mptcp: do not reset MP_CAPABLE subflow on mapping errors

When some mapping related errors occours we close the main
MPC subflow with a RST. We should instead fallback gracefully
to TCP, and do the reset only for MPJ subflows.

Fixes: d22f498 ("mptcp: process MP_CAPABLE data option")
Closes: multipath-tcp/mptcp_net-next#192
Signed-off-by: Paolo Abeni <pabeni@redhat.com>

net/mptcp/subflow.c

@@ -1109,10 +1109,9 @@ static bool subflow_check_data_avail(struct sock *ssk)
 
 		status = get_mapping_status(ssk, msk);
 		trace_subflow_check_data_avail(status, skb_peek(&ssk->sk_receive_queue));
-		if (unlikely(status == MAPPING_INVALID)) {
-			ssk->sk_err = EBADMSG;
-			goto fatal;
-		}
+		if (unlikely(status == MAPPING_INVALID))
+			goto fallback;
+
 		if (unlikely(status == MAPPING_DUMMY))
 			goto fallback;
 
@@ -1127,10 +1126,8 @@ static bool subflow_check_data_avail(struct sock *ssk)
 		 * MP_CAPABLE-based mapping
 		 */
 		if (unlikely(!READ_ONCE(msk->can_ack))) {
-			if (!subflow->mpc_map) {
-				ssk->sk_err = EBADMSG;
-				goto fatal;
-			}
+			if (!subflow->mpc_map)
+				goto fallback;
 			WRITE_ONCE(msk->remote_key, subflow->remote_key);
 			WRITE_ONCE(msk->ack_seq, subflow->map_seq);
 			WRITE_ONCE(msk->can_ack, true);
@@ -1159,19 +1156,22 @@ static bool subflow_check_data_avail(struct sock *ssk)
 	subflow_sched_work_if_closed(msk, ssk);
 	return false;
 
-fatal:
-	/* fatal protocol error, close the socket */
-	/* This barrier is coupled with smp_rmb() in tcp_poll() */
-	smp_wmb();
-	ssk->sk_error_report(ssk);
-	tcp_set_state(ssk, TCP_CLOSE);
-	subflow->reset_transient = 0;
-	subflow->reset_reason = MPTCP_RST_EMPTCP;
-	tcp_send_active_reset(ssk, GFP_ATOMIC);
-	subflow->data_avail = 0;
-	return false;
-
 fallback:
+	/* RFC 8684 section 3.7. */
+	if (subflow->mp_join || subflow->fully_established) {
+		/* fatal protocol error, close the socket.
+		 * subflow_error_report() will introduce the apprpriate barriers
+		*/
+		ssk->sk_err = EBADMSG;
+		ssk->sk_error_report(ssk);
+		tcp_set_state(ssk, TCP_CLOSE);
+		subflow->reset_transient = 0;
+		subflow->reset_reason = MPTCP_RST_EMPTCP;
+		tcp_send_active_reset(ssk, GFP_ATOMIC);
+		subflow->data_avail = 0;
+		return false;
+	}
+
 	__mptcp_do_fallback(msk);
 	skb = skb_peek(&ssk->sk_receive_queue);
 	subflow->map_valid = 1;