KVM: VMX: check descriptor table exits on instruction emulation

KVM emulates UMIP on hardware that doesn't support it by setting the
'descriptor table exiting' VM-execution control and performing
instruction emulation. When running nested, this emulation is broken as
KVM refuses to emulate L2 instructions by default.

Correct this regression by allowing the emulation of descriptor table
instructions if L1 hasn't requested 'descriptor table exiting'.

Fixes: 07721fe ("KVM: nVMX: Don't emulate instructions in guest mode")
Reported-by: Jan Kiszka <jan.kiszka@web.de>
Cc: stable@vger.kernel.org
Cc: Paolo Bonzini <pbonzini@redhat.com>
Cc: Jim Mattson <jmattson@google.com>
Signed-off-by: Oliver Upton <oupton@google.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

arch/x86/kvm/vmx/vmx.c

@@ -7177,6 +7177,7 @@ static int vmx_check_intercept_io(struct kvm_vcpu *vcpu,
 	else
 		intercept = nested_vmx_check_io_bitmaps(vcpu, port, size);
 
+	/* FIXME: produce nested vmexit and return X86EMUL_INTERCEPTED.  */
 	return intercept ? X86EMUL_UNHANDLEABLE : X86EMUL_CONTINUE;
 }
 
@@ -7206,6 +7207,20 @@ static int vmx_check_intercept(struct kvm_vcpu *vcpu,
 	case x86_intercept_outs:
 		return vmx_check_intercept_io(vcpu, info);
 
+	case x86_intercept_lgdt:
+	case x86_intercept_lidt:
+	case x86_intercept_lldt:
+	case x86_intercept_ltr:
+	case x86_intercept_sgdt:
+	case x86_intercept_sidt:
+	case x86_intercept_sldt:
+	case x86_intercept_str:
+		if (!nested_cpu_has2(vmcs12, SECONDARY_EXEC_DESC))
+			return X86EMUL_CONTINUE;
+
+		/* FIXME: produce nested vmexit and return X86EMUL_INTERCEPTED.  */
+		break;
+
 	/* TODO: check more intercepts... */
 	default:
 		break;