1 // SPDX-License-Identifier: GPL-2.0
3 * KFENCE guarded object allocator and fault handling.
5 * Copyright (C) 2020, Google LLC.
8 #define pr_fmt(fmt) "kfence: " fmt
10 #include <linux/atomic.h>
11 #include <linux/bug.h>
12 #include <linux/debugfs.h>
13 #include <linux/hash.h>
14 #include <linux/irq_work.h>
15 #include <linux/jhash.h>
16 #include <linux/kcsan-checks.h>
17 #include <linux/kfence.h>
18 #include <linux/kmemleak.h>
19 #include <linux/list.h>
20 #include <linux/lockdep.h>
21 #include <linux/log2.h>
22 #include <linux/memblock.h>
23 #include <linux/moduleparam.h>
24 #include <linux/random.h>
25 #include <linux/rcupdate.h>
26 #include <linux/sched/clock.h>
27 #include <linux/sched/sysctl.h>
28 #include <linux/seq_file.h>
29 #include <linux/slab.h>
30 #include <linux/spinlock.h>
31 #include <linux/string.h>
33 #include <asm/kfence.h>
37 /* Disables KFENCE on the first warning assuming an irrecoverable error. */
38 #define KFENCE_WARN_ON(cond) \
40 const bool __cond = WARN_ON(cond); \
41 if (unlikely(__cond)) \
42 WRITE_ONCE(kfence_enabled, false); \
46 /* === Data ================================================================= */
48 static bool kfence_enabled __read_mostly;
50 unsigned long kfence_sample_interval __read_mostly = CONFIG_KFENCE_SAMPLE_INTERVAL;
51 EXPORT_SYMBOL_GPL(kfence_sample_interval); /* Export for test modules. */
53 #ifdef MODULE_PARAM_PREFIX
54 #undef MODULE_PARAM_PREFIX
56 #define MODULE_PARAM_PREFIX "kfence."
58 static int param_set_sample_interval(const char *val, const struct kernel_param *kp)
61 int ret = kstrtoul(val, 0, &num);
66 if (!num) /* Using 0 to indicate KFENCE is disabled. */
67 WRITE_ONCE(kfence_enabled, false);
68 else if (!READ_ONCE(kfence_enabled) && system_state != SYSTEM_BOOTING)
69 return -EINVAL; /* Cannot (re-)enable KFENCE on-the-fly. */
71 *((unsigned long *)kp->arg) = num;
75 static int param_get_sample_interval(char *buffer, const struct kernel_param *kp)
77 if (!READ_ONCE(kfence_enabled))
78 return sprintf(buffer, "0\n");
80 return param_get_ulong(buffer, kp);
83 static const struct kernel_param_ops sample_interval_param_ops = {
84 .set = param_set_sample_interval,
85 .get = param_get_sample_interval,
87 module_param_cb(sample_interval, &sample_interval_param_ops, &kfence_sample_interval, 0600);
89 /* Pool usage% threshold when currently covered allocations are skipped. */
90 static unsigned long kfence_skip_covered_thresh __read_mostly = 75;
91 module_param_named(skip_covered_thresh, kfence_skip_covered_thresh, ulong, 0644);
93 /* The pool of pages used for guard pages and objects. */
94 char *__kfence_pool __ro_after_init;
95 EXPORT_SYMBOL(__kfence_pool); /* Export for test modules. */
98 * Per-object metadata, with one-to-one mapping of object metadata to
99 * backing pages (in __kfence_pool).
101 static_assert(CONFIG_KFENCE_NUM_OBJECTS > 0);
102 struct kfence_metadata kfence_metadata[CONFIG_KFENCE_NUM_OBJECTS];
104 /* Freelist with available objects. */
105 static struct list_head kfence_freelist = LIST_HEAD_INIT(kfence_freelist);
106 static DEFINE_RAW_SPINLOCK(kfence_freelist_lock); /* Lock protecting freelist. */
109 * The static key to set up a KFENCE allocation; or if static keys are not used
110 * to gate allocations, to avoid a load and compare if KFENCE is disabled.
112 DEFINE_STATIC_KEY_FALSE(kfence_allocation_key);
114 /* Gates the allocation, ensuring only one succeeds in a given period. */
115 atomic_t kfence_allocation_gate = ATOMIC_INIT(1);
118 * A Counting Bloom filter of allocation coverage: limits currently covered
119 * allocations of the same source filling up the pool.
