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

[Xiang Mei]

On Mon, Jul 13, 2026 at 6:09 AM Jan Kara <jack@xxxxxxx> wrote:
>
> On Thu 09-07-26 11:41:01, Xiang Mei 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 <bestswngs@xxxxxxxxx>
> > Assisted-by: Claude:claude-opus-4-8
> > Signed-off-by: Xiang Mei <xmei5@xxxxxxx>
>
> Looks good. Feel free to add:
>
> Reviewed-by: Jan Kara <jack@xxxxxxx>
>
>                                                                         Honza
>
Thanks for your review!

Xiang
> > ---
> >  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
> >
> --
> Jan Kara <jack@xxxxxxxx>
> SUSE Labs, CR