netns: fix double free at netns creation

This patch fix a double free when a network namespace fails.
The previous code does a kfree of the net_generic structure when
one of the init subsystem initialization fails.
The 'setup_net' function does kfree(ng) and returns an error.
The caller, 'copy_net_ns', call net_free on error, and this one
calls kfree(net->gen), making this pointer freed twice.

This patch make the code symetric, the net_alloc does the net_generic
allocation and the net_free frees the net_generic.
Signed-off-by: Daniel Lezcano <daniel.lezcano@free.fr>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/core/net_namespace.c

@@ -32,24 +32,14 @@ static __net_init int setup_net(struct net *net)
 {
 	/* Must be called with net_mutex held */
 	struct pernet_operations *ops;
-	int error;
-	struct net_generic *ng;
+	int error = 0;
 
 	atomic_set(&net->count, 1);
+
 #ifdef NETNS_REFCNT_DEBUG
 	atomic_set(&net->use_count, 0);
 #endif
 
-	error = -ENOMEM;
-	ng = kzalloc(sizeof(struct net_generic) +
-			INITIAL_NET_GEN_PTRS * sizeof(void *), GFP_KERNEL);
-	if (ng == NULL)
-		goto out;
-
-	ng->len = INITIAL_NET_GEN_PTRS;
-	rcu_assign_pointer(net->gen, ng);
-
-	error = 0;
 	list_for_each_entry(ops, &pernet_list, list) {
 		if (ops->init) {
 			error = ops->init(net);
@@ -70,7 +60,6 @@ out_undo:
 	}
 
 	rcu_barrier();
-	kfree(ng);
 	goto out;
 }
 
@@ -78,16 +67,43 @@ out_undo:
 static struct kmem_cache *net_cachep;
 static struct workqueue_struct *netns_wq;
 
-static struct net *net_alloc(void)
+static struct net_generic *net_alloc_generic(void)
 {
-	return kmem_cache_zalloc(net_cachep, GFP_KERNEL);
+	struct net_generic *ng;
+	size_t generic_size = sizeof(struct net_generic) +
+		INITIAL_NET_GEN_PTRS * sizeof(void *);
+
+	ng = kzalloc(generic_size, GFP_KERNEL);
+	if (ng)
+		ng->len = INITIAL_NET_GEN_PTRS;
+
+	return ng;
 }
 
-static void net_free(struct net *net)
+static struct net *net_alloc(void)
 {
+	struct net *net = NULL;
+	struct net_generic *ng;
+
+	ng = net_alloc_generic();
+	if (!ng)
+		goto out;
+
+	net = kmem_cache_zalloc(net_cachep, GFP_KERNEL);
 	if (!net)
-		return;
+		goto out_free;
+
+	rcu_assign_pointer(net->gen, ng);
+out:
+	return net;
+
+out_free:
+	kfree(ng);
+	goto out;
+}
 
+static void net_free(struct net *net)
+{
 #ifdef NETNS_REFCNT_DEBUG
 	if (unlikely(atomic_read(&net->use_count) != 0)) {
 		printk(KERN_EMERG "network namespace not free! Usage: %d\n",
@@ -112,27 +128,28 @@ struct net *copy_net_ns(unsigned long flags, struct net *old_net)
 	err = -ENOMEM;
 	new_net = net_alloc();
 	if (!new_net)
-		goto out;
+		goto out_err;
 
 	mutex_lock(&net_mutex);
 	err = setup_net(new_net);
-	if (err)
-		goto out_unlock;
-
-	rtnl_lock();
-	list_add_tail(&new_net->list, &net_namespace_list);
-	rtnl_unlock();
-
-
-out_unlock:
+	if (!err) {
+		rtnl_lock();
+		list_add_tail(&new_net->list, &net_namespace_list);
+		rtnl_unlock();
+	}
 	mutex_unlock(&net_mutex);
+
+	if (err)
+		goto out_free;
 out:
 	put_net(old_net);
-	if (err) {
-		net_free(new_net);
-		new_net = ERR_PTR(err);
-	}
 	return new_net;
+
+out_free:
+	net_free(new_net);
+out_err:
+	new_net = ERR_PTR(err);
+	goto out;
 }
 
 static void cleanup_net(struct work_struct *work)
@@ -188,6 +205,7 @@ struct net *copy_net_ns(unsigned long flags, struct net *old_net)
 
 static int __init net_ns_init(void)
 {
+	struct net_generic *ng;
 	int err;
 
 	printk(KERN_INFO "net_namespace: %zd bytes\n", sizeof(struct net));
@@ -202,6 +220,12 @@ static int __init net_ns_init(void)
 		panic("Could not create netns workq");
 #endif
 
+	ng = net_alloc_generic();
+	if (!ng)
+		panic("Could not allocate generic netns");
+
+	rcu_assign_pointer(init_net.gen, ng);
+
 	mutex_lock(&net_mutex);
 	err = setup_net(&init_net);