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); \
43 disabled_by_warn = true; \
48 /* === Data ================================================================= */
50 static bool kfence_enabled __read_mostly;
51 static bool disabled_by_warn __read_mostly;
53 unsigned long kfence_sample_interval __read_mostly = CONFIG_KFENCE_SAMPLE_INTERVAL;
54 EXPORT_SYMBOL_GPL(kfence_sample_interval); /* Export for test modules. */
56 #ifdef MODULE_PARAM_PREFIX
57 #undef MODULE_PARAM_PREFIX
59 #define MODULE_PARAM_PREFIX "kfence."
61 static int kfence_enable_late(void);
62 static int param_set_sample_interval(const char *val, const struct kernel_param *kp)
65 int ret = kstrtoul(val, 0, &num);
70 if (!num) /* Using 0 to indicate KFENCE is disabled. */
71 WRITE_ONCE(kfence_enabled, false);
73 *((unsigned long *)kp->arg) = num;
75 if (num && !READ_ONCE(kfence_enabled) && system_state != SYSTEM_BOOTING)
76 return disabled_by_warn ? -EINVAL : kfence_enable_late();
80 static int param_get_sample_interval(char *buffer, const struct kernel_param *kp)
82 if (!READ_ONCE(kfence_enabled))
83 return sprintf(buffer, "0\n");
85 return param_get_ulong(buffer, kp);
88 static const struct kernel_param_ops sample_interval_param_ops = {
89 .set = param_set_sample_interval,
90 .get = param_get_sample_interval,
92 module_param_cb(sample_interval, &sample_interval_param_ops, &kfence_sample_interval, 0600);
94 /* Pool usage% threshold when currently covered allocations are skipped. */
95 static unsigned long kfence_skip_covered_thresh __read_mostly = 75;
96 module_param_named(skip_covered_thresh, kfence_skip_covered_thresh, ulong, 0644);
98 /* If true, use a deferrable timer. */
99 static bool kfence_deferrable __read_mostly = IS_ENABLED(CONFIG_KFENCE_DEFERRABLE);
100 module_param_named(deferrable, kfence_deferrable, bool, 0444);
102 /* The pool of pages used for guard pages and objects. */
103 char *__kfence_pool __read_mostly;
104 EXPORT_SYMBOL(__kfence_pool); /* Export for test modules. */
107 * Per-object metadata, with one-to-one mapping of object metadata to
108 * backing pages (in __kfence_pool).
110 static_assert(CONFIG_KFENCE_NUM_OBJECTS > 0);
111 struct kfence_metadata kfence_metadata[CONFIG_KFENCE_NUM_OBJECTS];
113 /* Freelist with available objects. */
114 static struct list_head kfence_freelist = LIST_HEAD_INIT(kfence_freelist);
115 static DEFINE_RAW_SPINLOCK(kfence_freelist_lock); /* Lock protecting freelist. */
118 * The static key to set up a KFENCE allocation; or if static keys are not used
119 * to gate allocations, to avoid a load and compare if KFENCE is disabled.
121 DEFINE_STATIC_KEY_FALSE(kfence_allocation_key);
123 /* Gates the allocation, ensuring only one succeeds in a given period. */
124 atomic_t kfence_allocation_gate = ATOMIC_INIT(1);
127 * A Counting Bloom filter of allocation coverage: limits currently covered
128 * allocations of the same source filling up the pool.
130 * Assuming a range of 15%-85% unique allocations in the pool at any point in
131 * time, the below parameters provide a probablity of 0.02-0.33 for false
132 * positive hits respectively:
134 * P(alloc_traces) = (1 - e^(-HNUM * (alloc_traces / SIZE)) ^ HNUM
136 #define ALLOC_COVERED_HNUM 2
137 #define ALLOC_COVERED_ORDER (const_ilog2(CONFIG_KFENCE_NUM_OBJECTS) + 2)
138 #define ALLOC_COVERED_SIZE (1 << ALLOC_COVERED_ORDER)
139 #define ALLOC_COVERED_HNEXT(h) hash_32(h, ALLOC_COVERED_ORDER)
140 #define ALLOC_COVERED_MASK (ALLOC_COVERED_SIZE - 1)
141 static atomic_t alloc_covered[ALLOC_COVERED_SIZE];
143 /* Stack depth used to determine uniqueness of an allocation. */
144 #define UNIQUE_ALLOC_STACK_DEPTH ((size_t)8)
147 * Randomness for stack hashes, making the same collisions across reboots and
148 * different machines less likely.
