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

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Date:   Thu, 5 Jan 2017 15:16:05 -0800
From:   Thomas Garnier <thgarnie@...gle.com>
To:     Linus Torvalds <torvalds@...ux-foundation.org>
Cc:     Andy Lutomirski <luto@...nel.org>,
        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 Thu, Jan 5, 2017 at 3:05 PM, Linus Torvalds
<torvalds@...ux-foundation.org> wrote:
> On Thu, Jan 5, 2017 at 12:18 PM, Andy Lutomirski <luto@...nel.org> wrote:
>>
>> Hmm.  I bet that if we preset the accessed bits in all the segments
>> then we don't need it to be writable in general.
>
> I'm not sure that this is architecturally safe.
>
> IIRC, we do mark the IDT read-only - but that one we started doing due
> to the f00f bug, so we knew it was ok. I'm not sure you can do the
> same with the GDT/LDT.
>

I started testing a variant that make the GDT remapping read-only by
default and writeable only for LTR. Everything works fine, even
hibernation. I need to do more testing though on different
architectures.

To be on the safe side, I could separate the read-only part in a
separate patch so we can easily remove it if extended testing show
something.

>                    Linus



-- 
Thomas