On Mon 15-10-18 15:25:47, Kees Cook wrote: > On Wed, Oct 10, 2018 at 6:36 PM, Dan Williams <dan.j.williams@xxxxxxxxx> wrote: > > While SLAB_FREELIST_RANDOM reduces the predictability of some local slab > > caches it leaves vast bulk of memory to be predictably in order > > allocated. That ordering can be detected by a memory side-cache. > > > > The shuffling is done in terms of CONFIG_SHUFFLE_PAGE_ORDER sized free > > pages where the default CONFIG_SHUFFLE_PAGE_ORDER is MAX_ORDER-1 i.e. > > 10, 4MB this trades off randomization granularity for time spent > > shuffling. MAX_ORDER-1 was chosen to be minimally invasive to the page > > allocator while still showing memory-side cache behavior improvements, > > and the expectation that the security implications of finer granularity > > randomization is mitigated by CONFIG_SLAB_FREELIST_RANDOM. > > Perhaps it would help some of the detractors of this feature to make > this a runtime choice? Some benchmarks show improvements, some show > regressions. It could just be up to the admin to turn this on/off > given their paranoia levels? (i.e. the shuffling could become a no-op > with a given specific boot param?) Sure, making this a opt-in is really necessary but it would be even _better_ to actually evaluate how much security relevance it has as well. If for nothing else then to allow an educated decision rather than a fear driven one. And that pretty much involves evaluation on how hard it is to bypass the randomness. If I am going to pay some overhead I would like to know how much hardening I get in return, right? Something completely missing in the current evaluation so far. -- Michal Hocko SUSE Labs
