Randomized slab caches for kmalloc()

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 address 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 other words, the randomness of cache selection will
be with respect to the code address rather than time, i.e. allocations
in different code paths would most likely pick different caches,
although kmalloc() at each place would use the same cache copy whenever
it is executed. 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.

Meanwhile, the static random selection is further enhanced with a
per-boot random seed, which prevents the attacker from finding a usable
kmalloc that happens to pick the same cache with the vulnerable
subsystem/module by analyzing the open source code. In other words, with
the per-boot seed, the random selection is static during each time the
system starts and runs, but not across different system startups.

The overhead of performance has been tested on a 40-core x86 server by
comparing the results of `perf bench all` between the kernels with and
without this patch based on the latest linux-next kernel, which shows
minor difference. A subset of benchmarks are listed below:

                sched/  sched/  syscall/       mem/       mem/
             messaging    pipe     basic     memcpy     memset
                 (sec)   (sec)     (sec)   (GB/sec)   (GB/sec)

control1         0.019   5.459     0.733  15.258789  51.398026
control2         0.019   5.439     0.730  16.009221  48.828125
control3         0.019   5.282     0.735  16.009221  48.828125
control_avg      0.019   5.393     0.733  15.759077  49.684759

experiment1      0.019   5.374     0.741  15.500992  46.502976
experiment2      0.019   5.440     0.746  16.276042  51.398026
experiment3      0.019   5.242     0.752  15.258789  51.398026
experiment_avg   0.019   5.352     0.746  15.678608  49.766343

The overhead of memory usage was measured by executing `free` after boot
on a QEMU VM with 1GB total memory, and as expected, it's positively
correlated with # of cache copies:

           control  4 copies  8 copies  16 copies

total       969.8M    968.2M    968.2M     968.2M
used         20.0M     21.9M     24.1M      26.7M
free        936.9M    933.6M    931.4M     928.6M
available   932.2M    928.8M    926.6M     923.9M

Co-developed-by: Xiu Jianfeng <[email protected]>
Signed-off-by: Xiu Jianfeng <[email protected]>
Signed-off-by: GONG, Ruiqi <[email protected]>
Reviewed-by: Kees Cook <[email protected]>
Reviewed-by: Hyeonggon Yoo <[email protected]>
Acked-by: Dennis Zhou <[email protected]> # percpu
Signed-off-by: Vlastimil Babka <[email protected]>

Author: GONG, Ruiqi

include/linux/percpu.h

@@ -35,14 +35,20 @@
 #define PCPU_BITMAP_BLOCK_BITS		(PCPU_BITMAP_BLOCK_SIZE >>	\
 					 PCPU_MIN_ALLOC_SHIFT)
 
+#ifdef CONFIG_RANDOM_KMALLOC_CACHES
+#define PERCPU_DYNAMIC_SIZE_SHIFT      12
+#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.
  * The following parameter decide how much resource to preallocate
  * 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
@@ -56,9 +62,9 @@
  * 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;

include/linux/slab.h

@@ -19,6 +19,7 @@
 #include <linux/workqueue.h>
 #include <linux/percpu-refcount.h>
 #include <linux/cleanup.h>
+#include <linux/hash.h>
 
 
 /*
@@ -345,6 +346,12 @@ 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 RANDOM_KMALLOC_CACHES_NR	15 // # of cache copies
+#else
+#define RANDOM_KMALLOC_CACHES_NR	0
+#endif
+
 /*
  * Whenever changing this, take care of that kmalloc_type() and
  * create_kmalloc_caches() still work as intended.
@@ -361,6 +368,8 @@ enum kmalloc_cache_type {
 #ifndef CONFIG_MEMCG_KMEM
 	KMALLOC_CGROUP = KMALLOC_NORMAL,
 #endif
+	KMALLOC_RANDOM_START = KMALLOC_NORMAL,
+	KMALLOC_RANDOM_END = KMALLOC_RANDOM_START + RANDOM_KMALLOC_CACHES_NR,
 #ifdef CONFIG_SLUB_TINY
 	KMALLOC_RECLAIM = KMALLOC_NORMAL,
 #else
@@ -386,14 +395,22 @@ 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)
+extern unsigned long random_kmalloc_seed;
+
+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))
+#ifdef CONFIG_RANDOM_KMALLOC_CACHES
+		/* RANDOM_KMALLOC_CACHES_NR (=15) copies + the KMALLOC_NORMAL */
+		return KMALLOC_RANDOM_START + hash_64(caller ^ random_kmalloc_seed,
+						      ilog2(RANDOM_KMALLOC_CACHES_NR + 1));
+#else
 		return KMALLOC_NORMAL;
+#endif
 
