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)

[Baoquan He]

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]?

Thanks
Baoquan