Merge branch 'net-fix-race-of-rtnl_net_lock-dev_net-dev'

Kuniyuki Iwashima says:

====================
net: Fix race of rtnl_net_lock(dev_net(dev)).

Yael Chemla reported that commit 7fb1073300 ("net: Hold rtnl_net_lock()
in (un)?register_netdevice_notifier_dev_net().") started to trigger KASAN's
use-after-free splat.

The problem is that dev_net(dev) fetched before rtnl_net_lock() might be
different after rtnl_net_lock().

The patch 2 fixes the issue by checking dev_net(dev) after rtnl_net_lock(),
and the patch 3 fixes the same potential issue that would emerge once RTNL
is removed.

v4: https://lore.kernel.org/20250212064206.18159-1-kuniyu@amazon.com
v3: https://lore.kernel.org/20250211051217.12613-1-kuniyu@amazon.com
v2: https://lore.kernel.org/20250207044251.65421-1-kuniyu@amazon.com
v1: https://lore.kernel.org/20250130232435.43622-1-kuniyu@amazon.com
====================

Link: https://patch.msgid.link/20250217191129.19967-1-kuniyu@amazon.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>

include/net/net_namespace.h

@@ -297,6 +297,7 @@ static inline int check_net(const struct net *net)
 }
 
 void net_drop_ns(void *);
+void net_passive_dec(struct net *net);
 
 #else
 
@@ -326,8 +327,18 @@ static inline int check_net(const struct net *net)
 }
 
 #define net_drop_ns NULL
+
+static inline void net_passive_dec(struct net *net)
+{
+	refcount_dec(&net->passive);
+}
 #endif
 
+static inline void net_passive_inc(struct net *net)
+{
+	refcount_inc(&net->passive);
+}
+
 /* Returns true if the netns initialization is completed successfully */
 static inline bool net_initialized(const struct net *net)
 {

net/core/dev.c

@@ -2070,6 +2070,42 @@ static void __move_netdevice_notifier_net(struct net *src_net,
 	__register_netdevice_notifier_net(dst_net, nb, true);
 }
 
+static void rtnl_net_dev_lock(struct net_device *dev)
+{
+	bool again;
+
+	do {
+		struct net *net;
+
+		again = false;
+
+		/* netns might be being dismantled. */
+		rcu_read_lock();
+		net = dev_net_rcu(dev);
+		net_passive_inc(net);
+		rcu_read_unlock();
+
+		rtnl_net_lock(net);
+
+#ifdef CONFIG_NET_NS
+		/* dev might have been moved to another netns. */
+		if (!net_eq(net, rcu_access_pointer(dev->nd_net.net))) {
+			rtnl_net_unlock(net);
+			net_passive_dec(net);
+			again = true;
+		}
+#endif
+	} while (again);
+}
+
+static void rtnl_net_dev_unlock(struct net_device *dev)
+{
+	struct net *net = dev_net(dev);
+
+	rtnl_net_unlock(net);
+	net_passive_dec(net);
+}
+
 int register_netdevice_notifier_dev_net(struct net_device *dev,
 					struct notifier_block *nb,
 					struct netdev_net_notifier *nn)
@@ -2077,6 +2113,11 @@ int register_netdevice_notifier_dev_net(struct net_device *dev,
 	struct net *net = dev_net(dev);
 	int err;
 
+	/* rtnl_net_lock() assumes dev is not yet published by
+	 * register_netdevice().
+	 */
+	DEBUG_NET_WARN_ON_ONCE(!list_empty(&dev->dev_list));
+
 	rtnl_net_lock(net);
 	err = __register_netdevice_notifier_net(net, nb, false);
 	if (!err) {
@@ -2093,13 +2134,12 @@ int unregister_netdevice_notifier_dev_net(struct net_device *dev,
 					  struct notifier_block *nb,
 					  struct netdev_net_notifier *nn)
 {
-	struct net *net = dev_net(dev);
 	int err;
 
-	rtnl_net_lock(net);
+	rtnl_net_dev_lock(dev);
 	list_del(&nn->list);
-	err = __unregister_netdevice_notifier_net(net, nb);
+	err = __unregister_netdevice_notifier_net(dev_net(dev), nb);
-	rtnl_net_unlock(net);
+	rtnl_net_dev_unlock(dev);
 
 	return err;
 }
@@ -11880,11 +11920,9 @@ EXPORT_SYMBOL(unregister_netdevice_many);
  */
 void unregister_netdev(struct net_device *dev)
 {
-	struct net *net = dev_net(dev);
+	rtnl_net_dev_lock(dev);
-
-	rtnl_net_lock(net);
 	unregister_netdevice(dev);
-	rtnl_net_unlock(net);
+	rtnl_net_dev_unlock(dev);
 }
 EXPORT_SYMBOL(unregister_netdev);
 

net/core/net_namespace.c

@@ -464,7 +464,7 @@ static void net_complete_free(void)
 
 }
 
-static void net_free(struct net *net)
+void net_passive_dec(struct net *net)
 {
 	if (refcount_dec_and_test(&net->passive)) {
 		kfree(rcu_access_pointer(net->gen));
@@ -482,7 +482,7 @@ void net_drop_ns(void *p)
 	struct net *net = (struct net *)p;
 
 	if (net)
-		net_free(net);
+		net_passive_dec(net);
 }
 
 struct net *copy_net_ns(unsigned long flags,
@@ -523,7 +523,7 @@ put_userns:
 		key_remove_domain(net->key_domain);
 #endif
 		put_user_ns(user_ns);
-		net_free(net);
+		net_passive_dec(net);
 dec_ucounts:
 		dec_net_namespaces(ucounts);
 		return ERR_PTR(rv);
@@ -672,7 +672,7 @@ static void cleanup_net(struct work_struct *work)
 		key_remove_domain(net->key_domain);
 #endif
 		put_user_ns(net->user_ns);
-		net_free(net);
+		net_passive_dec(net);
 	}
 	cleanup_net_task = NULL;
 }