[PATCH net] ipv6: fix a race in ip6_sock_set_v6only()

[Eric Dumazet <edumazet@google.com>]

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: 9b115749acb24 ("ipv6: add ip6_sock_set_v6only")
Signed-off-by: Eric Dumazet <edumazet@google.com>
---
 include/net/ipv6.h | 11 +++++++----
 1 file changed, 7 insertions(+), 4 deletions(-)

diff --git a/include/net/ipv6.h b/include/net/ipv6.h
index cc56e09525d03b8e12dd92cec2751c2a13f16c10..53c5056508be543ae5d312e26dea6ed78fcc04ee 100644
--- a/include/net/ipv6.h
+++ b/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)
-- 
2.53.0.310.g728cabbaf7-goog