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Date: Thu, 5 Jan 2017 08:40:29 -0800
From: Thomas Garnier <thgarnie@...gle.com>
To: Arjan van de Ven <arjan@...ux.intel.com>
Cc: Ingo Molnar <mingo@...nel.org>, 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>, 
	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>, "the arch/x86 maintainers" <x86@...nel.org>, LKML <linux-kernel@...r.kernel.org>, 
	Kernel Hardening <kernel-hardening@...ts.openwall.com>, 
	Linus Torvalds <torvalds@...ux-foundation.org>, Peter Zijlstra <a.p.zijlstra@...llo.nl>
Subject: Re: [RFC] x86/mm/KASLR: Remap GDTs at fixed location

On Thu, Jan 5, 2017 at 7:08 AM, Arjan van de Ven <arjan@...ux.intel.com> wrote:
> On 1/5/2017 12:11 AM, Ingo Molnar wrote:
>>
>>
>> * 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.
>>>
>>> The document was changed to mention GDT remapping for KASLR. This patch
>>> also include dump page tables support.
>>>
>>> This patch was tested on multiple hardware configurations and for
>>> hibernation support.
>>
>>
>>>  void kernel_randomize_memory(void);
>>> +void kernel_randomize_smp(void);
>>> +void* kaslr_get_gdt_remap(int cpu);
>>
>>
>> Yeah, no fundamental objections from me to the principle, but I get some
>> bad vibes
>> from the naming here: seeing that kernel_randomize_smp() actually makes
>> things
>> less random.
>>
>
> kernel_unrandomize_smp() ...
>

That seems like a better name.

> one request.. can we make sure this unrandomization is optional?
>

Well, it happens only when KASLR memory randomization is enabled. Do
you think it should have a separate config option?

-- 
Thomas

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