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 <bhe@redhat.com>]

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.

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