linux-kernel - Re: [PATCH v11 05/20] x86/virt/tdx: Add SEAMCALL infrastructure

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Message-ID: <f7ef157e-8f26-8d7b-a9b8-cb8de7f7aa2b@intel.com>
Date:   Wed, 7 Jun 2023 12:27:33 -0700
From:   Dave Hansen <dave.hansen@...el.com>
To:     Isaku Yamahata <isaku.yamahata@...il.com>
Cc:     Kai Huang <kai.huang@...el.com>, linux-kernel@...r.kernel.org,
        kvm@...r.kernel.org, linux-mm@...ck.org,
        kirill.shutemov@...ux.intel.com, tony.luck@...el.com,
        peterz@...radead.org, tglx@...utronix.de, seanjc@...gle.com,
        pbonzini@...hat.com, david@...hat.com, dan.j.williams@...el.com,
        rafael.j.wysocki@...el.com, ying.huang@...el.com,
        reinette.chatre@...el.com, len.brown@...el.com, ak@...ux.intel.com,
        isaku.yamahata@...el.com, chao.gao@...el.com,
        sathyanarayanan.kuppuswamy@...ux.intel.com, bagasdotme@...il.com,
        sagis@...gle.com, imammedo@...hat.com
Subject: Re: [PATCH v11 05/20] x86/virt/tdx: Add SEAMCALL infrastructure

On 6/7/23 11:53, Isaku Yamahata wrote:
>>> VMX enabling, and KVM is the only user of TDX.  This implementation
>>> chooses to make KVM itself responsible for enabling VMX before using
>>> TDX and let the rest of the kernel stay blissfully unaware of VMX.
>>>
>>> The current TDX_MODULE_CALL macro handles neither #GP nor #UD.  The
>>> kernel would hit Oops if SEAMCALL were mistakenly made w/o enabling VMX
>>> first.  Architecturally, there is no CPU flag to check whether the CPU
>>> is in VMX operation.  Also, if a BIOS were buggy, it could still report
>>> valid TDX private KeyIDs when TDX actually couldn't be enabled.
>> I'm not sure this is a great justification.  If the BIOS is lying to the
>> OS, we _should_ oops.
>>
>> How else can this happen other than silly kernel bugs.  It's OK to oops
>> in the face of silly kernel bugs.
> TDX KVM + reboot can hit #UD.  On reboot, VMX is disabled (VMXOFF) via
> syscore.shutdown callback.  However, guest TD can be still running to issue
> SEAMCALL resulting in #UD.
> 
> Or we can postpone the change and make the TDX KVM patch series carry a patch
> for it.

How does the existing KVM use of VMLAUNCH/VMRESUME avoid that problem?