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

[Jim Mattson]

On Wed, Jul 19, 2017 at 7:31 PM, Wanpeng Li <kernellwp@xxxxxxxxx> wrote:
> Hi Jim,
> 2017-07-19 2:47 GMT+08:00 Jim Mattson <jmattson@xxxxxxxxxx>:
>> Why do we expect the VM_EXIT_INTR_INFO and EXIT_QUALIFICATION fields
>> of the VMCS to have the correct values for the injected exception?
>
> Good point, I think we should synthesize VM_EXIT_INTR_INFO and
> EXIT_QUALIFICATION manually, I will post a patch for it. Btw, how
> about setting EXIT_QULIFICATION to vcpu->arch.cr2 for the page fault
> exception and 0 for other exceptions?

>From the SDM, section 27.1:

    If an event causes a VM exit directly, it does not update
architectural state as it would have if it had it not caused the VM
exit:
      - A debug exception does not update DR6, DR7.GD, or
IA32_DEBUGCTL.LBR. (Information about the nature of the debug
exception is saved in the exit qualification field.)
      - A page fault does not update CR2. (The linear address causing
the page fault is saved in the exit-qualification field.)

This means that vcpu->arch.cr2 should not be set at this point for a
#PF injection (and vcpu->arch.dr6 should not be set at this point for
a #DB injection). For all other exceptions, yes, the exit
qualification should be cleared.

>
> Regards,
> Wanpeng Li
>
>>
>> On Mon, Jun 5, 2017 at 5:19 AM, Wanpeng Li <kernellwp@xxxxxxxxx> wrote:
>>> 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;

Here, we should consult vmcs12->page_fault_error_code_mask and
vmcs12->page_fault_error_code_match when nr==PF_VECTOR, as in
nested_vmx_is_page_fault_vmexit().

>>>>
>>>> -       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