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1// SPDX-License-Identifier: GPL-2.0
2/*
3 * KFENCE guarded object allocator and fault handling.
4 *
5 * Copyright (C) 2020, Google LLC.
6 */
7
8#define pr_fmt(fmt) "kfence: " fmt
9
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/notifier.h>
25#include <linux/panic_notifier.h>
26#include <linux/random.h>
27#include <linux/rcupdate.h>
28#include <linux/sched/clock.h>
29#include <linux/seq_file.h>
30#include <linux/slab.h>
31#include <linux/spinlock.h>
32#include <linux/string.h>
33
34#include <asm/kfence.h>
35
36#include "kfence.h"
37
38/* Disables KFENCE on the first warning assuming an irrecoverable error. */
39#define KFENCE_WARN_ON(cond) \
40 ({ \
41 const bool __cond = WARN_ON(cond); \
42 if (unlikely(__cond)) { \
43 WRITE_ONCE(kfence_enabled, false); \
44 disabled_by_warn = true; \
45 } \
46 __cond; \
47 })
48
49/* === Data ================================================================= */
50
51static bool kfence_enabled __read_mostly;
52static bool disabled_by_warn __read_mostly;
53
54unsigned long kfence_sample_interval __read_mostly = CONFIG_KFENCE_SAMPLE_INTERVAL;
55EXPORT_SYMBOL_GPL(kfence_sample_interval); /* Export for test modules. */
56
57#ifdef MODULE_PARAM_PREFIX
58#undef MODULE_PARAM_PREFIX
59#endif
60#define MODULE_PARAM_PREFIX "kfence."
61
62static int kfence_enable_late(void);
63static int param_set_sample_interval(const char *val, const struct kernel_param *kp)
64{
65 unsigned long num;
66 int ret = kstrtoul(val, 0, &num);
67
68 if (ret < 0)
69 return ret;
70
71 /* Using 0 to indicate KFENCE is disabled. */
72 if (!num && READ_ONCE(kfence_enabled)) {
73 pr_info("disabled\n");
74 WRITE_ONCE(kfence_enabled, false);
75 }
76
77 *((unsigned long *)kp->arg) = num;
78
79 if (num && !READ_ONCE(kfence_enabled) && system_state != SYSTEM_BOOTING)
80 return disabled_by_warn ? -EINVAL : kfence_enable_late();
81 return 0;
82}
83
84static int param_get_sample_interval(char *buffer, const struct kernel_param *kp)
85{
86 if (!READ_ONCE(kfence_enabled))
87 return sprintf(buffer, "0\n");
88
89 return param_get_ulong(buffer, kp);
90}
91
92static const struct kernel_param_ops sample_interval_param_ops = {
93 .set = param_set_sample_interval,
94 .get = param_get_sample_interval,
95};
96module_param_cb(sample_interval, &sample_interval_param_ops, &kfence_sample_interval, 0600);
97
98/* Pool usage% threshold when currently covered allocations are skipped. */
99static unsigned long kfence_skip_covered_thresh __read_mostly = 75;
100module_param_named(skip_covered_thresh, kfence_skip_covered_thresh, ulong, 0644);
101
102/* If true, use a deferrable timer. */
103static bool kfence_deferrable __read_mostly = IS_ENABLED(CONFIG_KFENCE_DEFERRABLE);
104module_param_named(deferrable, kfence_deferrable, bool, 0444);
105
106/* If true, check all canary bytes on panic. */
107static bool kfence_check_on_panic __read_mostly;
108module_param_named(check_on_panic, kfence_check_on_panic, bool, 0444);
109
110/* The pool of pages used for guard pages and objects. */
111char *__kfence_pool __read_mostly;
112EXPORT_SYMBOL(__kfence_pool); /* Export for test modules. */
113
114/*
115 * Per-object metadata, with one-to-one mapping of object metadata to
116 * backing pages (in __kfence_pool).
