[PATCH v2 1/4] fbcon: Disallow setting font bigger than screen size

[Helge Deller]

On 6/25/22 14:45, Daniel Vetter wrote:
> On Sat, Jun 25, 2022 at 02:24:59PM +0200, Helge Deller 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 at gmx.de>
>> Cc: stable at vger.kernel.org # v4.14+
>> ---
>>  drivers/video/fbdev/core/fbcon.c | 5 +++++
>>  1 file changed, 5 insertions(+)
>>
>> diff --git a/drivers/video/fbdev/core/fbcon.c b/drivers/video/fbdev/core/fbcon.c
>> index c4e91715ef00..e162d5e753e5 100644
>> --- 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))
>> +		return -EINVAL;
>
> Reviewed-by: Daniel Vetter <daniel.vetter at ffwll.ch>

Thanks!

> Maybe as a safety follow up patch, we have a few copies of the below:
>
> 	cols = FBCON_SWAP(ops->rotate, info->var.xres, info->var.yres);
> 	rows = FBCON_SWAP(ops->rotate, info->var.yres, info->var.xres);
> 	cols /= vc->vc_font.width;
> 	rows /= vc->vc_font.height;
>
> Might be good to extract that into a helper and also add
>
> 	WARN_ON(!cols);
> 	WARN_ON(!rows);

That's not needed then.
The checks I added above will ensure that cols and rows are both greater than 0.

> to make sure we really didn't screw this up and give syzkaller et all an
> easier time finding bugs - it doesn't need to discover the full exploit,
> only needs to get to this here.
>
> Also maybe even check that cols/rows is within reasons, like smaller than
> BIT(24) or so (so that we have a bunch of headroom for overflows).

Not needed either.
cols and rows is the screen size divided by an value between 1-32 (the max
font size). So, since screen size is already the higest limit, cols&rows
will always be smaller than screen size (and > 0).

Helge