[PATCH] drm/mediatek: fix device use-after-free on unbind

[CK Hu (胡俊光)]

On Mon, 2025-10-06 at 11:39 +0200, Johan Hovold wrote:
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> 
> 
> 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.

Reviewed-by: CK Hu <ck.hu at mediatek.com>

> 
> Fixes: 1f403699c40f ("drm/mediatek: Fix device/node reference count leaks in mtk_drm_get_all_drm_priv")
> Reported-by: Sjoerd Simons <sjoerd at collabora.com>
> Link: https://lore.kernel.org/r/20251003-mtk-drm-refcount-v1-1-3b3f2813b0db@collabora.com
> Cc: stable at vger.kernel.org
> Cc: Ma Ke <make24 at iscas.ac.cn>
> Cc: AngeloGioacchino Del Regno <angelogioacchino.delregno at collabora.com>
> Signed-off-by: Johan Hovold <johan at kernel.org>
> ---
>  drivers/gpu/drm/mediatek/mtk_drm_drv.c | 10 ----------
>  1 file changed, 10 deletions(-)
> 
> diff --git a/drivers/gpu/drm/mediatek/mtk_drm_drv.c b/drivers/gpu/drm/mediatek/mtk_drm_drv.c
> index 384b0510272c..a94c51a83261 100644
> --- a/drivers/gpu/drm/mediatek/mtk_drm_drv.c
> +++ b/drivers/gpu/drm/mediatek/mtk_drm_drv.c
> @@ -686,10 +686,6 @@ 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;
>  }
> @@ -697,18 +693,12 @@ static int mtk_drm_bind(struct device *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;
> --
> 2.49.1
>