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]>
---
v5:
- Rebase to the latest linux-next.
- Make CONFIG_RANDOM_KMALLOC_CACHES depends on !SLUB_TINY.
- Rephrase parts of CONFIG_RANDOM_KMALLOC_CACHES's help paragraph.
- Pass 0 as caller at places where the real caller is not needed.
- Restore KMALLOC_NORMAL to 0, and move KMALLOC_RANDOM_* after it.
- Change RANDOM_KMALLOC_CACHES_NR to 15, and adjust the kmalloc rnd
name accordingly.
- Replace RANDOM_KMALLOC_CACHES_BITS via ilog2(..._NR + 1).
v4:
- Set # of caches to 16 and remove config selection.
- Shorten "kmalloc-random-" to "kmalloc-rnd-".
- Update commit log and config's help paragraph.
- Fine-tune PERCPU_DYNAMIC_SIZE_SHIFT to 12 instead of 13 (enough to
pass compilation with allmodconfig and CONFIG_SLUB_TINY=n).
- Some cleanup and typo fixing.
- Link: https://lore.kernel.org/all/[email protected]/
v3:
- Replace SLAB_RANDOMSLAB with the new existing SLAB_NO_MERGE flag.
- Shorten long code lines by wrapping and renaming.
- Update commit message with latest perf benchmark and additional
theorectical explanation.
- Remove "RFC" from patch title and make it a formal patch
- Link: https://lore.kernel.org/all/[email protected]/
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.
- Link: https://lore.kernel.org/all/[email protected]/
v1:
- Link: https://lore.kernel.org/all/[email protected]/
include/linux/percpu.h | 12 +++++++---
include/linux/slab.h | 23 ++++++++++++++++---
mm/Kconfig | 17 ++++++++++++++
mm/kfence/kfence_test.c | 7 ++++--
mm/slab.c | 2 +-
mm/slab.h | 2 +-
mm/slab_common.c | 49 ++++++++++++++++++++++++++++++++++++-----
7 files changed, 97 insertions(+), 15 deletions(-)
diff --git a/include/linux/percpu.h b/include/linux/percpu.h
index b3b458442330..68fac2e7cbe6 100644
--- a/include/linux/percpu.h
+++ b/include/linux/percpu.h
@@ -35,6 +35,12 @@
#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.
@@ -42,7 +48,7 @@
* 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
@@ -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;
diff --git a/include/linux/slab.h b/include/linux/slab.h
index 848c7c82ad5a..8228d1276a2f 100644
--- a/include/linux/slab.h
+++ b/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);
diff --git a/mm/Kconfig b/mm/Kconfig
index 22acffd9009d..989ab72bbecc 100644
--- a/mm/Kconfig
+++ b/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
diff --git a/mm/kfence/kfence_test.c b/mm/kfence/kfence_test.c
index 9e008a336d9f..95b2b84c296d 100644
--- a/mm/kfence/kfence_test.c
+++ b/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
diff --git a/mm/slab.c b/mm/slab.c
index 88194391d553..9ad3d0f2d1a5 100644
--- a/mm/slab.c
+++ b/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;
diff --git a/mm/slab.h b/mm/slab.h
index 9c0e09d0f81f..799a315695c6 100644
--- a/mm/slab.h
+++ b/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,
diff --git a/mm/slab_common.c b/mm/slab_common.c
index d1555ea2981a..01cdbf122463 100644
--- a/mm/slab_common.c
+++ b/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;
--
2.25.1
On 7/14/23 08:44, 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 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]>
Thanks! Pushed to slab/for-6.6/random_kmalloc and for-next.
+CC also percpu maintainers for awareness/review of the percpu.h changes
On 7/14/23 08:44, 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 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]>
> ---
>
> v5:
> - Rebase to the latest linux-next.
> - Make CONFIG_RANDOM_KMALLOC_CACHES depends on !SLUB_TINY.
> - Rephrase parts of CONFIG_RANDOM_KMALLOC_CACHES's help paragraph.
> - Pass 0 as caller at places where the real caller is not needed.
> - Restore KMALLOC_NORMAL to 0, and move KMALLOC_RANDOM_* after it.
