linux-kernel - Re: [PATCH v3 4/4] platform/x86: intel_tdx_attest: Add TDX Guest attestation interface driver

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Message-ID: <20220421070444.GB1423762@private.email.ne.jp>
Date:   Thu, 21 Apr 2022 00:04:44 -0700
From:   Isaku Yamahata <isaku.yamahata@...il.com>
To:     Sathyanarayanan Kuppuswamy 
        <sathyanarayanan.kuppuswamy@...ux.intel.com>
Cc:     Isaku Yamahata <isaku.yamahata@...il.com>,
        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,
        Hans de Goede <hdegoede@...hat.com>,
        Mark Gross <mgross@...ux.intel.com>,
        "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>, linux-kernel@...r.kernel.org,
        platform-driver-x86@...r.kernel.org
Subject: Re: [PATCH v3 4/4] platform/x86: intel_tdx_attest: Add TDX Guest
 attestation interface driver

On Tue, Apr 19, 2022 at 06:26:43PM -0700,
Sathyanarayanan Kuppuswamy <sathyanarayanan.kuppuswamy@...ux.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.

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.
-- 
Isaku Yamahata <isaku.yamahata@...il.com>