Re: [PATCH RFC v2] Randomized slab caches for kmalloc()
From: xiujianfeng
Date: Thu May 11 2023 - 08:31:07 EST
On 2023/5/8 15:55, GONG, Ruiqi wrote:
> 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 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.
>
> 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:
>
> control experiment (avg of 3 samples)
> sched/messaging (sec) 0.019 0.019
> sched/pipe (sec) 5.253 5.340
> syscall/basic (sec) 0.741 0.742
> mem/memcpy (GB/sec) 15.258789 14.860495
> mem/memset (GB/sec) 48.828125 50.431069
>
Could this be a measurement error? otherwise it doesn't look reasonable
to just improve memcpy and worsen memset.
> 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
>
> Signed-off-by: GONG, Ruiqi <gongruiqi1@xxxxxxxxxx>
> ---
>
> v2:
> - Use hash_64() and a per-boot random seed to select kmalloc() caches.
> - Change acceptable # of caches from [4,16] to {2,4,8,16}, which is
> more compatible with hashing.
> - Supplement results of performance and memory overhead tests.
>
> include/linux/percpu.h | 12 ++++++---
> include/linux/slab.h | 25 +++++++++++++++---
> mm/Kconfig | 49 ++++++++++++++++++++++++++++++++++++
> mm/kfence/kfence_test.c | 4 +--
> mm/slab.c | 2 +-
> mm/slab.h | 3 ++-
> mm/slab_common.c | 56 +++++++++++++++++++++++++++++++++++++----
> 7 files changed, 135 insertions(+), 16 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 6b3e155b70bf..939c41c20600 100644
> --- a/include/linux/slab.h
> +++ b/include/linux/slab.h
> @@ -18,6 +18,9 @@
> #include <linux/workqueue.h>
> #include <linux/percpu-refcount.h>
>
> +#ifdef CONFIG_RANDOM_KMALLOC_CACHES
> +#include <linux/hash.h>
> +#endif
>
> /*
> * Flags to pass to kmem_cache_create().
> @@ -106,6 +109,12 @@
> /* 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)
> @@ -331,7 +340,9 @@ 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 + CONFIG_RANDOM_KMALLOC_CACHES_NR - 1,
> + KMALLOC_NORMAL = KMALLOC_RANDOM_END,
> #ifndef CONFIG_ZONE_DMA
> KMALLOC_DMA = KMALLOC_NORMAL,
> #endif
> @@ -363,14 +374,20 @@ 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
> + return KMALLOC_RANDOM_START + hash_64(caller ^ random_kmalloc_seed, CONFIG_RANDOM_KMALLOC_CACHES_BITS);
> +#else
> return KMALLOC_NORMAL;
> +#endif
>
> /*
> * At least one of the flags has to be set. Their priorities in
> @@ -557,7 +574,7 @@ 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);
> @@ -573,7 +590,7 @@ 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 7672a22647b4..e868da87d9cd 100644
> --- a/mm/Kconfig
> +++ b/mm/Kconfig
> @@ -311,6 +311,55 @@ 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.
> +
> +choice
> + prompt "Number of random slab caches copies"
> + depends on RANDOM_KMALLOC_CACHES
> + default RANDOM_KMALLOC_CACHES_16
> + 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.
> +
> +config RANDOM_KMALLOC_CACHES_2
> + bool "2"
> +
> +config RANDOM_KMALLOC_CACHES_4
> + bool "4"
> +
> +config RANDOM_KMALLOC_CACHES_8
> + bool "8"
> +
> +config RANDOM_KMALLOC_CACHES_16
> + bool "16"
> +
> +endchoice
> +
> +config RANDOM_KMALLOC_CACHES_BITS
> + int
> + default 0 if !RANDOM_KMALLOC_CACHES
> + default 1 if RANDOM_KMALLOC_CACHES_2
> + default 2 if RANDOM_KMALLOC_CACHES_4
> + default 3 if RANDOM_KMALLOC_CACHES_8
> + default 4 if RANDOM_KMALLOC_CACHES_16
> +
> +config RANDOM_KMALLOC_CACHES_NR
> + int
> + default 1 if !RANDOM_KMALLOC_CACHES
> + default 2 if RANDOM_KMALLOC_CACHES_2
> + default 4 if RANDOM_KMALLOC_CACHES_4
> + default 8 if RANDOM_KMALLOC_CACHES_8
> + default 16 if RANDOM_KMALLOC_CACHES_16
> +
> endmenu # SLAB allocator options
>
> config SHUFFLE_PAGE_ALLOCATOR
> diff --git a/mm/kfence/kfence_test.c b/mm/kfence/kfence_test.c
> index 6aee19a79236..8a95ef649d5e 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 bb57f7fdbae1..82e2a8d4cd9d 100644
> --- a/mm/slab.c
> +++ b/mm/slab.c
> @@ -1674,7 +1674,7 @@ 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 f01ac256a8f5..1e484af71c52 100644
> --- a/mm/slab.h
> +++ b/mm/slab.h
> @@ -243,7 +243,7 @@ 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,
> @@ -319,6 +319,7 @@ 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 607249785c07..70899b20a9a7 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 | \
> @@ -679,6 +680,11 @@ 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
> @@ -721,7 +727,7 @@ 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;
>
> @@ -736,7 +742,7 @@ 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)
> @@ -754,7 +760,7 @@ 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);
> @@ -777,12 +783,44 @@ 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)
> +#if CONFIG_RANDOM_KMALLOC_CACHES_BITS >= 1
> +#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,
> +#endif
> +#if CONFIG_RANDOM_KMALLOC_CACHES_BITS >= 2
> +#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,
> +#endif
> +#if CONFIG_RANDOM_KMALLOC_CACHES_BITS >= 3
> +#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,
> +#endif
> +#if CONFIG_RANDOM_KMALLOC_CACHES_BITS >= 4
> +#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,
> +#endif
> +#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(CONFIG_RANDOM_KMALLOC_CACHES_NR, __short_size) \
> .size = __size, \
> }
>
> @@ -878,6 +916,11 @@ 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,
> @@ -904,7 +947,7 @@ 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);
> @@ -922,6 +965,9 @@ 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;
> @@ -957,7 +1003,7 @@ 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;