121 * Assuming a range of 15%-85% unique allocations in the pool at any point in
122 * time, the below parameters provide a probablity of 0.02-0.33 for false
123 * positive hits respectively:
125 * P(alloc_traces) = (1 - e^(-HNUM * (alloc_traces / SIZE)) ^ HNUM
127 #define ALLOC_COVERED_HNUM 2
128 #define ALLOC_COVERED_ORDER (const_ilog2(CONFIG_KFENCE_NUM_OBJECTS) + 2)
129 #define ALLOC_COVERED_SIZE (1 << ALLOC_COVERED_ORDER)
130 #define ALLOC_COVERED_HNEXT(h) hash_32(h, ALLOC_COVERED_ORDER)
131 #define ALLOC_COVERED_MASK (ALLOC_COVERED_SIZE - 1)
132 static atomic_t alloc_covered[ALLOC_COVERED_SIZE];
134 /* Stack depth used to determine uniqueness of an allocation. */
135 #define UNIQUE_ALLOC_STACK_DEPTH ((size_t)8)
138 * Randomness for stack hashes, making the same collisions across reboots and
139 * different machines less likely.
141 static u32 stack_hash_seed __ro_after_init;
143 /* Statistics counters for debugfs. */
144 enum kfence_counter_id {
145 KFENCE_COUNTER_ALLOCATED,
146 KFENCE_COUNTER_ALLOCS,
147 KFENCE_COUNTER_FREES,
148 KFENCE_COUNTER_ZOMBIES,
150 KFENCE_COUNTER_SKIP_INCOMPAT,
151 KFENCE_COUNTER_SKIP_CAPACITY,
152 KFENCE_COUNTER_SKIP_COVERED,
153 KFENCE_COUNTER_COUNT,
155 static atomic_long_t counters[KFENCE_COUNTER_COUNT];
156 static const char *const counter_names[] = {
157 [KFENCE_COUNTER_ALLOCATED] = "currently allocated",
158 [KFENCE_COUNTER_ALLOCS] = "total allocations",
159 [KFENCE_COUNTER_FREES] = "total frees",
160 [KFENCE_COUNTER_ZOMBIES] = "zombie allocations",
161 [KFENCE_COUNTER_BUGS] = "total bugs",
162 [KFENCE_COUNTER_SKIP_INCOMPAT] = "skipped allocations (incompatible)",
163 [KFENCE_COUNTER_SKIP_CAPACITY] = "skipped allocations (capacity)",
164 [KFENCE_COUNTER_SKIP_COVERED] = "skipped allocations (covered)",
166 static_assert(ARRAY_SIZE(counter_names) == KFENCE_COUNTER_COUNT);
168 /* === Internals ============================================================ */
170 static inline bool should_skip_covered(void)
172 unsigned long thresh = (CONFIG_KFENCE_NUM_OBJECTS * kfence_skip_covered_thresh) / 100;
174 return atomic_long_read(&counters[KFENCE_COUNTER_ALLOCATED]) > thresh;
177 static u32 get_alloc_stack_hash(unsigned long *stack_entries, size_t num_entries)
179 num_entries = min(num_entries, UNIQUE_ALLOC_STACK_DEPTH);
180 num_entries = filter_irq_stacks(stack_entries, num_entries);
181 return jhash(stack_entries, num_entries * sizeof(stack_entries[0]), stack_hash_seed);
185 * Adds (or subtracts) count @val for allocation stack trace hash
186 * @alloc_stack_hash from Counting Bloom filter.
188 static void alloc_covered_add(u32 alloc_stack_hash, int val)
192 for (i = 0; i < ALLOC_COVERED_HNUM; i++) {
193 atomic_add(val, &alloc_covered[alloc_stack_hash & ALLOC_COVERED_MASK]);
194 alloc_stack_hash = ALLOC_COVERED_HNEXT(alloc_stack_hash);
199 * Returns true if the allocation stack trace hash @alloc_stack_hash is
200 * currently contained (non-zero count) in Counting Bloom filter.
