kvm: nVMX: Enforce cpl=0 for VMX instructions

VMX instructions executed inside a L1 VM will always trigger a VM exit
even when executed with cpl 3. This means we must perform the
privilege check in software.

Fixes: 70f3aac("kvm: nVMX: Remove superfluous VMX instruction fault checks")
Cc: stable@vger.kernel.org
Signed-off-by: Felix Wilhelm <fwilhelm@google.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

arch/x86/kvm/vmx.c

@@ -7905,6 +7905,12 @@ static int handle_vmon(struct kvm_vcpu *vcpu)
 		return 1;
 	}
 
+	/* CPL=0 must be checked manually. */
+	if (vmx_get_cpl(vcpu)) {
+		kvm_queue_exception(vcpu, UD_VECTOR);
+		return 1;
+	}
+
 	if (vmx->nested.vmxon) {
 		nested_vmx_failValid(vcpu, VMXERR_VMXON_IN_VMX_ROOT_OPERATION);
 		return kvm_skip_emulated_instruction(vcpu);
@@ -7964,6 +7970,11 @@ static int handle_vmon(struct kvm_vcpu *vcpu)
  */
 static int nested_vmx_check_permission(struct kvm_vcpu *vcpu)
 {
+	if (vmx_get_cpl(vcpu)) {
+		kvm_queue_exception(vcpu, UD_VECTOR);
+		return 0;
+	}
+
 	if (!to_vmx(vcpu)->nested.vmxon) {
 		kvm_queue_exception(vcpu, UD_VECTOR);
 		return 0;
@@ -8283,7 +8294,7 @@ static int handle_vmread(struct kvm_vcpu *vcpu)
 		if (get_vmx_mem_address(vcpu, exit_qualification,
 				vmx_instruction_info, true, &gva))
 			return 1;
-		/* _system ok, as hardware has verified cpl=0 */
+		/* _system ok, nested_vmx_check_permission has verified cpl=0 */
 		kvm_write_guest_virt_system(&vcpu->arch.emulate_ctxt, gva,
 			     &field_value, (is_long_mode(vcpu) ? 8 : 4), NULL);
 	}
@@ -8448,7 +8459,7 @@ static int handle_vmptrst(struct kvm_vcpu *vcpu)
 	if (get_vmx_mem_address(vcpu, exit_qualification,
 			vmx_instruction_info, true, &vmcs_gva))
 		return 1;
-	/* ok to use *_system, as hardware has verified cpl=0 */
+	/* *_system ok, nested_vmx_check_permission has verified cpl=0 */
 	if (kvm_write_guest_virt_system(&vcpu->arch.emulate_ctxt, vmcs_gva,
 				 (void *)&to_vmx(vcpu)->nested.current_vmptr,
 				 sizeof(u64), &e)) {