CVE-2026-64431: ntfs: avoid calling post_write_mst_fixup() for invalid index_block

From: Greg Kroah-Hartman <[email protected]>

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

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

ntfs: avoid calling post_write_mst_fixup() for invalid index_block

ntfs_icx_ib_sync_write() calls post_write_mst_fixup() when ntfs_ib_write()
returns an error, intending to restore the buffer after a failed write.

However, ntfs_ib_write() returns an error immediately if
pre_write_mst_fixup() validation fails. The caller,
ntfs_icx_ib_sync_write(), interprets any error as a write failure
requiring rollback. It does not differentiate between I/O errors and
validation failures, and calls post_write_mst_fixup() anyway.

Since post_write_mst_fixup() assumes that the index_block contents is
correct, it doesn't perform the boundary checks, which results in
out-of-bounds memory access.

An attacker can craft a malicious NTFS image with:
  - large index_block.usa_ofs offset, pointing outside the ntfs_record
  - index_block.usa_count = 0, causing integer underflow
  - or index_block.usa_count larger than actual number of sectors in the
    ntfs_record, causing out-of-bounds access

KASAN reports describing the memory corruption:
  ==================================================================
  BUG: KASAN: slab-out-of-bounds in post_write_mst_fixup+0x19c/0x1d0
  Read of size 2 at addr ffff8881586c9018 by task p/9428
  Call Trace:
   <TASK>
   dump_stack_lvl+0x100/0x190
   print_report+0x139/0x4ad
   ? post_write_mst_fixup+0x19c/0x1d0
   ? __virt_addr_valid+0x262/0x500
   ? post_write_mst_fixup+0x19c/0x1d0
   kasan_report+0xe4/0x1d0
   ? post_write_mst_fixup+0x19c/0x1d0
   post_write_mst_fixup+0x19c/0x1d0
   ntfs_icx_ib_sync_write+0x179/0x220
   ntfs_inode_sync_filename+0x83d/0x1080
   __ntfs_write_inode+0x1049/0x1480
   ntfs_file_fsync+0x131/0x9b0
  ==================================================================
  BUG: KASAN: slab-out-of-bounds in post_write_mst_fixup+0x1aa/0x1d0
  Write of size 2 at addr ffff8881586c91fe by task p/9428
  Call Trace:
   <TASK>
   dump_stack_lvl+0x100/0x190
   print_report+0x139/0x4ad
   ? post_write_mst_fixup+0x1aa/0x1d0
   ? __virt_addr_valid+0x262/0x500
   ? post_write_mst_fixup+0x1aa/0x1d0
   kasan_report+0xe4/0x1d0
   ? post_write_mst_fixup+0x1aa/0x1d0
   post_write_mst_fixup+0x1aa/0x1d0
   ntfs_icx_ib_sync_write+0x179/0x220
   ntfs_inode_sync_filename+0x83d/0x1080
   __ntfs_write_inode+0x1049/0x1480
   ntfs_file_fsync+0x131/0x9b0
  ==================================================================

Let's move the post_write_mst_fixup() call to ntfs_ib_write().
The ntfs_ib_write() function calls pre_write_mst_fixup() at the beginning.
If the index_block contents is invalid, pre_write_mst_fixup() fails and
ntfs_ib_write() returns early without calling post_write_mst_fixup() on
bad index_block.

The Linux kernel CVE team has assigned CVE-2026-64431 to this issue.


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

	Issue introduced in 7.1 with commit 0a8ac0c1fa0b99a5b29002bc7f232ed7eafddef0 and fixed in 7.1.4 with commit e2018628301a6d9f54e34b0cb417f1688c66df1d
	Issue introduced in 7.1 with commit 0a8ac0c1fa0b99a5b29002bc7f232ed7eafddef0 and fixed in 7.2-rc3 with commit 5b6eedd7cc2936f9238e852b553a1b326105bde8

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-2026-64431
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/index.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/e2018628301a6d9f54e34b0cb417f1688c66df1d
	https://git.kernel.org/stable/c/5b6eedd7cc2936f9238e852b553a1b326105bde8