Quoting Seth Forshee (seth.forshee@xxxxxxxxxxxxx): > On Mon, Jan 22, 2018 at 05:24:52PM +0100, Alban Crequy wrote: > > From: Alban Crequy <alban@xxxxxxxxxx> > > > > 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@xxxxxxxxxxxxxxx > > Cc: linux-integrity@xxxxxxxxxxxxxxx > > Cc: linux-security-module@xxxxxxxxxxxxxxx > > Cc: linux-fsdevel@xxxxxxxxxxxxxxx > > Cc: Miklos Szeredi <miklos@xxxxxxxxxx> > > Cc: Alexander Viro <viro@xxxxxxxxxxxxxxxxxx> > > Cc: Mimi Zohar <zohar@xxxxxxxxxxxxxxxxxx> > > Cc: Dmitry Kasatkin <dmitry.kasatkin@xxxxxxxxx> > > Cc: James Morris <james.l.morris@xxxxxxxxxx> > > Cc: "Serge E. Hallyn" <serge@xxxxxxxxxx> > > Cc: Seth Forshee <seth.forshee@xxxxxxxxxxxxx> > > Cc: Christoph Hellwig <hch@xxxxxxxxxxxxx> > > Tested-by: Dongsu Park <dongsu@xxxxxxxxxx> > > Signed-off-by: Alban Crequy <alban@xxxxxxxxxx> > > I like this approach as it doesn't require any IMA policy changes to be > effective. I'm not sure about the flag name (in my mind it would > describe the fs property and not be specifically giving instructions to > IMA), but if others are happy with it I won't complain. Purely internal name at least, so if we come up with a better name we can trivially change it.
