RE: [RFC] Randomness on confidential computing platforms

[Reshetova, Elena]

> On January 29, 2024 2:18:50 PM PST, Dave Hansen <dave.hansen@xxxxxxxxx>
> wrote:
> >On 1/29/24 13:33, Kirill A. Shutemov wrote:
> >>> Let's assume buggy userspace exists.  Is that userspace *uniquely*
> >>> exposed to a naughty VMM or is that VMM just added to the list of things
> >>> that can attack buggy userspace?
> >> This is good question.
> >>
> >> VMM has control over when a VCPU gets scheduled and on what CPU which
> >> gives it tighter control over the target workload. It can make a
> >> difference if there's small window for an attack before RDRAND is
> >> functional again.
> >
> >This is all a bit too theoretical for my taste.  I'm fine with doing
> >some generic mitigation (WARN_ON_ONCE(hardware_is_exhausted)), but we're
> >talking about a theoretical attack with theoretical buggy software when
> >in a theoretically unreachable hardware state.
> >
> >Until it's clearly much more practical, we have much bigger problems to
> >worry about.
> 
> Again, do we even have a problem with the "hold the boot until we have
> entropy"option?

Yes, we do have a problem. You cannot build a secure random number generator
in a situation when attacker controls/observes all your entropy sources. 
Linux RNG has many entropy sources (RDRAND/RDSEED is just one of them), and
as soon as we have at least some proper entropy input, you are ok (I am greatly 
oversimplifying the RNG theory now). 
What changes with confidential computing is that the entropy sources like
interrupts or timing-based information can be viewed as under attacker control
/observance. But this is *not* how Linux RNG views it by its threat model.
So, Linux RNG will boot and run just fine in a confidential guest in situations when
RDRAND/RDSEED always fails (it will use other entropy source like interrupts/timing info),
but the quality of its output becomes questionable assuming host/VMM is out of TCB. 

I hope we can get an opinion on this from maintainers of Linux RNG.

Best Regards,
Elena.