Re: [PATCH v2 0/4] Enable SMEP CPU Feature

[Ingo Molnar <mingo@elte.hu>]

* Avi Kivity <avi@redhat.com> wrote:

> On 05/17/2011 12:29 PM, Ingo Molnar wrote:
> >* Avi Kivity<avi@redhat.com>  wrote:
> >
> >>  >  Some programmable configurability seems necessary on the KVM side, as KVM
> >>  >  has no control over how sane the guest kernel is.
> >>
> >>  We should simply expose the cpuid bit and cr4.smep.  If the guest kernel
> >>  feels it is up to it, it can enable smep itself.
> >
> > Well, given that there's lots of legacy installations around it would be a 
> > neat KVM feature if it was possible to enable SMEP even if the guest kernel 
> > does not enable it. As an additional (optional) layer of security.
> >
> > For example legacy Linux guests will work just fine, even if they do not 
> > enable SMEP themselves.
> 
> It's certainly possible (set CR4.SMEP transparently and hide it from the 
> guest).  But there's no way to tell if it doesn't break something wierd.  The 
> host might not even know if the guest is Linux or something else.
> 
> We could support it as a non-default feature, but that reduces its utility.

It would be a nice touch for tools/kvm/: we would use KVM_GET_SREGS and 
KVM_GET_SREGS to twiddle CR4.SMEP, even without the guest explicitly doing it.

A quick glance suggests that it could be done straight away in 
tools/kvm/kvm-cpu.c::kvm_cpu__setup_sregs() during vcpu setup, and hopefully 
that cr4 value survives boot and ends up in the guest kernel's mmu_cr4_features 
mask shadow register.

Thanks,

	Ingo