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Date:   Tue, 23 May 2023 09:05:12 -0700
From:   Jakub Kicinski <kuba@kernel.org>
To:     "Jason A. Donenfeld" <Jason@zx2c4.com>, edumazet@google.com
Cc:     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: <20230523090512.19ca60b6@kernel.org>
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        <ZGzfzEs-vJcZAySI@zx2c4.com>
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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. 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?