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... -Kees -- Kees Cook Pixel Security
