[PATCH 3.16 240/305] ext2: fix potential use after free

[Ben Hutchings]

3.16.63-rc1 review patch.  If anyone has any objections, please let me know.

------------------

From: Pan Bian <bianpan2016@xxxxxxx>

commit ecebf55d27a11538ea84aee0be643dd953f830d5 upstream.

The function ext2_xattr_set calls brelse(bh) to drop the reference count
of bh. After that, bh may be freed. However, following brelse(bh),
it reads bh->b_data via macro HDR(bh). This may result in a
use-after-free bug. This patch moves brelse(bh) after reading field.

Signed-off-by: Pan Bian <bianpan2016@xxxxxxx>
Signed-off-by: Jan Kara <jack@xxxxxxx>
Signed-off-by: Ben Hutchings <ben@xxxxxxxxxxxxxxx>
---
 fs/ext2/xattr.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

--- a/fs/ext2/xattr.c
+++ b/fs/ext2/xattr.c
@@ -606,9 +606,9 @@ skip_replace:
 	}
 
 cleanup:
-	brelse(bh);
 	if (!(bh && header == HDR(bh)))
 		kfree(header);
+	brelse(bh);
 	up_write(&EXT2_I(inode)->xattr_sem);
 
 	return error;