Re: [PATCH v3 1/2] vhost: fix vhost_vq_access_ok() log check

[Michael S. Tsirkin]

On Wed, Apr 11, 2018 at 10:35:40AM +0800, Stefan Hajnoczi wrote:
> Commit d65026c6c62e7d9616c8ceb5a53b68bcdc050525 ("vhost: validate log
> when IOTLB is enabled") introduced a regression.  The logic was
> originally:
> 
>   if (vq->iotlb)
>       return 1;
>   return A && B;
> 
> After the patch the short-circuit logic for A was inverted:
> 
>   if (A || vq->iotlb)
>       return A;
>   return B;
> 
> This patch fixes the regression by rewriting the checks in the obvious
> way, no longer returning A when vq->iotlb is non-NULL (which is hard to
> understand).
> 
> Reported-by: syzbot+65a84dde0214b0387ccd@xxxxxxxxxxxxxxxxxxxxxxxxx
> Cc: Jason Wang <jasowang@xxxxxxxxxx>
> Signed-off-by: Stefan Hajnoczi <stefanha@xxxxxxxxxx>

Acked-by: Michael S. Tsirkin <mst@xxxxxxxxxx>

> ---
>  drivers/vhost/vhost.c | 8 +++++---
>  1 file changed, 5 insertions(+), 3 deletions(-)
> 
> diff --git a/drivers/vhost/vhost.c b/drivers/vhost/vhost.c
> index bec722e41f58..fc805b7fad9d 100644
> --- a/drivers/vhost/vhost.c
> +++ b/drivers/vhost/vhost.c
> @@ -1244,10 +1244,12 @@ static int vq_log_access_ok(struct vhost_virtqueue *vq,
>  /* Caller should have vq mutex and device mutex */
>  int vhost_vq_access_ok(struct vhost_virtqueue *vq)
>  {
> -	int ret = vq_log_access_ok(vq, vq->log_base);
> +	if (!vq_log_access_ok(vq, vq->log_base))
> +		return 0;
>  
> -	if (ret || vq->iotlb)
> -		return ret;
> +	/* Access validation occurs at prefetch time with IOTLB */
> +	if (vq->iotlb)
> +		return 1;
>  
>  	return vq_access_ok(vq, vq->num, vq->desc, vq->avail, vq->used);
>  }
> -- 
> 2.14.3