linux-kernel - Re: [RFC PATCH v2 4/4] tsm: Allow for extending and reading configured RTMRs

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Date: Mon, 13 May 2024 12:16:41 +0200
From: Samuel Ortiz <sameo@...osinc.com>
To: James Bottomley <James.Bottomley@...senpartnership.com>
Cc: Dan Williams <dan.j.williams@...el.com>,
	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 4/4] tsm: Allow for extending and reading
 configured RTMRs

On Fri, May 10, 2024 at 10:57:37PM -0400, James Bottomley wrote:
> I'm not really sure where to hang this, since there's no posted agenda
> or materials for the CCC meeting today.

The agenda was posted on the linux-coco ml [1]. I sent a link to the
presentation slides [2] to the thread.

> However, it struck me you missed a third option: use the ima log
> format.  This has the advantage that we can define additional events
> and have them published with a kernel patch (the IMA log format is
> defined in the kernel).  Thanks to the TCG, it's also CEL compatible
> but doesn't require any sort of TCG blessing of the events.  Plus we
> also have existing kernel infrastructure to log to that format.

That's an interesting idea. It may avoid having to extend the CEL spec
with a new Content Type, but otoh the current spec defines which IMA
events are supported. So adding new ones may require to also eventually
extend the spec. But I guess since IMA is a Linux kernel subsystem,
changing the kernel code and ABI would de-facto extend the TCG CEL IMA
spec.

Here I assume you're talking about the IMA_TEMPLATE CEL specified
format, which is designed to accomodate for the current kernel IMA log
format. The main drawback of this format is that the digest does not
include the whole content event, making the CEL content type, the IMA
tag name and both lengths (for the content event and the IMA content)
untrusted for event log verifiers.

CEL defines another IMA format (IMA_TLV), that hashes the whole event
content. I think we should at least use that format as our output ABI,
if we want to use a TCG specified IMA content type.

Cheers,
Samuel.

[1] https://lore.kernel.org/linux-coco/61b65115-5945-4e27-89e4-bb6cba657f7f@linux.intel.com/
[2] https://docs.google.com/presentation/d/1qMk-8TiMigVmVAEDWXqPu9Jd7OJ8AGvCR34Lp2WunhU/edit?usp=sharing

> Regards,
> 
> James
>