file capabilities: add no_file_caps switch (v4)

Hi James, do you mind adding this to the security-testing tree?

(It needs the recent file-caps-always-start-with-clear-bprm-caps patch which is upstream but not in security-testing:next - if it's more convenient for you I can resend that)

thanks,
-serge

From: Serge Hallyn <[email protected]>
Date: Tue, 23 Sep 2008 17:17:27 -0500
Subject: [PATCH 1/1] file capabilities: add no_file_caps switch (v4)

Add a no_file_caps boot option when file capabilities are compiled into the kernel (CONFIG_SECURITY_FILE_CAPABILITIES=y).

This allows distributions to ship a kernel with file capabilities compiled in, without forcing users to use (and understand and trust) them.

When no_file_caps is specified at boot, then when a process executes a file, any file capabilities stored with that file will not be used in the calculation of the process' new capability sets.

This means that booting with the no_file_caps boot option will not be the same as booting a kernel with file capabilities compiled out - in particular a task with CAP_SETPCAP will not have any chance of passing capabilities to another task (which isn't "really" possible anyway, and which may soon by killed altogether by David Howells in any case), and it will instead be able to put new capabilities in its pI. However since fI will always be empty and pI is masked with fI, it gains the task nothing.

We also support the extra prctl options, setting securebits and dropping capabilities from the per-process bounding set.

The other remaining difference is that killpriv, task_setscheduler, setioprio, and setnice will continue to be hooked. That will be noticable in the case where a root task changed its uid while keeping some caps, and another task owned by the new uid tries to change settings for the more privileged task.

Changelog:
Nov 05 2008: (v4) trivial port on top of always-start-\
with-clear-caps patch
Sep 23 2008: nixed file_caps_enabled when file caps are
not compiled in as it isn't used. Document no_file_caps in kernel-parameters.txt.

---
 Documentation/kernel-parameters.txt |    4 ++++
 include/linux/capability.h          |    3 +++
 kernel/capability.c                 |   11 +++++++++++
 security/commoncap.c                |    3 +++
 4 files changed, 21 insertions(+), 0 deletions(-)

index 329dcab..ed397f4 100644
@@ -1387,6 +1387,10 @@ and is between 256 and 4096 characters. It is defined in the file
 			instruction doesn't work correctly and not to
 			use it.
 
+	no_file_caps	Tells the kernel not to honor file capabilities.  The
+			only way then for a file to be executed with privilege
+			is to be setuid root or executed by root.
+
 	nohalt		[IA-64] Tells the kernel not to use the power saving
 			function PAL_HALT_LIGHT when idle. This increases
 			power-consumption. On the positive side, it reduces

index 9d1fe30..5bc145b 100644
@@ -68,6 +68,9 @@ typedef struct __user_cap_data_struct {
 #define VFS_CAP_U32             VFS_CAP_U32_2
 #define VFS_CAP_REVISION	VFS_CAP_REVISION_2
 
+#ifdef CONFIG_SECURITY_FILE_CAPABILITIES
+extern int file_caps_enabled;
+#endif
 
 struct vfs_cap_data {
 	__le32 magic_etc;            /* Little endian */

index 33e51e7..e13a685 100644
@@ -33,6 +33,17 @@ EXPORT_SYMBOL(__cap_empty_set);
 EXPORT_SYMBOL(__cap_full_set);
 EXPORT_SYMBOL(__cap_init_eff_set);
 
+#ifdef CONFIG_SECURITY_FILE_CAPABILITIES
+int file_caps_enabled = 1;
+
+static int __init file_caps_disable(char *str)
+{
+	file_caps_enabled = 0;
+	return 1;
+}
+__setup("no_file_caps", file_caps_disable);
+#endif
+
 /*
  * More recent versions of libcap are available from:
  *

index 3976613..f88119c 100644
@@ -281,6 +281,9 @@ static int get_file_caps(struct linux_binprm *bprm)
 
 	bprm_clear_caps(bprm);
 
+	if (!file_caps_enabled)
+		return 0;
+
 	if (bprm->file->f_vfsmnt->mnt_flags & MNT_NOSUID)
 		return 0;
 

1.5.6.3

I merged the tree with Linus and applied the patch to:
git://git.kernel.org/pub/scm/linux/kernel/git/jmorris/security-testing-2.6#next

James Morris
<[email protected]>