xfs: flush new eof page on truncate to avoid post-eof corruption

It is possible to expose non-zeroed post-EOF data in XFS if the new
EOF page is dirty, backed by an unwritten block and the truncate
happens to race with writeback. iomap_truncate_page() will not zero
the post-EOF portion of the page if the underlying block is
unwritten. The subsequent call to truncate_setsize() will, but
doesn't dirty the page. Therefore, if writeback happens to complete
after iomap_truncate_page() (so it still sees the unwritten block)
but before truncate_setsize(), the cached page becomes inconsistent
with the on-disk block. A mapped read after the associated page is
reclaimed or invalidated exposes non-zero post-EOF data.

For example, consider the following sequence when run on a kernel
modified to explicitly flush the new EOF page within the race
window:

$ xfs_io -fc "falloc 0 4k" -c fsync /mnt/file
$ xfs_io -c "pwrite 0 4k" -c "truncate 1k" /mnt/file
  ...
$ xfs_io -c "mmap 0 4k" -c "mread -v 1k 8" /mnt/file
00000400:  00 00 00 00 00 00 00 00  ........
$ umount /mnt/; mount <dev> /mnt/
$ xfs_io -c "mmap 0 4k" -c "mread -v 1k 8" /mnt/file
00000400:  cd cd cd cd cd cd cd cd  ........

Update xfs_setattr_size() to explicitly flush the new EOF page prior
to the page truncate to ensure iomap has the latest state of the
underlying block.

Fixes: 68a9f5e700 ("xfs: implement iomap based buffered write path")
Signed-off-by: Brian Foster <bfoster@redhat.com>
Reviewed-by: Darrick J. Wong <darrick.wong@oracle.com>
Signed-off-by: Darrick J. Wong <darrick.wong@oracle.com>

fs/xfs/xfs_iops.c

@@ -911,6 +911,16 @@ xfs_setattr_size(
 		error = iomap_zero_range(inode, oldsize, newsize - oldsize,
 				&did_zeroing, &xfs_buffered_write_iomap_ops);
 	} else {
+		/*
+		 * iomap won't detect a dirty page over an unwritten block (or a
+		 * cow block over a hole) and subsequently skips zeroing the
+		 * newly post-EOF portion of the page. Flush the new EOF to
+		 * convert the block before the pagecache truncate.
+		 */
+		error = filemap_write_and_wait_range(inode->i_mapping, newsize,
+						     newsize);
+		if (error)
+			return error;
 		error = iomap_truncate_page(inode, newsize, &did_zeroing,
 				&xfs_buffered_write_iomap_ops);
 	}