Re: [PATCH] HID: magicmouse: prevent division by 0 on scroll

[José Expósito]

On Sun, Nov 14, 2021 at 03:53:27AM +0100, Claudia Pellegrino wrote:
> In hid_magicmouse, if the user has set scroll_speed to a value between
> 55 and 63 and scrolls seven times in quick succession, the
> step_hr variable in the magicmouse_emit_touch function becomes 0.
> 
> That causes a division by zero further down in the function when
> it does `step_x_hr /= step_hr`.
> 
> To reproduce, create `/etc/modprobe.d/hid_magicmouse.conf` with the
> following content:
> 
> ```
> options hid_magicmouse scroll_acceleration=1 scroll_speed=55
> ```
> 
> Then reboot, connect a Magic Mouse and scroll seven times quickly.
> The system will freeze for a minute, and after that `dmesg` will
> confirm that a division by zero occurred.
> 
> Enforce a minimum of 1 for the variable so the high resolution
> step count can never reach 0 even at maximum scroll acceleration.
> 
> Fixes: d4b9f10a0eb6 ("HID: magicmouse: enable high-resolution scroll")
> 
> Signed-off-by: Claudia Pellegrino <linux@xxxxxxxxxxxxxx>
> ---
>  drivers/hid/hid-magicmouse.c | 7 +++++--
>  1 file changed, 5 insertions(+), 2 deletions(-)
> 
> diff --git a/drivers/hid/hid-magicmouse.c b/drivers/hid/hid-magicmouse.c
> index 686788ebf3e1..d7687ce70614 100644
> --- a/drivers/hid/hid-magicmouse.c
> +++ b/drivers/hid/hid-magicmouse.c
> @@ -256,8 +256,11 @@ static void magicmouse_emit_touch(struct magicmouse_sc *msc, int raw_id, u8 *tda
>  		unsigned long now = jiffies;
>  		int step_x = msc->touches[id].scroll_x - x;
>  		int step_y = msc->touches[id].scroll_y - y;
> -		int step_hr = ((64 - (int)scroll_speed) * msc->scroll_accel) /
> -			      SCROLL_HR_STEPS;
> +		int step_hr =
> +			max_t(int,
> +			      ((64 - (int)scroll_speed) * msc->scroll_accel) /
> +					SCROLL_HR_STEPS,
> +			      1);
>  		int step_x_hr = msc->touches[id].scroll_x_hr - x;
>  		int step_y_hr = msc->touches[id].scroll_y_hr - y;
>  
> -- 
> 2.33.1
> 

Hi Caludia,

Thanks for ccing me.

I can confirm both that the bug is present and that your patch fixes it.

Tested-by: José Expósito <jose.exposito89@xxxxxxxxx>

Best wishes,
Jose