From mboxrd@z Thu Jan  1 00:00:00 1970
From: Theodore Ts'o <tytso@mit.edu>
Subject: Re: [PATCH v4 2/5] random: Add and use arch_get_rng_seed
Date: Tue, 22 Jul 2014 09:59:15 -0400
Message-ID: <20140722135915.GB25291@thunk.org>
References: <cover.1405620944.git.luto@amacapital.net>
 <9c2a0549519b4eb5eee2d5d480f8e83a574273df.1405620944.git.luto@amacapital.net>
Mime-Version: 1.0
Content-Type: text/plain; charset=us-ascii
Cc: kvm@vger.kernel.org, "H. Peter Anvin" <hpa@zytor.com>,
	linux-kernel@vger.kernel.org, Kees Cook <keescook@chromium.org>,
	x86@kernel.org, Daniel Borkmann <dborkman@redhat.com>,
	Srivatsa Vaddagiri <vatsa@linux.vnet.ibm.com>,
	Raghavendra K T <raghavendra.kt@linux.vnet.ibm.com>,
	Gleb Natapov <gleb@kernel.org>,
	Paolo Bonzini <pbonzini@redhat.com>, bsd@redhat.com,
	Andrew Honig <ahonig@google.com>
To: Andy Lutomirski <luto@amacapital.net>
Return-path: <linux-kernel-owner@vger.kernel.org>
Content-Disposition: inline
In-Reply-To: <9c2a0549519b4eb5eee2d5d480f8e83a574273df.1405620944.git.luto@amacapital.net>
Sender: linux-kernel-owner@vger.kernel.org
List-Id: kvm.vger.kernel.org

On Thu, Jul 17, 2014 at 11:22:17AM -0700, Andy Lutomirski wrote:
> Currently, init_std_data contains its own logic for using arch
> random sources.  This logic is a bit strange: it reads one long of
> arch random data per byte of internal state.

This isn't true.  Check out the init_std_data() a bit more closely.

	unsigned long rv;

	...

	for (i = r->poolinfo->poolbytes; i > 0; i -= sizeof(rv)) {
	    ...

In particular, note the "i -= sizeof(rv)".  We are reading one bit per
bit of internal state beeing seeded.

> Assuming the arch sources are perfect, this is the right thing to
> do.  They're not, though, so the followup patch attempts to
> implement the correct logic on x86.

... and that's not a problem because we aren't giving any entropy
credit --- and this is deliberate, because we don't want to trust
un-auditable hardware.  We are deliberately trying to be conservative
here.

So I don't think either this patch or the next one is needed.  It adds
far more complexity than is warranted.

Regards,

						- Ted