Jason Wang
2021-Apr-16 03:02 UTC
[PATCH v6 10/10] Documentation: Add documentation for VDUSE
? 2021/4/16 ??10:58, Yongji Xie ??:> On Fri, Apr 16, 2021 at 10:20 AM Jason Wang <jasowang at redhat.com> wrote: >> >> ? 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. >> > 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 >