linux-kernel - Re: TPMs and random numbers

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Message-ID: <522FCC51.1010604@amacapital.net>
Date:	Tue, 10 Sep 2013 18:50:09 -0700
From:	Andy Lutomirski <luto@...capital.net>
To:	"H. Peter Anvin" <hpa@...or.com>
CC:	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>,
	Jeff Garzik <jgarzik@...ox.com>, Ted Ts'o <tytso@....edu>,
	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 09/09/2013 02:11 PM, H. Peter Anvin wrote:
> It recently came to my attention that there are no standards whatsoever
> for random number generated by TPMs.  In fact, there *are* TPMs where
> random numbers are generated by an encrypted nonvolatile counter (I do
> not know which ones); this is apparently considered acceptable for the
> uses of random numbers that TPMs produce.
> 
> There are two issues with this from a Linux point of view.  One, we
> harvest supposed entropy from the TPM for /dev/*random use via
> /dev/hwrng and rngd.  This was something I originally proposed because
> on a lot of platforms it is the only available entropy source with any
> significant bandwidth.  However, in light of the above it is
> questionable at best, at least with entropy being credited.

Presumably the "entropy" should be mixed in but not credited to the
available entropy.

> 
> The other issue is that we use tpm_get_random() *directly* in
> security/keys/trusted.c.

I don't know whether this makes sense, but all but one call seem to be
related to TPM transactions -- breaking the TPM's RNG won't have any
effects beyond, say, breaking the TPM's SRK.

The one that looks dangerous is the one just under case Opt_new: it's
using tpm_get_random to create an encryption key *that's used by the
kernel for software crypto*.  That's IMO bogus.

--Andy
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