linux-kernel - Re: random: Providing a seed value to VM guests

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Date:	Thu, 1 May 2014 15:56:16 -0700
From:	Andy Lutomirski <luto@...capital.net>
To:	"H. Peter Anvin" <hpa@...or.com>
Cc:	"Theodore Ts'o" <tytso@....edu>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	kvm list <kvm@...r.kernel.org>,
	Florian Weimer <fweimer@...hat.com>,
	Kees Cook <kees@...flux.net>
Subject: Re: random: Providing a seed value to VM guests

On Thu, May 1, 2014 at 3:46 PM, H. Peter Anvin <hpa@...or.com> wrote:
> On 05/01/2014 03:32 PM, Andy Lutomirski wrote:
>> On Thu, May 1, 2014 at 3:28 PM,  <tytso@....edu> wrote:
>>> On Thu, May 01, 2014 at 02:06:13PM -0700, Andy Lutomirski wrote:
>>>>
>>>> I still don't see the point.  What does this do better than virtio-rng?
>>>
>>> I believe you had been complaining about how complicated it was to set
>>> up virtio?  And this complexity is also an issue if we want to use it
>>> to initialize the RNG used for the kernel text ASLR --- which has to
>>> be done very early in the boot process, and where making something as
>>> simple as possible is a Good Thing.
>>
>> It's complicated, so it won't be up until much later in the boot
>> process.  This is completely fine for /dev/random, but it's a problem
>> for /dev/urandom, ASLR, and such.
>>
>>>
>>> And since we would want to use RDRAND/RDSEED if it is available
>>> *anyway*, perhaps in combination with other things, why not use the
>>> RDRAND/RDSEED interface?
>>
>> Because it's awkward.  I don't think it simplifies anything.
>>
>
> It greatly simplifies discovery, which is a Big Deal[TM] in early code.

I think we're comparing:

a) cpuid to detect rdrand *or* emulated rdrand followed by rdrand

to

b) cpuid to detect rdrand or the paravirt seed msr/cpuid call,
followed by rdrand or the msr or cpuid read

this seems like it barely makes a difference, especially since (a)
probably requires detecting KVM anyway.


For the real kernel code, it's probably even closer to making no
difference, since I don't think we'll want arch_get_random_long to use
emulated rdrand.

--Andy
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