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Subject: Re: [PATCH 1/3] tpm: Disable TCG_TPM2_HMAC by default
From: James Bottomley <James.Bottomley@HansenPartnership.com>
To: Jarkko Sakkinen <jarkko@kernel.org>, Vitor Soares <ivitro@gmail.com>, 
	linux-integrity@vger.kernel.org
Cc: keyrings@vger.kernel.org, Peter Huewe <peterhuewe@gmx.de>, Jason
 Gunthorpe <jgg@ziepe.ca>, Mimi Zohar <zohar@linux.ibm.com>, David Howells
 <dhowells@redhat.com>, Paul Moore <paul@paul-moore.com>, James Morris
 <jmorris@namei.org>, "Serge E. Hallyn" <serge@hallyn.com>, 
 linux-kernel@vger.kernel.org, linux-security-module@vger.kernel.org
Date: Mon, 27 May 2024 19:44:38 -0400
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On Tue, 2024-05-28 at 02:17 +0300, Jarkko Sakkinen wrote:
> On Tue May 28, 2024 at 12:36 AM EEST, James Bottomley wrote:
> > On Mon, 2024-05-27 at 22:53 +0300, Jarkko Sakkinen wrote:
> > > On Mon May 27, 2024 at 8:57 PM EEST, James Bottomley wrote:
> > > > On Mon, 2024-05-27 at 18:34 +0300, Jarkko Sakkinen wrote:
> > [...]
> > > > > While looking at code I started to wanted what was the
> > > > > reasoning for adding *undocumented* "TPM2_OA_TMPL" in
> > > > > include/linux/tpm.h.It should really be in tpm2-sessions.c
> > > > > and named something like TPM2_NULL_KEY_OA or similar.
> > > > 
> > > > Well, because you asked for it. I originally had all the flags
> > > > spelled out and I'm not a fan of this obscurity, but you have
> > > > to do stuff like this to get patches accepted:
> > > > 
> > > > https://lore.kernel.org/linux-integrity/CZCKTWU6ZCC9.2UTEQPEVICYHL@suppilovahvero/
> > > 
> > > I still think the constant does make sense.
> > 
> > I'm not so sure.  The TCG simply defines it as a collection of
> > flags and every TPM tool set I've seen simply uses a list of flags
> > as well.  The original design was that the template would be in
> > this one place and everything else would call into it.  I think the
> > reason all template construction looks similar is for ease of
> > auditing (it's easy to get things, particularly the flags, wrong).
> > 
> > If it only has one use case, it should be spelled out but if
> > someone else would use it then it should be in the tpm.h shared
> > header.
> 
> It is used only in tpm2-sessions.c and for the null key so there it
> should be. And it is also lacking the associated documentation. Now
> both name and context it is used is lost.

The comment above the whole thing says what it is and where it comes
from:

	/*
	 * create the template.  Note: in order for userspace to
	 * verify the security of the system, it will have to create
	 * and certify this NULL primary, meaning all the template
	 * parameters will have to be identical, so conform exactly to
	 * the TCG TPM v2.0 Provisioning Guidance for the SRK ECC
	 * key H template (H has zero size unique points)
	 */

If we put the broken out flags back it's all fully documented.

James