linux-kernel - Re: [RFC] x86/mm/KASLR: Remap GDTs at fixed location

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Date:   Sat, 7 Jan 2017 07:58:38 -0800
From:   Andy Lutomirski <luto@...capital.net>
To:     Ingo Molnar <mingo@...nel.org>
Cc:     Andy Lutomirski <luto@...nel.org>,
        Thomas Garnier <thgarnie@...gle.com>,
        Arjan van de Ven <arjan@...ux.intel.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>,
        "H . Peter Anvin" <hpa@...or.com>,
        Kees Cook <keescook@...omium.org>,
        Borislav Petkov <bp@...en8.de>, Dave Hansen <dave@...1.net>,
        Chen Yucong <slaoub@...il.com>,
        Paul Gortmaker <paul.gortmaker@...driver.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Masahiro Yamada <yamada.masahiro@...ionext.com>,
        Sebastian Andrzej Siewior <bigeasy@...utronix.de>,
        Anna-Maria Gleixner <anna-maria@...utronix.de>,
        Boris Ostrovsky <boris.ostrovsky@...cle.com>,
        Rasmus Villemoes <linux@...musvillemoes.dk>,
        Michael Ellerman <mpe@...erman.id.au>,
        Juergen Gross <jgross@...e.com>,
        Richard Weinberger <richard@....at>, X86 ML <x86@...nel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "kernel-hardening@...ts.openwall.com" 
        <kernel-hardening@...ts.openwall.com>
Subject: Re: [RFC] x86/mm/KASLR: Remap GDTs at fixed location

On Fri, Jan 6, 2017 at 11:45 PM, Ingo Molnar <mingo@...nel.org> wrote:
>
> * Andy Lutomirski <luto@...nel.org> wrote:

>> P.S. Let's do the move to the fixmap, read/write as a separate patch. That will
>> make bisecting much easier.
>
> Absolutely, but this has to be within the same series, as the interim fixmap-only
> step is less secure in some circumstances: we are moving the writable GDT from a
> previously randomized location to a fixed location.

True, but despite being randomized its location was never even
remotely secret.  (Except on Kaby Lake or Foobar Lake or whatever CPU
that is.)

--Andy