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

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Date:   Mon, 20 Aug 2018 17:25:56 -0400
From:   Konrad Rzeszutek Wilk <konrad.wilk@...cle.com>
To:     kernel-hardening@...ts.openwall.com,
        Liran Alon <liran.alon@...cle.com>,
        Deepa Srinivasan <deepa.srinivasan@...cle.com>,
        linux-mm@...ck.org, juerg.haefliger@....com,
        khalid.aziz@...cle.com, chris.hyser@...cle.com,
        tyhicks@...onical.com, dwmw@...zon.co.uk, keescook@...gle.com,
        andrew.cooper3@...rix.com, jcm@...hat.com,
        Boris Ostrovsky <boris.ostrovsky@...cle.com>,
        Kanth <kanth.ghatraju@...cle.com>,
        Joao Martins <joao.m.martins@...cle.com>, jmattson@...gle.com,
        pradeep.vincent@...cle.com,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        ak@...ux.intel.com, john.haxby@...cle.com,
        jsteckli@...inf.tu-dresden.de
Cc:     linux-kernel@...r.kernel.org, tglx@...utronix.de
Subject: Redoing eXclusive Page Frame Ownership (XPFO) with isolated CPUs in
 mind (for KVM to isolate its guests per CPU)

Hi!

See eXclusive Page Frame Ownership (https://lwn.net/Articles/700606/) which was posted
way back in in 2016..

In the last couple of months there has been a slew of CPU issues that have complicated
a lot of things. The latest - L1TF - is still fresh in folks's mind and it is
especially acute to virtualization workloads.

As such a bunch of various folks from different cloud companies (CCed) are looking
at a way to make Linux kernel be more resistant to hardware having these sort of 
bugs.

In particular we are looking at a way to "remove as many mappings from the global
kernel address space as possible. Specifically, while being in the
context of process A, memory of process B should not be visible in the
kernel." (email from Julian Stecklina). That is the high-level view and 
how this could get done, well, that is why posting this on
LKML/linux-hardening/kvm-devel/linux-mm to start the discussion.

Usually I would start with a draft of RFC patches so folks can rip it apart, but
thanks to other people (Juerg thank you!) it already exists:

(see https://www.mail-archive.com/linux-kernel@vger.kernel.org/msg1222756.html)

The idea would be to extend this to:

 1) Only do it for processes that run under CPUS which are in isolcpus list.

 2) Expand this to be a per-cpu page tables. That is each CPU has its own unique
    set of pagetables - naturally _START_KERNEL -> __end would be mapped but the
    rest would not.

Thoughts? Is this possible? Crazy? Better ideas?