drm: Fix race when checking for fb in the generic kms obj lookup

danvet · airlied · commit 168c02ec06f8 · 2014-08-05T11:06:52.000+10:00
In my review of commit 98f75de Author: Rob Clark <robdclark@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@gmail.com> Cc: Chris Wilson <chris@chris-wilson.co.uk> Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch> Reviewed-by: Rob Clark <robdclark@gmail.com> Signed-off-by: Dave Airlie <airlied@redhat.com>
diff --git a/drivers/gpu/drm/drm_crtc.c b/drivers/gpu/drm/drm_crtc.c
@@ -446,8 +446,12 @@ 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);
 
@@ -474,9 +478,6 @@ 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);
