CVE-2026-89787: ext4: check dir entry fits before reading the hash trailer in ext4_search_dir()

CVE-2026-89787: ext4: check dir entry fits before reading the hash trailer in ext4_search_dir()

[Greg Kroah-Hartman]

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

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

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

ext4: check dir entry fits before reading the hash trailer in ext4_search_dir()

For casefolded encrypted directories ext4 stores an 8-byte hash trailer
after the name (EXT4_DIRENT_HASHES()), at an offset derived from
de->name_len.  On the sb_no_casefold_compat_fallback() path ext4_match()
reads that trailer, but ext4_search_dir()'s by-hand pre-check only tests
de->name + de->name_len <= dlimit, which proves the name fits, not the
rounded trailer.  A crafted entry whose name ends at the block boundary
passes the check while EXT4_DIRENT_HASHES(de) lands past the block end,
so ext4_match() reads out of bounds on an ordinary lookup.  KASAN reports
it as a use-after-free when the page after the directory block holds a
freed object:

  BUG: KASAN: use-after-free in ext4_match (fs/ext4/namei.c:1435)
  Read of size 4 at addr ffff888010458000 by task exploit
  Call Trace:
   ext4_match (fs/ext4/namei.c:1435)
   ext4_search_dir (fs/ext4/namei.c:1470)
   __ext4_find_entry (fs/ext4/namei.c:1268 fs/ext4/namei.c:1632)
   ext4_lookup (fs/ext4/namei.c:1703 fs/ext4/namei.c:1769)
   ...
   filename_lookup (fs/namei.c:2842)
   vfs_statx (fs/stat.c:353)
   __do_sys_newfstatat (fs/stat.c:538)
   do_syscall_64 (arch/x86/entry/syscall_64.c:94)
   entry_SYSCALL_64_after_hwframe (arch/x86/entry/entry_64.S:121)

Require, for hash-in-dirent directories, that the whole entry including
the rounded trailer fits before calling ext4_match().  This is the same
bound ext4_check_dir_entry() already enforces via ext4_dir_rec_len(), so
no well-formed entry is rejected.  The other caller, ext4_find_dest_de(),
runs ext4_check_dir_entry() first and is unaffected.

The Linux kernel CVE team has assigned CVE-2026-89787 to this issue.


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

	Issue introduced in 5.13 with commit 471fbbea7ff7061b2d6474665cb5a2ceb4fd6500 and fixed in 5.15.221 with commit 4d20106c536b73c4a8a02652dc85e35898baebf7
	Issue introduced in 5.13 with commit 471fbbea7ff7061b2d6474665cb5a2ceb4fd6500 and fixed in 6.1.188 with commit 61a395967de06edba58760e81907a272db749faa
	Issue introduced in 5.13 with commit 471fbbea7ff7061b2d6474665cb5a2ceb4fd6500 and fixed in 6.6.157 with commit e94676a08af6312aa72d8a981232b281f9bcfcf5
	Issue introduced in 5.13 with commit 471fbbea7ff7061b2d6474665cb5a2ceb4fd6500 and fixed in 6.12.110 with commit d8c184bec24b5a00ae96d704856d935eaded1685
	Issue introduced in 5.13 with commit 471fbbea7ff7061b2d6474665cb5a2ceb4fd6500 and fixed in 6.18.52 with commit 3933884bc3102898b458c53fbd1ac52eb9cdb8a4
	Issue introduced in 5.13 with commit 471fbbea7ff7061b2d6474665cb5a2ceb4fd6500 and fixed in 7.2.6 with commit 83663c0b739480c00cfe785675db87520ec484ed
	Issue introduced in 5.13 with commit 471fbbea7ff7061b2d6474665cb5a2ceb4fd6500 and fixed in 7.3-rc1 with commit c7e6b863d298f56522d0d08554bbea7f142e6588

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-2026-89787
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/namei.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/4d20106c536b73c4a8a02652dc85e35898baebf7
	https://git.kernel.org/stable/c/61a395967de06edba58760e81907a272db749faa
	https://git.kernel.org/stable/c/e94676a08af6312aa72d8a981232b281f9bcfcf5
	https://git.kernel.org/stable/c/d8c184bec24b5a00ae96d704856d935eaded1685
	https://git.kernel.org/stable/c/3933884bc3102898b458c53fbd1ac52eb9cdb8a4
	https://git.kernel.org/stable/c/83663c0b739480c00cfe785675db87520ec484ed
	https://git.kernel.org/stable/c/c7e6b863d298f56522d0d08554bbea7f142e6588