linux-kernel - Re: [RFC 0/6] Enlightened VMCS support for KVM on Hyper-V

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Date:   Tue, 16 Jan 2018 16:03:27 +0100
From:   Paolo Bonzini <pbonzini@...hat.com>
To:     Vitaly Kuznetsov <vkuznets@...hat.com>
Cc:     David Hildenbrand <david@...hat.com>,
        Wanpeng Li <kernellwp@...il.com>, kvm <kvm@...r.kernel.org>,
        the arch/x86 maintainers <x86@...nel.org>,
        Radim Krčmář <rkrcmar@...hat.com>,
        "K. Y. Srinivasan" <kys@...rosoft.com>,
        Haiyang Zhang <haiyangz@...rosoft.com>,
        Stephen Hemminger <sthemmin@...rosoft.com>,
        "Michael Kelley (EOSG)" <Michael.H.Kelley@...rosoft.com>,
        Mohammed Gamal <mmorsy@...hat.com>,
        Cathy Avery <cavery@...hat.com>, Bandan Das <bsd@...hat.com>,
        linux-kernel@...r.kernel.org
Subject: Re: [RFC 0/6] Enlightened VMCS support for KVM on Hyper-V

On 16/01/2018 15:43, Vitaly Kuznetsov wrote:
> Paolo Bonzini <pbonzini@...hat.com> writes:
> 
>> On 16/01/2018 14:58, Vitaly Kuznetsov wrote:
>>>> Haven't looked into the details, but we have to watch out for other
>>>> VCPUs trying to modify that vmcs12.
>>>>
>>>> Basically because other VCPUs could try to modify values in vmcs12 while
>>>> we are currently building vmcs02. Nasty races could result in us copying
>>>> stuff (probably unchecked) into vmcs02 and therefore running something
>>>> that was not intended.
>>>>
>>> I don't think we share VMCS among vCPUs, do we?
>>
>> VMCS is just memory, so who knows what a malicious L1 guest will do.
>> But for vmread/vmwrite we can go through hypervisor memory, for
>> enlightened VMCS we cannot.
>>
> 
> True; not sure if Hyper-V actually copies the data to some internal
> storage, probably it does. TLFS explicitly forbids making the same
> enlightened VMCS active on several vCPUs simultaneously but again, this
> is just memory...

You don't even need to make them active, you can just scribble on it
simultaneously with a VMRESUME.

Paolo