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

[Isaku Yamahata]

On Wed, Jun 07, 2023 at 12:27:33PM -0700,
Dave Hansen <dave.hansen@xxxxxxxxx> 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@xxxxxxxxx>