202 static bool alloc_covered_contains(u32 alloc_stack_hash)
206 for (i = 0; i < ALLOC_COVERED_HNUM; i++) {
207 if (!atomic_read(&alloc_covered[alloc_stack_hash & ALLOC_COVERED_MASK]))
209 alloc_stack_hash = ALLOC_COVERED_HNEXT(alloc_stack_hash);
215 static bool kfence_protect(unsigned long addr)
217 return !KFENCE_WARN_ON(!kfence_protect_page(ALIGN_DOWN(addr, PAGE_SIZE), true));
220 static bool kfence_unprotect(unsigned long addr)
222 return !KFENCE_WARN_ON(!kfence_protect_page(ALIGN_DOWN(addr, PAGE_SIZE), false));
225 static inline struct kfence_metadata *addr_to_metadata(unsigned long addr)
229 /* The checks do not affect performance; only called from slow-paths. */
231 if (!is_kfence_address((void *)addr))
235 * May be an invalid index if called with an address at the edge of
236 * __kfence_pool, in which case we would report an "invalid access"
239 index = (addr - (unsigned long)__kfence_pool) / (PAGE_SIZE * 2) - 1;
240 if (index < 0 || index >= CONFIG_KFENCE_NUM_OBJECTS)
243 return &kfence_metadata[index];
246 static inline unsigned long metadata_to_pageaddr(const struct kfence_metadata *meta)
248 unsigned long offset = (meta - kfence_metadata + 1) * PAGE_SIZE * 2;
249 unsigned long pageaddr = (unsigned long)&__kfence_pool[offset];
251 /* The checks do not affect performance; only called from slow-paths. */
253 /* Only call with a pointer into kfence_metadata. */
254 if (KFENCE_WARN_ON(meta < kfence_metadata ||
255 meta >= kfence_metadata + CONFIG_KFENCE_NUM_OBJECTS))
259 * This metadata object only ever maps to 1 page; verify that the stored
260 * address is in the expected range.
262 if (KFENCE_WARN_ON(ALIGN_DOWN(meta->addr, PAGE_SIZE) != pageaddr))
269 * Update the object's metadata state, including updating the alloc/free stacks
270 * depending on the state transition.
273 metadata_update_state(struct kfence_metadata *meta, enum kfence_object_state next,
274 unsigned long *stack_entries, size_t num_stack_entries)
276 struct kfence_track *track =
277 next == KFENCE_OBJECT_FREED ? &meta->free_track : &meta->alloc_track;
279 lockdep_assert_held(&meta->lock);
282 memcpy(track->stack_entries, stack_entries,
283 num_stack_entries * sizeof(stack_entries[0]));
286 * Skip over 1 (this) functions; noinline ensures we do not
287 * accidentally skip over the caller by never inlining.
289 num_stack_entries = stack_trace_save(track->stack_entries, KFENCE_STACK_DEPTH, 1);
291 track->num_stack_entries = num_stack_entries;
292 track->pid = task_pid_nr(current);
293 track->cpu = raw_smp_processor_id();
294 track->ts_nsec = local_clock(); /* Same source as printk timestamps. */
297 * Pairs with READ_ONCE() in
298 * kfence_shutdown_cache(),
299 * kfence_handle_page_fault().
301 WRITE_ONCE(meta->state, next);
304 /* Write canary byte to @addr. */
305 static inline bool set_canary_byte(u8 *addr)
307 *addr = KFENCE_CANARY_PATTERN(addr);
311 /* Check canary byte at @addr. */
312 static inline bool check_canary_byte(u8 *addr)
314 struct kfence_metadata *meta;
317 if (likely(*addr == KFENCE_CANARY_PATTERN(addr)))
320 atomic_long_inc(&counters[KFENCE_COUNTER_BUGS]);
322 meta = addr_to_metadata((unsigned long)addr);
323 raw_spin_lock_irqsave(&meta->lock, flags);
324 kfence_report_error((unsigned long)addr, false, NULL, meta, KFENCE_ERROR_CORRUPTION);
325 raw_spin_unlock_irqrestore(&meta->lock, flags);
330 /* __always_inline this to ensure we won't do an indirect call to fn. */
331 static __always_inline void for_each_canary(const struct kfence_metadata *meta, bool (*fn)(u8 *))
333 const unsigned long pageaddr = ALIGN_DOWN(meta->addr, PAGE_SIZE);
337 * We'll iterate over each canary byte per-side until fn() returns
338 * false. However, we'll still iterate over the canary bytes to the
339 * right of the object even if there was an error in the canary bytes to
340 * the left of the object. Specifically, if check_canary_byte()
341 * generates an error, showing both sides might give more clues as to
342 * what the error is about when displaying which bytes were corrupted.
