Re: [PATCH 6/7] v4l2-device: v4l2_device_release_subdev_node can't reference sd

[Laurent Pinchart <laurent.pinchart@ideasonboard.com>]

Hi Hans,

Thank you for the patch.

On Thu, Feb 21, 2019 at 03:21:47PM +0100, Hans Verkuil wrote:
> When the v4l-subdev device node is released it calls the
> v4l2_device_release_subdev_node() function which sets sd->devnode
> to NULL.
> 
> However, the v4l2_subdev struct may already be released causing this
> to write in freed memory.
> 
> Instead just use the regular video_device_release release function
> (just calls kfree) and set sd->devnode to NULL right after the
> video_unregister_device() call.

This seems a bit of a workaround. The devnode can access the subdev in
multiple ways, it should really keep a reference to the subdev to ensure
it doesn't get freed early.

> Signed-off-by: Hans Verkuil <hverkuil-cisco@xs4all.nl>
> ---
>  drivers/media/v4l2-core/v4l2-device.c | 10 ++--------
>  1 file changed, 2 insertions(+), 8 deletions(-)
> 
> diff --git a/drivers/media/v4l2-core/v4l2-device.c b/drivers/media/v4l2-core/v4l2-device.c
> index e0ddb9a52bd1..57a7b220fa4d 100644
> --- a/drivers/media/v4l2-core/v4l2-device.c
> +++ b/drivers/media/v4l2-core/v4l2-device.c
> @@ -216,13 +216,6 @@ int v4l2_device_register_subdev(struct v4l2_device *v4l2_dev,
>  }
>  EXPORT_SYMBOL_GPL(v4l2_device_register_subdev);
>  
> -static void v4l2_device_release_subdev_node(struct video_device *vdev)
> -{
> -	struct v4l2_subdev *sd = video_get_drvdata(vdev);
> -	sd->devnode = NULL;
> -	kfree(vdev);
> -}
> -
>  int v4l2_device_register_subdev_nodes(struct v4l2_device *v4l2_dev)
>  {
>  	struct video_device *vdev;
> @@ -250,7 +243,7 @@ int v4l2_device_register_subdev_nodes(struct v4l2_device *v4l2_dev)
>  		vdev->dev_parent = sd->dev;
>  		vdev->v4l2_dev = v4l2_dev;
>  		vdev->fops = &v4l2_subdev_fops;
> -		vdev->release = v4l2_device_release_subdev_node;
> +		vdev->release = video_device_release;
>  		vdev->ctrl_handler = sd->ctrl_handler;
>  		err = __video_register_device(vdev, VFL_TYPE_SUBDEV, -1, 1,
>  					      sd->owner);
> @@ -319,6 +312,7 @@ void v4l2_device_unregister_subdev(struct v4l2_subdev *sd)
>  	}
>  #endif
>  	video_unregister_device(sd->devnode);
> +	sd->devnode = NULL;
>  	if (!sd->owner_v4l2_dev)
>  		module_put(sd->owner);
>  }

-- 
Regards,

Laurent Pinchart