On Mon, Oct 15, 2018 at 3:25 PM Kees Cook <keescook@xxxxxxxxxxxx> 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?) Yes, I think it's a valid concern to not turn this on for everybody given the potential for performance regression. For the next version I'll add some runtime detection for a memory-side-cache to set the default on/off, and include a command line override for the paranoid that want in on regardless of the presence of such a cache.