345 /* Apply to left of object. */
346 for (addr = pageaddr; addr < meta->addr; addr++) {
351 /* Apply to right of object. */
352 for (addr = meta->addr + meta->size; addr < pageaddr + PAGE_SIZE; addr++) {
358 static void *kfence_guarded_alloc(struct kmem_cache *cache, size_t size, gfp_t gfp,
359 unsigned long *stack_entries, size_t num_stack_entries,
360 u32 alloc_stack_hash)
362 struct kfence_metadata *meta = NULL;
367 /* Try to obtain a free object. */
368 raw_spin_lock_irqsave(&kfence_freelist_lock, flags);
369 if (!list_empty(&kfence_freelist)) {
370 meta = list_entry(kfence_freelist.next, struct kfence_metadata, list);
371 list_del_init(&meta->list);
373 raw_spin_unlock_irqrestore(&kfence_freelist_lock, flags);
375 atomic_long_inc(&counters[KFENCE_COUNTER_SKIP_CAPACITY]);
379 if (unlikely(!raw_spin_trylock_irqsave(&meta->lock, flags))) {
381 * This is extremely unlikely -- we are reporting on a
382 * use-after-free, which locked meta->lock, and the reporting
383 * code via printk calls kmalloc() which ends up in
384 * kfence_alloc() and tries to grab the same object that we're
385 * reporting on. While it has never been observed, lockdep does
386 * report that there is a possibility of deadlock. Fix it by
387 * using trylock and bailing out gracefully.
389 raw_spin_lock_irqsave(&kfence_freelist_lock, flags);
390 /* Put the object back on the freelist. */
391 list_add_tail(&meta->list, &kfence_freelist);
392 raw_spin_unlock_irqrestore(&kfence_freelist_lock, flags);
397 meta->addr = metadata_to_pageaddr(meta);
398 /* Unprotect if we're reusing this page. */
399 if (meta->state == KFENCE_OBJECT_FREED)
400 kfence_unprotect(meta->addr);
403 * Note: for allocations made before RNG initialization, will always
404 * return zero. We still benefit from enabling KFENCE as early as
405 * possible, even when the RNG is not yet available, as this will allow
406 * KFENCE to detect bugs due to earlier allocations. The only downside
407 * is that the out-of-bounds accesses detected are deterministic for
410 if (prandom_u32_max(2)) {
411 /* Allocate on the "right" side, re-calculate address. */
412 meta->addr += PAGE_SIZE - size;
413 meta->addr = ALIGN_DOWN(meta->addr, cache->align);
416 addr = (void *)meta->addr;
418 /* Update remaining metadata. */
419 metadata_update_state(meta, KFENCE_OBJECT_ALLOCATED, stack_entries, num_stack_entries);
420 /* Pairs with READ_ONCE() in kfence_shutdown_cache(). */
421 WRITE_ONCE(meta->cache, cache);
423 meta->alloc_stack_hash = alloc_stack_hash;
424 raw_spin_unlock_irqrestore(&meta->lock, flags);
426 alloc_covered_add(alloc_stack_hash, 1);
428 /* Set required slab fields. */
429 slab = virt_to_slab((void *)meta->addr);
430 slab->slab_cache = cache;
431 #if defined(CONFIG_SLUB)
433 #elif defined(CONFIG_SLAB)
437 /* Memory initialization. */
438 for_each_canary(meta, set_canary_byte);
441 * We check slab_want_init_on_alloc() ourselves, rather than letting
442 * SL*B do the initialization, as otherwise we might overwrite KFENCE's
445 if (unlikely(slab_want_init_on_alloc(gfp, cache)))
446 memzero_explicit(addr, size);
450 if (CONFIG_KFENCE_STRESS_TEST_FAULTS && !prandom_u32_max(CONFIG_KFENCE_STRESS_TEST_FAULTS))
451 kfence_protect(meta->addr); /* Random "faults" by protecting the object. */
453 atomic_long_inc(&counters[KFENCE_COUNTER_ALLOCATED]);
454 atomic_long_inc(&counters[KFENCE_COUNTER_ALLOCS]);
459 static void kfence_guarded_free(void *addr, struct kfence_metadata *meta, bool zombie)
461 struct kcsan_scoped_access assert_page_exclusive;
465 raw_spin_lock_irqsave(&meta->lock, flags);
467 if (meta->state != KFENCE_OBJECT_ALLOCATED || meta->addr != (unsigned long)addr) {
468 /* Invalid or double-free, bail out. */
469 atomic_long_inc(&counters[KFENCE_COUNTER_BUGS]);
470 kfence_report_error((unsigned long)addr, false, NULL, meta,
471 KFENCE_ERROR_INVALID_FREE);
472 raw_spin_unlock_irqrestore(&meta->lock, flags);
476 /* Detect racy use-after-free, or incorrect reallocation of this page by KFENCE. */
477 kcsan_begin_scoped_access((void *)ALIGN_DOWN((unsigned long)addr, PAGE_SIZE), PAGE_SIZE,
478 KCSAN_ACCESS_SCOPED | KCSAN_ACCESS_WRITE | KCSAN_ACCESS_ASSERT,
479 &assert_page_exclusive);
481 if (CONFIG_KFENCE_STRESS_TEST_FAULTS)
482 kfence_unprotect((unsigned long)addr); /* To check canary bytes. */
484 /* Restore page protection if there was an OOB access. */
485 if (meta->unprotected_page) {
486 memzero_explicit((void *)ALIGN_DOWN(meta->unprotected_page, PAGE_SIZE), PAGE_SIZE);
487 kfence_protect(meta->unprotected_page);
488 meta->unprotected_page = 0;
491 /* Mark the object as freed. */
492 metadata_update_state(meta, KFENCE_OBJECT_FREED, NULL, 0);
493 init = slab_want_init_on_free(meta->cache);
494 raw_spin_unlock_irqrestore(&meta->lock, flags);
496 alloc_covered_add(meta->alloc_stack_hash, -1);
498 /* Check canary bytes for memory corruption. */
499 for_each_canary(meta, check_canary_byte);
502 * Clear memory if init-on-free is set. While we protect the page, the
503 * data is still there, and after a use-after-free is detected, we
504 * unprotect the page, so the data is still accessible.
