Re: [PATCH v2 4/6] virtio: Initialize authorized attribute for confidential guest

On Sat, Oct 2, 2021 at 7:20 AM Andi Kleen [off-list ref] wrote:

On 10/2/2021 4:14 AM, Greg Kroah-Hartman wrote:

quoted

On Sat, Oct 02, 2021 at 07:04:28AM -0400, Michael S. Tsirkin wrote:

quoted

On Fri, Oct 01, 2021 at 08:49:28AM -0700, Andi Kleen wrote:

quoted

quoted

   Do you have a list of specific drivers and kernel options that you
feel you now "trust"?

For TDX it's currently only virtio net/block/console

But we expect this list to grow slightly over time, but not at a high rate
(so hopefully <10)

Well there are already >10 virtio drivers and I think it's reasonable
that all of these will be used with encrypted guests. The list will
grow.

What is keeping "all" drivers from being on this list?

It would be too much work to harden them all, and it would be pointless
because all these drivers are never legitimately needed in a virtualized
environment which only virtualize a very small number of devices.

quoted

  How exactly are
you determining what should, and should not, be allowed?

Everything that has had reasonable effort at hardening can be added. But
if someone proposes to add a driver that should trigger additional
scrutiny in code review. We should also request them to do some fuzzing.

It's a bit similar to someone trying to add a new syscall interface.
That also triggers much additional scrutiny for good reasons and people
start fuzzing it.

quoted

   How can
drivers move on, or off, of it over time?

Adding something is submitting a patch to the allow list.

I'm not sure the "off" case would happen, unless the driver is
completely removed, or maybe it has some unfixable security problem. But
that is all rather unlikely.

quoted

And why not just put all of that into userspace and have it pick and
choose?  That should be the end-goal here, you don't want to encode
policy like this in the kernel, right?

How would user space know what drivers have been hardened? This is
really something that the kernel needs to determine. I don't think we
can outsource it to anyone else.

How it is outsourcing by moving that same allow list over the kernel /
user boundary. It can be maintained by the same engineers and get
deployed by something like:

dracut --authorize-device-list=confidential-computing-default $kernel-version

With that distributions can deploy kernel-specific authorizations and
admins can deploy site-specific authorizations. Then the kernel
implementation is minimized to authorize just enough drivers by
default to let userspace take over the policy.

Also BTW of course user space can still override it, but really the
defaults should be a kernel policy.

The default is secure, trust nothing but bootstrap devices.

There's also the additional problem that one of the goals of
confidential guest is to just move existing guest virtual images into
them without much changes. So it's better for such a case if as much as
possible of the policy is in the kernel. But that's more a secondary
consideration, the first point is really the important part.

The same image can be used on host and guest in this "do it in
userspace" proposal.
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