Re: [RFC] Randomness on confidential computing platforms

[Kirill A. Shutemov]

On Mon, Jan 29, 2024 at 01:04:23PM -0800, Dave Hansen wrote:
> On 1/29/24 12:26, Kirill A. Shutemov wrote:
> >>> Do we care?
> >> I want to make sure I understand the scenario:
> >>
> >>  1. We're running in a guest under TDX (or SEV-SNP)
> >>  2. The VMM (or somebody) is attacking the guest by eating all the
> >>     hardware entropy and RDRAND is effectively busted
> >>  3. Assuming kernel-based panic_on_warn and WARN_ON() rdrand_long()
> >>     failure, that rdrand_long() never gets called.
> > Never gets called during attack. It can be used before and after.
> > 
> >>  4. Userspace is using RDRAND output in some critical place like key
> >>     generation and is not checking it for failure, nor mixing it with
> >>     entropy from any other source
> >>  5. Userspace uses the failed RDRAND output to generate a key
> >>  6. Someone exploits the horrible key
> >>
> >> Is that it?
> > Yes.
> 
> Is there something that fundamentally makes this a VMM vs. TDX guest
> problem?  If a malicious VMM can exhaust RDRAND, why can't malicious
> userspace do the same?
> 
> 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.

Admittedly, I don't find my own argument very convincing :)

-- 
  Kiryl Shutsemau / Kirill A. Shutemov