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

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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@xxxxxxx>
Sent: Friday, July 29, 2022 7:26 AM
To: Zeng Jingxiang <zengjx95@xxxxxxxxx>; Chrisanthus, Anitha
<anitha.chrisanthus@xxxxxxxxx>; edmund.j.dea@xxxxxxxxx; airlied@xxxxxxxx;
daniel@xxxxxxxx; laurent.pinchart@xxxxxxxxxxxxxxxx; maxime@xxxxxxxxxx;
ville.syrjala@xxxxxxxxxxxxxxx
Cc: Zeng Jingxiang <linuszeng@xxxxxxxxxxx>; linux-kernel@xxxxxxxxxxxxxxx;
dri-devel@xxxxxxxxxxxxxxxxxxxxx
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@xxxxxxxxxxx>

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@xxxxxxxxxxx>
---
   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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