When exploiting memory vulnerabilities, "heap spraying" is a common
technique targeting those related to dynamic memory allocation (i.e. the
"heap"), and it plays an important role in a successful exploitation.
Basically, it is to overwrite the memory area of vulnerable object by
triggering allocation in other subsystems or modules and therefore
getting a reference to the targeted memory location. It's usable on
various types of vulnerablity including use after free (UAF), heap out-
of-bound write and etc.
There are (at least) two reasons why the heap can be sprayed: 1) generic
slab caches are shared among different subsystems and modules, and
2) dedicated slab caches could be merged with the generic ones.
Currently these two factors cannot be prevented at a low cost: the first
one is a widely used memory allocation mechanism, and shutting down slab
merging completely via `slub_nomerge` would be overkill.
To efficiently prevent heap spraying, we propose the following approach:
to create multiple copies of generic slab caches that will never be
merged, and random one of them will be used at allocation. The random
selection is based on the location of code that calls `kmalloc()`, which
means it is static at runtime (rather than dynamically determined at
each time of allocation, which could be bypassed by repeatedly spraying
in brute force). In this way, the vulnerable object and memory allocated
in other subsystems and modules will (most probably) be on different
slab caches, which prevents the object from being sprayed.
Signed-off-by: GONG, Ruiqi <gongruiqi1@xxxxxxxxxx>
---
v0:
The current implementation only randomize slab caches for KMALLOC_NORMAL
allocation. Besides the patch itself, we would also like to know the
opinion of the community about whether or not it's necessary to extend
this randomization to all KMALLOC_*, and if so, if implementing a three-
dimensional `kmalloc_caches` is a better choice.
include/linux/percpu.h | 12 +++++++++---
include/linux/slab.h | 24 +++++++++++++++++++-----
mm/Kconfig | 20 ++++++++++++++++++++
mm/kfence/kfence_test.c | 4 ++--
mm/slab.c | 2 +-
mm/slab.h | 3 ++-
mm/slab_common.c | 40 +++++++++++++++++++++++++++++++++++-----
7 files changed, 88 insertions(+), 17 deletions(-)
diff --git a/include/linux/percpu.h b/include/linux/percpu.h
index 1338ea2aa720..6cee6425951f 100644
--- a/include/linux/percpu.h
+++ b/include/linux/percpu.h
@@ -34,6 +34,12 @@
#define PCPU_BITMAP_BLOCK_BITS (PCPU_BITMAP_BLOCK_SIZE >> \
PCPU_MIN_ALLOC_SHIFT)
+#ifdef CONFIG_RANDOM_KMALLOC_CACHES
+#define PERCPU_DYNAMIC_SIZE_SHIFT 13
+#else
+#define PERCPU_DYNAMIC_SIZE_SHIFT 10
+#endif
+
/*
* Percpu allocator can serve percpu allocations before slab is
* initialized which allows slab to depend on the percpu allocator.
@@ -41,7 +47,7 @@
* for this. Keep PERCPU_DYNAMIC_RESERVE equal to or larger than
* PERCPU_DYNAMIC_EARLY_SIZE.
*/
-#define PERCPU_DYNAMIC_EARLY_SIZE (20 << 10)
+#define PERCPU_DYNAMIC_EARLY_SIZE (20 << PERCPU_DYNAMIC_SIZE_SHIFT)
/*
* PERCPU_DYNAMIC_RESERVE indicates the amount of free area to piggy
@@ -55,9 +61,9 @@
* intelligent way to determine this would be nice.
