Re: [PATCH 2/2] xfs: fix data corruption w/ unaligned reflink ranges

["Darrick J. Wong" <darrick.wong@oracle.com>]

On Fri, Oct 05, 2018 at 11:23:36AM +1000, Dave Chinner wrote:
> When reflinking sub-file ranges, a data corruption can occur when
> the source file range includes a partial EOF block. This shares the
> unknown data beyond EOF into the second file at a position inside
> EOF, exposing stale data in the second file.
> 
> XFS only supports whole block sharing, but we still need to
> support whole file reflink correctly.  Hence if the reflink
> request includes the last block of the souce file, only proceed with
> the reflink operation if it lands at or past the destination file's
> current EOF. If it lands within the destination file EOF, reject the
> entire request with -EINVAL and make the caller go the hard way.
> 
> This avoids the data corruption vector, but also avoids disruption
> of returning EINVAL to userspace for the common case of whole file
> cloning.
> 
> Signed-off-by: Dave Chinner <dchinner@redhat.com>
> ---
>  fs/xfs/xfs_reflink.c | 39 +++++++++++++++++++++++++++++++--------
>  1 file changed, 31 insertions(+), 8 deletions(-)
> 
> diff --git a/fs/xfs/xfs_reflink.c b/fs/xfs/xfs_reflink.c
> index 6b0da1b80103..2615271603ce 100644
> --- a/fs/xfs/xfs_reflink.c
> +++ b/fs/xfs/xfs_reflink.c
> @@ -1229,12 +1229,24 @@ xfs_iolock_two_inodes_and_break_layout(
>   * hence can introduce a corruption into the file that has it's
>   * block replaced.
>   *
> - * Despite this issue, we still need to report that range as successfully
> - * deduped to avoid confusing userspace with EINVAL errors on completely
> - * matching file data. The only time that an unaligned length will be passed to
> - * us is when it spans the EOF block of the source file, so if we simply mask it
> - * down to be block aligned here the we will dedupe everything but that partial
> - * EOF block.
> + * In similar fashion, the VFS file cloning also allows partial EOF blocks to be
> + * "block aligned" for the purposes of cloning entire files. 
> + * However, if the source file range
> + * includes the EOF block and it lands within the existing EOF of the
> + * destination file, then we can expose stale data from beyond the source file
> + * EOF in the destination file.
> + *
> + * XFs doesn't support partial block sharing, so in both cases we have check
> + * these cases ourselves. For dedupe, we can simply round the length to dedupe
> + * down to the previous whole block and ignore the partial EOF block. While this
> + * means we can't dedupe the last block of a file, this is an acceptible
> + * tradeoff for simplicity on implementation.
> + *
> + * For cloning, we want to share the partial EOF block if it is also the new EOF
> + * block of the destination file. If the partial EOF blck lies inside the
> + * existing destination EOF, then we have to abort the clone to avoid exposing
> + * stale data int eh destination file. Hence we reject these clone attempts with
> + * -EINVAL in this case.
>   */
>  int
>  xfs_reflink_remap_range(
> @@ -1255,6 +1267,7 @@ xfs_reflink_remap_range(
>  	xfs_filblks_t		fsblen;
>  	xfs_extlen_t		cowextsize;
>  	ssize_t			ret;
> +	u64			blkmask = i_blocksize(inode_in) - 1;
>  
>  	if (!xfs_sb_version_hasreflink(&mp->m_sb))
>  		return -EOPNOTSUPP;
> @@ -1292,8 +1305,18 @@ xfs_reflink_remap_range(
>  	 * from the source file so we don't try to dedupe the partial
>  	 * EOF block.
>  	 */
> -	if (is_dedupe)
> -		len &= ~((u64)i_blocksize(inode_in) - 1);
> +	if (is_dedupe) {
> +		len &= ~blkmask;
> +	} else if (len & blkmask) {
> +		/*
> +		 * The user is attempting to share a partial EOF block,
> +		 * if it's inside the destination EOF then reject it
> +		 */
> +		if (pos_out + len < i_size_read(inode_out)) {
> +			ret = -EINVAL;
> +			goto out_unlock;

Same comment about reporting truncated lengths back to userspace as the
last patch, but nevertheless:

Reviewed-by: Darrick J. Wong <darrick.wong@oracle.com>

--D

> +		}
> +	}
>  
>  	/* Attach dquots to dest inode before changing block map */
>  	ret = xfs_qm_dqattach(dest);
> -- 
> 2.17.0
>