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

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Message-ID: <CAJcbSZHAtxbRzhTcZtBSabW0t+Cj7a1z-xJk8d310a2h8pkG=g@mail.gmail.com>
Date:   Thu, 5 Jan 2017 13:08:21 -0800
From:   Thomas Garnier <thgarnie@...gle.com>
To:     Andy Lutomirski <luto@...nel.org>
Cc:     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 12:18 PM, Andy Lutomirski <luto@...nel.org> wrote:
> On Thu, Jan 5, 2017 at 11:03 AM, Thomas Garnier <thgarnie@...gle.com> wrote:
>> On Thu, Jan 5, 2017 at 10:58 AM, Arjan van de Ven <arjan@...ux.intel.com> wrote:
>>> On 1/5/2017 9:54 AM, Thomas Garnier wrote:
>>>
>>>>
>>>> That's my goal too. I started by doing a RO remap and got couple
>>>> problems with hibernation. I can try again for the next iteration or
>>>> delay it for another patch. I also need to look at KVM GDT usage, I am
>>>> not familiar with it yet.
>>>
>>>
>>> don't we write to the GDT as part of the TLS segment stuff for glibc ?
>>>
>>
>> Not sure which glibc feature it is.
>>
>> In this design, you can write to the GDT per-cpu variable that will
>> remain read-write. You just need to make the remapping writeable when
>> we load task registers (ltr) then the processor use the current GDT
>> address. At least that the case I know, I might find more through
>> testing.
>
> 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.  But your point about
> set_thread_area (TLS) is well taken.  However, I strongly suspect that
> we could make set_thread_area unconditionally set the accessed bit and
> no one would ever notice.

Not sure I fully understood and I don't want to miss an important
point. Do you mean making GDT (remapping and per-cpu) read-only and
switch the writeable flag only when we write to the per-cpu entry?

-- 
Thomas