[NET]: rtnl_link: fix use-after-free

When unregistering the rtnl_link_ops, all existing devices using
the ops are destroyed. With nested devices this may lead to a
use-after-free despite the use of for_each_netdev_safe() in case
the upper device is next in the device list and is destroyed
by the NETDEV_UNREGISTER notifier.

The easy fix is to restart scanning the device list after removing
a device. Alternatively we could add new devices to the front of
the list to avoid having dependant devices follow the device they
depend on. A third option would be to only restart scanning if
dev->iflink of the next device matches dev->ifindex of the current
one. For now this seems like the safest solution.

With this patch, the veth rtnl_link_ops unregistration can use
rtnl_link_unregister() directly since it now also handles destruction
of multiple devices at once.
Signed-off-by: Patrick McHardy <kaber@trash.net>
Signed-off-by: David S. Miller <davem@davemloft.net>

drivers/net/veth.c

@@ -459,19 +459,7 @@ static __init int veth_init(void)
 
 static __exit void veth_exit(void)
 {
-	struct veth_priv *priv, *next;
-
-	rtnl_lock();
-	/*
-	 * cannot trust __rtnl_link_unregister() to unregister all
-	 * devices, as each ->dellink call will remove two devices
-	 * from the list at once.
-	 */
-	list_for_each_entry_safe(priv, next, &veth_list, list)
-		veth_dellink(priv->dev);
-
-	__rtnl_link_unregister(&veth_link_ops);
-	rtnl_unlock();
+	rtnl_link_unregister(&veth_link_ops);
 }
 
 module_init(veth_init);

net/core/rtnetlink.c

@@ -308,9 +308,12 @@ void __rtnl_link_unregister(struct rtnl_link_ops *ops)
 	struct net *net;
 
 	for_each_net(net) {
+restart:
 		for_each_netdev_safe(net, dev, n) {
-			if (dev->rtnl_link_ops == ops)
+			if (dev->rtnl_link_ops == ops) {
 				ops->dellink(dev);
+				goto restart;
+			}
 		}
 	}
 	list_del(&ops->list);