inet: fix NULL pointer Oops in fib(6)_rule_suppress

This changes ensures that the routing entry investigated by the suppress
function actually does point to a device struct before following that pointer,
fixing a possible kernel oops situation when verifying the interface group
associated with a routing table entry.

According to Daniel Golle, this Oops can be triggered by a user process trying
to establish an outgoing IPv6 connection while having no real IPv6 connectivity
set up (only autoassigned link-local addresses).

Fixes: 6ef94cfafba15 ("fib_rules: add route suppression based on ifgroup")

Reported-by: Daniel Golle <daniel.golle@gmail.com>
Tested-by: Daniel Golle <daniel.golle@gmail.com>
Signed-off-by: Stefan Tomanek <stefan.tomanek@wertarbyte.de>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/ipv4/fib_rules.c

@@ -104,7 +104,10 @@ errout:
 static bool fib4_rule_suppress(struct fib_rule *rule, struct fib_lookup_arg *arg)
 {
 	struct fib_result *result = (struct fib_result *) arg->result;
-	struct net_device *dev = result->fi->fib_dev;
+	struct net_device *dev = NULL;
+
+	if (result->fi)
+		dev = result->fi->fib_dev;
 
 	/* do not accept result if the route does
 	 * not meet the required prefix length

net/ipv6/fib6_rules.c

@@ -122,7 +122,11 @@ out:
 static bool fib6_rule_suppress(struct fib_rule *rule, struct fib_lookup_arg *arg)
 {
 	struct rt6_info *rt = (struct rt6_info *) arg->result;
-	struct net_device *dev = rt->rt6i_idev->dev;
+	struct net_device *dev = NULL;
+
+	if (rt->rt6i_idev)
+		dev = rt->rt6i_idev->dev;
+
 	/* do not accept result if the route does
 	 * not meet the required prefix length
 	 */