Re: [PATCH v2] mm: SLAB freelist randomization

[Thomas Garnier <thgarnie@google.com>]

On Mon, Apr 25, 2016 at 2:10 PM, Andrew Morton
<akpm@linux-foundation.org> wrote:
> On Mon, 25 Apr 2016 13:39:23 -0700 Thomas Garnier <thgarnie@google.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?
>


>
>> +     prandom_seed_state(&state, seed);
>> +
>> +     freelist_randomize(&state, cachep->random_seq, count);
>> +}
>> +
>

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