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

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Date:	Thu, 1 May 2014 14:06:13 -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 2:01 PM, H. Peter Anvin <hpa@...or.com> wrote:
> On 05/01/2014 01:56 PM, Andy Lutomirski wrote:
>>
>> Even if we could emulate RDSEED effectively**, I don't really
>> understand what the guest is expected to do with it.  And I generally
>> dislike defining an interface with no known sensible users, because it
>> means that there's a good chance that the interface won't end up
>> working.
>>
>> ** Doing this sensibly in the host will be awkward.  Is the host
>> supposed to use non-blocking reads of /dev/random?  Getting anything
>> remotely fair may be difficult.
>
> The host can use nonblocking reads of /dev/random.  Fairness would have
> to be implemented at the host level, but that is true for anything.
>

I still don't see the point.  What does this do better than virtio-rng?

The ASLR code doesn't even try to use RDSEED.  RDSEED is used in
add_interrupt_randomness, which should drain the host's /dev/random
even if it could, and it's used in init_std_data.  The logic there is:

                if (!arch_get_random_seed_long(&rv) &&
                    !arch_get_random_long(&rv))
                        rv = random_get_entropy();

I think this is better achieved by having the host try to supply the
highest quality data it can.

The third RDSEED use is arch_random_refill.  This purpose would be
much better served by the khwrng stuff and virtio-rng.

So I still claim that fancy emulated RDSEED support will have no users.

--Andy
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