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

[Jason Wang <jasowang@redhat.com>]

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).


>
>
>>   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.

Thanks


>
> Thanks,
> Yongji
>