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Date: Tue, 26 Apr 2016 09:40:22 +0900
From: Joonsoo Kim <iamjoonsoo.kim@....com>
To: Thomas Garnier <thgarnie@...gle.com>
Cc: Christoph Lameter <cl@...ux.com>, Pekka Enberg <penberg@...nel.org>,
	David Rientjes <rientjes@...gle.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Kees Cook <keescook@...omium.org>, gthelen@...gle.com,
	labbott@...oraproject.org, kernel-hardening@...ts.openwall.com,
	linux-kernel@...r.kernel.org, linux-mm@...ck.org
Subject: Re: [PATCH v2] mm: SLAB freelist randomization

On Mon, Apr 25, 2016 at 01:39:23PM -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 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.
> 
> Before:
> 
> Single thread testing
> =====================
> 1. Kmalloc: Repeatedly allocate then free test
> 10000 times kmalloc(8) -> 99 cycles kfree -> 112 cycles
> 10000 times kmalloc(16) -> 109 cycles kfree -> 140 cycles
> 10000 times kmalloc(32) -> 129 cycles kfree -> 137 cycles
> 10000 times kmalloc(64) -> 141 cycles kfree -> 141 cycles
> 10000 times kmalloc(128) -> 152 cycles kfree -> 148 cycles
> 10000 times kmalloc(256) -> 195 cycles kfree -> 167 cycles
> 10000 times kmalloc(512) -> 257 cycles kfree -> 199 cycles
> 10000 times kmalloc(1024) -> 393 cycles kfree -> 251 cycles
> 10000 times kmalloc(2048) -> 649 cycles kfree -> 228 cycles
> 10000 times kmalloc(4096) -> 806 cycles kfree -> 370 cycles
> 10000 times kmalloc(8192) -> 814 cycles kfree -> 411 cycles
> 10000 times kmalloc(16384) -> 892 cycles kfree -> 455 cycles
> 2. Kmalloc: alloc/free test
> 10000 times kmalloc(8)/kfree -> 121 cycles
> 10000 times kmalloc(16)/kfree -> 121 cycles
> 10000 times kmalloc(32)/kfree -> 121 cycles
> 10000 times kmalloc(64)/kfree -> 121 cycles
> 10000 times kmalloc(128)/kfree -> 121 cycles
> 10000 times kmalloc(256)/kfree -> 119 cycles
> 10000 times kmalloc(512)/kfree -> 119 cycles
> 10000 times kmalloc(1024)/kfree -> 119 cycles
> 10000 times kmalloc(2048)/kfree -> 119 cycles
> 10000 times kmalloc(4096)/kfree -> 121 cycles
> 10000 times kmalloc(8192)/kfree -> 119 cycles
> 10000 times kmalloc(16384)/kfree -> 119 cycles
> 
> After:
> 
> Single thread testing
> =====================
> 1. Kmalloc: Repeatedly allocate then free test
> 10000 times kmalloc(8) -> 130 cycles kfree -> 86 cycles
> 10000 times kmalloc(16) -> 118 cycles kfree -> 86 cycles
> 10000 times kmalloc(32) -> 121 cycles kfree -> 85 cycles
> 10000 times kmalloc(64) -> 176 cycles kfree -> 102 cycles
> 10000 times kmalloc(128) -> 178 cycles kfree -> 100 cycles
> 10000 times kmalloc(256) -> 205 cycles kfree -> 109 cycles
> 10000 times kmalloc(512) -> 262 cycles kfree -> 136 cycles
> 10000 times kmalloc(1024) -> 342 cycles kfree -> 157 cycles
> 10000 times kmalloc(2048) -> 701 cycles kfree -> 238 cycles
> 10000 times kmalloc(4096) -> 803 cycles kfree -> 364 cycles
> 10000 times kmalloc(8192) -> 835 cycles kfree -> 404 cycles
> 10000 times kmalloc(16384) -> 896 cycles kfree -> 441 cycles
> 2. Kmalloc: alloc/free test
> 10000 times kmalloc(8)/kfree -> 121 cycles
> 10000 times kmalloc(16)/kfree -> 121 cycles
> 10000 times kmalloc(32)/kfree -> 123 cycles
> 10000 times kmalloc(64)/kfree -> 142 cycles
> 10000 times kmalloc(128)/kfree -> 121 cycles
> 10000 times kmalloc(256)/kfree -> 119 cycles
> 10000 times kmalloc(512)/kfree -> 119 cycles
> 10000 times kmalloc(1024)/kfree -> 119 cycles
> 10000 times kmalloc(2048)/kfree -> 119 cycles
> 10000 times kmalloc(4096)/kfree -> 119 cycles
> 10000 times kmalloc(8192)/kfree -> 119 cycles
> 10000 times kmalloc(16384)/kfree -> 119 cycles
> 
> Signed-off-by: Thomas Garnier <thgarnie@...gle.com>
> ---
> Based on next-20160422
> ---
>  include/linux/slab_def.h |   4 +
>  init/Kconfig             |   9 ++
>  mm/slab.c                | 213 ++++++++++++++++++++++++++++++++++++++++++++++-
>  3 files changed, 224 insertions(+), 2 deletions(-)
> 
> diff --git a/include/linux/slab_def.h b/include/linux/slab_def.h
> index 9edbbf3..182ec26 100644
> --- 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.
> +
>  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);

Hello,

Please make function return int and propagate error to the cache creator.

