compute_creds race

In fs/exec.c, compute_creds does:

	task_lock(current);
	if (bprm->e_uid != current->uid || bprm->e_gid != current->gid) {
                current->mm->dumpable = 0;
		
		if (must_not_trace_exec(current)
		    || atomic_read(&current->fs->count) > 1
		    || atomic_read(&current->files->count) > 1
		    || atomic_read(&current->sighand->count) > 1) {
			if(!capable(CAP_SETUID)) {
				bprm->e_uid = current->uid;
				bprm->e_gid = current->gid;
			}
		}
	}

        current->suid = current->euid = current->fsuid = bprm->e_uid;
        current->sgid = current->egid = current->fsgid = bprm->e_gid;

	task_unlock(current);

	security_bprm_compute_creds(bprm);

I assume the task_lock is to prevent another process (on SMP or preempt) from ptracing the execing process between the check and the assignment. If that's the concern then the fact that the lock is dropped before the call to security_brpm_compute_creds means that, if security_bprm_compute_creds does anything interesting, there's a race.

For my (nearly complete) caps patch, I obviously need to fix this. But I think it may be exploitable now. Suppose there are two processes, A (the malicious code) and B (which uses exec). B starts out unprivileged (A and B have, e.g., uid and euid = 500).

1. A ptraces B.
2. B calls exec on some setuid-root program.
3. in cap_bprm_set_security, B sets bprm->cap_permitted to the full set.
4. B gets to compute_creds in exec.c, calls task_lock, and does not change its uid.
5. B calls task_unlock.
6. A detaches from B (on preempt or SMP).
7. B gets to task_lock in cap_bprm_compute_creds, changes its capabilities, and returns from compute_creds into load_elf_binary.
8. load_elf_binary calls create_elf_tables (line 852 in 2.6.5-mm1), which calls cap_bprm_secureexec (through LSM), which returns false (!).
9. exec finishes.

The setuid program is now running with uid=euid=500 but full permitted capabilities. There are two (or three) ways to effectively get local root now:

1. IIRC, linux 2.4 doesn't check capabilities in ptrace, so A could just ptrace B again.
2. LD_PRELOAD.
3. There are probably programs that will misbehave on their own under these circumstances.

Is there some reason why this is not doable?

The patch renames bprm_compute_creds to bprm_apply_creds and moves all uid logic into the hook. This way, out-of-tree LSMs will fail to compile instead of malfunctioning. It should also make life easier for LSMs and will certainly make it easier for me to finish the cap patch. Someone please check me on the selinux part -- I haven't compiled it, let alone tested it. It should work because whatever hook selinux calls will do uid evolution. On the other hand, selinux may be insecure even with this patch -- I'm not familiar enough with it to tell.

Patch against 2.6.5-mm1, applies (with offset in selinux) to -vanilla:

Fixes a race in compute_creds with LSM by moving uid/gid evolution logic into bprm_compute_creds and renaming the hook to brpm_apply_creds

 fs/exec.c                |   22 +---------------------
 include/linux/security.h |   18 +++++++++---------
 security/commoncap.c     |   24 +++++++++++++++++++++---
 security/dummy.c         |   26 ++++++++++++++++++++++----
 security/selinux/hooks.c |    8 ++++----
 5 files changed, 57 insertions(+), 41 deletions(-)

@@ -944,27 +944,7 @@
 
 void compute_creds(struct linux_binprm *bprm)
 {
-	task_lock(current);
-	if (bprm->e_uid != current->uid || bprm->e_gid != current->gid) {
-                current->mm->dumpable = 0;
-		
-		if (must_not_trace_exec(current)
-		    || atomic_read(&current->fs->count) > 1
-		    || atomic_read(&current->files->count) > 1
-		    || atomic_read(&current->sighand->count) > 1) {
-			if(!capable(CAP_SETUID)) {
-				bprm->e_uid = current->uid;
-				bprm->e_gid = current->gid;
-			}
-		}
-	}
-
-        current->suid = current->euid = current->fsuid = bprm->e_uid;
-        current->sgid = current->egid = current->fsgid = bprm->e_gid;
-
-	task_unlock(current);
-
-	security_bprm_compute_creds(bprm);
+	security_bprm_apply_creds(bprm);
 }
 
