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

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Date:   Thu, 8 Sep 2022 13:36:00 -0700
From:   Dave Hansen <dave.hansen@...el.com>
To:     Sathyanarayanan Kuppuswamy 
        <sathyanarayanan.kuppuswamy@...ux.intel.com>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>
Cc:     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>,
        "H . Peter Anvin" <hpa@...or.com>,
        "Kirill A . Shutemov" <kirill.shutemov@...ux.intel.com>,
        Tony Luck <tony.luck@...el.com>,
        Andi Kleen <ak@...ux.intel.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 v12 1/3] x86/tdx: Add TDX Guest attestation interface
 driver

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.