netfilter: nf_conntrack: fix BUG_ON while removing nf_conntrack with netns

canqun zhang reported that we're hitting BUG_ON in the
nf_conntrack_destroy path when calling kfree_skb while
rmmod'ing the nf_conntrack module.

Currently, the nf_ct_destroy hook is being set to NULL in the
destroy path of conntrack.init_net. However, this is a problem
since init_net may be destroyed before any other existing netns
(we cannot assume any specific ordering while releasing existing
netns according to what I read in recent emails).

Thanks to Gao feng for initial patch to address this issue.
Reported-by: canqun zhang <canqunzhang@gmail.com>
Acked-by: Gao feng <gaofeng@cn.fujitsu.com>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>

include/net/netfilter/nf_conntrack_core.h

@@ -31,6 +31,8 @@ extern void nf_conntrack_cleanup(struct net *net);
 extern int nf_conntrack_proto_init(struct net *net);
 extern void nf_conntrack_proto_fini(struct net *net);
 
+extern void nf_conntrack_cleanup_end(void);
+
 extern bool
 nf_ct_get_tuple(const struct sk_buff *skb,
 		unsigned int nhoff,

net/netfilter/nf_conntrack_core.c

@@ -1376,11 +1376,12 @@ void nf_conntrack_cleanup(struct net *net)
 	synchronize_net();
 	nf_conntrack_proto_fini(net);
 	nf_conntrack_cleanup_net(net);
+}
 
-	if (net_eq(net, &init_net)) {
+void nf_conntrack_cleanup_end(void)
+{
 	RCU_INIT_POINTER(nf_ct_destroy, NULL);
 	nf_conntrack_cleanup_init_net();
-	}
 }
 
 void *nf_ct_alloc_hashtable(unsigned int *sizep, int nulls)

net/netfilter/nf_conntrack_standalone.c

@@ -575,6 +575,7 @@ static int __init nf_conntrack_standalone_init(void)
 static void __exit nf_conntrack_standalone_fini(void)
 {
 	unregister_pernet_subsys(&nf_conntrack_net_ops);
+	nf_conntrack_cleanup_end();
 }
 
 module_init(nf_conntrack_standalone_init);