linux-kernel - Re: [PATCH] Input: mousedev - add a schedule point in mousedev

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Date:   Thu, 4 Oct 2018 11:59:49 -0700
From:   Dmitry Torokhov <dmitry.torokhov@...il.com>
To:     Eric Dumazet <edumazet@...gle.com>
Cc:     linux-kernel <linux-kernel@...r.kernel.org>,
        Eric Dumazet <eric.dumazet@...il.com>,
        linux-input@...r.kernel.org,
        "Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
Subject: Re: [PATCH] Input: mousedev - add a schedule point in
 mousedev_write()

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...

> 
> Link: https://lkml.org/lkml/2018/8/23/1106
> Signed-off-by: Eric Dumazet <edumazet@...gle.com>
> Reported-by: syzbot+9436b02171ac0894d33e@...kaller.appspotmail.com
> Cc: Dmitry Torokhov <dmitry.torokhov@...il.com>
> Cc: linux-input@...r.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