linux-kernel - Re: [PATCH v2] RANDOM: ATH9K RNG delivers zero bits of entropy

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Message-ID: <20160810234425.GG10523@thunk.org>
Date:	Wed, 10 Aug 2016 19:44:25 -0400
From:	Theodore Ts'o <tytso@....edu>
To:	Jason Cooper <jason@...edaemon.net>
Cc:	"Pan, Miaoqing" <miaoqing@....qualcomm.com>,
	Stephan Mueller <smueller@...onox.de>,
	"Sepehrdad, Pouyan" <pouyans@....qualcomm.com>,
	"herbert@...dor.apana.org.au" <herbert@...dor.apana.org.au>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"linux-crypto@...r.kernel.org" <linux-crypto@...r.kernel.org>,
	ath9k-devel <ath9k-devel@....qualcomm.com>,
	"linux-wireless@...r.kernel.org" <linux-wireless@...r.kernel.org>,
	"ath9k-devel@...ts.ath9k.org" <ath9k-devel@...ts.ath9k.org>,
	Kalle Valo <kvalo@...eaurora.org>
Subject: Re: [PATCH v2] RANDOM: ATH9K RNG delivers zero bits of entropy

On Tue, Aug 09, 2016 at 02:04:44PM +0000, Jason Cooper wrote:
> 
> iiuc, Ted, you're saying using the hw_random framework would be
> disasterous because despite most drivers having a default quality of 0,
> rngd assumes 1 bit of entropy for every bit read?

Sorry, what I was trying to say (but failed) was that bypassing the
hwrng framework and injecting entropy directly the entropy pool was
disatrous.

> Thankfully, most hw_random drivers don't set the quality.  So unless the
> user sets the default_quality param, it's zero.

The fact that this is "most" and not "all" does scare me a little.

As far as I'm concerned *all* hw_random drivers should set quality to
zero, since it should be up to the system administrator.  Perhaps the
one exception is virtio_rng, since if you don't trust the hypvervisor,
the security of the VM is hopeless.  That being said, I have seen
configurations of KVM which use:

	-object rng-random,filename=/dev/urandom,id=rng0 \
	-device virtio-rng-pci,rng=rng0

Which is somewhat non-ideal.  (Try running od -x /dev/random on such a
guest system....)

					- Ted