KVM: nVMX: WARN if subtly-impossible VMFUNC conditions occur

WARN and inject #UD when emulating VMFUNC for L2 if the function is
out-of-bounds or if VMFUNC is not enabled in vmcs12.  Neither condition
should occur in practice, as the CPU is supposed to prioritize the #UD
over VM-Exit for out-of-bounds input and KVM is supposed to enable
VMFUNC in vmcs02 if and only if it's enabled in vmcs12, but neither of
those dependencies is obvious.
Signed-off-by: Sean Christopherson <seanjc@google.com>
Message-Id: <20210609234235.1244004-15-seanjc@google.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

arch/x86/kvm/vmx/nested.c

@@ -5552,6 +5552,16 @@ static int handle_vmfunc(struct kvm_vcpu *vcpu)
 	}
 
 	vmcs12 = get_vmcs12(vcpu);
+
+	/*
+	 * #UD on out-of-bounds function has priority over VM-Exit, and VMFUNC
+	 * is enabled in vmcs02 if and only if it's enabled in vmcs12.
+	 */
+	if (WARN_ON_ONCE((function > 63) || !nested_cpu_has_vmfunc(vmcs12))) {
+		kvm_queue_exception(vcpu, UD_VECTOR);
+		return 1;
+	}
+
 	if (!(vmcs12->vm_function_control & BIT_ULL(function)))
 		goto fail;