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From:   Eric Dumazet <edumazet@google.com>
Date:   Thu, 4 Oct 2018 12:28:56 -0700
Message-ID: <CANn89iJMhGEkw13KrGbqrvGbznb45QZwdr7eLy_rUV6zitvs=Q@mail.gmail.com>
Subject: Re: [PATCH] Input: mousedev - add a schedule point in mousedev_write()
To:     Dmitry Torokhov <dmitry.torokhov@gmail.com>
Cc:     LKML <linux-kernel@vger.kernel.org>,
        Eric Dumazet <eric.dumazet@gmail.com>,
        linux-input@vger.kernel.org,
        "Paul E. McKenney" <paulmck@linux.vnet.ibm.com>
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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 ....

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

I dunno...

> >
> > 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