drm/mediatek: Fix device use-after-free on unbind

A recent change fixed device reference leaks when looking up drm
platform device driver data during bind() but failed to remove a partial
fix which had been added by commit 80805b6 ("drm/mediatek: Fix
kobject put for component sub-drivers").

This results in a reference imbalance on component bind() failures and
on unbind() which could lead to a user-after-free.

Make sure to only drop the references after retrieving the driver data
by effectively reverting the previous partial fix.

Note that holding a reference to a device does not prevent its driver
data from going away so there is no point in keeping the reference.

Fixes: 1f40369 ("drm/mediatek: Fix device/node reference count leaks in mtk_drm_get_all_drm_priv")
Reported-by: Sjoerd Simons <[email protected]>
Closes: https://lore.kernel.org/r/[email protected]
Cc: [email protected]
Cc: Ma Ke <[email protected]>
Cc: AngeloGioacchino Del Regno <[email protected]>
Signed-off-by: Johan Hovold <[email protected]>
Reviewed-by: AngeloGioacchino Del Regno <[email protected]>
Reviewed-by: Sjoerd Simons <[email protected]>
Tested-by: Sjoerd Simons <[email protected]>
Tested-by: Ritesh Raj Sarraf <[email protected]>
Reviewed-by: CK Hu <[email protected]>
Link: https://patchwork.kernel.org/project/dri-devel/patch/[email protected]/
Signed-off-by: Chun-Kuang Hu <[email protected]>

Author: jhovold

drivers/gpu/drm/mediatek/mtk_drm_drv.c

@@ -686,29 +686,19 @@ static int mtk_drm_bind(struct device *dev)
 	for (i = 0; i < private->data->mmsys_dev_num; i++)
 		private->all_drm_private[i]->drm = NULL;
 err_put_dev:
-	for (i = 0; i < private->data->mmsys_dev_num; i++) {
-		/* For device_find_child in mtk_drm_get_all_priv() */
-		put_device(private->all_drm_private[i]->dev);
-	}
 	put_device(private->mutex_dev);
 	return ret;
 }
 
 static void mtk_drm_unbind(struct device *dev)
 {
 	struct mtk_drm_private *private = dev_get_drvdata(dev);
-	int i;
 
 	/* for multi mmsys dev, unregister drm dev in mmsys master */
 	if (private->drm_master) {
 		drm_dev_unregister(private->drm);
 		mtk_drm_kms_deinit(private->drm);
 		drm_dev_put(private->drm);
-
-		for (i = 0; i < private->data->mmsys_dev_num; i++) {
-			/* For device_find_child in mtk_drm_get_all_priv() */
-			put_device(private->all_drm_private[i]->dev);
-		}
 		put_device(private->mutex_dev);
 	}
 	private->mtk_drm_bound = false;