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

[Isaku Yamahata]

On Wed, Jun 07, 2023 at 07:24:23AM -0700,
Dave Hansen <dave.hansen@xxxxxxxxx> wrote:

> On 6/4/23 07:27, Kai Huang wrote:
> > TDX introduces a new CPU mode: Secure Arbitration Mode (SEAM).  This
> > mode runs only the TDX module itself or other code to load the TDX
> > module.
> > 
> > The host kernel communicates with SEAM software via a new SEAMCALL
> > instruction.  This is conceptually similar to a guest->host hypercall,
> > except it is made from the host to SEAM software instead.  The TDX
> > module establishes a new SEAMCALL ABI which allows the host to
> > initialize the module and to manage VMs.
> > 
> > Add infrastructure to make SEAMCALLs.  The SEAMCALL ABI is very similar
> > to the TDCALL ABI and leverages much TDCALL infrastructure.
> > 
> > SEAMCALL instruction causes #GP when TDX isn't BIOS enabled, and #UD
> > when CPU is not in VMX operation.  Currently, only KVM code mocks with
> 
> "mocks"?  Did you mean "mucks"?
> 
> > 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.
-- 
Isaku Yamahata <isaku.yamahata@xxxxxxxxx>