Re: [PATCH v4] Randomized slab caches for kmalloc()
From: GONG, Ruiqi
Date: Fri Jul 14 2023 - 03:09:41 EST
On 2023/07/13 20:46, Vlastimil Babka wrote:
>
> On 6/26/23 05:18, 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.
>
> Interesting :) Here's a recent patch to make slub_nomerge a default:
> https://lore.kernel.org/all/20230629221910.359711-1-julian.pidancet@xxxxxxxxxx/
>
> In v1, Kees said he's been running with that for ages:
> https://lore.kernel.org/all/202306281358.E6E6C2759@keescook/
>
> So it's not universally accepted in the kernel hardening community?
>
Thanks for the information! I'm not sure if the slub merging is that
much unwelcomed in the hardening community, since I don't see hardening
folks complaining a lot about it. I personally don't hate it as it does
bring benefits for the performance, at least theoretically (e.g. keeping
cache hot, reducing internal fragments etc.).
> [...]
>
> Here's the renumbering diff. It assumes SLUB_TINY would be excluded as
> suggested, otherwise more adjustment would be needed for KMALLOC_RECLAIM.
>
> ----8<----
> diff --git a/include/linux/slab.h b/include/linux/slab.h
> index 96fdfd96b708..2f6337361515 100644
> --- a/include/linux/slab.h
> +++ b/include/linux/slab.h
> @@ -347,10 +347,10 @@ static inline unsigned int arch_slab_minalign(void)
> (KMALLOC_MIN_SIZE) : 16)
>
> #ifdef CONFIG_RANDOM_KMALLOC_CACHES
> -#define RANDOM_KMALLOC_CACHES_NR 16 // # of cache copies
> +#define RANDOM_KMALLOC_CACHES_NR 15 // # of extra cache copies
> #define RANDOM_KMALLOC_CACHES_BITS 4 // =log2(_NR), for hashing
> #else
> -#define RANDOM_KMALLOC_CACHES_NR 1
> +#define RANDOM_KMALLOC_CACHES_NR 0
> #endif
>
> /*
> @@ -362,15 +362,15 @@ static inline unsigned int arch_slab_minalign(void)
> * kmem caches can have both accounted and unaccounted objects.
> */
> enum kmalloc_cache_type {
> - KMALLOC_RANDOM_START = 0,
> - KMALLOC_RANDOM_END = KMALLOC_RANDOM_START + RANDOM_KMALLOC_CACHES_NR - 1,
> - KMALLOC_NORMAL = KMALLOC_RANDOM_END,
> + KMALLOC_NORMAL = 0,
> #ifndef CONFIG_ZONE_DMA
> KMALLOC_DMA = KMALLOC_NORMAL,
> #endif
> #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
> diff --git a/mm/slab_common.c b/mm/slab_common.c
> index a1a111ca229c..0479037b2959 100644
> --- a/mm/slab_common.c
> +++ b/mm/slab_common.c
> @@ -784,22 +784,21 @@ EXPORT_SYMBOL(kmalloc_size_roundup);
> #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 + 0] = "kmalloc-rnd-01-" #sz,
> -#define KMA_RAND_2(sz) KMA_RAND_1(sz) .name[KMALLOC_RANDOM_START + 1] = "kmalloc-rnd-02-" #sz,
> -#define KMA_RAND_3(sz) KMA_RAND_2(sz) .name[KMALLOC_RANDOM_START + 2] = "kmalloc-rnd-03-" #sz,
> -#define KMA_RAND_4(sz) KMA_RAND_3(sz) .name[KMALLOC_RANDOM_START + 3] = "kmalloc-rnd-04-" #sz,
> -#define KMA_RAND_5(sz) KMA_RAND_4(sz) .name[KMALLOC_RANDOM_START + 4] = "kmalloc-rnd-05-" #sz,
> -#define KMA_RAND_6(sz) KMA_RAND_5(sz) .name[KMALLOC_RANDOM_START + 5] = "kmalloc-rnd-06-" #sz,
> -#define KMA_RAND_7(sz) KMA_RAND_6(sz) .name[KMALLOC_RANDOM_START + 6] = "kmalloc-rnd-07-" #sz,
> -#define KMA_RAND_8(sz) KMA_RAND_7(sz) .name[KMALLOC_RANDOM_START + 7] = "kmalloc-rnd-08-" #sz,
> -#define KMA_RAND_9(sz) KMA_RAND_8(sz) .name[KMALLOC_RANDOM_START + 8] = "kmalloc-rnd-09-" #sz,
> -#define KMA_RAND_10(sz) KMA_RAND_9(sz) .name[KMALLOC_RANDOM_START + 9] = "kmalloc-rnd-10-" #sz,
> -#define KMA_RAND_11(sz) KMA_RAND_10(sz) .name[KMALLOC_RANDOM_START + 10] = "kmalloc-rnd-11-" #sz,
> -#define KMA_RAND_12(sz) KMA_RAND_11(sz) .name[KMALLOC_RANDOM_START + 11] = "kmalloc-rnd-12-" #sz,
> -#define KMA_RAND_13(sz) KMA_RAND_12(sz) .name[KMALLOC_RANDOM_START + 12] = "kmalloc-rnd-13-" #sz,
> -#define KMA_RAND_14(sz) KMA_RAND_13(sz) .name[KMALLOC_RANDOM_START + 13] = "kmalloc-rnd-14-" #sz,
> -#define KMA_RAND_15(sz) KMA_RAND_14(sz) .name[KMALLOC_RANDOM_START + 14] = "kmalloc-rnd-15-" #sz,
> -#define KMA_RAND_16(sz) KMA_RAND_15(sz) .name[KMALLOC_RANDOM_START + 15] = "kmalloc-rnd-16-" #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
> @@ -957,7 +956,7 @@ void __init create_kmalloc_caches(slab_flags_t flags)
> /*
> * Including KMALLOC_CGROUP if CONFIG_MEMCG_KMEM defined
> */
> - for (type = KMALLOC_RANDOM_START; type < NR_KMALLOC_TYPES; type++) {
> + for (type = KMALLOC_NORMAL; 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);
>
>
I've adjusted the code according to all suggestions I received and sent
out the v5 patch. Thanks for the comments!