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

Andy Lutomirski
2014-09-19T22:06:55Z

On Fri, Sep 19, 2014 at 3:05 PM, Theodore Ts'o <tytso@mit.edu> wrote:
> On Fri, Sep 19, 2014 at 09:40:42AM -0700, H. Peter Anvin wrote:
>>
>> There is a huge disadvantage to the fact that CPUID is a user space
>> instruction, though.
>
> But if the goal is to provide something like getrandom(2) direct from
> the Host OS, it's not necessarily harmful to allow the Guest ring 3
> code to be able to fetch randomness in that way. The hypervisor can
> implement rate limiting to protect against the guest using this too
> frequently, but this is something that you should be doing for guest
> ring 0 code anyway, since from the POV of the hypervisor Guest ring 0
> is not necessarily any more trusted than Guest ring 3.

On the other hand, the guest kernel might not want the guest ring 3 to
be able to get random numbers.

--Andy