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 11:41:26 -0500
From:   James Bottomley <jejb@...ux.ibm.com>
To:     Jarkko Sakkinen <jarkko@...nel.org>
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 Sun, 2022-11-27 at 18:33 +0200, Jarkko Sakkinen wrote:
> 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.


Um how?  The observation is in the above referenced email is that PCR23
is reserved in the TCG literature for application usage.  If any
application is actually using PCR23 based on that spec then revoking
access to user space will cause it to break.  This is an ABI change
which is not backwards compatible.  You can call it a distro problem if
it's command line configurable, but the default would be what most
distros take, so it's rather throwing them under the bus if there is an
application using it.

Of course, if no application is actually using PCR23, then it's
probably OK to use it in the kernel and make it invisible to user
space, but no evidence about this has actually been presented.

James