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

[Jason A. Donenfeld]

On Tue, May 23, 2023 at 6:41 PM Jakub Kicinski <kuba@xxxxxxxxxx> wrote:
>
> 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 :(

Worth noting that it could also be caused by adding to a dead timer
anywhere in priv_data of another netdev, not just the sole timer_list
in net_device.