net: avoid potential race between netdev_get_by_index_lock() and netns switch

netdev_get_by_index_lock() performs following steps:

rcu_lock();
dev = lookup(netns, ifindex);
dev_get(dev);
rcu_unlock();
[... lock & validate the dev ...]
return dev

Validation right now only checks if the device is registered but since
the lookup is netns-aware we must also protect against the device
switching netns right after we dropped the RCU lock. Otherwise
the caller in netns1 may get a pointer to a device which has just
switched to netns2.

We can't hold the lock for the entire netns change process (because of
the NETDEV_UNREGISTER notifier), and there's no existing marking to
indicate that the netns is unlisted because of netns move, so add one.

AFAIU none of the existing netdev_get_by_index_lock() callers can
suffer from this problem (NAPI code double checks the netns membership
and other callers are either under rtnl_lock or not ns-sensitive),
so this patch does not have to be treated as a fix.

Reviewed-by: Joe Damato <[e-mail]>
Acked-by: Stanislav Fomichev <[e-mail]>
Link: https://patch.msgid.link/[e-mail]
Signed-off-by: Jakub Kicinski <[e-mail]>

include/linux/netdevice.h

···
  *	@priv_destructor:	Called from unregister
  *	@npinfo:		XXX: need comments on this one
  * 	@nd_net:		Network namespace this network device is inside
+ *				protected by @lock
  *
  * 	@ml_priv:	Mid-layer private
  *	@ml_priv_type:  Mid-layer private type
···
 
 	bool dismantle;
 
+	/** @moving_ns: device is changing netns, protected by @lock */
+	bool moving_ns;
+
 	enum {
 		RTNL_LINK_INITIALIZED,
 		RTNL_LINK_INITIALIZING,
···
 	 *	@net_shaper_hierarchy, @reg_state, @threaded
 	 *
 	 * Double protects:
-	 *	@up
+	 *	@up, @moving_ns, @nd_net
 	 *
 	 * Double ops protects:
 	 *	@real_num_rx_queues, @real_num_tx_queues

net/core/dev.c

···
 	dev_hold(dev);
 	rcu_read_unlock();
 
-	dev = __netdev_put_lock(dev);
+	dev = __netdev_put_lock(dev, net);
 	if (!dev)
 		return NULL;
 
···
  * This helper is intended for locking net_device after it has been looked up
  * using a lockless lookup helper. Lock prevents the instance from going away.
  */
-struct net_device *__netdev_put_lock(struct net_device *dev)
+struct net_device *__netdev_put_lock(struct net_device *dev, struct net *net)
 {
 	netdev_lock(dev);
-	if (dev->reg_state > NETREG_REGISTERED) {
+	if (dev->reg_state > NETREG_REGISTERED ||
+	    dev->moving_ns || !net_eq(dev_net(dev), net)) {
 		netdev_unlock(dev);
 		dev_put(dev);
 		return NULL;
···
 	if (!dev)
 		return NULL;
 
-	return __netdev_put_lock(dev);
+	return __netdev_put_lock(dev, net);
 }
 
 struct net_device *
···
 		dev_hold(dev);
 		rcu_read_unlock();
 
-		dev = __netdev_put_lock(dev);
+		dev = __netdev_put_lock(dev, net);
 		if (dev)
 			return dev;
 
···
 	netif_close(dev);
 	/* And unlink it from device chain */
 	unlist_netdevice(dev);
-	netdev_unlock_ops(dev);
+
+	if (!netdev_need_ops_lock(dev))
+		netdev_lock(dev);
+	dev->moving_ns = true;
+	netdev_unlock(dev);
 
 	synchronize_net();
 
···
 	move_netdevice_notifiers_dev_net(dev, net);
 
 	/* Actually switch the network namespace */
+	netdev_lock(dev);
 	dev_net_set(dev, net);
+	netdev_unlock(dev);
 	dev->ifindex = new_ifindex;
 
 	if (new_name[0]) {
···
 	err = netdev_change_owner(dev, net_old, net);
 	WARN_ON(err);
 
-	netdev_lock_ops(dev);
+	netdev_lock(dev);
+	dev->moving_ns = false;
+	if (!netdev_need_ops_lock(dev))
+		netdev_unlock(dev);
+
 	/* Add the device back in the hashes */
 	list_netdevice(dev);
 	/* Notify protocols, that a new device appeared. */

net/core/dev.h

···
 struct net_device *dev_get_by_napi_id(unsigned int napi_id);
 
 struct net_device *netdev_get_by_index_lock(struct net *net, int ifindex);
-struct net_device *__netdev_put_lock(struct net_device *dev);
+struct net_device *__netdev_put_lock(struct net_device *dev, struct net *net);
 struct net_device *
 netdev_xa_find_lock(struct net *net, struct net_device *dev,
 		    unsigned long *index);