Re: [PATCH v2] fbdev: Detect integer underflow at "struct fbcon_ops"->clear_margins.

[Greg Kroah-Hartman <gregkh@linuxfoundation.org>]

On Wed, Jul 22, 2020 at 10:07:06AM +0200, Daniel Vetter wrote:
> On Tue, Jul 21, 2020 at 6:08 PM Greg Kroah-Hartman
> <gregkh@linuxfoundation.org> wrote:
> >
> > 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.
> 
> Just missed this weeks pull request train and feeling like not worth
> making this an exception (it's been broken forever after all), so
> maybe best if you just add this to vt.
> 
> Acked-by: Daniel Vetter <daniel.vetter@ffwll.ch>
> 
> Also this avoids the impression I know what's going on in fbdev code,
> maybe with sufficient abandon from my side someone will pop up who
> cares an fixes the bazillion of syzkaller issues we seem to have
> around console/vt and everything related.

Great, will go queue it up now, thanks!

greg k-h
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