Re: [PATCH] drm/fb-helper: add virtual screen size check to drm_fb_helper_check_var()

[Geert Uytterhoeven]

Hi Andrey,

On Thu, Aug 11, 2022 at 12:23 PM Andrey Strachuk <strochuk@xxxxxxxxx> wrote:
> Add virtual screen size check to drm_fb_helper_check_var() in
> order to validate userspace input.
>
> Found by Linux Verification Center (linuxtesting.org) with syzkaller.
>
> Signed-off-by: Andrey Strachuk <strochuk@xxxxxxxxx>

Thanks for your patch!

> Fixes: 6c11df58fd1a ("fbmem: Check virtual screen sizes in fb_set_var()")

This Fixes tag is misleading: the bug has existed since basically
forever.

> --- a/drivers/gpu/drm/drm_fb_helper.c
> +++ b/drivers/gpu/drm/drm_fb_helper.c
> @@ -1355,6 +1355,16 @@ int drm_fb_helper_check_var(struct fb_var_screeninfo *var,
>             (drm_format_info_block_height(fb->format, 0) > 1))
>                 return -EINVAL;
>
> +       /* verify that virtual resolution >= physical resolution */
> +       if (var->xres_virtual < var->xres ||
> +           var->yres_virtual < var->yres) {
> +               drm_dbg_kms(dev, "requested virtual screen size that is "
> +                               "below the physical size(%ux%u vs. %ux%u)\n",
> +                       var->xres_virtual, var->yres_virtual,
> +                       var->xres, var->yres);
> +               return -EINVAL;

According to the fbdev rounding rules, invalid values should be rounded
up, if possible.  Hence it is better to not return an error, but round up:

    if (var->xres_virtual < var->xres)
            var->xres_virtual = var->xres;
    if (var->yres_virtual < var->yres)
            var->yres_virtual = var->yres;

> +       }
> +
>         /*
>          * Changes struct fb_var_screeninfo are currently not pushed back
>          * to KMS, hence fail if different settings are requested.

Gr{oetje,eeting}s,

                        Geert

--
Geert Uytterhoeven -- There's lots of Linux beyond ia32 -- geert@xxxxxxxxxxxxxx

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