tcp: syncookies: create mptcp request socket for ACK cookies with MPTCP option

If SYN packet contains MP_CAPABLE option, keep it enabled.
Syncokie validation and cookie-based socket creation is changed to
instantiate an mptcp request sockets if the ACK contains an MPTCP
connection request.

Rather than extend both cookie_v4/6_check, add a common helper to create
the (mp)tcp request socket.

Suggested-by: Paolo Abeni <[e-mail]>
Signed-off-by: Florian Westphal <[e-mail]>
Signed-off-by: David S. Miller <[e-mail]>

include/net/tcp.h

···
 int __cookie_v4_check(const struct iphdr *iph, const struct tcphdr *th,
 		      u32 cookie);
 struct sock *cookie_v4_check(struct sock *sk, struct sk_buff *skb);
+struct request_sock *cookie_tcp_reqsk_alloc(const struct request_sock_ops *ops,
+					    struct sock *sk, struct sk_buff *skb);
 #ifdef CONFIG_SYN_COOKIES
 
 /* Syncookies use a monotonic timer which increments every 60 seconds.

net/ipv4/syncookies.c

···
 }
 EXPORT_SYMBOL(cookie_ecn_ok);
 
+struct request_sock *cookie_tcp_reqsk_alloc(const struct request_sock_ops *ops,
+					    struct sock *sk,
+					    struct sk_buff *skb)
+{
+	struct tcp_request_sock *treq;
+	struct request_sock *req;
+
+#ifdef CONFIG_MPTCP
+	if (sk_is_mptcp(sk))
+		ops = &mptcp_subflow_request_sock_ops;
+#endif
+
+	req = inet_reqsk_alloc(ops, sk, false);
+	if (!req)
+		return NULL;
+
+#if IS_ENABLED(CONFIG_MPTCP)
+	treq = tcp_rsk(req);
+	treq->is_mptcp = sk_is_mptcp(sk);
+	if (treq->is_mptcp) {
+		int err = mptcp_subflow_init_cookie_req(req, sk, skb);
+
+		if (err) {
+			reqsk_free(req);
+			return NULL;
+		}
+	}
+#endif
+
+	return req;
+}
+EXPORT_SYMBOL_GPL(cookie_tcp_reqsk_alloc);
+
 /* On input, sk is a listener.
  * Output is listener if incoming packet would not create a child
  *           NULL if memory could not be allocated.
···
 		goto out;
 
 	ret = NULL;
-	req = inet_reqsk_alloc(&tcp_request_sock_ops, sk, false); /* for safety */
+	req = cookie_tcp_reqsk_alloc(&tcp_request_sock_ops, sk, skb);
 	if (!req)
 		goto out;
 
···
 	req->ts_recent		= tcp_opt.saw_tstamp ? tcp_opt.rcv_tsval : 0;
 	treq->snt_synack	= 0;
 	treq->tfo_listener	= false;
-
-	if (IS_ENABLED(CONFIG_MPTCP))
-		treq->is_mptcp = 0;
 
 	if (IS_ENABLED(CONFIG_SMC))
 		ireq->smc_ok = 0;

net/ipv4/tcp_input.c

···
 
 	af_ops->init_req(req, sk, skb);
 
-	if (IS_ENABLED(CONFIG_MPTCP) && want_cookie)
-		tcp_rsk(req)->is_mptcp = 0;
-
 	if (security_inet_conn_request(sk, skb, req))
 		goto drop_and_free;
 

net/ipv6/syncookies.c

···
 		goto out;
 
 	ret = NULL;
-	req = inet_reqsk_alloc(&tcp6_request_sock_ops, sk, false);
+	req = cookie_tcp_reqsk_alloc(&tcp6_request_sock_ops, sk, skb);
 	if (!req)
 		goto out;
 
 	ireq = inet_rsk(req);
 	treq = tcp_rsk(req);
 	treq->tfo_listener = false;
-
-	if (IS_ENABLED(CONFIG_MPTCP))
-		treq->is_mptcp = 0;
 
 	if (security_inet_conn_request(sk, skb, req))
 		goto out_free;