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Date: Thu, 5 Jan 2017 07:08:05 -0800
From: Arjan van de Ven <>
To: Ingo Molnar <>, Thomas Garnier <>
Cc: Thomas Gleixner <>, Ingo Molnar <>,
 "H . Peter Anvin" <>, Kees Cook <>,
 Borislav Petkov <>, Andy Lutomirski <>,
 Dave Hansen <>, Chen Yucong <>,
 Paul Gortmaker <>,
 Andrew Morton <>,
 Masahiro Yamada <>,
 Sebastian Andrzej Siewior <>,
 Anna-Maria Gleixner <>,
 Boris Ostrovsky <>,
 Rasmus Villemoes <>,
 Michael Ellerman <>, Juergen Gross <>,
 Richard Weinberger <>,,,,
 Linus Torvalds <>,
 Peter Zijlstra <>
Subject: Re: [RFC] x86/mm/KASLR: Remap GDTs at fixed location

On 1/5/2017 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.

kernel_unrandomize_smp() ...

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

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