[PATCH 1/1] video: fbdev: fix divide error in fbcon_switch

[Thomas Zimmermann]

Hi

On 22.10.20 01:57, saeed.mirzamohammadi at oracle.com wrote:
> From: Saeed Mirzamohammadi <saeed.mirzamohammadi at oracle.com>
> 
> This patch fixes the issue due to:
> 
> [   89.572883] divide_error: 0000 [#1] SMP KASAN PTI
> [   89.572897] CPU: 3 PID: 16083 Comm: repro Not tainted 5.9.0-rc7.20200930.rc1.allarch-19-g3e32d0d.syzk #5
> [   89.572902] Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS 0.5.1 01/01/2011
> [   89.572934] RIP: 0010:cirrusfb_check_var+0x84/0x1260

BTW, if you run qemu with cirrus, there's also a DRM driver named
cirrus.ko. Might be a better choice than the old fbdev driver. If you
just care about qemu, but not the actual graphics device, take a look at

  https://www.kraxel.org/blog/2014/10/qemu-using-cirrus-considered-harmful/

Anyway, thanks for your patch.

Best regards
Thomas

> 
> The error happens when the pixels value is calculated before performing the sanity checks on bits_per_pixel.
> A bits_per_pixel set to zero causes divide by zero error.
> 
> This patch moves the calculation after the sanity check.
> 
> Signed-off-by: Saeed Mirzamohammadi <saeed.mirzamohammadi at oracle.com>
> Tested-by: Saeed Mirzamohammadi <saeed.mirzamohammadi at oracle.com>
> ---
>  drivers/video/fbdev/cirrusfb.c | 3 ++-
>  1 file changed, 2 insertions(+), 1 deletion(-)
> 
> diff --git a/drivers/video/fbdev/cirrusfb.c b/drivers/video/fbdev/cirrusfb.c
> index 15a9ee7cd734..a7749101b094 100644
> --- a/drivers/video/fbdev/cirrusfb.c
> +++ b/drivers/video/fbdev/cirrusfb.c
> @@ -531,7 +531,7 @@ static int cirrusfb_check_var(struct fb_var_screeninfo *var,
>  {
>  	int yres;
>  	/* memory size in pixels */
> -	unsigned pixels = info->screen_size * 8 / var->bits_per_pixel;
> +	unsigned int pixels;
>  	struct cirrusfb_info *cinfo = info->par;
>  
>  	switch (var->bits_per_pixel) {
> @@ -573,6 +573,7 @@ static int cirrusfb_check_var(struct fb_var_screeninfo *var,
>  		return -EINVAL;
>  	}
>  
> +	pixels = info->screen_size * 8 / var->bits_per_pixel;
>  	if (var->xres_virtual < var->xres)
>  		var->xres_virtual = var->xres;
>  	/* use highest possible virtual resolution */
> 

-- 
Thomas Zimmermann
Graphics Driver Developer
SUSE Software Solutions Germany GmbH
Maxfeldstr. 5, 90409 Nürnberg, Germany
(HRB 36809, AG Nürnberg)
Geschäftsführer: Felix Imendörffer