tcp: Fix data-races around sysctl_tcp_workaround_signed_windows.

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

Fixes: 15d99e02 ("[TCP]: sysctl to allow TCP window > 32767 sans wscale")
Signed-off-by: Kuniyuki Iwashima <kuniyu@amazon.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/ipv4/tcp_output.c

@@ -230,7 +230,7 @@ void tcp_select_initial_window(const struct sock *sk, int __space, __u32 mss,
 	 * which we interpret as a sign the remote TCP is not
 	 * misinterpreting the window field as a signed quantity.
 	 */
-	if (sock_net(sk)->ipv4.sysctl_tcp_workaround_signed_windows)
+	if (READ_ONCE(sock_net(sk)->ipv4.sysctl_tcp_workaround_signed_windows))
 		(*rcv_wnd) = min(space, MAX_TCP_WINDOW);
 	else
 		(*rcv_wnd) = min_t(u32, space, U16_MAX);
@@ -285,7 +285,7 @@ static u16 tcp_select_window(struct sock *sk)
 	 * scaled window.
 	 */
 	if (!tp->rx_opt.rcv_wscale &&
-	    sock_net(sk)->ipv4.sysctl_tcp_workaround_signed_windows)
+	    READ_ONCE(sock_net(sk)->ipv4.sysctl_tcp_workaround_signed_windows))
 		new_win = min(new_win, MAX_TCP_WINDOW);
 	else
 		new_win = min(new_win, (65535U << tp->rx_opt.rcv_wscale));

net/mptcp/options.c

@@ -1271,7 +1271,7 @@ static void mptcp_set_rwin(struct tcp_sock *tp, struct tcphdr *th)
 		if (unlikely(th->syn))
 			new_win = min(new_win, 65535U) << tp->rx_opt.rcv_wscale;
 		if (!tp->rx_opt.rcv_wscale &&
-		    sock_net(ssk)->ipv4.sysctl_tcp_workaround_signed_windows)
+		    READ_ONCE(sock_net(ssk)->ipv4.sysctl_tcp_workaround_signed_windows))
 			new_win = min(new_win, MAX_TCP_WINDOW);
 		else
 			new_win = min(new_win, (65535U << tp->rx_opt.rcv_wscale));