[PATCH 2/2] ima: turn on force option for FUSE in builtin policies

[Dongsu Park]

In case of FUSE filesystem, cached integrity results in IMA could be
reused, when the userspace FUSE process has changed the
underlying files. To be able to avoid such cases, we need to turn on
the force option in builtin policies, for actions of measure and
appraise. Then integrity values become re-measured and re-appraised.
In that way, cached integrity results won't be used.

This patch depends on the patch "ima: define a new policy option
named force" by Mimi. [1]

How to test the force option written by Alban:

====

The test I did was using a patched version of the memfs FUSE driver
[2][3] and two very simple "hello-world" programs [5] (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-userns-v5-2" [4],
including both this new force option and Sascha's patch ("ima: Use
i_version only when filesystem supports it").


Step by step test procedure:

1. Mount the memfs FUSE using [3]:
rm -f  /tmp/memfs-switch* ; memfs -L DEBUG  /mnt/memfs

2. Copy prog1 and prog2 using [5]
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 7ac5aed52061cb09120e977c6d04ee5c7b11c371 ima-ng
sha1:ac14c9268cd2811f7a5adea17b27d84f50e1122c /mnt/memfs/prog1
10 9acc17a9a32aec4a676b8f6558e17a3d6c9a78e6 ima-ng
sha1:799cb5d1e06d5c37ae7a76ba25ecd1bd01476383 /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 patches, on a vanilla kernel, there are no new
measurements, despite the FUSE driver having served different
executables.

However, with the "force" option enabled, I can see additional
measurements for prog1 and prog2 with the hashes reversed when the
FUSE driver served the alternative content.

====

[1] https://www.spinics.net/lists/linux-integrity/msg00948.html
[2] https://github.com/bbengfort/memfs
[3] https://github.com/kinvolk/memfs/commits/alban/switch-files
[4] https://github.com/kinvolk/linux/commits/dongsu/fuse-userns-v5-2
[5] https://github.com/kinvolk/fuse-userns-patches/commit/cf1f5750cab0

Cc: linux-integrity@xxxxxxxxxxxxxxx
Cc: linux-security-module@xxxxxxxxxxxxxxx
Cc: linux-kernel@xxxxxxxxxxxxxxx
Cc: Miklos Szeredi <miklos@xxxxxxxxxx>
Cc: Mimi Zohar <zohar@xxxxxxxxxxxxxxxxxx>
Cc: Seth Forshee <seth.forshee@xxxxxxxxxxxxx>
Tested-by: Alban Crequy <alban@xxxxxxxxxx>
Signed-off-by: Dongsu Park <dongsu@xxxxxxxxxx>
---
 security/integrity/ima/ima_policy.c | 2 ++
 1 file changed, 2 insertions(+)

diff --git a/security/integrity/ima/ima_policy.c b/security/integrity/ima/ima_policy.c
index fddef8f8..8de40d85 100644
--- a/security/integrity/ima/ima_policy.c
+++ b/security/integrity/ima/ima_policy.c
@@ -127,6 +127,7 @@ static struct ima_rule_entry default_measurement_rules[] __ro_after_init = {
 	{.action = MEASURE, .func = MODULE_CHECK, .flags = IMA_FUNC},
 	{.action = MEASURE, .func = FIRMWARE_CHECK, .flags = IMA_FUNC},
 	{.action = MEASURE, .func = POLICY_CHECK, .flags = IMA_FUNC},
+	{.action = MEASURE, .fsmagic = FUSE_SUPER_MAGIC, .flags = IMA_FSMAGIC | IMA_FORCE},
 };
 
 static struct ima_rule_entry default_appraise_rules[] __ro_after_init = {
@@ -154,6 +155,7 @@ static struct ima_rule_entry default_appraise_rules[] __ro_after_init = {
 	{.action = APPRAISE, .fowner = GLOBAL_ROOT_UID, .fowner_op = &uid_eq,
 	 .flags = IMA_FOWNER | IMA_DIGSIG_REQUIRED},
 #endif
+	{.action = APPRAISE, .fsmagic = FUSE_SUPER_MAGIC, .flags = IMA_FSMAGIC | IMA_FORCE},
 };
 
 static struct ima_rule_entry secure_boot_rules[] __ro_after_init = {
-- 
2.13.6