Re: [PATCH v5 1/3] KVM: X86: Save&restore the triple fault request

Chenyi Qiang
2022-04-07T06:13:07Z

On 4/7/2022 5:15 AM, Sean Christopherson wrote:
> On Tue, Apr 05, 2022, Sean Christopherson wrote:
>> On Tue, Apr 05, 2022, Sean Christopherson wrote:
>>> On Fri, Mar 18, 2022, Chenyi Qiang wrote:
>>>> @@ -4976,6 +4980,9 @@ static int kvm_vcpu_ioctl_x86_set_vcpu_events(struct kvm_vcpu *vcpu,
>>>> }
>>>> }
>>>>
>>>> + if (events->flags & KVM_VCPUEVENT_TRIPLE_FAULT)
>>>> + kvm_make_request(KVM_REQ_TRIPLE_FAULT, vcpu);
>>>> +
>>>> kvm_make_request(KVM_REQ_EVENT, vcpu);
>>>
>>> Looks correct, but this really needs a selftest, at least for the SET path since
>>> the intent is to use that for the NOTIFY handling. Doesn't need to be super fancy,
>>> e.g. do port I/O from L2, inject a triple fault, and verify L1 sees the appropriate
>>> exit.
>>>
>>> Aha! And for the GET path, abuse KVM_X86_SET_MCE with CR4.MCE=0 to coerce KVM into
>>> making a KVM_REQ_TRIPLE_FAULT, that way there's no need to try and hit a timing
>>> window to intercept the request.
>>
>> Drat, I bet that MCE path means the WARN in nested_vmx_vmexit() can be triggered
>> by userspace. If so, this patch makes it really, really easy to hit, e.g. queue the
>> request while L2 is active, then do KVM_SET_NESTED_STATE to force an "exit" without
>> bouncing through kvm_check_nested_events().
>>
>> WARN_ON_ONCE(kvm_check_request(KVM_REQ_TRIPLE_FAULT, vcpu))
>>
>> I don't think SVM has a user-triggerable WARN, but the request should still be
>> dropped on forced exit from L2, e.g. I believe this is the correct fix:
>
> Confirmed the WARN can be triggered by abusing this patch, I'll get a patch out
> once I figure out why kvm/queue is broken.
>
> diff --git a/tools/testing/selftests/kvm/x86_64/state_test.c b/tools/testing/selftests/kvm/x86_64/state_test.c
> index 2e0a92da8ff5..b7faeae3dcc4 100644
> --- a/tools/testing/selftests/kvm/x86_64/state_test.c
> +++ b/tools/testing/selftests/kvm/x86_64/state_test.c
> @@ -210,6 +210,12 @@ int main(int argc, char *argv[])
> memset(&regs1, 0, sizeof(regs1));
> vcpu_regs_get(vm, VCPU_ID, &regs1);
>
> + if (stage == 6) {
> + state->events.flags |= 0x20;
> + vcpu_events_set(vm, VCPU_ID, &state->events);
> + vcpu_nested_state_set(vm, VCPU_ID, &state->nested, false);
> + }
> +
> kvm_vm_release(vm);
>
> /* Restore state in a new VM. */

Also verified the WARN with this. Then, is it still necessary to add an
individual selftest about the working flow of save/restore triple fault
event?

>