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

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Message-ID: <20170106064900.GC28091@gmail.com>
Date:   Fri, 6 Jan 2017 07:49:00 +0100
From:   Ingo Molnar <mingo@...nel.org>
To:     Thomas Garnier <thgarnie@...gle.com>
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


* 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?

Thanks,

	Ingo