Re: [PATCH v2] ntfs: validate usa_ofs before preserving the update sequence number

On Thu, Aug 20, 2026 at 3:42 PM Dennis Tighe <[email protected]> wrote:
>
> When ntfs_mft_record_alloc() reuses a free mft record it reads the old
> update sequence number straight from the on-disk record:
>
>     usn = *(__le16 *)((u8 *)m + le16_to_cpu(m->usa_ofs));
>
> Here m points into the raw $MFT page-cache folio, which still holds
> unvalidated, MST-protected bytes: the folio is read by a plain
> iomap_read_folio() and neither post_read_mst_fixup() nor
> ntfs_mft_record_check() has run on it (both work on private copies).
> m->usa_ofs is therefore an untrusted u16, and a corrupted record can put
> it past the end of the record so the two-byte read lands outside the
> folio.  Reading such a record while creating a file gives, under KASAN:
>
>   BUG: KASAN: use-after-free in ntfs_mft_record_alloc+...
>   Read of size 2 at addr ...
>    ntfs_mft_record_alloc -> __ntfs_create -> ntfs_create -> path_openat
>
> Only preserve the old update sequence number when usa_ofs is even and in
> range, mirroring the check ntfs_mft_record_check() already applies;
> otherwise leave usn zero, which the existing restore below skips.
>
> Fixes: 495e90fa3348 ("ntfs: update attrib operations")
> Assisted-by: Claude:claude-opus-4-8
> Signed-off-by: Dennis Tighe <[email protected]>
Applied it to #ntfs-next.
Thanks!