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

[Hyunchul Lee]

On Sat, May 02, 2026 at 09:48:52AM +0900, DaeMyung Kang 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>

Looks good to me.

Reviewed-by: Hyunchul Lee <hyc.lee@xxxxxxxxx>

> ---
>  fs/ntfs/bdev-io.c | 7 +++++--
>  1 file changed, 5 insertions(+), 2 deletions(-)
> 
> diff --git a/fs/ntfs/bdev-io.c b/fs/ntfs/bdev-io.c
> index 67e65c88d681..27d7c2767a33 100644
> --- a/fs/ntfs/bdev-io.c
> +++ b/fs/ntfs/bdev-io.c
> @@ -97,6 +97,8 @@ int ntfs_bdev_write(struct super_block *sb, void *buf, loff_t start, size_t size
>  		idx_end++;
>  
>  	for (; idx < idx_end; idx++, from = 0) {
> +		u32 len;
> +
>  		folio = read_mapping_folio(sb->s_bdev->bd_mapping, idx, NULL);
>  		if (IS_ERR(folio)) {
>  			ntfs_error(sb, "Unable to read %ld page", idx);
> @@ -105,9 +107,10 @@ int ntfs_bdev_write(struct super_block *sb, void *buf, loff_t start, size_t size
>  
>  		offset = (loff_t)idx << PAGE_SHIFT;
>  		to = min_t(u32, end - offset, PAGE_SIZE);
> +		len = to - from;
>  
> -		memcpy_to_folio(folio, from, buf + buf_off, to);
> -		buf_off += to;
> +		memcpy_to_folio(folio, from, buf + buf_off, len);
> +		buf_off += len;
>  		folio_mark_uptodate(folio);
>  		folio_mark_dirty(folio);
>  		folio_put(folio);
> -- 
> 2.43.0
> 

-- 
Thanks,
Hyunchul