506 if (!zombie && unlikely(init))
507 memzero_explicit(addr, meta->size);
509 /* Protect to detect use-after-frees. */
510 kfence_protect((unsigned long)addr);
512 kcsan_end_scoped_access(&assert_page_exclusive);
514 /* Add it to the tail of the freelist for reuse. */
515 raw_spin_lock_irqsave(&kfence_freelist_lock, flags);
516 KFENCE_WARN_ON(!list_empty(&meta->list));
517 list_add_tail(&meta->list, &kfence_freelist);
518 raw_spin_unlock_irqrestore(&kfence_freelist_lock, flags);
520 atomic_long_dec(&counters[KFENCE_COUNTER_ALLOCATED]);
521 atomic_long_inc(&counters[KFENCE_COUNTER_FREES]);
523 /* See kfence_shutdown_cache(). */
524 atomic_long_inc(&counters[KFENCE_COUNTER_ZOMBIES]);
528 static void rcu_guarded_free(struct rcu_head *h)
530 struct kfence_metadata *meta = container_of(h, struct kfence_metadata, rcu_head);
532 kfence_guarded_free((void *)meta->addr, meta, false);
535 static bool __init kfence_init_pool(void)
537 unsigned long addr = (unsigned long)__kfence_pool;
544 if (!arch_kfence_init_pool())
547 pages = virt_to_page(addr);
550 * Set up object pages: they must have PG_slab set, to avoid freeing
551 * these as real pages.
553 * We also want to avoid inserting kfence_free() in the kfree()
554 * fast-path in SLUB, and therefore need to ensure kfree() correctly
555 * enters __slab_free() slow-path.
557 for (i = 0; i < KFENCE_POOL_SIZE / PAGE_SIZE; i++) {
561 /* Verify we do not have a compound head page. */
562 if (WARN_ON(compound_head(&pages[i]) != &pages[i]))
565 __SetPageSlab(&pages[i]);
569 * Protect the first 2 pages. The first page is mostly unnecessary, and
570 * merely serves as an extended guard page. However, adding one
571 * additional page in the beginning gives us an even number of pages,
572 * which simplifies the mapping of address to metadata index.
574 for (i = 0; i < 2; i++) {
575 if (unlikely(!kfence_protect(addr)))
581 for (i = 0; i < CONFIG_KFENCE_NUM_OBJECTS; i++) {
582 struct kfence_metadata *meta = &kfence_metadata[i];
584 /* Initialize metadata. */
585 INIT_LIST_HEAD(&meta->list);
586 raw_spin_lock_init(&meta->lock);
587 meta->state = KFENCE_OBJECT_UNUSED;
588 meta->addr = addr; /* Initialize for validation in metadata_to_pageaddr(). */
589 list_add_tail(&meta->list, &kfence_freelist);
591 /* Protect the right redzone. */
592 if (unlikely(!kfence_protect(addr + PAGE_SIZE)))
595 addr += 2 * PAGE_SIZE;
599 * The pool is live and will never be deallocated from this point on.
600 * Remove the pool object from the kmemleak object tree, as it would
601 * otherwise overlap with allocations returned by kfence_alloc(), which
602 * are registered with kmemleak through the slab post-alloc hook.
604 kmemleak_free(__kfence_pool);
610 * Only release unprotected pages, and do not try to go back and change
611 * page attributes due to risk of failing to do so as well. If changing
612 * page attributes for some pages fails, it is very likely that it also
613 * fails for the first page, and therefore expect addr==__kfence_pool in
614 * most failure cases.
