Re: [PATCH] drm/simple_kms_helper: Fix NULL pointer dereference with no active CRTC

[Oleksandr Andrushchenko <Oleksandr_Andrushchenko@epam.com>]

On 02/19/2018 04:30 PM, Daniel Vetter wrote:
> On Tue, Feb 13, 2018 at 10:44:16AM +0200, Oleksandr Andrushchenko wrote:
>> From: Oleksandr Andrushchenko <oleksandr_andrushchenko@epam.com>
>>
>> It is possible that drm_simple_kms_plane_atomic_check called
>> with no CRTC set, e.g. when user-space application sets CRTC_ID/FB_ID
>> to 0 before doing any actual drawing. This leads to NULL pointer
>> dereference because in this case new CRTC state is NULL and must be
>> checked before accessing.
>>
>> Signed-off-by: Oleksandr Andrushchenko <oleksandr_andrushchenko@epam.com>
>> ---
>>   drivers/gpu/drm/drm_simple_kms_helper.c | 6 ++++--
>>   1 file changed, 4 insertions(+), 2 deletions(-)
>>
>> diff --git a/drivers/gpu/drm/drm_simple_kms_helper.c b/drivers/gpu/drm/drm_simple_kms_helper.c
>> index 9ca8a4a59b74..a05eca9cec8b 100644
>> --- a/drivers/gpu/drm/drm_simple_kms_helper.c
>> +++ b/drivers/gpu/drm/drm_simple_kms_helper.c
>> @@ -121,8 +121,10 @@ static int drm_simple_kms_plane_atomic_check(struct drm_plane *plane,
>>   	pipe = container_of(plane, struct drm_simple_display_pipe, plane);
>>   	crtc_state = drm_atomic_get_new_crtc_state(plane_state->state,
>>   						   &pipe->crtc);
>> -	if (!crtc_state->enable)
>> -		return 0; /* nothing to check when disabling or disabled */
>> +
>> +	if (!crtc_state || !crtc_state->enable)
>> +		/* nothing to check when disabling or disabled or no CRTC set */
>> +		return 0;
>>   
>>   	if (crtc_state->enable)
>>   		drm_mode_get_hv_timing(&crtc_state->mode,
> Hm, this is a bit annoying, since the can_position = false parameter to
> drm_atomic_helper_check_plane_state is supposed to catch all this. Would
> moving all the checks after the call to that helper, and gating them on
> plane_state->visible also work?
Yes, it does work if this is what you mean:
diff --git a/drivers/gpu/drm/drm_simple_kms_helper.c 
b/drivers/gpu/drm/drm_simple_kms_helper.c
index a05eca9cec8b..c48858bb2823 100644
--- a/drivers/gpu/drm/drm_simple_kms_helper.c
+++ b/drivers/gpu/drm/drm_simple_kms_helper.c
@@ -122,14 +122,6 @@ static int drm_simple_kms_plane_atomic_check(struct 
drm_plane *plane,
         crtc_state = drm_atomic_get_new_crtc_state(plane_state->state,
&pipe->crtc);

-       if (!crtc_state || !crtc_state->enable)
-               /* nothing to check when disabling or disabled or no 
CRTC set */
-               return 0;
-
-       if (crtc_state->enable)
-               drm_mode_get_hv_timing(&crtc_state->mode,
-                                      &clip.x2, &clip.y2);
-
         ret = drm_atomic_helper_check_plane_state(plane_state, crtc_state,
                                                   &clip,
DRM_PLANE_HELPER_NO_SCALING,
@@ -138,6 +130,13 @@ static int drm_simple_kms_plane_atomic_check(struct 
drm_plane *plane,
         if (ret)
                 return ret;

+       if (!plane_state->visible || !crtc_state->enable)
+               return 0; /* nothing to check when disabling or disabled */
+
+       if (plane_state->visible && crtc_state->enable)
+               drm_mode_get_hv_timing(&crtc_state->mode,
+                                      &clip.x2, &clip.y2);
+
         if (!plane_state->visible)
                 return -EINVAL;

> We'd need to add a guarantee to drm_atomic_helper_check_plane_state that
> it can cope with crtc_state == NULL, but I think that's a good idea
> anyway. Atm it shouldn't end up looking at the crtc_state pointer in that
> case.
It doesn't look at it at the moment
> Otherwise we'll just go with your fix, but it feels all a bit too fragile,
> hence why I want to explore more robust options a bit.
At list with the change above it passes my test which failed
before. Although I cannot confirm it works for others, but it
certainly does for me.
> -Daniel
Do you want me to send v1 with the code above?

Thank you,
Oleksandr