 EXPORT_SYMBOL(compute_creds);

@@ -1746,7 +1746,7 @@
 	spin_unlock(&files->file_lock);
 }
 
-static void selinux_bprm_compute_creds(struct linux_binprm *bprm)
+static void selinux_bprm_apply_creds(struct linux_binprm *bprm)
 {
 	struct task_security_struct *tsec, *psec;
 	struct bprm_security_struct *bsec;

@@ -1756,7 +1756,7 @@
 	struct rlimit *rlim, *initrlim;
 	int rc, i;
 
-	secondary_ops->bprm_compute_creds(bprm);
+	secondary_ops->bprm_apply_creds(bprm);
 
 	tsec = current->security;
 

@@ -2561,7 +2561,7 @@
 	/* Control the ability to change the hard limit (whether
 	   lowering or raising it), so that the hard limit can
 	   later be used as a safe reset point for the soft limit
-	   upon context transitions. See selinux_bprm_compute_creds. */
+	   upon context transitions. See selinux_bprm_apply_creds. */
 	if (old_rlim->rlim_max != new_rlim->rlim_max)
 		return task_has_perm(current, current, PROCESS__SETRLIMIT);
 

@@ -3972,7 +3972,7 @@
 
 	.bprm_alloc_security =		selinux_bprm_alloc_security,
 	.bprm_free_security =		selinux_bprm_free_security,
-	.bprm_compute_creds =		selinux_bprm_compute_creds,
+	.bprm_apply_creds =		selinux_bprm_apply_creds,
 	.bprm_set_security =		selinux_bprm_set_security,
 	.bprm_check_security =		selinux_bprm_check_security,
 	.bprm_secureexec =		selinux_bprm_secureexec,

@@ -121,7 +121,7 @@
 	return (p->ptrace & PT_PTRACED) && !(p->ptrace & PT_PTRACE_CAP);
 }
 
-void cap_bprm_compute_creds (struct linux_binprm *bprm)
+void cap_bprm_apply_creds (struct linux_binprm *bprm)
 {
 	/* Derived from fs/exec.c:compute_creds. */
 	kernel_cap_t new_permitted, working;

@@ -132,6 +132,24 @@
 	new_permitted = cap_combine (new_permitted, working);
 
 	task_lock(current);
+
+	if (bprm->e_uid != current->uid || bprm->e_gid != current->gid) {
+		current->mm->dumpable = 0;
+	
+		if (must_not_trace_exec(current)
+		    || atomic_read(&current->fs->count) > 1
+		    || atomic_read(&current->files->count) > 1
+		    || atomic_read(&current->sighand->count) > 1) {
+			if(!capable(CAP_SETUID)) {
+				bprm->e_uid = current->uid;
+				bprm->e_gid = current->gid;
+			}
+		}
+	}
+
+	current->suid = current->euid = current->fsuid = bprm->e_uid;
+	current->sgid = current->egid = current->fsgid = bprm->e_gid;
+
 	if (!cap_issubset (new_permitted, current->cap_permitted)) {
 		current->mm->dumpable = 0;
 

@@ -315,7 +333,7 @@
 
 	vm_acct_memory(pages);
 
-        /*
+	/*
 	 * Sometimes we want to use more memory than we have
 	 */
 	if (sysctl_overcommit_memory == 1)

@@ -377,7 +395,7 @@
 EXPORT_SYMBOL(cap_capset_check);
 EXPORT_SYMBOL(cap_capset_set);
 EXPORT_SYMBOL(cap_bprm_set_security);
-EXPORT_SYMBOL(cap_bprm_compute_creds);
+EXPORT_SYMBOL(cap_bprm_apply_creds);
 EXPORT_SYMBOL(cap_bprm_secureexec);
 EXPORT_SYMBOL(cap_inode_setxattr);
 EXPORT_SYMBOL(cap_inode_removexattr);

