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

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Message-ID: <d9328f94-a6a9-4d98-8ea7-9b9e22bc1db6@intel.com>
Date: Wed, 15 Oct 2025 11:11:01 +0800
From: Xiaoyao Li <xiaoyao.li@...el.com>
To: Chao Gao <chao.gao@...el.com>, Sean Christopherson <seanjc@...gle.com>
Cc: 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 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.

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