linux-kernel - Re: [PATCH v17 1/3] x86/tdx: Add a wrapper to get TDREPORT from the TDX Module

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Date:   Mon, 14 Nov 2022 16:54:36 -0800
From:   Dave Hansen <dave.hansen@...el.com>
To:     Sathyanarayanan Kuppuswamy 
        <sathyanarayanan.kuppuswamy@...ux.intel.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
        Dave Hansen <dave.hansen@...ux.intel.com>, x86@...nel.org,
        Shuah Khan <shuah@...nel.org>, Jonathan Corbet <corbet@....net>
Cc:     "H . Peter Anvin" <hpa@...or.com>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        "Kirill A . Shutemov" <kirill.shutemov@...ux.intel.com>,
        Tony Luck <tony.luck@...el.com>,
        Kai Huang <kai.huang@...el.com>,
        Wander Lairson Costa <wander@...hat.com>,
        Isaku Yamahata <isaku.yamahata@...il.com>,
        marcelo.cerri@...onical.com, tim.gardner@...onical.com,
        khalid.elmously@...onical.com, philip.cox@...onical.com,
        linux-kernel@...r.kernel.org, linux-kselftest@...r.kernel.org,
        linux-doc@...r.kernel.org
Subject: Re: [PATCH v17 1/3] x86/tdx: Add a wrapper to get TDREPORT from the
 TDX Module

On 11/14/22 16:33, Sathyanarayanan Kuppuswamy wrote:
> On 11/11/22 10:35 AM, Dave Hansen wrote:
>> This is *NOT* "a wrapper to get TDREPORT from the TDX Module", this is
>> at best "a wrapper to get TDREPORT sub type 0 from the TDX Module".
> 
> In both the commit log and the comments, I can highlight the "subtype 0"
> information. Will that work for you, or do you prefer that this wrapper
> take the "subtype" option as argument and we pass 0 for the subtype value
> from the TDX guest driver?

I actually think it's a *lot* more clear if the User<->Kernel ABI just
takes the subtype.  But, I also heard Greg's concerns about making the
ABI _too_ open-ended.

So, I really don't care.  Just make it clear that, as is, this ABI is
not the "TDREPORT ABI".

>> It also occurs to me that "sub type 0" could use an actual name.  Could
>> we give it one, please?
> 
> Although the subtype option is mentioned in the TDX Module spec, it is not
> currently used (it expects this value to be zero), and the spec also does
> not explain why this option is required. According to TDX architects, this
> option was primarily added to handle any future requirements that may arise
> that require additional information to be added to the TDREPORT. However,
> they do not currently have any valid use cases for it. So the current
> version can only be described as "Type-0." Once a new use case for Subtype 1
> is defined, we may be able to come up with a suitable name. Are you okay
> with calling it "Type-0" for the time being?

That sounds like a cop out to me.  I'd really appreciate some effort on
your part to look deeply into the problem.

The blob that the kernel is passing back and forth here _has_ content.
I guess it's somewhat hard to name because it's got a bunch of inputs
(ATTRIBUTES, XFAM, MRTD, MRCONFIGID, MROWNER, MROWNERCONFIG and RTMRs)
and a fixed hash algorithm (SHA-384).

Any time that those inputs change or, for instance, the hash algorithm
changes, it would need a new subtype.  Right?

I guess we can't call "subtype 0" TDREPORT_SHA384 because "subtype 1"
might still use SHA-384, but have the set of inputs change.

But, it'll also get maddeningly inconsistent if we have a "TDREPORT"
ioctl() that does "subtype 0" and "TDREPORT1" that does "subtype 1".

So, let's at *least* call this thing "TDREPORT0" in the ABI, along with
a description of why we're numbering it that way as opposed to taking
'subtype' as a numeric ioctl() argument.

Any better ideas?