Re: [PATCH] Input: mousedev - add a schedule point in mousedev_write()

[Dmitry Torokhov]

On October 4, 2018 12:28:56 PM PDT, Eric Dumazet <edumazet@xxxxxxxxxx> wrote:
>On Thu, Oct 4, 2018 at 11:59 AM Dmitry Torokhov
><dmitry.torokhov@xxxxxxxxx> 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 ....

That's fine even if it takes a couple of years. We are not holding spinlock for the entirety of this time, so we should get bumped off CPU at some point.

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

Legitimate users write a single character at a time and read response, so exciting after, let's say, 32 bytes would be fine. But I still want to understand why we have to do that.

>
>I dunno...
>
>> >
>> > Link: https://lkml.org/lkml/2018/8/23/1106
>> > Signed-off-by: Eric Dumazet <edumazet@xxxxxxxxxx>
>> > Reported-by: syzbot+9436b02171ac0894d33e@xxxxxxxxxxxxxxxxxxxxxxxxx
>> > Cc: Dmitry Torokhov <dmitry.torokhov@xxxxxxxxx>
>> > Cc: linux-input@xxxxxxxxxxxxxxx
>> > ---
>> >  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


Thanks.

-- 
Dmitry