Re: [PATCH v2] KVM: VMX: Enable Notify VM exit

[Xiaoyao Li <xiaoyao.li@intel.com>]

On 9/10/2021 2:59 AM, Sean Christopherson wrote:
> On Tue, Sep 07, 2021, Xiaoyao Li wrote:
>> On 9/3/2021 12:36 AM, Sean Christopherson wrote:
>>> On Thu, Sep 02, 2021, Sean Christopherson wrote:
>>>> On Tue, Aug 03, 2021, Xiaoyao Li wrote:
>>>>> On 8/2/2021 11:46 PM, Sean Christopherson wrote:
>>>>>>>>> @@ -5642,6 +5653,31 @@ static int handle_bus_lock_vmexit(struct kvm_vcpu *vcpu)
>>>>>>>>>      	return 0;
>>>>>>>>>      }
>>>>>>>>> +static int handle_notify(struct kvm_vcpu *vcpu)
>>>>>>>>> +{
>>>>>>>>> +	unsigned long exit_qual = vmx_get_exit_qual(vcpu);
>>>>>>>>> +
>>>>>>>>> +	if (!(exit_qual & NOTIFY_VM_CONTEXT_INVALID)) {
>>>>>>>>
>>>>>>>> What does CONTEXT_INVALID mean?  The ISE doesn't provide any information whatsoever.
>>>>>>>
>>>>>>> It means whether the VM context is corrupted and not valid in the VMCS.
>>>>>>
>>>>>> Well that's a bit terrifying.  Under what conditions can the VM context become
>>>>>> corrupted?  E.g. if the context can be corrupted by an inopportune NOTIFY exit,
>>>>>> then KVM needs to be ultra conservative as a false positive could be fatal to a
>>>>>> guest.
>>>>>>
>>>>>
>>>>> Short answer is no case will set the VM_CONTEXT_INVALID bit.
>>>>
>>>> But something must set it, otherwise it wouldn't exist.
>>
>> For existing Intel silicon, no case will set it. Maybe in the future new
>> case will set it.
>>
>>> The condition(s) under
>>>> which it can be set matters because it affects how KVM should respond.  E.g. if
>>>> the guest can trigger VM_CONTEXT_INVALID at will, then we should probably treat
>>>> it as a shutdown and reset the VMCS.
>>>
>>> Oh, and "shutdown" would be relative to the VMCS, i.e. if L2 triggers a NOTIFY
>>> exit with VM_CONTEXT_INVALID then KVM shouldn't kill the entire VM.  The least
>>> awful option would probably be to synthesize a shutdown VM-Exit to L1.  That
>>> won't communicate to L1 that vmcs12 state is stale/bogus, but I don't see any way
>>> to handle that via an existing VM-Exit reason :-/
>>>
>>>> But if VM_CONTEXT_INVALID can occur if and only if there's a hardware/ucode
>>>> issue, then we can do:
>>>>
>>>> 	if (KVM_BUG_ON(exit_qual & NOTIFY_VM_CONTEXT_INVALID, vcpu->kvm))
>>>> 		return -EIO;
>>>>
>>>> Either way, to enable this by default we need some form of documentation that
>>>> describes what conditions lead to VM_CONTEXT_INVALID.
>>
>> I still don't know why the conditions lead to it matters. I think the
>> consensus is that once VM_CONTEXT_INVALID happens, the vcpu can no longer
>> run.
> 
> Yes, and no longer being able to run the vCPU is precisely the problem.  The
> condition(s) matters because if there's a possibility, however small, that enabling
> NOTIFY_WINDOW can kill a well-behaved guest then it absolutely cannot be enabled by
> default.

For now, no condition will set it. For future, I believe it will be set 
only for some fatal case. However, we cannot guarantee no silicon bug to 
break a well-behaved the guest. Maybe let's make it opt-in?

>> Either KVM_BUG_ON() or a specific EXIT to userspace should be OK?
> 
> Not if the VM_CONTEXT_INVALID happens while L2 is running.  If software can trigger
> VM_CONTEXT_INVALID at will, then killing the VM would open up the door to a
> malicious L2 killing L1 (which would be rather ironic since this is an anti-DoS
> feature).  IIUC, VM_CONTEXT_INVALID only means the current VMCS is garbage, thus
> an occurence while L2 is active means that vmcs02 is junk, but L1's state in vmcs01,
> vmcs12, etc... is still valid.
> 

Maybe we can kill the L2 when VM_CONTEXT_INVALID happens in L2.