150 static u32 stack_hash_seed __ro_after_init;
152 /* Statistics counters for debugfs. */
153 enum kfence_counter_id {
154 KFENCE_COUNTER_ALLOCATED,
155 KFENCE_COUNTER_ALLOCS,
156 KFENCE_COUNTER_FREES,
157 KFENCE_COUNTER_ZOMBIES,
159 KFENCE_COUNTER_SKIP_INCOMPAT,
160 KFENCE_COUNTER_SKIP_CAPACITY,
161 KFENCE_COUNTER_SKIP_COVERED,
162 KFENCE_COUNTER_COUNT,
164 static atomic_long_t counters[KFENCE_COUNTER_COUNT];
165 static const char *const counter_names[] = {
166 [KFENCE_COUNTER_ALLOCATED] = "currently allocated",
167 [KFENCE_COUNTER_ALLOCS] = "total allocations",
168 [KFENCE_COUNTER_FREES] = "total frees",
169 [KFENCE_COUNTER_ZOMBIES] = "zombie allocations",
170 [KFENCE_COUNTER_BUGS] = "total bugs",
171 [KFENCE_COUNTER_SKIP_INCOMPAT] = "skipped allocations (incompatible)",
172 [KFENCE_COUNTER_SKIP_CAPACITY] = "skipped allocations (capacity)",
173 [KFENCE_COUNTER_SKIP_COVERED] = "skipped allocations (covered)",
175 static_assert(ARRAY_SIZE(counter_names) == KFENCE_COUNTER_COUNT);
177 /* === Internals ============================================================ */
179 static inline bool should_skip_covered(void)
181 unsigned long thresh = (CONFIG_KFENCE_NUM_OBJECTS * kfence_skip_covered_thresh) / 100;
183 return atomic_long_read(&counters[KFENCE_COUNTER_ALLOCATED]) > thresh;
186 static u32 get_alloc_stack_hash(unsigned long *stack_entries, size_t num_entries)
188 num_entries = min(num_entries, UNIQUE_ALLOC_STACK_DEPTH);
189 num_entries = filter_irq_stacks(stack_entries, num_entries);
190 return jhash(stack_entries, num_entries * sizeof(stack_entries[0]), stack_hash_seed);
194 * Adds (or subtracts) count @val for allocation stack trace hash
195 * @alloc_stack_hash from Counting Bloom filter.
197 static void alloc_covered_add(u32 alloc_stack_hash, int val)
201 for (i = 0; i < ALLOC_COVERED_HNUM; i++) {
202 atomic_add(val, &alloc_covered[alloc_stack_hash & ALLOC_COVERED_MASK]);
203 alloc_stack_hash = ALLOC_COVERED_HNEXT(alloc_stack_hash);
208 * Returns true if the allocation stack trace hash @alloc_stack_hash is
209 * currently contained (non-zero count) in Counting Bloom filter.
211 static bool alloc_covered_contains(u32 alloc_stack_hash)
215 for (i = 0; i < ALLOC_COVERED_HNUM; i++) {
216 if (!atomic_read(&alloc_covered[alloc_stack_hash & ALLOC_COVERED_MASK]))
218 alloc_stack_hash = ALLOC_COVERED_HNEXT(alloc_stack_hash);
224 static bool kfence_protect(unsigned long addr)
226 return !KFENCE_WARN_ON(!kfence_protect_page(ALIGN_DOWN(addr, PAGE_SIZE), true));
229 static bool kfence_unprotect(unsigned long addr)
231 return !KFENCE_WARN_ON(!kfence_protect_page(ALIGN_DOWN(addr, PAGE_SIZE), false));
234 static inline struct kfence_metadata *addr_to_metadata(unsigned long addr)
238 /* The checks do not affect performance; only called from slow-paths. */
240 if (!is_kfence_address((void *)addr))
244 * May be an invalid index if called with an address at the edge of
245 * __kfence_pool, in which case we would report an "invalid access"
248 index = (addr - (unsigned long)__kfence_pool) / (PAGE_SIZE * 2) - 1;
249 if (index < 0 || index >= CONFIG_KFENCE_NUM_OBJECTS)
252 return &kfence_metadata[index];
255 static inline unsigned long metadata_to_pageaddr(const struct kfence_metadata *meta)
257 unsigned long offset = (meta - kfence_metadata + 1) * PAGE_SIZE * 2;
258 unsigned long pageaddr = (unsigned long)&__kfence_pool[offset];
260 /* The checks do not affect performance; only called from slow-paths. */
262 /* Only call with a pointer into kfence_metadata. */
263 if (KFENCE_WARN_ON(meta < kfence_metadata ||
264 meta >= kfence_metadata + CONFIG_KFENCE_NUM_OBJECTS))
268 * This metadata object only ever maps to 1 page; verify that the stored
269 * address is in the expected range.
