[RFC PATCH v3 2/2] ima: force re-appraisal on filesystems with FS_IMA_NO_CACHE
From: serge@hallyn.com (Serge E. Hallyn)
Date: 2018-01-24 17:52:36
Also in:
linux-fsdevel, linux-integrity, lkml
Quoting Alban Crequy (alban.crequy at gmail.com):
From: Alban Crequy <redacted>
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 "alban/fuse-flag-ima-nocache-v3" [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/alban/fuse-flag-ima-nocache-v3
[4] https://github.com/kinvolk/fuse-userns-patches/commit/cf1f5750cab0
Cc: linux-kernel at vger.kernel.org
Cc: linux-integrity at vger.kernel.org
Cc: linux-security-module at vger.kernel.org
Cc: linux-fsdevel at vger.kernel.org
Cc: Miklos Szeredi <miklos@szeredi.hu>
Cc: Alexander Viro <viro@zeniv.linux.org.uk>
Cc: Mimi Zohar <redacted>
Cc: Dmitry Kasatkin <dmitry.kasatkin@gmail.com>
Cc: James Morris <redacted>
Cc: "Serge E. Hallyn" <serge@hallyn.com>Acked-by: Serge Hallyn <serge@hallyn.com> to both.
quoted hunk ↗ jump to hunk
Cc: Seth Forshee <redacted> Cc: Christoph Hellwig <hch@infradead.org> Tested-by: Dongsu Park <redacted> Signed-off-by: Alban Crequy <redacted> --- security/integrity/ima/ima_main.c | 24 ++++++++++++++++++++++-- 1 file changed, 22 insertions(+), 2 deletions(-)diff --git a/security/integrity/ima/ima_main.c b/security/integrity/ima/ima_main.c index 6d78cb26784d..8870a7bbe9b9 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 +229,28 @@ 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) { + if (action & IMA_MEASURE) { + iint->measured_pcrs = 0; + iint->flags &= + ~(IMA_COLLECTED | IMA_MEASURE | IMA_MEASURED); + } + if (action & IMA_APPRAISE) + iint->flags &= + ~(IMA_COLLECTED | IMA_APPRAISE | IMA_APPRAISED | + IMA_APPRAISE_SUBMASK | IMA_APPRAISED_SUBMASK); + if (action & IMA_AUDIT) + iint->flags &= + ~(IMA_COLLECTED | IMA_AUDIT | IMA_AUDITED); + } /* Determine if already appraised/measured based on bitmask * (IMA_MEASURE, IMA_MEASURED, IMA_XXXX_APPRAISE, IMA_XXXX_APPRAISED,-- 2.13.6
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