Re: [PATCH v4] mm: SLAB freelist randomization

[Andrew Morton <akpm@linux-foundation.org>]

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.

>  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!

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;
> +}
> +
>
> ...
>