Re: [PATCH v4] mm: SLAB freelist randomization

[Thomas Garnier <thgarnie@google.com>]

On Tue, Apr 26, 2016 at 4:17 PM, Andrew Morton
<akpm@linux-foundation.org> wrote:
> On Tue, 26 Apr 2016 09:21:10 -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. 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.
>

It was discussed a bit before. The intent is to have a similar feature
for other kernel heap (I know it is possible for SLUB). That's why I
think it make sense to have a similar config name used for all
allocators.

>>  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 ;)
>

Yes, as Christophe asked.

> 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!
>
> 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.
>
> 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).
>
> 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?
>
>> +     /* Get best entropy at this stage */
>> +     get_random_bytes_arch(&seed, sizeof(seed));
>> +     prandom_seed_state(&state, seed);
>> +
>> +     freelist_randomize(&state, cachep->random_seq, count);
>> +     return 0;
>> +}
>> +
>>
>> ...
>>

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