locks: close potential race in lease_get_mtime

lease_get_mtime is called without the i_lock held, so there's no
guarantee about the stability of the list. Between the time when we
assign "flock" and then dereference it to check whether it's a lease
and for write, the lease could be freed.

Ensure that that doesn't occur by taking the i_lock before trying
to check the lease.

Cc: J. Bruce Fields <bfields@fieldses.org>
Signed-off-by: Jeff Layton <jlayton@primarydata.com>
Reviewed-by: Christoph Hellwig <hch@lst.de>

fs/locks.c

@@ -1456,8 +1456,18 @@ EXPORT_SYMBOL(__break_lease);
  */
 void lease_get_mtime(struct inode *inode, struct timespec *time)
 {
-	struct file_lock *flock = inode->i_flock;
+	bool has_lease = false;
+	struct file_lock *flock;
+
+	if (inode->i_flock) {
+		spin_lock(&inode->i_lock);
+		flock = inode->i_flock;
 		if (flock && IS_LEASE(flock) && (flock->fl_type == F_WRLCK))
+			has_lease = true;
+		spin_unlock(&inode->i_lock);
+	}
+
+	if (has_lease)
 		*time = current_fs_time(inode->i_sb);
 	else
 		*time = inode->i_mtime;