Date:   Mon, 28 Feb 2022 20:32:53 -0800
From:   Jim Mattson <jmattson@...gle.com>
To:     Xiaoyao Li <xiaoyao.li@...el.com>
Cc:     Paolo Bonzini <pbonzini@...hat.com>,
        Chenyi Qiang <chenyi.qiang@...el.com>,
        Sean Christopherson <seanjc@...gle.com>,
        Vitaly Kuznetsov <vkuznets@...hat.com>,
        Wanpeng Li <wanpengli@...cent.com>,
        Joerg Roedel <joro@...tes.org>, kvm@...r.kernel.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH v3] KVM: VMX: Enable Notify VM exit

On Mon, Feb 28, 2022 at 5:41 PM Xiaoyao Li <xiaoyao.li@...el.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@...el.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@...el.com> wrote:
> >>>>
> >>>> On 2/26/2022 12:53 PM, Jim Mattson wrote:
> >>>>> On Fri, Feb 25, 2022 at 8:25 PM Jim Mattson <jmattson@...gle.com> wrote:
> >>>>>>
> >>>>>> On Fri, Feb 25, 2022 at 8:07 PM Xiaoyao Li <xiaoyao.li@...el.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).

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