG(bprm_check, struct linux_binprm *bprm)

{
     return -EPERM;
}

Quoted 4 lines

This discussion would probably be a lot shorter if this series were sent with a proper explanation of how this supposed to work and what it's used for.

It's currently scoped to BPF LSM (albeit limited to LSM for now) but it won't just be used in LSM programs but some (allow-listed) tracing programs too.

We want to leave the flexibility to the implementer of the LSM hooks. If the implementer choses to retrieve posix_acl_* we can also expose posix_acl_fix_xattr_to_user or a different kfunc that adds this logic too but that would be a separate kfunc (and a separate use-case).

Quoted 4 lines

A series without a cover letter and no detailed explanation in the commit messages makes it quite hard to understand whether what is asked can be acked or not.

As I mentioned in

https://lore.kernel.org/bpf/CACYkzJ70uqVJr5EnM0i03Lu+zkuSsXOXcOLQoUS6HZPqH=skpQ@mail.gmail.com/T/#m74f32bae800a97d5c2caf08cee4199d3ba48d76c

I will resend with a cover letter that has more details.

Quoted 6 lines

I'm just adding Serge and Casey to double-check here as the LSM stuff is more up their alley. I can just look at this from the perspective of a vfs person.

If you have your eBPF meeting thing I'm also happy to jump on there next

Sure, we can discuss this during BPF office hours next week.

Quoted 1 line

week to get more context.

Quoted 3 lines · Christian Brauner

Quoted 1 line · Alexei Starovoitov

On Wed, Jun 29, 2022 at 2:56 AM Christian Brauner <[email protected]> wrote:

[...]

Quoted 25 lines

Quoted 25 lines

Quoted 25 lines

Quoted 25 lines

Quoted 25 lines

Quoted 25 lines

Quoted 4 lines

Quoted 4 lines

Signed-off-by KP Singh <[email protected]>

---
 .../testing/selftests/bpf/prog_tests/xattr.c  | 54 +++++++++++++++++++

[...]

Quoted 7 lines

Quoted 7 lines

(quoted diff) +7 -0

+SEC("lsm.s/bprm_committed_creds")
+void BPF_PROG(bprm_cc, struct linux_binprm *bprm)
+{
+     struct task_struct *current = bpf_get_current_task_btf();
+     char dir_xattr_value[64] = {0};
+     int xattr_sz = 0;
+

Quoted 8 lines

Review

Quoted 3 lines

(quoted diff) +3 -0

+     xattr_sz = bpf_getxattr(bprm->file->f_path.dentry,
+                             bprm->file->f_path.dentry->d_inode, XATTR_NAME,
+                             dir_xattr_value, 64);

Yeah, this isn't right. You're not accounting for the caller's userns nor for the idmapped mount. If this is supposed to work you will need a variant of vfs_getxattr() that takes the mount's idmapping into account afaict. See what needs to happen after do_getxattr().

Thanks for taking a look.

[...]

Quoted 25 lines

Quoted 25 lines

Quoted 25 lines

Quoted 25 lines

Quoted 6 lines

That will not be correct. posix_acl_fix_xattr_to_user checking current_user_ns() is checking random tasks that happen to be running when lsm hook got invoked.

Quoted 8 lines

KP,
we probably have to document clearly that neither 'current*' should not be used here.
xattr_permission also makes little sense in this context. If anything it can be a different kfunc if there is a use case, but I don't see it yet.
bpf-lsm prog calling __vfs_getxattr is just like other lsm-s that call it directly. It's the kernel that is doing its security thing.

Right, but LSMs usually only retrieve their own xattr namespace (ima, selinux, smack) or they calculate hashes for xattrs based on the raw filesystem xattr values (evm).

But this new bpf_getxattr() is different. It allows to retrieve _any_ xattr in any security hook it can be attached to. So someone can write a bpf program that retrieves filesystem capabilites or posix acls. And these are xattrs that require higher-level vfs involvement to be sensible in most contexts.

[...]

Quoted 67 lines

Quoted 23 lines

Quoted 17 lines

Quoted 17 lines

This hooks a bpf-lsm program to the security_bprm_committed_creds() hook. It then retrieves the extended attributes of the file to be executed. The hook currently always retrieves the raw filesystem values.

But for example any XATTR_NAME_CAPS filesystem capabilities that might've been stored will be taken into account during exec. And both the idmapping of the mount and the caller matter when determing whether they are used or not.

But the current implementation of bpf_getxattr() just ignores both. It will always retrieve the raw filesystem values. So if one invokes this hook they're not actually retrieving the values as they are seen by fs/exec.c. And I'm wondering why that is ok? And even if this is ok for some use-cases it might very well become a security issue in others if access decisions are always based on the raw values.

Quoted 5 lines

Quoted 1 line

I'm not well-versed in this so bear with me, please.

If this is really just about retrieving the "security.bpf" xattr and no other xattr then the bpf_getxattr() variant should somehow hard-code that to ensure that no other xattrs can be retrieved, imho.

Quoted 6 lines

All of these restrictions look very artificial to me. Especially the part "might very well become a security issue" just doesn't click.
We're talking about bpf-lsm progs here that implement security. Can somebody implement a poor bpf-lsm that doesn't enforce any actual security? Sure. It's a code.

The point is that with the current implementation of bpf_getxattr() you are able to retrieve any xattrs and we have way less control over a bpf-lsm program than we do over selinux which a simple git grep __vfs_getxattr() is all we need.

The thing is that with bpf_getxattr() as it stands it is currently impossible to retrieve xattr values - specifically filesystem capabilities and posix acls - and see them exactly like the code you're trying to supervise is. And that seems very strange from a security perspective. So if someone were to write

SEC("lsm.s/bprm_creds_from_file")
void BPF_PROG(bprm_cc, struct linux_binprm *bprm)

{
        struct task_struct *current = bpf_get_current_task_btf();

        xattr_sz = bpf_getxattr(bprm->file->f_path.dentry,
                                bprm->file->f_path.dentry->d_inode,
                                XATTR_NAME_POSIX_ACL_ACCESS, ..);
        // or
        xattr_sz = bpf_getxattr(bprm->file->f_path.dentry,
                                bprm->file->f_path.dentry->d_inode,
                                XATTR_NAME_CAPS, ..);

}

they'd get the raw nscaps and the raw xattrs back. But now, as just a tiny example, the nscaps->rootuid and the ->e_id fields in the posix ACLs make zero sense in this context.

And what's more there's no way for the bpf-lsm program to turn them into something that makes sense in the context of the hook they are retrieved in. It lacks all the necessary helpers to do so afaict.

Quoted 2 lines

No one complains about the usage of EXPORT_SYMBOL(__vfs_getxattr) in the existing LSMs like selinux.

Quoted 15 lines

Selinux only cares about its own xattr namespace. It doesn't retrieve fscaps or posix acls and it's not possible to write selinux programs that do so. With the bpf-lsm that's very much possible.

And if we'd notice selinux would start retrieving random xattrs we'd ask the same questions we do here.

Quoted 8 lines

No one complains about its usage in out of tree LSMs.
Is that a security issue? Of course not.
__vfs_getxattr is a kernel mechanism that LSMs use to implement the security features they need.
__vfs_getxattr as kfunc here is pretty much the same as EXPORT_SYMBOL with a big difference that it's EXPORT_SYMBOL_GPL.
BPF land doesn't have an equivalent of non-gpl export and is not going to get one.

I want to reiterate what Alexei is saying here:

*Please* consider this as a simple wrapper around __vfs_getxattr with a limited attach surface and extra verification checks and and nothing else.

What you are saying is __vfs_getxattr does not make sense in some contexts. But kernel modules can still use it right?

The user is implementing an LSM, if they chose to do things that don't make sense, then they can surely cause a lot more harm:

SEC("lsm/bprm_check_security")
int BPF_PROG(bprm_check, struct linux_binprm *bprm)

{
     return -EPERM;
}

Quoted 4 lines

This discussion would probably be a lot shorter if this series were sent with a proper explanation of how this supposed to work and what it's used for.

It's currently scoped to BPF LSM (albeit limited to LSM for now) but it won't just be used in LSM programs but some (allow-listed) tracing programs too.

We want to leave the flexibility to the implementer of the LSM hooks. If the implementer choses to retrieve posix_acl_* we can also expose posix_acl_fix_xattr_to_user or a different kfunc that adds this logic too but that would be a separate kfunc (and a separate use-case).

Quoted 5 lines

A series without a cover letter and no detailed explanation in the commit messages makes it quite hard to understand whether what is asked can be acked or not.

As I mentioned in

https://lore.kernel.org/bpf/CACYkzJ70uqVJr5EnM0i03Lu+zkuSsXOXcOLQoUS6HZPqH=skpQ@mail.gmail.com/T/#m74f32bae800a97d5c2caf08cee4199d3ba48d76c

I will resend with a cover letter that has more details.

Quoted 7 lines

I'm just adding Serge and Casey to double-check here as the LSM stuff is more up their alley. I can just look at this from the perspective of a vfs person.

If you have your eBPF meeting thing I'm also happy to jump on there next

Sure, we can discuss this during BPF office hours next week.

Quoted 169 lines · KP Singh

Quoted 3 lines · Christian Brauner

Quoted 1 line · Alexei Starovoitov

On Wed, Jun 29, 2022 at 2:56 AM Christian Brauner <[email protected]> wrote:

[...]

Quoted 25 lines

Quoted 25 lines

Quoted 25 lines

Quoted 25 lines

Quoted 25 lines

Quoted 25 lines

Quoted 4 lines

Quoted 4 lines

Signed-off-by KP Singh <[email protected]>

---
 .../testing/selftests/bpf/prog_tests/xattr.c  | 54 +++++++++++++++++++

[...]

Quoted 7 lines

Quoted 7 lines

(quoted diff) +7 -0

+SEC("lsm.s/bprm_committed_creds")
+void BPF_PROG(bprm_cc, struct linux_binprm *bprm)
+{
+     struct task_struct *current = bpf_get_current_task_btf();
+     char dir_xattr_value[64] = {0};
+     int xattr_sz = 0;
+

Quoted 8 lines

Review

Quoted 3 lines

(quoted diff) +3 -0

+     xattr_sz = bpf_getxattr(bprm->file->f_path.dentry,
+                             bprm->file->f_path.dentry->d_inode, XATTR_NAME,
+                             dir_xattr_value, 64);

Yeah, this isn't right. You're not accounting for the caller's userns nor for the idmapped mount. If this is supposed to work you will need a variant of vfs_getxattr() that takes the mount's idmapping into account afaict. See what needs to happen after do_getxattr().

Thanks for taking a look.

[...]

Quoted 25 lines

Quoted 25 lines

Quoted 25 lines

Quoted 25 lines

Quoted 6 lines

That will not be correct. posix_acl_fix_xattr_to_user checking current_user_ns() is checking random tasks that happen to be running when lsm hook got invoked.

Quoted 8 lines

KP,
we probably have to document clearly that neither 'current*' should not be used here.
xattr_permission also makes little sense in this context. If anything it can be a different kfunc if there is a use case, but I don't see it yet.
bpf-lsm prog calling __vfs_getxattr is just like other lsm-s that call it directly. It's the kernel that is doing its security thing.

Right, but LSMs usually only retrieve their own xattr namespace (ima, selinux, smack) or they calculate hashes for xattrs based on the raw filesystem xattr values (evm).

But this new bpf_getxattr() is different. It allows to retrieve _any_ xattr in any security hook it can be attached to. So someone can write a bpf program that retrieves filesystem capabilites or posix acls. And these are xattrs that require higher-level vfs involvement to be sensible in most contexts.