117 */
118static_assert(CONFIG_KFENCE_NUM_OBJECTS > 0);
119struct kfence_metadata kfence_metadata[CONFIG_KFENCE_NUM_OBJECTS];
120
121/* Freelist with available objects. */
122static struct list_head kfence_freelist = LIST_HEAD_INIT(kfence_freelist);
123static DEFINE_RAW_SPINLOCK(kfence_freelist_lock); /* Lock protecting freelist. */
124
125/*
126 * The static key to set up a KFENCE allocation; or if static keys are not used
127 * to gate allocations, to avoid a load and compare if KFENCE is disabled.
128 */
129DEFINE_STATIC_KEY_FALSE(kfence_allocation_key);
130
131/* Gates the allocation, ensuring only one succeeds in a given period. */
132atomic_t kfence_allocation_gate = ATOMIC_INIT(1);
133
134/*
135 * A Counting Bloom filter of allocation coverage: limits currently covered
136 * allocations of the same source filling up the pool.
137 *
138 * Assuming a range of 15%-85% unique allocations in the pool at any point in
139 * time, the below parameters provide a probablity of 0.02-0.33 for false
140 * positive hits respectively:
141 *
142 * P(alloc_traces) = (1 - e^(-HNUM * (alloc_traces / SIZE)) ^ HNUM
143 */
144#define ALLOC_COVERED_HNUM 2
145#define ALLOC_COVERED_ORDER (const_ilog2(CONFIG_KFENCE_NUM_OBJECTS) + 2)
146#define ALLOC_COVERED_SIZE (1 << ALLOC_COVERED_ORDER)
147#define ALLOC_COVERED_HNEXT(h) hash_32(h, ALLOC_COVERED_ORDER)
148#define ALLOC_COVERED_MASK (ALLOC_COVERED_SIZE - 1)
149static atomic_t alloc_covered[ALLOC_COVERED_SIZE];
150
151/* Stack depth used to determine uniqueness of an allocation. */
152#define UNIQUE_ALLOC_STACK_DEPTH ((size_t)8)
153
154/*
155 * Randomness for stack hashes, making the same collisions across reboots and
156 * different machines less likely.
157 */
158static u32 stack_hash_seed __ro_after_init;
159
160/* Statistics counters for debugfs. */
161enum kfence_counter_id {
162 KFENCE_COUNTER_ALLOCATED,
163 KFENCE_COUNTER_ALLOCS,
164 KFENCE_COUNTER_FREES,
165 KFENCE_COUNTER_ZOMBIES,
166 KFENCE_COUNTER_BUGS,
167 KFENCE_COUNTER_SKIP_INCOMPAT,
168 KFENCE_COUNTER_SKIP_CAPACITY,
169 KFENCE_COUNTER_SKIP_COVERED,
170 KFENCE_COUNTER_COUNT,
171};
172static atomic_long_t counters[KFENCE_COUNTER_COUNT];
173static const char *const counter_names[] = {
174 [KFENCE_COUNTER_ALLOCATED] = "currently allocated",
175 [KFENCE_COUNTER_ALLOCS] = "total allocations",
176 [KFENCE_COUNTER_FREES] = "total frees",
177 [KFENCE_COUNTER_ZOMBIES] = "zombie allocations",
178 [KFENCE_COUNTER_BUGS] = "total bugs",
179 [KFENCE_COUNTER_SKIP_INCOMPAT] = "skipped allocations (incompatible)",
180 [KFENCE_COUNTER_SKIP_CAPACITY] = "skipped allocations (capacity)",
181 [KFENCE_COUNTER_SKIP_COVERED] = "skipped allocations (covered)",
182};
183static_assert(ARRAY_SIZE(counter_names) == KFENCE_COUNTER_COUNT);
184
185/* === Internals ============================================================ */
186
187static inline bool should_skip_covered(void)
188{
189 unsigned long thresh = (CONFIG_KFENCE_NUM_OBJECTS * kfence_skip_covered_thresh) / 100;
190
191 return atomic_long_read(&counters[KFENCE_COUNTER_ALLOCATED]) > thresh;
192}
193
194static u32 get_alloc_stack_hash(unsigned long *stack_entries, size_t num_entries)
195{
196 num_entries = min(num_entries, UNIQUE_ALLOC_STACK_DEPTH);
197 num_entries = filter_irq_stacks(stack_entries, num_entries);
198 return jhash(stack_entries, num_entries * sizeof(stack_entries[0]), stack_hash_seed);
199}
200
201/*
202 * Adds (or subtracts) count @val for allocation stack trace hash
203 * @alloc_stack_hash from Counting Bloom filter.
