[PATCH 0/7] Do not read from descriptor ring

[Jason Wang]

Hi:

The virtio driver should not trust the device. This beame more urgent
for the case of encrtpyed VM or VDUSE[1]. In both cases, technology
like swiotlb/IOMMU 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/device 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 simple test like
pktgen and netperf TCP stream (slightly noticed in a 40GBE environment
with more CPU usage).

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

Note that this series tries to fix the attack via descriptor
ring. The other cases (used ring and config space) will be fixed by
other series or patches.

Please review.

Changes from RFC V2:
- no code change
- twaeak the commit log a little bit

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

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(-)

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2.25.1