*/
#if BITS_PER_LONG > 32
-#define PERCPU_DYNAMIC_RESERVE (28 << 10)
+#define PERCPU_DYNAMIC_RESERVE (28 << PERCPU_DYNAMIC_SIZE_SHIFT)
#else
-#define PERCPU_DYNAMIC_RESERVE (20 << 10)
+#define PERCPU_DYNAMIC_RESERVE (20 << PERCPU_DYNAMIC_SIZE_SHIFT)
#endif
extern void *pcpu_base_addr;
diff --git a/include/linux/slab.h b/include/linux/slab.h
index 87d687c43d8c..fea7644a1985 100644
--- a/include/linux/slab.h
+++ b/include/linux/slab.h
@@ -106,6 +106,12 @@
/* Avoid kmemleak tracing */
#define SLAB_NOLEAKTRACE ((slab_flags_t __force)0x00800000U)
+#ifdef CONFIG_RANDOM_KMALLOC_CACHES
+# define SLAB_RANDOMSLAB ((slab_flags_t __force)0x01000000U)
+#else
+# define SLAB_RANDOMSLAB 0
+#endif
+
/* Fault injection mark */
#ifdef CONFIG_FAILSLAB
# define SLAB_FAILSLAB ((slab_flags_t __force)0x02000000U)
@@ -336,6 +342,12 @@ static inline unsigned int arch_slab_minalign(void)
#define SLAB_OBJ_MIN_SIZE (KMALLOC_MIN_SIZE < 16 ? \
(KMALLOC_MIN_SIZE) : 16)
+#ifdef CONFIG_RANDOM_KMALLOC_CACHES
+#define KMALLOC_RANDOM_NR CONFIG_RANDOM_KMALLOC_CACHES_NR
+#else
+#define KMALLOC_RANDOM_NR 1
+#endif
+
/*
* Whenever changing this, take care of that kmalloc_type() and
* create_kmalloc_caches() still work as intended.
@@ -345,7 +357,9 @@ static inline unsigned int arch_slab_minalign(void)
* kmem caches can have both accounted and unaccounted objects.
*/
enum kmalloc_cache_type {
- KMALLOC_NORMAL = 0,
+ KMALLOC_RANDOM_START = 0,
+ KMALLOC_RANDOM_END = KMALLOC_RANDOM_START + KMALLOC_RANDOM_NR - 1,
+ KMALLOC_NORMAL = KMALLOC_RANDOM_END,
#ifndef CONFIG_ZONE_DMA
KMALLOC_DMA = KMALLOC_NORMAL,
#endif
@@ -378,14 +392,14 @@ kmalloc_caches[NR_KMALLOC_TYPES][KMALLOC_SHIFT_HIGH + 1];
(IS_ENABLED(CONFIG_ZONE_DMA) ? __GFP_DMA : 0) | \
(IS_ENABLED(CONFIG_MEMCG_KMEM) ? __GFP_ACCOUNT : 0))
-static __always_inline enum kmalloc_cache_type kmalloc_type(gfp_t flags)
+static __always_inline enum kmalloc_cache_type kmalloc_type(gfp_t flags, unsigned long caller)
{
/*
* The most common case is KMALLOC_NORMAL, so test for it
* with a single branch for all the relevant flags.
*/
if (likely((flags & KMALLOC_NOT_NORMAL_BITS) == 0))
- return KMALLOC_NORMAL;
+ return KMALLOC_RANDOM_START + caller % KMALLOC_RANDOM_NR;
/*
* At least one of the flags has to be set. Their priorities in
@@ -578,7 +592,7 @@ static __always_inline __alloc_size(1) void *kmalloc(size_t size, gfp_t flags)
index = kmalloc_index(size);
return kmalloc_trace(
- kmalloc_caches[kmalloc_type(flags)][index],
+ kmalloc_caches[kmalloc_type(flags, _RET_IP_)][index],
flags, size);
}
return __kmalloc(size, flags);
@@ -604,7 +618,7 @@ static __always_inline __alloc_size(1) void *kmalloc_node(size_t size, gfp_t fla
index = kmalloc_index(size);
return kmalloc_node_trace(
- kmalloc_caches[kmalloc_type(flags)][index],
+ kmalloc_caches[kmalloc_type(flags, _RET_IP_)][index],
flags, node, size);
}
return __kmalloc_node(size, flags, node);
diff --git a/mm/Kconfig b/mm/Kconfig
index bc828f640cd9..0b116bd8fdf0 100644
--- a/mm/Kconfig
+++ b/mm/Kconfig
@@ -333,6 +333,26 @@ config SLUB_CPU_PARTIAL
which requires the taking of locks that may cause latency spikes.
