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

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Date:   Fri, 6 Jan 2017 10:03:51 -0800
From:   Thomas Garnier <thgarnie@...gle.com>
To:     Ingo Molnar <mingo@...nel.org>
Cc:     Andy Lutomirski <luto@...capital.net>,
        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 10:49 PM, Ingo Molnar <mingo@...nel.org> wrote:
>
> * Thomas Garnier <thgarnie@...gle.com> wrote:
>
>> >> 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?
>> >
>> > What I mean is: write to the GDT through normal percpu access (or whatever the
>> > normal mapping is) but load a read-only alias into the GDT register.  As long
>> > as nothing ever tries to write through the GDTR alias, no page faults will be
>> > generated.  So we just need to make sure that nothing ever writes to it
>> > through GDTR.  AFAIK the only reason the CPU ever writes to the address in
>> > GDTR is to set an accessed bit.
>>
>> A write is made when we use load_TR_desc (ltr). I didn't see any other yet.
>
> Is this write to the GDT, generated by the LTR instruction, done unconditionally
> by the hardware?
>

That was my experience. I didn't look into details. Do you think we
could change something so that ltr never writes to the GDT? (just mark
the TSS entry busy).

> Thanks,
>
>         Ingo



-- 
Thomas