linux-kernel - Re: [PATCH v2 05/16] KVM: VMX: Drop initialization of IA32_FEATURE

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Date:   Thu, 14 Nov 2019 10:34:53 -0800
From:   Sean Christopherson <sean.j.christopherson@...el.com>
To:     Paolo Bonzini <pbonzini@...hat.com>
Cc:     Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
        x86@...nel.org, "H. Peter Anvin" <hpa@...or.com>,
        linux-kernel@...r.kernel.org,
        Radim Krčmář <rkrcmar@...hat.com>,
        Vitaly Kuznetsov <vkuznets@...hat.com>,
        Wanpeng Li <wanpengli@...cent.com>,
        Jim Mattson <jmattson@...gle.com>, kvm@...r.kernel.org
Subject: Re: [PATCH v2 05/16] KVM: VMX: Drop initialization of
 IA32_FEATURE_CONTROL MSR

On Tue, Oct 22, 2019 at 08:16:22AM -0700, Sean Christopherson wrote:
> On Tue, Oct 22, 2019 at 12:51:01PM +0200, Paolo Bonzini wrote:
> > On 22/10/19 02:08, Sean Christopherson wrote:
> > > Remove the code to initialize IA32_FEATURE_CONTROL MSR when KVM is
> > > loaded now that the MSR is initialized during boot on all CPUs that
> > > support VMX, i.e. can possibly load kvm_intel.
> > > 
> > > Reviewed-by: Jim Mattson <jmattson@...gle.com>
> > > Signed-off-by: Sean Christopherson <sean.j.christopherson@...el.com>
> > > ---
> > >  arch/x86/kvm/vmx/vmx.c | 48 +++++++++++++++++-------------------------
> > >  1 file changed, 19 insertions(+), 29 deletions(-)
> > 
> > I am still not sure about this...  Enabling VMX is adding a possible
> > attack vector for the kernel, we should not do it unless we plan to do a
> > VMXON.
> 
> An attacker would need arbitrary cpl0 access to toggle CR4.VMXE and do
> VMXON (and VMLAUNCH), would an extra WRMSR really slow them down?
> 
> And practically speaking, how often do you encounter systems whose
> firmware leaves IA32_FEATURE_CONTROL unlocked?
> 
> > Why is it so important to operate with locked
> > IA32_FEATURE_CONTROL (so that KVM can enable VMX and the kernel can
> > still enable SGX if desired).
> 
> For simplicity.  The alternative that comes to mind is to compute the
> desired MSR value and write/lock the MSR on demand, e.g. add a sequence
> similar to KVM's hardware_enable_all() for SGX, but that's a fair amount
> of complexity for marginal benefit (IMO).
> 
> If a user really doesn't want VMX enabled, they can clear the feature bit
> via the clearcpuid kernel param. 
> 
> That being said, enabling VMX in IA32_FEATURE_CONTROL if and only if
> IS_ENABLED(CONFIG_KVM) is true would be an easy enhancement.

Paolo, any follow up thoughts on this approach?