204 */
205static void alloc_covered_add(u32 alloc_stack_hash, int val)
206{
207 int i;
208
209 for (i = 0; i < ALLOC_COVERED_HNUM; i++) {
210 atomic_add(val, &alloc_covered[alloc_stack_hash & ALLOC_COVERED_MASK]);
211 alloc_stack_hash = ALLOC_COVERED_HNEXT(alloc_stack_hash);
212 }
213}
214
215/*
216 * Returns true if the allocation stack trace hash @alloc_stack_hash is
217 * currently contained (non-zero count) in Counting Bloom filter.
218 */
219static bool alloc_covered_contains(u32 alloc_stack_hash)
220{
221 int i;
222
223 for (i = 0; i < ALLOC_COVERED_HNUM; i++) {
224 if (!atomic_read(&alloc_covered[alloc_stack_hash & ALLOC_COVERED_MASK]))
225 return false;
226 alloc_stack_hash = ALLOC_COVERED_HNEXT(alloc_stack_hash);
227 }
228
229 return true;
230}
231
232static bool kfence_protect(unsigned long addr)
233{
234 return !KFENCE_WARN_ON(!kfence_protect_page(ALIGN_DOWN(addr, PAGE_SIZE), true));
235}
236
237static bool kfence_unprotect(unsigned long addr)
238{
239 return !KFENCE_WARN_ON(!kfence_protect_page(ALIGN_DOWN(addr, PAGE_SIZE), false));
240}
241
242static inline unsigned long metadata_to_pageaddr(const struct kfence_metadata *meta)
243{
244 unsigned long offset = (meta - kfence_metadata + 1) * PAGE_SIZE * 2;
245 unsigned long pageaddr = (unsigned long)&__kfence_pool[offset];
246
247 /* The checks do not affect performance; only called from slow-paths. */
248
249 /* Only call with a pointer into kfence_metadata. */
250 if (KFENCE_WARN_ON(meta < kfence_metadata ||
251 meta >= kfence_metadata + CONFIG_KFENCE_NUM_OBJECTS))
252 return 0;
253
254 /*
255 * This metadata object only ever maps to 1 page; verify that the stored
256 * address is in the expected range.
257 */
258 if (KFENCE_WARN_ON(ALIGN_DOWN(meta->addr, PAGE_SIZE) != pageaddr))
259 return 0;
260
261 return pageaddr;
262}
263
264/*
265 * Update the object's metadata state, including updating the alloc/free stacks
266 * depending on the state transition.
267 */
268static noinline void
269metadata_update_state(struct kfence_metadata *meta, enum kfence_object_state next,
270 unsigned long *stack_entries, size_t num_stack_entries)
271{
272 struct kfence_track *track =
273 next == KFENCE_OBJECT_FREED ? &meta->free_track : &meta->alloc_track;
274
275 lockdep_assert_held(&meta->lock);
276
277 if (stack_entries) {
278 memcpy(track->stack_entries, stack_entries,
279 num_stack_entries * sizeof(stack_entries[0]));
280 } else {
281 /*
282 * Skip over 1 (this) functions; noinline ensures we do not
283 * accidentally skip over the caller by never inlining.
284 */
285 num_stack_entries = stack_trace_save(track->stack_entries, KFENCE_STACK_DEPTH, 1);
286 }
287 track->num_stack_entries = num_stack_entries;
288 track->pid = task_pid_nr(current);
289 track->cpu = raw_smp_processor_id();
290 track->ts_nsec = local_clock(); /* Same source as printk timestamps. */
291
292 /*
293 * Pairs with READ_ONCE() in
294 * kfence_shutdown_cache(),
295 * kfence_handle_page_fault().
