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

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Date:   Mon, 17 Jan 2022 12:20:32 +0100
From:   Paolo Bonzini <pbonzini@...hat.com>
To:     Vitaly Kuznetsov <vkuznets@...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

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.

Paolo