[RFC,v4,2/2] ima: force re-appraisal on filesystems with FS_IMA_NO_CACHE

Commit Message

Dongsu Park Jan. 30, 2018, 6:06 p.m. UTC

From: Alban Crequy <[e-mail]>

This patch forces files to be re-measured, re-appraised and re-audited
on file systems with the feature flag FS_IMA_NO_CACHE. In that way,
cached integrity results won't be used.

How to test this:

The test I did was using a patched version of the memfs FUSE driver
[1][2] and two very simple "hello-world" programs [4] (prog1 prints
"hello world: 1" and prog2 prints "hello world: 2").

I copy prog1 and prog2 in the fuse-memfs mount point, execute them and
check the sha1 hash in
"/sys/kernel/security/ima/ascii_runtime_measurements".

My patch on the memfs FUSE driver added a backdoor command to serve
prog1 when the kernel asks for prog2 or vice-versa. In this way, I can
exec prog1 and get it to print "hello world: 2" without ever replacing
the file via the VFS, so the kernel is not aware of the change.

The test was done using the branch "dongsu/fuse-flag-ima-nocache-v4" [3].

Step by step test procedure:

1. Mount the memfs FUSE using [2]:
rm -f  /tmp/memfs-switch* ; memfs -L DEBUG  /mnt/memfs

2. Copy prog1 and prog2 using [4]
cp prog1 /mnt/memfs/prog1
cp prog2 /mnt/memfs/prog2

3. Lookup the files and let the FUSE driver to keep the handles open:
dd if=/mnt/memfs/prog1 bs=1 | (read -n 1 x ; sleep 3600 ) &
dd if=/mnt/memfs/prog2 bs=1 | (read -n 1 x ; sleep 3600 ) &

4. Check the 2 programs work correctly:
$ /mnt/memfs/prog1
hello world: 1
$ /mnt/memfs/prog2
hello world: 2

5. Check the measurements for prog1 and prog2:
$ sudo cat /sys/kernel/security/ima/ascii_runtime_measurements \
                | grep /mnt/memfs/prog
10 [...] ima-ng sha1:ac14c9268cd2[...] /mnt/memfs/prog1
10 [...] ima-ng sha1:799cb5d1e06d[...] /mnt/memfs/prog2

6. Use the backdoor command in my patched memfs to redirect file
operations on file handle 3 to file handle 2:
rm -f  /tmp/memfs-switch* ; touch /tmp/memfs-switch-3-2

7. Check how the FUSE driver serves different content for the files:
$ /mnt/memfs/prog1
hello world: 2
$ /mnt/memfs/prog2
hello world: 2

8. Check the measurements:
sudo cat /sys/kernel/security/ima/ascii_runtime_measurements \
                | grep /mnt/memfs/prog

Without the patch, there are no new measurements, despite the FUSE
driver having served different executables.

With the patch, I can see additional measurements for prog1 and prog2
with the hashes reversed when the FUSE driver served the alternative
content.

[1] https://github.com/bbengfort/memfs
[2] https://github.com/kinvolk/memfs/commits/alban/switch-files
[3] https://github.com/kinvolk/linux/commits/dongsu/fuse-flag-ima-nocache-v4
[4] https://github.com/kinvolk/fuse-userns-patches/commit/cf1f5750cab0

Cc: [e-mail]
Cc: [e-mail]
Cc: [e-mail]
Cc: [e-mail]
Cc: Miklos Szeredi <[e-mail]>
Cc: Alexander Viro <[e-mail]>
Cc: Mimi Zohar <[e-mail]>
Cc: Dmitry Kasatkin <[e-mail]>
Cc: James Morris <[e-mail]>
Cc: Christoph Hellwig <[e-mail]>
Acked-by: "Serge E. Hallyn" <[e-mail]>
Acked-by: Seth Forshee <[e-mail]>
Tested-by: Dongsu Park <[e-mail]>
Signed-off-by: Alban Crequy <[e-mail]>
[dongsu: optimized code to address review comments by Mimi]
Signed-off-by: Dongsu Park <[e-mail]>
---
 security/integrity/ima/ima_main.c | 15 +++++++++++++--
 1 file changed, 13 insertions(+), 2 deletions(-)

Patch

diff --git a/security/integrity/ima/ima_main.c b/security/integrity/ima/ima_main.c
index 6d78cb26..83edbad8 100644
--- a/security/integrity/ima/ima_main.c
+++ b/security/integrity/ima/ima_main.c
@@ -24,6 +24,7 @@ 
 #include <linux/slab.h>
 #include <linux/xattr.h>
 #include <linux/ima.h>
+#include <linux/fs.h>
 
 #include "ima.h"
 
@@ -228,9 +229,19 @@  static int process_measurement(struct file *file, char *buf, loff_t size,
 				 IMA_APPRAISE_SUBMASK | IMA_APPRAISED_SUBMASK |
 				 IMA_ACTION_FLAGS);
 
-	if (test_and_clear_bit(IMA_CHANGE_XATTR, &iint->atomic_flags))
-		/* reset all flags if ima_inode_setxattr was called */
+	/*
+	 * Reset the measure, appraise and audit cached flags either if:
+	 * - ima_inode_setxattr was called, or
+	 * - based on filesystem feature flag
+	 * forcing the file to be re-evaluated.
+	 */
+	if (test_and_clear_bit(IMA_CHANGE_XATTR, &iint->atomic_flags)) {
 		iint->flags &= ~IMA_DONE_MASK;
+	} else if (inode->i_sb->s_type->fs_flags & FS_IMA_NO_CACHE) {
+		iint->flags &= ~IMA_DONE_MASK;
+		if (action & IMA_MEASURE)
+			iint->measured_pcrs = 0;
+	}
 
 	/* Determine if already appraised/measured based on bitmask
 	 * (IMA_MEASURE, IMA_MEASURED, IMA_XXXX_APPRAISE, IMA_XXXX_APPRAISED,