Re: [PATCH v12 1/3] x86/tdx: Add TDX Guest attestation interface driver

[Sathyanarayanan Kuppuswamy]

On 9/8/22 1:36 PM, Dave Hansen wrote:
> On 9/8/22 12:07, Sathyanarayanan Kuppuswamy wrote:
>> On 9/7/22 10:31 PM, Greg Kroah-Hartman wrote:
>>> On Wed, Sep 07, 2022 at 05:27:20PM -0700, Kuppuswamy Sathyanarayanan wrote:
>>>> +	/*
>>>> +	 * Per TDX Module 1.0 specification, section titled
>>>> +	 * "TDG.MR.REPORT", REPORTDATA length is fixed as
>>>> +	 * TDX_REPORTDATA_LEN, TDREPORT length is fixed as
>>>> +	 * TDX_REPORT_LEN, and TDREPORT subtype is fixed as
>>>> +	 * 0. Also check for valid user pointers.
>>>> +	 */
>>>> +	if (!req.reportdata || !req.tdreport || req.subtype ||
>>>> +		req.rpd_len != TDX_REPORTDATA_LEN ||
>>>> +		req.tdr_len != TDX_REPORT_LEN)
>>>> +		return -EINVAL;
>>> You never verify that your reserved[7] fields are actually set to 0,
>>> which means you can never use them in the future :(
>> Currently, we don't use those fields in our code. Why do we have to
>> make sure they are set to zero?
> 
> Yes.
> 
>> Can't we add checks when we really use them in future?
> 
> No.
> 
> This has been a hard learned lesson both by people writing software and
> designing hardware interfaces: if you _let_ folks pass garbage you have
> to _keep_ letting them pass garbage forever.  It becomes part of the ABI.
> 
> I'm sorry you missed the memo on this one.  But, this is one million
> percent a best practice across the industry.  Please do it.

Ok. Thanks for clarifying it. I will fix it in next version.

-- 
Sathyanarayanan Kuppuswamy
Linux Kernel Developer