Re: [PATCH] KVM: nVMX: Fix exception injection

[Wanpeng Li]

2017-06-05 20:07 GMT+08:00 Paolo Bonzini <pbonzini@xxxxxxxxxx>:
>
>
> On 03/06/2017 05:21, Wanpeng Li wrote:
>> Commit 0b6ac343fc (KVM: nVMX: Correct handling of exception injection) mentioned
>> that "KVM wants to inject page-faults which it got to the guest. This function
>> assumes it is called with the exit reason in vmcs02 being a #PF exception".
>> Commit e011c663 (KVM: nVMX: Check all exceptions for intercept during delivery to
>> L2) allows to check all exceptions for intercept during delivery to L2. However,
>> there is no guarantee the exit reason is exception currently, when there is an
>> external interrupt occurred on host, maybe a time interrupt for host which should
>> not be injected to guest, and somewhere queues an exception, then the function
>> nested_vmx_check_exception() will be called and the vmexit emulation codes will
>> try to emulate the "Acknowledge interrupt on exit" behavior, the warning is
>> triggered.
>>
>> This patch fixes it by confirming to inject exception to the guest when the exit
>> reason in vmcs02 is exception.
>
> I am confused.  On one hand, the comment originally "this is the only
> case in which KVM injects a #PF when L2 is running", but I'm not sure
> it's true.  For example, KVM could emulate a movs while running L2.  If
> the source is MMIO and the destination is a missing page, the original
> failure could be an EPT misconfig, but the access to the destination
> would cause a #PF in the guest (could be a nice testcase for
> kvm-unit-tests, BTW :)).
>
> On the other hand, why would you reuse to_vmx(vcpu)->exit_reason in
> nested_vmx_check_exception?  Would the following fix the bug:
>
> diff --git a/arch/x86/kvm/vmx.c b/arch/x86/kvm/vmx.c
> index 9b4b5d6dcd34..ca5d2b93385c 100644
> --- a/arch/x86/kvm/vmx.c
> +++ b/arch/x86/kvm/vmx.c
> @@ -2425,7 +2425,7 @@ static int nested_vmx_check_exception(struct
> kvm_vcpu *vcpu, unsigned nr)
>         if (!(vmcs12->exception_bitmap & (1u << nr)))
>                 return 0;
>
> -       nested_vmx_vmexit(vcpu, to_vmx(vcpu)->exit_reason,
> +       nested_vmx_vmexit(vcpu, EXIT_REASON_EXCEPTION_NMI,
>                           vmcs_read32(VM_EXIT_INTR_INFO),
>                           vmcs_readl(EXIT_QUALIFICATION));
>         return 1;
>

You are right.

Regards,
Wanpeng Li