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Date:   Tue, 23 May 2023 09:41:08 -0700
From:   Jakub Kicinski <kuba@kernel.org>
To:     Eric Dumazet <edumazet@google.com>
Cc:     "Jason A. Donenfeld" <Jason@zx2c4.com>,
        syzbot <syzbot+c2775460db0e1c70018e@syzkaller.appspotmail.com>,
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        davem@davemloft.net, linux-kernel@vger.kernel.org,
        pabeni@redhat.com, wireguard@lists.zx2c4.com, jann@thejh.net
Subject: Re: [syzbot] [wireguard?] KASAN: slab-use-after-free Write in
 enqueue_timer
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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 :(