RE: [PATCH 1/2] x86/random: Retry on RDSEED failure

[Reshetova, Elena]

> Hi Kirill,
> 
> I've been following the other discussion closely thinking about the
> matter, but I suppose I'll jump in here directly on this patch, if
> this is the approach the discussion is congealing around.
> 
> A comment below:
> 
> On Tue, Jan 30, 2024 at 9:30 AM Kirill A. Shutemov
> <kirill.shutemov@xxxxxxxxxxxxxxx> wrote:
> >  static inline bool __must_check rdseed_long(unsigned long *v)
> >  {
> > +       unsigned int retry = RDRAND_RETRY_LOOPS;
> >         bool ok;
> > -       asm volatile("rdseed %[out]"
> > -                    CC_SET(c)
> > -                    : CC_OUT(c) (ok), [out] "=r" (*v));
> > -       return ok;
> > +
> > +       do {
> > +               asm volatile("rdseed %[out]"
> > +                            CC_SET(c)
> > +                            : CC_OUT(c) (ok), [out] "=r" (*v));
> > +
> > +               if (ok)
> > +                       return true;
> > +       } while (--retry);
> > +
> > +       return false;
> >  }
> 
> So, my understanding of RDRAND vs RDSEED -- deliberately leaving out
> any cryptographic discussion here -- is roughly that RDRAND will
> expand the seed material for longer, while RDSEED will mostly always
> try to sample more bits from the environment. AES is fast, while
> sampling is slow, so RDRAND gives better performance and is less
> likely to fail, whereas RDSEED always has to wait on the hardware to
> collect some bits, so is more likely to fail.

The internals of Intel DRBG behind RDRAND/RDSEED has been publicly
documented, so the structure is no secret. Please see [1] for overall
structure and other aspects. So, yes, your overall understanding is correct
(there are many more details though). 

[1] https://www.intel.com/content/www/us/en/developer/articles/guide/intel-digital-random-number-generator-drng-software-implementation-guide.html


> 
> For that reason, most of the usage of RDRAND and RDSEED inside of
> random.c is something to the tune of `if (!rdseed(out)) rdrand(out);`,
> first trying RDSEED but falling back to RDRAND if it's busy. That
> still seems to me like a reasonable approach, which this patch would
> partly undermine (in concert with the next patch, which I'll comment
> on in a follow up email there).

I agree that for the purpose of extracting entropy for Linux RNG falling
back to RDRAND (current behavior) is perfectly ok, so I think you are doing
the right thing. However, in principle it is not always the case, there are
situations when a fallback to RDRAND should not be used, but it is also
true that the user of this interface should know/understand this situation. 

> 
> So maybe this patch #1 (of 2) can be dropped?

Before we start debating this patchset, what is your opinion on the original
problem we raised for CoCo VMs when both RDRAND/RDSEED are made to
fail deliberately? 

Best Regards,
Elena.