linux-kernel - Re: [PATCH 0/3] Prevent out-of-bounds access for built-in font data buffers

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Date:   Thu, 24 Sep 2020 11:30:35 -0400
From:   Peilin Ye <yepeilin.cs@...il.com>
To:     David Laight <David.Laight@...LAB.COM>
Cc:     Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Bartlomiej Zolnierkiewicz <b.zolnierkie@...sung.com>,
        Jiri Slaby <jirislaby@...nel.org>,
        Daniel Vetter <daniel.vetter@...ll.ch>,
        dri-devel@...ts.freedesktop.org, linux-fbdev@...r.kernel.org,
        linux-kernel-mentees@...ts.linuxfoundation.org,
        syzkaller-bugs@...glegroups.com, linux-kernel@...r.kernel.org
Subject: Re: [PATCH 0/3] Prevent out-of-bounds access for built-in font data
 buffers

Hi!

On Thu, Sep 24, 2020 at 02:42:18PM +0000, David Laight wrote:
> > On Thu, Sep 24, 2020 at 09:38:22AM -0400, Peilin Ye wrote:
> > > Hi all,
> > >
> > > syzbot has reported [1] a global out-of-bounds read issue in
> > > fbcon_get_font(). A malicious user may resize `vc_font.height` to a large
> > > value in vt_ioctl(), causing fbcon_get_font() to overflow our built-in
> > > font data buffers, declared in lib/fonts/font_*.c:
> ...
> > > (drivers/video/fbdev/core/fbcon.c)
> > >  	if (font->width <= 8) {
> > >  		j = vc->vc_font.height;
> > > +		if (font->charcount * j > FNTSIZE(fontdata))
> > > +			return -EINVAL;
> 
> Can that still go wrong because the multiply wraps?

Thank you for bringing this up!

The resizing of `vc_font.height` happened in vt_resizex():

(drivers/tty/vt/vt_ioctl.c)
	if (v.v_clin > 32)
		return -EINVAL;
	[...]
	for (i = 0; i < MAX_NR_CONSOLES; i++) {
			[...]
			if (v.v_clin)
				vcp->vc_font.height = v.v_clin;
				     ^^^^^^^^^^^^^^

It does check if `v.v_clin` is greater than 32. And, currently, all
built-in fonts have a `charcount` of 256.

Therefore, for built-in fonts and resizing happened in vt_resizex(), it
cannot cause an interger overflow.

However I am not very sure about user-provided fonts, and if there are
other functions that can resize `height` or even `charcount` to a really
huge value, but I will do more investigation and think about it.

Thank you,
Peilin Ye