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

[Saeed Mirzamohammadi]

Thanks, adding stable.

Saeed

> On Oct 22, 2020, at 12:34 AM, Thomas Zimmermann <tzimmermann@xxxxxxx> wrote:
> 
> Hi
> 
> On 22.10.20 01:57, saeed.mirzamohammadi@xxxxxxxxxx wrote:
>> From: Saeed Mirzamohammadi <saeed.mirzamohammadi@xxxxxxxxxx>
>> 
>> 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://urldefense.com/v3/__https://www.kraxel.org/blog/2014/10/qemu-using-cirrus-considered-harmful/__;!!GqivPVa7Brio!LmgeM-pVBVH80uVELF1P1nBGAbAlhvnxKKE_ZrEc9d76AznvAAgP1FAp3_zNa2frKaIUZteK$ 
> 
> 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@xxxxxxxxxx>
>> Tested-by: Saeed Mirzamohammadi <saeed.mirzamohammadi@xxxxxxxxxx>
>> ---
>> 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)
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