linux-kernel - Re: [RFC PATCH 00/13] XOM for KVM guest userspace

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Date:   Fri, 4 Oct 2019 07:56:58 -0700
From:   Andy Lutomirski <luto@...nel.org>
To:     Rick Edgecombe <rick.p.edgecombe@...el.com>
Cc:     kvm list <kvm@...r.kernel.org>,
        LKML <linux-kernel@...r.kernel.org>, X86 ML <x86@...nel.org>,
        Linux-MM <linux-mm@...ck.org>,
        Andrew Lutomirski <luto@...nel.org>,
        Peter Zijlstra <peterz@...radead.org>,
        Dave Hansen <dave.hansen@...el.com>,
        Paolo Bonzini <pbonzini@...hat.com>,
        "Christopherson, Sean J" <sean.j.christopherson@...el.com>,
        Kees Cook <keescook@...omium.org>,
        Kristen Carlson Accardi <kristen@...ux.intel.com>,
        "Dock, Deneen T" <deneen.t.dock@...el.com>
Subject: Re: [RFC PATCH 00/13] XOM for KVM guest userspace

On Thu, Oct 3, 2019 at 2:38 PM Rick Edgecombe
<rick.p.edgecombe@...el.com> wrote:
>
> This patchset enables the ability for KVM guests to create execute-only (XO)
> memory by utilizing EPT based XO permissions. XO memory is currently supported
> on Intel hardware natively for CPU's with PKU, but this enables it on older
> platforms, and can support XO for kernel memory as well.

The patchset seems to sometimes call this feature "XO" and sometimes
call it "NR".  To me, XO implies no-read and no-write, whereas NR
implies just no-read.  Can you please clarify *exactly* what the new
bit does and be consistent?

I suggest that you make it NR, which allows for PROT_EXEC and
PROT_EXEC|PROT_WRITE and plain PROT_WRITE.  WX is of dubious value,
but I can imagine plain W being genuinely useful for logging and for
JITs that could maintain a W and a separate X mapping of some code.
In other words, with an NR bit, all 8 logical access modes are
possible.  Also, keeping the paging bits more orthogonal seems nice --
we already have a bit that controls write access.