linux-kernel - Re: [PATCH v3 01/10] tpm: Cap the number of PCR banks

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Message-ID: <cf3fb265dd70a23d598fc3d68562b4be5355e7ae.camel@HansenPartnership.com>
Date: Tue, 30 Sep 2025 10:17:22 -0400
From: James Bottomley <James.Bottomley@...senPartnership.com>
To: Jarkko Sakkinen <jarkko@...nel.org>, Jonathan McDowell <noodles@...th.li>
Cc: linux-integrity@...r.kernel.org, dpsmith@...rtussolutions.com, 
 ross.philipson@...cle.com, Jarkko Sakkinen <jarkko.sakkinen@...nsys.com>, 
 Roberto Sassu <roberto.sassu@...wei.com>, Peter Huewe <peterhuewe@....de>,
 Jason Gunthorpe <jgg@...pe.ca>,  David Howells <dhowells@...hat.com>, Paul
 Moore <paul@...l-moore.com>, James Morris <jmorris@...ei.org>,  "Serge E.
 Hallyn" <serge@...lyn.com>, open list <linux-kernel@...r.kernel.org>, "open
 list:KEYS/KEYRINGS" <keyrings@...r.kernel.org>, "open list:SECURITY
 SUBSYSTEM" <linux-security-module@...r.kernel.org>
Subject: Re: [PATCH v3 01/10] tpm: Cap the number of PCR banks

On Tue, 2025-09-30 at 15:36 +0300, Jarkko Sakkinen wrote:
> On Tue, Sep 30, 2025 at 12:09:15PM +0100, Jonathan McDowell wrote:
> > On Mon, Sep 29, 2025 at 10:48:23PM +0300, Jarkko Sakkinen wrote:
[...]
> > > +#define TPM2_MAX_DIGEST_SIZE	SHA512_DIGEST_SIZE
> > > +#define TPM2_MAX_BANKS		4
> > 
> > Where does this max come from? It matches what I see with swtpm by 
> > default (SHA1, SHA2-256, SHA2-384, SHA-512), so I haven't seen
> > anything that exceeds it myself.
> 
> I've never seen hardware TPM that would have more than one or two
> banks. We can double it to leave some room. This was tested with
> swtpm defaults.

I've got a hardware TPM that comes with 3 banks by default (it's a
chinese one which has sha1 sha256 and sm2).  swtpm isn't a good
indicator because it's default allocation is rather pejorative (it
disables sha1 whereas most field TPMs don't).

However, if you look at how the reference implementation works, the
user is allowed to define any number of banks they want, up to the
number of supported hashes.  The only limitation being there can't be
>1 bank for the same hash.  Field TPM implementations are allowed to
constrain this, but most don't.   The question you should be asking
here is not how many banks does a particular implementation allow by
default, but what's the maximum number a user could configure.

Regards,

James