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

[Dave Chinner <david@fromorbit.com>]

On Thu, Oct 04, 2018 at 06:40:14PM -0700, Darrick J. Wong wrote:
> 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;
> 
> Hmm... to integrate this with the new series I just posted, I think we'd
> decrease len to be block aligned (perhaps in generic_clone_checks) so
> that copy_file_range would be able to pagecache copy the last bit
> instead of failing the whole operation.  IOWs,
> 
> if (is_dedupe) {
> 	len &= ~blkmask;
> } else if (len & blkmask) {
> 	if (pos_out + len < size_out) {
> 		len &= ~blkmask;
> 	}
> }

OK. But if I'm going to push it with just the EOF zeroing and
ctime/suid fixes, then this doesn't change until the handling of
partial completion is added to XFS later in the patchset, right?

Cheers,

Dave.

-- 
Dave Chinner
david@fromorbit.com