271 if (KFENCE_WARN_ON(ALIGN_DOWN(meta->addr, PAGE_SIZE) != pageaddr))
278 * Update the object's metadata state, including updating the alloc/free stacks
279 * depending on the state transition.
282 metadata_update_state(struct kfence_metadata *meta, enum kfence_object_state next,
283 unsigned long *stack_entries, size_t num_stack_entries)
285 struct kfence_track *track =
286 next == KFENCE_OBJECT_FREED ? &meta->free_track : &meta->alloc_track;
288 lockdep_assert_held(&meta->lock);
291 memcpy(track->stack_entries, stack_entries,
292 num_stack_entries * sizeof(stack_entries[0]));
295 * Skip over 1 (this) functions; noinline ensures we do not
296 * accidentally skip over the caller by never inlining.
298 num_stack_entries = stack_trace_save(track->stack_entries, KFENCE_STACK_DEPTH, 1);
300 track->num_stack_entries = num_stack_entries;
301 track->pid = task_pid_nr(current);
302 track->cpu = raw_smp_processor_id();
303 track->ts_nsec = local_clock(); /* Same source as printk timestamps. */
306 * Pairs with READ_ONCE() in
307 * kfence_shutdown_cache(),
308 * kfence_handle_page_fault().
310 WRITE_ONCE(meta->state, next);
313 /* Write canary byte to @addr. */
314 static inline bool set_canary_byte(u8 *addr)
316 *addr = KFENCE_CANARY_PATTERN(addr);
320 /* Check canary byte at @addr. */
321 static inline bool check_canary_byte(u8 *addr)
323 struct kfence_metadata *meta;
326 if (likely(*addr == KFENCE_CANARY_PATTERN(addr)))
329 atomic_long_inc(&counters[KFENCE_COUNTER_BUGS]);
331 meta = addr_to_metadata((unsigned long)addr);
332 raw_spin_lock_irqsave(&meta->lock, flags);
333 kfence_report_error((unsigned long)addr, false, NULL, meta, KFENCE_ERROR_CORRUPTION);
334 raw_spin_unlock_irqrestore(&meta->lock, flags);
339 /* __always_inline this to ensure we won't do an indirect call to fn. */
340 static __always_inline void for_each_canary(const struct kfence_metadata *meta, bool (*fn)(u8 *))
342 const unsigned long pageaddr = ALIGN_DOWN(meta->addr, PAGE_SIZE);
346 * We'll iterate over each canary byte per-side until fn() returns
347 * false. However, we'll still iterate over the canary bytes to the
348 * right of the object even if there was an error in the canary bytes to
349 * the left of the object. Specifically, if check_canary_byte()
350 * generates an error, showing both sides might give more clues as to
351 * what the error is about when displaying which bytes were corrupted.
354 /* Apply to left of object. */
355 for (addr = pageaddr; addr < meta->addr; addr++) {
360 /* Apply to right of object. */
361 for (addr = meta->addr + meta->size; addr < pageaddr + PAGE_SIZE; addr++) {
367 static void *kfence_guarded_alloc(struct kmem_cache *cache, size_t size, gfp_t gfp,
368 unsigned long *stack_entries, size_t num_stack_entries,
369 u32 alloc_stack_hash)
371 struct kfence_metadata *meta = NULL;
376 /* Try to obtain a free object. */
377 raw_spin_lock_irqsave(&kfence_freelist_lock, flags);
378 if (!list_empty(&kfence_freelist)) {
379 meta = list_entry(kfence_freelist.next, struct kfence_metadata, list);
380 list_del_init(&meta->list);
382 raw_spin_unlock_irqrestore(&kfence_freelist_lock, flags);
384 atomic_long_inc(&counters[KFENCE_COUNTER_SKIP_CAPACITY]);
388 if (unlikely(!raw_spin_trylock_irqsave(&meta->lock, flags))) {
390 * This is extremely unlikely -- we are reporting on a
391 * use-after-free, which locked meta->lock, and the reporting
392 * code via printk calls kmalloc() which ends up in
393 * kfence_alloc() and tries to grab the same object that we're
394 * reporting on. While it has never been observed, lockdep does
395 * report that there is a possibility of deadlock. Fix it by
396 * using trylock and bailing out gracefully.
