fib_frontend: fix possible NULL pointer dereference

The two commits 0115e8e30d (net: remove delay at device dismantle) and
748e2d9396a (net: reinstate rtnl in call_netdevice_notifiers()) silently
removed a NULL pointer check for in_dev since Linux 3.7.

This patch re-introduces this check as it causes crashing the kernel when
setting small mtu values on non-ip capable netdevices.

Signed-off-by: Oliver Hartkopp <socketcan@hartkopp.net>
Acked-by: Eric Dumazet <edumazet@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

1 files changed, 2 insertions, 0 deletions

diff --git a/net/ipv4/fib_frontend.c b/net/ipv4/fib_frontend.c
index d846304b7b89..c7539e22868b 100644
--- a/net/ipv4/fib_frontend.c
+++ b/net/ipv4/fib_frontend.c
@@ -1047,6 +1047,8 @@ static int fib_netdev_event(struct notifier_block *this, unsigned long event, vo
         }
 
         in_dev = __in_dev_get_rtnl(dev);
+        if (!in_dev)
+                return NOTIFY_DONE;
 
         switch (event) {
         case NETDEV_UP: