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 80805b62ea5b ("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: 1f403699c40f ("drm/mediatek: Fix device/node reference count leaks in mtk_drm_get_all_drm_priv")
Reported-by: Sjoerd Simons <sjoerd@collabora.com>
Closes: https://lore.kernel.org/r/20251003-mtk-drm-refcount-v1-1-3b3f2813b0db@collabora.com
Cc: stable@vger.kernel.org
Cc: Ma Ke <make24@iscas.ac.cn>
Cc: AngeloGioacchino Del Regno <angelogioacchino.delregno@collabora.com>
Signed-off-by: Johan Hovold <johan@kernel.org>
Reviewed-by: AngeloGioacchino Del Regno <angelogioacchino.delregno@collabora.com>
Reviewed-by: Sjoerd Simons <sjoerd@collabora.com>
Tested-by: Sjoerd Simons <sjoerd@collabora.com>
Tested-by: Ritesh Raj Sarraf <ritesh.sarraf@collabora.com>
Reviewed-by: CK Hu <ck.hu@mediatek.com>
Link: https://patchwork.kernel.org/project/dri-devel/patch/20251006093937.27869-1-johan@kernel.org/
Signed-off-by: Chun-Kuang Hu <chunkuang.hu@kernel.org>

diff --git a/drivers/gpu/drm/mediatek/mtk_drm_drv.c b/drivers/gpu/drm/mediatek/mtk_drm_drv.c
index eb5537f0ac90d87f59133c8e42e206d9ac62e050..31ff2922758a4426c5f05f7cc1942db1754b8023 100644 (file)
--- a/drivers/gpu/drm/mediatek/mtk_drm_drv.c
+++ b/drivers/gpu/drm/mediatek/mtk_drm_drv.c
@@ -686,10 +686,6 @@ err_free:
        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;
 }
@@ -697,18 +693,12 @@ err_put_dev:
 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;