[1/5] fbcon: Disallow setting font bigger than screen size

Prevent that users set a font size which is bigger than the physical screen.
It's unlikely this may happen (because screens are usually much larger than the
fonts and each font char is limited to 32x32 pixels), but it may happen on
smaller screens/LCD displays.

Signed-off-by: Helge Deller <deller@gmx.de>
Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Cc: stable@vger.kernel.org # v4.14+
---
 drivers/video/fbdev/core/fbcon.c | 5 +++++
 1 file changed, 5 insertions(+)

--
2.35.3

Hi Helge,

On Wed, Jun 29, 2022 at 10:00 PM Helge Deller <deller@gmx.de> wrote:
> Prevent that users set a font size which is bigger than the physical screen.
> It's unlikely this may happen (because screens are usually much larger than the
> fonts and each font char is limited to 32x32 pixels), but it may happen on
> smaller screens/LCD displays.
>
> Signed-off-by: Helge Deller <deller@gmx.de>
> Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch>

Thanks for your patch!

Reviewed-by: Geert Uytterhoeven <geert@linux-m68k.org>

> --- a/drivers/video/fbdev/core/fbcon.c
> +++ b/drivers/video/fbdev/core/fbcon.c
> @@ -2469,6 +2469,11 @@ static int fbcon_set_font(struct vc_data *vc, struct console_font *font,
>         if (charcount != 256 && charcount != 512)
>                 return -EINVAL;
>
> +       /* font bigger than screen resolution ? */
> +       if (font->width  > FBCON_SWAP(info->var.rotate, info->var.xres, info->var.yres) ||
> +           font->height > FBCON_SWAP(info->var.rotate, info->var.yres, info->var.xres))

Note that we already have local vars w and h, albeit with the wrong
signedness.

> +               return -EINVAL;
> +
>         /* Make sure drawing engine can handle the font */
>         if (!(info->pixmap.blit_x & (1 << (font->width - 1))) ||
>             !(info->pixmap.blit_y & (1 << (font->height - 1))))

There were already more opportunities for using w and h before...

Gr{oetje,eeting}s,

                        Geert

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

On 6/30/22 20:51, Geert Uytterhoeven wrote:
> Hi Helge,
>
> On Wed, Jun 29, 2022 at 10:00 PM Helge Deller <deller@gmx.de> wrote:
>> Prevent that users set a font size which is bigger than the physical screen.
>> It's unlikely this may happen (because screens are usually much larger than the
>> fonts and each font char is limited to 32x32 pixels), but it may happen on
>> smaller screens/LCD displays.
>>
>> Signed-off-by: Helge Deller <deller@gmx.de>
>> Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch>
>
> Thanks for your patch!
>
> Reviewed-by: Geert Uytterhoeven <geert@linux-m68k.org>
>
>> --- a/drivers/video/fbdev/core/fbcon.c
>> +++ b/drivers/video/fbdev/core/fbcon.c
>> @@ -2469,6 +2469,11 @@ static int fbcon_set_font(struct vc_data *vc, struct console_font *font,
>>         if (charcount != 256 && charcount != 512)
>>                 return -EINVAL;
>>
>> +       /* font bigger than screen resolution ? */
>> +       if (font->width  > FBCON_SWAP(info->var.rotate, info->var.xres, info->var.yres) ||
>> +           font->height > FBCON_SWAP(info->var.rotate, info->var.yres, info->var.xres))
>
> Note that we already have local vars w and h, albeit with the wrong
> signedness.

I don't like the "h" and "w" variables. Maybe something like "fh" for "font-heigth"
would have been better explaining which kind of "h" is meant.
I assume that's why the patch below didn't used it either.

That said, I'd like to keep it as is (at least for now).

Helge


>> +               return -EINVAL;
>> +
>>         /* Make sure drawing engine can handle the font */
>>         if (!(info->pixmap.blit_x & (1 << (font->width - 1))) ||
>>             !(info->pixmap.blit_y & (1 << (font->height - 1))))
>
> There were already more opportunities for using w and h before...
>
> Gr{oetje,eeting}s,
>
>                         Geert
>
> --
> Geert Uytterhoeven -- There's lots of Linux beyond ia32 -- geert@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