Re: [DISCUSSION PATCH 00/41] random: possible ways towards NIST SP800-90B compliance

[Nicolai Stange <nstange@suse.de>]

Eric Biggers <ebiggers@kernel.org> writes:

> On Fri, Oct 02, 2020 at 02:38:36PM +0200, Torsten Duwe wrote:
>>
>> Would some maintainer please comment on potential problems or
>> shortcomings?
>> 
>
> Well, very people are experts in the Linux RNG *and* have time to review large
> patchsets, especially when three people are all proposing conflicting changes.
> And those that might be able to review these patches aren't necessarily
> interested in compliance with particular government standards.

To make it clear: I'm personally not really enthusiastic about some of
the restrictions imposed by SP800-90 either and Jason certainly has a
point with his concerns about "subpar crypto" ([1]). However, at the
same time I'm acknowledging that for some users FIPS compliance is
simply a necessity and I don't see a strong reason why that shouldn't be
supported, if doable without negatively affecting !fips_enabled users.


> Note that having multiple RNG implementations would cause fragmentation, more
> maintenance burden, etc.  So IMO, that should be a last resort.  Instead we
> should try to find an implementation that works for everyone.  I.e., at least to
> me, Nicolai's patchset seems more on the right track than Stephan's patchset...

I suppose that this concern about fragmentation is among the main
reasons for reservations against Stephan's LRNG patchset and that's why
I posted this RFC series here for comparison purposes. But note that, as
said ([2]), it's incomplete and the only intent was to provide at least
a rough idea on what it would take to move the current /dev/random
implementation towards SP800-90 -- I was hoping for either a hard NACK
or something along the lines of "maybe, go ahead and let's see".


> However, not everyone cares about "compliance".  So any changes for "compliance"
> either need to have a real technical argument for making the change, *or* need
> to be optional (e.g. controlled by fips_enabled).

Fully agreed.


> AFAICS, this patchset mostly just talks about NIST SP800-90B compliance, and
> doesn't make clear whether the changes make the RNG better, worse, or the same
> from an actual technical perspective.
>
> If that was properly explained, and if the answer was "better" or at least
> "not worse", I expect that people would be more interested.

The goal was not to negatively affect !fips_enabled users, but as
outlined in the cover letter ([2]), a performance impact had been
measured on ARMv7. This probably isn't something which couldn't get
sorted out, but I see no point in doing it at this stage, because
- there's still quite some stuff missing for full SP800-90 compliance
  anyway, c.f. the overview at the end of [2] and
- such optimizations would have bloated this patchset even more,
  e.g. for making fips_enabled a static_key, which should certainly go
  into a separate series.

User visible effects set aside, an obvious downside of SP800-90
compliance would be the increase in code size and the associated
maintenance burden.

That being said, I can imagine that those boot health tests could also
get enabled for !fips_enabled users in the future, if wanted: rather
than inhibiting /dev/random output on failure, a warning would get
logged instead.  Whether or not this would be seen as an improvement
is for others to judge though.

Thanks,

Nicolai


[1] https://lkml.kernel.org/r/CAHmME9rMXORFXtwDAc8yxj+h9gytJj6DpvCxA-JMAAgyOP+5Yw@mail.gmail.com
[2] https://lkml.kernel.org/r/20200921075857.4424-1-nstange@suse.de

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