Comment 0 for bug 1948470

aufs: kernel bug with apparmor and fuseblk

[Impact]

 * AppArmor-enabled applications on the aufs filesystem
   might hit a kernel bug when getting file attributes.

 * The aufs filesystem explicitly assigns a NULL pointer
   to `struct path.mnt` for `vfs_getattr()`, which calls
   into AppArmor that checks `struct path.mnt->mnt_flags`,
   triggering a kernel NULL pointer dereference.

 * This is almost 10 years old [1,2], reproducible w/ the
   Linux v3.2 kernel, but it's rare as apparently it needs
   a fuseblk mount as an aufs branch, and file creation/
   open (O_CREAT), with a filename that exists only in a
   lower aufs branch. On Linux v5.15-rc* it doesn't need
   AppArmor anymore.

[Fix]

 * The patch fixing this issue does set `struct path.mnt`
   properly, by taking `struct path` as parameter instead
   of just `struct dentry` (and making up an incomplete
   `struct path` w/ that `dentry` and `mnt = NULL`.)

 * Since it changes the signature of a key, leaf function
   with several callers, the patch is a bit long/refactor,
   but it has been tested by the upstream aufs maintainer
   with a private test-suite.

[Test Plan]

 * Synthetic reproducer available in [1] and comment #1.

[Regression Potential]

 * Regressions would probably manifest as kernel errors
   mostly in the lookup and open paths, but more subtle
   manifestations would be possible as well.

 * The patch modifies a fair number of functions, even if
   doing so in simple ways. The synthetic reproducer only
   covers one of those functions.

 * The other code paths have been tested by the maintainer
   w/ the mainline kernel, and should be equivalent to our
   kernel as none of such changed for cherry-pick/backport.

 * The upstream aufs maintainer runs a private test suite
   that covers several features and use cases of aufs, so
   hopefully that provides some relief to take this patch.

[Other Info]

 * Impish no longer ships aufs; no fix needed.
 * Hirsute/Focal/Bionic do/need it.
 * Hirsute/Focal are clean cherry-picks.
 * Bionic is a trivial backport.

[1] https://sourceforge.net/p/aufs/mailman/message/37363599/
[2] https://unix.stackexchange.com/questions/324571/docker-run-causing-kernel-panic

[Kernel Traces]

BUG: kernel NULL pointer dereference, address: 0000000000000010
...
CPU: 23 PID: 17623 Comm: drone-agent Not tainted 5.4.0-1058-azure #60~18.04.1-Ubuntu
Hardware name: Microsoft Corporation Virtual Machine/Virtual Machine, BIOS 090008 12/07/2018
RIP: 0010:aa_path_name+0x55/0x370
...
Call Trace:
? request_wait_answer+0xc4/0x200
path_name+0x60/0xe0
profile_path_perm.part.9+0x57/0xa0
aa_path_perm+0xe2/0x130
common_perm+0x59/0x130
common_perm_cond+0x4c/0x70
apparmor_inode_getattr+0x1d/0x20
security_inode_getattr+0x35/0x50
vfs_getattr+0x21/0x40
vfsub_update_h_iattr+0x95/0xb0 [aufs]
? lookup_dcache+0x44/0x70
? lookup_one_len+0x66/0x90
vfsub_lookup_one_len+0x50/0x70 [aufs]
au_sio_lkup_one+0x8e/0xa0 [aufs]
au_lkup_dentry+0x3fa/0x660 [aufs]
aufs_lookup.part.35+0x11c/0x210 [aufs]
aufs_atomic_open+0xec/0x3c0 [aufs]
path_openat+0xe30/0x16a0
? aufs_lookup+0x30/0x30 [aufs]
? path_openat+0xe30/0x16a0
? unlock_page_memcg+0x12/0x20
? filemap_map_pages+0x17d/0x3b0
do_filp_open+0x9b/0x110
? __check_object_size+0xdb/0x1b0
? __alloc_fd+0xb2/0x170
do_sys_open+0x1ba/0x2e0
? do_sys_open+0x1ba/0x2e0
__x64_sys_openat+0x20/0x30
do_syscall_64+0x5e/0x200
entry_SYSCALL_64_after_hwframe+0x44/0xa9
RIP: 0033:0x4a06fa