linux-kernel - Re: TPMs and random numbers

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Message-ID: <20130911184509.GB13397@thunk.org>
Date:	Wed, 11 Sep 2013 14:45:09 -0400
From:	Theodore Ts'o <tytso@....edu>
To:	Andy Lutomirski <luto@...capital.net>
Cc:	David Safford <safford@...ibm.com>,
	"H. Peter Anvin" <hpa@...or.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>,
	Jeff Garzik <jgarzik@...ox.com>,
	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 10:49:59AM -0700, Andy Lutomirski wrote:
> 
> A TPM that has an excellent internal entropy source and is FIPS 140-2
> compliant with no bugs whatsoever may still use Dual_EC_DRBG, which
> looks increasingly likely to be actively malicious.

To be fair, given the limited CPU found in most TPM's, using
Dual_EC_DRBG would be rather unlikely.  It's more likely that the TPM
would be using a real hardware RNG --- and if the TPM was compromised
by some evil spy agency, it would be doing using something like
AES_ENCRYPT(i++, NSA_KEY), not using Dual_EC_DRBG.

> I'd be *much* happier if my system read a few hundred random bytes
> from the TPM at startup and fed those bytes into the kernel's entropy
> pool.  This should IMO happen at startup as early as possible.

We should definitely do this.  If the TPM driver could fetch some
randomness and then call add_device_randomness() to feed this into the
random driver's entropy pool when it initializes itself, that would be
***really*** cool.

							- Ted
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