616 memblock_free_late(__pa(addr), KFENCE_POOL_SIZE - (addr - (unsigned long)__kfence_pool));
617 __kfence_pool = NULL;
621 /* === DebugFS Interface ==================================================== */
623 static int stats_show(struct seq_file *seq, void *v)
627 seq_printf(seq, "enabled: %i\n", READ_ONCE(kfence_enabled));
628 for (i = 0; i < KFENCE_COUNTER_COUNT; i++)
629 seq_printf(seq, "%s: %ld\n", counter_names[i], atomic_long_read(&counters[i]));
633 DEFINE_SHOW_ATTRIBUTE(stats);
636 * debugfs seq_file operations for /sys/kernel/debug/kfence/objects.
637 * start_object() and next_object() return the object index + 1, because NULL is used
640 static void *start_object(struct seq_file *seq, loff_t *pos)
642 if (*pos < CONFIG_KFENCE_NUM_OBJECTS)
643 return (void *)((long)*pos + 1);
647 static void stop_object(struct seq_file *seq, void *v)
651 static void *next_object(struct seq_file *seq, void *v, loff_t *pos)
654 if (*pos < CONFIG_KFENCE_NUM_OBJECTS)
655 return (void *)((long)*pos + 1);
659 static int show_object(struct seq_file *seq, void *v)
661 struct kfence_metadata *meta = &kfence_metadata[(long)v - 1];
664 raw_spin_lock_irqsave(&meta->lock, flags);
665 kfence_print_object(seq, meta);
666 raw_spin_unlock_irqrestore(&meta->lock, flags);
667 seq_puts(seq, "---------------------------------\n");
672 static const struct seq_operations object_seqops = {
673 .start = start_object,
679 static int open_objects(struct inode *inode, struct file *file)
681 return seq_open(file, &object_seqops);
684 static const struct file_operations objects_fops = {
685 .open = open_objects,
688 .release = seq_release,
691 static int __init kfence_debugfs_init(void)
693 struct dentry *kfence_dir = debugfs_create_dir("kfence", NULL);
695 debugfs_create_file("stats", 0444, kfence_dir, NULL, &stats_fops);
696 debugfs_create_file("objects", 0400, kfence_dir, NULL, &objects_fops);
700 late_initcall(kfence_debugfs_init);
702 /* === Allocation Gate Timer ================================================ */
704 #ifdef CONFIG_KFENCE_STATIC_KEYS
705 /* Wait queue to wake up allocation-gate timer task. */
706 static DECLARE_WAIT_QUEUE_HEAD(allocation_wait);
708 static void wake_up_kfence_timer(struct irq_work *work)
710 wake_up(&allocation_wait);
712 static DEFINE_IRQ_WORK(wake_up_kfence_timer_work, wake_up_kfence_timer);
716 * Set up delayed work, which will enable and disable the static key. We need to
717 * use a work queue (rather than a simple timer), since enabling and disabling a
718 * static key cannot be done from an interrupt.
720 * Note: Toggling a static branch currently causes IPIs, and here we'll end up
721 * with a total of 2 IPIs to all CPUs. If this ends up a problem in future (with
722 * more aggressive sampling intervals), we could get away with a variant that
723 * avoids IPIs, at the cost of not immediately capturing allocations if the
724 * instructions remain cached.
726 static struct delayed_work kfence_timer;
727 static void toggle_allocation_gate(struct work_struct *work)
729 if (!READ_ONCE(kfence_enabled))
732 atomic_set(&kfence_allocation_gate, 0);
733 #ifdef CONFIG_KFENCE_STATIC_KEYS
734 /* Enable static key, and await allocation to happen. */
735 static_branch_enable(&kfence_allocation_key);
737 if (sysctl_hung_task_timeout_secs) {
739 * During low activity with no allocations we might wait a
740 * while; let's avoid the hung task warning.
