linux-kernel - Re: [syzbot] WARNING: refcount bug in __linkwatch_run

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Date:   Wed, 17 Nov 2021 06:15:48 -0800
From:   Jakub Kicinski <kuba@...nel.org>
To:     Willy Tarreau <w@....eu>
Cc:     syzbot <syzbot+6f8ddb9f2ff4adf065cb@...kaller.appspotmail.com>,
        davem@...emloft.net, linux-kernel@...r.kernel.org,
        netdev@...r.kernel.org, syzkaller-bugs@...glegroups.com
Subject: Re: [syzbot] WARNING: refcount bug in __linkwatch_run_queue

On Wed, 17 Nov 2021 09:19:07 +0100 Willy Tarreau wrote:
> Thanks for the report. I'm seeing that linkwatch_do_dev() is also
> called in linkwatch_forget_dev(), and am wondering if we're not
> seeing a sequence like this one:
> 
>   linkwatch_forget_dev()
>     list_del_init()
>     linkwatch_do_dev()
>       netdev_state_change()
>         ... one of the notifiers
>            ... linkwatch_add_event() => adds to watch list
>       dev_put()
>   ...
>   
>   __linkwatch_run_queue()
>     linkwatch_do_dev()
>       dev_put()
>         => bang!  
> 
> Well, in theory, no, since linkwatch_add_event() will call dev_hold()
> when adding to the list, so we ought to leave the first call with a
> refcount still covering the list's presence, and I don't see how it
> can reach zero before reaching dev_put() in linkwatch_do_dev() as this
> function is only called when the event was picked from the list.
> 
> The only difference I'm seeing is that before the patch, a call to
> linkwatch_forget_dev() on a non-present device would call dev_put()
> without going through dev_activate(), dev_deactivate(), nor
> netdev_state_change(), but I'm not seeing how that could make a
> difference. linkwatch_forget_dev() is called from netdev_wait_allrefs()
> which will wait for the refcnt to be exactly 1, thus even if we queue
> an extra event we cant leave that function until the event has been
> processed.

The ref leak could come from anywhere, tho. Like:

https://lore.kernel.org/all/87a6i3t2zg.fsf@nvidia.com/