Provide an optional config (CONFIG_FREELIST_RANDOM) to randomize the
SLAB freelist. This security feature reduces the predictability of
the kernel slab allocator against heap overflows.
Randomized lists are pre-computed using a Fisher-Yates shuffle and
re-used on slab creation for performance.
---
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
+ 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]);
+ }
+ }
+}
+#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 */
+
/* 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);
+}
+#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 */
}
+
+#ifdef CONFIG_FREELIST_RANDOM
+ shuffle_freelist(cachep, page, cachep->num);
+#endif /* CONFIG_FREELIST_RANDOM */
}
static void kmem_flagcheck(struct kmem_cache *cachep, gfp_t flags)
--
2.8.0.rc3.226.g39d4020
