tcp: annotate data races around tp->mtu_info

While tp->mtu_info is read while socket is owned, the write
sides happen from err handlers (tcp_v[46]_mtu_reduced)
which only own the socket spinlock.

Fixes: 563d34d0 ("tcp: dont drop MTU reduction indications")
Signed-off-by: Eric Dumazet <edumazet@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/ipv4/tcp_ipv4.c

@@ -342,7 +342,7 @@ void tcp_v4_mtu_reduced(struct sock *sk)
 
 	if ((1 << sk->sk_state) & (TCPF_LISTEN | TCPF_CLOSE))
 		return;
-	mtu = tcp_sk(sk)->mtu_info;
+	mtu = READ_ONCE(tcp_sk(sk)->mtu_info);
 	dst = inet_csk_update_pmtu(sk, mtu);
 	if (!dst)
 		return;
@@ -546,7 +546,7 @@ int tcp_v4_err(struct sk_buff *skb, u32 info)
 			if (sk->sk_state == TCP_LISTEN)
 				goto out;
 
-			tp->mtu_info = info;
+			WRITE_ONCE(tp->mtu_info, info);
 			if (!sock_owned_by_user(sk)) {
 				tcp_v4_mtu_reduced(sk);
 			} else {

net/ipv6/tcp_ipv6.c

@@ -352,7 +352,7 @@ static void tcp_v6_mtu_reduced(struct sock *sk)
 	if ((1 << sk->sk_state) & (TCPF_LISTEN | TCPF_CLOSE))
 		return;
 
-	dst = inet6_csk_update_pmtu(sk, tcp_sk(sk)->mtu_info);
+	dst = inet6_csk_update_pmtu(sk, READ_ONCE(tcp_sk(sk)->mtu_info));
 	if (!dst)
 		return;
 
@@ -443,7 +443,7 @@ static int tcp_v6_err(struct sk_buff *skb, struct inet6_skb_parm *opt,
 		if (!ip6_sk_accept_pmtu(sk))
 			goto out;
 
-		tp->mtu_info = ntohl(info);
+		WRITE_ONCE(tp->mtu_info, ntohl(info));
 		if (!sock_owned_by_user(sk))
 			tcp_v6_mtu_reduced(sk);
 		else if (!test_and_set_bit(TCP_MTU_REDUCED_DEFERRED,