On Wed, Jan 10, 2018 at 3:48 PM, Mimi Zohar <zohar@xxxxxxxxxxxxxxxxxx> wrote: > On Wed, 2018-01-10 at 08:44 -0600, Seth Forshee wrote: >> On Wed, Jan 10, 2018 at 09:39:10AM -0500, Mimi Zohar wrote: >> > 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. >> >> Yes, I was working on a patch but need to update/test it. Was planning >> to do that a few weeks ago but then other things came up, hoping I can >> get back to it soon. But if someone beats me to it that's okay too :-) > > Thanks, just making sure you wouldn't object to someone else doing it ... Thanks. Dongsu is preparing a patch to add this "force" for FUSE by default. >> > > 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. Btw, I also tested the "force" option with the memfs FUSE mounted in a non-initial user namespace (with the FUSE patches that allow that) and it works as well. I don't see any reasons why it would not have worked, but I just mention it for completeness. I tested that using "unshare -U -r -m". Alban >> > > Longer explanation: > > This "Longer explanation" would make for a really good patch > description for changing the builtin policies. > > Mimi > >> > > >> > > 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 >> > > >> > >> >
