net: datagram: fix data-races in datagram_poll()

datagram_poll() runs locklessly, we should add READ_ONCE()
annotations while reading sk->sk_err, sk->sk_shutdown and sk->sk_state.

Fixes: 1da177e4 ("Linux-2.6.12-rc2")
Signed-off-by: Eric Dumazet <edumazet@google.com>
Reviewed-by: Kuniyuki Iwashima <kuniyu@amazon.com>
Link: https://lore.kernel.org/r/20230509173131.3263780-1-edumazet@google.comSigned-off-by: Jakub Kicinski <kuba@kernel.org>

net/core/datagram.c

@@ -807,18 +807,21 @@ __poll_t datagram_poll(struct file *file, struct socket *sock,
 {
 	struct sock *sk = sock->sk;
 	__poll_t mask;
+	u8 shutdown;
 
 	sock_poll_wait(file, sock, wait);
 	mask = 0;
 
 	/* exceptional events? */
-	if (sk->sk_err || !skb_queue_empty_lockless(&sk->sk_error_queue))
+	if (READ_ONCE(sk->sk_err) ||
+	    !skb_queue_empty_lockless(&sk->sk_error_queue))
 		mask |= EPOLLERR |
 			(sock_flag(sk, SOCK_SELECT_ERR_QUEUE) ? EPOLLPRI : 0);
 
-	if (sk->sk_shutdown & RCV_SHUTDOWN)
+	shutdown = READ_ONCE(sk->sk_shutdown);
+	if (shutdown & RCV_SHUTDOWN)
 		mask |= EPOLLRDHUP | EPOLLIN | EPOLLRDNORM;
-	if (sk->sk_shutdown == SHUTDOWN_MASK)
+	if (shutdown == SHUTDOWN_MASK)
 		mask |= EPOLLHUP;
 
 	/* readable? */
@@ -827,10 +830,12 @@ __poll_t datagram_poll(struct file *file, struct socket *sock,
 
 	/* Connection-based need to check for termination and startup */
 	if (connection_based(sk)) {
-		if (sk->sk_state == TCP_CLOSE)
+		int state = READ_ONCE(sk->sk_state);
+
+		if (state == TCP_CLOSE)
 			mask |= EPOLLHUP;
 		/* connection hasn't started yet? */
-		if (sk->sk_state == TCP_SYN_SENT)
+		if (state == TCP_SYN_SENT)
 			return mask;
 	}