From: Brian Foster bfoster@redhat.com
[ Upstream commit 869ae85dae64b5540e4362d7fe4cd520e10ec05c ]
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 Reviewed-by: Darrick J. Wong darrick.wong@oracle.com Signed-off-by: Darrick J. Wong darrick.wong@oracle.com Signed-off-by: Sasha Levin sashal@kernel.org --- 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 e427ad097e2e..948ac1290121 100644 --- a/fs/xfs/xfs_iops.c +++ b/fs/xfs/xfs_iops.c @@ -895,6 +895,16 @@ xfs_setattr_size( error = iomap_zero_range(inode, oldsize, newsize - oldsize, &did_zeroing, &xfs_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_iomap_ops); }