Subject: Re: [PATCH 1/1] vhost: Protect the virtqueue from being cleared whilst still in use

From: "Michael S. Tsirkin" <[email-masked]>
Date: Mon, 7 Mar 2022 17:37:18 -0500

On Mon, Mar 07, 2022 at 07:17:57PM +0000, Lee Jones wrote:
> vhost_vsock_handle_tx_kick() already holds the mutex during its call
> to vhost_get_vq_desc().  All we have to do here is take the same lock
> during virtqueue clean-up and we mitigate the reported issues.

Pls just basically copy the code comment here. this is just confuses.

> Also WARN() as a precautionary measure.  The purpose of this is to
> capture possible future race conditions which may pop up over time.
> 
> Link: https://syzkaller.appspot.com/bug?extid=279432d30d825e63ba00

And this is a bug we already fixed, right?

> Cc: <[email-masked]>
> Reported-by: [email-masked]

not really applicable anymore ...

> Signed-off-by: Lee Jones <[email-masked]>
> ---
>  drivers/vhost/vhost.c | 10 ++++++++++
>  1 file changed, 10 insertions(+)
> 
> diff --git a/drivers/vhost/vhost.c b/drivers/vhost/vhost.c
> index 59edb5a1ffe28..ef7e371e3e649 100644
> --- a/drivers/vhost/vhost.c
> +++ b/drivers/vhost/vhost.c
> @@ -693,6 +693,15 @@ void vhost_dev_cleanup(struct vhost_dev *dev)
>  	int i;
>  
>  	for (i = 0; i < dev->nvqs; ++i) {
> +		/* No workers should run here by design. However, races have
> +		 * previously occurred where drivers have been unable to flush
> +		 * all work properly prior to clean-up.  Without a successful
> +		 * flush the guest will malfunction, but avoiding host memory
> +		 * corruption in those cases does seem preferable.
> +		 */
> +		WARN_ON(mutex_is_locked(&dev->vqs[i]->mutex));
> +
> +		mutex_lock(&dev->vqs[i]->mutex);
>  		if (dev->vqs[i]->error_ctx)
>  			eventfd_ctx_put(dev->vqs[i]->error_ctx);
>  		if (dev->vqs[i]->kick)
> @@ -700,6 +709,7 @@ void vhost_dev_cleanup(struct vhost_dev *dev)
>  		if (dev->vqs[i]->call_ctx.ctx)
>  			eventfd_ctx_put(dev->vqs[i]->call_ctx.ctx);
>  		vhost_vq_reset(dev, dev->vqs[i]);
> +		mutex_unlock(&dev->vqs[i]->mutex);
>  	}
>  	vhost_dev_free_iovecs(dev);
>  	if (dev->log_ctx)
> -- 
> 2.35.1.616.g0bdcbb4464-goog