netfilter: ebtables: do not hook tables by default

If any of these modules is loaded, hooks get registered in all netns:

Before: 'unshare -n nft list hooks' shows:
family bridge hook prerouting {
	-2147483648 ebt_broute
	-0000000300 ebt_nat_hook
}
family bridge hook input {
	-0000000200 ebt_filter_hook
}
family bridge hook forward {
	-0000000200 ebt_filter_hook
}
family bridge hook output {
	+0000000100 ebt_nat_hook
	+0000000200 ebt_filter_hook
}
family bridge hook postrouting {
	+0000000300 ebt_nat_hook
}

This adds 'template 'tables' for ebtables.

Each ebtable_foo registers the table as a template, with an init function
that gets called once the first get/setsockopt call is made.

ebtables core then searches the (per netns) list of tables.
If no table is found, it searches the list of templates instead.
If a template entry exists, the init function is called which will
enable the table and register the hooks (so packets are diverted
to the table).

If no entry is found in the template list, request_module is called.

After this, hook registration is delayed until the 'ebtables'
(set/getsockopt) request is made for a given table and will only
happen in the specific namespace.

Signed-off-by: Florian Westphal <fw@strlen.de>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>

1 files changed, 95 insertions, 14 deletions

diff --git a/net/bridge/netfilter/ebtables.c b/net/bridge/netfilter/ebtables.c
index f022deb3721e79..83d1798dfbb444 100644
--- a/net/bridge/netfilter/ebtables.c
+++ b/net/bridge/netfilter/ebtables.c
@@ -44,7 +44,16 @@ struct ebt_pernet {
 	struct list_head tables;
 };
 
+struct ebt_template {
+	struct list_head list;
+	char name[EBT_TABLE_MAXNAMELEN];
+	struct module *owner;
+	/* called when table is needed in the given netns */
+	int (*table_init)(struct net *net);
+};
+
 static unsigned int ebt_pernet_id __read_mostly;
+static LIST_HEAD(template_tables);
 static DEFINE_MUTEX(ebt_mutex);
 
 #ifdef CONFIG_NETFILTER_XTABLES_COMPAT
@@ -309,30 +318,57 @@ letscontinue:
 
 /* If it succeeds, returns element and locks mutex */
 static inline void *
-find_inlist_lock_noload(struct list_head *head, const char *name, int *error,
+find_inlist_lock_noload(struct net *net, const char *name, int *error,
 			struct mutex *mutex)
 {
-	struct {
-		struct list_head list;
-		char name[EBT_FUNCTION_MAXNAMELEN];
-	} *e;
+	struct ebt_pernet *ebt_net = net_generic(net, ebt_pernet_id);
+	struct ebt_template *tmpl;
+	struct ebt_table *table;
 
 	mutex_lock(mutex);
-	list_for_each_entry(e, head, list) {
-		if (strcmp(e->name, name) == 0)
-			return e;
+	list_for_each_entry(table, &ebt_net->tables, list) {
+		if (strcmp(table->name, name) == 0)
+			return table;
 	}
+
+	list_for_each_entry(tmpl, &template_tables, list) {
+		if (strcmp(name, tmpl->name) == 0) {
+			struct module *owner = tmpl->owner;
+
+			if (!try_module_get(owner))
+				goto out;
+
+			mutex_unlock(mutex);
+
+			*error = tmpl->table_init(net);
+			if (*error) {
+				module_put(owner);
+				return NULL;
+			}
+
+			mutex_lock(mutex);
+			module_put(owner);
+			break;
+		}
+	}
+
+	list_for_each_entry(table, &ebt_net->tables, list) {
+		if (strcmp(table->name, name) == 0)
+			return table;
+	}
+
+out:
 	*error = -ENOENT;
 	mutex_unlock(mutex);
 	return NULL;
 }
 
 static void *
-find_inlist_lock(struct list_head *head, const char *name, const char *prefix,
+find_inlist_lock(struct net *net, const char *name, const char *prefix,
 		 int *error, struct mutex *mutex)
 {
 	return try_then_request_module(
-			find_inlist_lock_noload(head, name, error, mutex),
+			find_inlist_lock_noload(net, name, error, mutex),
 			"%s%s", prefix, name);
 }
 
@@ -340,10 +376,7 @@ static inline struct ebt_table *
 find_table_lock(struct net *net, const char *name, int *error,
 		struct mutex *mutex)
 {
-	struct ebt_pernet *ebt_net = net_generic(net, ebt_pernet_id);
-
-	return find_inlist_lock(&ebt_net->tables, name,
-				"ebtable_", error, mutex);
+	return find_inlist_lock(net, name, "ebtable_", error, mutex);
 }
 
 static inline void ebt_free_table_info(struct ebt_table_info *info)
@@ -1258,6 +1291,54 @@ out:
 	return ret;
 }
 
+int ebt_register_template(const struct ebt_table *t, int (*table_init)(struct net *net))
+{
+	struct ebt_template *tmpl;
+
+	mutex_lock(&ebt_mutex);
+	list_for_each_entry(tmpl, &template_tables, list) {
+		if (WARN_ON_ONCE(strcmp(t->name, tmpl->name) == 0)) {
+			mutex_unlock(&ebt_mutex);
+			return -EEXIST;
+		}
+	}
+
+	tmpl = kzalloc(sizeof(*tmpl), GFP_KERNEL);
+	if (!tmpl) {
+		mutex_unlock(&ebt_mutex);
+		return -ENOMEM;
+	}
+
+	tmpl->table_init = table_init;
+	strscpy(tmpl->name, t->name, sizeof(tmpl->name));
+	tmpl->owner = t->me;
+	list_add(&tmpl->list, &template_tables);
+
+	mutex_unlock(&ebt_mutex);
+	return 0;
+}
+EXPORT_SYMBOL(ebt_register_template);
+
+void ebt_unregister_template(const struct ebt_table *t)
+{
+	struct ebt_template *tmpl;
+
+	mutex_lock(&ebt_mutex);
+	list_for_each_entry(tmpl, &template_tables, list) {
+		if (strcmp(t->name, tmpl->name))
+			continue;
+
+		list_del(&tmpl->list);
+		mutex_unlock(&ebt_mutex);
+		kfree(tmpl);
+		return;
+	}
+
+	mutex_unlock(&ebt_mutex);
+	WARN_ON_ONCE(1);
+}
+EXPORT_SYMBOL(ebt_unregister_template);
+
 static struct ebt_table *__ebt_find_table(struct net *net, const char *name)
 {
 	struct ebt_pernet *ebt_net = net_generic(net, ebt_pernet_id);