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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 17:36:09 -0400
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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.

> The current constant does not really imply that it is for the null
> key,

Well, it isn't exactly: it's the required flag set for all primaries.

James

>  it is defined in the wrong file and has no actual legit
> documentation to go with it.