On 9/29/17 2:48 PM, Richard Relph wrote:
On 9/29/17 2:34 PM, Michael S. Tsirkin wrote:
On Wed, Sep 27, 2017 at 02:06:10PM -0500, Richard Relph wrote:
Whether the "BIOS" is a "static shim" as Michael suggests, or a full
BIOS,
or even a BIOS+kernel+initrd is really not too significant. What is
significant is that the GO has a basis for trusting all code that is
imported in to their VM by the CP. And that NONE of the code provided
by the
CP is "unknown" and unauditable by the GO. If the CP has a way to inject
code unknown to the GO in to the guest VM, the trust model is broken and
both GO and CP suffer the consequences.
Absolutely.
When the CP needs to update the BIOS image, they will have to inform
the GO
and allow the GO to establish trust in the CP's new BIOS image somehow.
This GO update on every BIOS change is imho is not a workable model. You
want something like checking the BIOS signature instead. And since
hardware is all hash based, you need the shim to do it in software.
A BIOS "signed" by the CP doesn't meet the security requirement. It is
code that is "unknown" to the GO.
The (legitimate) CP does NOT want to be in that position of trust. If
they are, then some government somewhere is going to insist that they
sign a BIOS that allows the government to spy on the GO's VMs, and steal
secrets from it. Or some hacker admin will do it "for fun".
How often do large public CPs really change their BIOSes? My sense is
that large public CPs prefer stability over "latest and greatest".
And, perhaps more importantly, if a CP are able to sell a "more secure"
VM, one that justifies a higher price per vCPU hour, wouldn't that
warrant some changes in the "insecure" model being used today?
Ultimately, I think both approaches are "doable". It will be a CP and GO
decision. If the GO trusts the CP, the shim+signed BIOS will work fine.
If GO requires a more secure VM and the CP wants to offer it, the CP
will figure out a way to satisfy the GO's "trust issue" that the BIOS
can't be used to circumvent SEV's protections. Depending on your level
of paranoia, that may require advance notice of BIOS changes, or even
allowing the GO to provide the BIOS themselves, written to a spec
supported by the CP's HV, and/or based on BIOS code provided by the CP.
It's a business decision and I think SEV can support both. That said,
AMD currently has no plans to write a shim that can verify the signature
on a CP-provided BIOS image.
Richard
Richard
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