742 wait_event_idle_timeout(allocation_wait, atomic_read(&kfence_allocation_gate),
743 sysctl_hung_task_timeout_secs * HZ / 2);
745 wait_event_idle(allocation_wait, atomic_read(&kfence_allocation_gate));
748 /* Disable static key and reset timer. */
749 static_branch_disable(&kfence_allocation_key);
751 queue_delayed_work(system_unbound_wq, &kfence_timer,
752 msecs_to_jiffies(kfence_sample_interval));
754 static DECLARE_DELAYED_WORK(kfence_timer, toggle_allocation_gate);
756 /* === Public interface ===================================================== */
758 void __init kfence_alloc_pool(void)
760 if (!kfence_sample_interval)
763 __kfence_pool = memblock_alloc(KFENCE_POOL_SIZE, PAGE_SIZE);
766 pr_err("failed to allocate pool\n");
769 void __init kfence_init(void)
771 /* Setting kfence_sample_interval to 0 on boot disables KFENCE. */
772 if (!kfence_sample_interval)
775 stack_hash_seed = (u32)random_get_entropy();
776 if (!kfence_init_pool()) {
777 pr_err("%s failed\n", __func__);
781 if (!IS_ENABLED(CONFIG_KFENCE_STATIC_KEYS))
782 static_branch_enable(&kfence_allocation_key);
783 WRITE_ONCE(kfence_enabled, true);
784 queue_delayed_work(system_unbound_wq, &kfence_timer, 0);
785 pr_info("initialized - using %lu bytes for %d objects at 0x%p-0x%p\n", KFENCE_POOL_SIZE,
786 CONFIG_KFENCE_NUM_OBJECTS, (void *)__kfence_pool,
787 (void *)(__kfence_pool + KFENCE_POOL_SIZE));
790 void kfence_shutdown_cache(struct kmem_cache *s)
793 struct kfence_metadata *meta;
796 for (i = 0; i < CONFIG_KFENCE_NUM_OBJECTS; i++) {
799 meta = &kfence_metadata[i];
802 * If we observe some inconsistent cache and state pair where we
803 * should have returned false here, cache destruction is racing
804 * with either kmem_cache_alloc() or kmem_cache_free(). Taking
805 * the lock will not help, as different critical section
806 * serialization will have the same outcome.
808 if (READ_ONCE(meta->cache) != s ||
809 READ_ONCE(meta->state) != KFENCE_OBJECT_ALLOCATED)
812 raw_spin_lock_irqsave(&meta->lock, flags);
813 in_use = meta->cache == s && meta->state == KFENCE_OBJECT_ALLOCATED;
814 raw_spin_unlock_irqrestore(&meta->lock, flags);
818 * This cache still has allocations, and we should not
819 * release them back into the freelist so they can still
820 * safely be used and retain the kernel's default
821 * behaviour of keeping the allocations alive (leak the
822 * cache); however, they effectively become "zombie
823 * allocations" as the KFENCE objects are the only ones
824 * still in use and the owning cache is being destroyed.
826 * We mark them freed, so that any subsequent use shows
827 * more useful error messages that will include stack
828 * traces of the user of the object, the original
829 * allocation, and caller to shutdown_cache().
831 kfence_guarded_free((void *)meta->addr, meta, /*zombie=*/true);
835 for (i = 0; i < CONFIG_KFENCE_NUM_OBJECTS; i++) {
836 meta = &kfence_metadata[i];
839 if (READ_ONCE(meta->cache) != s || READ_ONCE(meta->state) != KFENCE_OBJECT_FREED)
842 raw_spin_lock_irqsave(&meta->lock, flags);
843 if (meta->cache == s && meta->state == KFENCE_OBJECT_FREED)
845 raw_spin_unlock_irqrestore(&meta->lock, flags);
849 void *__kfence_alloc(struct kmem_cache *s, size_t size, gfp_t flags)
851 unsigned long stack_entries[KFENCE_STACK_DEPTH];
852 size_t num_stack_entries;
853 u32 alloc_stack_hash;
856 * Perform size check before switching kfence_allocation_gate, so that
857 * we don't disable KFENCE without making an allocation.
859 if (size > PAGE_SIZE) {
860 atomic_long_inc(&counters[KFENCE_COUNTER_SKIP_INCOMPAT]);
865 * Skip allocations from non-default zones, including DMA. We cannot
866 * guarantee that pages in the KFENCE pool will have the requested
867 * properties (e.g. reside in DMAable memory).
869 if ((flags & GFP_ZONEMASK) ||
870 (s->flags & (SLAB_CACHE_DMA | SLAB_CACHE_DMA32))) {
871 atomic_long_inc(&counters[KFENCE_COUNTER_SKIP_INCOMPAT]);
875 if (atomic_inc_return(&kfence_allocation_gate) > 1)
877 #ifdef CONFIG_KFENCE_STATIC_KEYS
879 * waitqueue_active() is fully ordered after the update of
880 * kfence_allocation_gate per atomic_inc_return().
882 if (waitqueue_active(&allocation_wait)) {
884 * Calling wake_up() here may deadlock when allocations happen
885 * from within timer code. Use an irq_work to defer it.
