linux-kernel - Re: [PATCH v11 18/20] tpm: Add sysfs interface to allow setting and querying the default locality

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Message-Id: <D5B6EQ3BE8HC.2BZ3UDY70CGKG@kernel.org>
Date: Fri, 01 Nov 2024 23:56:22 +0200
From: "Jarkko Sakkinen" <jarkko@...nel.org>
To: "Jarkko Sakkinen" <jarkko@...nel.org>, "Ross Philipson"
 <ross.philipson@...cle.com>, <linux-kernel@...r.kernel.org>,
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Cc: <dpsmith@...rtussolutions.com>, <tglx@...utronix.de>,
 <mingo@...hat.com>, <bp@...en8.de>, <hpa@...or.com>,
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Subject: Re: [PATCH v11 18/20] tpm: Add sysfs interface to allow setting and
 querying the default locality

On Fri Nov 1, 2024 at 11:50 PM EET, Jarkko Sakkinen wrote:
> On Fri Nov 1, 2024 at 12:06 PM EET, Jarkko Sakkinen wrote:
> > On Fri Sep 13, 2024 at 11:05 PM EEST, Ross Philipson wrote:
> > > Expose a sysfs interface to allow user mode to set and query the default
> > > locality set for the TPM chip.
> > >
> > > Signed-off-by: Ross Philipson <ross.philipson@...cle.com>
> >
> > Must be read-only. Should be decided per power cycle.
>
> I'm throwing one incomplete idea not all things considered...
>
> So one idea is would be to apply set operation to /dev/tpm0 as ioctl
> (would not be available for /dev/tpmrm0).
>
> Then at least access control rules would apply.
>
> The open here is that the IMA etc. will use a different locality during
> boot-time, like it would also with sysfs attribute.

Looking at [1] this would become a problem if TPM2_PolicyLocality based
policy is ever used during boot-time.

We can make a choice of not allowing such policies for in-kernel clients
if agree so, but it is a choice that needs to be locked in. With quick
thinking I'm not sure if that is horrible limitation.

Also does not obviously affect clients communicating with /dev/tpm0.
With that constrain it would not matter if during boot-time different
locale is used.

With that constraint and "set" in ioctl instead of sysfs attributes
that might even work out... Open for feedback.

[1] https://trustedcomputinggroup.org/wp-content/uploads/TPM-2.0-1.83-Part-3-Commands.pdf

BR, Jarkko