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

[Sjoerd Simons <sjoerd@collabora.com>]

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

Thanks for correcting my "fix". This looks better and i can confirm it fixes the issue :)

Reviewed-By: Sjoerd Simons <sjoerd@collabora.com>
Tested-By: Sjoerd Simons <sjoerd@collabora.com>
 
> 
> Fixes: 1f403699c40f ("drm/mediatek: Fix device/node reference count leaks in
> mtk_drm_get_all_drm_priv")
> Reported-by: Sjoerd Simons <sjoerd@collabora.com>
> Link: 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>
> ---
>  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;