Re: [PATCH] drm/atomic: fix out of bounds read in for_each_*_in_state helpers

[Jani Nikula]

On Mon, 25 May 2015, Jani Nikula <jani.nikula@xxxxxxxxxxxxxxx> wrote:
> On Mon, 25 May 2015, Jani Nikula <jani.nikula@xxxxxxxxxxxxxxx> wrote:
>> On Mon, 25 May 2015, Andrey Ryabinin <a.ryabinin@xxxxxxxxxxx> wrote:
>>> On 05/25/2015 04:12 PM, Jani Nikula wrote:
>>>> On Mon, 25 May 2015, Andrey Ryabinin <a.ryabinin@xxxxxxxxxxx> 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@xxxxxxxxxxx>
>>>>> ---
>>>>>  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.


So in the mean time, I was looking at doing the below, not because of
this patch or the bug it fixes, but because I think the construct

	for (...) \
        	if (...)

in a for_something() style macro is a dangling else disaster waiting to
happen.

It's a bit tedious, I admit, and apparently makes some gcc versions
whine about using uninitialized variables, because they're not smart
enough to realize the pointers are initialized when the loop condition
is met.

BR,
Jani.


diff --git a/include/drm/drm_atomic.h b/include/drm/drm_atomic.h
index e89db0c377ba..eb81f5930a8c 100644
--- a/include/drm/drm_atomic.h
+++ b/include/drm/drm_atomic.h
@@ -134,28 +134,70 @@ int __must_check drm_atomic_check_only(struct drm_atomic_state *state);
 int __must_check drm_atomic_commit(struct drm_atomic_state *state);
 int __must_check drm_atomic_async_commit(struct drm_atomic_state *state);
 
+static inline int next_connector(struct drm_connector **pconnector,
+				 struct drm_connector **connectors,
+				 struct drm_connector_state **pstate,
+				 struct drm_connector_state **states,
+				 int index, int max)
+{
+	while (index < max && (connectors[index] == NULL || states[index] == NULL))
+		index++;
+
+	if (index < max) {
+		*pconnector = connectors[index];
+		*pstate = states[index];
+	}
+
+	return index;
+}
+
+static inline int next_crtc(struct drm_crtc **pcrtc,
+			    struct drm_crtc **crtcs,
+			    struct drm_crtc_state **pstate,
+			    struct drm_crtc_state **states,
+			    int index, int max)
+{
+	while (index < max && (crtcs[index] == NULL || states[index] == NULL))
+		index++;
+
+	if (index < max) {
+		*pcrtc = crtcs[index];
+		*pstate = states[index];
+	}
+
+	return index;
+}
+
+static inline int next_plane(struct drm_plane **pplane,
+			    struct drm_plane **planes,
+			    struct drm_plane_state **pstate,
+			    struct drm_plane_state **states,
+			    int index, int max)
+{
+	while (index < max && (planes[index] == NULL || states[index] == NULL))
+		index++;
+
+	if (index < max) {
+		*pplane = planes[index];
+		*pstate = states[index];
+	}
+
+	return index;
+}
+
 #define for_each_connector_in_state(state, connector, connector_state, __i) \
-	for ((__i) = 0;							\
-	     (connector) = (state)->connectors[__i],			\
-	     (connector_state) = (state)->connector_states[__i],	\
+	for ((__i) = next_connector(&(connector), (state)->connectors, &(connector_state), (state)->connector_states, 0, (state)->num_connector); \
 	     (__i) < (state)->num_connector;				\
-	     (__i)++)							\
-		if (connector)
-
-#define for_each_crtc_in_state(state, crtc, crtc_state, __i)	\
-	for ((__i) = 0;						\
-	     (crtc) = (state)->crtcs[__i],			\
-	     (crtc_state) = (state)->crtc_states[__i],		\
-	     (__i) < (state)->dev->mode_config.num_crtc;	\
-	     (__i)++)						\
-		if (crtc_state)
-
-#define for_each_plane_in_state(state, plane, plane_state, __i)	\
-	for ((__i) = 0;						\
-	     (plane) = (state)->planes[__i],			\
-	     (plane_state) = (state)->plane_states[__i],	\
-	     (__i) < (state)->dev->mode_config.num_total_plane;	\
-	     (__i)++)						\
-		if (plane_state)
+	     (__i) = next_connector(&(connector), (state)->connectors, &(connector_state), (state)->connector_states, (__i), (state)->num_connector))
+
+#define for_each_crtc_in_state(state, crtc, crtc_state, __i)		\
+	for ((__i) = next_crtc(&(crtc), (state)->crtcs, &(crtc_state), (state)->crtc_states, 0, (state)->dev->mode_config.num_crtc); \
+	     (__i) < (state)->dev->mode_config.num_crtc;		\
+	     (__i) = next_crtc(&crtc, (state)->crtcs, &(crtc_state), (state)->crtc_states, (__i), (state)->dev->mode_config.num_crtc))
+
+#define for_each_plane_in_state(state, plane, plane_state, __i)		\
+	for ((__i) = next_plane(&(plane), (state)->planes, &(plane_state), (state)->plane_states, 0, (state)->dev->mode_config.num_total_plane); \
+	     (__i) < (state)->dev->mode_config.num_total_plane;		\
+	     (__i) = next_plane(&(plane), (state)->planes, &(plane_state), (state)->plane_states, (__i), (state)->dev->mode_config.num_total_plane))
 
 #endif /* DRM_ATOMIC_H_ */




-- 
Jani Nikula, Intel Open Source Technology Center
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