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

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Message-ID: <20230607194721.GI2244082@ls.amr.corp.intel.com>
Date:   Wed, 7 Jun 2023 12:47:21 -0700
From:   Isaku Yamahata <isaku.yamahata@...il.com>
To:     Dave Hansen <dave.hansen@...el.com>
Cc:     Isaku Yamahata <isaku.yamahata@...il.com>,
        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 Wed, Jun 07, 2023 at 12:27:33PM -0700,
Dave Hansen <dave.hansen@...el.com> wrote:

> 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?

extable. From arch/x86/kvm/vmx/vmenter.S

.Lvmresume:
        vmresume
        jmp .Lvmfail

.Lvmlaunch:
        vmlaunch
        jmp .Lvmfail

        _ASM_EXTABLE(.Lvmresume, .Lfixup)
        _ASM_EXTABLE(.Lvmlaunch, .Lfixup)


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