Typically one would choose no for a realtime system.
+config RANDOM_KMALLOC_CACHES
+ default n
+ depends on SLUB
+ bool "Random slab caches for normal kmalloc"
+ help
+ A hardening feature that creates multiple copies of slab caches for
+ normal kmalloc allocation and makes kmalloc randomly pick one based
+ on code address, which makes the attackers unable to spray vulnerable
+ memory objects on the heap for exploiting memory vulnerabilities.
+
+config RANDOM_KMALLOC_CACHES_NR
+ int "Number of random slab caches copies"
+ default 16
+ range 4 16
+ depends on RANDOM_KMALLOC_CACHES
+ help
+ The number of copies of random slab caches. Bigger value makes the
+ potentially vulnerable memory object less likely to collide with
+ objects allocated from other subsystems or modules.
+
endmenu # SLAB allocator options
config SHUFFLE_PAGE_ALLOCATOR
diff --git a/mm/kfence/kfence_test.c b/mm/kfence/kfence_test.c
index b5d66a69200d..316d12af7202 100644
--- a/mm/kfence/kfence_test.c
+++ b/mm/kfence/kfence_test.c
@@ -213,7 +213,7 @@ static void test_cache_destroy(void)
static inline size_t kmalloc_cache_alignment(size_t size)
{
- return kmalloc_caches[kmalloc_type(GFP_KERNEL)][__kmalloc_index(size, false)]->align;
+ return kmalloc_caches[kmalloc_type(GFP_KERNEL, _RET_IP_)][__kmalloc_index(size, false)]->align;
}
/* Must always inline to match stack trace against caller. */
@@ -284,7 +284,7 @@ static void *test_alloc(struct kunit *test, size_t size, gfp_t gfp, enum allocat
if (is_kfence_address(alloc)) {
struct slab *slab = virt_to_slab(alloc);
struct kmem_cache *s = test_cache ?:
- kmalloc_caches[kmalloc_type(GFP_KERNEL)][__kmalloc_index(size, false)];
+ kmalloc_caches[kmalloc_type(GFP_KERNEL, _RET_IP_)][__kmalloc_index(size, false)];
/*
* Verify that various helpers return the right values
diff --git a/mm/slab.c b/mm/slab.c
index dabc2a671fc6..8dc7e183dcc5 100644
--- a/mm/slab.c
+++ b/mm/slab.c
@@ -1675,7 +1675,7 @@ static size_t calculate_slab_order(struct kmem_cache *cachep,
if (freelist_size > KMALLOC_MAX_CACHE_SIZE) {
freelist_cache_size = PAGE_SIZE << get_order(freelist_size);
} else {
- freelist_cache = kmalloc_slab(freelist_size, 0u);
+ freelist_cache = kmalloc_slab(freelist_size, 0u, _RET_IP_);
if (!freelist_cache)
continue;
freelist_cache_size = freelist_cache->size;
diff --git a/mm/slab.h b/mm/slab.h
index 43966aa5fadf..4f4caf422b77 100644
--- a/mm/slab.h
+++ b/mm/slab.h
@@ -280,7 +280,7 @@ void setup_kmalloc_cache_index_table(void);
void create_kmalloc_caches(slab_flags_t);
/* Find the kmalloc slab corresponding for a certain size */
-struct kmem_cache *kmalloc_slab(size_t, gfp_t);