 	/*
 	 * At least one of the flags has to be set. Their priorities in
@@ -580,7 +597,7 @@ 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);
@@ -596,7 +613,7 @@ 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);

mm/Kconfig

@@ -337,6 +337,23 @@ 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 && !SLUB_TINY
+	bool "Randomize 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 more difficult to spray
+	  vulnerable memory objects on the heap for the purpose of exploiting
+	  memory vulnerabilities.
+
+	  Currently the number of copies is set to 16, a reasonably large value
+	  that effectively diverges the memory objects allocated for different
+	  subsystems or modules into different caches, at the expense of a
+	  limited degree of memory and CPU overhead that relates to hardware and
+	  system workload.
+
 endmenu # SLAB allocator options
 
 config SHUFFLE_PAGE_ALLOCATOR

mm/kfence/kfence_test.c

@@ -212,7 +212,9 @@ 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;
+	/* just to get ->align so no need to pass in the real caller */
+	enum kmalloc_cache_type type = kmalloc_type(GFP_KERNEL, 0);
+	return kmalloc_caches[type][__kmalloc_index(size, false)]->align;
 }
 
 /* Must always inline to match stack trace against caller. */
@@ -282,8 +284,9 @@ 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);
+			enum kmalloc_cache_type type = kmalloc_type(GFP_KERNEL, _RET_IP_);
 			struct kmem_cache *s = test_cache ?:
-					kmalloc_caches[kmalloc_type(GFP_KERNEL)][__kmalloc_index(size, false)];
+					kmalloc_caches[type][__kmalloc_index(size, false)];
 
 			/*
 			 * Verify that various helpers return the right values

mm/slab.c

@@ -1670,7 +1670,7 @@ 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;

mm/slab.h

@@ -282,7 +282,7 @@ 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 size, gfp_t flags, unsigned long caller);
 
 void *__kmem_cache_alloc_node(struct kmem_cache *s, gfp_t gfpflags,
 			      int node, size_t orig_size,

mm/slab_common.c

@@ -678,6 +678,11 @@ kmalloc_caches[NR_KMALLOC_TYPES][KMALLOC_SHIFT_HIGH + 1] __ro_after_init =
 { /* initialization for https://bugs.llvm.org/show_bug.cgi?id=42570 */ };
 EXPORT_SYMBOL(kmalloc_caches);
 
+#ifdef CONFIG_RANDOM_KMALLOC_CACHES
+unsigned long random_kmalloc_seed __ro_after_init;
+EXPORT_SYMBOL(random_kmalloc_seed);
+#endif
+
 /*
  * Conversion table for small slabs sizes / 8 to the index in the
  * kmalloc array. This is necessary for slabs < 192 since we have non power
@@ -720,7 +725,7 @@ 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;
 
@@ -735,7 +740,7 @@ 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)
@@ -752,8 +757,11 @@ size_t kmalloc_size_roundup(size_t size)
 	if (size > KMALLOC_MAX_CACHE_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);
+	/*
+	 * The flags don't matter since size_index is common to all.
+	 * Neither does the caller for just getting ->object_size.
+	 */
+	c = kmalloc_slab(size, GFP_KERNEL, 0);
 	return c ? c->object_size : 0;
 }
 EXPORT_SYMBOL(kmalloc_size_roundup);
@@ -776,12 +784,35 @@ EXPORT_SYMBOL(kmalloc_size_roundup);
 #define KMALLOC_RCL_NAME(sz)
 #endif
 