> - Change RANDOM_KMALLOC_CACHES_NR to 15, and adjust the kmalloc rnd
> name accordingly.
> - Replace RANDOM_KMALLOC_CACHES_BITS via ilog2(..._NR + 1).
>
> v4:
> - Set # of caches to 16 and remove config selection.
> - Shorten "kmalloc-random-" to "kmalloc-rnd-".
> - Update commit log and config's help paragraph.
> - Fine-tune PERCPU_DYNAMIC_SIZE_SHIFT to 12 instead of 13 (enough to
> pass compilation with allmodconfig and CONFIG_SLUB_TINY=n).
> - Some cleanup and typo fixing.
> - Link: https://lore.kernel.org/all/[email protected]/
>
> v3:
> - Replace SLAB_RANDOMSLAB with the new existing SLAB_NO_MERGE flag.
> - Shorten long code lines by wrapping and renaming.
> - Update commit message with latest perf benchmark and additional
> theorectical explanation.
> - Remove "RFC" from patch title and make it a formal patch
> - Link: https://lore.kernel.org/all/[email protected]/
>
> 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.
> - Link: https://lore.kernel.org/all/[email protected]/
>
> v1:
> - Link: https://lore.kernel.org/all/[email protected]/
>
> include/linux/percpu.h | 12 +++++++---
> include/linux/slab.h | 23 ++++++++++++++++---
> mm/Kconfig | 17 ++++++++++++++
> mm/kfence/kfence_test.c | 7 ++++--
> mm/slab.c | 2 +-
> mm/slab.h | 2 +-
> mm/slab_common.c | 49 ++++++++++++++++++++++++++++++++++++-----
> 7 files changed, 97 insertions(+), 15 deletions(-)
>
> diff --git a/include/linux/percpu.h b/include/linux/percpu.h
> index b3b458442330..68fac2e7cbe6 100644
> --- a/include/linux/percpu.h
> +++ b/include/linux/percpu.h
> @@ -35,6 +35,12 @@
> #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.
> @@ -42,7 +48,7 @@
> * 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
> @@ -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;
> diff --git a/include/linux/slab.h b/include/linux/slab.h
> index 848c7c82ad5a..8228d1276a2f 100644
> --- a/include/linux/slab.h
> +++ b/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);
> diff --git a/mm/Kconfig b/mm/Kconfig
> index 22acffd9009d..989ab72bbecc 100644
> --- a/mm/Kconfig
> +++ b/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
> diff --git a/mm/kfence/kfence_test.c b/mm/kfence/kfence_test.c
> index 9e008a336d9f..95b2b84c296d 100644
> --- a/mm/kfence/kfence_test.c
> +++ b/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
> diff --git a/mm/slab.c b/mm/slab.c
> index 88194391d553..9ad3d0f2d1a5 100644
> --- a/mm/slab.c
> +++ b/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;
> diff --git a/mm/slab.h b/mm/slab.h
> index 9c0e09d0f81f..799a315695c6 100644
> --- a/mm/slab.h
> +++ b/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,
> diff --git a/mm/slab_common.c b/mm/slab_common.c
> index d1555ea2981a..01cdbf122463 100644
> --- a/mm/slab_common.c
> +++ b/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;
Hello,
On Fri, Jul 14, 2023 at 10:31:35AM +0200, Vlastimil Babka wrote:
> +CC also percpu maintainers for awareness/review of the percpu.h changes
>
> On 7/14/23 08:44, 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 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]>
> > ---
> >
> > v5:
> > - Rebase to the latest linux-next.
> > - Make CONFIG_RANDOM_KMALLOC_CACHES depends on !SLUB_TINY.
> > - Rephrase parts of CONFIG_RANDOM_KMALLOC_CACHES's help paragraph.
> > - Pass 0 as caller at places where the real caller is not needed.
> > - Restore KMALLOC_NORMAL to 0, and move KMALLOC_RANDOM_* after it.
> > - Change RANDOM_KMALLOC_CACHES_NR to 15, and adjust the kmalloc rnd
> > name accordingly.
> > - Replace RANDOM_KMALLOC_CACHES_BITS via ilog2(..._NR + 1).
> >
> > v4:
> > - Set # of caches to 16 and remove config selection.
