RE: Standardizing an MSR or other hypercall to get an RNG seed?

Niels Ferguson
2014-09-18T18:54:22Z

Defining a standard way of transferring random numbers between the host and the guest is an excellent idea.

As the person who writes the RNG code in Windows, I have a few comments:

DETECTION:
It should be possible to detect this feature through CPUID or similar mechanism. That allows the code that uses this feature to be written without needing the ability to catch CPU exceptions. I could be wrong, but as far as I know there is no support for exception handling in the Windows OS loader where we gather our initial random state.

EFFICIENCY:
Is there a way we can transfer more bytes per interaction? With a single 64-bit MSR we always need multiple reads to get a seed, and each of them results in a context switch to the host, which is expensive. This is even worse for 32-bit guests. Windows would typically need to fetch 64 bytes of random data at boot and at regular intervals. It is not a show-stopper, but better efficiency would be nice.

GUEST-TO-HOST:
Can we also define a way to have random values flow from the guest to the host? Guests are also gathering entropy from their own sources, and if we allow the guests to send random data to the host, then the host can treat it as an entropy source and all the VMs on a single host can share their entropy. (This is not a security problem; any reasonable host RNG cannot be hurt even by maliciously chosen entropy inputs.)

I don't know much about how hypervisors work on the inside, but maybe we can define a mechanism for standardized hypervisor calls that work on all hypervisors that support this feature. Then we could define a function to do an entropy exchange: the guest provides N bytes of random data to the host, and the host replies with N bytes of random data. The data exchange can now be done through memory.

A standardized hypervisor-call mechanism also seems generally useful for future features, whereas the MSR solution is very limited in what it can do. We might end up with standardized hypervisor-calls in the future for some other reason, and then the MSR solution looks very odd.

Niels

-----Original Message-----
From: H. Peter Anvin [mailto:hpa@zytor.com]
Sent: Thursday, September 18, 2014 11:39 AM
To: Andy Lutomirski; Nakajima, Jun
Cc: KY Srinivasan; Paolo Bonzini; Mathew John; Theodore Ts'o; John Starks; kvm list; Gleb Natapov; Niels Ferguson; David Hepkin; Jake Oshins; Linux Virtualization
Subject: Re: Standardizing an MSR or other hypercall to get an RNG seed?

Quite frankly it might make more sense to define a cross-VM *cpuid* range. The cpuid leaf can just point to the MSR. The big question is who will be willing to be the registrar.

On September 18, 2014 11:35:39 AM PDT, Andy Lutomirski <luto@amacapital.net> wrote:
>On Thu, Sep 18, 2014 at 10:42 AM, Nakajima, Jun
><jun.nakajima@intel.com> wrote:
>> On Thu, Sep 18, 2014 at 10:20 AM, KY Srinivasan <kys@microsoft.com>
>wrote:
>>>
>>>
>>>> -----Original Message-----
>>>> From: Paolo Bonzini [mailto:paolo.bonzini@gmail.com] On Behalf Of
>Paolo
>>>> Bonzini
>>>> Sent: Thursday, September 18, 2014 10:18 AM
>>>> To: Nakajima, Jun; KY Srinivasan
>>>> Cc: Mathew John; Theodore Ts'o; John Starks; kvm list; Gleb
>Natapov; Niels
>>>> Ferguson; Andy Lutomirski; David Hepkin; H. Peter Anvin; Jake
>Oshins; Linux
>>>> Virtualization
>>>> Subject: Re: Standardizing an MSR or other hypercall to get an RNG
>seed?
>>>>
>>>> Il 18/09/2014 19:13, Nakajima, Jun ha scritto:
>>>> > In terms of the address for the MSR, I suggest that you choose
>one
>>>> > from the range between 40000000H - 400000FFH. The SDM (35.1
>>>> > ARCHITECTURAL MSRS) says "All existing and future processors will
>not
>>>> > implement any features using any MSR in this range." Hyper-V
>already
>>>> > defines many synthetic MSRs in this range, and I think it would
>be
>>>> > reasonable for you to pick one for this to avoid a conflict?
>>>>
>>>> KVM is not using any MSR in that range.
>>>>
>>>> However, I think it would be better to have the MSR (and perhaps
>CPUID)
>>>> outside the hypervisor-reserved ranges, so that it becomes
>architecturally
>>>> defined. In some sense it is similar to the HYPERVISOR CPUID
>feature.
>>>
>>> Yes, given that we want this to be hypervisor agnostic.
>>>
>>
>> Actually, that MSR address range has been reserved for that purpose,
>along with:
>> - CPUID.EAX=1 -> ECX bit 31 (always returns 0 on bare metal)
>> - CPUID.EAX=4000_00xxH leaves (i.e. HYPERVISOR CPUID)
>
>I don't know whether this is documented anywhere, but Linux tries to
>detect a hypervisor by searching CPUID leaves 0x400xyz00 for
>"KVMKVMKVM\0\0\0", so at least Linux can handle the KVM leaves being in
>a somewhat variable location.
>
>Do we consider this mechanism to work across all hypervisors and
>guests? That is, could we put something like "CrossHVPara\0"
>somewhere in that range, where each hypervisor would be free to decide
>exactly where it ends up?
>
>--Andy

--
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