398 raw_spin_lock_irqsave(&kfence_freelist_lock, flags);
399 /* Put the object back on the freelist. */
400 list_add_tail(&meta->list, &kfence_freelist);
401 raw_spin_unlock_irqrestore(&kfence_freelist_lock, flags);
406 meta->addr = metadata_to_pageaddr(meta);
407 /* Unprotect if we're reusing this page. */
408 if (meta->state == KFENCE_OBJECT_FREED)
409 kfence_unprotect(meta->addr);
412 * Note: for allocations made before RNG initialization, will always
413 * return zero. We still benefit from enabling KFENCE as early as
414 * possible, even when the RNG is not yet available, as this will allow
415 * KFENCE to detect bugs due to earlier allocations. The only downside
416 * is that the out-of-bounds accesses detected are deterministic for
419 if (prandom_u32_max(2)) {
420 /* Allocate on the "right" side, re-calculate address. */
421 meta->addr += PAGE_SIZE - size;
422 meta->addr = ALIGN_DOWN(meta->addr, cache->align);
425 addr = (void *)meta->addr;
427 /* Update remaining metadata. */
428 metadata_update_state(meta, KFENCE_OBJECT_ALLOCATED, stack_entries, num_stack_entries);
429 /* Pairs with READ_ONCE() in kfence_shutdown_cache(). */
430 WRITE_ONCE(meta->cache, cache);
432 meta->alloc_stack_hash = alloc_stack_hash;
433 raw_spin_unlock_irqrestore(&meta->lock, flags);
435 alloc_covered_add(alloc_stack_hash, 1);
437 /* Set required slab fields. */
438 slab = virt_to_slab((void *)meta->addr);
439 slab->slab_cache = cache;
440 #if defined(CONFIG_SLUB)
442 #elif defined(CONFIG_SLAB)
446 /* Memory initialization. */
447 for_each_canary(meta, set_canary_byte);
450 * We check slab_want_init_on_alloc() ourselves, rather than letting
451 * SL*B do the initialization, as otherwise we might overwrite KFENCE's
454 if (unlikely(slab_want_init_on_alloc(gfp, cache)))
455 memzero_explicit(addr, size);
459 if (CONFIG_KFENCE_STRESS_TEST_FAULTS && !prandom_u32_max(CONFIG_KFENCE_STRESS_TEST_FAULTS))
460 kfence_protect(meta->addr); /* Random "faults" by protecting the object. */
462 atomic_long_inc(&counters[KFENCE_COUNTER_ALLOCATED]);
463 atomic_long_inc(&counters[KFENCE_COUNTER_ALLOCS]);
468 static void kfence_guarded_free(void *addr, struct kfence_metadata *meta, bool zombie)
470 struct kcsan_scoped_access assert_page_exclusive;
474 raw_spin_lock_irqsave(&meta->lock, flags);
476 if (meta->state != KFENCE_OBJECT_ALLOCATED || meta->addr != (unsigned long)addr) {
477 /* Invalid or double-free, bail out. */
478 atomic_long_inc(&counters[KFENCE_COUNTER_BUGS]);
479 kfence_report_error((unsigned long)addr, false, NULL, meta,
480 KFENCE_ERROR_INVALID_FREE);
481 raw_spin_unlock_irqrestore(&meta->lock, flags);
485 /* Detect racy use-after-free, or incorrect reallocation of this page by KFENCE. */
486 kcsan_begin_scoped_access((void *)ALIGN_DOWN((unsigned long)addr, PAGE_SIZE), PAGE_SIZE,
487 KCSAN_ACCESS_SCOPED | KCSAN_ACCESS_WRITE | KCSAN_ACCESS_ASSERT,
488 &assert_page_exclusive);
490 if (CONFIG_KFENCE_STRESS_TEST_FAULTS)
491 kfence_unprotect((unsigned long)addr); /* To check canary bytes. */
493 /* Restore page protection if there was an OOB access. */
494 if (meta->unprotected_page) {
495 memzero_explicit((void *)ALIGN_DOWN(meta->unprotected_page, PAGE_SIZE), PAGE_SIZE);
496 kfence_protect(meta->unprotected_page);
497 meta->unprotected_page = 0;
500 /* Mark the object as freed. */
501 metadata_update_state(meta, KFENCE_OBJECT_FREED, NULL, 0);
502 init = slab_want_init_on_free(meta->cache);
503 raw_spin_unlock_irqrestore(&meta->lock, flags);
505 alloc_covered_add(meta->alloc_stack_hash, -1);
507 /* Check canary bytes for memory corruption. */
508 for_each_canary(meta, check_canary_byte);
511 * Clear memory if init-on-free is set. While we protect the page, the
512 * data is still there, and after a use-after-free is detected, we
513 * unprotect the page, so the data is still accessible.
