On Wed, Feb 3, 2016 at 10:46 AM, Laura Abbott <labbott@xxxxxxxxxx> wrote: > On 01/25/2016 11:03 PM, Joonsoo Kim wrote: >> >> On Mon, Jan 25, 2016 at 05:15:10PM -0800, Laura Abbott wrote: >>> >>> Hi, >>> >>> Based on the discussion from the series to add slab sanitization >>> (lkml.kernel.org/g/<1450755641-7856-1-git-send-email-laura@xxxxxxxxxxxx>) >>> the existing SLAB_POISON mechanism already covers similar behavior. >>> The performance of SLAB_POISON isn't very good. With hackbench -g 20 -l >>> 1000 >>> on QEMU with one cpu: >> >> >> I doesn't follow up that discussion, but, I think that reusing >> SLAB_POISON for slab sanitization needs more changes. I assume that >> completeness and performance is matter for slab sanitization. >> >> 1) SLAB_POISON isn't applied to specific kmem_cache which has >> constructor or SLAB_DESTROY_BY_RCU flag. For debug, it's not necessary >> to be applied, but, for slab sanitization, it is better to apply it to >> all caches. > > > The grsecurity patches get around this by calling the constructor again > after poisoning. It could be worth investigating doing that as well > although my focus was on the cases without the constructor. >> >> >> 2) SLAB_POISON makes object size bigger so natural alignment will be >> broken. For example, kmalloc(256) cache's size is 256 in normal >> case but it would be 264 when SLAB_POISON is enabled. This causes >> memory waste. > > > The grsecurity patches also bump the size up to put the free pointer > outside the object. For sanitization purposes it is cleaner to have > no pointers in the object after free > >> >> In fact, I'd prefer not reusing SLAB_POISON. It would make thing >> simpler. But, it's up to Christoph. >> >> Thanks. >> > > It basically looks like trying to poison on the fast path at all > will have a negative impact even with the feature is turned off. > Christoph has indicated this is not acceptable so we are forced > to limit it to the slow path only if we want runtime enablement. Is it possible to have both? i.e fast path via CONFIG, and slow path via runtime options? > If we're limited to the slow path only, we might as well work > with SLAB_POISON to make it faster. We can reevaluate if it turns > out the poisoning isn't fast enough to be useful. And since I'm new to this area, I know of fast/slow path in the syscall sense. What happens in the allocation/free fast/slow path that makes it fast or slow? -Kees -- Kees Cook Chrome OS & Brillo Security -- To unsubscribe, send a message with 'unsubscribe linux-mm' in the body to majordomo@xxxxxxxxx. For more info on Linux MM, see: http://www.linux-mm.org/ . Don't email: <a href=mailto:"dont@xxxxxxxxx";> email@xxxxxxxxx </a>