> > - Shorten "kmalloc-random-" to "kmalloc-rnd-".
> > - Update commit log and config's help paragraph.
> > - Fine-tune PERCPU_DYNAMIC_SIZE_SHIFT to 12 instead of 13 (enough to
> > pass compilation with allmodconfig and CONFIG_SLUB_TINY=n).
> > - Some cleanup and typo fixing.
> > - Link: https://lore.kernel.org/all/[email protected]/
> >
> > v3:
> > - Replace SLAB_RANDOMSLAB with the new existing SLAB_NO_MERGE flag.
> > - Shorten long code lines by wrapping and renaming.
> > - Update commit message with latest perf benchmark and additional
> > theorectical explanation.
> > - Remove "RFC" from patch title and make it a formal patch
> > - Link: https://lore.kernel.org/all/[email protected]/
> >
> > 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.
> > - Link: https://lore.kernel.org/all/[email protected]/
> >
> > v1:
> > - Link: https://lore.kernel.org/all/[email protected]/
> >
> > include/linux/percpu.h | 12 +++++++---
> > include/linux/slab.h | 23 ++++++++++++++++---
> > mm/Kconfig | 17 ++++++++++++++
> > mm/kfence/kfence_test.c | 7 ++++--
> > mm/slab.c | 2 +-
> > mm/slab.h | 2 +-
> > mm/slab_common.c | 49 ++++++++++++++++++++++++++++++++++++-----
> > 7 files changed, 97 insertions(+), 15 deletions(-)
> >
> > diff --git a/include/linux/percpu.h b/include/linux/percpu.h
> > index b3b458442330..68fac2e7cbe6 100644
> > --- a/include/linux/percpu.h
> > +++ b/include/linux/percpu.h
> > @@ -35,6 +35,12 @@
> > #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.
> > @@ -42,7 +48,7 @@
> > * 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
> > @@ -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
> >
I'm not in love with being dynamic against BITS_PER_LONG and now
CONFIG_RANDOM_KMALLOC_CACHES. It's been a while since these numbers have
been re-evaluated. I have some draft patches to do batch percpu
allocation. Once I get around to mailing those out, I can try and clean
this up too.
Acked-by: Dennis Zhou <[email protected]>
Thanks,
Dennis
> > extern void *pcpu_base_addr;
> > diff --git a/include/linux/slab.h b/include/linux/slab.h
> > index 848c7c82ad5a..8228d1276a2f 100644
> > --- a/include/linux/slab.h
> > +++ b/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);
> > diff --git a/mm/Kconfig b/mm/Kconfig
> > index 22acffd9009d..989ab72bbecc 100644
> > --- a/mm/Kconfig
> > +++ b/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
> > diff --git a/mm/kfence/kfence_test.c b/mm/kfence/kfence_test.c
> > index 9e008a336d9f..95b2b84c296d 100644
> > --- a/mm/kfence/kfence_test.c
> > +++ b/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
> > diff --git a/mm/slab.c b/mm/slab.c
> > index 88194391d553..9ad3d0f2d1a5 100644
> > --- a/mm/slab.c
> > +++ b/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;
> > diff --git a/mm/slab.h b/mm/slab.h
> > index 9c0e09d0f81f..799a315695c6 100644
> > --- a/mm/slab.h
> > +++ b/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,
> > diff --git a/mm/slab_common.c b/mm/slab_common.c
> > index d1555ea2981a..01cdbf122463 100644
> > --- a/mm/slab_common.c
> > +++ b/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;
>
>
On Fri, Jul 14, 2023 at 3:40 PM GONG, Ruiqi <[email protected]> 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 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]>
> ---
>
> v5:
> - Rebase to the latest linux-next.
> - Make CONFIG_RANDOM_KMALLOC_CACHES depends on !SLUB_TINY.
> - Rephrase parts of CONFIG_RANDOM_KMALLOC_CACHES's help paragraph.
> - Pass 0 as caller at places where the real caller is not needed.
> - Restore KMALLOC_NORMAL to 0, and move KMALLOC_RANDOM_* after it.
> - Change RANDOM_KMALLOC_CACHES_NR to 15, and adjust the kmalloc rnd
> name accordingly.
