net: ipconfig: ic_dev can be NULL in ic_close_devs

ic_close_dev contains a generalization of the logic to not close a
network interface if it's the host port for a DSA switch. This logic is
disguised behind an iteration through the lowers of ic_dev in
ic_close_dev.

When no interface for ipconfig can be found, ic_dev is NULL, and
ic_close_dev:
- dereferences a NULL pointer when assigning selected_dev
- would attempt to search through the lower interfaces of a NULL
  net_device pointer

So we should protect against that case.

The "lower_dev" iterator variable was shortened to "lower" in order to
keep the 80 character limit.

Fixes: f68cbaed ("net: ipconfig: avoid use-after-free in ic_close_devs")
Fixes: 46acf7bd ("Revert "net: ipv4: handle DSA enabled master network devices"")
Signed-off-by: Vladimir Oltean <vladimir.oltean@nxp.com>
Tested-by: Heiko Thiery <heiko.thiery@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/ipv4/ipconfig.c

@@ -309,7 +309,7 @@ static int __init ic_open_devs(void)
  */
 static void __init ic_close_devs(void)
 {
-	struct net_device *selected_dev = ic_dev->dev;
+	struct net_device *selected_dev = ic_dev ? ic_dev->dev : NULL;
 	struct ic_device *d, *next;
 	struct net_device *dev;
 
@@ -317,16 +317,18 @@ static void __init ic_close_devs(void)
 	next = ic_first_dev;
 	while ((d = next)) {
 		bool bring_down = (d != ic_dev);
-		struct net_device *lower_dev;
+		struct net_device *lower;
 		struct list_head *iter;
 
 		next = d->next;
 		dev = d->dev;
 
-		netdev_for_each_lower_dev(selected_dev, lower_dev, iter) {
-			if (dev == lower_dev) {
-				bring_down = false;
-				break;
+		if (selected_dev) {
+			netdev_for_each_lower_dev(selected_dev, lower, iter) {
+				if (dev == lower) {
+					bring_down = false;
+					break;
+				}
 			}
 		}
 		if (bring_down) {