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

[Xiaoyao Li <xiaoyao.li@intel.com>]

On 10/15/2025 9:57 PM, Sean Christopherson wrote:
> On Wed, Oct 15, 2025, Xiaoyao Li wrote:
>> On 10/15/2025 7:10 AM, Sean Christopherson wrote:
>>> diff --git a/arch/x86/kvm/vmx/nested.c b/arch/x86/kvm/vmx/nested.c
>>> index 76271962cb70..f64a1eb241b6 100644
>>> --- 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.
>>> +		 */
>>
>> The i.e. part is confusing? It is exactly forwarding the EXITs to L1, while
>> it says L1 should never want or expect such an exit.
> 
> Gah, the comment is right, the code is wrong.

So the intent was to return false here? to let L0 handle the exit?

Then I have a question, why not implement it in 
nested_vmx_l0_wants_exit()? what's the reason and rule here?


> /facepalm
> 
> I even tried to explicitly test this, but I put the TDCALL and SEAMCALL in L1
> instead of L2.
> 
> diff --git a/tools/testing/selftests/kvm/x86/vmx_invalid_nested_guest_state.c b/tools/testing/selftests/kvm/x86/vmx_invalid_nested_guest_state.c
> index a100ee5f0009..1d7ef7d2d381 100644
> --- a/tools/testing/selftests/kvm/x86/vmx_invalid_nested_guest_state.c
> +++ b/tools/testing/selftests/kvm/x86/vmx_invalid_nested_guest_state.c
> @@ -23,11 +23,17 @@ static void l2_guest_code(void)
>                       : : [port] "d" (ARBITRARY_IO_PORT) : "rax");
>   }
>   
> +#define tdcall         ".byte 0x66,0x0f,0x01,0xcc"
> +#define seamcall       ".byte 0x66,0x0f,0x01,0xcf"
> +
>   static void l1_guest_code(struct vmx_pages *vmx_pages)
>   {
>   #define L2_GUEST_STACK_SIZE 64
>          unsigned long l2_guest_stack[L2_GUEST_STACK_SIZE];
>   
> +       TEST_ASSERT_EQ(kvm_asm_safe(tdcall), UD_VECTOR);
> +       TEST_ASSERT_EQ(kvm_asm_safe(seamcall), UD_VECTOR);
> +
>          GUEST_ASSERT(prepare_for_vmx_operation(vmx_pages));
>          GUEST_ASSERT(load_vmcs(vmx_pages));
>