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

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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.

Fixes: 471fbbea7f ("ext4: handle casefolding with encryption")
Reported-by: Weiming Shi <bestswngs@gmail.com>
Assisted-by: Claude:claude-opus-4-8
Signed-off-by: Xiang Mei <xmei5@asu.edu>
Reviewed-by: Andreas Dilger <adilger@dilger.ca>
Reviewed-by: Jan Kara <jack@suse.cz>
Link: https://patch.msgid.link/20260709184101.441348-1-xmei5@asu.edu
Signed-off-by: Theodore Ts'o <tytso@mit.edu>
This commit is contained in:
Xiang Mei
2026-07-09 11:41:01 -07:00
committed by Theodore Ts'o
parent 0570433580
commit c7e6b863d2
1 changed files with 2 additions and 0 deletions
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2
fs/ext4/namei.c
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@@ -1467,6 +1467,8 @@ int ext4_search_dir(struct buffer_head *bh, char *search_buf, int buf_size,
/* this code is executed quadratically often */
/* do minimal checking `by hand' */
if (de->name + de->name_len <= dlimit &&
(!ext4_hash_in_dirent(dir) ||
(char *)de + ext4_dir_rec_len(de->name_len, dir) <= dlimit) &&
ext4_match(dir, fname, de)) {
/* found a match - just to be sure, do
* a full check */