On Fri, Apr 16, 2021 at 11:03 AM Jason Wang <jasowang@xxxxxxxxxx> 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@xxxxxxxxxx> 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@xxxxxxxxxx> 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@xxxxxxxxxx/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