296 */
297 WRITE_ONCE(meta->state, next);
298}
299
300/* Write canary byte to @addr. */
301static inline bool set_canary_byte(u8 *addr)
302{
303 *addr = KFENCE_CANARY_PATTERN(addr);
304 return true;
305}
306
307/* Check canary byte at @addr. */
308static inline bool check_canary_byte(u8 *addr)
309{
310 struct kfence_metadata *meta;
311 unsigned long flags;
312
313 if (likely(*addr == KFENCE_CANARY_PATTERN(addr)))
314 return true;
315
316 atomic_long_inc(&counters[KFENCE_COUNTER_BUGS]);
317
318 meta = addr_to_metadata((unsigned long)addr);
319 raw_spin_lock_irqsave(&meta->lock, flags);
320 kfence_report_error((unsigned long)addr, false, NULL, meta, KFENCE_ERROR_CORRUPTION);
321 raw_spin_unlock_irqrestore(&meta->lock, flags);
322
323 return false;
324}
325
326/* __always_inline this to ensure we won't do an indirect call to fn. */
327static __always_inline void for_each_canary(const struct kfence_metadata *meta, bool (*fn)(u8 *))
328{
329 const unsigned long pageaddr = ALIGN_DOWN(meta->addr, PAGE_SIZE);
330 unsigned long addr;
331
332 /*
333 * We'll iterate over each canary byte per-side until fn() returns
334 * false. However, we'll still iterate over the canary bytes to the
335 * right of the object even if there was an error in the canary bytes to
336 * the left of the object. Specifically, if check_canary_byte()
337 * generates an error, showing both sides might give more clues as to
338 * what the error is about when displaying which bytes were corrupted.
339 */
340
341 /* Apply to left of object. */
342 for (addr = pageaddr; addr < meta->addr; addr++) {
343 if (!fn((u8 *)addr))
344 break;
345 }
346
347 /* Apply to right of object. */
348 for (addr = meta->addr + meta->size; addr < pageaddr + PAGE_SIZE; addr++) {
349 if (!fn((u8 *)addr))
350 break;
351 }
352}
353
354static void *kfence_guarded_alloc(struct kmem_cache *cache, size_t size, gfp_t gfp,
355 unsigned long *stack_entries, size_t num_stack_entries,
356 u32 alloc_stack_hash)
357{
358 struct kfence_metadata *meta = NULL;
359 unsigned long flags;
360 struct slab *slab;
361 void *addr;
362 const bool random_right_allocate = get_random_u32_below(2);
363 const bool random_fault = CONFIG_KFENCE_STRESS_TEST_FAULTS &&
364 !get_random_u32_below(CONFIG_KFENCE_STRESS_TEST_FAULTS);
365
366 /* Try to obtain a free object. */
367 raw_spin_lock_irqsave(&kfence_freelist_lock, flags);
368 if (!list_empty(&kfence_freelist)) {
369 meta = list_entry(kfence_freelist.next, struct kfence_metadata, list);
370 list_del_init(&meta->list);
371 }
372 raw_spin_unlock_irqrestore(&kfence_freelist_lock, flags);
373 if (!meta) {
374 atomic_long_inc(&counters[KFENCE_COUNTER_SKIP_CAPACITY]);
375 return NULL;
376 }
377
378 if (unlikely(!raw_spin_trylock_irqsave(&meta->lock, flags))) {
379 /*
380 * This is extremely unlikely -- we are reporting on a
381 * use-after-free, which locked meta->lock, and the reporting
382 * code via printk calls kmalloc() which ends up in
383 * kfence_alloc() and tries to grab the same object that we're
384 * reporting on. While it has never been observed, lockdep does
385 * report that there is a possibility of deadlock. Fix it by
386 * using trylock and bailing out gracefully.
