Subject: Re: [PATCH net V2 1/4] vhost: make sure used idx is seen before log in vhost_add_used_n()

From: "Michael S. Tsirkin" <[email-masked]>
Date: Wed, 12 Dec 2018 09:33:37 -0500

On Wed, Dec 12, 2018 at 06:08:16PM +0800, Jason Wang wrote:
> We miss a write barrier that guarantees used idx is updated and seen
> before log. This will let userspace sync and copy used ring before
> used idx is update. Fix this by adding a barrier before log_write().
> 
> Fixes: 8dd014adfea6f ("vhost-net: mergeable buffers support")
> Signed-off-by: Jason Wang <[email-masked]>

Acked-by: Michael S. Tsirkin <[email-masked]>

also for 4.20 and seems like a stable candidate.

> ---
>  drivers/vhost/vhost.c | 2 ++
>  1 file changed, 2 insertions(+)
> 
> diff --git a/drivers/vhost/vhost.c b/drivers/vhost/vhost.c
> index 6b98d8e3a5bf..5915f240275a 100644
> --- a/drivers/vhost/vhost.c
> +++ b/drivers/vhost/vhost.c
> @@ -2220,6 +2220,8 @@ int vhost_add_used_n(struct vhost_virtqueue *vq, struct vring_used_elem *heads,
>  		return -EFAULT;
>  	}
>  	if (unlikely(vq->log_used)) {
> +		/* Make sure used idx is seen before log. */
> +		smp_wmb();
>  		/* Log used index update. */
>  		log_write(vq->log_base,
>  			  vq->log_addr + offsetof(struct vring_used, idx),
> -- 
> 2.17.1