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

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Message-ID: <CALCETrXMAWSC6B-irn4LeXFzB2tODqagsBboky=xnGv5ZTh-sg@mail.gmail.com>
Date:	Thu, 1 May 2014 12:02:49 -0700
From:	Andy Lutomirski <luto@...capital.net>
To:	"H. Peter Anvin" <hpa@...or.com>
Cc:	"Theodore Ts'o" <tytso@....edu>,
	Florian Weimer <fweimer@...hat.com>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	Kees Cook <kees@...flux.net>, kvm list <kvm@...r.kernel.org>
Subject: Re: random: Providing a seed value to VM guests

On Thu, May 1, 2014 at 11:59 AM, H. Peter Anvin <hpa@...or.com> wrote:
> On 05/01/2014 11:53 AM, Andy Lutomirski wrote:
>>
>> A CPUID leaf or an MSR advertised by a CPUID leaf has another
>> advantage: it's easy to use in the ASLR code -- I don't think there's
>> a real IDT, so there's nothing like rdmsr_safe available.  It also
>> avoids doing anything complicated with the boot process to allow the
>> same seed to be used for ASLR and random.c; it can just be invoked
>> twice on boot.
>>
>
> At that point we are talking an x86-specific interface, and so we might
> as well simply emulate RDRAND (urandom) and RDSEED (random) if the CPU
> doesn't support them.  I believe KVM already has a way to report CPUID
> features that are "emulated but supported anyway", i.e. they work but
> are slow.

Do existing kernels and userspace respect this?  If the normal bit for
RDRAND is unset, then we might be okay, but, if not, then I think this
may kill guest performance.

Is RDSEED really reasonable here?  Won't it slow down by several
orders of magnitude?

>
>> What's the right forum for this?  This thread is probably not it.
>
> Change the subject line?

:)

>
>         -hpa
>
>



-- 
Andy Lutomirski
AMA Capital Management, LLC
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