387 */
388 raw_spin_lock_irqsave(&kfence_freelist_lock, flags);
389 /* Put the object back on the freelist. */
390 list_add_tail(&meta->list, &kfence_freelist);
391 raw_spin_unlock_irqrestore(&kfence_freelist_lock, flags);
392
393 return NULL;
394 }
395
396 meta->addr = metadata_to_pageaddr(meta);
397 /* Unprotect if we're reusing this page. */
398 if (meta->state == KFENCE_OBJECT_FREED)
399 kfence_unprotect(meta->addr);
400
401 /*
402 * Note: for allocations made before RNG initialization, will always
403 * return zero. We still benefit from enabling KFENCE as early as
404 * possible, even when the RNG is not yet available, as this will allow
405 * KFENCE to detect bugs due to earlier allocations. The only downside
406 * is that the out-of-bounds accesses detected are deterministic for
407 * such allocations.
408 */
409 if (random_right_allocate) {
410 /* Allocate on the "right" side, re-calculate address. */
411 meta->addr += PAGE_SIZE - size;
412 meta->addr = ALIGN_DOWN(meta->addr, cache->align);
413 }
414
415 addr = (void *)meta->addr;
416
417 /* Update remaining metadata. */
418 metadata_update_state(meta, KFENCE_OBJECT_ALLOCATED, stack_entries, num_stack_entries);
419 /* Pairs with READ_ONCE() in kfence_shutdown_cache(). */
420 WRITE_ONCE(meta->cache, cache);
421 meta->size = size;
422 meta->alloc_stack_hash = alloc_stack_hash;
423 raw_spin_unlock_irqrestore(&meta->lock, flags);
424
425 alloc_covered_add(alloc_stack_hash, 1);
426
427 /* Set required slab fields. */
428 slab = virt_to_slab((void *)meta->addr);
429 slab->slab_cache = cache;
430#if defined(CONFIG_SLUB)
431 slab->objects = 1;
432#elif defined(CONFIG_SLAB)
433 slab->s_mem = addr;
434#endif
435
436 /* Memory initialization. */
437 for_each_canary(meta, set_canary_byte);
438
439 /*
440 * We check slab_want_init_on_alloc() ourselves, rather than letting
441 * SL*B do the initialization, as otherwise we might overwrite KFENCE's
442 * redzone.
443 */
444 if (unlikely(slab_want_init_on_alloc(gfp, cache)))
445 memzero_explicit(addr, size);
446 if (cache->ctor)
447 cache->ctor(addr);
448
449 if (random_fault)
450 kfence_protect(meta->addr); /* Random "faults" by protecting the object. */
451
452 atomic_long_inc(&counters[KFENCE_COUNTER_ALLOCATED]);
453 atomic_long_inc(&counters[KFENCE_COUNTER_ALLOCS]);
454
455 return addr;
456}
457
458static void kfence_guarded_free(void *addr, struct kfence_metadata *meta, bool zombie)
459{
460 struct kcsan_scoped_access assert_page_exclusive;
461 unsigned long flags;
462 bool init;
463
464 raw_spin_lock_irqsave(&meta->lock, flags);
465
466 if (meta->state != KFENCE_OBJECT_ALLOCATED || meta->addr != (unsigned long)addr) {
467 /* Invalid or double-free, bail out. */
468 atomic_long_inc(&counters[KFENCE_COUNTER_BUGS]);
469 kfence_report_error((unsigned long)addr, false, NULL, meta,
470 KFENCE_ERROR_INVALID_FREE);
471 raw_spin_unlock_irqrestore(&meta->lock, flags);
472 return;
473 }
474
475 /* Detect racy use-after-free, or incorrect reallocation of this page by KFENCE. */
476 kcsan_begin_scoped_access((void *)ALIGN_DOWN((unsigned long)addr, PAGE_SIZE), PAGE_SIZE,
477 KCSAN_ACCESS_SCOPED | KCSAN_ACCESS_WRITE | KCSAN_ACCESS_ASSERT,
478 &assert_page_exclusive);
479
480 if (CONFIG_KFENCE_STRESS_TEST_FAULTS)
481 kfence_unprotect((unsigned long)addr); /* To check canary bytes. */
482
483 /* Restore page protection if there was an OOB access. */
484 if (meta->unprotected_page) {
485 memzero_explicit((void *)ALIGN_DOWN(meta->unprotected_page, PAGE_SIZE), PAGE_SIZE);
486 kfence_protect(meta->unprotected_page);
487 meta->unprotected_page = 0;
488 }
489
490 /* Mark the object as freed. */
491 metadata_update_state(meta, KFENCE_OBJECT_FREED, NULL, 0);
492 init = slab_want_init_on_free(meta->cache);
493 raw_spin_unlock_irqrestore(&meta->lock, flags);
494
495 alloc_covered_add(meta->alloc_stack_hash, -1);
496
497 /* Check canary bytes for memory corruption. */
498 for_each_canary(meta, check_canary_byte);
499
500 /*
501 * Clear memory if init-on-free is set. While we protect the page, the
502 * data is still there, and after a use-after-free is detected, we
503 * unprotect the page, so the data is still accessible.
