Subject: Re: [syzbot] [wireguard?] KASAN: slab-use-after-free Write in enqueue_timer

From: Jakub Kicinski <[email-masked]>
Date: Tue, 23 May 2023 09:41:08 -0700

On Tue, 23 May 2023 18:12:32 +0200 Eric Dumazet wrote:
> > Your timer had the pleasure of getting queued _after_ a dead watchdog
> > timer, no? IOW it tries to update the ->next pointer of a queued
> > watchdog timer. We should probably do:
> >
> > diff --git a/net/core/dev.c b/net/core/dev.c
> > index 374d38fb8b9d..f3ed20ebcf5a 100644
> > --- a/net/core/dev.c
> > +++ b/net/core/dev.c
> > @@ -10389,6 +10389,8 @@ void netdev_run_todo(void)
> >                 WARN_ON(rcu_access_pointer(dev->ip_ptr));
> >                 WARN_ON(rcu_access_pointer(dev->ip6_ptr));
> >
> > +               WARN_ON(timer_shutdown_sync(&dev->watchdog_timer));
> > +
> >                 if (dev->priv_destructor)
> >                         dev->priv_destructor(dev);
> >                 if (dev->needs_free_netdev)
> >
> > to catch how that watchdog_timer is getting queued. Would that make
> > sense, Eric?  
> 
> Would this case be catched at the time the device is freed ?
> 
> (CONFIG_DEBUG_OBJECTS_FREE=y or something)

It should, no idea why it isn't. Looking thru the code now I don't see
any obvious gaps where timer object is on a list but not active :S
There's no way to get a vmcore from syzbot, right? :)

Also I thought the shutdown leads to a warning when someone tries to
schedule the dead timer but in fact add_timer() just exits cleanly.
So the shutdown won't help us find the culprit :(