linux-kernel - Re: [PATCH] drm/atomic: fix out of bounds read in for_each_*_in

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Date:	Mon, 25 May 2015 16:57:41 +0300
From:	Ville Syrjälä <ville.syrjala@...ux.intel.com>
To:	Jani Nikula <jani.nikula@...ux.intel.com>
Cc:	Andrey Ryabinin <a.ryabinin@...sung.com>,
	David Airlie <airlied@...ux.ie>,
	Ander Conselvan de Oliveira 
	<ander.conselvan.de.oliveira@...el.com>,
	linux-kernel@...r.kernel.org, dri-devel@...ts.freedesktop.org
Subject: Re: [PATCH] drm/atomic: fix out of bounds read in
 for_each_*_in_state helpers

On Mon, May 25, 2015 at 04:52:31PM +0300, Jani Nikula wrote:
> On Mon, 25 May 2015, Jani Nikula <jani.nikula@...ux.intel.com> wrote:
> > On Mon, 25 May 2015, Andrey Ryabinin <a.ryabinin@...sung.com> wrote:
> >> On 05/25/2015 04:12 PM, Jani Nikula wrote:
> >>> On Mon, 25 May 2015, Andrey Ryabinin <a.ryabinin@...sung.com> wrote:
> >>>> for_each_*_in_state validate array index after
> >>>> access to array elements, thus perform out of bounds read.
> >>>>
> >>>> Fix this by validating index in the first place and read
> >>>> array element iff validation was successful.
> >>>>
> >>>> Fixes: df63b9994eaf ("drm/atomic: Add for_each_{connector,crtc,plane}_in_state helper macros")
> >>>> Signed-off-by: Andrey Ryabinin <a.ryabinin@...sung.com>
> >>>> ---
> >>>>  include/drm/drm_atomic.h | 24 ++++++++++++------------
> >>>>  1 file changed, 12 insertions(+), 12 deletions(-)
> >>>>
> >>>> diff --git a/include/drm/drm_atomic.h b/include/drm/drm_atomic.h
> >>>> index c1571034..3f13b91 100644
> >>>> --- a/include/drm/drm_atomic.h
> >>>> +++ b/include/drm/drm_atomic.h
> >>>> @@ -77,26 +77,26 @@ int __must_check drm_atomic_async_commit(struct drm_atomic_state *state);
> >>>>  
> >>>>  #define for_each_connector_in_state(state, connector, connector_state, __i) \
> >>>>  	for ((__i) = 0;							\
> >>>> -	     (connector) = (state)->connectors[__i],			\
> >>>> -	     (connector_state) = (state)->connector_states[__i],	\
> >>>> -	     (__i) < (state)->num_connector;				\
> >>>> +	     (__i) < (state)->num_connector &&				\
> >>>> +	     ((connector) = (state)->connectors[__i],			\
> >>>> +	     (connector_state) = (state)->connector_states[__i], 1); 	\
> >>> 
> >>> This will stop at the first NULL connector/connector_state. Similarly
> >>> for the loops below.
> >>> 
> >>
> >> This will stop iff (__i) >= (state)->num_connector, because the result of expression:
> >> 	 ((connector) = (state)->connectors[__i], (connector_state) = (state)->connector_states[__i], 1)
> >> is always 1.
> >
> > Why do you think it'll always be 1?
> 
> That might be because there's the 1 at the end. *blush*.
> 
> I do wonder if this is too subtle in general, or if it's just too subtle
> for me.

s/1/true/ might make it a bit less subtle, but not by much.

-- 
Ville Syrjälä
Intel OTC
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