@@ -116,7 +116,7 @@
 
 	vm_acct_memory(pages);
 
-        /*
+	/*
 	 * Sometimes we want to use more memory than we have
 	 */
 	if (sysctl_overcommit_memory == 1)

@@ -169,9 +169,27 @@
 	return;
 }
 
-static void dummy_bprm_compute_creds (struct linux_binprm *bprm)
+static void dummy_bprm_apply_creds (struct linux_binprm *bprm)
 {
-	return;
+	task_lock(current);
+	if (bprm->e_uid != current->uid || bprm->e_gid != current->gid) {
+		current->mm->dumpable = 0;
+	
+		if (must_not_trace_exec(current)
+		    || atomic_read(&current->fs->count) > 1
+		    || atomic_read(&current->files->count) > 1
+		    || atomic_read(&current->sighand->count) > 1) {
+			if(!capable(CAP_SETUID)) {
+				bprm->e_uid = current->uid;
+				bprm->e_gid = current->gid;
+			}
+		}
+	}
+
+	current->suid = current->euid = current->fsuid = bprm->e_uid;
+	current->sgid = current->egid = current->fsgid = bprm->e_gid;
+
+	task_unlock(current);
 }
 
 static int dummy_bprm_set_security (struct linux_binprm *bprm)

@@ -887,7 +905,7 @@
 	set_to_dummy_if_null(ops, vm_enough_memory);
 	set_to_dummy_if_null(ops, bprm_alloc_security);
 	set_to_dummy_if_null(ops, bprm_free_security);
-	set_to_dummy_if_null(ops, bprm_compute_creds);
+	set_to_dummy_if_null(ops, bprm_apply_creds);
 	set_to_dummy_if_null(ops, bprm_set_security);
 	set_to_dummy_if_null(ops, bprm_check_security);
 	set_to_dummy_if_null(ops, bprm_secureexec);

@@ -44,7 +44,7 @@
 extern int cap_capset_check (struct task_struct *target, kernel_cap_t 

*effective, kernel_cap_t *inheritable, kernel_cap_t *permitted);
extern void cap_capset_set (struct task_struct *target, kernel_cap_t
*effective, kernel_cap_t *inheritable, kernel_cap_t *permitted);
extern int cap_bprm_set_security (struct linux_binprm *bprm);
-extern void cap_bprm_compute_creds (struct linux_binprm *bprm); +extern void cap_bprm_apply_creds (struct linux_binprm *bprm);
extern int cap_bprm_secureexec(struct linux_binprm *bprm); extern int cap_inode_setxattr(struct dentry *dentry, char *name, void *value,
size_t size, int flags);
extern int cap_inode_removexattr(struct dentry *dentry, char *name);
@@ -102,7 +102,7 @@
* @bprm_free_security:
* @bprm contains the linux_binprm structure to be modified.
* Deallocate and clear the @bprm->security field.
- * @bprm_compute_creds:
+ * @bprm_apply_creds:
* Compute and set the security attributes of a process being transformed
* by an execve operation based on the old attributes (current->security)
* and the information saved in @bprm->security by the set_security hook.
@@ -115,7 +115,7 @@
* @bprm contains the linux_binprm structure.
* @bprm_set_security:
* Save security information in the bprm->security field, typically based
- * on information about the bprm->file, for later use by the compute_creds
+ * on information about the bprm->file, for later use by the apply_creds
* hook. This hook may also optionally check permissions (e.g. for
* transitions between security domains).
* This hook may be called multiple times during a single execve, e.g. for
@@ -924,7 +924,7 @@
* Check permission before allowing the @parent process to trace the
* @child process.
* Security modules may also want to perform a process tracing check
- * during an execve in the set_security or compute_creds hooks of
+ * during an execve in the set_security or apply_creds hooks of
* binprm_security_ops if the process is being traced and its security
* attributes would be changed by the execve.
* @parent contains the task_struct structure for parent process.
@@ -1026,7 +1026,7 @@

