netdev - Re: [PATCH RFC] uaccess: user_access_begin_after_access

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Date:   Wed, 3 Jun 2020 02:30:51 -0400
From:   "Michael S. Tsirkin" <mst@...hat.com>
To:     Jason Wang <jasowang@...hat.com>
Cc:     Al Viro <viro@...iv.linux.org.uk>, linux-kernel@...r.kernel.org,
        netdev@...r.kernel.org
Subject: Re: [PATCH RFC] uaccess: user_access_begin_after_access_ok()

On Wed, Jun 03, 2020 at 02:23:08PM +0800, Jason Wang wrote:
> > 
> > BTW now I re-read it I don't understand __vhost_get_user_slow:
> > 
> > 
> > static void __user *__vhost_get_user_slow(struct vhost_virtqueue *vq,
> >                                            void __user *addr, unsigned int size,
> >                                            int type)
> > {
> >          int ret;
> > 
> >          ret = translate_desc(vq, (u64)(uintptr_t)addr, size, vq->iotlb_iov,
> >                               ARRAY_SIZE(vq->iotlb_iov),
> >                               VHOST_ACCESS_RO);
> > 
> > ..
> > }
> > 
> > how does this work? how can we cast a pointer to guest address without
> > adding any offsets?
> 
> 
> I'm not sure I get you here. What kind of offset did you mean?
> 
> Thanks

OK so points:

1. type argument seems unused. Right?
2. Second argument to translate_desc is a GPA, isn't it?
   Here we cast a userspace address to this type. What if it
   matches a valid GPA by mistake?

-- 
MST