Bug 2535075 (CVE-2026-89787)

Summary:	CVE-2026-89787 kernel: ext4: check dir entry fits before reading the hash trailer in ext4_search_dir()
Product:	[Other] Security Response	Reporter:	OSIDB Bzimport <bzimport>
Component:	vulnerability	Assignee:	Product Security DevOps Team <prodsec-dev>
Status:	NEW ---	QA Contact:
Severity:	medium	Docs Contact:
Priority:	medium
Version:	unspecified	CC:	akhatavk, aos-team-art-private, asdas, dpaolell, jdelft, jupierce, lgarciaa, mbiarnes, ppalepu, ppostler, prdhamdh, rhel-process-autobot, sghai, sidsharm, suppawar, vlaad, watson-tool-maintainers
Target Milestone:	---	Keywords:	Security
Target Release:	---
Hardware:	All
OS:	Linux
Whiteboard:
Fixed In Version:		Doc Type:	---
Doc Text:	A flaw was found in the Linux kernel's ext4 filesystem. A local attacker could craft a directory entry that, when processed during a directory search, leads to an out-of-bounds read. This vulnerability can result in a use-after-free condition, potentially causing a denial of service or information disclosure.	Story Points:	---
Clone Of:		Environment:
Last Closed:		Type:	---
Regression:	---	Mount Type:	---
Documentation:	---	CRM:
Verified Versions:		Category:	---
oVirt Team:	---	RHEL 7.3 requirements from Atomic Host:
Cloudforms Team:	---	Target Upstream Version:
Embargoed:

Description OSIDB Bzimport 2026-09-16 09:12:05 UTC

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.

Comment 1 Keith Grant 2026-09-21 20:14:03 UTC

Upstream advisory:
https://git.kernel.org/pub/scm/linux/security/vulns.git/plain/cve/published/2026/CVE-2026-89787.mbox