// SPDX-License-Identifier: GPL-2.0 /* * KFENCE guarded object allocator and fault handling. * * Copyright (C) 2020, Google LLC. */ #define pr_fmt(fmt) "kfence: " fmt #include #include #include #include #include #include #include #include #include #include #include #include #include #include #include #include #include #include #include #include #include #include #include #include #include "kfence.h" /* Disables KFENCE on the first warning assuming an irrecoverable error. */ #define KFENCE_WARN_ON(cond) \ ({ \ const bool __cond = WARN_ON(cond); \ if (unlikely(__cond)) { \ WRITE_ONCE(kfence_enabled, false); \ disabled_by_warn = true; \ } \ __cond; \ }) /* === Data ================================================================= */ static bool kfence_enabled __read_mostly; static bool disabled_by_warn __read_mostly; unsigned long kfence_sample_interval __read_mostly = CONFIG_KFENCE_SAMPLE_INTERVAL; EXPORT_SYMBOL_GPL(kfence_sample_interval); /* Export for test modules. */ #ifdef MODULE_PARAM_PREFIX #undef MODULE_PARAM_PREFIX #endif #define MODULE_PARAM_PREFIX "kfence." static int kfence_enable_late(void); static int param_set_sample_interval(const char *val, const struct kernel_param *kp) { unsigned long num; int ret = kstrtoul(val, 0, &num); if (ret < 0) return ret; /* Using 0 to indicate KFENCE is disabled. */ if (!num && READ_ONCE(kfence_enabled)) { pr_info("disabled\n"); WRITE_ONCE(kfence_enabled, false); } *((unsigned long *)kp->arg) = num; if (num && !READ_ONCE(kfence_enabled) && system_state != SYSTEM_BOOTING) return disabled_by_warn ? -EINVAL : kfence_enable_late(); return 0; } static int param_get_sample_interval(char *buffer, const struct kernel_param *kp) { if (!READ_ONCE(kfence_enabled)) return sprintf(buffer, "0\n"); return param_get_ulong(buffer, kp); } static const struct kernel_param_ops sample_interval_param_ops = { .set = param_set_sample_interval, .get = param_get_sample_interval, }; module_param_cb(sample_interval, &sample_interval_param_ops, &kfence_sample_interval, 0600); /* Pool usage% threshold when currently covered allocations are skipped. */ static unsigned long kfence_skip_covered_thresh __read_mostly = 75; module_param_named(skip_covered_thresh, kfence_skip_covered_thresh, ulong, 0644); /* Allocation burst count: number of excess KFENCE allocations per sample. */ static unsigned int kfence_burst __read_mostly; module_param_named(burst, kfence_burst, uint, 0644); /* If true, use a deferrable timer. */ static bool kfence_deferrable __read_mostly = IS_ENABLED(CONFIG_KFENCE_DEFERRABLE); module_param_named(deferrable, kfence_deferrable, bool, 0444); /* If true, check all canary bytes on panic. */ static bool kfence_check_on_panic __read_mostly; module_param_named(check_on_panic, kfence_check_on_panic, bool, 0444); /* The pool of pages used for guard pages and objects. */ char *__kfence_pool __read_mostly; EXPORT_SYMBOL(__kfence_pool); /* Export for test modules. */ /* * Per-object metadata, with one-to-one mapping of object metadata to * backing pages (in __kfence_pool). */ static_assert(CONFIG_KFENCE_NUM_OBJECTS > 0); struct kfence_metadata *kfence_metadata __read_mostly; /* * If kfence_metadata is not NULL, it may be accessed by kfence_shutdown_cache(). * So introduce kfence_metadata_init to initialize metadata, and then make * kfence_metadata visible after initialization is successful. This prevents * potential UAF or access to uninitialized metadata. */ static struct kfence_metadata *kfence_metadata_init __read_mostly; /* Freelist with available objects. */ static struct list_head kfence_freelist = LIST_HEAD_INIT(kfence_freelist); static DEFINE_RAW_SPINLOCK(kfence_freelist_lock); /* Lock protecting freelist. */ /* * The static key to set up a KFENCE allocation; or if static keys are not used * to gate allocations, to avoid a load and compare if KFENCE is disabled. */ DEFINE_STATIC_KEY_FALSE(kfence_allocation_key); /* Gates the allocation, ensuring only one succeeds in a given period. */ atomic_t kfence_allocation_gate = ATOMIC_INIT(1); /* * A Counting Bloom filter of allocation coverage: limits currently covered * allocations of the same source filling up the pool. * * Assuming a range of 15%-85% unique allocations in the pool at any point in * time, the below parameters provide a probablity of 0.02-0.33 for false * positive hits respectively: * * P(alloc_traces) = (1 - e^(-HNUM * (alloc_traces / SIZE)) ^ HNUM */ #define ALLOC_COVERED_HNUM 2 #define ALLOC_COVERED_ORDER (const_ilog2(CONFIG_KFENCE_NUM_OBJECTS) + 