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

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Message-ID: <CALCETrV1HUA-JRAj9UicD3woqec7mSJLoT9CkQq=dOCpdPg4hw@mail.gmail.com>
Date:	Thu, 1 May 2014 13:32:55 -0700
From:	Andy Lutomirski <luto@...capital.net>
To:	"H. Peter Anvin" <hpa@...or.com>
Cc:	"Ted 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 1:30 PM, H. Peter Anvin <hpa@...or.com> wrote:
> RDSEED is not synchronous.  It is, however, nonblocking.

What I mean is: IIUC it's reasonable to call RDSEED a few times in a
loop and hope it works.  It makes no sense to do that with
/dev/random.

>
> On May 1, 2014 1:16:40 PM PDT, Andy Lutomirski <luto@...capital.net> wrote:
>>On May 1, 2014 12:26 PM, <tytso@....edu> wrote:
>>>
>>> On Thu, May 01, 2014 at 12:02:49PM -0700, Andy Lutomirski wrote:
>>> >
>>> > Is RDSEED really reasonable here?  Won't it slow down by several
>>> > orders of magnitude?
>>>
>>> That is I think the biggest problem; RDRAND and RDSEED are fast if
>>> they are native, but they will involve a VM exit if they need to be
>>> emulated.  So when an OS might want to use RDRAND and RDSEED might be
>>> quite different if we know they are being emulated.
>>>
>>> Using the RDRAND and RDSEED "api" certainly makes sense, at least for
>>> x86, but I suspect we might want to use a different way of signalling
>>> that a VM guest can use RDRAND and RDSEED if they are running on a
>>CPU
>>> which doesn't provide that kind of access.  Maybe a CPUID extended
>>> function parameter, if one could be allocated for use by a Linux
>>> hypervisor?
>>>
>>
>>I'm still not convinced.  This will affect userspace as well as the
>>guest kernel, and I don't see why guest user code should be able to
>>access this API.  RDRAND for CPL0 only would work, but that seems odd.
>>
>>And I think that RDSEED emulation is asking for trouble.  RDSEED is
>>synchronous, but /dev/random is asynchronous.  And making bootup wait
>>for even a single byte from /dev/random seems bad.  In any event,
>>virtio-rng should be a better interface for this.
>>
>>>                                                 - Ted
>>>
>
> --
> Sent from my mobile phone.  Please pardon brevity and lack of formatting.



-- 
Andy Lutomirski
AMA Capital Management, LLC
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