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

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Message-ID: <CALCETrX=0uA6CCg4UeeV8KgGAq6fxUSiGOw5Gzte4OSHrDWMqA@mail.gmail.com>
Date:	Thu, 1 May 2014 13:16:40 -0700
From:	Andy Lutomirski <luto@...capital.net>
To:	"Ted Ts'o" <tytso@....edu>
Cc:	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"H. Peter Anvin" <hpa@...or.com>, 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 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
>
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