[...]

Quoted 67 lines

Quoted 23 lines

Quoted 17 lines

Quoted 17 lines

This hooks a bpf-lsm program to the security_bprm_committed_creds() hook. It then retrieves the extended attributes of the file to be executed. The hook currently always retrieves the raw filesystem values.

But for example any XATTR_NAME_CAPS filesystem capabilities that might've been stored will be taken into account during exec. And both the idmapping of the mount and the caller matter when determing whether they are used or not.

But the current implementation of bpf_getxattr() just ignores both. It will always retrieve the raw filesystem values. So if one invokes this hook they're not actually retrieving the values as they are seen by fs/exec.c. And I'm wondering why that is ok? And even if this is ok for some use-cases it might very well become a security issue in others if access decisions are always based on the raw values.

Quoted 5 lines

Quoted 1 line

I'm not well-versed in this so bear with me, please.

If this is really just about retrieving the "security.bpf" xattr and no other xattr then the bpf_getxattr() variant should somehow hard-code that to ensure that no other xattrs can be retrieved, imho.

Quoted 6 lines

All of these restrictions look very artificial to me. Especially the part "might very well become a security issue" just doesn't click.
We're talking about bpf-lsm progs here that implement security. Can somebody implement a poor bpf-lsm that doesn't enforce any actual security? Sure. It's a code.

The point is that with the current implementation of bpf_getxattr() you are able to retrieve any xattrs and we have way less control over a bpf-lsm program than we do over selinux which a simple git grep __vfs_getxattr() is all we need.

The thing is that with bpf_getxattr() as it stands it is currently impossible to retrieve xattr values - specifically filesystem capabilities and posix acls - and see them exactly like the code you're trying to supervise is. And that seems very strange from a security perspective. So if someone were to write

SEC("lsm.s/bprm_creds_from_file")
void BPF_PROG(bprm_cc, struct linux_binprm *bprm)

{
        struct task_struct *current = bpf_get_current_task_btf();

        xattr_sz = bpf_getxattr(bprm->file->f_path.dentry,
                                bprm->file->f_path.dentry->d_inode,
                                XATTR_NAME_POSIX_ACL_ACCESS, ..);
        // or
        xattr_sz = bpf_getxattr(bprm->file->f_path.dentry,
                                bprm->file->f_path.dentry->d_inode,
                                XATTR_NAME_CAPS, ..);

}

they'd get the raw nscaps and the raw xattrs back. But now, as just a tiny example, the nscaps->rootuid and the ->e_id fields in the posix ACLs make zero sense in this context.

And what's more there's no way for the bpf-lsm program to turn them into something that makes sense in the context of the hook they are retrieved in. It lacks all the necessary helpers to do so afaict.

Quoted 2 lines

No one complains about the usage of EXPORT_SYMBOL(__vfs_getxattr) in the existing LSMs like selinux.

Quoted 15 lines

Selinux only cares about its own xattr namespace. It doesn't retrieve fscaps or posix acls and it's not possible to write selinux programs that do so. With the bpf-lsm that's very much possible.

And if we'd notice selinux would start retrieving random xattrs we'd ask the same questions we do here.

Quoted 8 lines

No one complains about its usage in out of tree LSMs.
Is that a security issue? Of course not.
__vfs_getxattr is a kernel mechanism that LSMs use to implement the security features they need.
__vfs_getxattr as kfunc here is pretty much the same as EXPORT_SYMBOL with a big difference that it's EXPORT_SYMBOL_GPL.
BPF land doesn't have an equivalent of non-gpl export and is not going to get one.

I want to reiterate what Alexei is saying here:

*Please* consider this as a simple wrapper around __vfs_getxattr with a limited attach surface and extra verification checks and and nothing else.

What you are saying is __vfs_getxattr does not make sense in some contexts. But kernel modules can still use it right?

The user is implementing an LSM, if they chose to do things that don't make sense, then they can surely cause a lot more harm:

SEC("lsm/bprm_check_security")
int BPF_PROG(bprm_check, struct linux_binprm *bprm)

{
     return -EPERM;
}

Quoted 4 lines

This discussion would probably be a lot shorter if this series were sent with a proper explanation of how this supposed to work and what it's used for.

Quoted 8 lines

It's currently scoped to BPF LSM (albeit limited to LSM for now) but it won't just be used in LSM programs but some (allow-listed) tracing programs too.

We want to leave the flexibility to the implementer of the LSM hooks. If the implementer choses to retrieve posix_acl_* we can also expose posix_acl_fix_xattr_to_user or a different kfunc that adds this logic too but that would be a separate kfunc (and a separate use-case).

No, sorry. That's what I feared and that's why I think this low-level exposure of __vfs_getxattr() is wrong:
The posix_acl_fix_xattr_*() helpers, as well as the helpers like get_file_caps() will not be exported. We're not going to export that

deeply internal vfs machinery. So I would NACK that. If you want that - and that's what I'm saying here - you need to encapsulate this into your vfs_*xattr() helper that you can call from your kfuncs.

Quoted 7 lines

Quoted 6 lines

A series without a cover letter and no detailed explanation in the commit messages makes it quite hard to understand whether what is asked can be acked or not.

Quoted 5 lines

As I mentioned in

https://lore.kernel.org/bpf/CACYkzJ70uqVJr5EnM0i03Lu+zkuSsXOXcOLQoUS6HZPqH=skpQ@mail.gmail.com/T/#m74f32bae800a97d5c2caf08cee4199d3ba48d76c

I will resend with a cover letter that has more details.

Thank you!

Quoted 9 lines

Quoted 7 lines

I'm just adding Serge and Casey to double-check here as the LSM stuff is more up their alley. I can just look at this from the perspective of a vfs person.

If you have your eBPF meeting thing I'm also happy to jump on there next

Sure, we can discuss this during BPF office hours next week.

Sounds good.

Quoted 180 lines · Christian Brauner

Quoted 169 lines · KP Singh

Quoted 3 lines · Christian Brauner

Quoted 1 line · Alexei Starovoitov

On Wed, Jun 29, 2022 at 2:56 AM Christian Brauner <[email protected]> wrote:

[...]

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Quoted 25 lines

Quoted 25 lines

Quoted 4 lines

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Signed-off-by KP Singh <[email protected]>

---
 .../testing/selftests/bpf/prog_tests/xattr.c  | 54 +++++++++++++++++++

[...]

Quoted 7 lines

Quoted 7 lines

(quoted diff) +7 -0

+SEC("lsm.s/bprm_committed_creds")
+void BPF_PROG(bprm_cc, struct linux_binprm *bprm)
+{
+     struct task_struct *current = bpf_get_current_task_btf();
+     char dir_xattr_value[64] = {0};
+     int xattr_sz = 0;
+

Quoted 8 lines

Review

Quoted 3 lines

(quoted diff) +3 -0

+     xattr_sz = bpf_getxattr(bprm->file->f_path.dentry,
+                             bprm->file->f_path.dentry->d_inode, XATTR_NAME,
+                             dir_xattr_value, 64);

Yeah, this isn't right. You're not accounting for the caller's userns nor for the idmapped mount. If this is supposed to work you will need a variant of vfs_getxattr() that takes the mount's idmapping into account afaict. See what needs to happen after do_getxattr().

Thanks for taking a look.

[...]

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That will not be correct. posix_acl_fix_xattr_to_user checking current_user_ns() is checking random tasks that happen to be running when lsm hook got invoked.

Quoted 8 lines

KP,
we probably have to document clearly that neither 'current*' should not be used here.
xattr_permission also makes little sense in this context. If anything it can be a different kfunc if there is a use case, but I don't see it yet.
bpf-lsm prog calling __vfs_getxattr is just like other lsm-s that call it directly. It's the kernel that is doing its security thing.

Right, but LSMs usually only retrieve their own xattr namespace (ima, selinux, smack) or they calculate hashes for xattrs based on the raw filesystem xattr values (evm).

But this new bpf_getxattr() is different. It allows to retrieve _any_ xattr in any security hook it can be attached to. So someone can write a bpf program that retrieves filesystem capabilites or posix acls. And these are xattrs that require higher-level vfs involvement to be sensible in most contexts.

[...]

Quoted 67 lines

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This hooks a bpf-lsm program to the security_bprm_committed_creds() hook. It then retrieves the extended attributes of the file to be executed. The hook currently always retrieves the raw filesystem values.

But for example any XATTR_NAME_CAPS filesystem capabilities that might've been stored will be taken into account during exec. And both the idmapping of the mount and the caller matter when determing whether they are used or not.

But the current implementation of bpf_getxattr() just ignores both. It will always retrieve the raw filesystem values. So if one invokes this hook they're not actually retrieving the values as they are seen by fs/exec.c. And I'm wondering why that is ok? And even if this is ok for some use-cases it might very well become a security issue in others if access decisions are always based on the raw values.

Quoted 5 lines

Quoted 1 line

I'm not well-versed in this so bear with me, please.

If this is really just about retrieving the "security.bpf" xattr and no other xattr then the bpf_getxattr() variant should somehow hard-code that to ensure that no other xattrs can be retrieved, imho.

Quoted 6 lines

All of these restrictions look very artificial to me. Especially the part "might very well become a security issue" just doesn't click.
We're talking about bpf-lsm progs here that implement security. Can somebody implement a poor bpf-lsm that doesn't enforce any actual security? Sure. It's a code.

The point is that with the current implementation of bpf_getxattr() you are able to retrieve any xattrs and we have way less control over a bpf-lsm program than we do over selinux which a simple git grep __vfs_getxattr() is all we need.

The thing is that with bpf_getxattr() as it stands it is currently impossible to retrieve xattr values - specifically filesystem capabilities and posix acls - and see them exactly like the code you're trying to supervise is. And that seems very strange from a security perspective. So if someone were to write

SEC("lsm.s/bprm_creds_from_file")
void BPF_PROG(bprm_cc, struct linux_binprm *bprm)

{
        struct task_struct *current = bpf_get_current_task_btf();

        xattr_sz = bpf_getxattr(bprm->file->f_path.dentry,
                                bprm->file->f_path.dentry->d_inode,
                                XATTR_NAME_POSIX_ACL_ACCESS, ..);
        // or
        xattr_sz = bpf_getxattr(bprm->file->f_path.dentry,
                                bprm->file->f_path.dentry->d_inode,
                                XATTR_NAME_CAPS, ..);

}

they'd get the raw nscaps and the raw xattrs back. But now, as just a tiny example, the nscaps->rootuid and the ->e_id fields in the posix ACLs make zero sense in this context.

And what's more there's no way for the bpf-lsm program to turn them into something that makes sense in the context of the hook they are retrieved in. It lacks all the necessary helpers to do so afaict.

Quoted 2 lines

No one complains about the usage of EXPORT_SYMBOL(__vfs_getxattr) in the existing LSMs like selinux.

Quoted 15 lines

Selinux only cares about its own xattr namespace. It doesn't retrieve fscaps or posix acls and it's not possible to write selinux programs that do so. With the bpf-lsm that's very much possible.

And if we'd notice selinux would start retrieving random xattrs we'd ask the same questions we do here.

Quoted 8 lines

No one complains about its usage in out of tree LSMs.
Is that a security issue? Of course not.
__vfs_getxattr is a kernel mechanism that LSMs use to implement the security features they need.
__vfs_getxattr as kfunc here is pretty much the same as EXPORT_SYMBOL with a big difference that it's EXPORT_SYMBOL_GPL.
BPF land doesn't have an equivalent of non-gpl export and is not going to get one.

I want to reiterate what Alexei is saying here:

*Please* consider this as a simple wrapper around __vfs_getxattr with a limited attach surface and extra verification checks and and nothing else.

