Date:   Mon, 28 Feb 2022 06:30:15 -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 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. In addition, to
avoid breaking legacy userspace, the NOTIFY VM-exit should be opt-in.

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