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Date:	Tue, 22 Jul 2014 14:16:43 -0700
From:	"H. Peter Anvin" <>
To:	Andy Lutomirski <>
CC:	"Theodore Ts'o" <>, kvm list <>,
	"" <>,
	Kees Cook <>, X86 ML <>,
	Daniel Borkmann <>,
	Srivatsa Vaddagiri <>,
	Raghavendra K T <>,
	Gleb Natapov <>,
	Paolo Bonzini <>,
	Bandan Das <>, Andrew Honig <>
Subject: Re: [PATCH v4 2/5] random: Add and use arch_get_rng_seed

On 07/22/2014 02:10 PM, Andy Lutomirski wrote:
> On Tue, Jul 22, 2014 at 2:08 PM, H. Peter Anvin <> wrote:
>> On 07/22/2014 02:04 PM, Andy Lutomirski wrote:
>>> Just to check: do you mean the RDRAND is very likely to work (i.e.
>>> arch_get_random_long will return true) or that RDRAND will actually
>>> reseed several times during initialization?
>> I mean that RDRAND will actually reseed several times during
>> initialization.  The documented architectural limit is actually
>> extremely conservative.
>> Either way, it isn't really different from seeding from a VM hosts
>> /dev/urandom...
> Sure it is.  The VM host's /dev/urandom makes no guarantee (or AFAIK
> even any particular effort) to reseed such that the output has some
> minimum entropy per bit, so there would be no point to reading extra
> data from it.

Depends on what you define as "extra data".  If the data pulled is less
than the size of the output pool, it *may* be fully entropic.

(Fun fact: it may even have been fully entropic at the time you pull it,
but then turn out not to be later because *another* process consumed
data from /dev/urandom without adequate reseeding.)

> Anyway, I'd be willing to drop the conservative RDRAND logic, but I
> *still* think that arch_get_rng_seed is a much better interface than
> arch_get_slow_rng_u64.

That I will leave up to you and Ted.


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