Re: [PATCH] ntfs: fix u16 truncation of the update sequence array count check

[Namjae Jeon <linkinjeon@kernel.org>]

On Tue, Oct 20, 2026 at 10:44 PM Zhan Xusheng <zhanxusheng1024@gmail.com> wrote:
>
> From: Zhan Xusheng <zhanxusheng@xiaomi.com>
>
> ntfs_check_restart_page_header() bounds the on-disk system page size by
> recomputing the update sequence array count from it:
>
>         u16 ra_ofs, usa_count, usa_ofs, usa_end = 0;
>         ...
>         usa_count = 1 + (logfile_system_page_size >> NTFS_BLOCK_SIZE_BITS);
>         if (usa_count != le16_to_cpu(rp->usa_count))
>                 return false;
>
> The right hand side is computed in u32 but stored in a u16, so the count
> wraps once the system page size reaches 0x2000000.  An image claiming any
> larger power of two passes the check by also claiming usa_count == 1.
>
> Nothing else limits the value, so ntfs_check_and_load_restart_page() goes
> on to kvzalloc() up to 2 GiB for the restart page and copy that many bytes
> a page at a time.  The size is only rejected afterwards, by the
> (size >> NTFS_BLOCK_SIZE_BITS) != usa_count test in post_read_mst_fixup().
>
> Compute the count in a u32 so the existing check rejects an over-large
> system page size instead of being defeated by the truncation.  This adds
> no new limit: the array holds one u16 per protected sector plus the update
> sequence number and must end within the first sector, which caps usa_count
> at 240 and the system page size at 64 KiB.
>
> Widen ra_ofs and usa_ofs as well, as ntfs_check_restart_area() already
> does: both are loaded from __le16 on-disk fields and every comparison is
> against a non-negative value, so no result changes.
>
> Fixes: 1e9ea7e04472 ("Revert "fs: Remove NTFS classic"")
> Signed-off-by: Zhan Xusheng <zhanxusheng@xiaomi.com>
Applied it to #ntfs-next.
Thanks!