> +
> +	/* 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);
> +}
> +
> +/* Destroy the per-cache random freelist sequence */
> +static void cache_random_seq_destroy(struct kmem_cache *cachep)
> +{
> +	kfree(cachep->random_seq);
> +	cachep->random_seq = NULL;
> +}
> +
> +/*
> + * Global static list are used when pre-computed cache list are not yet
> + * available. Lists of different sizes are created to optimize performance on
> + * SLABS with different object counts.
> + */
> +static freelist_idx_t freelist_random_seq_2[2];
> +static freelist_idx_t freelist_random_seq_4[4];
> +static freelist_idx_t freelist_random_seq_8[8];
> +static freelist_idx_t freelist_random_seq_16[16];
> +static freelist_idx_t freelist_random_seq_32[32];
> +static freelist_idx_t freelist_random_seq_64[64];
> +static freelist_idx_t freelist_random_seq_128[128];
> +static freelist_idx_t freelist_random_seq_256[256];
> +const static struct m_list {
> +	size_t count;
> +	freelist_idx_t *list;
> +} freelist_random_seqs[] = {
> +	{ ARRAY_SIZE(freelist_random_seq_2), freelist_random_seq_2 },
> +	{ ARRAY_SIZE(freelist_random_seq_4), freelist_random_seq_4 },
> +	{ ARRAY_SIZE(freelist_random_seq_8), freelist_random_seq_8 },
> +	{ ARRAY_SIZE(freelist_random_seq_16), freelist_random_seq_16 },
> +	{ ARRAY_SIZE(freelist_random_seq_32), freelist_random_seq_32 },
> +	{ ARRAY_SIZE(freelist_random_seq_64), freelist_random_seq_64 },
> +	{ ARRAY_SIZE(freelist_random_seq_128), freelist_random_seq_128 },
> +	{ ARRAY_SIZE(freelist_random_seq_256), freelist_random_seq_256 },
> +};

I'd like to remove this global static list even if we can't get random
sequence in early boot-up process. In this stage that kernel is not
yet initialized, malicious user cannot do anything so random sequence
doesn't give any more security. After kernel initialization, we will
use per cache random sequence so problem suface is really small. If you
want to randomize freelist sequence even in this case, you can manually
permute the sequence with calling prandom_u32_state(). But, I don't
think it is necessary.

Thanks.

