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Date: Thu, 5 Jan 2017 09:51:15 -0800
From: Andy Lutomirski <>
To: Thomas Garnier <>
Cc: Thomas Gleixner <>, Ingo Molnar <>, 
	"H . Peter Anvin" <>, Kees Cook <>, Borislav Petkov <>, 
	Andy Lutomirski <>, Dave Hansen <>, Chen Yucong <>, 
	Arjan van de Ven <>, Paul Gortmaker <>, 
	Andrew Morton <>, Masahiro Yamada <>, 
	Sebastian Andrzej Siewior <>, Anna-Maria Gleixner <>, 
	Boris Ostrovsky <>, Rasmus Villemoes <>, 
	Michael Ellerman <>, Juergen Gross <>, 
	Richard Weinberger <>, X86 ML <>, 
	"" <>, 
	"" <>
Subject: Re: [RFC] x86/mm/KASLR: Remap GDTs at fixed location

On Wed, Jan 4, 2017 at 2:16 PM, Thomas Garnier <> 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

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