Re: [PATCH v1 1/8] virtio: Force only split mode with protected guest

From: Jason Wang
Date: Thu Jun 03 2021 - 21:22:56 EST

在 2021/6/4 上午3:31, Andy Lutomirski 写道:

On Thu, Jun 3, 2021, at 11:00 AM, Andi Kleen wrote:
On 6/3/2021 10:33 AM, Andy Lutomirski wrote:
On 6/2/21 5:41 PM, Andi Kleen wrote:
Only allow split mode when in a protected guest. Followon
patches harden the split mode code paths, and we don't want
an malicious host to force anything else. Also disallow
indirect mode for similar reasons.
I read this as "the virtio driver is buggy. Let's disable most of the
buggy code in one special case in which we need a driver without bugs.
In all the other cases (e.g. hardware virtio device connected over
USB-C), driver bugs are still allowed."
My understanding is most of the other modes (except for split with
separate descriptors) are obsolete and just there for compatibility. As
long as they're deprecated they won't harm anyone.

Tell that to every crypto downgrade attack ever.

I see two credible solutions:

1. Actually harden the virtio driver.

2. Have a new virtio-modern driver and use it for modern use cases. Maybe rename the old driver virtio-legacy or virtio-insecure. They can share code.

Note that we had already split legacy driver out which can be turned off via Kconfig.

Another snag you may hit: virtio’s heuristic for whether to use proper DMA ops or to bypass them is a giant kludge. I’m very slightly optimistic that getting the heuristic wrong will make the driver fail to operate but won’t allow the host to take over the guest, but I’m not really convinced. And I wrote that code! A virtio-modern mode probably should not have a heuristic, and the various iommu-bypassing modes should be fixed to work at the bus level, not the device level.

I remember there's a very long discussion about this and probably without any conclusion. Fortunately, the management layer has been taught to enforce VIRTIO_F_ACCESS_PLATFORM for encrypted guests.

A possible way to fix this is without any conflicts is to mandate the VIRTIO_F_ACCESS_PLATFORM in version 1.2.