[libvirt] [Qemu-devel] libvirt/QEMU/SEV interaction

[Daniel P. Berrange]

On Mon, Sep 18, 2017 at 11:43:57AM +0200, Erik Skultety wrote:
> [...]
> 
> > >
> > >     > c) what existing communicate interface can be used between libvirt and qemu
> > >     > to get the measurement ? can we add a new qemu monitor command
> > >     > 'get_sev_measurement' to get the measurement ? (step 10)
> > >
> > >     Yes, QMP commands seeem most likely.
> > >
> > >     > d) how to pass the secret blob from libvirt to qemu ? should we consider
> > >     > adding a new object (sev-guest-secret) -- libvirt can add the object through
> > >     > qemu monitor.
> > >
> > >     Yeah, that looks like a viable option too.
> >
> > So I could see a flow like the following:
> >
> >
> >   1. mgmt tool calls  virConnectGetCapabilities. This returns an XML
> >      document that includes the following
> >
> >       <host>
> >          ...other bits...
> >         <sev>
> > 	  <platform-key>...hex encoded PDH key...</platform-key>
> > 	</sev>
> >       </host>
> >
> >   2. mgmt tool requests to start a guest calling virCreateXML(),
> >      passing VIR_DOMAIN_START_PAUSED. The XML would include
> >
> >       <sev>
> >         <owner-key>...hex encode DH key...</owner-key>
> > 	<session-info>..hex encode info...</session-info>
> > 	<policy>...int32 value..</policy>
> >       </sev>
> >
> >
> >      if <sev> is provided and VIR_DOMAIN_START_PAUSED is missing,
> >      libvirt would report an error and refuse to start the guest
> >
> >   3. Libvirt generates the QEMU cli arg to enable SEV using
> >      the XML data and starts QEMU, leaving CPUs paused
> >
> >   4. QEMU emits a SEV_MEASURE event containing the measurement
> >      blob
> 
> Speaking of which, I expect QEMU to have a QMP command to retrieve the
> measurement, in which case I think libvirt has to provide an API for the user
> to retrieve the measurement in case libvirtd crashes somewhere between setting
> up QEMU and waiting for the measurement event from QEMU, or simply because the
> GO missed the event for some unspecified reason.

Yeah, that's a good point - we also ought to have a pause-reason that
reflects that it is paused due to waiting for SEV secrets.

> 
> >
> >   5. Libvirt catches the QEMU event and emits its own
> >      VIR_CONNECT_DOMAIN_EVENT_SEV_MEASURE event containing
> >      the measurement blob
> >
> >   6. GO does its validation of the measurement
> >
> >   7a  If validation failed, then virDomainDestroy() to stop QEMU
> >
> >   7b  If validation succeeed
> >
> >      Optionally call
> >
> >          virDomainSetSEVSecret()
> 
> Given the fact that we're likely introducing a new <sev> element to the XML
> config, I'm more inclined to utilizing the existing virSecret interfaces (as
> was originally suggested) instead of creating a vendor-specific API. You could
> have an optional secret sub-element within the <sev> element and libvirt would
> simply check if that secret has a value set, once the GO issues
> virDomainResume(). Any particular reason for having a specific API for this that
> I'm missing?

Initially I was intending to suggest extensive use of virSecret, but it
turns out that despite being called a "secret", none of the SEV data we are
passing around needs protection. Either it is safe to be public, or it is
already encrypted.  So essentially we just have some data blobs we need to
pass into QEMU. I didn't feel we ought to be abusing virSecret as a
general purpose mechanism for passing in opaque data blobs which do not
need any kind of protection.

Regards,
Daniel
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