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

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Date:   Thu, 7 Apr 2022 14:12:59 +0800
From:   Chenyi Qiang <chenyi.qiang@...el.com>
To:     Sean Christopherson <seanjc@...gle.com>
Cc:     Paolo Bonzini <pbonzini@...hat.com>,
        Vitaly Kuznetsov <vkuznets@...hat.com>,
        Wanpeng Li <wanpengli@...cent.com>,
        Jim Mattson <jmattson@...gle.com>,
        Joerg Roedel <joro@...tes.org>,
        Xiaoyao Li <xiaoyao.li@...el.com>, kvm@...r.kernel.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH v5 1/3] KVM: X86: Save&restore the triple fault request



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?

>