Messages in this thread | | | Date | Tue, 1 Mar 2022 13:30:21 +0800 | Subject | Re: [PATCH v3] KVM: VMX: Enable Notify VM exit | From | Xiaoyao Li <> |
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On 3/1/2022 12:32 PM, Jim Mattson wrote: > On Mon, Feb 28, 2022 at 5:41 PM Xiaoyao Li <xiaoyao.li@intel.com> wrote: >> >> On 2/28/2022 10:30 PM, Jim Mattson wrote: >>> On Sun, Feb 27, 2022 at 11:10 PM Xiaoyao Li <xiaoyao.li@intel.com> wrote: >>>> >>>> On 2/26/2022 10:24 PM, Jim Mattson wrote: >>>>> On Fri, Feb 25, 2022 at 10:24 PM Xiaoyao Li <xiaoyao.li@intel.com> wrote: >>>>>> >>>>>> On 2/26/2022 12:53 PM, Jim Mattson wrote: >>>>>>> On Fri, Feb 25, 2022 at 8:25 PM Jim Mattson <jmattson@google.com> wrote: >>>>>>>> >>>>>>>> On Fri, Feb 25, 2022 at 8:07 PM Xiaoyao Li <xiaoyao.li@intel.com> wrote: >>>>>>>>> >>>>>>>>> On 2/25/2022 11:13 PM, Paolo Bonzini wrote: >>>>>>>>>> On 2/25/22 16:12, Xiaoyao Li wrote: >>>>>>>>>>>>>> >>>>>>>>>>>>> >>>>>>>>>>>>> I don't like the idea of making things up without notifying userspace >>>>>>>>>>>>> that this is fictional. How is my customer running nested VMs supposed >>>>>>>>>>>>> to know that L2 didn't actually shutdown, but L0 killed it because the >>>>>>>>>>>>> notify window was exceeded? If this information isn't reported to >>>>>>>>>>>>> userspace, I have no way of getting the information to the customer. >>>>>>>>>>>> >>>>>>>>>>>> Then, maybe a dedicated software define VM exit for it instead of >>>>>>>>>>>> reusing triple fault? >>>>>>>>>>>> >>>>>>>>>>> >>>>>>>>>>> Second thought, we can even just return Notify VM exit to L1 to tell >>>>>>>>>>> L2 causes Notify VM exit, even thought Notify VM exit is not exposed >>>>>>>>>>> to L1. >>>>>>>>>> >>>>>>>>>> That might cause NULL pointer dereferences or other nasty occurrences. >>>>>>>>> >>>>>>>>> IMO, a well written VMM (in L1) should handle it correctly. >>>>>>>>> >>>>>>>>> L0 KVM reports no Notify VM Exit support to L1, so L1 runs without >>>>>>>>> setting Notify VM exit. If a L2 causes notify_vm_exit with >>>>>>>>> invalid_vm_context, L0 just reflects it to L1. In L1's view, there is no >>>>>>>>> support of Notify VM Exit from VMX MSR capability. Following L1 handler >>>>>>>>> is possible: >>>>>>>>> >>>>>>>>> a) if (notify_vm_exit available & notify_vm_exit enabled) { >>>>>>>>> handle in b) >>>>>>>>> } else { >>>>>>>>> report unexpected vm exit reason to userspace; >>>>>>>>> } >>>>>>>>> >>>>>>>>> b) similar handler like we implement in KVM: >>>>>>>>> if (!vm_context_invalid) >>>>>>>>> re-enter guest; >>>>>>>>> else >>>>>>>>> report to userspace; >>>>>>>>> >>>>>>>>> c) no Notify VM Exit related code (e.g. old KVM), it's treated as >>>>>>>>> unsupported exit reason >>>>>>>>> >>>>>>>>> As long as it belongs to any case above, I think L1 can handle it >>>>>>>>> correctly. Any nasty occurrence should be caused by incorrect handler in >>>>>>>>> L1 VMM, in my opinion. >>>>>>>> >>>>>>>> Please test some common hypervisors (e.g. ESXi and Hyper-V). >>>>>>> >>>>>>> I took a look at KVM in Linux v4.9 (one of our more popular guests), >>>>>>> and it will not handle this case well: >>>>>>> >>>>>>> if (exit_reason < kvm_vmx_max_exit_handlers >>>>>>> && kvm_vmx_exit_handlers[exit_reason]) >>>>>>> return kvm_vmx_exit_handlers[exit_reason](vcpu); >>>>>>> else { >>>>>>> WARN_ONCE(1, "vmx: unexpected exit reason 0x%x\n", exit_reason); >>>>>>> kvm_queue_exception(vcpu, UD_VECTOR); >>>>>>> return 1; >>>>>>> } >>>>>>> >>>>>>> At least there's an L1 kernel log message for the first unexpected >>>>>>> NOTIFY VM-exit, but after that, there is silence. Just a completely >>>>>>> inexplicable #UD in L2, assuming that L2 is resumable at this point. >>>>>> >>>>>> At least there is a message to tell L1 a notify VM exit is triggered in >>>>>> L2. Yes, the inexplicable #UD won't be hit unless L2 triggers Notify VM >>>>>> exit with invalid_context, which is malicious to L0 and L1. >>>>> >>>>> There is only an L1 kernel log message *the first time*. That's not >>>>> good enough. And this is just one of the myriad of possible L1 >>>>> hypervisors. >>>>> >>>>>> If we use triple_fault (i.e., shutdown), then no info to tell L1 that >>>>>> it's caused by Notify VM exit with invalid context. Triple