linux-kernel - Re: Redoing eXclusive Page Frame Ownership (XPFO) with isolated CPUs in mind (for KVM to isolate its guests per CPU)

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Date:   Wed, 19 Sep 2018 11:03:37 +1000
From:   Balbir Singh <bsingharora@...il.com>
To:     "Woodhouse, David" <dwmw@...zon.co.uk>
Cc:     "torvalds@...ux-foundation.org" <torvalds@...ux-foundation.org>,
        "konrad.wilk@...cle.com" <konrad.wilk@...cle.com>,
        "juerg.haefliger@....com" <juerg.haefliger@....com>,
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        "jmattson@...gle.com" <jmattson@...gle.com>,
        "andrew.cooper3@...rix.com" <andrew.cooper3@...rix.com>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "boris.ostrovsky@...cle.com" <boris.ostrovsky@...cle.com>,
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        "jcm@...hat.com" <jcm@...hat.com>
Subject: Re: Redoing eXclusive Page Frame Ownership (XPFO) with isolated CPUs
 in mind (for KVM to isolate its guests per CPU)

On Mon, Aug 20, 2018 at 09:52:19PM +0000, Woodhouse, David wrote:
> On Mon, 2018-08-20 at 14:48 -0700, Linus Torvalds wrote:
> > 
> > Of course, after the long (and entirely unrelated) discussion about
> > the TLB flushing bug we had, I'm starting to worry about my own
> > competence, and maybe I'm missing something really fundamental, and
> > the XPFO patches do something else than what I think they do, or my
> > "hey, let's use our Meltdown code" idea has some fundamental weakness
> > that I'm missing.
> 
> The interesting part is taking the user (and other) pages out of the
> kernel's 1:1 physmap.
> 
> It's the *kernel* we don't want being able to access those pages,
> because of the multitude of unfixable cache load gadgets.

I am missing why we need this since the kernel can't access
(SMAP) unless we go through to the copy/to/from interface
or execute any of the user pages. Is it because of the dependency
on the availability of those features?

Balbir Singh.