Re: [PATCH 2/2] x86/random: Issue a warning if RDRAND or RDSEED fails

Jason A. Donenfeld
2024-01-30T15:00:49Z

On Tue, Jan 30, 2024 at 02:55:08PM +0000, Reshetova, Elena wrote:
>
>
> > On Tue, Jan 30, 2024 at 2:45 PM Reshetova, Elena
> > <elena.reshetova@intel.com> wrote:
> > > No, this is not the case per Intel SDM. I think we can live under a simple
> > > assumption that both of these instructions can fail not just due to broken
> > > HW, but also due to enough pressure put into the whole DRBG construction
> > > that supplies random numbers via RDRAND/RDSEED.
> >
> > Yea, thought so.
> >
> > > I guess your concern about DoS here is for the case when we don’t
> > > trust the host/VMM *and* assume malicious userspace, correct?
> > > Because in non-confidential computing case, the Linux RNG in such
> > > case will just use non-RDRAND fallbacks, no DoS will happen and we
> > > should have enough entropy that is outside of userspace control.
> >
> > Don't think about the RNG for just one second. The basic principle is
> > simpler: if you have a
> > `WARN_ON(unprivd_userspace_triggerable_condition)`, that's usually
> > considered a DoS - panic_on_warn and such.
>
> Ok, agree, you do bring a valid point that we should not create new
> DoS attack vectors from userspace in such cases.
>
> >
> > > >
> > > > And if the DoS thing _is_ a concern, and the use case for this WARN_ON
> > > > in the first place is the trusted computing scenario, so we basically
> > > > only care about early boot, then one addendum would be to only warn if
> > > > we're in early boot, which would work because seeding via RDRAND is
> > > > attempted pretty early on in init.c.
> > >
> > > I don’t think we are only concerned with initial early boot and initial seeding.
> > > What about periodic reseeding of ChaCha CSPRNG? If you don’t get
> > > RDRAND/RDSEED output during this step, don’t we formally loose the forward
> > > prediction resistance property of Linux RNG assuming this is the only source
> > > of entropy that is outside of attacker control?
> >
> > If you never add new material, and you have the initial seed, then
> > it's deterministic. But you still mostly can't backtrack if the state
> > leaks at some future point in time.
>
> I am not talking about backtrack resistance, i.e. when attacker learns about
> RNG state and then can recover the past output. I was talking about an attacker
> learning the RNG state at some point of time (RNG compromise) and
> then for RNG being able to recover over time from this state to a secure state using
> fresh entropy input that is outside of attacker control/observance.
> Does Linux RNG aim to provide this property? Do people care about this?
> If noone cares about this one and Linux RNG doesn’t aim to provide it anyhow,
> then I agree that we should just ensure that early entropy collection includes
> RDRAND/RDSEED input for confidential VMs one way or another.

That's the first thing I mentioned -- "If you never add new material,
and you have the initial seed, then it's deterministic." The property
you mention is a good one to have and Linux usually has it.

>
> Best Regards,
> Elena.
>
> >
> > Jason