504 */
505 if (!zombie && unlikely(init))
506 memzero_explicit(addr, meta->size);
507
508 /* Protect to detect use-after-frees. */
509 kfence_protect((unsigned long)addr);
510
511 kcsan_end_scoped_access(&assert_page_exclusive);
512 if (!zombie) {
513 /* Add it to the tail of the freelist for reuse. */
514 raw_spin_lock_irqsave(&kfence_freelist_lock, flags);
515 KFENCE_WARN_ON(!list_empty(&meta->list));
516 list_add_tail(&meta->list, &kfence_freelist);
517 raw_spin_unlock_irqrestore(&kfence_freelist_lock, flags);
518
519 atomic_long_dec(&counters[KFENCE_COUNTER_ALLOCATED]);
520 atomic_long_inc(&counters[KFENCE_COUNTER_FREES]);
521 } else {
522 /* See kfence_shutdown_cache(). */
523 atomic_long_inc(&counters[KFENCE_COUNTER_ZOMBIES]);
524 }
525}
526
527static void rcu_guarded_free(struct rcu_head *h)
528{
529 struct kfence_metadata *meta = container_of(h, struct kfence_metadata, rcu_head);
530
531 kfence_guarded_free((void *)meta->addr, meta, false);
532}
533
534/*
535 * Initialization of the KFENCE pool after its allocation.
536 * Returns 0 on success; otherwise returns the address up to
537 * which partial initialization succeeded.
538 */
539static unsigned long kfence_init_pool(void)
540{
541 unsigned long addr = (unsigned long)__kfence_pool;
542 struct page *pages;
543 int i;
544
545 if (!arch_kfence_init_pool())
546 return addr;
547
548 pages = virt_to_page(__kfence_pool);
549
550 /*
551 * Set up object pages: they must have PG_slab set, to avoid freeing
552 * these as real pages.
553 *
554 * We also want to avoid inserting kfence_free() in the kfree()
555 * fast-path in SLUB, and therefore need to ensure kfree() correctly
556 * enters __slab_free() slow-path.
557 */
558 for (i = 0; i < KFENCE_POOL_SIZE / PAGE_SIZE; i++) {
559 struct slab *slab = page_slab(&pages[i]);
560
561 if (!i || (i % 2))
562 continue;
563
564 /* Verify we do not have a compound head page. */
565 if (WARN_ON(compound_head(&pages[i]) != &pages[i]))
566 return addr;
567
568 __folio_set_slab(slab_folio(slab));
569#ifdef CONFIG_MEMCG
570 slab->memcg_data = (unsigned long)&kfence_metadata[i / 2 - 1].objcg |
571 MEMCG_DATA_OBJCGS;
572#endif
573 }
574
575 /*
576 * Protect the first 2 pages. The first page is mostly unnecessary, and
577 * merely serves as an extended guard page. However, adding one
578 * additional page in the beginning gives us an even number of pages,
579 * which simplifies the mapping of address to metadata index.
