[PATCH v2 1/3] xfs: flush new eof page on truncate to avoid post-eof corruption

Brian Foster
2020-10-29T13:24:09Z

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: 68a9f5e7007c ("xfs: implement iomap based buffered write path")
Signed-off-by: Brian Foster <bfoster@redhat.com>
---
fs/xfs/xfs_iops.c | 10 ++++++++++
1 file changed, 10 insertions(+)

diff --git a/fs/xfs/xfs_iops.c b/fs/xfs/xfs_iops.c
index 5e165456da68..1414ab79eacf 100644
--- a/fs/xfs/xfs_iops.c
+++ b/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);
}
--
2.25.4