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ntfs: validate usa_ofs before preserving the update sequence number

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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: 495e90fa33 ("ntfs: update attrib operations")
Assisted-by: Claude:claude-opus-4-8
Signed-off-by: Dennis Tighe <dennis.tighe@gmail.com>
Reviewed-by: Hyunchul Lee <hyc.lee@gmail.com>
Signed-off-by: Namjae Jeon <linkinjeon@kernel.org>
This commit is contained in:
Dennis Tighe 2026-08-20 19:28:39 +09:00 committed by Namjae Jeon
parent 98716c9fce
commit 8168434096
1 changed files with 11 additions and 1 deletions
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12
fs/ntfs/mft.c
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@ -2333,7 +2333,17 @@ 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));
/*
* The mft record still holds unvalidated, MST-protected on-disk
* bytes, so m->usa_ofs is untrusted here. Only preserve the old
* update sequence number if that offset is in bounds; otherwise
* leave usn zero so it is not restored below.
*/
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.",