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

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Date:	Sun, 17 Mar 2013 20:50:03 +0100
From:	Daniel Vetter <daniel.vetter@...ll.ch>
To:	Chris Wilson <chris@...is-wilson.co.uk>,
	Ben Widawsky <ben@...dawsk.net>,
	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 Sat, Mar 16, 2013 at 11:19 AM, Chris Wilson <chris@...is-wilson.co.uk> wrote:
> 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.

Doesn't that mean that we need these checks everywhere? Or at least a
fixup in drm core proper?

And I think we need to add trinity to our test setup eventually ;-)
-Daniel
-- 
Daniel Vetter
Software Engineer, Intel Corporation
+41 (0) 79 365 57 48 - http://blog.ffwll.ch
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