net: sk_clone_lock() should only do get_net() if the parent is not a kernel socket

The newsk returned by sk_clone_lock should hold a get_net()
reference if, and only if, the parent is not a kernel socket
(making this similar to sk_alloc()).

E.g,. for the SYN_RECV path, tcp_v4_syn_recv_sock->..inet_csk_clone_lock
sets up the syn_recv newsk from sk_clone_lock. When the parent (listen)
socket is a kernel socket (defined in sk_alloc() as having
sk_net_refcnt == 0), then the newsk should also have a 0 sk_net_refcnt
and should not hold a get_net() reference.

Fixes: 26abe143 ("net: Modify sk_alloc to not reference count the
      netns of kernel sockets.")
Acked-by: Eric Dumazet <edumazet@google.com>
Cc: Eric W. Biederman <ebiederm@xmission.com>
Signed-off-by: Sowmini Varadhan <sowmini.varadhan@oracle.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/core/sock.c

@@ -1497,7 +1497,8 @@ struct sock *sk_clone_lock(const struct sock *sk, const gfp_t priority)
 		sock_copy(newsk, sk);
 
 		/* SANITY */
-		get_net(sock_net(newsk));
+		if (likely(newsk->sk_net_refcnt))
+			get_net(sock_net(newsk));
 		sk_node_init(&newsk->sk_node);
 		sock_lock_init(newsk);
 		bh_lock_sock(newsk);