Re: [RFC PATCH 13/13] x86/Kconfig: Add Kconfig for KVM based XO

From: Kees Cook <keescook@chromium.org>
To: "Edgecombe, Rick P" <rick.p.edgecombe@intel.com>
Cc: "kvm@vger.kernel.org" <kvm@vger.kernel.org>,
	"linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
	"peterz@infradead.org" <peterz@infradead.org>,
	"Dock, Deneen T" <deneen.t.dock@intel.com>,
	"Christopherson, Sean J" <sean.j.christopherson@intel.com>,
	"linux-mm@kvack.org" <linux-mm@kvack.org>,
	"x86@kernel.org" <x86@kernel.org>,
	"kristen@linux.intel.com" <kristen@linux.intel.com>,
	"luto@kernel.org" <luto@kernel.org>,
	"pbonzini@redhat.com" <pbonzini@redhat.com>,
	"Hansen, Dave" <dave.hansen@intel.com>
Subject: Re: [RFC PATCH 13/13] x86/Kconfig: Add Kconfig for KVM based XO
Date: Wed, 30 Oct 2019 11:36:20 -0700	[thread overview]
Message-ID: <201910301135.BDC4C7696@keescook> (raw)
In-Reply-To: <d645473f01c445a70bc1f2472217f1ae426b7020.camel@intel.com>

On Wed, Oct 30, 2019 at 12:01:18AM +0000, Edgecombe, Rick P wrote:
> On Tue, 2019-10-29 at 16:36 -0700, Kees Cook wrote:
> > On Thu, Oct 03, 2019 at 02:24:00PM -0700, Rick Edgecombe wrote:
> > > Add CONFIG_KVM_XO for supporting KVM based execute only memory.
> > 
> > I would expect this config to be added earlier in the series so that the
> > code being added that depends on it can be incrementally build tested...
> > 
> > (Also, if this is default=y, why have a Kconfig for it at all? Guests
> > need to know to use this already, yes?)
> > 
> > -Kees
> Hmm, good point. One reason could be that this requires SPARSEMEM_VMEMMAP due to
> some pre-processor tricks that need a compile time known max physical address
> size. So maybe someone could want KVM_GUEST and !SPARSEMEM_VMEMMAP. I'm not
> sure.

Good point about the combination of other CONFIGs. All the more reason
to move it earlier, though.

-Kees

> 
> > > 
> > > Signed-off-by: Rick Edgecombe <rick.p.edgecombe@intel.com>
> > > ---
> > >  arch/x86/Kconfig | 13 +++++++++++++
> > >  1 file changed, 13 insertions(+)
> > > 
> > > diff --git a/arch/x86/Kconfig b/arch/x86/Kconfig
> > > index 222855cc0158..3a3af2a456e8 100644
> > > --- a/arch/x86/Kconfig
> > > +++ b/arch/x86/Kconfig
> > > @@ -802,6 +802,19 @@ config KVM_GUEST
> > >  	  underlying device model, the host provides the guest with
> > >  	  timing infrastructure such as time of day, and system time
> > >  
> > > +config KVM_XO
> > > +	bool "Support for KVM based execute only virtual memory permissions"
> > > +	select DYNAMIC_PHYSICAL_MASK
> > > +	select SPARSEMEM_VMEMMAP
> > > +	depends on KVM_GUEST && X86_64
> > > +	default y
> > > +	help
> > > +	  This option enables support for execute only memory for KVM guests. If
> > > +	  support from the underlying VMM is not detected at boot, this
> > > +	  capability will automatically disable.
> > > +
> > > +	  If you are unsure how to answer this question, answer Y.
> > > +
> > >  config PVH
> > >  	bool "Support for running PVH guests"
> > >  	---help---
> > > -- 
> > > 2.17.1
> > > 
> > 
> > 

-- 
Kees Cook