What you are saying is __vfs_getxattr does not make sense in some contexts. But kernel modules can still use it right?

The user is implementing an LSM, if they chose to do things that don't make sense, then they can surely cause a lot more harm:

SEC("lsm/bprm_check_security")
int BPF_PROG(bprm_check, struct linux_binprm *bprm)

{
     return -EPERM;
}

Quoted 4 lines

This discussion would probably be a lot shorter if this series were sent with a proper explanation of how this supposed to work and what it's used for.

Quoted 8 lines

It's currently scoped to BPF LSM (albeit limited to LSM for now) but it won't just be used in LSM programs but some (allow-listed) tracing programs too.

We want to leave the flexibility to the implementer of the LSM hooks. If the implementer choses to retrieve posix_acl_* we can also expose posix_acl_fix_xattr_to_user or a different kfunc that adds this logic too but that would be a separate kfunc (and a separate use-case).

Quoted 4 lines

No, sorry. That's what I feared and that's why I think this low-level exposure of __vfs_getxattr() is wrong:
The posix_acl_fix_xattr_*() helpers, as well as the helpers like get_file_caps() will not be exported. We're not going to export that

I don't want to expose them and I don't want any others to be exposed either.

Quoted 3 lines

deeply internal vfs machinery. So I would NACK that. If you want that - and that's what I'm saying here - you need to encapsulate this into your vfs_*xattr() helper that you can call from your kfuncs.

It seems like __vfs_getxattr is already exposed and does the wrong thing in some contexts, why can't we just "fix" __vfs_getxattr then?

Quoted 189 lines · KP Singh

Quoted 180 lines · Christian Brauner

Quoted 169 lines · KP Singh

Quoted 3 lines · Christian Brauner

Quoted 1 line · Alexei Starovoitov

On Wed, Jun 29, 2022 at 2:56 AM Christian Brauner <[email protected]> wrote:

[...]

Quoted 25 lines

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Quoted 4 lines

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Signed-off-by KP Singh <[email protected]>

---
 .../testing/selftests/bpf/prog_tests/xattr.c  | 54 +++++++++++++++++++

[...]

Quoted 7 lines

Quoted 7 lines

(quoted diff) +7 -0

+SEC("lsm.s/bprm_committed_creds")
+void BPF_PROG(bprm_cc, struct linux_binprm *bprm)
+{
+     struct task_struct *current = bpf_get_current_task_btf();
+     char dir_xattr_value[64] = {0};
+     int xattr_sz = 0;
+

Quoted 8 lines

Review

Quoted 3 lines

(quoted diff) +3 -0

+     xattr_sz = bpf_getxattr(bprm->file->f_path.dentry,
+                             bprm->file->f_path.dentry->d_inode, XATTR_NAME,
+                             dir_xattr_value, 64);

Yeah, this isn't right. You're not accounting for the caller's userns nor for the idmapped mount. If this is supposed to work you will need a variant of vfs_getxattr() that takes the mount's idmapping into account afaict. See what needs to happen after do_getxattr().

Thanks for taking a look.

[...]

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That will not be correct. posix_acl_fix_xattr_to_user checking current_user_ns() is checking random tasks that happen to be running when lsm hook got invoked.

Quoted 8 lines

KP,
we probably have to document clearly that neither 'current*' should not be used here.
xattr_permission also makes little sense in this context. If anything it can be a different kfunc if there is a use case, but I don't see it yet.
bpf-lsm prog calling __vfs_getxattr is just like other lsm-s that call it directly. It's the kernel that is doing its security thing.

Right, but LSMs usually only retrieve their own xattr namespace (ima, selinux, smack) or they calculate hashes for xattrs based on the raw filesystem xattr values (evm).

But this new bpf_getxattr() is different. It allows to retrieve _any_ xattr in any security hook it can be attached to. So someone can write a bpf program that retrieves filesystem capabilites or posix acls. And these are xattrs that require higher-level vfs involvement to be sensible in most contexts.

[...]

Quoted 67 lines

Quoted 23 lines

Quoted 17 lines

Quoted 17 lines

This hooks a bpf-lsm program to the security_bprm_committed_creds() hook. It then retrieves the extended attributes of the file to be executed. The hook currently always retrieves the raw filesystem values.

But for example any XATTR_NAME_CAPS filesystem capabilities that might've been stored will be taken into account during exec. And both the idmapping of the mount and the caller matter when determing whether they are used or not.

But the current implementation of bpf_getxattr() just ignores both. It will always retrieve the raw filesystem values. So if one invokes this hook they're not actually retrieving the values as they are seen by fs/exec.c. And I'm wondering why that is ok? And even if this is ok for some use-cases it might very well become a security issue in others if access decisions are always based on the raw values.

Quoted 5 lines

Quoted 1 line

I'm not well-versed in this so bear with me, please.

If this is really just about retrieving the "security.bpf" xattr and no other xattr then the bpf_getxattr() variant should somehow hard-code that to ensure that no other xattrs can be retrieved, imho.

Quoted 6 lines

All of these restrictions look very artificial to me. Especially the part "might very well become a security issue" just doesn't click.
We're talking about bpf-lsm progs here that implement security. Can somebody implement a poor bpf-lsm that doesn't enforce any actual security? Sure. It's a code.

The point is that with the current implementation of bpf_getxattr() you are able to retrieve any xattrs and we have way less control over a bpf-lsm program than we do over selinux which a simple git grep __vfs_getxattr() is all we need.

The thing is that with bpf_getxattr() as it stands it is currently impossible to retrieve xattr values - specifically filesystem capabilities and posix acls - and see them exactly like the code you're trying to supervise is. And that seems very strange from a security perspective. So if someone were to write

SEC("lsm.s/bprm_creds_from_file")
void BPF_PROG(bprm_cc, struct linux_binprm *bprm)

{
        struct task_struct *current = bpf_get_current_task_btf();

        xattr_sz = bpf_getxattr(bprm->file->f_path.dentry,
                                bprm->file->f_path.dentry->d_inode,
                                XATTR_NAME_POSIX_ACL_ACCESS, ..);
        // or
        xattr_sz = bpf_getxattr(bprm->file->f_path.dentry,
                                bprm->file->f_path.dentry->d_inode,
                                XATTR_NAME_CAPS, ..);

}

they'd get the raw nscaps and the raw xattrs back. But now, as just a tiny example, the nscaps->rootuid and the ->e_id fields in the posix ACLs make zero sense in this context.

And what's more there's no way for the bpf-lsm program to turn them into something that makes sense in the context of the hook they are retrieved in. It lacks all the necessary helpers to do so afaict.

Quoted 2 lines

No one complains about the usage of EXPORT_SYMBOL(__vfs_getxattr) in the existing LSMs like selinux.

Quoted 15 lines

Selinux only cares about its own xattr namespace. It doesn't retrieve fscaps or posix acls and it's not possible to write selinux programs that do so. With the bpf-lsm that's very much possible.

And if we'd notice selinux would start retrieving random xattrs we'd ask the same questions we do here.

Quoted 8 lines

No one complains about its usage in out of tree LSMs.
Is that a security issue? Of course not.
__vfs_getxattr is a kernel mechanism that LSMs use to implement the security features they need.
__vfs_getxattr as kfunc here is pretty much the same as EXPORT_SYMBOL with a big difference that it's EXPORT_SYMBOL_GPL.
BPF land doesn't have an equivalent of non-gpl export and is not going to get one.

I want to reiterate what Alexei is saying here:

*Please* consider this as a simple wrapper around __vfs_getxattr with a limited attach surface and extra verification checks and and nothing else.

What you are saying is __vfs_getxattr does not make sense in some contexts. But kernel modules can still use it right?

The user is implementing an LSM, if they chose to do things that don't make sense, then they can surely cause a lot more harm:

SEC("lsm/bprm_check_security")
int BPF_PROG(bprm_check, struct linux_binprm *bprm)

{
     return -EPERM;
}

Quoted 4 lines

This discussion would probably be a lot shorter if this series were sent with a proper explanation of how this supposed to work and what it's used for.

Quoted 8 lines

It's currently scoped to BPF LSM (albeit limited to LSM for now) but it won't just be used in LSM programs but some (allow-listed) tracing programs too.

We want to leave the flexibility to the implementer of the LSM hooks. If the implementer choses to retrieve posix_acl_* we can also expose posix_acl_fix_xattr_to_user or a different kfunc that adds this logic too but that would be a separate kfunc (and a separate use-case).

Quoted 4 lines

No, sorry. That's what I feared and that's why I think this low-level exposure of __vfs_getxattr() is wrong:
The posix_acl_fix_xattr_*() helpers, as well as the helpers like get_file_caps() will not be exported. We're not going to export that

I don't want to expose them and I don't want any others to be exposed either.

Quoted 6 lines

Quoted 3 lines

deeply internal vfs machinery. So I would NACK that. If you want that - and that's what I'm saying here - you need to encapsulate this into your vfs_*xattr() helper that you can call from your kfuncs.

It seems like __vfs_getxattr is already exposed and does the wrong thing in some contexts, why can't we just "fix" __vfs_getxattr then?

To me having either a version of bpf_getxattr() that restricts access to certain xattrs or a version that takes care to perform the neccesary translations is what seems to make the most sense. I suggested that in one of my first mails.

The one thing where the way the xattrs are retrieved really matters is for vfscaps (see get_vfs_caps_from_disk()) you really need something like that function in order for vfs caps to make any sense and be interpretable by the user of the hook.

But again, I might just misunderstand the context here and for the bpf-lsm all of this isn't really a concern. If your new series comes out I'll try to get more into the wider context.
If the security folks are happy with this then I won't argue.

Quoted 165 lines · KP Singh

Quoted 156 lines · Christian Brauner

Quoted 155 lines · KP Singh

Quoted 2 lines · Christian Brauner

Quoted 1 line · Alexei Starovoitov

On Wed, Jun 29, 2022 at 2:56 AM Christian Brauner <[email protected]> wrote:

[...]

Quoted 21 lines

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Quoted 3 lines

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Signed-off-by KP Singh <[email protected]>

---
 .../testing/selftests/bpf/prog_tests/xattr.c  | 54 +++++++++++++++++++

[...]

Quoted 6 lines

Quoted 6 lines

(quoted diff) +6 -0

+SEC("lsm.s/bprm_committed_creds")
+void BPF_PROG(bprm_cc, struct linux_binprm *bprm)
+{
+     struct task_struct *current = bpf_get_current_task_btf();
+     char dir_xattr_value[64] = {0};
+     int xattr_sz = 0;

Quoted 8 lines

Review

Quoted 4 lines

(quoted diff) +4 -0

+
+     xattr_sz = bpf_getxattr(bprm->file->f_path.dentry,
+                             bprm->file->f_path.dentry->d_inode, XATTR_NAME,
+                             dir_xattr_value, 64);

Yeah, this isn't right. You're not accounting for the caller's userns nor for the idmapped mount. If this is supposed to work you will need a variant of vfs_getxattr() that takes the mount's idmapping into account afaict. See what needs to happen after do_getxattr().

Thanks for taking a look.

[...]

Quoted 23 lines

Quoted 23 lines

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Quoted 5 lines

That will not be correct. posix_acl_fix_xattr_to_user checking current_user_ns() is checking random tasks that happen to be running when lsm hook got invoked.

Quoted 8 lines

KP,
we probably have to document clearly that neither 'current*' should not be used here.
xattr_permission also makes little sense in this context. If anything it can be a different kfunc if there is a use case, but I don't see it yet.
bpf-lsm prog calling __vfs_getxattr is just like other lsm-s that call it directly. It's the kernel that is doing its security thing.

