Re: SEV guest attestation

[Daniel P . Berrangé]

On Thu, Nov 25, 2021 at 08:14:28AM +0100, Sergio Lopez wrote:
> 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
> all.

Libvirt already reports a wide variety of information about a
compute host, that is used for placement decisions by mgmt apps,
and this includes SEV data (obtained from qemu's query-sev-capabilities)

> 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
> case).
> 
> 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
> infrastructure.
> 
> 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.

Is that really 'must' be shut down or merely 'should' be shut down.
I'm expecting the latter, as if we're faced with an untrustworthy
compute host, we can't guarantee it will ever be shut dwon.

> 
> 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.

Nova must always talk with libvirt, never QEMU, because it needs to
be insulated from low level implementation details that change over
time. The QEMU binary we are invoking today, might be completely
replaced with something new tomorrow, with totally different CLI /
APIs, and livirt will isolate apps from this.

Also we need to bear in mind the complexity we're putting on users
and mgmt apps.

Regards,
Daniel
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