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

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Date:   Fri, 10 Sep 2021 15:39:03 +0800
From:   Xiaoyao Li <xiaoyao.li@...el.com>
To:     Sean Christopherson <seanjc@...gle.com>
Cc:     Chenyi Qiang <chenyi.qiang@...el.com>, Tao Xu <tao3.xu@...el.com>,
        pbonzini@...hat.com, vkuznets@...hat.com, wanpengli@...cent.com,
        jmattson@...gle.com, joro@...tes.org, tglx@...utronix.de,
        mingo@...hat.com, bp@...en8.de, hpa@...or.com, x86@...nel.org,
        kvm@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2] KVM: VMX: Enable Notify VM exit

On 9/10/2021 2:47 AM, Sean Christopherson wrote:
> On Tue, Sep 07, 2021, Xiaoyao Li wrote:
>> On 9/3/2021 12:29 AM, Sean Christopherson wrote:
>>>> After syncing internally, we know that the internal threshold is not
>>>> architectural but a model-specific value. It will be published in some place
>>>> in future.
>>>
>>> Any chance it will also be discoverable, e.g. via an MSR?
>>
>> I also hope we can expose it via MSR. If not, we can maintain a table per
>> FMS in KVM to get the internal threshold. However, per FMS info is not
>> friendly to be virtualized (when we are going to enable the nested support).
> 
> Yeah, FMS is awful.  If the built-in buffer isn't discoverable, my vote is to
> assume the worst, i.e. a built-in buffer of '0', and have the notify_window
> param default to a safe value, e.g. 25k or maybe even 150k (to go above what the
> hardware folks apparently deemed safe for SPR).  It's obviously not idea, but
> it's better than playing FMS guessing games.
> 
>> I'll try to persuade internal to expose it via MSR, but I guarantee nothing.
> 
> ...
> 
>>> On a related topic, this needs tests.  One thought would be to stop unconditionally
>>> intercepting #AC if NOTIFY_WINDOW is enabled, and then have the test set up the
>>> infinite #AC vectoring scenario.
>>>
>>
>> yes, we have already tested with this case with notify_window set to 0. No
>> false positive.
> 
> Can you send a selftest or kvm-unit-test?
> 

Actually we implement the attacking case of CVE-2015-5307 with 
kvm-unit-test, while manually disabling the intercept of #AC.

First, it requires modification of KVM that only posting the 
kvm-unit-test doesn't help.

Second, release the attacking case is not the correct action.