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

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Date: Sat, 24 Feb 2024 03:34:12 +0100
From: Lino Sanfilippo <l.sanfilippo@...bus.com>
To: "Daniel P. Smith" <dpsmith@...rtussolutions.com>,
 Alexander Steffen <Alexander.Steffen@...ineon.com>,
 Jarkko Sakkinen <jarkko@...nel.org>, 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 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. 

BR,
Lino