In the Linux kernel, the following vulnerability has been resolved:

ext4: check dot and dotdot of dx_root before making dir indexed

Syzbot reports a issue as follows:

BUG: unable to handle page fault for address: ffffed11022e24fe PGD 23ffee067 P4D 23ffee067 PUD 0 Oops: Oops: 0000 [#1] PREEMPT SMP KASAN PTI CPU: 0 PID: 5079 Comm: syz-executor306 Not tainted 6.10.0-rc5-g55027e689933 #0 Call Trace: make_indexed_dir+0xdaf/0x13c0 fs/ext4/namei.c:2341 ext4_add_entry+0x222a/0x25d0 fs/ext4/namei.c:2451 ext4_rename fs/ext4/namei.c:3936 [inline] ext4_rename2+0x26e5/0x4370 fs/ext4/namei.c:4214 […]

The immediate cause of this problem is that there is only one valid dentry for the block to be split during do_split, so split==0 results in out of bounds accesses to the map triggering the issue.

do_split
  unsigned split
  dx_make_map
   count = 1
  split = count/2 = 0;
  continued = hash2 == map[split - 1].hash;
   ---> map[4294967295]

The maximum length of a filename is 255 and the minimum block size is 1024, so it is always guaranteed that the number of entries is greater than or equal to 2 when do_split() is called.

But syzbots crafted image has no dot and dotdot in dir, and the dentry distribution in dirblock is as follows:

bus dentry1 hole dentry2 free |xx–|xx————-|……………|xx————-|……………| 0 12 (8+248)=256 268 256 524 (8+256)=264 788 236 1024

So when renaming dentry1 increases its name_len length by 1, neither hole nor free is sufficient to hold the new dentry, and make_indexed_dir() is called.

In make_indexed_dir() it is assumed that the first two entries of the dirblock must be dot and dotdot, so bus and dentry1 are left in dx_root because they are treated as dot and dotdot, and only dentry2 is moved to the new leaf block. Thats why count is equal to 1.

Therefore add the ext4_check_dx_root() helper function to add more sanity checks to dot and dotdot before starting the conversion to avoid the above issue.

Affected Software

References

• https://git.kernel.org/stable/c/19e13b4d7f0303186fcc891aba8d0de7c8fdbda8
• https://git.kernel.org/stable/c/42d420517072028fb0eb852c358056b7717ba5aa
• https://git.kernel.org/stable/c/50ea741def587a64e08879ce6c6a30131f7111e7
• https://git.kernel.org/stable/c/8afe06ed3be7a874b3cd82ef5f8959aca8d6429a
• https://git.kernel.org/stable/c/9d241b7a39af192d1bb422714a458982c7cc67a2
• https://git.kernel.org/stable/c/abb411ac991810c0bcbe51c2e76d2502bf611b5c
• https://git.kernel.org/stable/c/b80575ffa98b5bb3a5d4d392bfe4c2e03e9557db
• https://git.kernel.org/stable/c/cdd345321699042ece4a9d2e70754d2397d378c5