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@xxxxxxxxxx> 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 >