> - Replace RANDOM_KMALLOC_CACHES_BITS via ilog2(..._NR + 1).
>
> v4:
> - Set # of caches to 16 and remove config selection.
> - Shorten "kmalloc-random-" to "kmalloc-rnd-".
> - Update commit log and config's help paragraph.
> - Fine-tune PERCPU_DYNAMIC_SIZE_SHIFT to 12 instead of 13 (enough to
> pass compilation with allmodconfig and CONFIG_SLUB_TINY=n).
> - Some cleanup and typo fixing.
> - Link: https://lore.kernel.org/all/[email protected]/
>
> v3:
> - Replace SLAB_RANDOMSLAB with the new existing SLAB_NO_MERGE flag.
> - Shorten long code lines by wrapping and renaming.
> - Update commit message with latest perf benchmark and additional
> theorectical explanation.
> - Remove "RFC" from patch title and make it a formal patch
> - Link: https://lore.kernel.org/all/[email protected]/
>
> 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.
> - Link: https://lore.kernel.org/all/[email protected]/
>
> v1:
> - Link: https://lore.kernel.org/all/[email protected]/
>
> include/linux/percpu.h | 12 +++++++---
> include/linux/slab.h | 23 ++++++++++++++++---
> mm/Kconfig | 17 ++++++++++++++
> mm/kfence/kfence_test.c | 7 ++++--
> mm/slab.c | 2 +-
> mm/slab.h | 2 +-
> mm/slab_common.c | 49 ++++++++++++++++++++++++++++++++++++-----
> 7 files changed, 97 insertions(+), 15 deletions(-)
>
> diff --git a/include/linux/percpu.h b/include/linux/percpu.h
> index b3b458442330..68fac2e7cbe6 100644
> --- a/include/linux/percpu.h
> +++ b/include/linux/percpu.h
> @@ -35,6 +35,12 @@
> #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.
> @@ -42,7 +48,7 @@
> * 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
> @@ -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;
> diff --git a/include/linux/slab.h b/include/linux/slab.h
> index 848c7c82ad5a..8228d1276a2f 100644
> --- a/include/linux/slab.h
> +++ b/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);
> diff --git a/mm/Kconfig b/mm/Kconfig
> index 22acffd9009d..989ab72bbecc 100644
> --- a/mm/Kconfig
> +++ b/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
> diff --git a/mm/kfence/kfence_test.c b/mm/kfence/kfence_test.c
> index 9e008a336d9f..95b2b84c296d 100644
> --- a/mm/kfence/kfence_test.c
> +++ b/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
> diff --git a/mm/slab.c b/mm/slab.c
> index 88194391d553..9ad3d0f2d1a5 100644
> --- a/mm/slab.c
> +++ b/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;
> diff --git a/mm/slab.h b/mm/slab.h
> index 9c0e09d0f81f..799a315695c6 100644
> --- a/mm/slab.h
> +++ b/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,
> diff --git a/mm/slab_common.c b/mm/slab_common.c
> index d1555ea2981a..01cdbf122463 100644
> --- a/mm/slab_common.c
> +++ b/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;
> --
> 2.25.1
Looks good to me,
Reviewed-by: Hyeonggon Yoo <[email protected]>
Thanks,
Hyeonggon
On Mon, Sep 11, 2023 at 11:18:15PM +0200, jvoisin wrote:
> I wrote a small blogpost[1] about this series, and was told[2] that it
> would be interesting to share it on this thread, so here it is, copied
> verbatim:
Thanks for posting!
> Ruiqi Gong and Xiu Jianfeng got their
> [Randomized slab caches for
> kmalloc()](https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/commit/?id=3c6152940584290668b35fa0800026f6a1ae05fe)
> patch series merged upstream, and I've and enough discussions about it to
> warrant summarising them into a small blogpost.
>
> The main idea is to have multiple slab caches, and pick one at random
> based on
> the address of code calling `kmalloc()` and a per-boot seed, to make
> heap-spraying harder.
> It's a great idea, but comes with some shortcomings for now:
>
> - Objects being allocated via wrappers around `kmalloc()`, like
> `sock_kmalloc`,
> `f2fs_kmalloc`, `aligned_kmalloc`, … will end up in the same slab cache.
