net/x25: Fix to not accept on connected socket

If you call listen() and accept() on an already connect()ed x25 socket, accept() can successfully connect. This is because when the peer socket sends data to sendmsg, the skb with its own sk stored in the connected socket's sk->sk_receive_queue is connected, and x25_accept() dequeues the skb waiting in the sk->sk_receive_queue.

This creates a child socket with the sk of the parent x25 socket, which can cause confusion.

Fix x25_listen() to return -EINVAL if the socket has already been successfully connected, and add lock_sock to prevent this issue.

---
v1 -> v2 : Change the flag to check to SS_UNCONNECTED v2 -> v3 : Fix wrong patch description

---
 net/x25/af_x25.c | 6 ++++++
 1 file changed, 6 insertions(+)

index 3b55502b2965..5c7ad301d742 100644
@@ -482,6 +482,12 @@ static int x25_listen(struct socket *sock, int backlog)
 	int rc = -EOPNOTSUPP;
 
 	lock_sock(sk);
+	if (sock->state != SS_UNCONNECTED) {
+		rc = -EINVAL;
+		release_sock(sk);
+		return rc;
+	}
+
 	if (sk->sk_state != TCP_LISTEN) {
 		memset(&x25_sk(sk)->dest_addr, 0, X25_ADDR_LEN);
 		sk->sk_max_ack_backlog = backlog;

2.25.1