linux-kernel - Re: [PATCH] fbcon: Check font dimension limits

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Date:   Sun, 29 Jan 2023 16:13:07 +0100
From:   Samuel Thibault <samuel.thibault@...-lyon.org>
To:     Greg KH <gregkh@...uxfoundation.org>
Cc:     Daniel Vetter <daniel@...ll.ch>, Helge Deller <deller@....de>,
        linux-fbdev@...r.kernel.org, dri-devel@...ts.freedesktop.org,
        linux-kernel@...r.kernel.org, stable@...r.kernel.org,
        Sanan Hasanov <sanan.hasanov@...ghts.ucf.edu>
Subject: Re: [PATCH] fbcon: Check font dimension limits

Greg KH, le jeu. 26 janv. 2023 08:43:02 +0100, a ecrit:
> On Thu, Jan 26, 2023 at 01:49:12AM +0100, Samuel Thibault wrote:
> > blit_x and blit_y are uint32_t, so fbcon currently cannot support fonts
> > larger than 32x32.
> 
> "u32" you mean, right?

Right :)

> > The 32x32 case also needs shifting an unsigned int, to properly set bit
> > 31, otherwise we get "UBSAN: shift-out-of-bounds in fbcon_set_font",
> > as reported on
> > 
> > http://lore.kernel.org/all/IA1PR07MB98308653E259A6F2CE94A4AFABCE9@IA1PR07MB9830.namprd07.prod.outlook.com
> 
> Odd blank line?

Not sure what you mean? I found it easier to read to put a blank line
between the text and the references.

> > Kernel Branch: 6.2.0-rc5-next-20230124
> > Kernel config: https://drive.google.com/file/d/1F-LszDAizEEH0ZX0HcSR06v5q8FPl2Uv/view?usp=sharing
> > Reproducer: https://drive.google.com/file/d/1mP1jcLBY7vWCNM60OMf-ogw-urQRjNrm/view?usp=sharing
> 
> What are all of these lines for?

They provide the references that were set in the original report

http://lore.kernel.org/all/IA1PR07MB98308653E259A6F2CE94A4AFABCE9@IA1PR07MB9830.namprd07.prod.outlook.com

Arguably the "branch" and "config" are not that useful since the bug has
been there for more than a dozen years, but notably the "Reproducer" is
useful to provide a userland program that triggers the UB.

> > Reported-by: Sanan Hasanov <sanan.hasanov@...ghts.ucf.edu>
> > Signed-off-by: Samuel Thibault <samuel.thibault@...-lyon.org>
> 
> What commit id does this fix?

I though it was there forever, but it seems the check was added in 2007,
so I'll add it in next version.

> Should it go to stable kernels?

Yes. I added stable in Cc of the mail but missed adding it in the text,
I will add it.

> > Index: linux-6.0/drivers/video/fbdev/core/fbcon.c
> > ===================================================================
> > --- linux-6.0.orig/drivers/video/fbdev/core/fbcon.c
> > +++ linux-6.0/drivers/video/fbdev/core/fbcon.c
> > @@ -2489,9 +2489,12 @@ static int fbcon_set_font(struct vc_data
> >  	    h > FBCON_SWAP(info->var.rotate, info->var.yres, info->var.xres))
> >  		return -EINVAL;
> >  
> > +	if (font->width > 32 || font->height > 32)
> > +		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))))
> > +	if (!(info->pixmap.blit_x & (1U << (font->width - 1))) ||
> > +	    !(info->pixmap.blit_y & (1U << (font->height - 1))))
> 
> Are you sure this is still needed with the above check added?  If so,
> why?  What is the difference in the compiled code?

As mentioned by Jiri, yes in the 32 case it's needed otherwise it's UB.

Samuel