515 if (!zombie && unlikely(init))
516 memzero_explicit(addr, meta->size);
518 /* Protect to detect use-after-frees. */
519 kfence_protect((unsigned long)addr);
521 kcsan_end_scoped_access(&assert_page_exclusive);
523 /* Add it to the tail of the freelist for reuse. */
524 raw_spin_lock_irqsave(&kfence_freelist_lock, flags);
525 KFENCE_WARN_ON(!list_empty(&meta->list));
526 list_add_tail(&meta->list, &kfence_freelist);
527 raw_spin_unlock_irqrestore(&kfence_freelist_lock, flags);
529 atomic_long_dec(&counters[KFENCE_COUNTER_ALLOCATED]);
530 atomic_long_inc(&counters[KFENCE_COUNTER_FREES]);
532 /* See kfence_shutdown_cache(). */
533 atomic_long_inc(&counters[KFENCE_COUNTER_ZOMBIES]);
537 static void rcu_guarded_free(struct rcu_head *h)
539 struct kfence_metadata *meta = container_of(h, struct kfence_metadata, rcu_head);
541 kfence_guarded_free((void *)meta->addr, meta, false);
545 * Initialization of the KFENCE pool after its allocation.
546 * Returns 0 on success; otherwise returns the address up to
547 * which partial initialization succeeded.
549 static unsigned long kfence_init_pool(void)
551 unsigned long addr = (unsigned long)__kfence_pool;
555 if (!arch_kfence_init_pool())
558 pages = virt_to_page(addr);
561 * Set up object pages: they must have PG_slab set, to avoid freeing
562 * these as real pages.
564 * We also want to avoid inserting kfence_free() in the kfree()
565 * fast-path in SLUB, and therefore need to ensure kfree() correctly
566 * enters __slab_free() slow-path.
568 for (i = 0; i < KFENCE_POOL_SIZE / PAGE_SIZE; i++) {
569 struct slab *slab = page_slab(&pages[i]);
574 /* Verify we do not have a compound head page. */
575 if (WARN_ON(compound_head(&pages[i]) != &pages[i]))
578 __folio_set_slab(slab_folio(slab));
580 slab->memcg_data = (unsigned long)&kfence_metadata[i / 2 - 1].objcg |
586 * Protect the first 2 pages. The first page is mostly unnecessary, and
587 * merely serves as an extended guard page. However, adding one
588 * additional page in the beginning gives us an even number of pages,
589 * which simplifies the mapping of address to metadata index.
591 for (i = 0; i < 2; i++) {
592 if (unlikely(!kfence_protect(addr)))
598 for (i = 0; i < CONFIG_KFENCE_NUM_OBJECTS; i++) {
599 struct kfence_metadata *meta = &kfence_metadata[i];
601 /* Initialize metadata. */
602 INIT_LIST_HEAD(&meta->list);
603 raw_spin_lock_init(&meta->lock);
604 meta->state = KFENCE_OBJECT_UNUSED;
605 meta->addr = addr; /* Initialize for validation in metadata_to_pageaddr(). */
606 list_add_tail(&meta->list, &kfence_freelist);
608 /* Protect the right redzone. */
609 if (unlikely(!kfence_protect(addr + PAGE_SIZE)))
612 addr += 2 * PAGE_SIZE;
616 * The pool is live and will never be deallocated from this point on.
617 * Remove the pool object from the kmemleak object tree, as it would
618 * otherwise overlap with allocations returned by kfence_alloc(), which
619 * are registered with kmemleak through the slab post-alloc hook.
621 kmemleak_free(__kfence_pool);
626 static bool __init kfence_init_pool_early(void)
633 addr = kfence_init_pool();
639 * Only release unprotected pages, and do not try to go back and change
640 * page attributes due to risk of failing to do so as well. If changing
641 * page attributes for some pages fails, it is very likely that it also
642 * fails for the first page, and therefore expect addr==__kfence_pool in
643 * most failure cases.
645 memblock_free_late(__pa(addr), KFENCE_POOL_SIZE - (addr - (unsigned long)__kfence_pool));
646 __kfence_pool = NULL;
650 static bool kfence_init_pool_late(void)
652 unsigned long addr, free_size;
654 addr = kfence_init_pool();
660 free_size = KFENCE_POOL_SIZE - (addr - (unsigned long)__kfence_pool);
661 #ifdef CONFIG_CONTIG_ALLOC
662 free_contig_range(page_to_pfn(virt_to_page(addr)), free_size / PAGE_SIZE);
664 free_pages_exact((void *)addr, free_size);
666 __kfence_pool = NULL;
670 /* === DebugFS Interface ==================================================== */
672 static int stats_show(struct seq_file *seq, void *v)
676 seq_printf(seq, "enabled: %i\n", READ_ONCE(kfence_enabled));
677 for (i = 0; i < KFENCE_COUNTER_COUNT; i++)
678 seq_printf(seq, "%s: %ld\n", counter_names[i], atomic_long_read(&counters[i]));
682 DEFINE_SHOW_ATTRIBUTE(stats);
685 * debugfs seq_file operations for /sys/kernel/debug/kfence/objects.
