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)

[Chao Fan]

On Thu, Jan 11, 2018 at 10:04:56AM -0800, Kees Cook wrote:
>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...
>

Hi Kees,

So this issue can be figured out in a independent patch.
And does this patch have any problems? If so, please tell me, I will try
my best to improve it.

Thanks,
Chao Fan

>-Kees
>
>-- 
>Kees Cook
>Pixel Security
>
>