udp: ipv4: must add synchronization in udp_sk_rx_dst_set()

Unlike TCP, UDP input path does not hold the socket lock.

Before messing with sk->sk_rx_dst, we must use a spinlock, otherwise
multiple cpus could leak a refcount.

This patch also takes care of renewing a stale dst entry.
(When the sk->sk_rx_dst would not be used by IP early demux)

Fixes: 421b3885bf6d ("udp: ipv4: Add udp early demux")
Signed-off-by: Eric Dumazet <edumazet@google.com>
Cc: Shawn Bohrer <sbohrer@rgmadvisors.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/ipv4/udp.c

@@ -1599,12 +1599,21 @@ static void flush_stack(struct sock **stack, unsigned int count,
 		kfree_skb(skb1);
 }
 
-static void udp_sk_rx_dst_set(struct sock *sk, const struct sk_buff *skb)
+/* For TCP sockets, sk_rx_dst is protected by socket lock
+ * For UDP, we use sk_dst_lock to guard against concurrent changes.
+ */
+static void udp_sk_rx_dst_set(struct sock *sk, struct dst_entry *dst)
 {
-	struct dst_entry *dst = skb_dst(skb);
+	struct dst_entry *old;
 
-	dst_hold(dst);
-	sk->sk_rx_dst = dst;
+	spin_lock(&sk->sk_dst_lock);
+	old = sk->sk_rx_dst;
+	if (likely(old != dst)) {
+		dst_hold(dst);
+		sk->sk_rx_dst = dst;
+		dst_release(old);
+	}
+	spin_unlock(&sk->sk_dst_lock);
 }
 
 /*
@@ -1737,10 +1746,11 @@ int __udp4_lib_rcv(struct sk_buff *skb, struct udp_table *udptable,
 
 	sk = skb_steal_sock(skb);
 	if (sk) {
+		struct dst_entry *dst = skb_dst(skb);
 		int ret;
 
-		if (unlikely(sk->sk_rx_dst == NULL))
-			udp_sk_rx_dst_set(sk, skb);
+		if (unlikely(sk->sk_rx_dst != dst))
+			udp_sk_rx_dst_set(sk, dst);
 
 		ret = udp_queue_rcv_skb(sk, skb);
 		sock_put(sk);