Subject: Re: [PATCH 3/3] caif_virtio: fix the race between reset and netdev unregister

From: "Michael S. Tsirkin" <[email-masked]>
Date: Mon, 20 Jun 2022 05:09:19 -0400

On Mon, Jun 20, 2022 at 01:11:15PM +0800, Jason Wang wrote:
> We use to do the following steps during .remove():

We currently do

> static void cfv_remove(struct virtio_device *vdev)
> {
> 	struct cfv_info *cfv = vdev->priv;
> 
> 	rtnl_lock();
> 	dev_close(cfv->ndev);
> 	rtnl_unlock();
> 
> 	tasklet_kill(&cfv->tx_release_tasklet);
> 	debugfs_remove_recursive(cfv->debugfs);
> 
> 	vringh_kiov_cleanup(&cfv->ctx.riov);
> 	virtio_reset_device(vdev);
> 	vdev->vringh_config->del_vrhs(cfv->vdev);
> 	cfv->vr_rx = NULL;
> 	vdev->config->del_vqs(cfv->vdev);
> 	unregister_netdev(cfv->ndev);
> }
> This is racy since device could be re-opened after dev_close() but
> before unregister_netdevice():
> 
> 1) RX vringh is cleaned before resetting the device, rx callbacks that
>    is called after the vringh_kiov_cleanup() will result a UAF
> 2) Network stack can still try to use TX virtqueue even if it has been
>    deleted after dev_vqs()
> 
> Fixing this by unregistering the network device first to make sure not
> device access from both TX and RX side.
> 
> Fixes: 0d2e1a2926b18 ("caif_virtio: Introduce caif over virtio")
> Signed-off-by: Jason Wang <[email-masked]>
> ---
>  drivers/net/caif/caif_virtio.c | 6 ++----
>  1 file changed, 2 insertions(+), 4 deletions(-)
> 
> diff --git a/drivers/net/caif/caif_virtio.c b/drivers/net/caif/caif_virtio.c
> index 66375bea2fcd..a29f9b2df5b1 100644
> --- a/drivers/net/caif/caif_virtio.c
> +++ b/drivers/net/caif/caif_virtio.c
> @@ -752,9 +752,8 @@ static void cfv_remove(struct virtio_device *vdev)
>  {
>  	struct cfv_info *cfv = vdev->priv;
>  
> -	rtnl_lock();
> -	dev_close(cfv->ndev);
> -	rtnl_unlock();
> +	/* Make sure NAPI/TX won't try to access the device */
> +	unregister_netdev(cfv->ndev);
>  
>  	tasklet_kill(&cfv->tx_release_tasklet);
>  	debugfs_remove_recursive(cfv->debugfs);
> @@ -764,7 +763,6 @@ static void cfv_remove(struct virtio_device *vdev)
>  	vdev->vringh_config->del_vrhs(cfv->vdev);
>  	cfv->vr_rx = NULL;
>  	vdev->config->del_vqs(cfv->vdev);
> -	unregister_netdev(cfv->ndev);
>  }

This gives me pause, callbacks can now trigger after device
has been unregistered. Are we sure this is safe?
Won't it be safer to just keep the rtnl_lock around
the whole process?

>  static struct virtio_device_id id_table[] = {
> -- 
> 2.25.1