[RFC PATCH V2 0/7] Do not read from descripto ring

[Jason Wang]

Hi:

Sometimes, the driver doesn't trust the device. This is usually
happens for the encrtpyed VM or VDUSE[1]. In both cases, technology
like swiotlb is used to prevent the poking/mangling of memory from the
device. But this is not sufficient since current virtio driver may
trust what is stored in the descriptor table (coherent mapping) for
performing the DMA operations like unmap and bounce so the device may
choose to utilize the behaviour of swiotlb to perform attacks[2].

To protect from a malicous device, this series store and use the
descriptor metadata in an auxiliay structure which can not be accessed
via swiotlb instead of the ones in the descriptor table. This means
the descriptor table is write-only from the view of the driver.

Actually, we've almost achieved that through packed virtqueue and we
just need to fix a corner case of handling mapping errors. For split
virtqueue we just follow what's done in the packed.

Note that we don't duplicate descriptor medata for indirect
descriptors since it uses stream mapping which is read only so it's
safe if the metadata of non-indirect descriptors are correct.

For split virtqueue, the change increase the footprint due the the
auxiliary metadata but it's almost neglectlable in the simple test
like pktgen or netpef.

Slightly tested with packed on/off, iommu on/of, swiotlb force/off in
the guest.

Please review.

Changes from V1:
- Always use auxiliary metadata for split virtqueue
- Don't read from descripto when detaching indirect descriptor

[1]
https://lore.kernel.org/netdev/fab615ce-5e13-a3b3-3715-a4203b4ab010@xxxxxxxxxx/T/
[2]
https://yhbt.net/lore/all/c3629a27-3590-1d9f-211b-c0b7be152b32@xxxxxxxxxx/T/#mc6b6e2343cbeffca68ca7a97e0f473aaa871c95b

Jason Wang (7):
  virtio-ring: maintain next in extra state for packed virtqueue
  virtio_ring: rename vring_desc_extra_packed
  virtio-ring: factor out desc_extra allocation
  virtio_ring: secure handling of mapping errors
  virtio_ring: introduce virtqueue_desc_add_split()
  virtio: use err label in __vring_new_virtqueue()
  virtio-ring: store DMA metadata in desc_extra for split virtqueue

 drivers/virtio/virtio_ring.c | 201 +++++++++++++++++++++++++----------
 1 file changed, 144 insertions(+), 57 deletions(-)

-- 
2.25.1