Re: Re: [PATCH v6 10/10] Documentation: Add documentation for VDUSE

[Yongji Xie <xieyongji@bytedance.com>]

On Fri, Apr 16, 2021 at 11:03 AM Jason Wang <jasowang@redhat.com> wrote:
>
>
> 在 2021/4/16 上午10:58, Yongji Xie 写道:
> > On Fri, Apr 16, 2021 at 10:20 AM Jason Wang <jasowang@redhat.com> wrote:
> >>
> >> 在 2021/4/15 下午7:17, Yongji Xie 写道:
> >>> On Thu, Apr 15, 2021 at 5:05 PM Jason Wang <jasowang@redhat.com> wrote:
> >>>> 在 2021/4/15 下午4:36, Jason Wang 写道:
> >>>>>> Please state this explicitly at the start of the document. Existing
> >>>>>> interfaces like FUSE are designed to avoid trusting userspace.
> >>>>> There're some subtle difference here. VDUSE present a device to kernel
> >>>>> which means IOMMU is probably the only thing to prevent a malicous
> >>>>> device.
> >>>>>
> >>>>>
> >>>>>> Therefore
> >>>>>> people might think the same is the case here. It's critical that people
> >>>>>> are aware of this before deploying VDUSE with virtio-vdpa.
> >>>>>>
> >>>>>> We should probably pause here and think about whether it's possible to
> >>>>>> avoid trusting userspace. Even if it takes some effort and costs some
> >>>>>> performance it would probably be worthwhile.
> >>>>> Since the bounce buffer is used the only attack surface is the
> >>>>> coherent area, if we want to enforce stronger isolation we need to use
> >>>>> shadow virtqueue (which is proposed in earlier version by me) in this
> >>>>> case. But I'm not sure it's worth to do that.
> >>>>
> >>>> So this reminds me the discussion in the end of last year. We need to
> >>>> make sure we don't suffer from the same issues for VDUSE at least
> >>>>
> >>>> https://yhbt.net/lore/all/c3629a27-3590-1d9f-211b-c0b7be152b32@redhat.com/T/#mc6b6e2343cbeffca68ca7a97e0f473aaa871c95b
> >>>>
> >>>> Or we can solve it at virtio level, e.g remember the dma address instead
> >>>> of depending on the addr in the descriptor ring
> >>>>
> >>> I might miss something. But VDUSE has recorded the dma address during
> >>> dma mapping, so we would not do bouncing if the addr/length is invalid
> >>> during dma unmapping. Is it enough?
> >>
> >> E.g malicous device write a buggy dma address in the descriptor ring, so
> >> we had:
> >>
> >> vring_unmap_one_split(desc->addr, desc->len)
> >>       dma_unmap_single()
> >>           vduse_dev_unmap_page()
> >>               vduse_domain_bounce()
> >>
> >> And in vduse_domain_bounce() we had:
> >>
> >>           while (size) {
> >>                   map = &domain->bounce_maps[iova >> PAGE_SHIFT];
> >>                   offset = offset_in_page(iova);
> >>                   sz = min_t(size_t, PAGE_SIZE - offset, size);
> >>
> >> This means we trust the iova which is dangerous and exacly the issue
> >> mentioned in the above link.
> >>
> >>   From VDUSE level need to make sure iova is legal.
> >>
> > I think we already do that in vduse_domain_bounce():
> >
> >      while (size) {
> >          map = &domain->bounce_maps[iova >> PAGE_SHIFT];
> >
> >          if (WARN_ON(!map->bounce_page ||
> >              map->orig_phys == INVALID_PHYS_ADDR))
> >              return;
>
>
> So you don't check whether iova is legal before using it, so it's at
> least a possible out of bound access of the bounce_maps[] isn't it? (e.g
> what happens if iova is ULLONG_MAX).
>

Oh, yes. Will do it!

>
> >
> >
> >>   From virtio level, we should not truse desc->addr.
> >>
> > We would not touch desc->addr after vring_unmap_one_split(). So I'm
> > not sure what we need to do at the virtio level.
>
>
> I think the point is to record the dma addres/len somewhere instead of
> reading them from descriptor ring.
>

OK, I see.

Thanks,
Yongji