CVE-2026-90117: ntfs: validate usa_ofs before preserving the update sequence number

[Greg Kroah-Hartman <gregkh@linuxfoundation.org>]

From: Greg Kroah-Hartman <gregkh@kernel.org>

Description
===========

In the Linux kernel, the following vulnerability has been resolved:

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.

The Linux kernel CVE team has assigned CVE-2026-90117 to this issue.


Affected and fixed versions
===========================

	Issue introduced in 7.1 with commit 495e90fa334828d4119061e2726af51d0a0fb4ed and fixed in 7.2.6 with commit ea5a3c30710710c1dc5e483d3313309ec2e868f7
	Issue introduced in 7.1 with commit 495e90fa334828d4119061e2726af51d0a0fb4ed and fixed in 7.3-rc1 with commit 81684340963da2e898eabb8c1e274433d9375bc6

Please see https://www.kernel.org for a full list of currently supported
kernel versions by the kernel community.

Unaffected versions might change over time as fixes are backported to
older supported kernel versions.  The official CVE entry at
	https://cve.org/CVERecord/?id=CVE-2026-90117
will be updated if fixes are backported, please check that for the most
up to date information about this issue.


Affected files
==============

The file(s) affected by this issue are:
	fs/ntfs/mft.c


Mitigation
==========

The Linux kernel CVE team recommends that you update to the latest
stable kernel version for this, and many other bugfixes.  Individual
changes are never tested alone, but rather are part of a larger kernel
release.  Cherry-picking individual commits is not recommended or
supported by the Linux kernel community at all.  If however, updating to
the latest release is impossible, the individual changes to resolve this
issue can be found at these commits:
	https://git.kernel.org/stable/c/ea5a3c30710710c1dc5e483d3313309ec2e868f7
	https://git.kernel.org/stable/c/81684340963da2e898eabb8c1e274433d9375bc6