linux-kernel - Re: [PATCH v3] KVM: VMX: Enable Notify VM exit

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Date:   Sat, 26 Feb 2022 06:24:54 -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 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.