Re: [PATCH] exfat: fix potential use-after-free in exfat_find_dir_entry()

[Namjae Jeon]

On Thu, Apr 23, 2026 at 12:59 AM Michael Bommarito
<michael.bommarito@xxxxxxxxx> wrote:
>
> In exfat_find_dir_entry(), the buffer_head obtained from
> exfat_get_dentry() is released with brelse(bh) before the fall-through
> TYPE_EXTEND branch reads the directory entry through ep (which points
> into bh->b_data):
>
>         brelse(bh);
>         if (entry_type == TYPE_EXTEND) {
>                 ...
>                 len = exfat_extract_uni_name(ep, entry_uniname);
>                 ...
>         }
>
> After brelse() drops our reference, nothing guarantees that the
> underlying page backing bh->b_data remains valid for the subsequent
> exfat_extract_uni_name() read. This is the same pattern fixed in
> commit fc961522ddbd ("exfat: Fix potential use after free in
> exfat_load_upcase_table()").
>
> Move brelse(bh) so it runs after ep is no longer dereferenced on
> each branch.
>
> Confirmed on QEMU x86_64 with CONFIG_KASAN=y + CONFIG_DEBUG_PAGEALLOC=y
> + CONFIG_PAGE_POISONING=y on linux-next, using a crafted exFAT image
> (long filename with same-hash collisions forcing the TYPE_EXTEND path).
> With a debug-only invalidate_bdev() inserted between brelse(bh) and
> the ep read to make the stale-deref window deterministic, the
> unpatched kernel faults:
>
>   BUG: KASAN: use-after-free in exfat_find_dir_entry+0x133b/0x15a0
>   BUG: unable to handle page fault for address: ffff88801a5fa0c2
>   Oops: 0000 [#1] SMP DEBUG_PAGEALLOC KASAN NOPTI
>   RIP: 0010:exfat_find_dir_entry+0x1188/0x15a0
>
> With this patch applied, the same instrumented harness completes
> cleanly under the same sanitizer stack. I have not reproduced a
> crash on an uninstrumented kernel under ordinary reclaim; the
> instrumented A/B establishes the lifetime violation and that the
> patch closes it, not an unaided triggerability claim.
>
> Fixes: ca06197382bd ("exfat: add directory operations")
> Cc: stable@xxxxxxxxxxxxxxx
> Signed-off-by: Michael Bommarito <michael.bommarito@xxxxxxxxx>
> Assisted-by: Claude:claude-opus-4-7
Applied it to #dev.
Thanks!