netfilter: arp_tables: add pre_exit hook for table unregister

Same problem that also existed in iptables/ip(6)tables, when
arptable_filter is removed there is no longer a wait period before the
table/ruleset is free'd.

Unregister the hook in pre_exit, then remove the table in the exit
function.
This used to work correctly because the old nf_hook_unregister API
did unconditional synchronize_net.

The per-net hook unregister function uses call_rcu instead.

Fixes: b9e69e1 ("netfilter: xtables: don't hook tables by default")
Signed-off-by: Florian Westphal <fw@strlen.de>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>

include/linux/netfilter_arp/arp_tables.h

@@ -52,8 +52,9 @@ extern void *arpt_alloc_initial_table(const struct xt_table *);
 int arpt_register_table(struct net *net, const struct xt_table *table,
 			const struct arpt_replace *repl,
 			const struct nf_hook_ops *ops, struct xt_table **res);
-void arpt_unregister_table(struct net *net, struct xt_table *table,
-			   const struct nf_hook_ops *ops);
+void arpt_unregister_table(struct net *net, struct xt_table *table);
+void arpt_unregister_table_pre_exit(struct net *net, struct xt_table *table,
+				    const struct nf_hook_ops *ops);
 extern unsigned int arpt_do_table(struct sk_buff *skb,
 				  const struct nf_hook_state *state,
 				  struct xt_table *table);

net/ipv4/netfilter/arp_tables.c

@@ -1539,10 +1539,15 @@ int arpt_register_table(struct net *net,
 	return ret;
 }
 
-void arpt_unregister_table(struct net *net, struct xt_table *table,
-			   const struct nf_hook_ops *ops)
+void arpt_unregister_table_pre_exit(struct net *net, struct xt_table *table,
+				    const struct nf_hook_ops *ops)
 {
 	nf_unregister_net_hooks(net, ops, hweight32(table->valid_hooks));
+}
+EXPORT_SYMBOL(arpt_unregister_table_pre_exit);
+
+void arpt_unregister_table(struct net *net, struct xt_table *table)
+{
 	__arpt_unregister_table(net, table);
 }
 

net/ipv4/netfilter/arptable_filter.c

@@ -56,16 +56,24 @@ static int __net_init arptable_filter_table_init(struct net *net)
 	return err;
 }
 
+static void __net_exit arptable_filter_net_pre_exit(struct net *net)
+{
+	if (net->ipv4.arptable_filter)
+		arpt_unregister_table_pre_exit(net, net->ipv4.arptable_filter,
+					       arpfilter_ops);
+}
+
 static void __net_exit arptable_filter_net_exit(struct net *net)
 {
 	if (!net->ipv4.arptable_filter)
 		return;
-	arpt_unregister_table(net, net->ipv4.arptable_filter, arpfilter_ops);
+	arpt_unregister_table(net, net->ipv4.arptable_filter);
 	net->ipv4.arptable_filter = NULL;
 }
 
 static struct pernet_operations arptable_filter_net_ops = {
 	.exit = arptable_filter_net_exit,
+	.pre_exit = arptable_filter_net_pre_exit,
 };
 
 static int __init arptable_filter_init(void)