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Date: Thu, 5 Jan 2017 09:51:15 -0800
From: Andy Lutomirski <luto@...capital.net>
To: Thomas Garnier <thgarnie@...gle.com>
Cc: 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>, 
	Andy Lutomirski <luto@...nel.org>, Dave Hansen <dave@...1.net>, Chen Yucong <slaoub@...il.com>, 
	Arjan van de Ven <arjan@...ux.intel.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 Wed, Jan 4, 2017 at 2:16 PM, Thomas Garnier <thgarnie@...gle.com> wrote:
> Each processor holds a GDT in its per-cpu structure. The sgdt
> instruction gives the base address of the current GDT. This address can
> be used to bypass KASLR memory randomization. With another bug, an
> attacker could target other per-cpu structures or deduce the base of the
> main memory section (PAGE_OFFSET).
>
> In this change, a space is reserved at the end of the memory range
> available for KASLR memory randomization. The space is big enough to hold
> the maximum number of CPUs (as defined by setup_max_cpus). Each GDT is
> mapped at specific offset based on the target CPU. Note that if there is
> not enough space available, the GDTs are not remapped.

Can we remap it read-only?  I.e. use PAGE_KERNEL_RO instead of
PAGE_KERNEL.  After all, the ability to modify the GDT is instant
root.

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