> +
> +/* Pre-compute the global pre-computed lists early at boot */
> +static void __init freelist_random_init(void)
> +{
> +	unsigned int seed;
> +	size_t i;
> +	struct rnd_state state;
> +
> +	/* Get best entropy available at this stage */
> +	get_random_bytes_arch(&seed, sizeof(seed));
> +	prandom_seed_state(&state, seed);
> +
> +	for (i = 0; i < ARRAY_SIZE(freelist_random_seqs); i++) {
> +		freelist_randomize(&state, freelist_random_seqs[i].list,
> +				freelist_random_seqs[i].count);
> +	}
> +}
> +#else
> +static inline void __init freelist_random_init(void) { }
> +static inline void cache_random_seq_create(struct kmem_cache *cachep) { }
> +static inline void cache_random_seq_destroy(struct kmem_cache *cachep) { }
> +#endif /* CONFIG_FREELIST_RANDOM */
> +
> +
>  /*
>   * Initialisation.  Called after the page allocator have been initialised and
>   * before smp_init().
> @@ -1256,6 +1351,8 @@ void __init kmem_cache_init(void)
>  	if (!slab_max_order_set && totalram_pages > (32 << 20) >> PAGE_SHIFT)
>  		slab_max_order = SLAB_MAX_ORDER_HI;
>  
> +	freelist_random_init();
> +
>  	/* Bootstrap is tricky, because several objects are allocated
>  	 * from caches that do not exist yet:
>  	 * 1) initialize the kmem_cache cache: it contains the struct
> @@ -2337,6 +2434,8 @@ void __kmem_cache_release(struct kmem_cache *cachep)
>  	int i;
>  	struct kmem_cache_node *n;
>  
> +	cache_random_seq_destroy(cachep);
> +
>  	free_percpu(cachep->cpu_cache);
>  
>  	/* NUMA: free the node structures */
> @@ -2443,15 +2542,122 @@ static void cache_init_objs_debug(struct kmem_cache *cachep, struct page *page)
>  #endif
>  }
>  
> +#ifdef CONFIG_FREELIST_RANDOM
> +/* Hold information during a freelist initialization */
> +struct freelist_init_state {
> +	unsigned int padding;
> +	unsigned int pos;
> +	unsigned int count;
> +	unsigned int rand;
> +	struct m_list freelist_random_seq;
> +};
> +
> +/* Select the right pre-computed list and initialize state */
> +static void freelist_state_initialize(struct freelist_init_state *state,
> +				struct kmem_cache *cachep,
> +				unsigned int count)
> +{
> +	unsigned int idx;
> +	const unsigned int last_idx = ARRAY_SIZE(freelist_random_seqs) - 1;
> +
> +	memset(state, 0, sizeof(*state));
> +	state->count = count;
> +	state->pos = 0;
> +
> +	/* Use best entropy available to define a random shift */
> +	get_random_bytes_arch(&state->rand, sizeof(state->rand));
> +
> +	if (cachep->random_seq) {
> +		state->freelist_random_seq.list = cachep->random_seq;
> +		state->freelist_random_seq.count = count;
> +	} else {
> +		/* count is always >= 2 */
> +		idx = ilog2(count) - 1;
> +		if (idx >= last_idx)
> +			idx = last_idx;
> +		else if (roundup_pow_of_two(idx + 1) != count)
> +			idx++;
> +		state->freelist_random_seq = freelist_random_seqs[idx];
> +	}
> +}
> +
> +/* Get the next entry on the list depending on the target list size */
> +static freelist_idx_t get_next_entry(struct freelist_init_state *state)
> +{
> +	freelist_idx_t ret;
> +
> +	if (state->pos == state->freelist_random_seq.count) {
> +		state->padding += state->pos;
> +		state->pos = 0;
> +	}
> +
> +	/* Randomize the entry using the random shift */
> +	ret = state->freelist_random_seq.list[state->pos++];
> +	ret = (ret + state->rand) % state->freelist_random_seq.count;
> +	return ret;
> +}
> +
> +static freelist_idx_t next_random_slot(struct freelist_init_state *state)
> +{
> +	freelist_idx_t entry;
> +
> +	do {
> +		entry = get_next_entry(state);
> +	} while ((entry + state->padding) >= state->count);
> +
> +	return entry + state->padding;
> +}
> +
> +/*
> + * Shuffle the freelist initialization state based on pre-computed lists.
> + * return true if the list was successfully shuffled, false otherwise.
> + */
> +static bool shuffle_freelist(struct kmem_cache *cachep, struct page *page)
> +{
> +	unsigned int objfreelist, i, count = cachep->num;
> +	struct freelist_init_state state;
> +
> +	if (count < 2)
> +		return false;
> +
> +	objfreelist = 0;
> +	freelist_state_initialize(&state, cachep, count);
> +
> +	/* Take the first random entry as the objfreelist */
> +	if (OBJFREELIST_SLAB(cachep)) {
> +		objfreelist = next_random_slot(&state);
> +		page->freelist = index_to_obj(cachep, page, objfreelist) +
> +						obj_offset(cachep);
> +		count--;
> +	}
> +	for (i = 0; i < count; i++)
> +		set_free_obj(page, i, next_random_slot(&state));
> +
> +	if (OBJFREELIST_SLAB(cachep))
> +		set_free_obj(page, i, objfreelist);
> +	return true;
> +}
> +#else
> +static inline bool shuffle_freelist(struct kmem_cache *cachep,
> +				struct page *page)
> +{
> +	return false;
> +}
> +#endif /* CONFIG_FREELIST_RANDOM */
> +
>  static void cache_init_objs(struct kmem_cache *cachep,
>  			    struct page *page)
>  {
>  	int i;
>  	void *objp;
> +	bool shuffled;
>  
>  	cache_init_objs_debug(cachep, page);
>  
> -	if (OBJFREELIST_SLAB(cachep)) {
> +	/* Try to randomize the freelist if enabled */
> +	shuffled = shuffle_freelist(cachep, page);
> +
> +	if (!shuffled && OBJFREELIST_SLAB(cachep)) {
>  		page->freelist = index_to_obj(cachep, page, cachep->num - 1) +
>  						obj_offset(cachep);
>  	}
> @@ -2465,7 +2671,8 @@ static void cache_init_objs(struct kmem_cache *cachep,
>  			kasan_poison_object_data(cachep, objp);
>  		}
>  
> -		set_free_obj(page, i, i);
> +		if (!shuffled)
> +			set_free_obj(page, i, i);
>  	}
>  }
>  
> @@ -3815,6 +4022,8 @@ static int enable_cpucache(struct kmem_cache *cachep, gfp_t gfp)
>  	int shared = 0;
>  	int batchcount = 0;
>  
> +	cache_random_seq_create(cachep);
> +
>  	if (!is_root_cache(cachep)) {
>  		struct kmem_cache *root = memcg_root_cache(cachep);
>  		limit = root->limit;
> -- 
> 2.8.0.rc3.226.g39d4020
> 
> --
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