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!
