Re: [RFC] x86/mm/KASLR: Remap GDTs at fixed location

[Thomas Garnier]

On Thu, Jan 5, 2017 at 7:08 AM, Arjan van de Ven <arjan@xxxxxxxxxxxxxxx> wrote:
> On 1/5/2017 12:11 AM, Ingo Molnar wrote:
>>
>>
>> * Thomas Garnier <thgarnie@xxxxxxxxxx> 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