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Re: SEV guest attestation

From: Sergio Lopez
Subject: Re: SEV guest attestation
Date: Thu, 25 Nov 2021 08:14:28 +0100

On Wed, Nov 24, 2021 at 06:29:07PM +0000, Dr. David Alan Gilbert wrote:
> * Daniel P. Berrangé (berrange@redhat.com) wrote:
> > On Wed, Nov 24, 2021 at 11:34:16AM -0500, Tyler Fanelli wrote:
> > > Hi,
> > > 
> > > We recently discussed a way for remote SEV guest attestation through QEMU.
> > > My initial approach was to get data needed for attestation through 
> > > different
> > > QMP commands (all of which are already available, so no changes required
> > > there), deriving hashes and certificate data; and collecting all of this
> > > into a new QMP struct (SevLaunchStart, which would include the VM's 
> > > policy,
> > > secret, and GPA) which would need to be upstreamed into QEMU. Once this is
> > > provided, QEMU would then need to have support for attestation before a VM
> > > is started. Upon speaking to Dave about this proposal, he mentioned that
> > > this may not be the best approach, as some situations would render the
> > > attestation unavailable, such as the instance where a VM is running in a
> > > cloud, and a guest owner would like to perform attestation via QMP (a 
> > > likely
> > > scenario), yet a cloud provider cannot simply let anyone pass arbitrary 
> > > QMP
> > > commands, as this could be an issue.
> > 
> > As a general point, QMP is a low level QEMU implementation detail,
> > which is generally expected to be consumed exclusively on the host
> > by a privileged mgmt layer, which will in turn expose its own higher
> > level APIs to users or other apps. I would not expect to see QMP
> > exposed to anything outside of the privileged host layer.
> > 
> > We also use the QAPI protocol for QEMU guest agent commmunication,
> > however, that is a distinct service from QMP on the host. It shares
> > most infra with QMP but has a completely diffent command set. On the
> > host it is not consumed inside QEMU, but instead consumed by a
> > mgmt app like libvirt. 
> > 
> > > So I ask, does anyone involved in QEMU's SEV implementation have any input
> > > on a quality way to perform guest attestation? If so, I'd be interested.
> > 
> > I think what's missing is some clearer illustrations of how this
> > feature is expected to be consumed in some real world application
> > and the use cases we're trying to solve.
> > 
> > I'd like to understand how it should fit in with common libvirt
> > applications across the different virtualization management
> > scenarios - eg virsh (command line),  virt-manger (local desktop
> > GUI), cockpit (single host web mgmt), OpenStack (cloud mgmt), etc.
> > And of course any non-traditional virt use cases that might be
> > relevant such as Kata.
> That's still not that clear; I know Alice and Sergio have some ideas
> (cc'd).
> There's also some standardisation efforts (e.g. 
> https://www.potaroo.net/ietf/html/ids-wg-rats.html 
> and https://www.ietf.org/archive/id/draft-ietf-rats-architecture-00.html
> ) - that I can't claim to fully understand.
> However, there are some themes that are emerging:
>   a) One use is to only allow a VM to access some private data once we
> prove it's the VM we expect running in a secure/confidential system
>   b) (a) normally involves requesting some proof from the VM and then
> providing it some confidential data/a key if it's OK
>   c) RATs splits the problem up:
> https://www.ietf.org/archive/id/draft-ietf-rats-architecture-00.html#name-architectural-overview
>     I don't fully understand the split yet, but in principal there are
> at least a few different things:
>   d) The comms layer
>   e) Something that validates the attestation message (i.e. the
> signatures are valid, the hashes all add up etc)
>   f) Something that knows what hashes to expect (i.e. oh that's a RHEL
> 8.4 kernel, or that's a valid kernel command line)
>   g) Something that holds some secrets that can be handed out if e & f
> are happy.
>   There have also been proposals (e.g. Intel HTTPA) for an attestable
> connection after a VM is running; that's probably quite different from
> (g) but still involves (e) & (f).
> In the simpler setups d,e,f,g probably live in one place; but it's not
> clear where they live - for example one scenario says that your cloud
> management layer holds some of them, another says you don't trust your
> cloud management layer and you keep them separate.
> So I think all we're actually interested in at the moment, is (d) and
> (e) and the way for (g) to get the secret back to the guest.
> Unfortunately the comms and the contents of them varies heavily with
> technology; in some you're talking to the qemu/hypervisor (SEV/SEV-ES)
> while in some you're talking to the guest after boot (SEV-SNP/TDX maybe
> SEV-ES in some cases).
> So my expectation at the moment is libvirt needs to provide a transport
> layer for the comms, to enable an external validator to retrieve the
> measurements from the guest/hypervisor and provide data back if
> necessary.  Once this shakes out a bit, we might want libvirt to be
> able to invoke the validator; however I expect (f) and (g) to be much
> more complex things that don't feel like they belong in libvirt.

We experimented with the attestation flow quite a bit while working on
SEV-ES support for libkrun-tee. One important aspect we noticed quite
early, is that there's more data that's needed to be exchange of top
of the attestation itself.

For instance, even before you start the VM, the management layer in
charge of coordinating the confidential VM launch needs to obtain the
Virtualization TEE capabilities of the Host (SEV-ES vs. SEV-SNP
vs. TDX) and the platform version, to know which features are
available and whether that host is a candidate for running the VM at

With that information, the mgmt layer can build a guest policy (this
is SEV's terminology, but I guess we'll have something similar in
TDX) and feed it to component launching the VMM (libvirt, in this

For SEV-SNP, this is pretty much the end of the story, because the
attestation exchange is driven by an agent inside the guest. Well,
there's also the need to have in the VM a well-known vNIC bridged to a
network that's routed to the Attestation Server, that everyone seems
to consider a given, but to me, from a CSP perspective, looks like
quite a headache. In fact, I'd go as far as to suggest this
communication should happen through an alternative channel, such as
vsock, having a proxy on the Host, but I guess that depends on the CSP

For SEV/SEV-ES, as the attestation happens at the VMM level, there's
still the need to have some interactions with it. As Tyler pointed
out, we basically need to retrieve the measurement and, if valid,
inject the secret. If the measurement isn't valid, the VM must be shut
down immediately.

In libkrun-tee, this operation is driven by the VMM in libkrun, which
contacts the Attestation Server with the measurement and receives the
secret in exchange. I guess for QEMU/libvirt we expect this to be
driven by the upper management layer through a delegated component in
the Host, such as NOVA. In this case, NOVA would need to:

 - Based on the upper management layer info and the Host properties,
   generate a guest policy and use it while generating the compute
   instance XML.

 - Ask libvirt to launch the VM.

 - Wait for the VM to be in SEV_STATE_LAUNCH_SECRET state *.

 - Retrieve the measurement *.

 - Contact the Attestation Server and provide it with some kind of
   information to uniquely identify the VM (needed to determine what's
   the expected measurement) and the measurement itself.

   * If the measurement if valid, inject the secret *.

     + The secret is pre-encrypted with a key that only the PSP has,
       so there's no need to do any special handling of it.

 - Ask libvirt to either destroy the VM (if the measurement wasn't
   valid or there was some kind of communication error with the
   Attestation Server) or continue the execution of the VM (this will
   be the first time kvm_vcpu_run() is entered).

The operations marked with (*) are the ones that I'm not sure if
NOVA should communicate with libvirt or talk directly to QEMU.


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