linux-kernel - Re: [Intel-gfx] [PATCH] drm/i915: Sanity check incoming ioctl data for a NULL pointer

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Date:	Sat, 16 Mar 2013 10:19:20 +0000
From:	Chris Wilson <chris@...is-wilson.co.uk>
To:	Ben Widawsky <ben@...dawsk.net>
Cc:	Tommi Rantala <tt.rantala@...il.com>,
	David Airlie <airlied@...ux.ie>,
	Daniel Vetter <daniel.vetter@...ll.ch>,
	intel-gfx@...ts.freedesktop.org, linux-kernel@...r.kernel.org,
	dri-devel@...ts.freedesktop.org, Dave Jones <davej@...hat.com>
Subject: Re: [Intel-gfx] [PATCH] drm/i915: Sanity check incoming ioctl data
 for a NULL pointer

On Fri, Mar 15, 2013 at 04:49:42PM -0700, Ben Widawsky wrote:
> On Fri, Mar 15, 2013 at 10:06:19PM +0000, Chris Wilson wrote:
> > On Fri, Mar 15, 2013 at 09:36:07AM -0700, Ben Widawsky wrote:
> > > On Fri, Mar 15, 2013 at 08:24:03AM +0000, Chris Wilson wrote:
> > > > That's what I thought too. Looking at the stack trace, the empirical
> > > > evidence is that we need the check.
> > > > -Chris
> > > 
> > > I think we need to investigate the issue more then, or put a BUG_ON() in
> > > the drm code and run it through trinity. We have other places where arg
> > > can't/shouldn't be NULL and we don't check.
> > 
> > Actually we are both wrong. drm_ioctl() does not validate that the
> > user struct matches the expected size, just ensures that if that user
> > cmd specifies that the arg is to be used that it only up to the known
> > size is copied.
> > 
> > A hostile userspace can bludgen a NULL pointer through drm_ioctl() into
> > the driver->ioctl->func().
> 
> > > > +   if (args == NULL)
> > > > +           return -EINVAL;
> > > > +
> 
> I must be failing to see the obvious, but I'm still not getting how args
> can ever be NULL. kdata which is passed to us as "data" and cast to
> "args' is either always some stack variable, or some kmalloc'd memory. I
> see how the arguments themselves can be crap which is really only when
> user size != drv_size.
> 
> So tell me, which case can result in a NULL arg?
> 1. user size == drv_size // better not be this one
> 2. user size < driver size
> 3. user size > driver size
> 
> It seems to me we still must [simply] be missing something in our
> parameter validation.

If *userspace* doesn't request either IOC_IN | IOC_OUT in their ioctl
command (which are seperate from the ioctl number), then kdata is set to
NULL.
-Chris

-- 
Chris Wilson, Intel Open Source Technology Centre
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