linux-kernel - Re: [PATCH 2/2] KVM: x86: Forbid KVM_SET_CPUID{,2} after KVM

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Date:   Mon, 17 Jan 2022 14:02:31 +0100
From:   Vitaly Kuznetsov <vkuznets@...hat.com>
To:     Paolo Bonzini <pbonzini@...hat.com>,
        Sean Christopherson <seanjc@...gle.com>
Cc:     Igor Mammedov <imammedo@...hat.com>, kvm@...r.kernel.org,
        Wanpeng Li <wanpengli@...cent.com>,
        Jim Mattson <jmattson@...gle.com>, linux-kernel@...r.kernel.org
Subject: Re: [PATCH 2/2] KVM: x86: Forbid KVM_SET_CPUID{,2} after KVM_RUN

Paolo Bonzini <pbonzini@...hat.com> writes:

> On 1/17/22 10:55, Vitaly Kuznetsov wrote:
>> No, honestly I was thinking about something much simpler: instead of
>> forbidding KVM_SET_CPUID{,2} after KVM_RUN completely (what we have now
>> in 5.16), we only forbid to change certain data which we know breaks
>> some assumptions in MMU, from the comment:
>> "
>>           * KVM does not correctly handle changing guest CPUID after KVM_RUN, as
>>           * MAXPHYADDR, GBPAGES support, AMD reserved bit behavior, etc.. aren't
>>           * tracked in kvm_mmu_page_role.  As a result, KVM may miss guest page
>>           * faults due to reusing SPs/SPTEs.
>> "
>> It seems that CPU hotplug path doesn't need to change these so we don't
>> need an opt-in/opt-out, we can just forbid changing certain things for
>> the time being. Alternatively, we can silently ignore such changes but I
>> don't quite like it because it would mask bugs in VMMs.
>
> I think the version that only allows exactly the same CPUID is the best, 
> as it leaves less room for future bugs.
>

Ok, I hear your vote) Will prepare v2.

-- 
Vitaly