On Wed, Apr 6, 2016 at 12:35 PM, Thomas Garnier <thgarnie@xxxxxxxxxx> wrote:
> Provide an optional config (CONFIG_FREELIST_RANDOM) to randomize the
> SLAB freelist.
It may be useful to describe _how_ it randomizes it (i.e. a high-level
description of what needed changing).
> This security feature reduces the predictability of
> the kernel slab allocator against heap overflows.
I would add "... rendering attacks much less stable." And if you can
find a specific example exploit that is foiled by this, I would refer
to it.
> Randomized lists are pre-computed using a Fisher-Yates shuffle and
Should the use of Fisher-Yates (over other things) be justified?
> re-used on slab creation for performance.
I'd like to see some benchmark results for this so the Kconfig can
include the performance characteristics. I recommend using hackbench
and kernel build times with a before/after comparison.
> ---
> Based on next-20160405
> ---
> init/Kconfig | 9 ++++
> mm/slab.c | 155 +++++++++++++++++++++++++++++++++++++++++++++++++++++++++++
> 2 files changed, 164 insertions(+)
>
> diff --git a/init/Kconfig b/init/Kconfig
> index 0dfd09d..ee35418 100644
> --- a/init/Kconfig
> +++ b/init/Kconfig
> @@ -1742,6 +1742,15 @@ config SLOB
>
> endchoice
>
> +config FREELIST_RANDOM
I think I would name this "SLAB_FREELIST_RANDOM" since it's
SLAB-specific, unless you think it could be extended to the other
allocators in the future too? (If so, I'd mention the naming choice in
the commit log.)
> + 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 b70aabf..6f0d7be 100644
> --- a/mm/slab.c
> +++ b/mm/slab.c
> @@ -1229,6 +1229,59 @@ static void __init set_up_node(struct kmem_cache *cachep, int index)
> }
> }
>
> +#ifdef CONFIG_FREELIST_RANDOM
> +/*
> + * Master lists are pre-computed random lists
> + * Lists of different sizes are used to optimize performance on different
> + * SLAB object sizes per pages.
> + */
> +static freelist_idx_t master_list_2[2];
> +static freelist_idx_t master_list_4[4];
> +static freelist_idx_t master_list_8[8];
> +static freelist_idx_t master_list_16[16];
> +static freelist_idx_t master_list_32[32];
> +static freelist_idx_t master_list_64[64];
> +static freelist_idx_t master_list_128[128];
> +static freelist_idx_t master_list_256[256];
> +static struct m_list {
> + size_t count;
> + freelist_idx_t *list;
> +} master_lists[] = {
> + { ARRAY_SIZE(master_list_2), master_list_2 },
> + { ARRAY_SIZE(master_list_4), master_list_4 },
> + { ARRAY_SIZE(master_list_8), master_list_8 },
> + { ARRAY_SIZE(master_list_16), master_list_16 },
> + { ARRAY_SIZE(master_list_32), master_list_32 },
> + { ARRAY_SIZE(master_list_64), master_list_64 },
> + { ARRAY_SIZE(master_list_128), master_list_128 },
> + { ARRAY_SIZE(master_list_256), master_list_256 },
> +};
> +
> +void __init freelist_random_init(void)
> +{
> + unsigned int seed;
> + size_t z, i, rand;
> + struct rnd_state slab_rand;
> +
> + get_random_bytes_arch(&seed, sizeof(seed));
> + prandom_seed_state(&slab_rand, seed);
> +
> + for (z = 0; z < ARRAY_SIZE(master_lists); z++) {
> + for (i = 0; i < master_lists[z].count; i++)
> + master_lists[z].list[i] = i;
> +
> + /* Fisher-Yates shuffle */
> + for (i = master_lists[z].count - 1; i > 0; i--) {
> + rand = prandom_u32_state(&slab_rand);
> + rand %= (i + 1);
> + swap(master_lists[z].list[i],
> + master_lists[z].list[rand]);
> + }
> + }
> +}
For below...
#else
static inline freelist_random_init(void) { }
> +#endif /* CONFIG_FREELIST_RANDOM */
> +
> +
> /*
> * Initialisation. Called after the page allocator have been initialised and
> * before smp_init().