2) #define ALLOC_COVERED_SIZE (1 << ALLOC_COVERED_ORDER) #define ALLOC_COVERED_HNEXT(h) hash_32(h, ALLOC_COVERED_ORDER) #define ALLOC_COVERED_MASK (ALLOC_COVERED_SIZE - 1) static atomic_t alloc_covered[ALLOC_COVERED_SIZE]; /* Stack depth used to determine uniqueness of an allocation. */ #define UNIQUE_ALLOC_STACK_DEPTH ((size_t)8) /* * Randomness for stack hashes, making the same collisions across reboots and * different machines less likely. */ static u32 stack_hash_seed __ro_after_init; /* Statistics counters for debugfs. */ enum kfence_counter_id { KFENCE_COUNTER_ALLOCATED, KFENCE_COUNTER_ALLOCS, KFENCE_COUNTER_FREES, KFENCE_COUNTER_ZOMBIES, KFENCE_COUNTER_BUGS, KFENCE_COUNTER_SKIP_INCOMPAT, KFENCE_COUNTER_SKIP_CAPACITY, KFENCE_COUNTER_SKIP_COVERED, KFENCE_COUNTER_COUNT, }; static atomic_long_t counters[KFENCE_COUNTER_COUNT]; static const char *const counter_names[] = { [KFENCE_COUNTER_ALLOCATED] = "currently allocated", [KFENCE_COUNTER_ALLOCS] = "total allocations", [KFENCE_COUNTER_FREES] = "total frees", [KFENCE_COUNTER_ZOMBIES] = "zombie allocations", [KFENCE_COUNTER_BUGS] = "total bugs", [KFENCE_COUNTER_SKIP_INCOMPAT] = "skipped allocations (incompatible)", [KFENCE_COUNTER_SKIP_CAPACITY] = "skipped allocations (capacity)", [KFENCE_COUNTER_SKIP_COVERED] = "skipped allocations (covered)", }; static_assert(ARRAY_SIZE(counter_names) == KFENCE_COUNTER_COUNT); /* === Internals ============================================================ */ static inline bool should_skip_covered(void) { unsigned long thresh = (CONFIG_KFENCE_NUM_OBJECTS * kfence_skip_covered_thresh) / 100; return atomic_long_read(&counters[KFENCE_COUNTER_ALLOCATED]) > thresh; } static u32 get_alloc_stack_hash(unsigned long *stack_entries, size_t num_entries) { num_entries = min(num_entries, UNIQUE_ALLOC_STACK_DEPTH); num_entries = filter_irq_stacks(stack_entries, num_entries); return jhash(stack_entries, num_entries * sizeof(stack_entries[0]), stack_hash_seed); } /* * Adds (or subtracts) count @val for allocation stack trace hash * @alloc_stack_hash from Counting Bloom filter. */ static void alloc_covered_add(u32 alloc_stack_hash, int val) { int i; for (i = 0; i < ALLOC_COVERED_HNUM; i++) { atomic_add(val, &alloc_covered[alloc_stack_hash & ALLOC_COVERED_MASK]); alloc_stack_hash = ALLOC_COVERED_HNEXT(alloc_stack_hash); } } /* * Returns true if the allocation stack trace hash @alloc_stack_hash is * currently contained (non-zero count) in Counting Bloom filter. */ static bool alloc_covered_contains(u32 alloc_stack_hash) { int i; for (i = 0; i < ALLOC_COVERED_HNUM; i++) { if (!atomic_read(&alloc_covered[alloc_stack_hash & ALLOC_COVERED_MASK])) return false; alloc_stack_hash = ALLOC_COVERED_HNEXT(alloc_stack_hash); } return true; } static bool kfence_protect(unsigned long addr) { return !KFENCE_WARN_ON(!kfence_protect_page(ALIGN_DOWN(addr, PAGE_SIZE), true)); } static bool kfence_unprotect(unsigned long addr) { return !KFENCE_WARN_ON(!kfence_protect_page(ALIGN_DOWN(addr, PAGE_SIZE), false)); } static inline unsigned long metadata_to_pageaddr(const struct kfence_metadata *meta) { unsigned long offset = (meta - kfence_metadata + 1) * PAGE_SIZE * 2; unsigned long pageaddr = (unsigned long)&__kfence_pool[offset]; /* The checks do not affect performance; only called from slow-paths. */ /* Only call with a pointer into kfence_metadata. */ if (KFENCE_WARN_ON(meta < kfence_metadata || meta >= kfence_metadata + CONFIG_KFENCE_NUM_OBJECTS)) return 0; /* * This metadata object only ever maps to 1 page; verify that the stored * address is in the expected range. */ if (KFENCE_WARN_ON(ALIGN_DOWN(meta->addr, PAGE_SIZE) != pageaddr)) return 0; return pageaddr; } static inline bool kfence_obj_allocated(const struct kfence_metadata *meta) { enum kfence_object_state state = READ_ONCE(meta->state); return state == KFENCE_OBJECT_ALLOCATED || state == KFENCE_OBJECT_RCU_FREEING; } /* * Update the object's metadata state, including updating the alloc/free stacks * depending on the state transition. */ static noinline void metadata_update_state(struct kfence_metadata *meta, enum kfence_object_state next, unsigned long *stack_entries, size_t num_stack_entries) { struct kfence_track *track = next == KFENCE_OBJECT_ALLOCATED ? &meta->alloc_track : &meta->free_track; lockdep_assert_held(&meta->lock); /* Stack has been saved when calling rcu, skip. */ if (READ_ONCE(meta->state) == KFENCE_OBJECT_RCU_FREEING) goto out; if (stack_entries) { memcpy(track->stack_entries, stack_entries, num_stack_entries * sizeof(stack_entries[0])); } else { /* * Skip over 1 (this) functions; noinline ensures we do not * accidentally skip over the caller by never inlining. */ num_stack_entries = stack_trace_save(track->stack_entries, KFENCE_STACK_DEPTH, 1); } track->num_stack_entries = num_stack_entries; track->pid = task_pid_nr(current); track->cpu = raw_smp_processor_id(); track->ts_nsec = local_clock(); /* Same source as printk timestamps. */ out: /* * Pairs with READ_ONCE() in * kfence_shutdown_cache(), * kfence_handle_page_fault(). */ WRITE_ONCE(meta->state, next); } #ifdef CONFIG_KMSAN #define check_canary_attributes noinline __no_kmsan_checks #else #define check_canary_attributes inline #endif /* Check canary byte at @addr. */ static check_canary_attributes bool check_canary_byte(u8 *addr) { struct kfence_metadata *meta; unsigned long flags; if (likely(*addr == KFENCE_CANARY_PATTERN_U8(addr))) return true; atomic_long_inc(&counters[KFENCE_COUNTER_BUGS]); meta = addr_to_metadata((unsigned long)addr); raw_spin_lock_irqsave(&meta->lock, flags); kfence_report_error((unsigned long)addr, false, NULL, meta, KFENCE_ERROR_CORRUPTION); raw_spin_unlock_irqrestore(&meta->lock, flags); return false; } static inline void set_canary(const struct kfence_metadata *meta) { const unsigned long pageaddr = ALIGN_DOWN(meta->addr, PAGE_SIZE); unsigned long addr = pageaddr; /* * The canary may be written to part of the object memory, but it does * not affect it. The user should initialize the object before using it. */ for (; addr < meta->addr; addr += sizeof(u64)) *((u64 *)addr) = KFENCE_CANARY_PATTERN_U64; addr = ALIGN_DOWN(meta->addr + meta->size, sizeof(u64)); for (; addr - pageaddr < PAGE_SIZE; addr += sizeof(u64)) *((u64 *)addr) = KFENCE_CANARY_PATTERN_U64; } static check_canary_attributes void check_canary(const struct kfence_metadata *meta) { const unsigned long pageaddr = ALIGN_DOWN(meta->addr, PAGE_SIZE); unsigned long addr = pageaddr; /* * We'll iterate over each canary byte per-side until a corrupted byte * is found. However, we'll still iterate over the canary bytes to the * right of the object even if there was an error in the canary bytes to * the left of the object. Specifically, if check_canary_byte() * generates an error, showing both sides might give more clues as to * what the error is about when displaying which bytes were corrupted. */ /* Apply to left of object. */ for (; meta->addr - addr >= sizeof(u64); addr += sizeof(u64)) { if (unlikely(*((u64 *)addr) != KFENCE_CANARY_PATTERN_U64)) break; } /* * If the canary is corrupted in a certain 64 bytes, or the canary * memory cannot be completely covered by multiple consecutive 64 bytes, * it needs to be checked one by one. */ for (; addr < meta->addr; addr++) { if (unlikely(!check_canary_byte((u8 *)addr))) break; } /* Apply to right of object. */ for (addr = meta->addr + meta->size; addr % sizeof(u64) != 0; addr++) { if (unlikely(!check_canary_byte((u8 *)addr))) return; } for (; addr - pageaddr < PAGE_SIZE; addr += sizeof(u64)) { if (unlikely(*((u64 *)addr) != KFENCE_CANARY_PATTERN_U64)) { for (; addr - pageaddr < PAGE_SIZE; addr++) { if (!check_canary_byte((u8 *)addr)) return; } } } } static void *kfence_guarded_alloc(struct kmem_cache *cache, size_t size, gfp_t gfp, unsigned long *stack_entries, size_t num_stack_entries, u32 alloc_stack_hash) { struct kfence_metadata *meta = NULL; unsigned long flags; struct slab *slab; void *addr; const bool random_right_allocate = get_random_u32_below(2); const bool random_fault = CONFIG_KFENCE_STRESS_TEST_FAULTS && !get_random_u32_below(CONFIG_KFENCE_STRESS_TEST_FAULTS); /* Try to obtain a free object. */ raw_spin_lock_irqsave(&kfence_freelist_lock, flags); if (!list_empty(&kfence_freelist)) { meta = list_entry(kfence_freelist.next, struct kfence_metadata, list); list_del_init(&meta->list); } raw_spin_unlock_irqrestore(&kfence_freelist_lock, flags); if (!meta) { atomic_long_inc(&counters[KFENCE_COUNTER_SKIP_CAPACITY]); return NULL; } if (unlikely(!raw_spin_trylock_irqsave(&meta->lock, flags))) { /* * This is extremely unlikely -- we are reporting on a * use-after-free, which locked meta->lock, and the reporting * code via printk calls kmalloc() which ends up in * kfence_alloc() and tries to grab the same object that we're * reporting on. While it has never been observed, lockdep does * report that there is a possibility of deadlock. Fix it by * using trylock and bailing out gracefully. */ raw_spin_lock_irqsave(&kfence_freelist_lock, flags); /* Put the object back on the freelist. */ list_add_tail(&meta->list, &kfence_freelist); raw_spin_unlock_irqrestore(&kfence_freelist_lock, flags); return NULL; } meta->addr = metadata_to_pageaddr(meta); /* Unprotect if we're reusing this page. */ if (meta->state == KFENCE_OBJECT_FREED) kfence_unprotect(meta->addr); /* * Note: for allocations made before RNG initialization, will always * return zero. We still benefit from enabling KFENCE as early as * possible, even when the RNG is not yet available, as this will allow * KFENCE to detect bugs due to earlier allocations. The only downside * is that the out-of-bounds accesses detected are deterministic for * such allocations. */ if (random_right_allocate) { /* Allocate on the "right" side, re-calculate address. */ meta->addr += PAGE_SIZE - size; meta->addr = ALIGN_DOWN(meta->addr, cache->align); } addr = (void *)meta->addr; /* Update remaining metadata. */ metadata_update_state(meta, KFENCE_OBJECT_ALLOCATED, stack_entries, num_stack_entries); /* Pairs with READ_ONCE() in kfence_shutdown_cache(). */ WRITE_ONCE(meta->cache, cache); meta->size = size; meta->alloc_stack_hash = alloc_stack_hash; raw_spin_unlock_irqrestore(&meta->lock, flags); alloc_covered_add(alloc_stack_hash, 1); /* Set required slab fields. */ slab = virt_to_slab((void *)meta->addr); slab->slab_cache = cache; slab->objects = 1; /* Memory initialization. */ set_canary(meta); /* * We check slab_want_init_on_alloc() ourselves, rather than letting * SL*B do the initialization, as otherwise we might overwrite KFENCE's * redzone. */ if (unlikely(slab_want_init_on_alloc(gfp, cache))) memzero_explicit(addr, size); if (cache->ctor) cache->ctor(addr); if (random_fault) kfence_protect(meta->addr); /* Random "faults" by protecting the object. */ atomic_long_inc(&counters[KFENCE_COUNTER_ALLOCATED]); atomic_long_inc(&counters[KFENCE_COUNTER_ALLOCS]); return addr; } static void kfence_guarded_free(void *addr, struct kfence_metadata *meta, bool zombie) { struct kcsan_scoped_access assert_page_exclusive; unsigned long flags; bool init; raw_spin_lock_irqsave(&meta->lock, flags); if (!kfence_obj_allocated(meta) || meta->addr != (unsigned long)addr) { /* Invalid or double-free, bail out. */ atomic_long_inc(&counters[KFENCE_COUNTER_BUGS]); kfence_report_error((unsigned long)addr, false, NULL, meta, KFENCE_ERROR_INVALID_FREE); raw_spin_unlock_irqrestore(&meta->lock, flags); return; } /* Detect racy use-after-free, or incorrect reallocation of this page by KFENCE. */ kcsan_begin_scoped_access((void *)ALIGN_DOWN((unsigned long)addr, PAGE_SIZE), PAGE_SIZE, KCSAN_ACCESS_SCOPED | KCSAN_ACCESS_WRITE | KCSAN_ACCESS_ASSERT, &assert_page_exclusive); if (CONFIG_KFENCE_STRESS_TEST_FAULTS) kfence_unprotect((unsigned long)addr); /* To check canary bytes. */ /* Restore page protection if there was an OOB access. */ if (meta->unprotected_page) { memzero_explicit((void *)ALIGN_DOWN(meta->unprotected_page, PAGE_SIZE), PAGE_SIZE); kfence_protect(meta->unprotected_page); meta->unprotected_page = 0; } /* Mark the object as freed. */ metadata_update_state(meta, KFENCE_OBJECT_FREED, NULL, 0); init = slab_want_init_on_free(meta->cache); raw_spin_unlock_irqrestore(&meta->lock, flags); alloc_covered_add(meta->alloc_stack_hash, -1); /* Check canary bytes for memory corruption. */ check_canary(meta); /* * Clear memory if init-on-free is set. While we protect the page, the * data is still there, and after a use-after-free is detected, we * unprotect the page, so the data is still accessible. */ if (!zombie && unlikely(init)) memzero_explicit(addr, meta->size); /* Protect to detect use-after-frees. */ kfence_protect((unsigned long)addr); kcsan_end_scoped_access(&assert_page_exclusive); if (!zombie) { /* Add it to the tail of the freelist for reuse. */ raw_spin_lock_irqsave(&kfence_freelist_lock, flags); KFENCE_WARN_ON(!list_empty(&meta->list)); list_add_tail(&meta->list, &kfence_freelist); raw_spin_unlock_irqrestore(&kfence_freelist_lock, flags); atomic_long_dec(&counters[KFENCE_COUNTER_ALLOCATED]); atomic_long_inc(&counters[KFENCE_COUNTER_FREES]); } else { /* See kfence_shutdown_cache(). */ atomic_long_inc(&counters[KFENCE_COUNTER_ZOMBIES]); } } static void rcu_guarded_free(struct rcu_head *h) { struct kfence_metadata *meta = container_of(h, struct kfence_metadata, rcu_head); kfence_guarded_free((void *)meta->addr, meta, false); } /* * Initialization of the KFENCE pool after its allocation. * Returns 0 on success; otherwise returns the address up to * which partial initialization succeeded. */ static unsigned long