veth: prevent oops caused by netdev destructor

From: Stephen Hemminger <[e-mail]>

The veth driver will oops if sysfs hooks are open while module is removed.

The net device destructor can not point to code in a module; basically
there are only two possible safe values: NULL - no destructor, or
free_netdev - free on last use

Signed-off-by: David S. Miller <[e-mail]>

drivers/net/veth.c

···
 
 static struct net_device_stats *veth_get_stats(struct net_device *dev)
 {
-	struct veth_priv *priv;
-	struct net_device_stats *dev_stats;
-	int cpu;
+	struct veth_priv *priv = netdev_priv(dev);
+	struct net_device_stats *dev_stats = &dev->stats;
+	unsigned int cpu;
 	struct veth_net_stats *stats;
-
-	priv = netdev_priv(dev);
-	dev_stats = &dev->stats;
 
 	dev_stats->rx_packets = 0;
 	dev_stats->tx_packets = 0;
···
 	dev_stats->tx_dropped = 0;
 	dev_stats->rx_dropped = 0;
 
-	for_each_online_cpu(cpu) {
-		stats = per_cpu_ptr(priv->stats, cpu);
+	if (priv->stats)
+		for_each_online_cpu(cpu) {
+			stats = per_cpu_ptr(priv->stats, cpu);
 
-		dev_stats->rx_packets += stats->rx_packets;
-		dev_stats->tx_packets += stats->tx_packets;
-		dev_stats->rx_bytes += stats->rx_bytes;
-		dev_stats->tx_bytes += stats->tx_bytes;
-		dev_stats->tx_dropped += stats->tx_dropped;
-		dev_stats->rx_dropped += stats->rx_dropped;
-	}
+			dev_stats->rx_packets += stats->rx_packets;
+			dev_stats->tx_packets += stats->tx_packets;
+			dev_stats->rx_bytes += stats->rx_bytes;
+			dev_stats->tx_bytes += stats->tx_bytes;
+			dev_stats->tx_dropped += stats->tx_dropped;
+			dev_stats->rx_dropped += stats->rx_dropped;
+		}
 
 	return dev_stats;
 }
···
 	netif_carrier_off(dev);
 	netif_carrier_off(priv->peer);
 
+	free_percpu(priv->stats);
+	priv->stats = NULL;
 	return 0;
 }
 
···
 	return 0;
 }
 
-static void veth_dev_free(struct net_device *dev)
-{
-	struct veth_priv *priv;
-
-	priv = netdev_priv(dev);
-	free_percpu(priv->stats);
-	free_netdev(dev);
-}
-
 static const struct net_device_ops veth_netdev_ops = {
 	.ndo_init            = veth_dev_init,
 	.ndo_open            = veth_open,
···
 	dev->netdev_ops = &veth_netdev_ops;
 	dev->ethtool_ops = &veth_ethtool_ops;
 	dev->features |= NETIF_F_LLTX;
-	dev->destructor = veth_dev_free;
+	dev->destructor = free_netdev;
 }
 
 /*