686 * start_object() and next_object() return the object index + 1, because NULL is used
689 static void *start_object(struct seq_file *seq, loff_t *pos)
691 if (*pos < CONFIG_KFENCE_NUM_OBJECTS)
692 return (void *)((long)*pos + 1);
696 static void stop_object(struct seq_file *seq, void *v)
700 static void *next_object(struct seq_file *seq, void *v, loff_t *pos)
703 if (*pos < CONFIG_KFENCE_NUM_OBJECTS)
704 return (void *)((long)*pos + 1);
708 static int show_object(struct seq_file *seq, void *v)
710 struct kfence_metadata *meta = &kfence_metadata[(long)v - 1];
713 raw_spin_lock_irqsave(&meta->lock, flags);
714 kfence_print_object(seq, meta);
715 raw_spin_unlock_irqrestore(&meta->lock, flags);
716 seq_puts(seq, "---------------------------------\n");
721 static const struct seq_operations object_seqops = {
722 .start = start_object,
728 static int open_objects(struct inode *inode, struct file *file)
730 return seq_open(file, &object_seqops);
733 static const struct file_operations objects_fops = {
734 .open = open_objects,
737 .release = seq_release,
740 static int __init kfence_debugfs_init(void)
742 struct dentry *kfence_dir = debugfs_create_dir("kfence", NULL);
744 debugfs_create_file("stats", 0444, kfence_dir, NULL, &stats_fops);
745 debugfs_create_file("objects", 0400, kfence_dir, NULL, &objects_fops);
749 late_initcall(kfence_debugfs_init);
751 /* === Allocation Gate Timer ================================================ */
753 static struct delayed_work kfence_timer;
755 #ifdef CONFIG_KFENCE_STATIC_KEYS
756 /* Wait queue to wake up allocation-gate timer task. */
757 static DECLARE_WAIT_QUEUE_HEAD(allocation_wait);
759 static void wake_up_kfence_timer(struct irq_work *work)
761 wake_up(&allocation_wait);
763 static DEFINE_IRQ_WORK(wake_up_kfence_timer_work, wake_up_kfence_timer);
767 * Set up delayed work, which will enable and disable the static key. We need to
768 * use a work queue (rather than a simple timer), since enabling and disabling a
769 * static key cannot be done from an interrupt.
771 * Note: Toggling a static branch currently causes IPIs, and here we'll end up
772 * with a total of 2 IPIs to all CPUs. If this ends up a problem in future (with
773 * more aggressive sampling intervals), we could get away with a variant that
774 * avoids IPIs, at the cost of not immediately capturing allocations if the
775 * instructions remain cached.
777 static void toggle_allocation_gate(struct work_struct *work)
779 if (!READ_ONCE(kfence_enabled))
782 atomic_set(&kfence_allocation_gate, 0);
783 #ifdef CONFIG_KFENCE_STATIC_KEYS
784 /* Enable static key, and await allocation to happen. */
785 static_branch_enable(&kfence_allocation_key);
787 if (sysctl_hung_task_timeout_secs) {
789 * During low activity with no allocations we might wait a
790 * while; let's avoid the hung task warning.
792 wait_event_idle_timeout(allocation_wait, atomic_read(&kfence_allocation_gate),
793 sysctl_hung_task_timeout_secs * HZ / 2);
795 wait_event_idle(allocation_wait, atomic_read(&kfence_allocation_gate));
798 /* Disable static key and reset timer. */
799 static_branch_disable(&kfence_allocation_key);
801 queue_delayed_work(system_unbound_wq, &kfence_timer,
802 msecs_to_jiffies(kfence_sample_interval));
805 /* === Public interface ===================================================== */
807 void __init kfence_alloc_pool(void)
809 if (!kfence_sample_interval)
812 __kfence_pool = memblock_alloc(KFENCE_POOL_SIZE, PAGE_SIZE);
815 pr_err("failed to allocate pool\n");
818 static void kfence_init_enable(void)
820 if (!IS_ENABLED(CONFIG_KFENCE_STATIC_KEYS))
821 static_branch_enable(&kfence_allocation_key);
823 if (kfence_deferrable)
824 INIT_DEFERRABLE_WORK(&kfence_timer, toggle_allocation_gate);
826 INIT_DELAYED_WORK(&kfence_timer, toggle_allocation_gate);
828 WRITE_ONCE(kfence_enabled, true);
829 queue_delayed_work(system_unbound_wq, &kfence_timer, 0);
831 pr_info("initialized - using %lu bytes for %d objects at 0x%p-0x%p\n", KFENCE_POOL_SIZE,
