Re: [PATCH v2] KVM: VMX: Enable Notify VM exit

[Xiaoyao Li]

On 6/3/2021 9:52 PM, Vitaly Kuznetsov wrote:
> Xiaoyao Li <xiaoyao.li@xxxxxxxxx> writes:
>
> > On 6/2/2021 6:31 PM, Vitaly Kuznetsov wrote:
> > > Tao Xu <tao3.xu@xxxxxxxxx> writes:
> > >
> > > > There are some cases that malicious virtual machines can cause CPU stuck
> > > > (event windows don't open up), e.g., infinite loop in microcode when
> > > > nested #AC (CVE-2015-5307). No event window obviously means no events,
> > > > e.g. NMIs, SMIs, and IRQs will all be blocked, may cause the related
> > > > hardware CPU can't be used by host or other VM.
> > > >
> > > > To resolve those cases, it can enable a notify VM exit if no event
> > > > window occur in VMX non-root mode for a specified amount of time
> > > > (notify window). Since CPU is first observed the risk of not causing
> > > > forward progress, after notify window time in a units of crystal clock,
> > > > Notify VM exit will happen. Notify VM exit can happen incident to delivery
> > > > of a vectored event.
> > > >
> > > > Expose a module param for configuring notify window, which is in unit of
> > > > crystal clock cycle.
> > > > - A negative value (e.g. -1) is to disable this feature.
> > > > - Make the default as 0. It is safe because an internal threshold is added
> > > > to notify window to ensure all the normal instructions being coverd.
> > > > - User can set it to a large value when they want to give more cycles to
> > > > wait for some reasons, e.g., silicon wrongly kill some normal instruction
> > > > due to internal threshold is too small.
> > > >
> > > > Notify VM exit is defined in latest Intel Architecture Instruction Set
> > > > Extensions Programming Reference, chapter 9.2.
> > > >
> > > > Co-developed-by: Xiaoyao Li <xiaoyao.li@xxxxxxxxx>
> > > > Signed-off-by: Xiaoyao Li <xiaoyao.li@xxxxxxxxx>
> > > > Signed-off-by: Tao Xu <tao3.xu@xxxxxxxxx>
> > > > ---
> > > >
> > > > Changelog:
> > > > v2:
> > > >        Default set notify window to 0, less than 0 to disable.
> > > >        Add more description in commit message.
> > >
> > > Sorry if this was already discussed, but in case of nested
> > > virtualization and when L1 also enables
> > > SECONDARY_EXEC_NOTIFY_VM_EXITING, shouldn't we just reflect NOTIFY exits
> > > during L2 execution to L1 instead of crashing the whole L1?
> >
> > yes. If we expose it to nested, it should reflect the Notify VM exit to
> > L1 when L1 enables it.
> >
> > But regarding nested, there are more things need to be discussed. e.g.,
> > 1) It has dependence between L0 and L1, for security consideration. When
> > L0 enables it, it shouldn't be turned off during L2 VM is running.
> >      a. Don't expose to L1 but enable for L1 when L2 VM is running.
> >      b. expose it to L1 and force it enabled.
>
> Could you please elaborate on the 'security' concern? 

I mean the case that if we expose this feature to L1 VMM, L1 VMM cannot 
en/dis-able this feature on its own purpose when L0 turns it on.

i.e., vmcs02.settings has to be (L0's | L1's)

otherwise L1 guest can escape by creating an nested guest and disabling it.

> My understanding
> that during L2 execution:
> If L0 enables the feature and L1 doesn't, vmexit goes to L0.
> If L1 enables the feature and L0 doesn't, vmexit goes to L1.

> If both L0 and L1 enable the feature, vmexit can probably (I didn't put
> enough though in it I'm afraid) go to the one which has smaller window.

It sounds reasonable.

> > 2) When expose it to L1, vmcs02.notify_window needs to be
> > min(L0.notify_window, L1.nofity_window)
> >
> > We don't deal with nested to make this Patch simple.
>
> Sure, I just wanted to check with you what's the future plan and if the
> behavior you introduce is desireable in nested case.