linux-kernel - Re: [RFC PATCH v1 3/4] tsm: Allow for mapping RTMRs to TCG TPM PCRs

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Message-ID: <14dffda2-f413-4304-9932-3ac8ddfb30e4@intel.com>
Date: Tue, 23 Jan 2024 10:48:00 -0800
From: "Xing, Cedric" <cedric.xing@...el.com>
To: Dan Williams <dan.j.williams@...el.com>, "Yao, Jiewen"
	<jiewen.yao@...el.com>, Qinkun Bao <qinkun@...gle.com>, Samuel Ortiz
	<sameo@...osinc.com>, "Lu, Ken" <ken.lu@...el.com>
CC: Kuppuswamy Sathyanarayanan <sathyanarayanan.kuppuswamy@...ux.intel.com>,
	"linux-coco@...ts.linux.dev" <linux-coco@...ts.linux.dev>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [RFC PATCH v1 3/4] tsm: Allow for mapping RTMRs to TCG TPM PCRs

On 1/22/2024 2:32 PM, Dan Williams wrote:
> Xing, Cedric wrote:
> [..]
>>> So, yes, the mapping should be allowed to specified by the low-level
>>> driver, but at the same time every vendor should not reinvent their own
>>> enumeration method when we have EFI for that.
>>
>> Given PCR->RTMR mapping is static, I just wonder why it needs to be kept
>> in kernel. Given that PCRs can never be 1:1 mapped to RTMRs, and that
>> TDX quotes are never TPM quotes, applications used to extend PCRs would
>> have to be changed/recompiled. Then wouldn't it suffice to define the
>> mappings as macros in an architecture specific header file?
> 
> I think something is wrong if applications are exposed to the PCR->RTMR
> mapping thrash. I would hope / expect that detail is hidden behind a TPM
> proxy layer sitting in front of this mapping on behalf of TPM-client
> applications.

Hi Dan,

My apology for the confusion! I think we are talking about 2 different 
scenarios - (1) this patch alone; and (2) this patch + vTPM.

Scenario 1: This patch provides RTMR access only. My assumption is, 
there are existing application (and/or kernel modules) that extend to 
PCRs today and would like to work in TDs where only RTMRs are available. 
Changes are of course necessary in those applications as TPMs/PCRs are 
no longer available, but from security perspective they would like to 
keep the same activity log and just change to use RTMRs (in lieu of 
PCRs) as the secure storage. Hence a PCR->RTMR mapping is necessary and 
must be agreed upon by all those applications and relying parties. IIUC, 
this is the intention of having PCR->RTMR mapping config maintained by 
the kernel, as proposed by Sam O. originally.

Scenario 2: A vTPM is implemented on top of this patch, in which case 
the existing applications don't have to change as they can continue 
extending to the same PCRs, which will then be emulated by the 
underlying vTPM implementation. PCR->RTMR mapping in this scenario is 
obviously internal to the vTPM and I agree with you completely that it 
should be hidden inside the vTPM.

My comment in my previous email was regarding Scenario 1. I hope the 
clarification above helps.

-Cedric