Subject: Re: [PATCH] net: Fix NETDEV_CHANGE notifier usage causing spurious arp flush

From: Dan Aloni <[email-masked]>
Date: Wed, 2 Jul 2014 10:03:49 +0300

On Tue, Jul 01, 2014 at 09:39:43PM -0700, Loic Prylli wrote:
> A bug was introduced in NETDEV_CHANGE notifier sequence causing the
> arp table to be sometimes spuriously cleared (including manual arp
> entries marked permanent), upon network link carrier changes.
> 
> The changed argument for the notifier was applied only to a single
> caller of NETDEV_CHANGE, missing among others netdev_state_change().
> So upon net_carrier events induced by the network, which are
> triggering a call to netdev_state_change(), arp_netdev_event() would
> decide whether to clear or not arp cache based on random/junk stack
> values (a kind of read buffer overflow).
[..]
>  {
>  	if (dev->flags & IFF_UP) {
> -		call_netdevice_notifiers(NETDEV_CHANGE, dev);
> +		struct netdev_notifier_change_info change_info;
> +
> +		change_info.flags_changed = 0;

I think it would be safer to do:

   struct netdev_notifier_change_info change_info = {};

So that when future fields are added to the struct and this call-site
happens to be forgotten, they will get 0 by default rather than
random stack values.

-- 
Dan Aloni