Right, but LSMs usually only retrieve their own xattr namespace (ima, selinux, smack) or they calculate hashes for xattrs based on the raw filesystem xattr values (evm).

But this new bpf_getxattr() is different. It allows to retrieve _any_ xattr in any security hook it can be attached to. So someone can write a bpf program that retrieves filesystem capabilites or posix acls. And these are xattrs that require higher-level vfs involvement to be sensible in most contexts.

[...]

Quoted 60 lines

Quoted 20 lines

Quoted 16 lines

Quoted 16 lines

This hooks a bpf-lsm program to the security_bprm_committed_creds() hook. It then retrieves the extended attributes of the file to be executed. The hook currently always retrieves the raw filesystem values.

But for example any XATTR_NAME_CAPS filesystem capabilities that might've been stored will be taken into account during exec. And both the idmapping of the mount and the caller matter when determing whether they are used or not.

But the current implementation of bpf_getxattr() just ignores both. It will always retrieve the raw filesystem values. So if one invokes this hook they're not actually retrieving the values as they are seen by fs/exec.c. And I'm wondering why that is ok? And even if this is ok for some use-cases it might very well become a security issue in others if access decisions are always based on the raw values.

Quoted 4 lines

Quoted 1 line

I'm not well-versed in this so bear with me, please.

If this is really just about retrieving the "security.bpf" xattr and no other xattr then the bpf_getxattr() variant should somehow hard-code that to ensure that no other xattrs can be retrieved, imho.

Quoted 6 lines

All of these restrictions look very artificial to me. Especially the part "might very well become a security issue" just doesn't click.
We're talking about bpf-lsm progs here that implement security. Can somebody implement a poor bpf-lsm that doesn't enforce any actual security? Sure. It's a code.

The point is that with the current implementation of bpf_getxattr() you are able to retrieve any xattrs and we have way less control over a bpf-lsm program than we do over selinux which a simple git grep __vfs_getxattr() is all we need.

The thing is that with bpf_getxattr() as it stands it is currently impossible to retrieve xattr values - specifically filesystem capabilities and posix acls - and see them exactly like the code you're trying to supervise is. And that seems very strange from a security perspective. So if someone were to write

SEC("lsm.s/bprm_creds_from_file")
void BPF_PROG(bprm_cc, struct linux_binprm *bprm)

{
        struct task_struct *current = bpf_get_current_task_btf();

        xattr_sz = bpf_getxattr(bprm->file->f_path.dentry,
                                bprm->file->f_path.dentry->d_inode,
                                XATTR_NAME_POSIX_ACL_ACCESS, ..);
        // or
        xattr_sz = bpf_getxattr(bprm->file->f_path.dentry,
                                bprm->file->f_path.dentry->d_inode,
                                XATTR_NAME_CAPS, ..);

}

they'd get the raw nscaps and the raw xattrs back. But now, as just a tiny example, the nscaps->rootuid and the ->e_id fields in the posix ACLs make zero sense in this context.

And what's more there's no way for the bpf-lsm program to turn them into something that makes sense in the context of the hook they are retrieved in. It lacks all the necessary helpers to do so afaict.

Quoted 17 lines

Quoted 2 lines

No one complains about the usage of EXPORT_SYMBOL(__vfs_getxattr) in the existing LSMs like selinux.

Selinux only cares about its own xattr namespace. It doesn't retrieve fscaps or posix acls and it's not possible to write selinux programs that do so. With the bpf-lsm that's very much possible.

And if we'd notice selinux would start retrieving random xattrs we'd ask the same questions we do here.

Quoted 8 lines

No one complains about its usage in out of tree LSMs.
Is that a security issue? Of course not.
__vfs_getxattr is a kernel mechanism that LSMs use to implement the security features they need.
__vfs_getxattr as kfunc here is pretty much the same as EXPORT_SYMBOL with a big difference that it's EXPORT_SYMBOL_GPL.
BPF land doesn't have an equivalent of non-gpl export and is not going to get one.

I want to reiterate what Alexei is saying here:

*Please* consider this as a simple wrapper around __vfs_getxattr with a limited attach surface and extra verification checks and and nothing else.

What you are saying is __vfs_getxattr does not make sense in some contexts. But kernel modules can still use it right?

The user is implementing an LSM, if they chose to do things that don't make sense, then they can surely cause a lot more harm:

SEC("lsm/bprm_check_security")
int BPF_PROG(bprm_check, struct linux_binprm *bprm)

{
     return -EPERM;
}

Quoted 3 lines

This discussion would probably be a lot shorter if this series were sent with a proper explanation of how this supposed to work and what it's used for.

It's currently scoped to BPF LSM (albeit limited to LSM for now) but it won't just be used in LSM programs but some (allow-listed) tracing programs too.

Quoted 8 lines

Quoted 4 lines

We want to leave the flexibility to the implementer of the LSM hooks. If the implementer choses to retrieve posix_acl_* we can also expose posix_acl_fix_xattr_to_user or a different kfunc that adds this logic too but that would be a separate kfunc (and a separate use-case).

No, sorry. That's what I feared and that's why I think this low-level exposure of __vfs_getxattr() is wrong:
The posix_acl_fix_xattr_*() helpers, as well as the helpers like get_file_caps() will not be exported. We're not going to export that

Quoted 8 lines

I don't want to expose them and I don't want any others to be exposed either.

Quoted 3 lines

deeply internal vfs machinery. So I would NACK that. If you want that - and that's what I'm saying here - you need to encapsulate this into your vfs_*xattr() helper that you can call from your kfuncs.

It seems like __vfs_getxattr is already exposed and does the wrong thing in some contexts, why can't we just "fix" __vfs_getxattr then?

To me having either a version of bpf_getxattr() that restricts access to certain xattrs or a version that takes care to perform the neccesary translations is what seems to make the most sense. I suggested that in one of my first mails.

The one thing where the way the xattrs are retrieved really matters is for vfscaps (see get_vfs_caps_from_disk()) you really need something like that function in order for vfs caps to make any sense and be interpretable by the user of the hook.

But again, I might just misunderstand the context here and for the bpf-lsm all of this isn't really a concern. If your new series comes out I'll try to get more into the wider context.
If the security folks are happy with this then I won't argue.

Quoted 184 lines · Christian Brauner

Quoted 165 lines · KP Singh

Quoted 156 lines · Christian Brauner

Quoted 155 lines · KP Singh

Quoted 2 lines · Christian Brauner

Quoted 1 line · Alexei Starovoitov

On Wed, Jun 29, 2022 at 2:56 AM Christian Brauner <[email protected]> wrote:

[...]

Quoted 21 lines

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Quoted 21 lines

Quoted 21 lines

Quoted 21 lines

Quoted 21 lines

Quoted 3 lines

Quoted 3 lines

Signed-off-by: KP Singh <[email protected]>
---
.../testing/selftests/bpf/prog_tests/xattr.c | 54 +++++++++++++++++++

[...]

Quoted 6 lines

Quoted 6 lines

+SEC("lsm.s/bprm_committed_creds")
+void BPF_PROG(bprm_cc, struct linux_binprm *bprm)
+{
+ struct task_struct *current = bpf_get_current_task_btf();
+ char dir_xattr_value[64] = {0};
+ int xattr_sz = 0;

Quoted 8 lines

Quoted 4 lines

+
+ xattr_sz = bpf_getxattr(bprm->file->f_path.dentry,
+ bprm->file->f_path.dentry->d_inode, XATTR_NAME,
+ dir_xattr_value, 64);

Yeah, this isn't right. You're not accounting for the caller's userns nor for the idmapped mount. If this is supposed to work you will need a variant of vfs_getxattr() that takes the mount's idmapping into account afaict. See what needs to happen after do_getxattr().

Thanks for taking a look.

[...]

Quoted 23 lines

Quoted 23 lines

Quoted 23 lines

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That will not be correct. posix_acl_fix_xattr_to_user checking current_user_ns() is checking random tasks that happen to be running when lsm hook got invoked.

Quoted 8 lines

KP,
we probably have to document clearly that neither 'current*' should not be used here.
xattr_permission also makes little sense in this context. If anything it can be a different kfunc if there is a use case, but I don't see it yet.
bpf-lsm prog calling __vfs_getxattr is just like other lsm-s that call it directly. It's the kernel that is doing its security thing.

Right, but LSMs usually only retrieve their own xattr namespace (ima, selinux, smack) or they calculate hashes for xattrs based on the raw filesystem xattr values (evm).

But this new bpf_getxattr() is different. It allows to retrieve _any_ xattr in any security hook it can be attached to. So someone can write a bpf program that retrieves filesystem capabilites or posix acls. And these are xattrs that require higher-level vfs involvement to be sensible in most contexts.

[...]

Quoted 60 lines

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Quoted 16 lines

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This hooks a bpf-lsm program to the security_bprm_committed_creds() hook. It then retrieves the extended attributes of the file to be executed. The hook currently always retrieves the raw filesystem values.

But for example any XATTR_NAME_CAPS filesystem capabilities that might've been stored will be taken into account during exec. And both the idmapping of the mount and the caller matter when determing whether they are used or not.

But the current implementation of bpf_getxattr() just ignores both. It will always retrieve the raw filesystem values. So if one invokes this hook they're not actually retrieving the values as they are seen by fs/exec.c. And I'm wondering why that is ok? And even if this is ok for some use-cases it might very well become a security issue in others if access decisions are always based on the raw values.

Quoted 4 lines

Quoted 1 line

I'm not well-versed in this so bear with me, please.

If this is really just about retrieving the "security.bpf" xattr and no other xattr then the bpf_getxattr() variant should somehow hard-code that to ensure that no other xattrs can be retrieved, imho.

Quoted 6 lines

All of these restrictions look very artificial to me. Especially the part "might very well become a security issue" just doesn't click.
We're talking about bpf-lsm progs here that implement security. Can somebody implement a poor bpf-lsm that doesn't enforce any actual security? Sure. It's a code.

The point is that with the current implementation of bpf_getxattr() you are able to retrieve any xattrs and we have way less control over a bpf-lsm program than we do over selinux which a simple git grep __vfs_getxattr() is all we need.

The thing is that with bpf_getxattr() as it stands it is currently impossible to retrieve xattr values - specifically filesystem capabilities and posix acls - and see them exactly like the code you're trying to supervise is. And that seems very strange from a security perspective. So if someone were to write

SEC("lsm.s/bprm_creds_from_file")
void BPF_PROG(bprm_cc, struct linux_binprm *bprm)

{
        struct task_struct *current = bpf_get_current_task_btf();

        xattr_sz = bpf_getxattr(bprm->file->f_path.dentry,
                                bprm->file->f_path.dentry->d_inode,
                                XATTR_NAME_POSIX_ACL_ACCESS, ..);
        // or
        xattr_sz = bpf_getxattr(bprm->file->f_path.dentry,
                                bprm->file->f_path.dentry->d_inode,
                                XATTR_NAME_CAPS, ..);

}

they'd get the raw nscaps and the raw xattrs back. But now, as just a tiny example, the nscaps->rootuid and the ->e_id fields in the posix ACLs make zero sense in this context.

And what's more there's no way for the bpf-lsm program to turn them into something that makes sense in the context of the hook they are retrieved in. It lacks all the necessary helpers to do so afaict.

Quoted 17 lines

Quoted 2 lines

No one complains about the usage of EXPORT_SYMBOL(__vfs_getxattr) in the existing LSMs like selinux.

Selinux only cares about its own xattr namespace. It doesn't retrieve fscaps or posix acls and it's not possible to write selinux programs that do so. With the bpf-lsm that's very much possible.