int (*bprm_alloc_security) (struct linux_binprm * bprm); void (*bprm_free_security) (struct linux_binprm * bprm);
- void (*bprm_compute_creds) (struct linux_binprm * bprm);
+ void (*bprm_apply_creds) (struct linux_binprm * bprm);
int (*bprm_set_security) (struct linux_binprm * bprm); int (*bprm_check_security) (struct linux_binprm * bprm); int (*bprm_secureexec) (struct linux_binprm * bprm);
@@ -1290,9 +1290,9 @@
{
security_ops->bprm_free_security (bprm);
}
-static inline void security_bprm_compute_creds (struct linux_binprm *bprm) +static inline void security_bprm_apply_creds (struct linux_binprm *bprm)
{
- security_ops->bprm_compute_creds (bprm);
+ security_ops->bprm_apply_creds (bprm);
}
static inline int security_bprm_set (struct linux_binprm *bprm) {
@@ -1962,9 +1962,9 @@
static inline void security_bprm_free (struct linux_binprm *bprm) { }

-static inline void security_bprm_compute_creds (struct linux_binprm *bprm) +static inline void security_bprm_apply_creds (struct linux_binprm *bprm)
{
- cap_bprm_compute_creds (bprm);
+ cap_bprm_apply_creds (bprm);
}

static inline int security_bprm_set (struct linux_binprm *bprm)

Yes, dropping and regaining the lock is asking for trouble. Thank you for catching this. I don't have an issue with changing the interface name. I guess the only question I have is if it's better to leave the setuid handling in the core, and move the newly named hook under the task_lock()?

thanks,
-chris

Linux Security Modules     http://lsm.immunix.org     http://lsm.bkbits.net

I imagine some LSM might want to do something complex, and holding the task lock might become a problem. Also, an LSM might want to change the setuid handling, and this makes it easier.

--Andy

What about this slightly shorter fix?

diff -urN a/fs/exec.c b/fs/exec.c
@@ -942,6 +942,9 @@
 			if(!capable(CAP_SETUID)) {
 				bprm->e_uid = current->uid;
 				bprm->e_gid = current->gid;
+				cap_clear (bprm->cap_inheritable);
+				cap_clear (bprm->cap_permitted);
+				cap_clear (bprm->cap_effective);
 			}
 		}
 	}

Regards, Olaf.

This patch fixes the problem without moving and renaming huge amounts of code. And the hook is still in place, so I don't see your problems.

If you look at the code - as seen in 2.4 and early 2.5 - that's (more or less) the place where the fix should be.

Anyway, I don't really object against moving code around.

Regards, Olaf.

With your patch, compute_creds tries assumes that, if a program is not setuid, it should clear all capabilities. This would be very wrong if capabilities worked right.

It may also be wrong with the current caps code. If root execs a setuid-nonroot program from a thread, your patch looks like the program will start up as root but without capabilities. I don't see a trivial fix for it, short of moving some code.

--Andy

In linux 2.4.25 there is no LSM and thus no vulnerability.

linux-2.4.25/fs/exec.c:

		lock_kernel();
		if (must_not_trace_exec(current)
		    || atomic_read(&current->fs->count) > 1
		    || atomic_read(&current->files->count) > 1
		    || atomic_read(&current->sig->count) > 1) {
			if(!capable(CAP_SETUID)) {
				bprm->e_uid = current->uid;
				bprm->e_gid = current->gid;
			}
			if(!capable(CAP_SETPCAP)) {
				new_permitted = cap_intersect(new_permitted,
							current->cap_permitted);
			}
		}
		do_unlock = 1;

Regards, Olaf.

And, more specifically, the locking is sufficient.

...

thanks,
-chris

Linux Security Modules     http://lsm.immunix.org     http://lsm.bkbits.net