+struct kmem_cache *kmalloc_slab(size_t, gfp_t, unsigned long);
void *__kmem_cache_alloc_node(struct kmem_cache *s, gfp_t gfpflags,
int node, size_t orig_size,
@@ -374,6 +374,7 @@ static inline bool is_kmalloc_cache(struct kmem_cache *s)
SLAB_TEMPORARY | \
SLAB_ACCOUNT | \
SLAB_KMALLOC | \
+ SLAB_RANDOMSLAB | \
SLAB_NO_USER_FLAGS)
bool __kmem_cache_empty(struct kmem_cache *);
diff --git a/mm/slab_common.c b/mm/slab_common.c
index bf4e777cfe90..895a3edb82d4 100644
--- a/mm/slab_common.c
+++ b/mm/slab_common.c
@@ -47,6 +47,7 @@ static DECLARE_WORK(slab_caches_to_rcu_destroy_work,
*/
#define SLAB_NEVER_MERGE (SLAB_RED_ZONE | SLAB_POISON | SLAB_STORE_USER | \
SLAB_TRACE | SLAB_TYPESAFE_BY_RCU | SLAB_NOLEAKTRACE | \
+ SLAB_RANDOMSLAB | \
SLAB_FAILSLAB | kasan_never_merge())
#define SLAB_MERGE_SAME (SLAB_RECLAIM_ACCOUNT | SLAB_CACHE_DMA | \
@@ -722,7 +723,7 @@ static inline unsigned int size_index_elem(unsigned int bytes)
* Find the kmem_cache structure that serves a given size of
* allocation
*/
-struct kmem_cache *kmalloc_slab(size_t size, gfp_t flags)
+struct kmem_cache *kmalloc_slab(size_t size, gfp_t flags, unsigned long caller)
{
unsigned int index;
@@ -737,7 +738,7 @@ struct kmem_cache *kmalloc_slab(size_t size, gfp_t flags)
index = fls(size - 1);
}
- return kmalloc_caches[kmalloc_type(flags)][index];
+ return kmalloc_caches[kmalloc_type(flags, caller)][index];
}
size_t kmalloc_size_roundup(size_t size)
@@ -755,7 +756,7 @@ size_t kmalloc_size_roundup(size_t size)
return PAGE_SIZE << get_order(size);
/* The flags don't matter since size_index is common to all. */
- c = kmalloc_slab(size, GFP_KERNEL);
+ c = kmalloc_slab(size, GFP_KERNEL, _RET_IP_);
return c ? c->object_size : 0;
}
EXPORT_SYMBOL(kmalloc_size_roundup);
@@ -778,12 +779,36 @@ EXPORT_SYMBOL(kmalloc_size_roundup);
#define KMALLOC_RCL_NAME(sz)
#endif
+#ifdef CONFIG_RANDOM_KMALLOC_CACHES
+#define __KMALLOC_RANDOM_CONCAT(a, b, c) a ## b ## c
+#define KMALLOC_RANDOM_NAME(N, sz) __KMALLOC_RANDOM_CONCAT(KMALLOC_RANDOM_, N, _NAME)(sz)
+#define KMALLOC_RANDOM_1_NAME(sz) .name[KMALLOC_RANDOM_START + 0] = "kmalloc-random-01-" #sz,
+#define KMALLOC_RANDOM_2_NAME(sz) KMALLOC_RANDOM_1_NAME(sz) .name[KMALLOC_RANDOM_START + 1] = "kmalloc-random-02-" #sz,
+#define KMALLOC_RANDOM_3_NAME(sz) KMALLOC_RANDOM_2_NAME(sz) .name[KMALLOC_RANDOM_START + 2] = "kmalloc-random-03-" #sz,
+#define KMALLOC_RANDOM_4_NAME(sz) KMALLOC_RANDOM_3_NAME(sz) .name[KMALLOC_RANDOM_START + 3] = "kmalloc-random-04-" #sz,
+#define KMALLOC_RANDOM_5_NAME(sz) KMALLOC_RANDOM_4_NAME(sz) .name[KMALLOC_RANDOM_START + 4] = "kmalloc-random-05-" #sz,
+#define KMALLOC_RANDOM_6_NAME(sz) KMALLOC_RANDOM_5_NAME(sz) .name[KMALLOC_RANDOM_START + 5] = "kmalloc-random-06-" #sz,
