linux-kernel - Re: [RFC PATCH v1 0/4] tsm: Runtime measurement registers ABI

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Date: Sun, 21 Jan 2024 11:15:09 -0800
From: Dan Williams <dan.j.williams@...el.com>
To: Samuel Ortiz <sameo@...osinc.com>, <biao.lu@...el.com>
CC: <dan.j.williams@...el.com>, <linux-coco@...ts.linux.dev>,
	<linux-kernel@...r.kernel.org>
Subject: Re: [RFC PATCH v1 0/4] tsm: Runtime measurement registers ABI

Samuel Ortiz wrote:
> On Thu, Jan 18, 2024 at 11:35:15AM +0800, biao.lu@...el.com wrote:
> > Samuel Ortiz wrote:
> > > Some confidential computing architectures (Intel TDX, ARM CCA, RISC-V
> > > CoVE) provide their guests with a set of measurements registers that can
> > > be extended at runtime, i.e. after the initial, host-initiated
> > > measurements of the TVM are finalized. Those runtime measurement
> > > registers (RTMR) are isolated from the host accessible ones but TSMs
> > > include them in their signed attestation reports.
> > >
> > > 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.
> > 
> > Hi, Samuel
> > The ABI does not include eventlog, but eventlog is usually used with RTMR.
> > What do you think about how to implement eventlog?
> 
> Since the event log is typically maintained in the firmware and not in
> the TSM itself, I don't think we should expose e.g. an event log
> extension ABI through the config-tsm one.
> We could decide to check for an EFI CC protocol availability and extend
> the event log when any RTMR gets extended, and that would be an
> internal, not userspace visible operation. I'm not sure that this
> would scale well with e.g. IMA (a lot more events than pre-OS boot
> afaik).

Another observation after chatting with my colleague Cedric is that the 
TPM layer that builds on RTMR can maintain an event log that forks from
the RTMR log. I.e. instead of the TPM event log containig pre-OS events
starting from 0, it would start from a golden point in the RTMR
measurements.