Messages in this thread | | | Subject | Re: [PATCH v2] KVM: VMX: Enable Notify VM exit | From | Xiaoyao Li <> | Date | Mon, 13 Sep 2021 10:58:20 +0800 |
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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.
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