Received-SPF: pass (google.com: domain of linux-kernel-owner@vger.kernel.org designates 2620:137:e000::1:20 as permitted sender) client-ip=2620:137:e000::1:20;
Message-ID: <ac10ec37-91c8-031e-b3d3-843eaf28f0ee@intel.com>
Date:   Thu, 8 Sep 2022 13:36:00 -0700
MIME-Version: 1.0
User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:91.0) Gecko/20100101
 Thunderbird/91.11.0
Subject: Re: [PATCH v12 1/3] x86/tdx: Add TDX Guest attestation interface
 driver
Content-Language: en-US
To:     Sathyanarayanan Kuppuswamy 
        <sathyanarayanan.kuppuswamy@linux.intel.com>,
        Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Cc:     Thomas Gleixner <tglx@linutronix.de>,
        Ingo Molnar <mingo@redhat.com>, Borislav Petkov <bp@alien8.de>,
        Dave Hansen <dave.hansen@linux.intel.com>, x86@kernel.org,
        Shuah Khan <shuah@kernel.org>,
        "H . Peter Anvin" <hpa@zytor.com>,
        "Kirill A . Shutemov" <kirill.shutemov@linux.intel.com>,
        Tony Luck <tony.luck@intel.com>,
        Andi Kleen <ak@linux.intel.com>,
        Kai Huang <kai.huang@intel.com>,
        Wander Lairson Costa <wander@redhat.com>,
        Isaku Yamahata <isaku.yamahata@gmail.com>,
        marcelo.cerri@canonical.com, tim.gardner@canonical.com,
        khalid.elmously@canonical.com, philip.cox@canonical.com,
        linux-kernel@vger.kernel.org, linux-kselftest@vger.kernel.org,
        linux-doc@vger.kernel.org
References: <20220908002723.923241-1-sathyanarayanan.kuppuswamy@linux.intel.com>
 <20220908002723.923241-2-sathyanarayanan.kuppuswamy@linux.intel.com>
 <Yxl+PE4A+WUfQ7bl@kroah.com>
 <6cf407ed-95c7-0db4-d581-b85efad13239@linux.intel.com>
From:   Dave Hansen <dave.hansen@intel.com>
In-Reply-To: <6cf407ed-95c7-0db4-d581-b85efad13239@linux.intel.com>
Content-Type: text/plain; charset=UTF-8
Content-Transfer-Encoding: 7bit
Precedence: bulk

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.