Re: [PATCH] vhost: Don't call vq_access_ok() when using IOTLB

[Greg Kurz <groug@kaod.org>]

On Tue, 29 Sep 2020 03:45:28 -0400
"Michael S. Tsirkin" <mst@redhat.com> wrote:

> On Mon, Sep 28, 2020 at 02:35:04PM +0200, Greg Kurz wrote:
> > When the IOTLB device is enabled, the vring addresses we get from
> > userspace are GIOVAs. It is thus wrong to pass them to vq_access_ok()
> > which only takes HVAs. The IOTLB map is likely empty at this stage,
> > so there isn't much that can be done with these GIOVAs. Access validation
> > will be performed at IOTLB prefetch time anyway.
> > 
> > BugLink: https://bugzilla.redhat.com/show_bug.cgi?id=1883084
> > Fixes: 6b1e6cc7855b ("vhost: new device IOTLB API")
> > Cc: jasowang@redhat.com
> > CC: stable@vger.kernel.org # 4.14+
> > Signed-off-by: Greg Kurz <groug@kaod.org>
> > ---
> >  drivers/vhost/vhost.c |    5 ++++-
> >  1 file changed, 4 insertions(+), 1 deletion(-)
> > 
> > diff --git a/drivers/vhost/vhost.c b/drivers/vhost/vhost.c
> > index b45519ca66a7..6296e33df31d 100644
> > --- a/drivers/vhost/vhost.c
> > +++ b/drivers/vhost/vhost.c
> > @@ -1509,7 +1509,10 @@ static long vhost_vring_set_addr(struct vhost_dev *d,
> >  	 * If it is not, we don't as size might not have been setup.
> >  	 * We will verify when backend is configured. */
> >  	if (vq->private_data) {
> > -		if (!vq_access_ok(vq, vq->num,
> > +		/* If an IOTLB device is present, the vring addresses are
> > +		 * GIOVAs. Access will be validated during IOTLB prefetch. */
> > +		if (!vq->iotlb &&
> > +		    !vq_access_ok(vq, vq->num,
> >  			(void __user *)(unsigned long)a.desc_user_addr,
> >  			(void __user *)(unsigned long)a.avail_user_addr,
> >  			(void __user *)(unsigned long)a.used_user_addr))
> 
> OK I think you are right here.
> 
> Jason, can you ack pls?
> 
> However, I think a cleaner way to check this is by moving
> the following check from vhost_vq_access_ok to vq_access_ok:
> 
>         /* Access validation occurs at prefetch time with IOTLB */
>         if (vq->iotlb)
>                 return true;
> 

Yes I agree. I'll do that in v2.

> 
> > 
>