fault needs >>>>>> to be extended and L1 kernel needs to be enlightened. It doesn't help >>>>>> old guest kernel. >>>>>> >>>>>> If we use Machine Check, it's somewhat same inexplicable to L2 unless >>>>>> it's enlightened. But it doesn't help old guest kernel. >>>>>> >>>>>> Anyway, for Notify VM exit with invalid context from L2, I don't see a >>>>>> good solution to tell L1 VMM it's a "Notify VM exit with invalid context >>>>>> from L2" and keep all kinds of L1 VMM happy, especially for those with >>>>>> old kernel versions. >>>>> >>>>> I agree that there is no way to make every conceivable L1 happy. >>>>> That's why the information needs to be surfaced to the L0 userspace. I >>>>> contend that any time L0 kvm violates the architectural specification >>>>> in its emulation of L1 or L2, the L0 userspace *must* be informed. >>>> >>>> We can make the design to exit to userspace on notify vm exit >>>> unconditionally with exit_qualification passed, then userspace can take >>>> the same action like what this patch does in KVM that >>>> >>>> - re-enter guest when context_invalid is false; >>>> - stop running the guest if context_invalid is true; (userspace can >>>> definitely re-enter the guest in this case, but it needs to take the >>>> fall on this) >>>> >>>> Then, for nested case, L0 needs to enable it transparently for L2 if >>>> this feature is enabled for L1 guest (the reason as we all agreed that >>>> cannot allow L1 to escape just by creating a L2). Then what should KVM >>>> do when notify vm exit from L2? >>>> >>>> - Exit to L0 userspace on L2's notify vm exit. L0 userspace takes the >>>> same action: >>>> - re-enter if context-invalid is false; >>>> - kill L1 if context-invalid is true; (I don't know if there is any >>>> interface for L0 userspace to kill L2). Then it opens the potential door >>>> for malicious user to kill L1 by creating a L2 to trigger fatal notify >>>> vm exit. If you guys accept it, we can implement in this way. >>>> >>>> >>>> in conclusion, we have below solution: >>>> >>>> 1. Take this patch as is. The drawback is L1 VMM receives a triple_fault >>>> from L2 when L2 triggers notify vm exit with invalid context. Neither of >>>> L1 VMM, L1 userspace, nor L2 kernel know it's caused due to notify vm >>>> exit. There is only kernel log in L0, which seems not accessible for L1 >>>> user or L2 guest. >>> >>> You are correct on that last point, and I feel that I cannot stress it >>> enough. In a typical environment, the L0 kernel log is only available >>> to the administrator of the L0 host. >>> >>>> 2. a) Inject notify vm exit back to L1 if L2 triggers notify vm exit >>>> with invalid context. The drawback is, old L1 hypervisor is not >>>> enlightened of it and maybe misbehave on it. >>>> >>>> b) Inject a synthesized SHUTDOWN exit to L1, with additional info to >>>> tell it's caused by fatal notify vm exit from L2. It has the same >>>> drawback that old hypervisor has no idea of it and maybe misbehave on it. >>>> >>>> 3. Exit to L0 usersapce unconditionally no matter it's caused from L1 or >>>> L2. Then it may open the door for L1 user to kill L1. >>>> >>>> Do you have any better solution other than above? If no, we need to pick> >> one from above though it cannot make everyone happy. >>> >>> Yes, I believe I have a better solution. We obviously need an API for >>> userspace to synthesize a SHUTDOWN event for a vCPU. >> >> Can you elaborate on it? Do you mean userspace to inject a synthesized >> SHUTDOWN to guest? If so, I have no idea how it will work. > > It can probably be implemented as an extension of KVM_SET_VCPU_EVENTS > that invokes kvm_make_request(KVM_REQ_TRIPLE_FAULT).
Then, you mean
1. notify vm exit from guest; 2. exit to userspace on notify vm exit; 3. a. if context_invalid, inject SHUTDOWN to vcpu from userspace to request KVM_REQ_TRIPLE_FAULT; goto step 4; b. if !context_invalid, re-run vcpu; no step 4 and 5; 4. exit to userspace again with KVM_EXIT_SHUTDOWN due to triple fault; 5. userspace stop running the vcpu/VM
Then why not handle it as KVM_EXIT_SHUTDOWN directly in 3.a ? I don't get the point of userspace to inject TRIPLE_FAULT to KVM.
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