And if we'd notice selinux would start retrieving random xattrs we'd ask the same questions we do here.

Quoted 8 lines

No one complains about its usage in out of tree LSMs.
Is that a security issue? Of course not.
__vfs_getxattr is a kernel mechanism that LSMs use to implement the security features they need.
__vfs_getxattr as kfunc here is pretty much the same as EXPORT_SYMBOL with a big difference that it's EXPORT_SYMBOL_GPL.
BPF land doesn't have an equivalent of non-gpl export and is not going to get one.

I want to reiterate what Alexei is saying here:

*Please* consider this as a simple wrapper around __vfs_getxattr with a limited attach surface and extra verification checks and and nothing else.

What you are saying is __vfs_getxattr does not make sense in some contexts. But kernel modules can still use it right?

The user is implementing an LSM, if they chose to do things that don't make sense, then they can surely cause a lot more harm:

SEC("lsm/bprm_check_security")
int BPF_PROG(bprm_check, struct linux_binprm *bprm)

{
     return -EPERM;
}

Quoted 3 lines

This discussion would probably be a lot shorter if this series were sent with a proper explanation of how this supposed to work and what it's used for.

It's currently scoped to BPF LSM (albeit limited to LSM for now) but it won't just be used in LSM programs but some (allow-listed) tracing programs too.

Quoted 8 lines

Quoted 4 lines

We want to leave the flexibility to the implementer of the LSM hooks. If the implementer choses to retrieve posix_acl_* we can also expose posix_acl_fix_xattr_to_user or a different kfunc that adds this logic too but that would be a separate kfunc (and a separate use-case).

No, sorry. That's what I feared and that's why I think this low-level exposure of __vfs_getxattr() is wrong:
The posix_acl_fix_xattr_*() helpers, as well as the helpers like get_file_caps() will not be exported. We're not going to export that

Quoted 8 lines

I don't want to expose them and I don't want any others to be exposed either.

Quoted 3 lines

deeply internal vfs machinery. So I would NACK that. If you want that - and that's what I'm saying here - you need to encapsulate this into your vfs_*xattr() helper that you can call from your kfuncs.

It seems like __vfs_getxattr is already exposed and does the wrong thing in some contexts, why can't we just "fix" __vfs_getxattr then?

To me having either a version of bpf_getxattr() that restricts access to certain xattrs or a version that takes care to perform the neccesary translations is what seems to make the most sense. I suggested that in one of my first mails.

The one thing where the way the xattrs are retrieved really matters is for vfscaps (see get_vfs_caps_from_disk()) you really need something like that function in order for vfs caps to make any sense and be interpretable by the user of the hook.

Quoted 4 lines

But again, I might just misunderstand the context here and for the bpf-lsm all of this isn't really a concern. If your new series comes out I'll try to get more into the wider context.
If the security folks are happy with this then I won't argue.

A security module (BPF) using another security module's (Smack) xattrs without that module's (Smack) explicit approval would be considered extremely rude. Smack and SELinux use published interfaces of the capability security module, but never access the capability attributes directly. The details of a security module's implementation are not a factor. The fact that BPF uses loadable programs as opposed to loadable policy is not relevant. The only security.xattr values that the BPF security module should allow the programs it runs to access are the ones it is managing. If you decided to create an eBPF

implementation of SELinux you would still have to use attributes specific to the BPF security module. If, on the other hand, you wanted to extend Smack using eBPF programs, and the Smack maintainer liked the idea, it would be OK for the BPF security module to access some of the security.SMACK64 attributes.

I want it to be clear that BPF is a Linux Security Module (LSM) and a collection of eBPF programs is *not* an LSM. BPF is responsible for being a good kernel citizen, and must ensure that it does not allow a set of configuration data that violates proper behavior. You can't write an SELinux policy that monster-mashes an ACL. You can't allow BPF to permit that either. You can't count on the good intentions, wisdom or skill of the author of an unreviewed, out of tree, eBPF program. I believe that this was understood during the review process of the BPF LSM.

Quoted 189 lines · Casey Schaufler

Quoted 184 lines · Christian Brauner

Quoted 165 lines · KP Singh

Quoted 156 lines · Christian Brauner

Quoted 155 lines · KP Singh

Quoted 2 lines · Christian Brauner

Quoted 1 line · Alexei Starovoitov

On Wed, Jun 29, 2022 at 2:56 AM Christian Brauner <[email protected]> wrote:

[...]

Quoted 21 lines

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Quoted 21 lines

Quoted 21 lines

Quoted 21 lines

Quoted 3 lines

Signed-off-by: KP Singh <[email protected]>
---
.../testing/selftests/bpf/prog_tests/xattr.c | 54 +++++++++++++++++++

[...]

Quoted 14 lines

+SEC("lsm.s/bprm_committed_creds")
+void BPF_PROG(bprm_cc, struct linux_binprm *bprm)
+{
+ struct task_struct *current = bpf_get_current_task_btf();
+ char dir_xattr_value[64] = {0};
+ int xattr_sz = 0;
+
+ xattr_sz = bpf_getxattr(bprm->file->f_path.dentry,
+ bprm->file->f_path.dentry->d_inode, XATTR_NAME,
+ dir_xattr_value, 64);
Yeah, this isn't right. You're not accounting for the caller's userns
nor for the idmapped mount. If this is supposed to work you will need a
variant of vfs_getxattr() that takes the mount's idmapping into account
afaict. See what needs to happen after do_getxattr().

Thanks for taking a look.

[...]

Quoted 23 lines

Quoted 23 lines

Quoted 23 lines

Quoted 23 lines

Quoted 5 lines

That will not be correct. posix_acl_fix_xattr_to_user checking current_user_ns() is checking random tasks that happen to be running when lsm hook got invoked.

Quoted 8 lines

KP,
we probably have to document clearly that neither 'current*' should not be used here.
xattr_permission also makes little sense in this context. If anything it can be a different kfunc if there is a use case, but I don't see it yet.
bpf-lsm prog calling __vfs_getxattr is just like other lsm-s that call it directly. It's the kernel that is doing its security thing.

Right, but LSMs usually only retrieve their own xattr namespace (ima, selinux, smack) or they calculate hashes for xattrs based on the raw filesystem xattr values (evm).

But this new bpf_getxattr() is different. It allows to retrieve _any_ xattr in any security hook it can be attached to. So someone can write a bpf program that retrieves filesystem capabilites or posix acls. And these are xattrs that require higher-level vfs involvement to be sensible in most contexts.

[...]

Quoted 60 lines

Quoted 20 lines

Quoted 16 lines

Quoted 16 lines

This hooks a bpf-lsm program to the security_bprm_committed_creds() hook. It then retrieves the extended attributes of the file to be executed. The hook currently always retrieves the raw filesystem values.

But for example any XATTR_NAME_CAPS filesystem capabilities that might've been stored will be taken into account during exec. And both the idmapping of the mount and the caller matter when determing whether they are used or not.

But the current implementation of bpf_getxattr() just ignores both. It will always retrieve the raw filesystem values. So if one invokes this hook they're not actually retrieving the values as they are seen by fs/exec.c. And I'm wondering why that is ok? And even if this is ok for some use-cases it might very well become a security issue in others if access decisions are always based on the raw values.

Quoted 4 lines

Quoted 1 line

I'm not well-versed in this so bear with me, please.

If this is really just about retrieving the "security.bpf" xattr and no other xattr then the bpf_getxattr() variant should somehow hard-code that to ensure that no other xattrs can be retrieved, imho.

Quoted 6 lines

All of these restrictions look very artificial to me. Especially the part "might very well become a security issue" just doesn't click.
We're talking about bpf-lsm progs here that implement security. Can somebody implement a poor bpf-lsm that doesn't enforce any actual security? Sure. It's a code.

The point is that with the current implementation of bpf_getxattr() you are able to retrieve any xattrs and we have way less control over a bpf-lsm program than we do over selinux which a simple git grep __vfs_getxattr() is all we need.

The thing is that with bpf_getxattr() as it stands it is currently impossible to retrieve xattr values - specifically filesystem capabilities and posix acls - and see them exactly like the code you're trying to supervise is. And that seems very strange from a security perspective. So if someone were to write

SEC("lsm.s/bprm_creds_from_file")
void BPF_PROG(bprm_cc, struct linux_binprm *bprm)

{
        struct task_struct *current = bpf_get_current_task_btf();

        xattr_sz = bpf_getxattr(bprm->file->f_path.dentry,
                                bprm->file->f_path.dentry->d_inode,
                                XATTR_NAME_POSIX_ACL_ACCESS, ..);
        // or
        xattr_sz = bpf_getxattr(bprm->file->f_path.dentry,
                                bprm->file->f_path.dentry->d_inode,
                                XATTR_NAME_CAPS, ..);

}

they'd get the raw nscaps and the raw xattrs back. But now, as just a tiny example, the nscaps->rootuid and the ->e_id fields in the posix ACLs make zero sense in this context.

And what's more there's no way for the bpf-lsm program to turn them into something that makes sense in the context of the hook they are retrieved in. It lacks all the necessary helpers to do so afaict.

Quoted 17 lines

Quoted 2 lines

No one complains about the usage of EXPORT_SYMBOL(__vfs_getxattr) in the existing LSMs like selinux.

Selinux only cares about its own xattr namespace. It doesn't retrieve fscaps or posix acls and it's not possible to write selinux programs that do so. With the bpf-lsm that's very much possible.

And if we'd notice selinux would start retrieving random xattrs we'd ask the same questions we do here.

Quoted 8 lines

No one complains about its usage in out of tree LSMs.
Is that a security issue? Of course not.
__vfs_getxattr is a kernel mechanism that LSMs use to implement the security features they need.
__vfs_getxattr as kfunc here is pretty much the same as EXPORT_SYMBOL with a big difference that it's EXPORT_SYMBOL_GPL.
BPF land doesn't have an equivalent of non-gpl export and is not going to get one.

I want to reiterate what Alexei is saying here:

*Please* consider this as a simple wrapper around __vfs_getxattr with a limited attach surface and extra verification checks and and nothing else.

What you are saying is __vfs_getxattr does not make sense in some contexts. But kernel modules can still use it right?

The user is implementing an LSM, if they chose to do things that don't make sense, then they can surely cause a lot more harm:

SEC("lsm/bprm_check_security")
int BPF_PROG(bprm_check, struct linux_binprm *bprm)

{
     return -EPERM;
}

Quoted 3 lines

This discussion would probably be a lot shorter if this series were sent with a proper explanation of how this supposed to work and what it's used for.

It's currently scoped to BPF LSM (albeit limited to LSM for now) but it won't just be used in LSM programs but some (allow-listed) tracing programs too.

Quoted 8 lines

Quoted 4 lines

We want to leave the flexibility to the implementer of the LSM hooks. If the implementer choses to retrieve posix_acl_* we can also expose posix_acl_fix_xattr_to_user or a different kfunc that adds this logic too but that would be a separate kfunc (and a separate use-case).

No, sorry. That's what I feared and that's why I think this low-level exposure of __vfs_getxattr() is wrong:
The posix_acl_fix_xattr_*() helpers, as well as the helpers like get_file_caps() will not be exported. We're not going to export that

Quoted 8 lines

I don't want to expose them and I don't want any others to be exposed either.

Quoted 3 lines

deeply internal vfs machinery. So I would NACK that. If you want that - and that's what I'm saying here - you need to encapsulate this into your vfs_*xattr() helper that you can call from your kfuncs.

It seems like __vfs_getxattr is already exposed and does the wrong thing in some contexts, why can't we just "fix" __vfs_getxattr then?