887 irq_work_queue(&wake_up_kfence_timer_work);
891 if (!READ_ONCE(kfence_enabled))
894 num_stack_entries = stack_trace_save(stack_entries, KFENCE_STACK_DEPTH, 0);
897 * Do expensive check for coverage of allocation in slow-path after
898 * allocation_gate has already become non-zero, even though it might
899 * mean not making any allocation within a given sample interval.
901 * This ensures reasonable allocation coverage when the pool is almost
902 * full, including avoiding long-lived allocations of the same source
903 * filling up the pool (e.g. pagecache allocations).
905 alloc_stack_hash = get_alloc_stack_hash(stack_entries, num_stack_entries);
906 if (should_skip_covered() && alloc_covered_contains(alloc_stack_hash)) {
907 atomic_long_inc(&counters[KFENCE_COUNTER_SKIP_COVERED]);
911 return kfence_guarded_alloc(s, size, flags, stack_entries, num_stack_entries,
915 size_t kfence_ksize(const void *addr)
917 const struct kfence_metadata *meta = addr_to_metadata((unsigned long)addr);
920 * Read locklessly -- if there is a race with __kfence_alloc(), this is
921 * either a use-after-free or invalid access.
923 return meta ? meta->size : 0;
926 void *kfence_object_start(const void *addr)
928 const struct kfence_metadata *meta = addr_to_metadata((unsigned long)addr);
931 * Read locklessly -- if there is a race with __kfence_alloc(), this is
932 * either a use-after-free or invalid access.
934 return meta ? (void *)meta->addr : NULL;
937 void __kfence_free(void *addr)
939 struct kfence_metadata *meta = addr_to_metadata((unsigned long)addr);
942 * If the objects of the cache are SLAB_TYPESAFE_BY_RCU, defer freeing
943 * the object, as the object page may be recycled for other-typed
944 * objects once it has been freed. meta->cache may be NULL if the cache
947 if (unlikely(meta->cache && (meta->cache->flags & SLAB_TYPESAFE_BY_RCU)))
948 call_rcu(&meta->rcu_head, rcu_guarded_free);
950 kfence_guarded_free(addr, meta, false);
953 bool kfence_handle_page_fault(unsigned long addr, bool is_write, struct pt_regs *regs)
955 const int page_index = (addr - (unsigned long)__kfence_pool) / PAGE_SIZE;
956 struct kfence_metadata *to_report = NULL;
957 enum kfence_error_type error_type;
960 if (!is_kfence_address((void *)addr))
963 if (!READ_ONCE(kfence_enabled)) /* If disabled at runtime ... */
964 return kfence_unprotect(addr); /* ... unprotect and proceed. */
966 atomic_long_inc(&counters[KFENCE_COUNTER_BUGS]);
968 if (page_index % 2) {
969 /* This is a redzone, report a buffer overflow. */
970 struct kfence_metadata *meta;
973 meta = addr_to_metadata(addr - PAGE_SIZE);
974 if (meta && READ_ONCE(meta->state) == KFENCE_OBJECT_ALLOCATED) {
976 /* Data race ok; distance calculation approximate. */
977 distance = addr - data_race(meta->addr + meta->size);
980 meta = addr_to_metadata(addr + PAGE_SIZE);
981 if (meta && READ_ONCE(meta->state) == KFENCE_OBJECT_ALLOCATED) {
982 /* Data race ok; distance calculation approximate. */
983 if (!to_report || distance > data_race(meta->addr) - addr)
990 raw_spin_lock_irqsave(&to_report->lock, flags);
991 to_report->unprotected_page = addr;
992 error_type = KFENCE_ERROR_OOB;
995 * If the object was freed before we took the look we can still
996 * report this as an OOB -- the report will simply show the
997 * stacktrace of the free as well.
1000 to_report = addr_to_metadata(addr);
1004 raw_spin_lock_irqsave(&to_report->lock, flags);
1005 error_type = KFENCE_ERROR_UAF;
1007 * We may race with __kfence_alloc(), and it is possible that a
1008 * freed object may be reallocated. We simply report this as a
1009 * use-after-free, with the stack trace showing the place where
1010 * the object was re-allocated.
1016 kfence_report_error(addr, is_write, regs, to_report, error_type);
1017 raw_spin_unlock_irqrestore(&to_report->lock, flags);
1019 /* This may be a UAF or OOB access, but we can't be sure. */
1020 kfence_report_error(addr, is_write, regs, NULL, KFENCE_ERROR_INVALID);
1023 return kfence_unprotect(addr); /* Unprotect and let access proceed. */