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Date: Thu, 5 Jan 2017 08:39:05 -0800
From: Thomas Garnier <>
To: Ingo Molnar <>
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 <>, "the arch/x86 maintainers" <>, LKML <>, 
	Kernel Hardening <>, 
	Linus Torvalds <>, Peter Zijlstra <>
Subject: Re: [RFC] x86/mm/KASLR: Remap GDTs at fixed location

On Thu, Jan 5, 2017 at 12:11 AM, Ingo Molnar <> wrote:
> * 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.
>> 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.

I agree, I went back and forth on the name. I will change it to
something better.

> Also, don't we want to do this unconditionally and not allow remapping failures?
> The GDT is fairly small, plus making the SGDT instruction expose fewer kernel
> internals would be (marginally) useful on non-randomized kernels as well.
> It also makes the code more common, more predictable, more debuggable and less
> complex overall - which is pretty valuable in terms of long term security as well.

Okay, I will add BUG_ON on failures to remap.

> Thanks,
>         Ingo

Ingo: I saw the 5-level page table support being sent through. Do you
want me to wait for it to be -next? (Given it will need to be changed


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