linux-kernel - Re: [PATCH v5 02/22] cc_platform: Add new attribute to prevent ACPI CPU hotplug

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Message-ID: <02f40786caee3ecf9b2bfd90c70317c282dd87e5.camel@intel.com>
Date:   Wed, 03 Aug 2022 21:20:32 +1200
From:   Kai Huang <kai.huang@...el.com>
To:     Binbin Wu <binbin.wu@...ux.intel.com>,
        Dave Hansen <dave.hansen@...el.com>,
        linux-kernel@...r.kernel.org, kvm@...r.kernel.org
Cc:     linux-acpi@...r.kernel.org, seanjc@...gle.com, pbonzini@...hat.com,
        len.brown@...el.com, tony.luck@...el.com,
        rafael.j.wysocki@...el.com, reinette.chatre@...el.com,
        dan.j.williams@...el.com, peterz@...radead.org, ak@...ux.intel.com,
        kirill.shutemov@...ux.intel.com,
        sathyanarayanan.kuppuswamy@...ux.intel.com,
        isaku.yamahata@...el.com, thomas.lendacky@....com,
        Tianyu.Lan@...rosoft.com, rdunlap@...radead.org, Jason@...c4.com,
        juri.lelli@...hat.com, mark.rutland@....com, frederic@...nel.org,
        yuehaibing@...wei.com, dongli.zhang@...cle.com
Subject: Re: [PATCH v5 02/22] cc_platform: Add new attribute to prevent ACPI
 CPU hotplug

On Wed, 2022-08-03 at 11:40 +0800, Binbin Wu wrote:
> host kernel is also not in TDX's TCB either, what would happen if kernel 
> doesn't
> do anything in case of buggy BIOS? How does TDX handle the case to 
> enforce the
> secure of TDs?

TDX doesn't support hot-add or hot-removal CPU from TDX' security perimeter at
runtime.  Even BIOS/kernel can ever bring up new CPUs at runtime, the new CPUs
cannot run within TDX's security domain, in which case TDX's security isn't
compromised.  If kernel schedules a TD to a new added CPU, then AFAICT the
behaviour is TDX module implementation specific but not architectural.  A
reasonable behaviour would be the TDENTER should refuse to run when the CPU
isn't verified by TDX during boot.

If any CPU is hot-removed, then the security's TDX isn't compromised, but TDX is
not guaranteed to functionally work anymore.

-- 
Thanks,
-Kai