Re: [PATCH v4] Randomized slab caches for kmalloc()

["GONG, Ruiqi" <gongruiqi@huaweicloud.com>]

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@oracle.com/
> 
> 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!