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

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Date:   Mon, 7 Jun 2021 17:23:35 +0800
From:   Xiaoyao Li <xiaoyao.li@...el.com>
To:     Vitaly Kuznetsov <vkuznets@...hat.com>, Tao Xu <tao3.xu@...el.com>
Cc:     x86@...nel.org, kvm@...r.kernel.org, linux-kernel@...r.kernel.org,
        pbonzini@...hat.com, seanjc@...gle.com, wanpengli@...cent.com,
        jmattson@...gle.com, joro@...tes.org, tglx@...utronix.de,
        mingo@...hat.com, bp@...en8.de, hpa@...or.com
Subject: Re: [PATCH v2] KVM: VMX: Enable Notify VM exit

On 6/3/2021 9:52 PM, Vitaly Kuznetsov wrote:
> Xiaoyao Li <xiaoyao.li@...el.com> writes:
> 
>> On 6/2/2021 6:31 PM, Vitaly Kuznetsov wrote:
>>> Tao Xu <tao3.xu@...el.com> 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@...el.com>
>>>> Signed-off-by: Xiaoyao Li <xiaoyao.li@...el.com>
>>>> Signed-off-by: Tao Xu <tao3.xu@...el.com>
>>>> ---
>>>>
>>>> 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.
>