Messages in this thread Patch in this message | | | From | Alban Crequy <> | Subject | [RFC PATCH v3 2/2] ima: force re-appraisal on filesystems with FS_IMA_NO_CACHE | Date | Mon, 22 Jan 2018 17:24:52 +0100 |
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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.
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@vger.kernel.org Cc: linux-integrity@vger.kernel.org Cc: linux-security-module@vger.kernel.org Cc: linux-fsdevel@vger.kernel.org Cc: Miklos Szeredi <miklos@szeredi.hu> 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 <james.l.morris@oracle.com> Cc: "Serge E. Hallyn" <serge@hallyn.com> Cc: Seth Forshee <seth.forshee@canonical.com> Cc: Christoph Hellwig <hch@infradead.org> Tested-by: Dongsu Park <dongsu@kinvolk.io> Signed-off-by: Alban Crequy <alban@kinvolk.io> --- 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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