ipv6: annotate data races around np->min_hopcount

No report yet from KCSAN, yet worth documenting the races.
Signed-off-by: Eric Dumazet <edumazet@google.com>
Acked-by: Soheil Hassas Yeganeh <soheil@google.com>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>

net/ipv6/ipv6_sockglue.c

@@ -950,7 +950,10 @@ static int do_ipv6_setsockopt(struct sock *sk, int level, int optname,
 			goto e_inval;
 		if (val < 0 || val > 255)
 			goto e_inval;
-		np->min_hopcount = val;
+		/* tcp_v6_err() and tcp_v6_rcv() might read min_hopcount
+		 * while we are changing it.
+		 */
+		WRITE_ONCE(np->min_hopcount, val);
 		retv = 0;
 		break;
 	case IPV6_DONTFRAG:

net/ipv6/tcp_ipv6.c

@@ -414,7 +414,8 @@ static int tcp_v6_err(struct sk_buff *skb, struct inet6_skb_parm *opt,
 	if (sk->sk_state == TCP_CLOSE)
 		goto out;
 
-	if (ipv6_hdr(skb)->hop_limit < tcp_inet6_sk(sk)->min_hopcount) {
+	/* min_hopcount can be changed concurrently from do_ipv6_setsockopt() */
+	if (ipv6_hdr(skb)->hop_limit < READ_ONCE(tcp_inet6_sk(sk)->min_hopcount)) {
 		__NET_INC_STATS(net, LINUX_MIB_TCPMINTTLDROP);
 		goto out;
 	}
@@ -1726,7 +1727,8 @@ INDIRECT_CALLABLE_SCOPE int tcp_v6_rcv(struct sk_buff *skb)
 			return 0;
 		}
 	}
-	if (hdr->hop_limit < tcp_inet6_sk(sk)->min_hopcount) {
+	/* min_hopcount can be changed concurrently from do_ipv6_setsockopt() */
+	if (hdr->hop_limit < READ_ONCE(tcp_inet6_sk(sk)->min_hopcount)) {
 		__NET_INC_STATS(net, LINUX_MIB_TCPMINTTLDROP);
 		goto discard_and_relse;
 	}