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

[Daniel Vetter]

On Sun, Jun 26, 2022 at 12:27:51AM +0200, Daniel Vetter wrote:
> On Sat, Jun 25, 2022 at 04:53:25PM +0200, Helge Deller wrote:
> > 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.
> 
> Yeah I reviewed it too, but I don't trust review all over the place.
> Especially with something like fbcon with entry points from everywhere.

Also the other motivation is that a bit of common code extraction refactor
for non-trivial math like the above is simply the right thing to do.
-Daniel
-- 
Daniel Vetter
Software Engineer, Intel Corporation
http://blog.ffwll.ch