[Intel-gfx] [PATCH] drm: Fix race when checking for fb in the generic kms obj lookup

[Daniel Vetter]

In my review of

commit 98f75de40e9d83c3a90d294b8fd25fa2874212a9
Author: Rob Clark <robdclark at gmail.com>
Date:   Fri May 30 11:37:03 2014 -0400

    drm: add object property typ

I asked for a check to make sure that we never leak an fb from the
generic mode object lookup since those have completely different
lifetime rules. Rob added it, but outside of the idr mutex, which
means that our dereference of obj->type can already chase free'd
memory.

Somehow I didn't spot this, so fix this asap.

v2: Simplify the conditionals as suggested by Chris.

Cc: Rob Clark <robdclark at gmail.com>
Cc: Chris Wilson <chris at chris-wilson.co.uk>
Signed-off-by: Daniel Vetter <daniel.vetter at ffwll.ch>
---
 drivers/gpu/drm/drm_crtc.c | 11 ++++++-----
 1 file changed, 6 insertions(+), 5 deletions(-)

diff --git a/drivers/gpu/drm/drm_crtc.c b/drivers/gpu/drm/drm_crtc.c
index f0a777747907..d87df8836aa5 100644
--- a/drivers/gpu/drm/drm_crtc.c
+++ b/drivers/gpu/drm/drm_crtc.c
@@ -426,8 +426,12 @@ static struct drm_mode_object *_object_find(struct drm_device *dev,
 
 	mutex_lock(&dev->mode_config.idr_mutex);
 	obj = idr_find(&dev->mode_config.crtc_idr, id);
-	if (!obj || (type != DRM_MODE_OBJECT_ANY && obj->type != type) ||
-	    (obj->id != id))
+	if (obj && type != DRM_MODE_OBJECT_ANY && obj->type != type)
+		obj = NULL;
+	if (obj && obj->id != id)
+		obj = NULL;
+	/* don't leak out unref'd fb's */
+	if (obj && (obj->type == DRM_MODE_OBJECT_FB))
 		obj = NULL;
 	mutex_unlock(&dev->mode_config.idr_mutex);
 
@@ -454,9 +458,6 @@ struct drm_mode_object *drm_mode_object_find(struct drm_device *dev,
 	 * function.*/
 	WARN_ON(type == DRM_MODE_OBJECT_FB);
 	obj = _object_find(dev, id, type);
-	/* don't leak out unref'd fb's */
-	if (obj && (obj->type == DRM_MODE_OBJECT_FB))
-		obj = NULL;
 	return obj;
 }
 EXPORT_SYMBOL(drm_mode_object_find);
-- 
2.0.1