Re: [PATCH] ntfs: fix copy length in ntfs_bdev_write() for non-page-aligned start

[Namjae Jeon]

On Sat, May 2, 2026 at 9:49 AM DaeMyung Kang <charsyam@xxxxxxxxx> wrote:
>
> This is not a normal data I/O hot path.  The single in-tree caller is
> the $LogFile emptying path used during read-write mount/remount, and
> the bug only becomes visible on NTFS volumes whose cluster_size is
> strictly smaller than the kernel's PAGE_SIZE (typically 4 KiB on
> x86_64).  Per Microsoft's format command documentation, NTFS supports
> allocation unit sizes starting at 512 bytes, so 512 B, 1 KiB and 2 KiB
> clusters are uncommon but valid on-disk configurations.  When
> cluster_size >= PAGE_SIZE every "start" passed in is page-aligned and
> the buggy "from != 0" path is never taken.
>
> ntfs_bdev_write() splits the write across one or more block-device
> folios.  Inside the loop, "to" is computed as the *end byte offset*
> within the current page (0..PAGE_SIZE), and "from" is the start byte
> offset within the page (reset to 0 from the second iteration onward).
> The copy length should therefore be "to - from", but the current code
> uses "to" directly:
>
>         to = min_t(u32, end - offset, PAGE_SIZE);
>         memcpy_to_folio(folio, from, buf + buf_off, to);
>         buf_off += to;
>
> When "from != 0" (i.e. "start" is not page-aligned) memcpy_to_folio()
> copies "from" extra bytes:
>
>   - it reads "from" bytes past the source buffer into kernel heap;
>   - it writes "from" bytes past the requested range into the next part
>     of the block-device page (or, if "from + to > PAGE_SIZE", past the
>     folio boundary entirely, which trips the VM_BUG_ON inside
>     memcpy_to_folio() on CONFIG_DEBUG_VM=y kernels).
>
> "buf_off" is then advanced by the wrong amount, so every subsequent
> iteration also reads the source buffer at the wrong offset and writes
> the wrong content to disk.
>
> ntfs_empty_logfile() calls
>
>         ntfs_bdev_write(sb, empty_buf, NTFS_CLU_TO_B(vol, lcn),
>                         vol->cluster_size);
>
> with empty_buf sized to vol->cluster_size.  On a sub-PAGE_SIZE-cluster
> volume, any $LogFile run whose LCN is not aligned to
> PAGE_SIZE / cluster_size reaches the non-page-aligned path.  The
> over-copy can read beyond empty_buf and overwrite the sectors following
> the requested cluster in the block-device page with unrelated kernel
> heap contents while $LogFile is being emptied.
>
> A userspace reducer of the same arithmetic and copy loop confirms the
> bug under AddressSanitizer: ASan reports a heap-buffer-overflow read
> past the source buffer for the buggy length, and the fixed version is
> ASan-clean.
>
> Compute the copy length as "to - from" and advance buf_off by the same
> amount.
>
> Fixes: 5218cd102aec ("ntfs: update misc operations")
> Link: https://learn.microsoft.com/en-us/windows-server/administration/windows-commands/format
> Signed-off-by: DaeMyung Kang <charsyam@xxxxxxxxx>
Applied it to #ntfs-next
Thanks!