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.

Weakness

The product reads data past the end, or before the beginning, of the intended buffer.

Affected Software

Potential Mitigations

• Assume all input is malicious. Use an “accept known good” input validation strategy, i.e., use a list of acceptable inputs that strictly conform to specifications. Reject any input that does not strictly conform to specifications, or transform it into something that does.
• When performing input validation, consider all potentially relevant properties, including length, type of input, the full range of acceptable values, missing or extra inputs, syntax, consistency across related fields, and conformance to business rules. As an example of business rule logic, “boat” may be syntactically valid because it only contains alphanumeric characters, but it is not valid if the input is only expected to contain colors such as “red” or “blue.”
• Do not rely exclusively on looking for malicious or malformed inputs. This is likely to miss at least one undesirable input, especially if the code’s environment changes. This can give attackers enough room to bypass the intended validation. However, denylists can be useful for detecting potential attacks or determining which inputs are so malformed that they should be rejected outright.
• To reduce the likelihood of introducing an out-of-bounds read, ensure that you validate and ensure correct calculations for any length argument, buffer size calculation, or offset. Be especially careful of relying on a sentinel (i.e. special character such as NUL) in untrusted inputs.

Related Attack Patterns

• https://cwe.mitre.org/data/definitions/540.html

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
• https://lists.debian.org/debian-lts-announce/2024/10/msg00003.html
• https://lists.debian.org/debian-lts-announce/2025/01/msg00001.html