linux-kernel - Re: [PATCH 03/15] KVM: SVM: Inject #UD on RDTSCP when it should be disabled in the guest

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Date:   Tue, 18 May 2021 12:59:46 +0200
From:   Paolo Bonzini <pbonzini@...hat.com>
To:     Sean Christopherson <seanjc@...gle.com>,
        Maxim Levitsky <mlevitsk@...hat.com>
Cc:     Jim Mattson <jmattson@...gle.com>,
        Vitaly Kuznetsov <vkuznets@...hat.com>,
        Wanpeng Li <wanpengli@...cent.com>,
        Joerg Roedel <joro@...tes.org>, kvm list <kvm@...r.kernel.org>,
        LKML <linux-kernel@...r.kernel.org>,
        Xiaoyao Li <xiaoyao.li@...el.com>,
        Reiji Watanabe <reijiw@...gle.com>
Subject: Re: [PATCH 03/15] KVM: SVM: Inject #UD on RDTSCP when it should be
 disabled in the guest

On 10/05/21 18:56, Sean Christopherson wrote:
> On Mon, May 10, 2021, Maxim Levitsky wrote:
>> On Tue, 2021-05-04 at 14:58 -0700, Jim Mattson wrote:
>>> On Tue, May 4, 2021 at 2:57 PM Paolo Bonzini <pbonzini@...hat.com> wrote:
>>>> On 04/05/21 23:53, Sean Christopherson wrote:
>>>>>> Does the right thing happen here if the vCPU is in guest mode when
>>>>>> userspace decides to toggle the CPUID.80000001H:EDX.RDTSCP bit on or
>>>>>> off?
>>>>> I hate our terminology.  By "guest mode", do you mean running the vCPU, or do
>>>>> you specifically mean running in L2?
>>>>>
>>>>
>>>> Guest mode should mean L2.
>>>>
>>>> (I wonder if we should have a capability that says "KVM_SET_CPUID2 can
>>>> only be called prior to KVM_RUN").
>>>
>>> It would certainly make it easier to reason about potential security issues.
>>>
>> I vote too for this.
> 
> Alternatively, what about adding KVM_VCPU_RESET to let userspace explicitly
> pull RESET#, and defining that ioctl() to freeze the vCPU model?  I.e. after
> userspace resets the vCPU, KVM_SET_CPUID (and any other relevant ioctls() is
> disallowed.
> 
> Lack of proper RESET emulation is annoying, e.g. userspace has to manually stuff
> EDX after vCPU creation to get the right value at RESET.  A dedicated ioctl()
> would kill two birds with one stone, without having to add yet another "2"
> ioctl().

That has a disadvantage of opting into the more secure behavior, but we 
can do both (forbidding KVM_SET_CPUID2 after both KVM_RUN and KVM_RESET).

Paolo