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.

Fixes: 471fbbea7ff7 ("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>

0 files changed, 0 insertions, 0 deletions