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Date: Mon, 25 Apr 2016 14:14:33 -0700
From: Thomas Garnier <thgarnie@...gle.com>
To: Andrew Morton <akpm@...ux-foundation.org>
Cc: Christoph Lameter <cl@...ux.com>, Pekka Enberg <penberg@...nel.org>,
David Rientjes <rientjes@...gle.com>, Joonsoo Kim <iamjoonsoo.kim@....com>,
Kees Cook <keescook@...omium.org>, Greg Thelen <gthelen@...gle.com>,
Laura Abbott <labbott@...oraproject.org>, kernel-hardening@...ts.openwall.com,
LKML <linux-kernel@...r.kernel.org>, Linux-MM <linux-mm@...ck.org>
Subject: Re: [PATCH v2] mm: SLAB freelist randomization
On Mon, Apr 25, 2016 at 2:13 PM, Thomas Garnier <thgarnie@...gle.com> wrote:
> On Mon, Apr 25, 2016 at 2:10 PM, Andrew Morton
> <akpm@...ux-foundation.org> wrote:
>> On Mon, 25 Apr 2016 13:39:23 -0700 Thomas Garnier <thgarnie@...gle.com> 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 except
>>> early on boot where global lists are used. 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:
>>>
>>> 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
>>
>> 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?
>>
>>> + 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.
>>> +
>>> config SLUB_CPU_PARTIAL
>>> default y
>>> depends on SLUB && SMP
>>> diff --git a/mm/slab.c b/mm/slab.c
>>> index b82ee6b..89eb617 100644
>>> --- a/mm/slab.c
>>> +++ b/mm/slab.c
>>> @@ -116,6 +116,7 @@
>>> #include <linux/kmemcheck.h>
>>> #include <linux/memory.h>
>>> #include <linux/prefetch.h>
>>> +#include <linux/log2.h>
>>>
>>> #include <net/sock.h>
>>>
>>> @@ -1230,6 +1231,100 @@ 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 void cache_random_seq_create(struct kmem_cache *cachep)
>>> +{
>>> + unsigned int seed, count = cachep->num;
>>> + struct rnd_state state;
>>> +
>>> + if (count < 2)
>>> + return;
>>> +
>>> + cachep->random_seq = kcalloc(count, sizeof(freelist_idx_t), GFP_KERNEL);
>>> + BUG_ON(cachep->random_seq == NULL);
>
> On your previous email. (trying to stay in one thread). I added a
> comment on this
> version to explain that we need best entropy at this boot stage.
>
>>
>> Yikes, that's a bit rude. Is there no way of recovering from this? If
>> the answer to that is really really "no" then I guess we should put a
>> __GFP_NOFAIL in there. Add a comment explaining why (apologetically -
>> __GFP_NOFAIL is unpopular!) and remove the now-unneeded BUG_ON.
>>
>>
>
> We can always use the static. I will update on next iteration to remove the
> BUG_ON.
>
>>> + /* Get best entropy at this stage */
>>> + get_random_bytes_arch(&seed, sizeof(seed));
>>
>> See concerns in other email - isn't this a no-op if CONFIG_ARCH_RANDOM=n?
>>
The arch_* functions will return 0 which will break the loop in
get_random_bytes_arch and make it uses extract_entropy (as does
get_random_bytes).
(cf http://lxr.free-electrons.com/source/drivers/char/random.c#L1335)
I might be missing something.
>
>
>>
>>> + prandom_seed_state(&state, seed);
>>> +
>>> + freelist_randomize(&state, cachep->random_seq, count);
>>> +}
>>> +
>>
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