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

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Date:   Wed, 17 Nov 2021 15:20:12 +0100
From:   Willy Tarreau <w@....eu>
To:     Jakub Kicinski <kuba@...nel.org>
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, Nov 17, 2021 at 06:15:48AM -0800, Jakub Kicinski wrote:
> 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/

OK thanks for the link, so better wait for this part to clarify itself
and see if the issue magically disappears ?

Willy