linux-kernel - Re: [PATCH 4/4] KVM: x86: lapic: don't allow to set non default apic id when not using x2apic api

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Date:   Thu, 3 Mar 2022 16:51:10 +0000
From:   Sean Christopherson <seanjc@...gle.com>
To:     Maxim Levitsky <mlevitsk@...hat.com>
Cc:     kvm@...r.kernel.org, Jim Mattson <jmattson@...gle.com>,
        "H. Peter Anvin" <hpa@...or.com>, linux-kernel@...r.kernel.org,
        Vitaly Kuznetsov <vkuznets@...hat.com>,
        Paolo Bonzini <pbonzini@...hat.com>,
        Joerg Roedel <joro@...tes.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        Wanpeng Li <wanpengli@...cent.com>,
        Borislav Petkov <bp@...en8.de>, x86@...nel.org
Subject: Re: [PATCH 4/4] KVM: x86: lapic: don't allow to set non default apic
 id when not using x2apic api

On Wed, Mar 02, 2022, Maxim Levitsky wrote:
> When APIC state is loading while APIC is in *x2apic* mode it does enforce that
> value in this 0x20 offset is initial apic id if KVM_CAP_X2APIC_API.
>  
> I think that it is fair to also enforce this when KVM_CAP_X2APIC_API is not used,
> especially if we make apic id read-only.

I don't disagree in principle.  But, (a) this loophole as existing for nearly 6
years, (b) closing the loophole could break userspace, (c) false positive are
possible due to truncation, and (d) KVM gains nothing meaningful by closing the
loophole.

(d) changes when we add a knob to make xAPIC ID read-only, but we can simply
require userspace to enable KVM_CAP_X2APIC_API (or force it).  That approach
avoids (c) by eliminating truncation, and avoids (b) by virtue of being opt-in.