[PATCH 4/5] fbmem: Prevent invalid virtual screen sizes in fb_set_var()

[Michel Dänzer]

On 2022-07-01 16:49, Geert Uytterhoeven wrote:
> On Thu, Jun 30, 2022 at 9:38 PM Geert Uytterhoeven <geert at linux-m68k.org> wrote:
>> On Thu, Jun 30, 2022 at 9:17 PM Helge Deller <deller at gmx.de> wrote:
>>> On 6/30/22 21:11, Geert Uytterhoeven wrote:
>>>> On Wed, Jun 29, 2022 at 10:00 PM Helge Deller <deller at gmx.de> wrote:
>>>>> Prevent that drivers configure a virtual screen resolution smaller than
>>>>> the physical screen resolution.  This is important, because otherwise we
>>>>> may access memory outside of the graphics memory area.
>>>>>
>>>>> Signed-off-by: Helge Deller <deller at gmx.de>
>>>>> Cc: stable at vger.kernel.org # v5.4+
>>>>
>>>> Thanks for your patch!
>>>>
>>>>> --- a/drivers/video/fbdev/core/fbmem.c
>>>>> +++ b/drivers/video/fbdev/core/fbmem.c
>>>>> @@ -1006,6 +1006,12 @@ fb_set_var(struct fb_info *info, struct fb_var_screeninfo *var)
>>>>>         if (var->xres < 8 || var->yres < 8)
>>>>>                 return -EINVAL;
>>>>>
>>>>> +       /* make sure virtual resolution >= physical resolution */
>>>>> +       if (WARN_ON(var->xres_virtual < var->xres))
>>>>> +               var->xres_virtual = var->xres;
>>>>> +       if (WARN_ON(var->yres_virtual < var->yres))
>>>>> +               var->yres_virtual = var->yres;
>>>>
>>>> This should be moved below the call to info->fbops->fb_check_var(),
>>>> so the WARN_ON() catches buggy fbdev drivers, not userspace fuzzers.
>>>
>>> Yes, makes sense.
>>
>> And print the name of the frame buffer device driver, so people know
>> who to blame.
> 
> Or better, do not continue, but return with a failure:
> 
>     if (WARN(var->xres_virtual < var->xres || var->yres_virtual < var->yres,
>         "%ps for %s is broken\n", info->fbops->fb_check_var, info->fix.id)
>             return -EINVAL;

I'd also recommend WARN(_ON)_ONCE, or users with a broken driver might get spammed.


-- 
Earthling Michel Dänzer            |                  https://redhat.com
Libre software enthusiast          |         Mesa and Xwayland developer