Re: [RFC 1/2] virtio_net: timeout control virtqueue commands

["Michael S. Tsirkin" <mst@redhat.com>]

On Tue, Oct 07, 2025 at 03:06:21PM +0200, Eugenio Pérez wrote:
> An userland device implemented through VDUSE could take rtnl forever if
> the virtio-net driver is running on top of virtio_vdpa.  Let's break the
> device if it does not return the buffer in a longer-than-assumible
> timeout.

So now I can't debug qemu with gdb because guest dies :(
Let's not break valid use-cases please.


Instead, solve it in vduse, probably by handling cvq within
kernel.


> A less agressive path can be taken to recover the device, like only
> resetting the control virtqueue.  However, the state of the device after
> this action is taken races, as the vq could be reset after the device
> writes the OK.  Leaving TODO anyway.
> 
> Signed-off-by: Eugenio Pérez <eperezma@redhat.com>
> ---
>  drivers/net/virtio_net.c | 10 ++++++++++
>  1 file changed, 10 insertions(+)
> 
> diff --git a/drivers/net/virtio_net.c b/drivers/net/virtio_net.c
> index 31bd32bdecaf..ed68ad69a019 100644
> --- a/drivers/net/virtio_net.c
> +++ b/drivers/net/virtio_net.c
> @@ -3576,6 +3576,7 @@ static bool virtnet_send_command_reply(struct virtnet_info *vi, u8 class, u8 cmd
>  {
>  	struct scatterlist *sgs[5], hdr, stat;
>  	u32 out_num = 0, tmp, in_num = 0;
> +	unsigned long end_time;
>  	bool ok;
>  	int ret;
>  
> @@ -3614,11 +3615,20 @@ static bool virtnet_send_command_reply(struct virtnet_info *vi, u8 class, u8 cmd
>  
>  	/* Spin for a response, the kick causes an ioport write, trapping
>  	 * into the hypervisor, so the request should be handled immediately.
> +	 *
> +	 * Long timeout so a malicious device is not able to lock rtnl forever.
>  	 */
> +	end_time = jiffies + 30 * HZ;
>  	while (!virtqueue_get_buf(vi->cvq, &tmp) &&
>  	       !virtqueue_is_broken(vi->cvq)) {
>  		cond_resched();
>  		cpu_relax();
> +
> +		if (time_after(end_time, jiffies)) {
> +			/* TODO Reset vq if possible? */
> +			virtio_break_device(vi->vdev);
> +			break;
> +		}
>  	}
>  
>  unlock:
> -- 
> 2.51.0