Re: [PATCH] drm/kmb: fix dereference before NULL check in kmb_plane_atomic_update()

[Thomas Zimmermann <tzimmermann@suse.de>]

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Hi

Am 29.07.22 um 18:40 schrieb Chrisanthus, Anitha:
> Agree with Thomas, drm_atomic_commit() will not call kmb_atomic_update() with a NULL plane. This is not an actual bug.

Indeed, it's the atomic_update function. I didn't notice at first.

> 
> Thanks,
> Anitha
> 
>> -----Original Message-----
>> From: Thomas Zimmermann <tzimmermann@suse.de>
>> Sent: Friday, July 29, 2022 7:26 AM
>> To: Zeng Jingxiang <zengjx95@gmail.com>; Chrisanthus, Anitha
>> <anitha.chrisanthus@intel.com>; edmund.j.dea@intel.com; airlied@linux.ie;
>> daniel@ffwll.ch; laurent.pinchart@ideasonboard.com; maxime@cerno.tech;
>> ville.syrjala@linux.intel.com
>> Cc: Zeng Jingxiang <linuszeng@tencent.com>; linux-kernel@vger.kernel.org;
>> dri-devel@lists.freedesktop.org
>> Subject: Re: [PATCH] drm/kmb: fix dereference before NULL check in
>> kmb_plane_atomic_update()
>>
>> Hi
>>
>> Am 29.07.22 um 16:23 schrieb Thomas Zimmermann:
>>> Hi
>>>
>>> Am 29.07.22 um 05:07 schrieb Zeng Jingxiang:
>>>> From: Zeng Jingxiang <linuszeng@tencent.com>
>>>>
>>>> The "plane" pointer was access before checking if it was NULL.
>>>>
>>>> The drm_atomic_get_old_plane_state() function will dereference
>>>> the pointer "plane".
>>>> 345    struct drm_plane_state *old_plane_state =
>>>>          drm_atomic_get_old_plane_state(state, plane);
>>>> 346    struct drm_plane_state *new_plane_state =
>>>>          drm_atomic_get_new_plane_state(state, plane);
>>>>
>>>> A NULL check for "plane" indicates that it may be NULL
>>>> 363    if (!plane || !new_plane_state || !old_plane_state)
>>>
>>> Is this an actual bug that happens?
>>>
>>> All planes should always have a state. Therefore the tests for
>>> !new_plane_state and !old_plane_state can be removed, I'd say.
>>
>> Just to clarify: moving the test for !plane before using one of the
>> get_plane_state functions is a correct bugfix.
>>
>> Best regards
>> Thomas
>>
>>>
>>> Best regards
>>> Thomas
>>>
>>>>
>>>> Fixes: 977697e20b3d ("drm/atomic: Pass the full state to planes atomic
>>>> disable and update")
>>>> Fixes: 37418bf14c13 ("drm: Use state helper instead of the plane state
>>>> pointer")
>>>> Signed-off-by: Zeng Jingxiang <linuszeng@tencent.com>
>>>> ---
>>>>    drivers/gpu/drm/kmb/kmb_plane.c | 13 ++++++++-----
>>>>    1 file changed, 8 insertions(+), 5 deletions(-)
>>>>
>>>> diff --git a/drivers/gpu/drm/kmb/kmb_plane.c
>>>> b/drivers/gpu/drm/kmb/kmb_plane.c
>>>> index 2735b8eb3537..d2bc998b65ce 100644
>>>> --- a/drivers/gpu/drm/kmb/kmb_plane.c
>>>> +++ b/drivers/gpu/drm/kmb/kmb_plane.c
>>>> @@ -342,10 +342,7 @@ static void kmb_plane_set_alpha(struct
>>>> kmb_drm_private *kmb,
>>>>    static void kmb_plane_atomic_update(struct drm_plane *plane,
>>>>                        struct drm_atomic_state *state)
>>>>    {
>>>> -    struct drm_plane_state *old_plane_state =
>>>> drm_atomic_get_old_plane_state(state,
>>>> -                                         plane);
>>>> -    struct drm_plane_state *new_plane_state =
>>>> drm_atomic_get_new_plane_state(state,
>>>> -                                         plane);
>>>> +    struct drm_plane_state *old_plane_state, *new_plane_state;
>>>>        struct drm_framebuffer *fb;
>>>>        struct kmb_drm_private *kmb;
>>>>        unsigned int width;
>>>> @@ -360,7 +357,13 @@ static void kmb_plane_atomic_update(struct
>>>> drm_plane *plane,
>>>>        static dma_addr_t addr[MAX_SUB_PLANES];
>>>>        struct disp_cfg *init_disp_cfg;
>>>> -    if (!plane || !new_plane_state || !old_plane_state)
>>>> +    if (!plane)
>>>> +        return;
>>>> +
>>>> +    old_plane_state = drm_atomic_get_old_plane_state(state, plane);
>>>> +    new_plane_state = drm_atomic_get_new_plane_state(state, plane);
>>>> +
>>>> +    if (!new_plane_state || !old_plane_state)

To add to my previous reply, none of these variables can legally be NULL 
here. If plane if NULL, DRM helpers would run atomic_disable instead.

If you want to fix anything, just remove the tests entirely and save a 
few CPU cycles.

Best regards
Thomas

>>>>            return;
>>>>        fb = new_plane_state->fb;
>>>
>>
>> --
>> Thomas Zimmermann
>> Graphics Driver Developer
>> SUSE Software Solutions Germany GmbH
>> Maxfeldstr. 5, 90409 Nürnberg, Germany
>> (HRB 36809, AG Nürnberg)
>> Geschäftsführer: Ivo Totev

-- 
Thomas Zimmermann
Graphics Driver Developer
SUSE Software Solutions Germany GmbH
Maxfeldstr. 5, 90409 Nürnberg, Germany
(HRB 36809, AG Nürnberg)
Geschäftsführer: Ivo Totev

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