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

Jason Wang
Fri, 16 Apr 2021 02:20:25 -0000

? 2021/4/15 ??7:17, Yongji Xie ??:
> On Thu, Apr 15, 2021 at 5:05 PM Jason Wang <jasowang at redhat.com>
wrote:
>>
>> ? 2021/4/15 ??4:36, Jason Wang ??:
>>>> 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 at
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.

 From virtio level, we should not truse desc->addr.

Thanks


>
> Thanks,
> Yongji
>