linux-kernel - Re: [RFC PATCH v2 0/4] tsm: Runtime measurement registers ABI

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Date: Fri, 02 Feb 2024 07:24:06 +0100
From: James Bottomley <James.Bottomley@...senPartnership.com>
To: Samuel Ortiz <sameo@...osinc.com>, Dan Williams
 <dan.j.williams@...el.com>
Cc: Kuppuswamy Sathyanarayanan <sathyanarayanan.kuppuswamy@...ux.intel.com>,
  Qinkun Bao <qinkun@...gle.com>, "Yao, Jiewen" <jiewen.yao@...el.com>,
 "Xing, Cedric" <cedric.xing@...el.com>, Dionna Amalie Glaze
 <dionnaglaze@...gle.com>,  biao.lu@...el.com, linux-coco@...ts.linux.dev,
 linux-integrity@...r.kernel.org,  linux-kernel@...r.kernel.org
Subject: Re: [RFC PATCH v2 0/4] tsm: Runtime measurement registers ABI

On Sun, 2024-01-28 at 22:25 +0100, Samuel Ortiz wrote:
> All architectures supporting RTMRs expose a similar interface to
> their TVMs: An extension command/call that takes a measurement value
> and an RTMR index to extend it with, and a readback command for
> reading an RTMR value back (taking an RTMR index as an argument as
> well). This patch series builds an architecture agnostic, configfs-
> based ABI for userspace to extend and read RTMR values back. It
> extends the current TSM ops structure and each confidential computing
> architecture can implement this extension to provide RTMR support.

What's the actual use case for this?  At the moment the TPM PCRs only
provide a read interface to userspace (via /sys/class/tpm/tpmX/pcr-
shaY/Z) and don't have any extension ability becuase nothing in
userspace currently extends them.

The only current runtime use for TPM PCRs is IMA, which is in-kernel
(and which this patch doesn't enable).

Without the ability to log, this interface is unusable anyway, but even
with that it's not clear that you need the ability separately to extend
PCRs because the extension and log entry should be done atomically to
prevent the log going out of sync with the PCRs, so it would seem a log
first interface would be the correct way of doing this rather than a
PCR first one.

James