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 <fanc.fnst@cn.fujitsu.com>]

On Fri, Jan 12, 2018 at 10:31:52AM +0800, Baoquan He wrote:
>On 01/11/18 at 10:04am, Kees Cook wrote:
>> On Thu, Jan 11, 2018 at 1:00 AM, Baoquan He <bhe@redhat.com> 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...
>
>Seems KASLR doesn't have an solution which allow user to specify avoided
>range for kernel text KASLR stage only. The memmap="!#$" can add range to
>mem_avoid, while it will make them not added to e820.
>

How about adding a new option, like "huge_page=nn@ss". Fill the regions
to mem_avoid. But this parameter will only be parsed in kaslr period.
The followed handlling of memmap will not be excuted.

Thanks,
Chao Fan

>Here like this hugetlb case, Luiz wants kernel to avoid the [2G, 3G)
>candidate position for hugetlb allocation, meanwhile wants it to be
>added to mm subsystem later.
>
>Thanks
>Baoquan
>
>
>