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

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Date:   Fri, 14 Jan 2022 17:00:26 +0000
From:   Sean Christopherson <seanjc@...gle.com>
To:     Vitaly Kuznetsov <vkuznets@...hat.com>
Cc:     Igor Mammedov <imammedo@...hat.com>,
        Paolo Bonzini <pbonzini@...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 Fri, Jan 14, 2022, Vitaly Kuznetsov wrote:
> Igor Mammedov <imammedo@...hat.com> writes:
> 
> > On Fri, 14 Jan 2022 10:31:50 +0100
> > Vitaly Kuznetsov <vkuznets@...hat.com> wrote:
> >
> >> Igor Mammedov <imammedo@...hat.com> writes:
> >> 
> >> 
> >> > However, a problem of failing KVM_SET_CPUID2 during VCPU re-plug
> >> > is still there and re-plug will fail if KVM rejects repeated KVM_SET_CPUID2
> >> > even if ioctl called with exactly the same CPUID leafs as the 1st call.
> >> >  
> >> 
> >> Assuming APIC id change doesn not need to be supported, I can send v2
> >> here with an empty allowlist.
> > As you mentioned in another thread black list would be better
> > to address Sean's concerns or just revert problematic commit.
> >
> 
> Personally, I'm leaning towards the blocklist approach even if just for
> 'documenting' the fact that KVM doesn't correctly handle the
> change. Compared to a comment in the code, such approach could help
> someone save tons of debugging time (if anyone ever decides do something
> weird, like changing MAXPHYADDR on the fly).

I assume the blocklist approach is let userspace opt into rejecting KVM_SET_CPUID{,2},
but allow all CPUID leafs and sub-leafs to be modified at will by default?  I don't
dislike the idea, but I wonder if it's unnecessarily fancy.

What if we instead provide an ioctl/capability to let userspace toggle disabling
of KVM_SET_CPUID{,2}, a la STAC/CLAC to override SMAP?  E.g. QEMU could enable
protections after initially creating the vCPU, then temporarily disable protections
only for the hotplug path?

That'd provide solid protections for minimal effort, and if userspace can restrict
the danger zone to one specific path, then userspace can easily do its own auditing
for that one path.