CVE-2026-92499: ext4: validate readdir offset before accessing dirent

CVE-2026-92499: ext4: validate readdir offset before accessing dirent

[Greg Kroah-Hartman]

From: Greg Kroah-Hartman <gregkh@kernel.org>

Description
===========

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

ext4: validate readdir offset before accessing dirent

A corrupted directory can trigger the following KASAN report when
ext4_readdir() resumes from an invalid position:

  BUG: KASAN: use-after-free in __ext4_check_dir_entry+0x5ef/0x820
  Read of size 2 at addr ffff88810a646000 by task repro_linear/509

  Call Trace:
   <TASK>
   dump_stack_lvl+0x53/0x70
   print_report+0xd0/0x630
   kasan_report+0xce/0x100
   __ext4_check_dir_entry+0x5ef/0x820
   ext4_readdir+0xcde/0x2b70
   iterate_dir+0x1a1/0x520
   __x64_sys_getdents64+0x12b/0x220
   do_syscall_64+0xf9/0x540
   entry_SYSCALL_64_after_hwframe+0x77/0x7f
   </TASK>

KASAN reports use-after-free because the out-of-bounds access lands in an
adjacent freed page. The directory buffer itself is still referenced.

ext4_dir_llseek() invalidates the directory cookie so that ext4_readdir()
rescans directory entries from the start of the block. The rescan checks
only the lower bound of rec_len before advancing. A corrupted rec_len can
therefore place the offset where the block has insufficient space for a
complete directory entry. The rescan itself may dereference that truncated
entry, or the main loop may pass it to __ext4_check_dir_entry(). The latter
reads de->rec_len before validating the range. For example:

  block offset  0                           4092  4096
                |---- de1.rec_len = 4092 -----|----|
                                               de2.inode
                                                    | de2.rec_len
                                                    ^ OOB, reported as UAF

de2 starts at offset 4092 in this 4 KiB block. Its four-byte inode fits in
the block, but its rec_len starts at offset 4096 and crosses the boundary.

The minimum safe length is inode-dependent. Encrypted and casefolded
directory entries need eight additional hash bytes, while a valid metadata
checksum tail is only 12 bytes.

Cache the metadata checksum feature state and derive the minimum directory
entry length from the on-disk format. Use it to bound both the rescan and
the offset passed to the main loop. Report an offset in a truncated block
tail and skip the remainder of the block, while continuing to accept an
offset exactly at the block boundary.

The Linux kernel CVE team has assigned CVE-2026-92499 to this issue.


Affected and fixed versions
===========================

	Issue introduced in 2.6.19 with commit ac27a0ec112a089f1a5102bc8dffc79c8c815571 and fixed in 6.18.52 with commit 64d445d40e5ea4c4d4d88880db753b370cb69161
	Issue introduced in 2.6.19 with commit ac27a0ec112a089f1a5102bc8dffc79c8c815571 and fixed in 7.2.6 with commit b4c728577933753180e3e97f08424d5bcaff705e
	Issue introduced in 2.6.19 with commit ac27a0ec112a089f1a5102bc8dffc79c8c815571 and fixed in 7.3-rc1 with commit bc4b7b0414c33b2c8898eb04386df0d21a13dad8

Please see https://www.kernel.org for a full list of currently supported
kernel versions by the kernel community.

Unaffected versions might change over time as fixes are backported to
older supported kernel versions.  The official CVE entry at
	https://cve.org/CVERecord/?id=CVE-2026-92499
will be updated if fixes are backported, please check that for the most
up to date information about this issue.


Affected files
==============

The file(s) affected by this issue are:
	fs/ext4/dir.c


Mitigation
==========

The Linux kernel CVE team recommends that you update to the latest
stable kernel version for this, and many other bugfixes.  Individual
changes are never tested alone, but rather are part of a larger kernel
release.  Cherry-picking individual commits is not recommended or
supported by the Linux kernel community at all.  If however, updating to
the latest release is impossible, the individual changes to resolve this
issue can be found at these commits:
	https://git.kernel.org/stable/c/64d445d40e5ea4c4d4d88880db753b370cb69161
	https://git.kernel.org/stable/c/b4c728577933753180e3e97f08424d5bcaff705e
	https://git.kernel.org/stable/c/bc4b7b0414c33b2c8898eb04386df0d21a13dad8