linux-kernel - Re: [PATCH 1/3] tpm: protect against locality counter underflow

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Date: Mon, 26 Feb 2024 11:38:39 +0200
From: "Jarkko Sakkinen" <jarkko@...nel.org>
To: "Lino Sanfilippo" <l.sanfilippo@...bus.com>, "Daniel P. Smith"
 <dpsmith@...rtussolutions.com>, "Alexander Steffen"
 <Alexander.Steffen@...ineon.com>, "Jason Gunthorpe" <jgg@...pe.ca>, "Sasha
 Levin" <sashal@...nel.org>, <linux-integrity@...r.kernel.org>,
 <linux-kernel@...r.kernel.org>
Cc: "Ross Philipson" <ross.philipson@...cle.com>, "Kanth Ghatraju"
 <kanth.ghatraju@...cle.com>, "Peter Huewe" <peterhuewe@....de>
Subject: Re: [PATCH 1/3] tpm: protect against locality counter underflow

On Sat Feb 24, 2024 at 4:34 AM EET, Lino Sanfilippo wrote:
>
>
> On 23.02.24 02:56, Daniel P. Smith wrote:
>
> >>
> >> Will the TPM TIS CORE ever (have to) request another locality than 0? Maybe the best would
> >> be to hardcode TPM_ACCESS(0) and get rid of all the locality parameters that are
> >> passed from one function to another.
> >> But this is rather code optimization and not really required to fix the reported bug.
> > 
> > Actually, doing so will break the TPM API. The function
> > tpm_tis_request_locality() is registered as the locality handler,
> >  int (*request_locality)(struct tpm_chip *chip, int loc), in the tis
> > instance of struct tpm_class_ops{}. This is the API used by the Secure
> > Launch series to open Locality2 for the measurements it must record.
> > 
>
> I dont understand this. How do you use locality 2 with the current mainline
> API? Do you adjust the mainline code to use locality 2 instead of 0? This would 
> at least explain how you ran into the underflow issue which from
> the source code seems to be impossible when using locality 0. But then I wonder why
> this has not been made clear in this discussion. And then we are talking
> about fixing a bug that does not even exist in the upstream code. 

Thanks for bringing this up, now I finally figured out what confuses me
in this series.

Daniel, I also have troubles understanding why locality_count would ever
be greater than zero exactly in the mainline kernel, *without* [1]?

[1] https://lore.kernel.org/linux-integrity/20240214221847.2066632-1-ross.philipson@oracle.com/

BR, Jarkko