832 CONFIG_KFENCE_NUM_OBJECTS, (void *)__kfence_pool,
833 (void *)(__kfence_pool + KFENCE_POOL_SIZE));
836 void __init kfence_init(void)
838 stack_hash_seed = (u32)random_get_entropy();
840 /* Setting kfence_sample_interval to 0 on boot disables KFENCE. */
841 if (!kfence_sample_interval)
844 if (!kfence_init_pool_early()) {
845 pr_err("%s failed\n", __func__);
849 kfence_init_enable();
852 static int kfence_init_late(void)
854 const unsigned long nr_pages = KFENCE_POOL_SIZE / PAGE_SIZE;
855 #ifdef CONFIG_CONTIG_ALLOC
858 pages = alloc_contig_pages(nr_pages, GFP_KERNEL, first_online_node, NULL);
861 __kfence_pool = page_to_virt(pages);
863 if (nr_pages > MAX_ORDER_NR_PAGES) {
864 pr_warn("KFENCE_NUM_OBJECTS too large for buddy allocator\n");
867 __kfence_pool = alloc_pages_exact(KFENCE_POOL_SIZE, GFP_KERNEL);
872 if (!kfence_init_pool_late()) {
873 pr_err("%s failed\n", __func__);
877 kfence_init_enable();
881 static int kfence_enable_late(void)
884 return kfence_init_late();
886 WRITE_ONCE(kfence_enabled, true);
887 queue_delayed_work(system_unbound_wq, &kfence_timer, 0);
891 void kfence_shutdown_cache(struct kmem_cache *s)
894 struct kfence_metadata *meta;
897 for (i = 0; i < CONFIG_KFENCE_NUM_OBJECTS; i++) {
900 meta = &kfence_metadata[i];
903 * If we observe some inconsistent cache and state pair where we
904 * should have returned false here, cache destruction is racing
905 * with either kmem_cache_alloc() or kmem_cache_free(). Taking
906 * the lock will not help, as different critical section
907 * serialization will have the same outcome.
909 if (READ_ONCE(meta->cache) != s ||
910 READ_ONCE(meta->state) != KFENCE_OBJECT_ALLOCATED)
913 raw_spin_lock_irqsave(&meta->lock, flags);
914 in_use = meta->cache == s && meta->state == KFENCE_OBJECT_ALLOCATED;
915 raw_spin_unlock_irqrestore(&meta->lock, flags);
919 * This cache still has allocations, and we should not
920 * release them back into the freelist so they can still
921 * safely be used and retain the kernel's default
922 * behaviour of keeping the allocations alive (leak the
923 * cache); however, they effectively become "zombie
924 * allocations" as the KFENCE objects are the only ones
925 * still in use and the owning cache is being destroyed.
927 * We mark them freed, so that any subsequent use shows
928 * more useful error messages that will include stack
929 * traces of the user of the object, the original
930 * allocation, and caller to shutdown_cache().
932 kfence_guarded_free((void *)meta->addr, meta, /*zombie=*/true);
936 for (i = 0; i < CONFIG_KFENCE_NUM_OBJECTS; i++) {
937 meta = &kfence_metadata[i];
940 if (READ_ONCE(meta->cache) != s || READ_ONCE(meta->state) != KFENCE_OBJECT_FREED)
943 raw_spin_lock_irqsave(&meta->lock, flags);
944 if (meta->cache == s && meta->state == KFENCE_OBJECT_FREED)
946 raw_spin_unlock_irqrestore(&meta->lock, flags);
950 void *__kfence_alloc(struct kmem_cache *s, size_t size, gfp_t flags)
952 unsigned long stack_entries[KFENCE_STACK_DEPTH];
953 size_t num_stack_entries;
954 u32 alloc_stack_hash;
957 * Perform size check before switching kfence_allocation_gate, so that
958 * we don't disable KFENCE without making an allocation.
960 if (size > PAGE_SIZE) {
961 atomic_long_inc(&counters[KFENCE_COUNTER_SKIP_INCOMPAT]);
966 * Skip allocations from non-default zones, including DMA. We cannot
967 * guarantee that pages in the KFENCE pool will have the requested
968 * properties (e.g. reside in DMAable memory).
970 if ((flags & GFP_ZONEMASK) ||
971 (s->flags & (SLAB_CACHE_DMA | SLAB_CACHE_DMA32))) {
972 atomic_long_inc(&counters[KFENCE_COUNTER_SKIP_INCOMPAT]);
976 if (atomic_inc_return(&kfence_allocation_gate) > 1)
978 #ifdef CONFIG_KFENCE_STATIC_KEYS
980 * waitqueue_active() is fully ordered after the update of
981 * kfence_allocation_gate per atomic_inc_return().
983 if (waitqueue_active(&allocation_wait)) {
985 * Calling wake_up() here may deadlock when allocations happen
986 * from within timer code. Use an irq_work to defer it.