To me having either a version of bpf_getxattr() that restricts access to certain xattrs or a version that takes care to perform the neccesary translations is what seems to make the most sense. I suggested that in one of my first mails.

The one thing where the way the xattrs are retrieved really matters is for vfscaps (see get_vfs_caps_from_disk()) you really need something like that function in order for vfs caps to make any sense and be interpretable by the user of the hook.

Quoted 4 lines

But again, I might just misunderstand the context here and for the bpf-lsm all of this isn't really a concern. If your new series comes out I'll try to get more into the wider context.
If the security folks are happy with this then I won't argue.

Quoted 9 lines

A security module (BPF) using another security module's (Smack) xattrs without that module's (Smack) explicit approval would be considered extremely rude. Smack and SELinux use published interfaces of the capability security module, but never access the capability attributes directly. The details of a security module's implementation are not a factor. The fact that BPF uses loadable programs as opposed to loadable policy is not relevant. The only security.xattr values that the BPF security module should allow the programs it runs to access are the ones it is managing. If you decided to create an eBPF

What about kernel modules who can use __vfs_getxattr already as it's an exported symbol? This can still end up influencing security policy or using them in any way they like.

Anyways, I think, for now, for the use case we have, it can work with a restriction to security.bpf xattrs.

Quoted 199 lines · KP Singh

Quoted 189 lines · Casey Schaufler

Quoted 184 lines · Christian Brauner

Quoted 165 lines · KP Singh

Quoted 156 lines · Christian Brauner

Quoted 155 lines · KP Singh

Quoted 2 lines · Christian Brauner

Quoted 1 line · Alexei Starovoitov

On Wed, Jun 29, 2022 at 2:56 AM Christian Brauner <[email protected]> wrote:

[...]

Quoted 21 lines

Quoted 21 lines

Quoted 21 lines

Quoted 21 lines

Quoted 21 lines

Quoted 21 lines

Signed-off-by: KP Singh <[email protected]>
---
.../testing/selftests/bpf/prog_tests/xattr.c | 54 +++++++++++++++++++
[...]

+SEC("lsm.s/bprm_committed_creds")
+void BPF_PROG(bprm_cc, struct linux_binprm *bprm)
+{
+ struct task_struct *current = bpf_get_current_task_btf();
+ char dir_xattr_value[64] = {0};
+ int xattr_sz = 0;
+
+ xattr_sz = bpf_getxattr(bprm->file->f_path.dentry,
+ bprm->file->f_path.dentry->d_inode, XATTR_NAME,
+ dir_xattr_value, 64);
Yeah, this isn't right. You're not accounting for the caller's userns
nor for the idmapped mount. If this is supposed to work you will need a
variant of vfs_getxattr() that takes the mount's idmapping into account
afaict. See what needs to happen after do_getxattr().
Thanks for taking a look.

[...]

Quoted 23 lines

Quoted 23 lines

Quoted 23 lines

Quoted 23 lines

Quoted 5 lines

That will not be correct. posix_acl_fix_xattr_to_user checking current_user_ns() is checking random tasks that happen to be running when lsm hook got invoked.

Quoted 8 lines

KP,
we probably have to document clearly that neither 'current*' should not be used here.
xattr_permission also makes little sense in this context. If anything it can be a different kfunc if there is a use case, but I don't see it yet.
bpf-lsm prog calling __vfs_getxattr is just like other lsm-s that call it directly. It's the kernel that is doing its security thing.

Right, but LSMs usually only retrieve their own xattr namespace (ima, selinux, smack) or they calculate hashes for xattrs based on the raw filesystem xattr values (evm).

But this new bpf_getxattr() is different. It allows to retrieve _any_ xattr in any security hook it can be attached to. So someone can write a bpf program that retrieves filesystem capabilites or posix acls. And these are xattrs that require higher-level vfs involvement to be sensible in most contexts.

[...]

Quoted 60 lines

Quoted 20 lines

Quoted 16 lines

Quoted 16 lines

This hooks a bpf-lsm program to the security_bprm_committed_creds() hook. It then retrieves the extended attributes of the file to be executed. The hook currently always retrieves the raw filesystem values.

But for example any XATTR_NAME_CAPS filesystem capabilities that might've been stored will be taken into account during exec. And both the idmapping of the mount and the caller matter when determing whether they are used or not.

But the current implementation of bpf_getxattr() just ignores both. It will always retrieve the raw filesystem values. So if one invokes this hook they're not actually retrieving the values as they are seen by fs/exec.c. And I'm wondering why that is ok? And even if this is ok for some use-cases it might very well become a security issue in others if access decisions are always based on the raw values.

Quoted 4 lines

Quoted 1 line

I'm not well-versed in this so bear with me, please.

If this is really just about retrieving the "security.bpf" xattr and no other xattr then the bpf_getxattr() variant should somehow hard-code that to ensure that no other xattrs can be retrieved, imho.

Quoted 6 lines

All of these restrictions look very artificial to me. Especially the part "might very well become a security issue" just doesn't click.
We're talking about bpf-lsm progs here that implement security. Can somebody implement a poor bpf-lsm that doesn't enforce any actual security? Sure. It's a code.

The point is that with the current implementation of bpf_getxattr() you are able to retrieve any xattrs and we have way less control over a bpf-lsm program than we do over selinux which a simple git grep __vfs_getxattr() is all we need.

The thing is that with bpf_getxattr() as it stands it is currently impossible to retrieve xattr values - specifically filesystem capabilities and posix acls - and see them exactly like the code you're trying to supervise is. And that seems very strange from a security perspective. So if someone were to write

SEC("lsm.s/bprm_creds_from_file")
void BPF_PROG(bprm_cc, struct linux_binprm *bprm)

{
        struct task_struct *current = bpf_get_current_task_btf();

        xattr_sz = bpf_getxattr(bprm->file->f_path.dentry,
                                bprm->file->f_path.dentry->d_inode,
                                XATTR_NAME_POSIX_ACL_ACCESS, ..);
        // or
        xattr_sz = bpf_getxattr(bprm->file->f_path.dentry,
                                bprm->file->f_path.dentry->d_inode,
                                XATTR_NAME_CAPS, ..);

}

they'd get the raw nscaps and the raw xattrs back. But now, as just a tiny example, the nscaps->rootuid and the ->e_id fields in the posix ACLs make zero sense in this context.

And what's more there's no way for the bpf-lsm program to turn them into something that makes sense in the context of the hook they are retrieved in. It lacks all the necessary helpers to do so afaict.

Quoted 17 lines

Quoted 2 lines

No one complains about the usage of EXPORT_SYMBOL(__vfs_getxattr) in the existing LSMs like selinux.

Selinux only cares about its own xattr namespace. It doesn't retrieve fscaps or posix acls and it's not possible to write selinux programs that do so. With the bpf-lsm that's very much possible.

And if we'd notice selinux would start retrieving random xattrs we'd ask the same questions we do here.

Quoted 8 lines

No one complains about its usage in out of tree LSMs.
Is that a security issue? Of course not.
__vfs_getxattr is a kernel mechanism that LSMs use to implement the security features they need.
__vfs_getxattr as kfunc here is pretty much the same as EXPORT_SYMBOL with a big difference that it's EXPORT_SYMBOL_GPL.
BPF land doesn't have an equivalent of non-gpl export and is not going to get one.

I want to reiterate what Alexei is saying here:

*Please* consider this as a simple wrapper around __vfs_getxattr with a limited attach surface and extra verification checks and and nothing else.

What you are saying is __vfs_getxattr does not make sense in some contexts. But kernel modules can still use it right?

The user is implementing an LSM, if they chose to do things that don't make sense, then they can surely cause a lot more harm:

SEC("lsm/bprm_check_security")
int BPF_PROG(bprm_check, struct linux_binprm *bprm)

{
     return -EPERM;
}

Quoted 3 lines

This discussion would probably be a lot shorter if this series were sent with a proper explanation of how this supposed to work and what it's used for.

It's currently scoped to BPF LSM (albeit limited to LSM for now) but it won't just be used in LSM programs but some (allow-listed) tracing programs too.

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We want to leave the flexibility to the implementer of the LSM hooks. If the implementer choses to retrieve posix_acl_* we can also expose posix_acl_fix_xattr_to_user or a different kfunc that adds this logic too but that would be a separate kfunc (and a separate use-case).

No, sorry. That's what I feared and that's why I think this low-level exposure of __vfs_getxattr() is wrong:
The posix_acl_fix_xattr_*() helpers, as well as the helpers like get_file_caps() will not be exported. We're not going to export that

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I don't want to expose them and I don't want any others to be exposed either.

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deeply internal vfs machinery. So I would NACK that. If you want that - and that's what I'm saying here - you need to encapsulate this into your vfs_*xattr() helper that you can call from your kfuncs.

It seems like __vfs_getxattr is already exposed and does the wrong thing in some contexts, why can't we just "fix" __vfs_getxattr then?

To me having either a version of bpf_getxattr() that restricts access to certain xattrs or a version that takes care to perform the neccesary translations is what seems to make the most sense. I suggested that in one of my first mails.

The one thing where the way the xattrs are retrieved really matters is for vfscaps (see get_vfs_caps_from_disk()) you really need something like that function in order for vfs caps to make any sense and be interpretable by the user of the hook.

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But again, I might just misunderstand the context here and for the bpf-lsm all of this isn't really a concern. If your new series comes out I'll try to get more into the wider context.
If the security folks are happy with this then I won't argue.

Quoted 9 lines

A security module (BPF) using another security module's (Smack) xattrs without that module's (Smack) explicit approval would be considered extremely rude. Smack and SELinux use published interfaces of the capability security module, but never access the capability attributes directly. The details of a security module's implementation are not a factor. The fact that BPF uses loadable programs as opposed to loadable policy is not relevant. The only security.xattr values that the BPF security module should allow the programs it runs to access are the ones it is managing. If you decided to create an eBPF

Quoted 3 lines

What about kernel modules who can use __vfs_getxattr already as it's an exported symbol? This can still end up influencing security policy or using them in any way they like.

If I put code in Smack to read SELinux attributes I would expect to get a possibly polite but definitely strongly worded email from Paul Moore regarding that behavior. The integrity subsystem looks at Smack and SELinux attributes, but that's upstream and we can see what nefarious things are being done with them. Because I can see the upstream kernel code I can convince myself that regardless of the SELinux policy loaded SELinux isn't going to muck with the Smack attributes. I can't say the same for eBPF programs that aren't going to be in Linus' tree.

Quoted 2 lines

Anyways, I think, for now, for the use case we have, it can work with a restriction to security.bpf xattrs.

I can't say that this whole discussion is making me feel better about the BPF LSM concept. The approval was based on the notion that eBPF programs were restricted to "safe" behavior. It's hard to see how allowing access to security.selinux could be guaranteed to be in support of safe behavior.

Quoted 215 lines · Casey Schaufler

Quoted 199 lines · KP Singh

Quoted 189 lines · Casey Schaufler

Quoted 184 lines · Christian Brauner

Quoted 165 lines · KP Singh

Quoted 156 lines · Christian Brauner

Quoted 155 lines · KP Singh

Quoted 2 lines · Christian Brauner

Quoted 1 line · Alexei Starovoitov

On Wed, Jun 29, 2022 at 2:56 AM Christian Brauner <[email protected]> wrote:

[...]

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Signed-off-by: KP Singh <[email protected]>
---
.../testing/selftests/bpf/prog_tests/xattr.c | 54 +++++++++++++++++++
[...]