kfence_init_pool(void) { unsigned long addr; struct page *pages; int i; if (!arch_kfence_init_pool()) return (unsigned long)__kfence_pool; addr = (unsigned long)__kfence_pool; pages = virt_to_page(__kfence_pool); /* * Set up object pages: they must have PG_slab set, to avoid freeing * these as real pages. * * We also want to avoid inserting kfence_free() in the kfree() * fast-path in SLUB, and therefore need to ensure kfree() correctly * enters __slab_free() slow-path. */ for (i = 0; i < KFENCE_POOL_SIZE / PAGE_SIZE; i++) { struct slab *slab = page_slab(nth_page(pages, i)); if (!i || (i % 2)) continue; __folio_set_slab(slab_folio(slab)); #ifdef CONFIG_MEMCG slab->obj_exts = (unsigned long)&kfence_metadata_init[i / 2 - 1].obj_exts | MEMCG_DATA_OBJEXTS; #endif } /* * Protect the first 2 pages. The first page is mostly unnecessary, and * merely serves as an extended guard page. However, adding one * additional page in the beginning gives us an even number of pages, * which simplifies the mapping of address to metadata index. */ for (i = 0; i < 2; i++) { if (unlikely(!kfence_protect(addr))) return addr; addr += PAGE_SIZE; } for (i = 0; i < CONFIG_KFENCE_NUM_OBJECTS; i++) { struct kfence_metadata *meta = &kfence_metadata_init[i]; /* Initialize metadata. */ INIT_LIST_HEAD(&meta->list); raw_spin_lock_init(&meta->lock); meta->state = KFENCE_OBJECT_UNUSED; meta->addr = addr; /* Initialize for validation in metadata_to_pageaddr(). */ list_add_tail(&meta->list, &kfence_freelist); /* Protect the right redzone. */ if (unlikely(!kfence_protect(addr + PAGE_SIZE))) goto reset_slab; addr += 2 * PAGE_SIZE; } /* * Make kfence_metadata visible only when initialization is successful. * Otherwise, if the initialization fails and kfence_metadata is freed, * it may cause UAF in kfence_shutdown_cache(). */ smp_store_release(&kfence_metadata, kfence_metadata_init); return 0; reset_slab: for (i = 0; i < KFENCE_POOL_SIZE / PAGE_SIZE; i++) { struct slab *slab = page_slab(nth_page(pages, i)); if (!i || (i % 2)) continue; #ifdef CONFIG_MEMCG slab->obj_exts = 0; #endif __folio_clear_slab(slab_folio(slab)); } return addr; } static bool __init kfence_init_pool_early(void) { unsigned long addr; if (!__kfence_pool) return false; addr = kfence_init_pool(); if (!addr) { /* * The pool is live and will never be deallocated from this point on. * Ignore the pool object from the kmemleak phys object tree, as it would * otherwise overlap with allocations returned by kfence_alloc(), which * are registered with kmemleak through the slab post-alloc hook. */ kmemleak_ignore_phys(__pa(__kfence_pool)); return true; } /* * Only release unprotected pages, and do not try to go back and change * page attributes due to risk of failing to do so as well. If changing * page attributes for some pages fails, it is very likely that it also * fails for the first page, and therefore expect addr==__kfence_pool in * most failure cases. */ memblock_free_late(__pa(addr), KFENCE_POOL_SIZE - (addr - (unsigned long)__kfence_pool)); __kfence_pool = NULL; memblock_free_late(__pa(kfence_metadata_init), KFENCE_METADATA_SIZE); kfence_metadata_init = NULL; return false; } /* === DebugFS Interface ==================================================== */ static int stats_show(struct seq_file *seq, void *v) { int i; seq_printf(seq, "enabled: %i\n", READ_ONCE(kfence_enabled)); for (i = 0; i < KFENCE_COUNTER_COUNT; i++) seq_printf(seq, "%s: %ld\n", counter_names[i], atomic_long_read(&counters[i])); return 0; } DEFINE_SHOW_ATTRIBUTE(stats); /* * debugfs seq_file operations for /sys/kernel/debug/kfence/objects. * start_object() and next_object() return the object index + 1, because NULL is used * to stop iteration. */ static void *start_object(struct seq_file *seq, loff_t *pos) { if (*pos < CONFIG_KFENCE_NUM_OBJECTS) return (void *)((long)*pos + 1); return NULL; } static void stop_object(struct seq_file *seq, void *v) { } static void *next_object(struct seq_file *seq, void *v, loff_t *pos) { ++*pos; if (*pos < CONFIG_KFENCE_NUM_OBJECTS) return (void *)((long)*pos + 1); return NULL; } static int show_object(struct seq_file *seq, void *v) { struct kfence_metadata *meta = &kfence_metadata[(long)v - 1]; unsigned long flags; raw_spin_lock_irqsave(&meta->lock, flags); kfence_print_object(seq, meta); raw_spin_unlock_irqrestore(&meta->lock, flags); seq_puts(seq, "---------------------------------\n"); return 0; } static const struct seq_operations objects_sops = { .start = start_object, .next = next_object, .stop = stop_object, .show = show_object, }; DEFINE_SEQ_ATTRIBUTE(objects); static int kfence_debugfs_init(void) { struct dentry *kfence_dir; if (!READ_ONCE(kfence_enabled)) return 0; kfence_dir = debugfs_create_dir("kfence", NULL); debugfs_create_file("stats", 0444, kfence_dir, NULL, &stats_fops); debugfs_create_file("objects", 0400, kfence_dir, NULL, &objects_fops); return 0; } late_initcall(kfence_debugfs_init); /* === Panic Notifier ====================================================== */ static void kfence_check_all_canary(void) { int i; for (i = 0; i < CONFIG_KFENCE_NUM_OBJECTS; i++) { struct kfence_metadata *meta = &kfence_metadata[i]; if (kfence_obj_allocated(meta)) check_canary(meta); } } static int kfence_check_canary_callback(struct notifier_block *nb, unsigned long reason, void *arg) { kfence_check_all_canary(); return NOTIFY_OK; } static struct notifier_block kfence_check_canary_notifier = { .notifier_call = kfence_check_canary_callback, }; /* === Allocation Gate Timer ================================================ */ static struct delayed_work kfence_timer; #ifdef CONFIG_KFENCE_STATIC_KEYS /* Wait queue to wake up allocation-gate timer task. */ static DECLARE_WAIT_QUEUE_HEAD(allocation_wait); static void wake_up_kfence_timer(struct irq_work *work) { wake_up(&allocation_wait); } static DEFINE_IRQ_WORK(wake_up_kfence_timer_work, wake_up_kfence_timer); #endif /* * Set up delayed work, which will enable and disable the static key. We need to * use a work queue (rather than a simple timer), since enabling and disabling a * static key cannot be done from an interrupt. * * Note: Toggling a static branch currently causes IPIs, and here we'll end up * with a total of 2 IPIs to all CPUs. If this ends up a problem in future (with * more aggressive sampling intervals), we could get away with a variant that * avoids IPIs, at the cost of not immediately capturing allocations if the * instructions remain cached. */ static void toggle_allocation_gate(struct work_struct *work) { if (!READ_ONCE(kfence_enabled)) return; atomic_set(&kfence_allocation_gate, -kfence_burst); #ifdef CONFIG_KFENCE_STATIC_KEYS /* Enable static key, and await allocation to happen. */ static_branch_enable(&kfence_allocation_key); wait_event_idle(allocation_wait, atomic_read(&kfence_allocation_gate) > 0); /* Disable static key and reset timer. */ static_branch_disable(&kfence_allocation_key); #endif queue_delayed_work(system_unbound_wq, &kfence_timer, msecs_to_jiffies(kfence_sample_interval)); } /* === Public interface ===================================================== */ void __init kfence_alloc_pool_and_metadata(void) { if (!kfence_sample_interval) return; /* * If the pool has already been initialized by arch, there is no need to * re-allocate the memory pool. */ if (!__kfence_pool) __kfence_pool = memblock_alloc(KFENCE_POOL_SIZE, PAGE_SIZE); if (!__kfence_pool) { pr_err("failed to allocate pool\n"); return; } /* The memory allocated by memblock has been zeroed out. */ kfence_metadata_init = memblock_alloc(KFENCE_METADATA_SIZE, PAGE_SIZE); if (!kfence_metadata_init) { pr_err("failed to allocate metadata\n"); memblock_free(__kfence_pool, KFENCE_POOL_SIZE); __kfence_pool = NULL; } } static void kfence_init_enable(void) { if (!IS_ENABLED(CONFIG_KFENCE_STATIC_KEYS)) static_branch_enable(&kfence_allocation_key); if (kfence_deferrable) INIT_DEFERRABLE_WORK(&kfence_timer, toggle_allocation_gate); else INIT_DELAYED_WORK(&kfence_timer, toggle_allocation_gate); if (kfence_check_on_panic) atomic_notifier_chain_register(&panic_notifier_list, &kfence_check_canary_notifier); WRITE_ONCE(kfence_enabled, true); queue_delayed_work(system_unbound_wq, &kfence_timer, 0); pr_info("initialized - using %lu bytes for %d objects at 0x%p-0x%p\n", KFENCE_POOL_SIZE, CONFIG_KFENCE_NUM_OBJECTS, (void *)__kfence_pool, (void *)(__kfence_pool + KFENCE_POOL_SIZE)); } void __init kfence_init(void) { stack_hash_seed = get_random_u32(); /* Setting kfence_sample_interval to 0 on boot disables KFENCE. */ if (!kfence_sample_interval) return; if (!kfence_init_pool_early()) { pr_err("%s failed\n", __func__); return; } kfence_init_enable(); } static int kfence_init_late(void) { const unsigned long nr_pages_pool = KFENCE_POOL_SIZE / PAGE_SIZE; const unsigned long nr_pages_meta = KFENCE_METADATA_SIZE / PAGE_SIZE; unsigned long addr = (unsigned long)__kfence_pool; unsigned long free_size = KFENCE_POOL_SIZE; int err = -ENOMEM; #ifdef CONFIG_CONTIG_ALLOC struct page *pages; pages = alloc_contig_pages(nr_pages_pool, GFP_KERNEL, first_online_node, NULL); if (!pages) return -ENOMEM; __kfence_pool = page_to_virt(pages); pages = alloc_contig_pages(nr_pages_meta, GFP_KERNEL, first_online_node, NULL); if (pages) kfence_metadata_init = page_to_virt(pages); #else if (nr_pages_pool > MAX_ORDER_NR_PAGES || nr_pages_meta > MAX_ORDER_NR_PAGES) { pr_warn("KFENCE_NUM_OBJECTS too large for buddy allocator\n"); return -EINVAL; } __kfence_pool = alloc_pages_exact(KFENCE_POOL_SIZE, GFP_KERNEL); if (!