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

[Yongji Xie <xieyongji@bytedance.com>]

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;


>  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.

Thanks,
Yongji