CHROMIUM: LSM: Convert symlink checks to MNT_NOSYMFOLLOW
The chromiumos LSM previously tracked a nosymfollow mount parameter that
it removed from the mount options before passing them through to the fs.
It shared the security page with SELinux by assuming that it could
control page allocation and write to an offset larger than the size
needed by SELinux. After the VFS security hooks were refactored, these
assumptions no longer apply. Mounting a filesystem with SELinux options
triggers a kernel panic when the chromiumos lsm structure isn't found at
the expected offset.
The VFS gained a nosymfollow mount option that enforces the same
behavior in commit 03e64555f1a0 ("Add a "nosymfollow" mount option."),
so keeping separate track of this flag is no longer necessary. This
change removes the lsm nosymfollow functionality and simply enforces
that the flag can't be removed with a remount instead.
There are three effective differences in behavior:
1. The nosymfollow can be added with mount -oremount. Once on, it can't
be removed, so this shouldn't be a decrease in security.
2. Warnings are no longer emitted to the logs when symlink traversal is
blocked because the VFS checks MNT_NOSYMFOLLOW before calling
security hooks. I didn't find any bugs with the logged text in them,
so I think this isn't an important loss.
3. The error is now -ELOOP instead of -EPERM because that's what
upstream used.
This change is similar to crrev.com/c/2122916, but backported to account
for the lsm differences in 4.19 and with the fix in crrev.com/c/2151712
applied.
BUG=b:152074038
TEST=test_that security_NosymfollowMountOption.baseline
test_that security_NosymfollowMountOption.test_selinux_interaction
test_that cheets_CTS_P.{arm,x86}.CtsOsTestCases
tast run security.StatefulPartitionHardening
Signed-off-by: Benjamin Gordon <bmgordon@chromium.org>
Change-Id: I01fc8ce512ff51dfd387e3f15f8eb201318ea2d3
(cherry picked from commit ba5681dc23a0aadbc3d9f76500ea68d82b1e6db3)
Signed-off-by: Sarthak Kukreti <sarthakkukreti@google.com>
Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/third_party/kernel/+/3261326
Commit-Queue: Sarthak Kukreti <sarthakkukreti@chromium.org>
Tested-by: Sarthak Kukreti <sarthakkukreti@chromium.org>
1 file changed
