mptcp: emit tcp reset when a join request fails

RFC 8684 says:
 If the token is unknown or the host wants to refuse subflow establishment
 (for example, due to a limit on the number of subflows it will permit),
 the receiver will send back a reset (RST) signal, analogous to an unknown
 port in TCP, containing an MP_TCPRST option (Section 3.6) with an
 "MPTCP specific error" reason code.

mptcp-next doesn't support MP_TCPRST yet, this can be added in another
change.
Signed-off-by: Florian Westphal <fw@strlen.de>
Reviewed-by: Mat Martineau <mathew.j.martineau@linux.intel.com>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>

net/mptcp/subflow.c

@@ -112,7 +112,12 @@ static int __subflow_init_req(struct request_sock *req, const struct sock *sk_li
 	return 0;
 }
 
-static void subflow_init_req(struct request_sock *req,
+/* Init mptcp request socket.
+ *
+ * Returns an error code if a JOIN has failed and a TCP reset
+ * should be sent.
+ */
+static int subflow_init_req(struct request_sock *req,
 			    const struct sock *sk_listener,
 			    struct sk_buff *skb)
 {
@@ -125,7 +130,7 @@ static void subflow_init_req(struct request_sock *req,
 
 	ret = __subflow_init_req(req, sk_listener);
 	if (ret)
-		return;
+		return 0;
 
 	mptcp_get_options(skb, &mp_opt);
 
@@ -133,7 +138,7 @@ static void subflow_init_req(struct request_sock *req,
 		SUBFLOW_REQ_INC_STATS(req, MPTCP_MIB_MPCAPABLEPASSIVE);
 
 		if (mp_opt.mp_join)
-			return;
+			return 0;
 	} else if (mp_opt.mp_join) {
 		SUBFLOW_REQ_INC_STATS(req, MPTCP_MIB_JOINSYNRX);
 	}
@@ -157,7 +162,7 @@ static void subflow_init_req(struct request_sock *req,
 			} else {
 				subflow_req->mp_capable = 1;
 			}
-			return;
+			return 0;
 		}
 
 		err = mptcp_token_new_request(req);
@@ -175,7 +180,11 @@ static void subflow_init_req(struct request_sock *req,
 		subflow_req->remote_nonce = mp_opt.nonce;
 		subflow_req->msk = subflow_token_join_request(req, skb);
 
-		if (unlikely(req->syncookie) && subflow_req->msk) {
+		/* Can't fall back to TCP in this case. */
+		if (!subflow_req->msk)
+			return -EPERM;
+
+		if (unlikely(req->syncookie)) {
 			if (mptcp_can_accept_new_subflow(subflow_req->msk))
 				subflow_init_req_cookie_join_save(subflow_req, skb);
 		}
@@ -183,6 +192,8 @@ static void subflow_init_req(struct request_sock *req,
 		pr_debug("token=%u, remote_nonce=%u msk=%p", subflow_req->token,
 			 subflow_req->remote_nonce, subflow_req->msk);
 	}
+
+	return 0;
 }
 
 int mptcp_subflow_init_cookie_req(struct request_sock *req,
@@ -234,6 +245,7 @@ static struct dst_entry *subflow_v4_route_req(const struct sock *sk,
 					      struct request_sock *req)
 {
 	struct dst_entry *dst;
+	int err;
 
 	tcp_rsk(req)->is_mptcp = 1;
 
@@ -241,8 +253,14 @@ static struct dst_entry *subflow_v4_route_req(const struct sock *sk,
 	if (!dst)
 		return NULL;
 
-	subflow_init_req(req, sk, skb);
+	err = subflow_init_req(req, sk, skb);
+	if (err == 0)
 		return dst;
+
+	dst_release(dst);
+	if (!req->syncookie)
+		tcp_request_sock_ops.send_reset(sk, skb);
+	return NULL;
 }
 
 #if IS_ENABLED(CONFIG_MPTCP_IPV6)
@@ -252,6 +270,7 @@ static struct dst_entry *subflow_v6_route_req(const struct sock *sk,
 					      struct request_sock *req)
 {
 	struct dst_entry *dst;
+	int err;
 
 	tcp_rsk(req)->is_mptcp = 1;
 
@@ -259,8 +278,14 @@ static struct dst_entry *subflow_v6_route_req(const struct sock *sk,
 	if (!dst)
 		return NULL;
 
-	subflow_init_req(req, sk, skb);
+	err = subflow_init_req(req, sk, skb);
+	if (err == 0)
 		return dst;
+
+	dst_release(dst);
+	if (!req->syncookie)
+		tcp6_request_sock_ops.send_reset(sk, skb);
+	return NULL;
 }
 #endif