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Date:   Thu, 4 Oct 2018 12:36:16 -0700
From:   "Paul E. McKenney" <paulmck@linux.ibm.com>
To:     Eric Dumazet <edumazet@google.com>
Cc:     Dmitry Torokhov <dmitry.torokhov@gmail.com>,
        LKML <linux-kernel@vger.kernel.org>,
        Eric Dumazet <eric.dumazet@gmail.com>,
        linux-input@vger.kernel.org
Subject: Re: [PATCH] Input: mousedev - add a schedule point in
 mousedev_write()
Reply-To: paulmck@linux.ibm.com
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On Thu, Oct 04, 2018 at 12:28:56PM -0700, Eric Dumazet wrote:
> On Thu, Oct 4, 2018 at 11:59 AM Dmitry Torokhov
> <dmitry.torokhov@gmail.com> wrote:
> >
> > Hi Eric,
> >
> > On Thu, Oct 04, 2018 at 08:47:49AM -0700, Eric Dumazet wrote:
> > > syzbot was able to trigger rcu stalls by calling write()
> > > with large number of bytes.
> > >
> > > Add a cond_resched() in the loop to avoid this.
> >
> > I think this simply masks a deeper issue. The code fetches characters
> > from userspace in a loop, takes a lock, quickly places response in an
> > output buffer, and releases interrupt. I do not see why this should
> > cause stalls as we do not hold spinlock/interrupts off for extended
> > period of time.
> >
> > Adding Paul so he can straighten me out...
> >
> 
> Well...
> 
> write(fd, buffer, 0x7FFF0000);
> 
> Takes between 20 seconds and 2 minutes depending on CONFIG options ....

And two minutes would get you an RCU CPU stall warning, even on distro
kernels that set the stall-warning time to a full minute (as opposed
to 21 seconds in mainline).

> So either apply my patch, or add a limit on the max count, and
> possibly break legitimate user space ?
> 
> I dunno...

I vote for Eric's patch.  In fact:

Reviewed-by: Paul E. McKenney <paulmck@linux.ibm.com>

> > > Link: https://lkml.org/lkml/2018/8/23/1106
> > > Signed-off-by: Eric Dumazet <edumazet@google.com>
> > > Reported-by: syzbot+9436b02171ac0894d33e@syzkaller.appspotmail.com
> > > Cc: Dmitry Torokhov <dmitry.torokhov@gmail.com>
> > > Cc: linux-input@vger.kernel.org
> > > ---
> > >  drivers/input/mousedev.c | 1 +
> > >  1 file changed, 1 insertion(+)
> > >
> > > diff --git a/drivers/input/mousedev.c b/drivers/input/mousedev.c
> > > index e08228061bcdd2f97aaadece31d6c83eb7539ae5..412fa71245afe26a7a8ad75705566f83633ba347 100644
> > > --- a/drivers/input/mousedev.c
> > > +++ b/drivers/input/mousedev.c
> > > @@ -707,6 +707,7 @@ static ssize_t mousedev_write(struct file *file, const char __user *buffer,
> > >               mousedev_generate_response(client, c);
> > >
> > >               spin_unlock_irq(&client->packet_lock);
> > > +             cond_resched();
> > >       }
> > >
> > >       kill_fasync(&client->fasync, SIGIO, POLL_IN);
> > > --
> > > 2.19.0.605.g01d371f741-goog
> > >
> >
> > Thanks.
> >
> > --
> > Dmitry
>