[RFC PATCH v5 0/2] ima,fuse: introduce new fs flag FS_IMA_NO_CACHE

[RFC PATCH v5 0/2] ima,fuse: introduce new fs flag FS_IMA_NO_CACHE

[Dongsu Park]

This patchset v5 introduces a new fs flag FS_IMA_NO_CACHE and uses it in
FUSE. This 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.

There was a previous attempt (unmerged) with a IMA option named "force" and using
that option for FUSE filesystems. These patches use a different approach
so that the IMA subsystem does not need to know about FUSE.
- https://www.spinics.net/lists/linux-integrity/msg00948.html
- https://www.mail-archive.com/linux-kernel@vger.kernel.org/msg1584131.html

Changes since v1: https://www.mail-archive.com/linux-kernel@vger.kernel.org/msg1587390.html
- include linux-fsdevel mailing list in cc
- mark patch as RFC
- based on next-integrity, without other unmerged FUSE / IMA patches

Changes since v2: https://www.mail-archive.com/linux-kernel@vger.kernel.org/msg1587678.html
- rename flag to FS_IMA_NO_CACHE
- split patch into 2

Changes since v3: https://www.mail-archive.com/linux-kernel@vger.kernel.org/msg1592393.html
- make the code simpler by resetting IMA_DONE_MASK

Changes since v4: https://www.mail-archive.com/linux-kernel@vger.kernel.org/msg1598387.html
- add Acked-by from Miklos
- change ordering of patches as suggested by Miklos
- improve commit messages
- code diff since v4 is empty: only commit messages, ordering were changed

The patchset is also available in our github repo:
  https://github.com/kinvolk/linux/tree/dongsu/fuse-flag-ima-nocache-v5


Alban Crequy (2):
  ima: force re-appraisal on filesystems with FS_IMA_NO_CACHE
  fuse: introduce new fs_type flag FS_IMA_NO_CACHE

 fs/fuse/inode.c                   |  2 +-
 include/linux/fs.h                |  1 +
 security/integrity/ima/ima_main.c | 15 +++++++++++++--
 3 files changed, 15 insertions(+), 3 deletions(-)

-- 
2.13.6

[RFC PATCH v5 1/2] ima: force re-appraisal on filesystems with FS_IMA_NO_CACHE

[Dongsu Park]

From: Alban Crequy <alban@kinvolk.io>

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.

Cc: linux-kernel@vger.kernel.org
Cc: linux-integrity@vger.kernel.org
Cc: linux-security-module@vger.kernel.org
Cc: linux-fsdevel@vger.kernel.org
Cc: Alexander Viro <viro@zeniv.linux.org.uk>
Cc: Miklos Szeredi <miklos@szeredi.hu>
Cc: Mimi Zohar <zohar@linux.vnet.ibm.com>
Cc: Dmitry Kasatkin <dmitry.kasatkin@gmail.com>
Cc: James Morris <jmorris@namei.org>
Cc: Christoph Hellwig <hch@infradead.org>
Acked-by: "Serge E. Hallyn" <serge@hallyn.com>
Acked-by: Seth Forshee <seth.forshee@canonical.com>
Tested-by: Dongsu Park <dongsu@kinvolk.io>
Signed-off-by: Alban Crequy <alban@kinvolk.io>
Signed-off-by: Dongsu Park <dongsu@kinvolk.io>
---
 include/linux/fs.h                |  1 +
 security/integrity/ima/ima_main.c | 15 +++++++++++++--
 2 files changed, 14 insertions(+), 2 deletions(-)

diff --git a/include/linux/fs.h b/include/linux/fs.h
index 511fbaab..ced841ba 100644
--- a/include/linux/fs.h
+++ b/include/linux/fs.h
@@ -2075,6 +2075,7 @@ struct file_system_type {
 #define FS_BINARY_MOUNTDATA	2
 #define FS_HAS_SUBTYPE		4
 #define FS_USERNS_MOUNT		8	/* Can be mounted by userns root */
+#define FS_IMA_NO_CACHE		16	/* Force IMA to re-measure, re-appraise, re-audit files */
 #define FS_RENAME_DOES_D_MOVE	32768	/* FS will handle d_move() during rename() internally. */
 	struct dentry *(*mount) (struct file_system_type *, int,
 		       const char *, void *);
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,
-- 
2.13.6

[RFC PATCH v5 2/2] fuse: introduce new fs_type flag FS_IMA_NO_CACHE

[Dongsu Park]

From: Alban Crequy <alban@kinvolk.io>

This new fs_type flag FS_IMA_NO_CACHE means files should be re-measured,
re-appraised and re-audited each time. Cached integrity results should
not be used.

It is useful in FUSE because the userspace FUSE process can change the
underlying files at any time without notifying the kernel. FUSE can be
mounted by unprivileged users either today with fusermount installed
with setuid, or soon with the upcoming patches to allow FUSE mounts in
a non-init user namespace. That makes the issue more visible than for
network filesystems where unprivileged users cannot mount.

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-v5" [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-v5
[4] https://github.com/kinvolk/fuse-userns-patches/commit/cf1f5750cab0

Cc: linux-kernel@vger.kernel.org
Cc: linux-integrity@vger.kernel.org
Cc: linux-security-module@vger.kernel.org
Cc: linux-fsdevel@vger.kernel.org
Cc: Alexander Viro <viro@zeniv.linux.org.uk>
Cc: Mimi Zohar <zohar@linux.vnet.ibm.com>
Cc: Dmitry Kasatkin <dmitry.kasatkin@gmail.com>
Cc: James Morris <jmorris@namei.org>
Cc: Christoph Hellwig <hch@infradead.org>
Acked-by: Miklos Szeredi <miklos@szeredi.hu>
Acked-by: "Serge E. Hallyn" <serge@hallyn.com>
Acked-by: Seth Forshee <seth.forshee@canonical.com>
Tested-by: Dongsu Park <dongsu@kinvolk.io>
Signed-off-by: Alban Crequy <alban@kinvolk.io>
---
 fs/fuse/inode.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/fs/fuse/inode.c b/fs/fuse/inode.c
index 624f18bb..0a9e5164 100644
--- a/fs/fuse/inode.c
+++ b/fs/fuse/inode.c
@@ -1205,7 +1205,7 @@ static void fuse_kill_sb_anon(struct super_block *sb)
 static struct file_system_type fuse_fs_type = {
 	.owner		= THIS_MODULE,
 	.name		= "fuse",
-	.fs_flags	= FS_HAS_SUBTYPE,
+	.fs_flags	= FS_HAS_SUBTYPE | FS_IMA_NO_CACHE,
 	.mount		= fuse_mount,
 	.kill_sb	= fuse_kill_sb_anon,
 };
-- 
2.13.6