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

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Date:	Fri, 15 Mar 2013 22:06:19 +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 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 we used driver->ioctl->cmd instead of the user supplied cmd, we would
have a whole other can of worms to deal with (as we suddenly get a
struct of zeroes). However, those worms should already be treated as
invalid. Choose your poison.
-Chris

-- 
Chris Wilson, Intel Open Source Technology Centre
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