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

[Daniel P. Berrangé]

On Tue, Jan 30, 2024 at 06:49:15PM +0100, Jason A. Donenfeld wrote:
> On Tue, Jan 30, 2024 at 6:32 PM Dave Hansen <dave.hansen@xxxxxxxxx> wrote:
> >
> > On 1/30/24 05:45, Reshetova, Elena wrote:
> > >> You're the Intel employee so you can find out about this with much
> > >> more assurance than me, but I understand the sentence above to be _way
> > >> more_ true for RDRAND than for RDSEED. If your informed opinion is,
> > >> "RDRAND failing can only be due to totally broken hardware"
> > > 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.
> >
> > I don't think the SDM is the right thing to look at for guidance here.
> >
> > Despite the SDM allowing it, we (software) need RDRAND/RDSEED failures
> > to be exceedingly rare by design.  If they're not, we're going to get
> > our trusty torches and pitchforks and go after the folks who built the
> > broken hardware.
> >
> > Repeat after me:
> >
> >         Regular RDRAND/RDSEED failures only occur on broken hardware
> >
> > If it's nice hardware that's gone bad, then we WARN() and try to make
> > the best of it.  If it turns out that WARN() was because of a broken
> > hardware _design_ then we go sharpen the pitchforks.
> >
> > Anybody disagree?
> 
> Yes, I disagree. I made a trivial test that shows RDSEED breaks easily
> in a busy loop. So at the very least, your statement holds true only
> for RDRAND.
> 
> But, anyway, if the statement "RDRAND failures only occur on broken
> hardware" is true, then a WARN() in the failure path there presents no
> DoS potential of any kind, and so that's a straightforward conclusion
> to this discussion. However, that really hinges on  "RDRAND failures
> only occur on broken hardware" being a true statement.

There's a useful comment here from an Intel engineer

https://web.archive.org/web/20190219074642/https://software.intel.com/en-us/blogs/2012/11/17/the-difference-between-rdrand-and-rdseed

  "RDRAND is, indeed, faster than RDSEED because it comes
   from a hardware-based pseudorandom number generator.
   One seed value (effectively, the output from one RDSEED
   command) can provide up to 511 128-bit random values
   before forcing a reseed"

We know we can exhaust RDSEED directly pretty trivially. Making your
test program run in parallel across 20 cpus, I got a mere 3% success
rate from RDSEED.

If RDRAND is reseeding every 511 values, RDRAND output would have
to be consumed significantly faster than RDSEED in order that the
reseed will happen frequently enough to exhaust the seeds.

This looks pretty hard, but maybe with a large enough CPU count
this will be possible in extreme load ?

So I'm not convinced we can blindly wave away RDRAND failures as
guaranteed to mean broken hardware.

With regards,
Daniel
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