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

[Helge Deller]

On 6/30/22 21:11, Geert Uytterhoeven wrote:
> Hi Helge,
>
> 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.

THX,
Helge


>> +
>>         /* Too huge resolution causes multiplication overflow. */
>>         if (check_mul_overflow(var->xres, var->yres, &unused) ||
>>             check_mul_overflow(var->xres_virtual, var->yres_virtual, &unused))
>
> Gr{oetje,eeting}s,
>
>                         Geert
>
> --
> Geert Uytterhoeven -- There's lots of Linux beyond ia32 -- geert at linux-m68k.org
>
> In personal conversations with technical people, I call myself a hacker. But
> when I'm talking to journalists I just say "programmer" or something like that.
>                                 -- Linus Torvalds