Date: Thu, 17 Dec 2015 12:15:29 -0800 From: Laura Abbott <laura@...bott.name> To: kernel-hardening@...ts.openwall.com Subject: Re: Looking at PAX_MEMORY_SANITIZE On 12/16/15 5:29 PM, Kees Cook wrote: > > > On Wed, Dec 16, 2015 at 10:46 AM, Laura Abbott <laura@...bott.name <mailto:laura@...bott.name>> wrote: > > Hi, > > I started looking at PAX_MEMORY_SANITIZE for bringing into the kernel. I thought > I would give a short update on what I've found so far for feedback and the like. > > PAX_MEMORY_SANITIZE is used for clearing both the SL*B allocators and the > buddy allocator on free. Arguably, similar behavior exists already as debug > features (SLUB_DEBUG poison, DEBUG_PAGEALLOC for some arches). Given what we're > already finding with features like DEBUG_RODATA though, the sanitization really > needs to be a separate Kconfig not tied to debugging. I debated trying to make > those Kconfigs non-debug but they were tied to other features besides poison/ > sanitization. > > > Sounds great! As for CONFIGs, one thing that was request was that we ultimately put all >the hardening configs under a common Kconfig heading so they can be found easily. I view >this as a late-stage tweak, and mostly we just need to worry about functionality first. > Sure. I was going to put the Kconfig somewhere I think is reasonable and see if there are better suggestions. > For upstreaming, we'll need a comparison between PAX_MEMORY_SANITIZE and the existing >upstream things that overlap with it. What kind of comparison are you thinking of? Just a list of what differs featurewise or performance characteristics or something else? > > I've been focusing my efforts on the SL*B allocators. As it stands, the feature > is fairly self-contained and mostly just needs some refactoring. I plan on > expanding the command line option to give a bit more control on where the > sanitization happens. The sanitization currently always happens on the fast path > so my thought was to allow the option of sanitizing only on the slow path. > > > Seems like it should happen on all paths to be a true always-on hardening feature, though? The existing PaX code has an option to skip caches marked with SLAB_NO_SANITIZE. I was thinking skipping on the fast path might be a similar expansion of that. There is still a boot commandline option for full sanitization. I'm going to profile before I send anything out to get a better idea if this option would make a difference. > > The existing PaX code also disables cache merging. It's not clear if this is an > additional security measure but the sanitization as written doesn't work with > merging. For at least the first version, slab merging will be disabled when > sanitization is enabled on a slab. > > > That seems fine to me. These trade-offs are to be expected. > > Another thing we should call attention to are past bugs and exploits or exploit methods > that would have been hampered or eliminated with this feature. e.g. How does this >play with classic use-after-free, etc? Good point. I'll find some examples. > > I'm hoping to post actual patches before I go on vacation for the holidays next > week. Early feedback is appreciated as well if I missed anything. > > > Fantastic! I look forward to testing it. Which gets to a third piece: adding an lkdtm > test for the feature so it's easy to test the kernel's reaction to the state that is >being protected. I'll look into this as well. Thanks, Laura
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