ipv6: tcp: fix race in IPV6_2292PKTOPTIONS

IPv6 TCP sockets store in np->pktoptions skbs, and use skb_set_owner_r()
to charge the skb to socket.

It means that destructor must be called while socket is locked.

Therefore, we cannot use skb_get() or atomic_inc(&skb->users)
to protect ourselves : kfree_skb() might race with other users
manipulating sk->sk_forward_alloc

Fix this race by holding socket lock for the duration of
ip6_datagram_recv_ctl()
Signed-off-by: Eric Dumazet <edumazet@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/ipv6/ipv6_sockglue.c

@@ -996,13 +996,9 @@ static int do_ipv6_getsockopt(struct sock *sk, int level, int optname,
 		lock_sock(sk);
 		skb = np->pktoptions;
 		if (skb)
-			atomic_inc(&skb->users);
-		release_sock(sk);
-
-		if (skb) {
 			ip6_datagram_recv_ctl(sk, &msg, skb);
-			kfree_skb(skb);
-		} else {
+		release_sock(sk);
+		if (!skb) {
 			if (np->rxopt.bits.rxinfo) {
 				struct in6_pktinfo src_info;
 				src_info.ipi6_ifindex = np->mcast_oif ? np->mcast_oif :