__kfence_pool) return -ENOMEM; kfence_metadata_init = alloc_pages_exact(KFENCE_METADATA_SIZE, GFP_KERNEL); #endif if (!kfence_metadata_init) goto free_pool; memzero_explicit(kfence_metadata_init, KFENCE_METADATA_SIZE); addr = kfence_init_pool(); if (!addr) { kfence_init_enable(); kfence_debugfs_init(); return 0; } pr_err("%s failed\n", __func__); free_size = KFENCE_POOL_SIZE - (addr - (unsigned long)__kfence_pool); err = -EBUSY; #ifdef CONFIG_CONTIG_ALLOC free_contig_range(page_to_pfn(virt_to_page((void *)kfence_metadata_init)), nr_pages_meta); free_pool: free_contig_range(page_to_pfn(virt_to_page((void *)addr)), free_size / PAGE_SIZE); #else free_pages_exact((void *)kfence_metadata_init, KFENCE_METADATA_SIZE); free_pool: free_pages_exact((void *)addr, free_size); #endif kfence_metadata_init = NULL; __kfence_pool = NULL; return err; } static int kfence_enable_late(void) { if (!__kfence_pool) return kfence_init_late(); WRITE_ONCE(kfence_enabled, true); queue_delayed_work(system_unbound_wq, &kfence_timer, 0); pr_info("re-enabled\n"); return 0; } void kfence_shutdown_cache(struct kmem_cache *s) { unsigned long flags; struct kfence_metadata *meta; int i; /* Pairs with release in kfence_init_pool(). */ if (!smp_load_acquire(&kfence_metadata)) return; for (i = 0; i < CONFIG_KFENCE_NUM_OBJECTS; i++) { bool in_use; meta = &kfence_metadata[i]; /* * If we observe some inconsistent cache and state pair where we * should have returned false here, cache destruction is racing * with either kmem_cache_alloc() or kmem_cache_free(). Taking * the lock will not help, as different critical section * serialization will have the same outcome. */ if (READ_ONCE(meta->cache) != s || !kfence_obj_allocated(meta)) continue; raw_spin_lock_irqsave(&meta->lock, flags); in_use = meta->cache == s && kfence_obj_allocated(meta); raw_spin_unlock_irqrestore(&meta->lock, flags); if (in_use) { /* * This cache still has allocations, and we should not * release them back into the freelist so they can still * safely be used and retain the kernel's default * behaviour of keeping the allocations alive (leak the * cache); however, they effectively become "zombie * allocations" as the KFENCE objects are the only ones * still in use and the owning cache is being destroyed. * * We mark them freed, so that any subsequent use shows * more useful error messages that will include stack * traces of the user of the object, the original * allocation, and caller to shutdown_cache(). */ kfence_guarded_free((void *)meta->addr, meta, /*zombie=*/true); } } for (i = 0; i < CONFIG_KFENCE_NUM_OBJECTS; i++) { meta = &kfence_metadata[i]; /* See above. */ if (READ_ONCE(meta->cache) != s || READ_ONCE(meta->state) != KFENCE_OBJECT_FREED) continue; raw_spin_lock_irqsave(&meta->lock, flags); if (meta->cache == s && meta->state == KFENCE_OBJECT_FREED) meta->cache = NULL; raw_spin_unlock_irqrestore(&meta->lock, flags); } } void *__kfence_alloc(struct kmem_cache *s, size_t size, gfp_t flags) { unsigned long stack_entries[KFENCE_STACK_DEPTH]; size_t num_stack_entries; u32 alloc_stack_hash; int allocation_gate; /* * Perform size check before switching kfence_allocation_gate, so that * we don't disable KFENCE without making an allocation. */ if (size > PAGE_SIZE) { atomic_long_inc(&counters[KFENCE_COUNTER_SKIP_INCOMPAT]); return NULL; } /* * Skip allocations from non-default zones, including DMA. We cannot * guarantee that pages in the KFENCE pool will have the requested * properties (e.g. reside in DMAable memory). */ if ((flags & GFP_ZONEMASK) || (s->flags & (SLAB_CACHE_DMA | SLAB_CACHE_DMA32))) { atomic_long_inc(&counters[KFENCE_COUNTER_SKIP_INCOMPAT]); return NULL; } /* * Skip allocations for this slab, if KFENCE has been disabled for * this slab. */ if (s->flags & SLAB_SKIP_KFENCE) return NULL; allocation_gate = atomic_inc_return(&kfence_allocation_gate); if (allocation_gate > 1) return NULL; #ifdef CONFIG_KFENCE_STATIC_KEYS /* * waitqueue_active() is fully ordered after the update of * kfence_allocation_gate per atomic_inc_return(). */ if (allocation_gate == 1 && waitqueue_active(&allocation_wait)) { /* * Calling wake_up() here may deadlock when allocations happen * from within timer code. Use an irq_work to defer it. */ irq_work_queue(&wake_up_kfence_timer_work); } #endif if (!READ_ONCE(kfence_enabled)) return NULL; num_stack_entries = stack_trace_save(stack_entries, KFENCE_STACK_DEPTH, 0); /* * Do expensive check for coverage of allocation in slow-path after * allocation_gate has already become non-zero, even though it might * mean not making any allocation within a given sample interval. * * This ensures reasonable allocation coverage when the pool is almost * full, including avoiding long-lived allocations of the same source * filling up the pool (e.g. pagecache allocations). */ alloc_stack_hash = get_alloc_stack_hash(stack_entries, num_stack_entries); if (should_skip_covered() && alloc_covered_contains(alloc_stack_hash)) { atomic_long_inc(&counters[KFENCE_COUNTER_SKIP_COVERED]); return NULL; } return kfence_guarded_alloc(s, size, flags, stack_entries, num_stack_entries, alloc_stack_hash); } size_t kfence_ksize(const void *addr) { const struct kfence_metadata *meta = addr_to_metadata((unsigned long)addr); /* * Read locklessly -- if there is a race with __kfence_alloc(), this is * either a use-after-free or invalid access. */ return meta ? meta->size : 0; } void *kfence_object_start(const void *addr) { const struct kfence_metadata *meta = addr_to_metadata((unsigned long)addr); /* * Read locklessly -- if there is a race with __kfence_alloc(), this is * either a use-after-free or invalid access. */ return meta ? (void *)meta->addr : NULL; } void __kfence_free(void *addr) { struct kfence_metadata *meta = addr_to_metadata((unsigned long)addr); #ifdef CONFIG_MEMCG KFENCE_WARN_ON(meta->obj_exts.objcg); #endif /* * If the objects of the cache are SLAB_TYPESAFE_BY_RCU, defer freeing * the object, as the object page may be recycled for other-typed * objects once it has been freed. meta->cache may be NULL if the cache * was destroyed. * Save the stack trace here so that reports show where the user freed * the object. */ if (unlikely(meta->cache && (meta->cache->flags & SLAB_TYPESAFE_BY_RCU))) { unsigned long flags; raw_spin_lock_irqsave(&meta->lock, flags); metadata_update_state(meta, KFENCE_OBJECT_RCU_FREEING, NULL, 0); raw_spin_unlock_irqrestore(&meta->lock, flags); call_rcu(&meta->rcu_head, rcu_guarded_free); } else { kfence_guarded_free(addr, meta, false); } } bool kfence_handle_page_fault(unsigned long addr, bool is_write, struct pt_regs *regs) { const int page_index = (addr - (unsigned long)__kfence_pool) / PAGE_SIZE; struct kfence_metadata *to_report = NULL; enum kfence_error_type error_type; unsigned long flags; if (!is_kfence_address((void *)addr)) return false; if (!READ_ONCE(kfence_enabled)) /* If disabled at runtime ... */ return kfence_unprotect(addr); /* ... unprotect and proceed. */ atomic_long_inc(&counters[KFENCE_COUNTER_BUGS]); if (page_index % 2) { /* This is a redzone, report a buffer overflow. */ struct kfence_metadata *meta; int distance = 0; meta = addr_to_metadata(addr - PAGE_SIZE); if (meta && kfence_obj_allocated(meta)) { to_report = meta; /* Data race ok; distance calculation approximate. */ distance = addr - data_race(meta->addr + meta->size); } meta = addr_to_metadata(addr + PAGE_SIZE); if (meta && kfence_obj_allocated(meta)) { /* Data race ok; distance calculation approximate. */ if (!to_report || distance > data_race(meta->addr) - addr) to_report = meta; } if (!to_report) goto out; raw_spin_lock_irqsave(&to_report->lock, flags); to_report->unprotected_page = addr; error_type = KFENCE_ERROR_OOB; /* * If the object was freed before we took the look we can still * report this as an OOB -- the report will simply show the * stacktrace of the free as well. */ } else { to_report = addr_to_metadata(addr); if (!to_report) goto out; raw_spin_lock_irqsave(&to_report->lock, flags); error_type = KFENCE_ERROR_UAF; /* * We may race with __kfence_alloc(), and it is possible that a * freed object may be reallocated. We simply report this as a * use-after-free, with the stack trace showing the place where * the object was re-allocated. */ } out: if (to_report) { kfence_report_error(addr, is_write, regs, to_report, error_type); raw_spin_unlock_irqrestore(&to_report->lock, flags); } else { /* This may be a UAF or OOB access, but we can't be sure. */ kfence_report_error(addr, is_write, regs, NULL, KFENCE_ERROR_INVALID); } return kfence_unprotect(addr); /* Unprotect and let access proceed. */ }