linux-kernel - Re: [PATCH v5 03/11] tpm: Allow PCR 23 to be restricted to kernel-only use

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Date:   Sun, 27 Nov 2022 18:33:41 +0200
From:   Jarkko Sakkinen <jarkko@...nel.org>
To:     James Bottomley <jejb@...ux.ibm.com>
Cc:     Evan Green <evgreen@...omium.org>, linux-kernel@...r.kernel.org,
        corbet@....net, linux-integrity@...r.kernel.org,
        Eric Biggers <ebiggers@...nel.org>, gwendal@...omium.org,
        dianders@...omium.org, apronin@...omium.org,
        Pavel Machek <pavel@....cz>, Ben Boeckel <me@...boeckel.net>,
        rjw@...ysocki.net, Kees Cook <keescook@...omium.org>,
        dlunev@...gle.com, zohar@...ux.ibm.com,
        Matthew Garrett <mgarrett@...ora.tech>,
        linux-pm@...r.kernel.org, Matthew Garrett <mjg59@...gle.com>,
        Jason Gunthorpe <jgg@...pe.ca>, Peter Huewe <peterhuewe@....de>
Subject: Re: [PATCH v5 03/11] tpm: Allow PCR 23 to be restricted to
 kernel-only use

On Mon, Nov 14, 2022 at 12:11:20PM -0500, James Bottomley wrote:
> On Fri, 2022-11-11 at 15:16 -0800, Evan Green wrote:
> > Introduce a new Kconfig, TCG_TPM_RESTRICT_PCR, which if enabled
> > restricts usermode's ability to extend or reset PCR 23.
> 
> Could I re ask the question here that I asked of Matthew's patch set:
> 
> https://lore.kernel.org/all/b0c4980c8fad14115daa3040979c52f07f7fbe2c.camel@linux.ibm.com/
> 
> Which was could we use an NVRAM index in the TPM instead of a PCR?  The
> reason for asking was that PCRs are rather precious and might get more
> so now that Lennart has some grand scheme for using more of them in his
> unified boot project.  Matthew promised to play with the idea but never
> got back to the patch set to say whether he investigated this or not.

Even for PCR case it would be better to have it configurable through
kernel command-line, including a disabled state, which would the
default.

This would be backwards compatible, and if designed properly, could
more easily extended for NV index later on.

BR, Jarkko