tcp: fix potential double free issue for fastopen_req

tp->fastopen_req could potentially be double freed if a malicious
user does the following:
1. Enable TCP_FASTOPEN_CONNECT sockopt and do a connect() on the socket.
2. Call connect() with AF_UNSPEC to disconnect the socket.
3. Make this socket a listening socket by calling listen().
4. Accept incoming connections and generate child sockets. All child
   sockets will get a copy of the pointer of fastopen_req.
5. Call close() on all sockets. fastopen_req will get freed multiple
   times.

Fixes: 19f6d3f3 ("net/tcp-fastopen: Add new API support")
Reported-by: NAndrey Konovalov <andreyknvl@google.com>
Signed-off-by: NWei Wang <weiwan@google.com>
Signed-off-by: NEric Dumazet <edumazet@google.com>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>

net/ipv4/tcp.c

@@ -1110,9 +1110,14 @@ static int tcp_sendmsg_fastopen(struct sock *sk, struct msghdr *msg,
 	flags = (msg->msg_flags & MSG_DONTWAIT) ? O_NONBLOCK : 0;
 	err = __inet_stream_connect(sk->sk_socket, msg->msg_name,
 				    msg->msg_namelen, flags, 1);
-	inet->defer_connect = 0;
-	*copied = tp->fastopen_req->copied;
-	tcp_free_fastopen_req(tp);
+	/* fastopen_req could already be freed in __inet_stream_connect
+	 * if the connection times out or gets rst
+	 */
+	if (tp->fastopen_req) {
+		*copied = tp->fastopen_req->copied;
+		tcp_free_fastopen_req(tp);
+		inet->defer_connect = 0;
+	}
 	return err;
 }
 
@@ -2318,6 +2323,10 @@ int tcp_disconnect(struct sock *sk, int flags)
 	memset(&tp->rx_opt, 0, sizeof(tp->rx_opt));
 	__sk_dst_reset(sk);
 
+	/* Clean up fastopen related fields */
+	tcp_free_fastopen_req(tp);
+	inet->defer_connect = 0;
+
 	WARN_ON(inet->inet_num && !icsk->icsk_bind_hash);
 
 	sk->sk_error_report(sk);