linux-kernel - Re: [PATCH 2/2] KVM: VMX: Add knob to allow rejecting kvm

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Message-ID: <CALMp9eQTYY3NiP-3fZgnMn9-nGWTtx7aA3T0-tTvdfeZkhdNrA@mail.gmail.com>
Date:   Wed, 25 May 2022 18:00:24 -0700
From:   Jim Mattson <jmattson@...gle.com>
To:     Sean Christopherson <seanjc@...gle.com>
Cc:     Paolo Bonzini <pbonzini@...hat.com>,
        Vitaly Kuznetsov <vkuznets@...hat.com>,
        Wanpeng Li <wanpengli@...cent.com>,
        Joerg Roedel <joro@...tes.org>, kvm@...r.kernel.org,
        linux-kernel@...r.kernel.org,
        Chenyi Qiang <chenyi.qiang@...el.com>,
        Lei Wang <lei4.wang@...el.com>
Subject: Re: [PATCH 2/2] KVM: VMX: Add knob to allow rejecting kvm_intel on
 inconsistent VMCS config

On Wed, May 25, 2022 at 5:45 PM Sean Christopherson <seanjc@...gle.com> wrote:
>
> On Wed, May 25, 2022, Jim Mattson wrote:
> > On Wed, May 25, 2022 at 2:04 PM Sean Christopherson <seanjc@...gle.com> wrote:
> > >
> > > Add an off-by-default module param, reject_inconsistent_vmcs_config, to
> > > allow rejecting the load of kvm_intel if an inconsistent VMCS config is
> > > detected.  Continuing on with an inconsistent, degraded config is
> > > undesirable when the CPU is expected to support a given set of features,
> > > e.g. can result in a misconfigured VM if userspace doesn't cross-check
> > > KVM_GET_SUPPORTED_CPUID, and/or can result in poor performance due to
> > > lack of fast MSR switching.
> > >
> > > Signed-off-by: Sean Christopherson <seanjc@...gle.com>
> > > ---
> > There are several inconsistent VMCS configs that are not rejected here
> > (e.g. "enable XSAVES/XRSTORS" on a CPU that doesn't support XSAVES).
> > Do you plan to include more checks in the future, or should this be,
> > "reject_some_inconsistent_vmcs_configs"? :-)
>
> I have no plan, it was purely a reaction to continuing on with a known bad entry/exit
> pair handling being awful.  I hesitated to even apply it to the EPT/VPID stuff, but
> again it seemed silly to detect an inconsistency and do nothing about it.
>
> I'm not opposed to adding more checks, though there is definitely a point of
> diminishing returns.  I'm just picking the really low hanging fruit :-)

The usual KVM approach to a misconfigured guest is to let userspace
shoot itself in the foot, as long as it doesn't put the host at risk.
This change seems to run counter to that.