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Date: Thu, 5 Jan 2017 09:54:33 -0800
From: Thomas Garnier <>
To: Andy Lutomirski <>
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 Thu, Jan 5, 2017 at 9:51 AM, Andy Lutomirski <> wrote:
> 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
> root.

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.


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