> @@ -1255,6 +1308,10 @@ void __init kmem_cache_init(void)
> if (!slab_max_order_set && totalram_pages > (32 << 20) >> PAGE_SHIFT)
> slab_max_order = SLAB_MAX_ORDER_HI;
>
> +#ifdef CONFIG_FREELIST_RANDOM
> + freelist_random_init();
> +#endif /* CONFIG_FREELIST_RANDOM */
Rather than these embedded ifdefs, I would create stub function at the
top, as above.
> +
> /* 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
> @@ -2442,6 +2499,98 @@ static void cache_init_objs_debug(struct kmem_cache *cachep, struct page *page)
> #endif
> }
>
> +#ifdef CONFIG_FREELIST_RANDOM
> +enum master_type {
> + match,
> + less,
> + more
> +};
> +
> +struct random_mng {
> + unsigned int padding;
> + unsigned int pos;
> + unsigned int count;
> + struct m_list master_list;
> + unsigned int master_count;
> + enum master_type type;
> +};
> +
> +static void random_mng_initialize(struct random_mng *mng, unsigned int count)
> +{
> + unsigned int idx;
> + const unsigned int last_idx = ARRAY_SIZE(master_lists) - 1;
> +
> + memset(mng, 0, sizeof(*mng));
> + mng->count = count;
> + mng->pos = 0;
> + /* count is >= 2 */
> + idx = ilog2(count) - 1;
> + if (idx >= last_idx)
> + idx = last_idx;
> + else if (roundup_pow_of_two(idx + 1) != count)
> + idx++;
> + mng->master_list = master_lists[idx];
> + if (mng->master_list.count == mng->count)
> + mng->type = match;
> + else if (mng->master_list.count > mng->count)
> + mng->type = more;
> + else
> + mng->type = less;
> +}
> +
> +static freelist_idx_t get_next_entry(struct random_mng *mng)
> +{
> + if (mng->type == less && mng->pos == mng->master_list.count) {
> + mng->padding += mng->pos;
> + mng->pos = 0;
> + }
> + BUG_ON(mng->pos >= mng->master_list.count);
> + return mng->master_list.list[mng->pos++];
> +}
> +
> +static freelist_idx_t next_random_slot(struct random_mng *mng)
> +{
> + freelist_idx_t cur, entry;
> +
> + entry = get_next_entry(mng);
> +
> + if (mng->type != match) {
> + while ((entry + mng->padding) >= mng->count)
> + entry = get_next_entry(mng);
> + cur = entry + mng->padding;
> + BUG_ON(cur >= mng->count);
> + } else {
> + cur = entry;
> + }
> +
> + return cur;
> +}
> +
> +static void shuffle_freelist(struct kmem_cache *cachep, struct page *page,
> + unsigned int count)
> +{
> + unsigned int i;
> + struct random_mng mng;
> +
> + if (count < 2) {
> + for (i = 0; i < count; i++)
> + set_free_obj(page, i, i);
> + return;
> + }
> +
> + /* Last chunk is used already in this case */
> + if (OBJFREELIST_SLAB(cachep))
> + count--;
> +
> + random_mng_initialize(&mng, count);
> + for (i = 0; i < count; i++)
> + set_free_obj(page, i, next_random_slot(&mng));
> +
> + if (OBJFREELIST_SLAB(cachep))
> + set_free_obj(page, i, i);
> +}
Same thing here...
#else
static inline void set_free_obj(...) { }
static inline void shuffle_freelist(struct kmem_cache *cachep,
struct page *page, unsigned int count) { }
> +#endif /* CONFIG_FREELIST_RANDOM */
> +
> static void cache_init_objs(struct kmem_cache *cachep,
> struct page *page)
> {
> @@ -2464,8 +2613,14 @@ static void cache_init_objs(struct kmem_cache *cachep,
> kasan_poison_object_data(cachep, objp);
> }
>
> +#ifndef CONFIG_FREELIST_RANDOM
> set_free_obj(page, i, i);
> +#endif /* CONFIG_FREELIST_RANDOM */
For this one, I'd use:
if (config_enabled(CONFIG_FREELIST_RANDOM))
set_free_obj(page, i, i);
> }
> +
> +#ifdef CONFIG_FREELIST_RANDOM
> + shuffle_freelist(cachep, page, cachep->num);
> +#endif /* CONFIG_FREELIST_RANDOM */
This one can drop the ifdef in favor of using the stub function too.
> }
>
> static void kmem_flagcheck(struct kmem_cache *cachep, gfp_t flags)
> --
> 2.8.0.rc3.226.g39d4020
>
Exciting!
-Kees
--
Kees Cook
Chrome OS & Brillo Security
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