linux-kernel - Re: [PATCH v4 2/5] random: Add and use arch_get_rng

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Message-ID: <20140722135915.GB25291@thunk.org>
Date:	Tue, 22 Jul 2014 09:59:15 -0400
From:	Theodore Ts'o <tytso@....edu>
To:	Andy Lutomirski <luto@...capital.net>
Cc:	kvm@...r.kernel.org, "H. Peter Anvin" <hpa@...or.com>,
	linux-kernel@...r.kernel.org, Kees Cook <keescook@...omium.org>,
	x86@...nel.org, Daniel Borkmann <dborkman@...hat.com>,
	Srivatsa Vaddagiri <vatsa@...ux.vnet.ibm.com>,
	Raghavendra K T <raghavendra.kt@...ux.vnet.ibm.com>,
	Gleb Natapov <gleb@...nel.org>,
	Paolo Bonzini <pbonzini@...hat.com>, bsd@...hat.com,
	Andrew Honig <ahonig@...gle.com>
Subject: Re: [PATCH v4 2/5] random: Add and use arch_get_rng_seed

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
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