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

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Message-ID: <aN0NcIlyrUsejMXW@kernel.org>
Date: Wed, 1 Oct 2025 14:16:00 +0300
From: Jarkko Sakkinen <jarkko@...nel.org>
To: James Bottomley <James.Bottomley@...senpartnership.com>
Cc: Jonathan McDowell <noodles@...th.li>, 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, Sep 30, 2025 at 10:17:22AM -0400, James Bottomley wrote:
> 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.

It needs some compilation time cap as the value comes from external
device. If someone hits to that value, then it needs to be increased
but as unconstrained it's a bug.


> Regards,
> 
> James
> 

BR, Jarkko