linux-kernel - Re: [PATCH] KVM: VMX: Inject #UD if guest tries to execute SEAMCALL or TDCALL

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Message-ID: <aO-fqYO_vmwtCn8l@google.com>
Date: Wed, 15 Oct 2025 06:20:41 -0700
From: Sean Christopherson <seanjc@...gle.com>
To: Xiaoyao Li <xiaoyao.li@...el.com>
Cc: Chao Gao <chao.gao@...el.com>, Paolo Bonzini <pbonzini@...hat.com>, kvm@...r.kernel.org, 
	linux-kernel@...r.kernel.org, Kai Huang <kai.huang@...el.com>, 
	Rick Edgecombe <rick.p.edgecombe@...el.com>
Subject: Re: [PATCH] KVM: VMX: Inject #UD if guest tries to execute SEAMCALL
 or TDCALL

On Wed, Oct 15, 2025, Xiaoyao Li wrote:
> On 10/15/2025 9:13 AM, Chao Gao wrote:
> > > --- a/arch/x86/kvm/vmx/nested.c
> > > +++ b/arch/x86/kvm/vmx/nested.c
> > > @@ -6728,6 +6728,14 @@ static bool nested_vmx_l1_wants_exit(struct kvm_vcpu *vcpu,
> > > 	case EXIT_REASON_NOTIFY:
> > > 		/* Notify VM exit is not exposed to L1 */
> > > 		return false;
> > > +	case EXIT_REASON_SEAMCALL:
> > > +	case EXIT_REASON_TDCALL:
> > > +		/*
> > > +		 * SEAMCALL and TDCALL unconditionally VM-Exit, but aren't
> > > +		 * virtualized by KVM for L1 hypervisors, i.e. L1 should
> > > +		 * never want or expect such an exit.
> > > +		 */
> > > +		return true;
> > > 	default:
> > > 		return true;
> > > 	}
> > > diff --git a/arch/x86/kvm/vmx/tdx.c b/arch/x86/kvm/vmx/tdx.c
> > > index 097304bf1e1d..7326c68f9909 100644
> > > --- a/arch/x86/kvm/vmx/tdx.c
> > > +++ b/arch/x86/kvm/vmx/tdx.c
> > > @@ -2127,6 +2127,9 @@ int tdx_handle_exit(struct kvm_vcpu *vcpu, fastpath_t fastpath)
> > > 		return tdx_emulate_mmio(vcpu);
> > > 	case EXIT_REASON_EPT_VIOLATION:
> > > 		return tdx_handle_ept_violation(vcpu);
> > > +	case EXIT_REASON_SEAMCALL:
> > > +		kvm_queue_exception(vcpu, UD_VECTOR);
> > 
> > Emm, the host cannot inject exceptions to TDX guests. Right?
> 
> Right.

Oh, yeah, duh.

> I also tested in TD guest. Actually, the TDX module injects #UD to TD guest
> on SEAMCALL exit.
> 
> The behavior is documented in 11.7.1 "Unconditionally Blocked Instructions"
> of TDX module base spec:
> 
>   Instructions that causes a #UD Unconditionally
> 
>   - SEAMCALL, SEAMRET

Nice!  I'll add a WARN_ON_ONCE() if KVM sees a SEAMCALL TD-Exit.  The SEAMRET
documentation in the TDX-Module spec is amusing since the CPU is supposed to
generate a #UD for SEAMRET, i.e. it's not like the TDX-Module has any choice in
the matter :-)

Thanks!