Re: [PATCH] ima: define a new policy option named "force"

[Mimi Zohar <zohar@linux.vnet.ibm.com>]

On Wed, 2018-01-10 at 15:13 +0100, Alban Crequy wrote:
> > There are times instead of relying on previously cached status
> > information we want to force the file to be re-measured, re-appraised,
> > and re-audited.
> >
> > This patch defines a new policy option named "force", which forces
> > files to be re-measured, re-appraised or re-audited.
> >
> > Signed-off-by: Mimi Zohar <zohar@xxxxxxxxxxxxxxxxxx>
> 
> Tested-by: Alban Crequy <alban@kinvolk.io>

Thanks!  The builtin policies should be updated to require force for
fuse filesystems.  I was expecting to receive patches (from Seth) to
update the builtin policies and upstream them together.

Mimi

> 
> tl;dr: without the patch, the measurements don't get all the changes
> on FUSE. With the patch and when enabling the "force" option,
> ascii_runtime_measurements gets the updated measurements.
> 
> 
> Longer explanation:
> 
> 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 Dongsu's branch "fuse-userns-v5-2" [3],
> 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 [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 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. Same thing with the patch but without enabling the new
> force option.
> 
> 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://github.com/bbengfort/memfs
> [2] https://github.com/kinvolk/memfs/commits/alban/switch-files
> [3] https://github.com/kinvolk/linux/commits/dongsu/fuse-userns-v5-2
> [4] https://github.com/kinvolk/fuse-userns-patches/commit/cf1f5750cab0
>