net: macvlan: fix potential UAF problem for lowerdev

Add the reference operation to lowerdev of macvlan to avoid
the potential UAF problem under the following known scenario:

Someone module puts the NETDEV_UNREGISTER event handler to a
work, and lowerdev is accessed in the work handler. But when
the work is excuted, lowerdev has been destroyed because upper
macvlan did not get reference to lowerdev correctly.

That likes as the scenario occurred by
commit 563bcbae ("net: vlan: fix a UAF in vlan_dev_real_dev()").
Signed-off-by: Ziyang Xuan <william.xuanziyang@huawei.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

drivers/net/macvlan.c

@@ -889,7 +889,7 @@ static void macvlan_set_lockdep_class(struct net_device *dev)
 static int macvlan_init(struct net_device *dev)
 {
 	struct macvlan_dev *vlan = netdev_priv(dev);
-	const struct net_device *lowerdev = vlan->lowerdev;
+	struct net_device *lowerdev = vlan->lowerdev;
 	struct macvlan_port *port = vlan->port;
 
 	dev->state		= (dev->state & ~MACVLAN_STATE_MASK) |
@@ -911,6 +911,9 @@ static int macvlan_init(struct net_device *dev)
 
 	port->count += 1;
 
+	/* Get macvlan's reference to lowerdev */
+	dev_hold(lowerdev);
+
 	return 0;
 }
 
@@ -1173,6 +1176,14 @@ static const struct net_device_ops macvlan_netdev_ops = {
 	.ndo_features_check	= passthru_features_check,
 };
 
+static void macvlan_dev_free(struct net_device *dev)
+{
+	struct macvlan_dev *vlan = netdev_priv(dev);
+
+	/* Get rid of the macvlan's reference to lowerdev */
+	dev_put(vlan->lowerdev);
+}
+
 void macvlan_common_setup(struct net_device *dev)
 {
 	ether_setup(dev);
@@ -1184,6 +1195,7 @@ void macvlan_common_setup(struct net_device *dev)
 	dev->priv_flags	       |= IFF_UNICAST_FLT | IFF_CHANGE_PROTO_DOWN;
 	dev->netdev_ops		= &macvlan_netdev_ops;
 	dev->needs_free_netdev	= true;
+	dev->priv_destructor	= macvlan_dev_free;
 	dev->header_ops		= &macvlan_hard_header_ops;
 	dev->ethtool_ops	= &macvlan_ethtool_ops;
 }