+#ifdef CONFIG_RANDOM_KMALLOC_CACHES
+#define __KMALLOC_RANDOM_CONCAT(a, b) a ## b
+#define KMALLOC_RANDOM_NAME(N, sz) __KMALLOC_RANDOM_CONCAT(KMA_RAND_, N)(sz)
+#define KMA_RAND_1(sz)                  .name[KMALLOC_RANDOM_START +  1] = "kmalloc-rnd-01-" #sz,
+#define KMA_RAND_2(sz)  KMA_RAND_1(sz)  .name[KMALLOC_RANDOM_START +  2] = "kmalloc-rnd-02-" #sz,
+#define KMA_RAND_3(sz)  KMA_RAND_2(sz)  .name[KMALLOC_RANDOM_START +  3] = "kmalloc-rnd-03-" #sz,
+#define KMA_RAND_4(sz)  KMA_RAND_3(sz)  .name[KMALLOC_RANDOM_START +  4] = "kmalloc-rnd-04-" #sz,
+#define KMA_RAND_5(sz)  KMA_RAND_4(sz)  .name[KMALLOC_RANDOM_START +  5] = "kmalloc-rnd-05-" #sz,
+#define KMA_RAND_6(sz)  KMA_RAND_5(sz)  .name[KMALLOC_RANDOM_START +  6] = "kmalloc-rnd-06-" #sz,
+#define KMA_RAND_7(sz)  KMA_RAND_6(sz)  .name[KMALLOC_RANDOM_START +  7] = "kmalloc-rnd-07-" #sz,
+#define KMA_RAND_8(sz)  KMA_RAND_7(sz)  .name[KMALLOC_RANDOM_START +  8] = "kmalloc-rnd-08-" #sz,
+#define KMA_RAND_9(sz)  KMA_RAND_8(sz)  .name[KMALLOC_RANDOM_START +  9] = "kmalloc-rnd-09-" #sz,
+#define KMA_RAND_10(sz) KMA_RAND_9(sz)  .name[KMALLOC_RANDOM_START + 10] = "kmalloc-rnd-10-" #sz,
+#define KMA_RAND_11(sz) KMA_RAND_10(sz) .name[KMALLOC_RANDOM_START + 11] = "kmalloc-rnd-11-" #sz,
+#define KMA_RAND_12(sz) KMA_RAND_11(sz) .name[KMALLOC_RANDOM_START + 12] = "kmalloc-rnd-12-" #sz,
+#define KMA_RAND_13(sz) KMA_RAND_12(sz) .name[KMALLOC_RANDOM_START + 13] = "kmalloc-rnd-13-" #sz,
+#define KMA_RAND_14(sz) KMA_RAND_13(sz) .name[KMALLOC_RANDOM_START + 14] = "kmalloc-rnd-14-" #sz,
+#define KMA_RAND_15(sz) KMA_RAND_14(sz) .name[KMALLOC_RANDOM_START + 15] = "kmalloc-rnd-15-" #sz,
+#else // CONFIG_RANDOM_KMALLOC_CACHES
+#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(RANDOM_KMALLOC_CACHES_NR, __short_size)	\
 	.size = __size,						\
 }
 
@@ -890,6 +921,11 @@ 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_NO_MERGE;
+#endif
+
 	/*
 	 * If CONFIG_MEMCG_KMEM is enabled, disable cache merging for
 	 * KMALLOC_NORMAL caches.
@@ -941,6 +977,9 @@ void __init create_kmalloc_caches(slab_flags_t flags)
 				new_kmalloc_cache(2, type, flags);
 		}
 	}
+#ifdef CONFIG_RANDOM_KMALLOC_CACHES
+	random_kmalloc_seed = get_random_u64();
+#endif
 
 	/* Kmalloc array is now usable */
 	slab_state = UP;
@@ -976,7 +1015,7 @@ 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;