tcp: Fix data-races around sysctl_tcp_fastopen.

While reading sysctl_tcp_fastopen, it can be changed concurrently.
Thus, we need to add READ_ONCE() to its readers.

Fixes: 2100c8d2 ("net-tcp: Fast Open base")
Signed-off-by: Kuniyuki Iwashima <kuniyu@amazon.com>
Acked-by: Yuchung Cheng <ycheng@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/ipv4/af_inet.c

@@ -217,7 +217,7 @@ int inet_listen(struct socket *sock, int backlog)
 		 * because the socket was in TCP_LISTEN state previously but
 		 * was shutdown() rather than close().
 		 */
-		tcp_fastopen = sock_net(sk)->ipv4.sysctl_tcp_fastopen;
+		tcp_fastopen = READ_ONCE(sock_net(sk)->ipv4.sysctl_tcp_fastopen);
 		if ((tcp_fastopen & TFO_SERVER_WO_SOCKOPT1) &&
 		    (tcp_fastopen & TFO_SERVER_ENABLE) &&
 		    !inet_csk(sk)->icsk_accept_queue.fastopenq.max_qlen) {

net/ipv4/tcp.c

@@ -1150,7 +1150,8 @@ static int tcp_sendmsg_fastopen(struct sock *sk, struct msghdr *msg,
 	struct sockaddr *uaddr = msg->msg_name;
 	int err, flags;
 
-	if (!(sock_net(sk)->ipv4.sysctl_tcp_fastopen & TFO_CLIENT_ENABLE) ||
+	if (!(READ_ONCE(sock_net(sk)->ipv4.sysctl_tcp_fastopen) &
+	      TFO_CLIENT_ENABLE) ||
 	    (uaddr && msg->msg_namelen >= sizeof(uaddr->sa_family) &&
 	     uaddr->sa_family == AF_UNSPEC))
 		return -EOPNOTSUPP;
@@ -3617,7 +3618,8 @@ static int do_tcp_setsockopt(struct sock *sk, int level, int optname,
 	case TCP_FASTOPEN_CONNECT:
 		if (val > 1 || val < 0) {
 			err = -EINVAL;
-		} else if (net->ipv4.sysctl_tcp_fastopen & TFO_CLIENT_ENABLE) {
+		} else if (READ_ONCE(net->ipv4.sysctl_tcp_fastopen) &
+			   TFO_CLIENT_ENABLE) {
 			if (sk->sk_state == TCP_CLOSE)
 				tp->fastopen_connect = val;
 			else

net/ipv4/tcp_fastopen.c

@@ -332,7 +332,7 @@ static bool tcp_fastopen_no_cookie(const struct sock *sk,
 				   const struct dst_entry *dst,
 				   int flag)
 {
-	return (sock_net(sk)->ipv4.sysctl_tcp_fastopen & flag) ||
+	return (READ_ONCE(sock_net(sk)->ipv4.sysctl_tcp_fastopen) & flag) ||
 	       tcp_sk(sk)->fastopen_no_cookie ||
 	       (dst && dst_metric(dst, RTAX_FASTOPEN_NO_COOKIE));
 }
@@ -347,7 +347,7 @@ struct sock *tcp_try_fastopen(struct sock *sk, struct sk_buff *skb,
 			      const struct dst_entry *dst)
 {
 	bool syn_data = TCP_SKB_CB(skb)->end_seq != TCP_SKB_CB(skb)->seq + 1;
-	int tcp_fastopen = sock_net(sk)->ipv4.sysctl_tcp_fastopen;
+	int tcp_fastopen = READ_ONCE(sock_net(sk)->ipv4.sysctl_tcp_fastopen);
 	struct tcp_fastopen_cookie valid_foc = { .len = -1 };
 	struct sock *child;
 	int ret = 0;