linux-kernel - Re: [PATCH] x86/kvm/nVMCS: fix VMCLEAR when Enlightened VMCS is in use

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Date:   Tue, 25 Jun 2019 14:18:05 +0300
From:   Liran Alon <liran.alon@...cle.com>
To:     Vitaly Kuznetsov <vkuznets@...hat.com>
Cc:     kvm@...r.kernel.org, linux-kernel@...r.kernel.org,
        Paolo Bonzini <pbonzini@...hat.com>,
        Radim Krčmář <rkrcmar@...hat.com>
Subject: Re: [PATCH] x86/kvm/nVMCS: fix VMCLEAR when Enlightened VMCS is in
 use



> On 25 Jun 2019, at 14:15, Vitaly Kuznetsov <vkuznets@...hat.com> wrote:
> 
> Liran Alon <liran.alon@...cle.com> writes:
> 
>>> On 25 Jun 2019, at 11:51, Vitaly Kuznetsov <vkuznets@...hat.com> wrote:
>>> 
>>> Liran Alon <liran.alon@...cle.com> writes:
>>> 
>>>>> On 24 Jun 2019, at 16:30, Vitaly Kuznetsov <vkuznets@...hat.com> wrote:
>>>>> 
>>>>> 
>>>>> +bool nested_enlightened_vmentry(struct kvm_vcpu *vcpu, u64 *evmptr)
>>>> 
>>>> I prefer to rename evmptr to evmcs_ptr. I think it’s more readable and sufficiently short.
>>>> In addition, I think you should return either -1ull or assist_page.current_nested_vmcs.
>>>> i.e. Don’t return evmcs_ptr by pointer but instead as a return-value
>>>> and get rid of the bool.
>>> 
>>> Actually no, sorry, I'm having second thoughts here: in handle_vmclear()
>>> we don't care about the value of evmcs_ptr, we only want to check that
>>> enlightened vmentry bit is enabled in assist page. If we switch to
>>> checking evmcs_ptr against '-1', for example, we will make '-1' a magic
>>> value which is not in the TLFS. Windows may decide to use it for
>>> something else - and we will get a hard-to-debug bug again.
>> 
>> I’m not sure I understand.
>> You are worried that when guest have setup a valid assist-page and set
>> enlighten_vmentry to true,
>> that assist_page.current_nested_vmcs can be -1ull and still be considered a valid eVMCS?
>> I don't think that's reasonable.
> 
> No, -1ull is not a valid eVMCS - but this shouldn't change VMCLEAR
> semantics as VMCLEAR has it's own argument. It's perfectly valid to try
> to put a eVMCS which was previously used on a different vCPU (and thus
> which is 'active') to non-active state. The fact that we don't have an
> active eVMCS on the vCPU doing VMCLEAR shouldn't matter at all.
> 
> -- 
> Vitaly

Oh oops sure. Yes you are right.
I forgot about the larger context here for a moment.
Sorry for the confusion. :)

-Liran