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Date:   Tue, 23 May 2023 18:14:18 +0200
From:   "Jason A. Donenfeld" <Jason@zx2c4.com>
To:     Jakub Kicinski <kuba@kernel.org>
Cc:     edumazet@google.com,
        syzbot <syzbot+c2775460db0e1c70018e@syzkaller.appspotmail.com>,
        netdev@vger.kernel.org, syzkaller-bugs@googlegroups.com,
        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
Message-ID: <ZGzmWtd7itw6oFsI@zx2c4.com>
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 <20230523090512.19ca60b6@kernel.org>
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On Tue, May 23, 2023 at 09:05:12AM -0700, Jakub Kicinski wrote:
> On Tue, 23 May 2023 17:46:20 +0200 Jason A. Donenfeld wrote:
> > > Freed by task 41:
> > >  __kmem_cache_free+0x264/0x3c0 mm/slub.c:3799
> > >  device_release+0x95/0x1c0
> > >  kobject_cleanup lib/kobject.c:683 [inline]
> > >  kobject_release lib/kobject.c:714 [inline]
> > >  kref_put include/linux/kref.h:65 [inline]
> > >  kobject_put+0x228/0x470 lib/kobject.c:731
> > >  netdev_run_todo+0xe5a/0xf50 net/core/dev.c:10400  
> > 
> > So that means the memory in question is actually the one that's
> > allocated and freed by the networking stack. Specifically, dev.c:10626
> > is allocating a struct net_device with a trailing struct wg_device (its
> > priv_data). However, wg_device does not have any struct timer_lists in
> > it, and I don't see how net_device's watchdog_timer would be related to
> > the stacktrace which is clearly operating over a wg_peer timer.
> > 
> > So what on earth is going on here?
> 
> 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. 

Ahh, you're right! Specifically,

> hlist_add_head include/linux/list.h:945 [inline]
> enqueue_timer+0xad/0x560 kernel/time/timer.c:605

The write on line 945 refers to the side of the timer base, not the
peer's timer_list being queued. So indeed, the wireguard netdev is still
alive at this point, but it's being queued to a timer in a different
netdev that's already been freed (whether watchdog or otherwise in some
privdata). So, IOW, not a wireguard bug, right?

Jason