I'd love to see some way to "unwrap" these kinds of allocators. Right
now we try to manually mark them so the debugging options can figure out
what did the allocation, but it's not complete by any means.
I'd kind of like to see a common front end that specified some set of
"do stuff" routines. e.g. to replace devm_kmalloc(), we could have:
void *alloc(size_t usable, gfp_t flags,
size_t (*prepare)(size_t, gfp_t, void *ctx),
void * (*finish)(size_t, gfp_t, void *ctx, void *allocated)
void * ctx)
ssize_t devm_prep(size_t usable, gfp_t *flags, void *ctx)
{
ssize_t tot_size;
if (unlikely(check_add_overflow(sizeof(struct devres),
size, &tot_size)))
return -ENOMEM;
tot_size = kmalloc_size_roundup(tot_size);
*flags |= __GFP_ZERO;
return tot_size;
}
void *devm_finish(size_t usable, gfp_t flags, void *ctx, void *allocated)
{
struct devres *dr = allocated;
struct device *dev = ctx;
INIT_LIST_HEAD(&dr->node.entry);
dr->node.release = devm_kmalloc_release;
set_node_dbginfo(&dr->node, "devm_kzalloc_release", usable);
devres_add(dev, dr->data);
return dr->data;
}
#define devm_kmalloc(dev, size, gfp) \
alloc(size, gfp, devm_prep, devm_finish, dev)
And now there's no wrapper any more, just a routine to get the actual
size, and a routine to set up the memory and return the "usable"
pointer.
> - The slabs needs to be pinned, otherwise an attacker could
> [feng-shui](https://en.wikipedia.org/wiki/Heap_feng_shui) their way
> into having the whole slab free'ed, garbage-collected, and have a slab for
> another type allocated at the same VA. [Jann Horn](https://thejh.net/)
> and [Matteo Rizzo](https://infosec.exchange/@nspace) have a [nice
> set of
> patches](https://github.com/torvalds/linux/compare/master...thejh:linux:slub-virtual-upstream),
> discussed a bit in [this Project Zero
> blogpost](https://googleprojectzero.blogspot.com/2021/10/how-simple-linux-kernel-memory.html),
> for a feature called [`SLAB_VIRTUAL`](
> https://github.com/torvalds/linux/commit/f3afd3a2152353be355b90f5fd4367adbf6a955e),
> implementing precisely this.
I'm hoping this will get posted to LKML soon.
> - There are 16 slabs by default, so one chance out of 16 to end up in
> the same
> slab cache as the target.
Future work can make this more deterministic.
> - There are no guard pages between caches, so inter-caches overflows are
> possible.
This may be addressed by SLAB_VIRTUAL.
> - As pointed by
> [andreyknvl](https://twitter.com/andreyknvl/status/1700267669336080678)
> and [minipli](https://infosec.exchange/@minipli/111045336853055793),
> the fewer allocations hitting a given cache means less noise,
> so it might even help with some heap feng-shui.
That may be true, but I suspect it'll be mitigated by the overall
reduction shared caches.
> - minipli also pointed that "randomized caches still freely
> mix kernel allocations with user controlled ones (`xattr`, `keyctl`,
> `msg_msg`, …).
> So even though merging is disabled for these caches, i.e. no direct
> overlap
> with `cred_jar` etc., other object types can still be targeted (`struct
> pipe_buffer`, BPF maps, its verifier state objects,…). It’s just a
> matter of
> probing which allocation index the targeted object falls into.",
> but I considered this out of scope, since it's much more involved;
> albeit something like
> [`CONFIG_KMALLOC_SPLIT_VARSIZE`](https://github.com/thejh/linux/blob/slub-virtual/MITIGATION_README)
> wouldn't significantly increase complexity.
Now that we have a mechanism to easily deal with "many kmalloc buckets",
I think we can easily start carving out specific variable-sized caches
(like msg_msg). Basically doing a manual type-based separation.
So, yeah, we're in a better place that we were before, and better
positioned to continue to make improvements here. I think an easy win
would be doing this last one: separate out the user controlled
variable-sized caches and give them their own distinct buckets outside
of the 16 random ones. Can you give that a try and send patches?
-Kees
--
Kees Cook