ipv6: fix a race in ip6_sock_set_v6only()

It is unlikely that this function will be ever called
with isk->inet_num being not zero.

Perform the check on isk->inet_num inside the locked section
for complete safety.

Fixes: 9b11574 ("ipv6: add ip6_sock_set_v6only")
Signed-off-by: Eric Dumazet <[email protected]>
Reviewed-by: Simon Horman <[email protected]>
Reviewed-by: Fernando Fernandez Mancera <[email protected]>
Link: https://patch.msgid.link/[email protected]
Signed-off-by: Jakub Kicinski <[email protected]>

Author: edumazet

include/net/ipv6.h

@@ -1213,12 +1213,15 @@ int ipv6_sock_mc_drop(struct sock *sk, int ifindex,
 
 static inline int ip6_sock_set_v6only(struct sock *sk)
 {
-	if (inet_sk(sk)->inet_num)
-		return -EINVAL;
+	int ret = 0;
+
 	lock_sock(sk);
-	sk->sk_ipv6only = true;
+	if (inet_sk(sk)->inet_num)
+		ret = -EINVAL;
+	else
+		sk->sk_ipv6only = true;
 	release_sock(sk);
-	return 0;
+	return ret;
 }
 
 static inline void ip6_sock_set_recverr(struct sock *sk)