[PATCH 3.16 090/366] ext4: force revalidation of directory pointer after seekdir(2)

[Ben Hutchings]

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

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

From: Theodore Ts'o <tytso@xxxxxxx>

commit e40ff213898502d299351cc2fe1e350cd186f0d3 upstream.

A malicious user could force the directory pointer to be in an invalid
spot by using seekdir(2).  Use the mechanism we already have to notice
if the directory has changed since the last time we called
ext4_readdir() to force a revalidation of the pointer.

Reported-by: syzbot+1236ce66f79263e8a862@xxxxxxxxxxxxxxxxxxxxxxxxx
Signed-off-by: Theodore Ts'o <tytso@xxxxxxx>
[bwh: Backported to 3.16: open-code inode_peek_iversion()]
Signed-off-by: Ben Hutchings <ben@xxxxxxxxxxxxxxx>
---
 fs/ext4/dir.c | 8 +++++---
 1 file changed, 5 insertions(+), 3 deletions(-)

--- a/fs/ext4/dir.c
+++ b/fs/ext4/dir.c
@@ -322,13 +322,15 @@ static loff_t ext4_dir_llseek(struct fil
 {
 	struct inode *inode = file->f_mapping->host;
 	int dx_dir = is_dx_dir(inode);
-	loff_t htree_max = ext4_get_htree_eof(file);
+	loff_t ret, htree_max = ext4_get_htree_eof(file);
 
 	if (likely(dx_dir))
-		return generic_file_llseek_size(file, offset, whence,
+		ret = generic_file_llseek_size(file, offset, whence,
 						    htree_max, htree_max);
 	else
-		return ext4_llseek(file, offset, whence);
+		ret = ext4_llseek(file, offset, whence);
+	file->f_version = inode->i_version - 1;
+	return ret;
 }
 
 /*