[NETNS]: Double free in netlink_release.

Netlink protocol table is global for all namespaces. Some netlink
protocols have been virtualized, i.e. they have per/namespace netlink
socket. This difference can easily lead to double free if more than 1
namespace is started. Count the number of kernel netlink sockets to
track that this table is not used any more.
Signed-off-by: Denis V. Lunev <den@openvz.org>
Tested-by: Alexey Dobriyan <adobriyan@openvz.org>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/netlink/af_netlink.c

@@ -498,9 +498,12 @@ static int netlink_release(struct socket *sock)
 
 	netlink_table_grab();
 	if (netlink_is_kernel(sk)) {
-		kfree(nl_table[sk->sk_protocol].listeners);
-		nl_table[sk->sk_protocol].module = NULL;
-		nl_table[sk->sk_protocol].registered = 0;
+		BUG_ON(nl_table[sk->sk_protocol].registered == 0);
+		if (--nl_table[sk->sk_protocol].registered == 0) {
+			kfree(nl_table[sk->sk_protocol].listeners);
+			nl_table[sk->sk_protocol].module = NULL;
+			nl_table[sk->sk_protocol].registered = 0;
+		}
 	} else if (nlk->subscriptions)
 		netlink_update_listeners(sk);
 	netlink_table_ungrab();
@@ -1389,6 +1392,7 @@ netlink_kernel_create(struct net *net, int unit, unsigned int groups,
 		nl_table[unit].registered = 1;
 	} else {
 		kfree(listeners);
+		nl_table[unit].registered++;
 	}
 	netlink_table_ungrab();