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

[Dave Hansen <dave.hansen@intel.com>]

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.