Re: [PATCH] ntfs: avoid calling post_write_mst_fixup() for invalid index_block

[Namjae Jeon]

On Sat, Jul 4, 2026 at 6:39 PM Valeriy Yashnikov
<yashnikov.valeriy@xxxxxxxxx> wrote:
>
> 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.
>
> Fixes: 0a8ac0c1fa0b ("ntfs: update directory operations")
> Cc: stable@xxxxxxxxxxxxxxx
> Signed-off-by: Valeriy Yashnikov <yashnikov.valeriy@xxxxxxxxx>
Applied it to #ntfs-next.
Thanks!