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 03:21:21PM +1000, Dave Chinner wrote:
> 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?

Right.  If you add this series before the partial completion patches
I'll fix things up when I rebase that part of my series.

--D

> Cheers,
> 
> Dave.
> 
> -- 
> Dave Chinner
> david@fromorbit.com