+SEC("lsm.s/bprm_committed_creds")
+void BPF_PROG(bprm_cc, struct linux_binprm *bprm)
+{
+ struct task_struct *current = bpf_get_current_task_btf();
+ char dir_xattr_value[64] = {0};
+ int xattr_sz = 0;
+
+ xattr_sz = bpf_getxattr(bprm->file->f_path.dentry,
+ bprm->file->f_path.dentry->d_inode, XATTR_NAME,
+ dir_xattr_value, 64);
Yeah, this isn't right. You're not accounting for the caller's userns
nor for the idmapped mount. If this is supposed to work you will need a
variant of vfs_getxattr() that takes the mount's idmapping into account
afaict. See what needs to happen after do_getxattr().
Thanks for taking a look.

[...]

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That will not be correct.
posix_acl_fix_xattr_to_user checking current_user_ns()
is checking random tasks that happen to be running
when lsm hook got invoked.

KP,
we probably have to document clearly that neither 'current*'
should not be used here.
xattr_permission also makes little sense in this context.
If anything it can be a different kfunc if there is a use case,
but I don't see it yet.
bpf-lsm prog calling __vfs_getxattr is just like other lsm-s that
call it directly. It's the kernel that is doing its security thing.

Right, but LSMs usually only retrieve their own xattr namespace (ima, selinux, smack) or they calculate hashes for xattrs based on the raw filesystem xattr values (evm).

But this new bpf_getxattr() is different. It allows to retrieve _any_ xattr in any security hook it can be attached to. So someone can write a bpf program that retrieves filesystem capabilites or posix acls. And these are xattrs that require higher-level vfs involvement to be sensible in most contexts.

[...]

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This hooks a bpf-lsm program to the security_bprm_committed_creds() hook. It then retrieves the extended attributes of the file to be executed. The hook currently always retrieves the raw filesystem values.

But for example any XATTR_NAME_CAPS filesystem capabilities that might've been stored will be taken into account during exec. And both the idmapping of the mount and the caller matter when determing whether they are used or not.

But the current implementation of bpf_getxattr() just ignores both. It will always retrieve the raw filesystem values. So if one invokes this hook they're not actually retrieving the values as they are seen by fs/exec.c. And I'm wondering why that is ok? And even if this is ok for some use-cases it might very well become a security issue in others if access decisions are always based on the raw values.

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I'm not well-versed in this so bear with me, please.

If this is really just about retrieving the "security.bpf" xattr and no other xattr then the bpf_getxattr() variant should somehow hard-code that to ensure that no other xattrs can be retrieved, imho.

Quoted 6 lines

All of these restrictions look very artificial to me. Especially the part "might very well become a security issue" just doesn't click.
We're talking about bpf-lsm progs here that implement security. Can somebody implement a poor bpf-lsm that doesn't enforce any actual security? Sure. It's a code.

The point is that with the current implementation of bpf_getxattr() you are able to retrieve any xattrs and we have way less control over a bpf-lsm program than we do over selinux which a simple git grep __vfs_getxattr() is all we need.

The thing is that with bpf_getxattr() as it stands it is currently impossible to retrieve xattr values - specifically filesystem capabilities and posix acls - and see them exactly like the code you're trying to supervise is. And that seems very strange from a security perspective. So if someone were to write

SEC("lsm.s/bprm_creds_from_file")
void BPF_PROG(bprm_cc, struct linux_binprm *bprm)

{
        struct task_struct *current = bpf_get_current_task_btf();

        xattr_sz = bpf_getxattr(bprm->file->f_path.dentry,
                                bprm->file->f_path.dentry->d_inode,
                                XATTR_NAME_POSIX_ACL_ACCESS, ..);
        // or
        xattr_sz = bpf_getxattr(bprm->file->f_path.dentry,
                                bprm->file->f_path.dentry->d_inode,
                                XATTR_NAME_CAPS, ..);

}

they'd get the raw nscaps and the raw xattrs back. But now, as just a tiny example, the nscaps->rootuid and the ->e_id fields in the posix ACLs make zero sense in this context.

And what's more there's no way for the bpf-lsm program to turn them into something that makes sense in the context of the hook they are retrieved in. It lacks all the necessary helpers to do so afaict.

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No one complains about the usage of EXPORT_SYMBOL(__vfs_getxattr) in the existing LSMs like selinux.

Selinux only cares about its own xattr namespace. It doesn't retrieve fscaps or posix acls and it's not possible to write selinux programs that do so. With the bpf-lsm that's very much possible.

And if we'd notice selinux would start retrieving random xattrs we'd ask the same questions we do here.

Quoted 8 lines

No one complains about its usage in out of tree LSMs.
Is that a security issue? Of course not.
__vfs_getxattr is a kernel mechanism that LSMs use to implement the security features they need.
__vfs_getxattr as kfunc here is pretty much the same as EXPORT_SYMBOL with a big difference that it's EXPORT_SYMBOL_GPL.
BPF land doesn't have an equivalent of non-gpl export and is not going to get one.

I want to reiterate what Alexei is saying here:

*Please* consider this as a simple wrapper around __vfs_getxattr with a limited attach surface and extra verification checks and and nothing else.

What you are saying is __vfs_getxattr does not make sense in some contexts. But kernel modules can still use it right?

The user is implementing an LSM, if they chose to do things that don't make sense, then they can surely cause a lot more harm:

SEC("lsm/bprm_check_security")
int BPF_PROG(bprm_check, struct linux_binprm *bprm)

{
     return -EPERM;
}

Quoted 3 lines

This discussion would probably be a lot shorter if this series were sent with a proper explanation of how this supposed to work and what it's used for.

It's currently scoped to BPF LSM (albeit limited to LSM for now) but it won't just be used in LSM programs but some (allow-listed) tracing programs too.

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We want to leave the flexibility to the implementer of the LSM hooks. If the implementer choses to retrieve posix_acl_* we can also expose posix_acl_fix_xattr_to_user or a different kfunc that adds this logic too but that would be a separate kfunc (and a separate use-case).

No, sorry. That's what I feared and that's why I think this low-level exposure of __vfs_getxattr() is wrong:
The posix_acl_fix_xattr_*() helpers, as well as the helpers like get_file_caps() will not be exported. We're not going to export that

Quoted 8 lines

I don't want to expose them and I don't want any others to be exposed either.

Quoted 3 lines

deeply internal vfs machinery. So I would NACK that. If you want that - and that's what I'm saying here - you need to encapsulate this into your vfs_*xattr() helper that you can call from your kfuncs.

It seems like __vfs_getxattr is already exposed and does the wrong thing in some contexts, why can't we just "fix" __vfs_getxattr then?

To me having either a version of bpf_getxattr() that restricts access to certain xattrs or a version that takes care to perform the neccesary translations is what seems to make the most sense. I suggested that in one of my first mails.

The one thing where the way the xattrs are retrieved really matters is for vfscaps (see get_vfs_caps_from_disk()) you really need something like that function in order for vfs caps to make any sense and be interpretable by the user of the hook.

Quoted 4 lines

But again, I might just misunderstand the context here and for the bpf-lsm all of this isn't really a concern. If your new series comes out I'll try to get more into the wider context.
If the security folks are happy with this then I won't argue.

Quoted 9 lines

A security module (BPF) using another security module's (Smack) xattrs without that module's (Smack) explicit approval would be considered extremely rude. Smack and SELinux use published interfaces of the capability security module, but never access the capability attributes directly. The details of a security module's implementation are not a factor. The fact that BPF uses loadable programs as opposed to loadable policy is not relevant. The only security.xattr values that the BPF security module should allow the programs it runs to access are the ones it is managing. If you decided to create an eBPF

Quoted 3 lines

What about kernel modules who can use __vfs_getxattr already as it's an exported symbol? This can still end up influencing security policy or using them in any way they like.

If I put code in Smack to read SELinux attributes I would expect to get a possibly polite but definitely strongly worded email from Paul Moore regarding that behavior. The integrity subsystem looks at Smack and SELinux attributes, but that's upstream and we can see what nefarious things are being done with them. Because I can see the upstream kernel code I can convince myself that regardless of the SELinux policy loaded SELinux isn't going to muck with the Smack attributes. I can't say the same for eBPF programs that aren't going to be in Linus' tree.

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Anyways, I think, for now, for the use case we have, it can work with a restriction to security.bpf xattrs.

I can't say that this whole discussion is making me feel better about the BPF LSM concept. The approval was based on the notion that eBPF programs were restricted to "safe" behavior. It's hard to see how allowing access to security.selinux could be guaranteed to be in support of safe behavior.

Apropos __vfs_getxattr(), looks like ecryptfs_getxattr_lower() is abusing it.

Christian, not sure if you intend to spend time of idmapped mount support of ecryptfs lower layer, but anyway that's that.

Quoted 3 lines

Apropos __vfs_getxattr(), looks like ecryptfs_getxattr_lower() is abusing it.

Heh, quoting what I wrote to KP yesterday off-list about __vfs_getxattr():

"it's [__vfs_getxattr()] exported but [afaict] it's not used in kernel modules. afaict it's only exposed because of ecryptfs"

So right at the beginning I had already pondered whether we should just rip out __vfs_getxattr() from ecryptfs and unexport the helper completely because there's barely a reason to use it. Module/driver code should not use something as low-level as __vfs_getxattr() imho.

Overlayfs does it correctly and uses vfs_getxattr() but maybe ecryptfs needs to use it for for some reason?. I haven't looked yet.

Quoted 2 lines

Christian, not sure if you intend to spend time of idmapped mount support of ecryptfs lower layer, but anyway that's that.

Not really. Remember the conversation we had with Tyler at LSFMM where he considered marking it deprecated. I don't think it's worth putting in the work.

Quoted 4 lines

This discussion would probably be a lot shorter if this series were sent with a proper explanation of how this supposed to work and what it's used for.

It's currently scoped to BPF LSM (albeit limited to LSM for now) but it won't just be used in LSM programs but some (allow-listed) tracing programs too.

Quoted 10 lines

Quoted 5 lines

This discussion would probably be a lot shorter if this series were sent with a proper explanation of how this supposed to work and what it's used for.

It's currently scoped to BPF LSM (albeit limited to LSM for now) but it won't just be used in LSM programs but some (allow-listed) tracing programs too.

KP,

Without taking sides in the discussion about the security aspect of bpf_getxattr(),
I wanted to say that we have plans to add BPF hooks for fanotify event filters and
AFAIK Alessio's team is working on adding BPF hooks for FUSE bypass decisions.

In both those cases, being able to tag files with some xattr and use that as part of
criteria in the hook would be very useful IMO, but I don't think that it should be a
problem to limit the scope of the allowed namespace to security.bpf.* for these use cases.

Thanks,
Amir.

v4 -> v5

- Fixes suggested by Andrii

v3 -> v4

- Fixed issue incorrect increment of arg counter
- Removed __weak and noinline from kfunc definiton
- Some other minor fixes.

v2 -> v3

- Fixed missing prototype error
- Fixes suggested by other Joanne and Kumar.

v1 -> v2

- Used kfuncs as suggested by Alexei
- Used Benjamin Tissoires' patch from the HID v4 series to add a sleepable kfunc set (I sent the patch as a part of this series as it seems to have been dropped from v5) and acked it. Hope this is okay.
- Added support for verifying string constants to kfuncs

Benjamin Tissoires (1):
btf: Add a new kfunc set which allows to mark a function to be
sleepable

KP Singh (4):
bpf: kfunc support for ARG_PTR_TO_CONST_STR
bpf: Allow kfuncs to be used in LSM programs
bpf: Add a bpf_getxattr kfunc
bpf/selftests: Add a selftest for bpf_getxattr

 include/linux/bpf_verifier.h                  |  2 +
 include/linux/btf.h                           |  2 +
 kernel/bpf/btf.c                              | 43 ++++++++-
 kernel/bpf/verifier.c                         | 89 +++++++++++--------
 kernel/trace/bpf_trace.c                      | 42 +++++++++
 .../testing/selftests/bpf/prog_tests/xattr.c  | 54 +++++++++++
 tools/testing/selftests/bpf/progs/xattr.c     | 37 ++++++++
 7 files changed, 229 insertions(+), 40 deletions(-)
 create mode 100644 tools/testing/selftests/bpf/prog_tests/xattr.c
 create mode 100644 tools/testing/selftests/bpf/progs/xattr.c

Signature · 1 line

2.37.0.rc0.161.g10f37bed90-goog

Quoted 15 lines · KP Singh

v4 -> v5

- Fixes suggested by Andrii

v3 -> v4

- Fixed issue incorrect increment of arg counter
- Removed __weak and noinline from kfunc definiton
- Some other minor fixes.

v2 -> v3

- Fixed missing prototype error
- Fixes suggested by other Joanne and Kumar.

