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Date:   Tue, 21 Jul 2020 18:08:36 +0200
From:   Greg Kroah-Hartman <gregkh@linuxfoundation.org>
To:     Tetsuo Handa <penguin-kernel@i-love.sakura.ne.jp>
Cc:     Daniel Vetter <daniel@ffwll.ch>,
        Dan Carpenter <dan.carpenter@oracle.com>,
        linux-fbdev@vger.kernel.org,
        Bartlomiej Zolnierkiewicz <b.zolnierkie@samsung.com>,
        syzbot <syzbot+e5fd3e65515b48c02a30@syzkaller.appspotmail.com>,
        linux-kernel@vger.kernel.org, dri-devel@lists.freedesktop.org,
        George Kennedy <george.kennedy@oracle.com>,
        Jiri Slaby <jslaby@suse.com>,
        Dmitry Vyukov <dvyukov@google.com>
Subject: Re: [PATCH v2] fbdev: Detect integer underflow at "struct
 fbcon_ops"->clear_margins.
Message-ID: <20200721160836.GA2109047@kroah.com>
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On Thu, Jul 16, 2020 at 08:27:21PM +0900, Tetsuo Handa wrote:
> On 2020/07/16 19:00, Daniel Vetter wrote:
> > On Thu, Jul 16, 2020 at 12:29:00AM +0900, Tetsuo Handa wrote:
> >> On 2020/07/16 0:12, Dan Carpenter wrote:
> >>> I've complained about integer overflows in fbdev for a long time...
> >>>
> >>> What I'd like to see is something like the following maybe.  I don't
> >>> know how to get the vc_data in fbmem.c so it doesn't include your checks
> >>> for negative.
> >>
> >> Yes. Like I said "Thus, I consider that we need more sanity/constraints checks." at
> >> https://lore.kernel.org/lkml/b1e7dd6a-fc22-bba8-0abb-d3e779329bce@i-love.sakura.ne.jp/ ,
> >> we want basic checks. That's a task for fbdev people who should be familiar with
> >> necessary constraints.
> > 
> > I think the worldwide supply of people who understand fbdev and willing to
> > work on it is roughly 0. So if someone wants to fix this mess properly
> > (which likely means adding tons of over/underflow checks at entry points,
> > since you're never going to catch the driver bugs, there's too many and
> > not enough people who care) they need to fix this themselves.
> 
> But I think we can enforce reasonable constraint which is much stricter than Dan's basic_checks()
> (which used INT_MAX). For example, do we need to accept var->{xres,yres} >= 1048576, for
> "32768 rows or cols" * "32 pixels per character" = 1045876 and vc_do_resize() accepts only
> rows and cols < 32768 ?
> 
> > 
> > Just to avoid confusion here.
> > 
> >> Anyway, my two patches are small and low cost; can we apply these patches regardless
> >> of basic checks?
> > 
> > Which two patches where?
> 
> [PATCH v3] vt: Reject zero-sized screen buffer size.
>  from https://lkml.kernel.org/r/20200712111013.11881-1-penguin-kernel@I-love.SAKURA.ne.jp

This is now in my tree.

> [PATCH v2] fbdev: Detect integer underflow at "struct fbcon_ops"->clear_margins.
>  from https://lkml.kernel.org/r/20200715015102.3814-1-penguin-kernel@I-love.SAKURA.ne.jp

That should be taken by the fbdev maintainer, but I can take it too if
people want.

thanks,

greg k-h