From mboxrd@z Thu Jan 1 00:00:00 1970 Reply-To: kernel-hardening@lists.openwall.com References: <1450755641-7856-1-git-send-email-laura@labbott.name> <1450755641-7856-7-git-send-email-laura@labbott.name> <567964F3.2020402@intel.com> <567986E7.50107@intel.com> <56798851.60906@intel.com> From: Dave Hansen Message-ID: <5679943C.1050604@intel.com> Date: Tue, 22 Dec 2015 10:19:40 -0800 MIME-Version: 1.0 In-Reply-To: Content-Type: text/plain; charset=windows-1252 Content-Transfer-Encoding: 7bit Subject: Re: [kernel-hardening] [RFC][PATCH 6/7] mm: Add Kconfig option for slab sanitization To: Christoph Lameter Cc: kernel-hardening@lists.openwall.com, Pekka Enberg , David Rientjes , Joonsoo Kim , Andrew Morton , Laura Abbott , linux-mm@kvack.org, linux-kernel@vger.kernel.org, Kees Cook List-ID: On 12/22/2015 10:08 AM, Christoph Lameter wrote: > On Tue, 22 Dec 2015, Dave Hansen wrote: >>> Why would you use zeros? The point is just to clear the information right? >>> The regular poisoning does that. >> >> It then allows you to avoid the zeroing at allocation time. > > Well much of the code is expecting a zeroed object from the allocator and > its zeroed at that time. Zeroing makes the object cache hot which is an > important performance aspect. Yes, modifying this behavior has a performance impact. It absolutely needs to be evaluated, and I wouldn't want to speculate too much on how good or bad any of the choices are. Just to reiterate, I think we have 3 real choices here: 1. Zero at alloc, only when __GFP_ZERO (behavior today) 2. Poison at free, also Zero at alloc (when __GFP_ZERO) (this patch's proposed behavior, also what current poisoning does, doubles writes) 3. Zero at free, *don't* Zero at alloc (when __GFP_ZERO) (what I'm suggesting, possibly less perf impact vs. #2)