988 irq_work_queue(&wake_up_kfence_timer_work);
992 if (!READ_ONCE(kfence_enabled))
995 num_stack_entries = stack_trace_save(stack_entries, KFENCE_STACK_DEPTH, 0);
998 * Do expensive check for coverage of allocation in slow-path after
999 * allocation_gate has already become non-zero, even though it might
1000 * mean not making any allocation within a given sample interval.
1002 * This ensures reasonable allocation coverage when the pool is almost
1003 * full, including avoiding long-lived allocations of the same source
1004 * filling up the pool (e.g. pagecache allocations).
1006 alloc_stack_hash = get_alloc_stack_hash(stack_entries, num_stack_entries);
1007 if (should_skip_covered() && alloc_covered_contains(alloc_stack_hash)) {
1008 atomic_long_inc(&counters[KFENCE_COUNTER_SKIP_COVERED]);
1012 return kfence_guarded_alloc(s, size, flags, stack_entries, num_stack_entries,
1016 size_t kfence_ksize(const void *addr)
1018 const struct kfence_metadata *meta = addr_to_metadata((unsigned long)addr);
1021 * Read locklessly -- if there is a race with __kfence_alloc(), this is
1022 * either a use-after-free or invalid access.
1024 return meta ? meta->size : 0;
1027 void *kfence_object_start(const void *addr)
1029 const struct kfence_metadata *meta = addr_to_metadata((unsigned long)addr);
1032 * Read locklessly -- if there is a race with __kfence_alloc(), this is
1033 * either a use-after-free or invalid access.
1035 return meta ? (void *)meta->addr : NULL;
1038 void __kfence_free(void *addr)
1040 struct kfence_metadata *meta = addr_to_metadata((unsigned long)addr);
1043 KFENCE_WARN_ON(meta->objcg);
1046 * If the objects of the cache are SLAB_TYPESAFE_BY_RCU, defer freeing
1047 * the object, as the object page may be recycled for other-typed
1048 * objects once it has been freed. meta->cache may be NULL if the cache
1051 if (unlikely(meta->cache && (meta->cache->flags & SLAB_TYPESAFE_BY_RCU)))
1052 call_rcu(&meta->rcu_head, rcu_guarded_free);
1054 kfence_guarded_free(addr, meta, false);
1057 bool kfence_handle_page_fault(unsigned long addr, bool is_write, struct pt_regs *regs)
1059 const int page_index = (addr - (unsigned long)__kfence_pool) / PAGE_SIZE;
1060 struct kfence_metadata *to_report = NULL;
1061 enum kfence_error_type error_type;
1062 unsigned long flags;
1064 if (!is_kfence_address((void *)addr))
1067 if (!READ_ONCE(kfence_enabled)) /* If disabled at runtime ... */
1068 return kfence_unprotect(addr); /* ... unprotect and proceed. */
1070 atomic_long_inc(&counters[KFENCE_COUNTER_BUGS]);
1072 if (page_index % 2) {
1073 /* This is a redzone, report a buffer overflow. */
1074 struct kfence_metadata *meta;
1077 meta = addr_to_metadata(addr - PAGE_SIZE);
1078 if (meta && READ_ONCE(meta->state) == KFENCE_OBJECT_ALLOCATED) {
1080 /* Data race ok; distance calculation approximate. */
1081 distance = addr - data_race(meta->addr + meta->size);
1084 meta = addr_to_metadata(addr + PAGE_SIZE);
1085 if (meta && READ_ONCE(meta->state) == KFENCE_OBJECT_ALLOCATED) {
1086 /* Data race ok; distance calculation approximate. */
1087 if (!to_report || distance > data_race(meta->addr) - addr)
1094 raw_spin_lock_irqsave(&to_report->lock, flags);
1095 to_report->unprotected_page = addr;
1096 error_type = KFENCE_ERROR_OOB;
1099 * If the object was freed before we took the look we can still
1100 * report this as an OOB -- the report will simply show the
1101 * stacktrace of the free as well.
1104 to_report = addr_to_metadata(addr);
1108 raw_spin_lock_irqsave(&to_report->lock, flags);
1109 error_type = KFENCE_ERROR_UAF;
1111 * We may race with __kfence_alloc(), and it is possible that a
1112 * freed object may be reallocated. We simply report this as a
1113 * use-after-free, with the stack trace showing the place where
1114 * the object was re-allocated.
1120 kfence_report_error(addr, is_write, regs, to_report, error_type);
1121 raw_spin_unlock_irqrestore(&to_report->lock, flags);
1123 /* This may be a UAF or OOB access, but we can't be sure. */
1124 kfence_report_error(addr, is_write, regs, NULL, KFENCE_ERROR_INVALID);
1127 return kfence_unprotect(addr); /* Unprotect and let access proceed. */