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

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Date:	Tue, 17 May 2011 12:46:54 +0200
From:	Ingo Molnar <mingo@...e.hu>
To:	Avi Kivity <avi@...hat.com>
Cc:	"H. Peter Anvin" <hpa@...or.com>,
	Fenghua Yu <fenghua.yu@...el.com>,
	Thomas Gleixner <tglx@...utronix.de>,
	Asit K Mallick <asit.k.mallick@...el.com>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Arjan van de Ven <arjan@...radead.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Andi Kleen <andi@...stfloor.org>,
	linux-kernel <linux-kernel@...r.kernel.org>,
	Pekka Enberg <penberg@...helsinki.fi>
Subject: Re: [PATCH v2 0/4] Enable SMEP CPU Feature


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

> On 05/17/2011 12:29 PM, Ingo Molnar wrote:
> >* Avi Kivity<avi@...hat.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
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