Re: [PATCH] ext4: check dir entry fits before reading the hash trailer in ext4_search_dir()

On Jul 9, 2026, at 12:41, Xiang Mei <[email protected]> wrote:
> 
> 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 <[email protected]>
> Assisted-by: Claude:claude-opus-4-8
> Signed-off-by: Xiang Mei <[email protected]>

Reviewed-by: Andreas Dilger <[email protected] <mailto:[email protected]>>

> ---
> fs/ext4/namei.c | 2 ++
> 1 file changed, 2 insertions(+)
> 
> diff --git a/fs/ext4/namei.c b/fs/ext4/namei.c
> index cc49ae04a6f6..3b9740c1c16d 100644
> --- a/fs/ext4/namei.c
> +++ b/fs/ext4/namei.c
> @@ -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 */
> -- 
> 2.43.0
> 


Cheers, Andreas