ksmbd: remove stale channels from all sessions on teardown

ksmbd_sessions_deregister() removes a connection's channels from other
sessions' channel lists only while conn->binding is still set:

	if (conn->binding) {
		hash_for_each_safe(sessions_table, ...)
			ksmbd_chann_del(conn, sess);
	}

conn->binding is a transient flag: it is cleared once a binding
SESSION_SETUP completes, and also by a subsequent non-binding
SESSION_SETUP on the same connection (a reauthentication on a bound
channel, or a new SessionId==0 setup). A connection that has bound a
channel into another session's ksmbd_chann_list and then clears
conn->binding leaves that channel behind when it disconnects: the
channel, whose chann->conn points at the now freed struct ksmbd_conn,
stays on the owner session's list.

When the owning connection later tears down, the second loop
dereferences the stale channel:

	xa_for_each(&sess->ksmbd_chann_list, chann_id, chann)
		if (chann->conn != conn)
			ksmbd_conn_set_exiting(chann->conn);   /* freed */

which is a use-after-free write into the freed ksmbd_conn (the same
stale channel is also walked by show_proc_session() through /proc). The
session is leaked as well, because its channel list never empties.

Remove the conn->binding gate so a connection always removes its
channels from every session on teardown.

Fixes: faf8578 ("ksmbd: find bound sessions during reauthentication")
Signed-off-by: Gil Portnoy <dddhkts1@gmail.com>
Acked-by: Namjae Jeon <linkinjeon@kernel.org>
Signed-off-by: Steve French <stfrench@microsoft.com>

Author: dhkts1

fs/smb/server/mgmt/user_session.c

@@ -457,22 +457,19 @@ void ksmbd_sessions_deregister(struct ksmbd_conn *conn)
 {
 	struct ksmbd_session *sess;
 	unsigned long id;
+	struct hlist_node *tmp;
+	int bkt;
 
 	down_write(&sessions_table_lock);
-	if (conn->binding) {
-		int bkt;
-		struct hlist_node *tmp;
-
-		hash_for_each_safe(sessions_table, bkt, tmp, sess, hlist) {
-			if (!ksmbd_chann_del(conn, sess) &&
-			    xa_empty(&sess->ksmbd_chann_list)) {
-				hash_del(&sess->hlist);
-				down_write(&conn->session_lock);
-				xa_erase(&conn->sessions, sess->id);
-				up_write(&conn->session_lock);
-				if (atomic_dec_and_test(&sess->refcnt))
-					ksmbd_session_destroy(sess);
-			}
+	hash_for_each_safe(sessions_table, bkt, tmp, sess, hlist) {
+		if (!ksmbd_chann_del(conn, sess) &&
+		    xa_empty(&sess->ksmbd_chann_list)) {
+			hash_del(&sess->hlist);
+			down_write(&conn->session_lock);
+			xa_erase(&conn->sessions, sess->id);
+			up_write(&conn->session_lock);
+			if (atomic_dec_and_test(&sess->refcnt))
+				ksmbd_session_destroy(sess);
 		}
 	}