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Date:   Sun, 27 Nov 2022 18:33:41 +0200
From:   Jarkko Sakkinen <jarkko@kernel.org>
To:     James Bottomley <jejb@linux.ibm.com>
Cc:     Evan Green <evgreen@chromium.org>, linux-kernel@vger.kernel.org,
        corbet@lwn.net, linux-integrity@vger.kernel.org,
        Eric Biggers <ebiggers@kernel.org>, gwendal@chromium.org,
        dianders@chromium.org, apronin@chromium.org,
        Pavel Machek <pavel@ucw.cz>, Ben Boeckel <me@benboeckel.net>,
        rjw@rjwysocki.net, Kees Cook <keescook@chromium.org>,
        dlunev@google.com, zohar@linux.ibm.com,
        Matthew Garrett <mgarrett@aurora.tech>,
        linux-pm@vger.kernel.org, Matthew Garrett <mjg59@google.com>,
        Jason Gunthorpe <jgg@ziepe.ca>, Peter Huewe <peterhuewe@gmx.de>
Subject: Re: [PATCH v5 03/11] tpm: Allow PCR 23 to be restricted to
 kernel-only use
Message-ID: <Y4ORZT2t/KhL5jfn@kernel.org>
References: <20221111231636.3748636-1-evgreen@chromium.org>
 <20221111151451.v5.3.I9ded8c8caad27403e9284dfc78ad6cbd845bc98d@changeid>
 <8ae56656a461d7b957b93778d716c6161070383a.camel@linux.ibm.com>
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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