sctp: create udp6 sock and set its encap_rcv

This patch is to add the udp6 sock part in sctp_udp_sock_start/stop().
udp_conf.use_udp6_rx_checksums is set to true, as:

   "The SCTP checksum MUST be computed for IPv4 and IPv6, and the UDP
    checksum SHOULD be computed for IPv4 and IPv6"

says in rfc6951#section-5.3.

v1->v2:
  - Add pr_err() when fails to create udp v6 sock.
  - Add #if IS_ENABLED(CONFIG_IPV6) not to create v6 sock when ipv6 is
    disabled.
Signed-off-by: Xin Long <lucien.xin@gmail.com>
Acked-by: Marcelo Ricardo Leitner <marcelo.leitner@gmail.com>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>

include/net/netns/sctp.h

@@ -24,6 +24,7 @@ struct netns_sctp {
 
 	/* UDP tunneling listening sock. */
 	struct sock *udp4_sock;
+	struct sock *udp6_sock;
 	/* UDP tunneling listening port. */
 	int udp_port;
 

net/sctp/protocol.c

@@ -869,6 +869,28 @@ int sctp_udp_sock_start(struct net *net)
 	setup_udp_tunnel_sock(net, sock, &tuncfg);
 	net->sctp.udp4_sock = sock->sk;
 
+#if IS_ENABLED(CONFIG_IPV6)
+	memset(&udp_conf, 0, sizeof(udp_conf));
+
+	udp_conf.family = AF_INET6;
+	udp_conf.local_ip6 = in6addr_any;
+	udp_conf.local_udp_port = htons(net->sctp.udp_port);
+	udp_conf.use_udp6_rx_checksums = true;
+	udp_conf.ipv6_v6only = true;
+	err = udp_sock_create(net, &udp_conf, &sock);
+	if (err) {
+		pr_err("Failed to create the SCTP UDP tunneling v6 sock\n");
+		udp_tunnel_sock_release(net->sctp.udp4_sock->sk_socket);
+		net->sctp.udp4_sock = NULL;
+		return err;
+	}
+
+	tuncfg.encap_type = 1;
+	tuncfg.encap_rcv = sctp_udp_rcv;
+	setup_udp_tunnel_sock(net, sock, &tuncfg);
+	net->sctp.udp6_sock = sock->sk;
+#endif
+
 	return 0;
 }
 
@@ -878,6 +900,10 @@ void sctp_udp_sock_stop(struct net *net)
 		udp_tunnel_sock_release(net->sctp.udp4_sock->sk_socket);
 		net->sctp.udp4_sock = NULL;
 	}
+	if (net->sctp.udp6_sock) {
+		udp_tunnel_sock_release(net->sctp.udp6_sock->sk_socket);
+		net->sctp.udp6_sock = NULL;
+	}
 }
 
 /* Register address family specific functions. */