linux-kernel - Re: TPMs and random numbers

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Message-ID: <20130911202831.GC13397@thunk.org>
Date:	Wed, 11 Sep 2013 16:28:31 -0400
From:	Theodore Ts'o <tytso@....edu>
To:	"H. Peter Anvin" <hpa@...or.com>
Cc:	Andy Lutomirski <luto@...capital.net>,
	Jeff Garzik <jgarzik@...ox.com>,
	David Safford <safford@...ibm.com>,
	Leonidas Da Silva Barbosa <leosilva@...ux.vnet.ibm.com>,
	Ashley Lai <ashley@...leylai.com>,
	Rajiv Andrade <mail@...jiv.net>,
	Marcel Selhorst <tpmdd@...horst.net>,
	Sirrix AG <tpmdd@...rix.com>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	Kent Yoder <key@...ux.vnet.ibm.com>,
	David Safford <safford@...son.ibm.com>,
	Mimi Zohar <zohar@...ibm.com>,
	"Johnston, DJ" <dj.johnston@...el.com>
Subject: Re: TPMs and random numbers

On Wed, Sep 11, 2013 at 12:25:48PM -0700, H. Peter Anvin wrote:
> This of course has been a long-running debate.  Similarly, we could
> make much better use of RDRAND if instead of doing data reduction in
> rngd we could feed it to the pool and just credit fractional bits.
> The FIPS tests that rngd runs are weak and obsoleted, but perhaps
> better than nothing (now when we don't shut down rngd due to false
> positives.)

/dev/urandom is using RDRAND already, and that's what most of the
applications which are generating ssh host keys, session keys, etc.,
are using.

/dev/random is using RDRAND as well, but we're not giving any entropy
credit, so it will take longer to get the necessary randomness to
generate a GPG key.

The rason why it would be good to use TPM to fetch randomness is for
those platforms is (a) for pre-RDRAND capable x86 systems, and (c)
non-x86 platforms that might be using a TPM which don't have a RDRAND
function.

Also, in general, it's better to use as many entropy sources as
possible.

Cheers,

					- Ted
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