netfilter: nf_tables: allow netdevice to be used only once per flowtable

Allow netdevice only once per flowtable, otherwise hit EEXIST.
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>

net/netfilter/nf_tables_api.c

@@ -1538,6 +1538,19 @@ static struct nft_hook *nft_netdev_hook_alloc(struct net *net,
 	return ERR_PTR(err);
 }
 
+static bool nft_hook_list_find(struct list_head *hook_list,
+			       const struct nft_hook *this)
+{
+	struct nft_hook *hook;
+
+	list_for_each_entry(hook, hook_list, list) {
+		if (this->ops.dev == hook->ops.dev)
+			return true;
+	}
+
+	return false;
+}
+
 static int nf_tables_parse_netdev_hooks(struct net *net,
 					const struct nlattr *attr,
 					struct list_head *hook_list)
@@ -1557,6 +1570,10 @@ static int nf_tables_parse_netdev_hooks(struct net *net,
 			err = PTR_ERR(hook);
 			goto err_hook;
 		}
+		if (nft_hook_list_find(hook_list, hook)) {
+			err = -EEXIST;
+			goto err_hook;
+		}
 		list_add_tail(&hook->list, hook_list);
 		n++;