Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1752760AbcD0AzO (ORCPT ); Tue, 26 Apr 2016 20:55:14 -0400 Received: from LGEAMRELO12.lge.com ([156.147.23.52]:44312 "EHLO lgeamrelo12.lge.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1752216AbcD0AzM (ORCPT ); Tue, 26 Apr 2016 20:55:12 -0400 X-Original-SENDERIP: 156.147.1.125 X-Original-MAILFROM: iamjoonsoo.kim@lge.com X-Original-SENDERIP: 10.177.222.138 X-Original-MAILFROM: iamjoonsoo.kim@lge.com Date: Wed, 27 Apr 2016 09:55:05 +0900 From: Joonsoo Kim To: Andrew Morton Cc: Thomas Garnier , Christoph Lameter , Pekka Enberg , David Rientjes , Kees Cook , gthelen@google.com, labbott@fedoraproject.org, kernel-hardening@lists.openwall.com, linux-kernel@vger.kernel.org, linux-mm@kvack.org Subject: Re: [PATCH v4] mm: SLAB freelist randomization Message-ID: <20160427005505.GA6336@js1304-P5Q-DELUXE> References: <1461687670-47585-1-git-send-email-thgarnie@google.com> <20160426161743.f831225a4efb3eb04debe402@linux-foundation.org> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20160426161743.f831225a4efb3eb04debe402@linux-foundation.org> User-Agent: Mutt/1.5.21 (2010-09-15) Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 6223 Lines: 174 On Tue, Apr 26, 2016 at 04:17:43PM -0700, Andrew Morton wrote: > On Tue, 26 Apr 2016 09:21:10 -0700 Thomas Garnier wrote: > > > Provides an optional config (CONFIG_FREELIST_RANDOM) to randomize the > > SLAB freelist. The list is randomized during initialization of a new set > > of pages. The order on different freelist sizes is pre-computed at boot > > for performance. Each kmem_cache has its own randomized freelist. Before > > pre-computed lists are available freelists are generated > > dynamically. This security feature reduces the predictability of the > > kernel SLAB allocator against heap overflows rendering attacks much less > > stable. > > > > For example this attack against SLUB (also applicable against SLAB) > > would be affected: > > https://jon.oberheide.org/blog/2010/09/10/linux-kernel-can-slub-overflow/ > > > > Also, since v4.6 the freelist was moved at the end of the SLAB. It means > > a controllable heap is opened to new attacks not yet publicly discussed. > > A kernel heap overflow can be transformed to multiple use-after-free. > > This feature makes this type of attack harder too. > > > > To generate entropy, we use get_random_bytes_arch because 0 bits of > > entropy is available in the boot stage. In the worse case this function > > will fallback to the get_random_bytes sub API. We also generate a shift > > random number to shift pre-computed freelist for each new set of pages. > > > > The config option name is not specific to the SLAB as this approach will > > be extended to other allocators like SLUB. > > > > Performance results highlighted no major changes: > > > > Hackbench (running 90 10 times): > > > > Before average: 0.0698 > > After average: 0.0663 (-5.01%) > > > > slab_test 1 run on boot. Difference only seen on the 2048 size test > > being the worse case scenario covered by freelist randomization. New > > slab pages are constantly being created on the 10000 allocations. > > Variance should be mainly due to getting new pages every few > > allocations. > > > > ... > > > > --- a/include/linux/slab_def.h > > +++ b/include/linux/slab_def.h > > @@ -80,6 +80,10 @@ struct kmem_cache { > > struct kasan_cache kasan_info; > > #endif > > > > +#ifdef CONFIG_FREELIST_RANDOM > > + void *random_seq; > > +#endif > > + > > struct kmem_cache_node *node[MAX_NUMNODES]; > > }; > > > > diff --git a/init/Kconfig b/init/Kconfig > > index 0c66640..73453d0 100644 > > --- a/init/Kconfig > > +++ b/init/Kconfig > > @@ -1742,6 +1742,15 @@ config SLOB > > > > endchoice > > > > +config FREELIST_RANDOM > > + default n > > + depends on SLAB > > + bool "SLAB freelist randomization" > > + help > > + Randomizes the freelist order used on creating new SLABs. This > > + security feature reduces the predictability of the kernel slab > > + allocator against heap overflows. > > Against the v2 patch I didst observe: > > : CONFIG_FREELIST_RANDOM bugs me a bit - "freelist" is so vague. > : CONFIG_SLAB_FREELIST_RANDOM would be better. I mean, what Kconfig > : identifier could be used for implementing randomisation in > : slub/slob/etc once CONFIG_FREELIST_RANDOM is used up? > > but this pearl appeared to pass unnoticed. > > > config SLUB_CPU_PARTIAL > > default y > > depends on SLUB && SMP > > diff --git a/mm/slab.c b/mm/slab.c > > index b82ee6b..0ed728a 100644 > > --- a/mm/slab.c > > +++ b/mm/slab.c > > @@ -1230,6 +1230,61 @@ static void __init set_up_node(struct kmem_cache *cachep, int index) > > } > > } > > > > +#ifdef CONFIG_FREELIST_RANDOM > > +static void freelist_randomize(struct rnd_state *state, freelist_idx_t *list, > > + size_t count) > > +{ > > + size_t i; > > + unsigned int rand; > > + > > + for (i = 0; i < count; i++) > > + list[i] = i; > > + > > + /* Fisher-Yates shuffle */ > > + for (i = count - 1; i > 0; i--) { > > + rand = prandom_u32_state(state); > > + rand %= (i + 1); > > + swap(list[i], list[rand]); > > + } > > +} > > + > > +/* Create a random sequence per cache */ > > +static int cache_random_seq_create(struct kmem_cache *cachep) > > +{ > > + unsigned int seed, count = cachep->num; > > + struct rnd_state state; > > + > > + if (count < 2) > > + return 0; > > + > > + /* If it fails, we will just use the global lists */ > > + cachep->random_seq = kcalloc(count, sizeof(freelist_idx_t), GFP_KERNEL); > > + if (!cachep->random_seq) > > + return -ENOMEM; > > OK, no BUG. If this happens, kmem_cache_init_late() will go BUG > instead ;) > > Questions for slab maintainers: > > What's going on with the gfp_flags in there? kmem_cache_init_late() > passes GFP_NOWAIT into enable_cpucache(). > > a) why the heck does it do that? It's __init code! Until some boot-up point, we should not enable interrupt. In slab subsystem, If we use __GFP_DIRECT_RECLAIM, it will cause to enable interrupt when allocating new slab page. GFP_NOWAIT is to prevent that situation. Anyway, I audit the code and kmem_cache_init_late() could use __GFP_DIRECT_RECLAIM because it is called after interrupt is enabled which means that that's safe time to manipulate interrupt. (See kmem_cache_init_late() in start_kernel()). > > b) if there's a legit reason then your new cache_random_seq_create() > should be getting its gfp_t from its caller, rather than blindly > assuming GFP_KERNEL. In any case, ignoring provided gfp argument isn't good practice. > c) kmem_cache_init_late() goes BUG on ENOMEM. Generally that's OK in > __init code: we assume infinite memory during bootup. But it's really > quite weird to use GFP_NOWAIT and then to go BUG if GFP_NOWAIT had its > predictable outcome (ie: failure). I don't think BUG() here is weird code. It just means that if we can't initialize slab subsystem properly, machine cannot run properly so BUG(). > Finally, all callers of enable_cpucache() (and hence of > cache_random_seq_create()) are __init, so we're unnecessarily bloating > up vmlinux. Could someone please take a look at this as a separate > thing? That's not true. It is called whenever new kmem_cache is created. I don't know concrete reason why setup_cpu_cache() is defined with __init_refok tag but looks like it needs to be fixed. I will look at it soon. Thanks.