Re: KASLR may break some kernel features (was Re: [PATCH v5 1/4] kaslr: add immovable_mem=nn[KMG]@ss[KMG] to specify extracting memory)

[Kees Cook]

On Thu, Jan 11, 2018 at 1:00 AM, Baoquan He <bhe@xxxxxxxxxx> wrote:
> Hi Luiz,
>
> On 01/04/18 at 11:21am, Luiz Capitulino wrote:
>> Having a generic kaslr parameter to control where the kernel is extracted
>> is one solution for this problem.
>>
>> The general problem statement is that KASLR may break some kernel features
>> depending on where the kernel is extracted. Two examples are hot-plugged
>> memory (this series) and 1GB HugeTLB pages.
>>
>> The 1GB HugeTLB page issue is not specific to KVM guests. It just happens
>> that there's a bunch of people running guests with up to 5GB of memory and
>> with that amount of memory you have one or two 1GB pages and is easier for
>> KASLR to extract the kernel into a 1GB region and split a 1GB page. So,
>> you may not get any 1GB pages at all when this happens. However, I can also
>> reproduce this on bare-metal with lots of memory where I can loose a 1GB
>> page from time to time.
>>
>> Having a kaslr_range= parameter solves both issues, but two major drawbacks
>> is that it breaks existing setups and I guess users will have a very hard
>> time choosing good ranges.
>>
>> Another idea would be to have a CONFIG_KASLR_RANGES, where each arch
>> could have a list of ranges known to contain holes and/or immovable
>> memory and only extract the kernel into those ranges.
>
> If add CONFIG_KASLR_RANGES, then a distro like RHEL will have this range
> always, whether people need hugetlb or not.
>
> So in this case, what range do we need to avoid? Only [1G, 2G]?

Any ranges like that that need to be avoided should be known at build
time, so they should simply be added to the mem_avoid list that is
already present in the KASLR code...

-Kees

-- 
Kees Cook
Pixel Security