netfilter: nftables: fix possible UAF over chains from packet path in netns

Although hooks are released via call_rcu(), chain and rule objects are
immediately released while packets are still walking over these bits.

This patch adds the .pre_exit callback which is invoked before
synchronize_rcu() in the netns framework to stay safe.

Remove a comment which is not valid anymore since the core does not use
synchronize_net() anymore since 8c873e21 ("netfilter: core: free
hooks with call_rcu").
Suggested-by: Florian Westphal <fw@strlen.de>
Fixes: df05ef87 ("netfilter: nf_tables: release objects on netns destruction")
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>

net/netfilter/nf_tables_api.c

@@ -8949,6 +8949,17 @@ int __nft_release_basechain(struct nft_ctx *ctx)
 }
 EXPORT_SYMBOL_GPL(__nft_release_basechain);
 
+static void __nft_release_hooks(struct net *net)
+{
+	struct nft_table *table;
+	struct nft_chain *chain;
+
+	list_for_each_entry(table, &net->nft.tables, list) {
+		list_for_each_entry(chain, &table->chains, list)
+			nf_tables_unregister_hook(net, table, chain);
+	}
+}
+
 static void __nft_release_tables(struct net *net)
 {
 	struct nft_flowtable *flowtable, *nf;
@@ -8964,10 +8975,6 @@ static void __nft_release_tables(struct net *net)
 
 	list_for_each_entry_safe(table, nt, &net->nft.tables, list) {
 		ctx.family = table->family;
-
-		list_for_each_entry(chain, &table->chains, list)
-			nf_tables_unregister_hook(net, table, chain);
-		/* No packets are walking on these chains anymore. */
 		ctx.table = table;
 		list_for_each_entry(chain, &table->chains, list) {
 			ctx.chain = chain;
@@ -9016,6 +9023,11 @@ static int __net_init nf_tables_init_net(struct net *net)
 	return 0;
 }
 
+static void __net_exit nf_tables_pre_exit_net(struct net *net)
+{
+	__nft_release_hooks(net);
+}
+
 static void __net_exit nf_tables_exit_net(struct net *net)
 {
 	mutex_lock(&net->nft.commit_mutex);
@@ -9029,8 +9041,9 @@ static void __net_exit nf_tables_exit_net(struct net *net)
 }
 
 static struct pernet_operations nf_tables_net_ops = {
-	.init	= nf_tables_init_net,
-	.exit	= nf_tables_exit_net,
+	.init		= nf_tables_init_net,
+	.pre_exit	= nf_tables_pre_exit_net,
+	.exit		= nf_tables_exit_net,
 };
 
 static int __init nf_tables_module_init(void)