linux-kernel - Re: [PATCH 3/9] KVM: x86: SVM: Pass through shadow stack MSRs

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Message-ID: <ZUrDyqXAQZsQzCzl@google.com>
Date:   Tue, 7 Nov 2023 15:10:02 -0800
From:   Sean Christopherson <seanjc@...gle.com>
To:     Maxim Levitsky <mlevitsk@...hat.com>
Cc:     nikunj@....com, John Allen <john.allen@....com>,
        kvm@...r.kernel.org, linux-kernel@...r.kernel.org,
        pbonzini@...hat.com, weijiang.yang@...el.com,
        rick.p.edgecombe@...el.com, x86@...nel.org,
        thomas.lendacky@....com, bp@...en8.de
Subject: Re: [PATCH 3/9] KVM: x86: SVM: Pass through shadow stack MSRs

On Tue, Nov 07, 2023, Maxim Levitsky wrote:
> Since no sane L1 hypervisor will ever allow access to all its msrs from L2,
> it might make sense to always use a dedicated MSR bitmap for L2.

Hmm, there might be a full passthrough use case out there, but in general, yeah,
I agree.  I think even kernel hardening use cases where the "hypervisor" is just
a lowvisor would utilize MSR bitmaps to prevent modifying the de-privileged
kernel from modifying select MSRs.

> Also since all sane L1 hypervisors do use a msr bitmap means that
> dedicated code path that doesn't use it is not well tested.
> 
> On VMX if I am not mistaken, this is not an issue because either all
> MSRS are intercepted or a bitmap is used.

Yep, if the MSR bitmaps aren't used then all MSR accesses are intercepted.