cifs: verify that tcon is valid before dereference in cifs_kill_sb

BugLink: https://bugs.launchpad.net/bugs/2036450

On umount, cifs_sb->tlink_tree might contain entries that do not represent
a valid tcon.
Check the tcon for error before we dereference it.

Signed-off-by: Ronnie Sahlberg <[email protected]>
Cc: [email protected]
Reviewed-by: Shyam Prasad N <[email protected]>
Reported-by: Xiaoli Feng <[email protected]>
Signed-off-by: Steve French <[email protected]>
(cherry picked from commit 8b6c584)
Signed-off-by: Tim Gardner <[email protected]>

Author: Ronnie Sahlberg

fs/cifs/cifsfs.c

@@ -266,18 +266,18 @@ static void cifs_kill_sb(struct super_block *sb)
 	 * before we kill the sb.
 	 */
 	if (cifs_sb->root) {
-		node = rb_first(root);
-		while (node != NULL) {
+		for (node = rb_first(root); node; node = rb_next(node)) {
 			tlink = rb_entry(node, struct tcon_link, tl_rbnode);
 			tcon = tlink_tcon(tlink);
+			if (IS_ERR(tcon))
+				continue;
 			cfid = &tcon->crfid;
 			mutex_lock(&cfid->fid_mutex);
 			if (cfid->dentry) {
 				dput(cfid->dentry);
 				cfid->dentry = NULL;
 			}
 			mutex_unlock(&cfid->fid_mutex);
-			node = rb_next(node);
 		}
 
 		/* finally release root dentry */