l2tp: fix l2tp_ip_sendmsg() route handling

l2tp_ip_sendmsg() in non connected mode incorrectly calls
sk_setup_caps(). Subsequent send() calls send data to wrong destination.

We can also avoid changing dst refcount in connected mode, using
appropriate rcu locking. Once output route lookups can also be done
under rcu, sendto() calls wont change dst refcounts too.
Signed-off-by: NEric Dumazet <eric.dumazet@gmail.com>
CC: James Chapman <jchapman@katalix.com>
Signed-off-by: NDavid S. Miller <davem@conan.davemloft.net>

net/l2tp/l2tp_ip.c

@@ -480,18 +480,16 @@ static int l2tp_ip_sendmsg(struct kiocb *iocb, struct sock *sk, struct msghdr *m
 	if (connected)
 		rt = (struct rtable *) __sk_dst_check(sk, 0);
 
+	rcu_read_lock();
 	if (rt == NULL) {
-		struct ip_options_rcu *inet_opt;
+		const struct ip_options_rcu *inet_opt;
 
-		rcu_read_lock();
 		inet_opt = rcu_dereference(inet->inet_opt);
 
 		/* Use correct destination address if we have options. */
 		if (inet_opt && inet_opt->opt.srr)
 			daddr = inet_opt->opt.faddr;
 
-		rcu_read_unlock();
-
 		/* If this fails, retransmit mechanism of transport layer will
 		 * keep trying until route appears or the connection times
 		 * itself out.
@@ -503,12 +501,20 @@ static int l2tp_ip_sendmsg(struct kiocb *iocb, struct sock *sk, struct msghdr *m
 					   sk->sk_bound_dev_if);
 		if (IS_ERR(rt))
 			goto no_route;
-		sk_setup_caps(sk, &rt->dst);
+		if (connected)
+			sk_setup_caps(sk, &rt->dst);
+		else
+			dst_release(&rt->dst); /* safe since we hold rcu_read_lock */
 	}
-	skb_dst_set(skb, dst_clone(&rt->dst));
+
+	/* We dont need to clone dst here, it is guaranteed to not disappear.
+	 *  __dev_xmit_skb() might force a refcount if needed.
+	 */
+	skb_dst_set_noref(skb, &rt->dst);
 
 	/* Queue the packet to IP for output */
 	rc = ip_queue_xmit(skb, &inet->cork.fl);
+	rcu_read_unlock();
 
 error:
 	/* Update stats */
@@ -525,6 +531,7 @@ static int l2tp_ip_sendmsg(struct kiocb *iocb, struct sock *sk, struct msghdr *m
 	return rc;
 
 no_route:
+	rcu_read_unlock();
 	IP_INC_STATS(sock_net(sk), IPSTATS_MIB_OUTNOROUTES);
 	kfree_skb(skb);
 	rc = -EHOSTUNREACH;