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

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Message-ID: <87ftnxex1g.fsf@vitty.brq.redhat.com>
Date:   Tue, 25 Jun 2019 13:15:07 +0200
From:   Vitaly Kuznetsov <vkuznets@...hat.com>
To:     Liran Alon <liran.alon@...cle.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

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