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Message-ID: <70436892-0b4f-30c5-051b-2c86f7387101@linux.intel.com>
Date:   Thu, 21 Apr 2022 07:44:19 -0700
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Subject: Re: [PATCH v3 4/4] platform/x86: intel_tdx_attest: Add TDX Guest
 attestation interface driver
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To:     Isaku Yamahata <isaku.yamahata@gmail.com>
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,
        Hans de Goede <hdegoede@redhat.com>,
        Mark Gross <mgross@linux.intel.com>,
        "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>, linux-kernel@vger.kernel.org,
        platform-driver-x86@vger.kernel.org
References: <20220415220109.282834-1-sathyanarayanan.kuppuswamy@linux.intel.com>
 <20220415220109.282834-5-sathyanarayanan.kuppuswamy@linux.intel.com>
 <20220420012032.GA2224031@ls.amr.corp.intel.com>
 <dd4a2b16-397e-8866-0fd5-b5c5dfd453ab@linux.intel.com>
 <20220421070444.GB1423762@private.email.ne.jp>
From:   Sathyanarayanan Kuppuswamy 
        <sathyanarayanan.kuppuswamy@linux.intel.com>
In-Reply-To: <20220421070444.GB1423762@private.email.ne.jp>
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On 4/21/22 12:04 AM, Isaku Yamahata wrote:
> On Tue, Apr 19, 2022 at 06:26:43PM -0700,
> Sathyanarayanan Kuppuswamy <sathyanarayanan.kuppuswamy@linux.intel.com> wrote:
> 
>> On 4/19/22 6:20 PM, Isaku Yamahata wrote:
>>> If timeout occurs, the state of adev->tdquote_buf is unknown.  It's not safe
>>> to continue to using adev->tdquote_buf.  VMM would continue to processing
>>> getquote request with this buffer.  What if TDX_CMD_GEN_QUOTE is issued again,
>>> and tdquote_buf is re-used?
>>
>> This part is not clearly discussed in the specification. May be spec
>> should define some reasonable timeout and teardown details.
>>
>> Regarding not using this buffer again, what happens if we de-allocate
>> it on timeout and the host still updates it?
> 
> Until GET_QUOTE_IN_FLIGHT is cleared, the shared page is owned by VMM, TD
> attestation driver shouldn't reuse/free the pages.
> 
> In the case of this driver, I think of two options
> - don't timeout. wait for interrupt to arrive and check the shared GPA state.
> - allow timeout. When the next request comes, check the shared GPA state.
>    If it's still GET_QUOTE_IN_FLIGHT, return EBUSY.

Out of the above two options, I think option 1 is better. It is simpler
to serialize them using mutex compared to checking the shared buffer of
previous request.

> 
> It's possible for VMM to keep the shared GPA forever maliciously(DoS) or
> unintentionally due to bug.  TD can't do much about it.

I will add a note about it in commit log.

-- 
Sathyanarayanan Kuppuswamy
Linux Kernel Developer