Quoted 7 lines

v1 -> v2

- Used kfuncs as suggested by Alexei
- Used Benjamin Tissoires' patch from the HID v4 series to add a sleepable kfunc set (I sent the patch as a part of this series as it seems to have been dropped from v5) and acked it. Hope this is okay.
- Added support for verifying string constants to kfuncs

Hm, I mean this isn't really giving any explanation as to why you are doing this. There's literally not a single sentence about the rationale? Did you accidently forget to put that into the cover letter? :)

Quoted 28 lines

Benjamin Tissoires (1):
btf: Add a new kfunc set which allows to mark a function to be
sleepable

KP Singh (4):
bpf: kfunc support for ARG_PTR_TO_CONST_STR
bpf: Allow kfuncs to be used in LSM programs
bpf: Add a bpf_getxattr kfunc
bpf/selftests: Add a selftest for bpf_getxattr

 include/linux/bpf_verifier.h                  |  2 +
 include/linux/btf.h                           |  2 +
 kernel/bpf/btf.c                              | 43 ++++++++-
 kernel/bpf/verifier.c                         | 89 +++++++++++--------
 kernel/trace/bpf_trace.c                      | 42 +++++++++
 .../testing/selftests/bpf/prog_tests/xattr.c  | 54 +++++++++++
 tools/testing/selftests/bpf/progs/xattr.c     | 37 ++++++++
 7 files changed, 229 insertions(+), 40 deletions(-)
 create mode 100644 tools/testing/selftests/bpf/prog_tests/xattr.c
 create mode 100644 tools/testing/selftests/bpf/progs/xattr.c

--
2.37.0.rc0.161.g10f37bed90-goog

Quoted 25 lines · Christian Brauner

Quoted 15 lines · KP Singh

v4 -> v5

- Fixes suggested by Andrii

v3 -> v4

- Fixed issue incorrect increment of arg counter
- Removed __weak and noinline from kfunc definiton
- Some other minor fixes.

v2 -> v3

- Fixed missing prototype error
- Fixes suggested by other Joanne and Kumar.

Quoted 7 lines

v1 -> v2

- Used kfuncs as suggested by Alexei
- Used Benjamin Tissoires' patch from the HID v4 series to add a sleepable kfunc set (I sent the patch as a part of this series as it seems to have been dropped from v5) and acked it. Hope this is okay.
- Added support for verifying string constants to kfuncs

Quoted 3 lines

Hm, I mean this isn't really giving any explanation as to why you are doing this. There's literally not a single sentence about the rationale? Did you accidently forget to put that into the cover letter? :)

Yes, actually I did forget to copy paste :)

Foundation for building more complex security policies using the BPF LSM as presented in LSF/MM/BPF:

http://vger.kernel.org/bpfconf2022_material/lsfmmbpf2022-xattr.pdf\

See: https://lore.kernel.org/bpf/[email protected]/

Quoted 28 lines

Quoted 28 lines

Benjamin Tissoires (1):
btf: Add a new kfunc set which allows to mark a function to be
sleepable

KP Singh (4):
bpf: kfunc support for ARG_PTR_TO_CONST_STR
bpf: Allow kfuncs to be used in LSM programs
bpf: Add a bpf_getxattr kfunc
bpf/selftests: Add a selftest for bpf_getxattr

 include/linux/bpf_verifier.h                  |  2 +
 include/linux/btf.h                           |  2 +
 kernel/bpf/btf.c                              | 43 ++++++++-
 kernel/bpf/verifier.c                         | 89 +++++++++++--------
 kernel/trace/bpf_trace.c                      | 42 +++++++++
 .../testing/selftests/bpf/prog_tests/xattr.c  | 54 +++++++++++
 tools/testing/selftests/bpf/progs/xattr.c     | 37 ++++++++
 7 files changed, 229 insertions(+), 40 deletions(-)
 create mode 100644 tools/testing/selftests/bpf/prog_tests/xattr.c
 create mode 100644 tools/testing/selftests/bpf/progs/xattr.c

--
2.37.0.rc0.161.g10f37bed90-goog

Quoted 32 lines · KP Singh

Quoted 24 lines · Christian Brauner

Quoted 15 lines · KP Singh

v4 -> v5

- Fixes suggested by Andrii

v3 -> v4

- Fixed issue incorrect increment of arg counter
- Removed __weak and noinline from kfunc definiton
- Some other minor fixes.

v2 -> v3

- Fixed missing prototype error
- Fixes suggested by other Joanne and Kumar.

Quoted 7 lines

v1 -> v2

- Used kfuncs as suggested by Alexei
- Used Benjamin Tissoires' patch from the HID v4 series to add a sleepable kfunc set (I sent the patch as a part of this series as it seems to have been dropped from v5) and acked it. Hope this is okay.
- Added support for verifying string constants to kfuncs

Quoted 3 lines

Hm, I mean this isn't really giving any explanation as to why you are doing this. There's literally not a single sentence about the rationale? Did you accidently forget to put that into the cover letter? :)

Yes, actually I did forget to copy paste :)

Quoted 4 lines

Foundation for building more complex security policies using the BPF LSM as presented in LSF/MM/BPF:

http://vger.kernel.org/bpfconf2022_material/lsfmmbpf2022-xattr.pdf\

And my copy paste skills are getting worse (with the back-slash removed):

http://vger.kernel.org/bpfconf2022_material/lsfmmbpf2022-xattr.pdf

Quoted 38 lines · KP Singh

Quoted 36 lines · KP Singh

Quoted 24 lines · Christian Brauner

Quoted 15 lines · KP Singh

v4 -> v5

- Fixes suggested by Andrii

v3 -> v4

- Fixed issue incorrect increment of arg counter
- Removed __weak and noinline from kfunc definiton
- Some other minor fixes.

v2 -> v3

- Fixed missing prototype error
- Fixes suggested by other Joanne and Kumar.

Quoted 7 lines

v1 -> v2

- Used kfuncs as suggested by Alexei
- Used Benjamin Tissoires' patch from the HID v4 series to add a sleepable kfunc set (I sent the patch as a part of this series as it seems to have been dropped from v5) and acked it. Hope this is okay.
- Added support for verifying string constants to kfuncs

Quoted 3 lines

Hm, I mean this isn't really giving any explanation as to why you are doing this. There's literally not a single sentence about the rationale? Did you accidently forget to put that into the cover letter? :)

Yes, actually I did forget to copy paste :)

Foundation for building more complex security policies using the BPF LSM as presented in LSF/MM/BPF:

http://vger.kernel.org/bpfconf2022_material/lsfmmbpf2022-xattr.pdf\

Quoted 3 lines

And my copy paste skills are getting worse (with the back-slash removed):

http://vger.kernel.org/bpfconf2022_material/lsfmmbpf2022-xattr.pdf

There's literally zero information in that link, so I still have no clue on what this does and how it interacts with filesystem xattr code.

So for those of us who have zero clue as to what you are trying to do, please write a cover letter containing a non-zero amount of information. i.e. a description of the problem, the threat model being addressed, the design of the infrastructure that needs this hook, document assumptions that have been made (e.g. for accessing inode metadata atomically from random bpf contexts), what

xattr namespace(s) this hook should belong/be constrained to, whether you're going to ask for a setxattr hook next, etc.

At minimum this is going to need a bunch of documentation for people to understand how to use this - where can I find that?

Cheers,

Dave.
--
Dave Chinner
[email protected]

Quoted 40 lines · Dave Chinner

Quoted 38 lines · KP Singh

Quoted 36 lines · KP Singh

Quoted 24 lines · Christian Brauner

Quoted 15 lines · KP Singh

v4 -> v5

- Fixes suggested by Andrii

v3 -> v4

- Fixed issue incorrect increment of arg counter
- Removed __weak and noinline from kfunc definiton
- Some other minor fixes.

v2 -> v3

- Fixed missing prototype error
- Fixes suggested by other Joanne and Kumar.

Quoted 7 lines

v1 -> v2

- Used kfuncs as suggested by Alexei
- Used Benjamin Tissoires' patch from the HID v4 series to add a sleepable kfunc set (I sent the patch as a part of this series as it seems to have been dropped from v5) and acked it. Hope this is okay.
- Added support for verifying string constants to kfuncs

Quoted 3 lines

Hm, I mean this isn't really giving any explanation as to why you are doing this. There's literally not a single sentence about the rationale? Did you accidently forget to put that into the cover letter? :)

Yes, actually I did forget to copy paste :)

Foundation for building more complex security policies using the BPF LSM as presented in LSF/MM/BPF:

http://vger.kernel.org/bpfconf2022_material/lsfmmbpf2022-xattr.pdf\

Quoted 7 lines

Quoted 3 lines

And my copy paste skills are getting worse (with the back-slash removed):

http://vger.kernel.org/bpfconf2022_material/lsfmmbpf2022-xattr.pdf

There's literally zero information in that link, so I still have no clue on what this does and how it interacts with filesystem xattr code.

This is literally a wrapper around __vfs_getxattr which is an exported symbol. So, the interaction with the xattr code is the same as __vfs_getxattr interacts currently.

ssize_t bpf_getxattr(struct dentry *dentry, struct inode *inode, const char *name, void *value, int value__sz)
{
return __vfs_getxattr(dentry, inode, name, value, value__sz); }

The reason for the wrapper is that the BPF verifier offers extra checks on the arguments passed.

https://lore.kernel.org/bpf/[email protected]/T/

has more information on the kfunc support.

Quoted 7 lines

So for those of us who have zero clue as to what you are trying to do, please write a cover letter containing a non-zero amount of information. i.e. a description of the problem, the threat model being addressed, the design of the infrastructure that needs this hook, document assumptions that have been made (e.g. for accessing inode metadata atomically from random bpf contexts), what

The intention is to use this in BPF programs which can only be loaded with CAP_SYS_ADMIN.
We are currently planning on limiting the usage of this kfunc to the sleepable LSM hooks listed here:

https://git.kernel.org/pub/scm/linux/kernel/git/bpf/bpf-next.git/tree/kernel/bpf/bpf_lsm.c#n169

Quoted 2 lines

xattr namespace(s) this hook should belong/be constrained to, whether you're going to ask for a setxattr hook next, etc.

Fair point, I will resend the series with the details.

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At minimum this is going to need a bunch of documentation for people to understand how to use this - where can I find that?

There are a bunch of examples in selftests on how to use kfuncs in BPF and we added a selftests (there is a simple selftests added with this patch too).

As to how we will use xattrs to create security policies or use this functionality for
logging, this is work in progress.

Cheers,
- KP

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Cheers,

Dave.
--
Dave Chinner
[email protected]