tcp: Fix data-races around sysctl_tcp_mtu_probing.

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

Fixes: 5d424d5a ("[TCP]: MTU probing")
Signed-off-by: Kuniyuki Iwashima <kuniyu@amazon.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/ipv4/tcp_output.c

@@ -1762,7 +1762,7 @@ void tcp_mtup_init(struct sock *sk)
 	struct inet_connection_sock *icsk = inet_csk(sk);
 	struct net *net = sock_net(sk);
 
-	icsk->icsk_mtup.enabled = net->ipv4.sysctl_tcp_mtu_probing > 1;
+	icsk->icsk_mtup.enabled = READ_ONCE(net->ipv4.sysctl_tcp_mtu_probing) > 1;
 	icsk->icsk_mtup.search_high = tp->rx_opt.mss_clamp + sizeof(struct tcphdr) +
 			       icsk->icsk_af_ops->net_header_len;
 	icsk->icsk_mtup.search_low = tcp_mss_to_mtu(sk, net->ipv4.sysctl_tcp_base_mss);

net/ipv4/tcp_timer.c

@@ -163,7 +163,7 @@ static void tcp_mtu_probing(struct inet_connection_sock *icsk, struct sock *sk)
 	int mss;
 
 	/* Black hole detection */
-	if (!net->ipv4.sysctl_tcp_mtu_probing)
+	if (!READ_ONCE(net->ipv4.sysctl_tcp_mtu_probing))
 		return;
 
 	if (!icsk->icsk_mtup.enabled) {