CVE-2026-89787

Summary

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.

Affected Software

Weaknesses

References

• 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

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