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

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]>
---
For v2: 
- drop the explanatory comment above the check (per review).
- fix blankspace issues

 fs/ntfs/mft.c | 6 +++++-
 1 file changed, 5 insertions(+), 1 deletion(-)

diff --git a/fs/ntfs/mft.c b/fs/ntfs/mft.c
index fd20d7a..82d2190 100644
--- a/fs/ntfs/mft.c
+++ b/fs/ntfs/mft.c
@@ -2333,7 +2333,11 @@ int ntfs_mft_record_alloc(struct ntfs_volume *vol, const int mode,
 		 * wrong with the previous mft record.
 		 */
 		seq_no = m->sequence_number;
-		usn = *(__le16 *)((u8 *)m + le16_to_cpu(m->usa_ofs));
+		if (!(le16_to_cpu(m->usa_ofs) & 1) &&
+		    le16_to_cpu(m->usa_ofs) + sizeof(usn) <= vol->mft_record_size)
+			usn = *(__le16 *)((u8 *)m + le16_to_cpu(m->usa_ofs));
+		else
+			usn = 0;
 		err = ntfs_mft_record_layout(vol, bit, m);
 		if (unlikely(err)) {
 			ntfs_error(vol->sb, "Failed to layout allocated mft record 0x%llx.",
-- 
2.47.3