On 01/11/18 at 10:04am, Kees Cook wrote: > On Thu, Jan 11, 2018 at 1:00 AM, Baoquan He <b...@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