+#define KMALLOC_RANDOM_7_NAME(sz) KMALLOC_RANDOM_6_NAME(sz) .name[KMALLOC_RANDOM_START + 6] = "kmalloc-random-07-" #sz,
+#define KMALLOC_RANDOM_8_NAME(sz) KMALLOC_RANDOM_7_NAME(sz) .name[KMALLOC_RANDOM_START + 7] = "kmalloc-random-08-" #sz,
+#define KMALLOC_RANDOM_9_NAME(sz) KMALLOC_RANDOM_8_NAME(sz) .name[KMALLOC_RANDOM_START + 8] = "kmalloc-random-09-" #sz,
+#define KMALLOC_RANDOM_10_NAME(sz) KMALLOC_RANDOM_9_NAME(sz) .name[KMALLOC_RANDOM_START + 9] = "kmalloc-random-10-" #sz,
+#define KMALLOC_RANDOM_11_NAME(sz) KMALLOC_RANDOM_10_NAME(sz) .name[KMALLOC_RANDOM_START + 10] = "kmalloc-random-11-" #sz,
+#define KMALLOC_RANDOM_12_NAME(sz) KMALLOC_RANDOM_11_NAME(sz) .name[KMALLOC_RANDOM_START + 11] = "kmalloc-random-12-" #sz,
+#define KMALLOC_RANDOM_13_NAME(sz) KMALLOC_RANDOM_12_NAME(sz) .name[KMALLOC_RANDOM_START + 12] = "kmalloc-random-13-" #sz,
+#define KMALLOC_RANDOM_14_NAME(sz) KMALLOC_RANDOM_13_NAME(sz) .name[KMALLOC_RANDOM_START + 13] = "kmalloc-random-14-" #sz,
+#define KMALLOC_RANDOM_15_NAME(sz) KMALLOC_RANDOM_14_NAME(sz) .name[KMALLOC_RANDOM_START + 14] = "kmalloc-random-15-" #sz,
+#define KMALLOC_RANDOM_16_NAME(sz) KMALLOC_RANDOM_15_NAME(sz) .name[KMALLOC_RANDOM_START + 15] = "kmalloc-random-16-" #sz,
+#else
+#define KMALLOC_RANDOM_NAME(N, sz)
+#endif
+
#define INIT_KMALLOC_INFO(__size, __short_size) \
{ \
.name[KMALLOC_NORMAL] = "kmalloc-" #__short_size, \
KMALLOC_RCL_NAME(__short_size) \
KMALLOC_CGROUP_NAME(__short_size) \
KMALLOC_DMA_NAME(__short_size) \
+ KMALLOC_RANDOM_NAME(CONFIG_RANDOM_KMALLOC_CACHES_NR, __short_size) \
.size = __size, \
}
@@ -879,6 +904,11 @@ new_kmalloc_cache(int idx, enum kmalloc_cache_type type, slab_flags_t flags)
flags |= SLAB_CACHE_DMA;
}
+#ifdef CONFIG_RANDOM_KMALLOC_CACHES
+ if (type >= KMALLOC_RANDOM_START && type <= KMALLOC_RANDOM_END)
+ flags |= SLAB_RANDOMSLAB;
+#endif
+
kmalloc_caches[type][idx] = create_kmalloc_cache(
kmalloc_info[idx].name[type],
kmalloc_info[idx].size, flags, 0,
@@ -905,7 +935,7 @@ void __init create_kmalloc_caches(slab_flags_t flags)
/*
* Including KMALLOC_CGROUP if CONFIG_MEMCG_KMEM defined
*/
- for (type = KMALLOC_NORMAL; type < NR_KMALLOC_TYPES; type++) {
+ for (type = KMALLOC_RANDOM_START; type < NR_KMALLOC_TYPES; type++) {
for (i = KMALLOC_SHIFT_LOW; i <= KMALLOC_SHIFT_HIGH; i++) {
if (!kmalloc_caches[type][i])
new_kmalloc_cache(i, type, flags);
@@ -958,7 +988,7 @@ void *__do_kmalloc_node(size_t size, gfp_t flags, int node, unsigned long caller
return ret;
}
- s = kmalloc_slab(size, flags);
+ s = kmalloc_slab(size, flags, caller);
if (unlikely(ZERO_OR_NULL_PTR(s)))
return s;
--
2.25.1
