Re: [PATCH RFC] uaccess: user_access_begin_after_access_ok()

[Jason Wang]

On 2020/6/3 äå2:30, Michael S. Tsirkin wrote:
> 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?


Yes, we can remove that.


> 2. Second argument to translate_desc is a GPA, isn't it?


No, it's IOVA, this function will be called only when IOTLB is enabled.

Thanks


>     Here we cast a userspace address to this type. What if it
>     matches a valid GPA by mistake?