580 */
581 for (i = 0; i < 2; i++) {
582 if (unlikely(!kfence_protect(addr)))
583 return addr;
584
585 addr += PAGE_SIZE;
586 }
587
588 for (i = 0; i < CONFIG_KFENCE_NUM_OBJECTS; i++) {
589 struct kfence_metadata *meta = &kfence_metadata[i];
590
591 /* Initialize metadata. */
592 INIT_LIST_HEAD(&meta->list);
593 raw_spin_lock_init(&meta->lock);
594 meta->state = KFENCE_OBJECT_UNUSED;
595 meta->addr = addr; /* Initialize for validation in metadata_to_pageaddr(). */
596 list_add_tail(&meta->list, &kfence_freelist);
597
598 /* Protect the right redzone. */
599 if (unlikely(!kfence_protect(addr + PAGE_SIZE)))
600 return addr;
601
602 addr += 2 * PAGE_SIZE;
603 }
604
605 return 0;
606}
607
608static bool __init kfence_init_pool_early(void)
609{
610 unsigned long addr;
611
612 if (!__kfence_pool)
613 return false;
614
615 addr = kfence_init_pool();
616
617 if (!addr) {
618 /*
619 * The pool is live and will never be deallocated from this point on.
620 * Ignore the pool object from the kmemleak phys object tree, as it would
621 * otherwise overlap with allocations returned by kfence_alloc(), which
622 * are registered with kmemleak through the slab post-alloc hook.
623 */
624 kmemleak_ignore_phys(__pa(__kfence_pool));
625 return true;
626 }
627
628 /*
629 * Only release unprotected pages, and do not try to go back and change
630 * page attributes due to risk of failing to do so as well. If changing
631 * page attributes for some pages fails, it is very likely that it also
632 * fails for the first page, and therefore expect addr==__kfence_pool in
633 * most failure cases.
634 */
635 for (char *p = (char *)addr; p < __kfence_pool + KFENCE_POOL_SIZE; p += PAGE_SIZE) {
636 struct slab *slab = virt_to_slab(p);
637
638 if (!slab)
639 continue;
640#ifdef CONFIG_MEMCG
641 slab->memcg_data = 0;
642#endif
643 __folio_clear_slab(slab_folio(slab));
644 }
645 memblock_free_late(__pa(addr), KFENCE_POOL_SIZE - (addr - (unsigned long)__kfence_pool));
646 __kfence_pool = NULL;
647 return false;
648}
649
650static bool kfence_init_pool_late(void)
651{
652 unsigned long addr, free_size;
653
654 addr = kfence_init_pool();
655
656 if (!addr)
657 return true;
658
659 /* Same as above. */
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((void *)addr)), free_size / PAGE_SIZE);
663#else
664 free_pages_exact((void *)addr, free_size);
665#endif
666 __kfence_pool = NULL;
667 return false;
668}
669
670/* === DebugFS Interface ==================================================== */
671
672static int stats_show(struct seq_file *seq, void *v)
673{
674 int i;
675
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]));
679
680 return 0;
681}
682DEFINE_SHOW_ATTRIBUTE(stats);
683
684/*
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
687 * to stop iteration.
688 */
689static void *start_object(struct seq_file *seq, loff_t *pos)
690{
691 if (*pos < CONFIG_KFENCE_NUM_OBJECTS)
692 return (void *)((long)*pos + 1);
693 return NULL;
694}
695
696static void stop_object(struct seq_file *seq, void *v)
697{
698}
699
700static void *next_object(struct seq_file *seq, void *v, loff_t *pos)
701{
702 ++*pos;
703 if (*pos < CONFIG_KFENCE_NUM_OBJECTS)
704 return (void *)((long)*pos + 1);
705 return NULL;
706}
707
708static int show_object(struct seq_file *seq, void *v)
709{
710 struct kfence_metadata *meta = &kfence_metadata[(long)v - 1];
711 unsigned long flags;
712
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");
717
718 return 0;
719}
720
721static const struct seq_operations objects_sops = {
722 .start = start_object,
723 .next = next_object,
724 .stop = stop_object,
725 .show = show_object,
726};
727DEFINE_SEQ_ATTRIBUTE(objects);
728
729static int __init kfence_debugfs_init(void)
730{
731 struct dentry *kfence_dir = debugfs_create_dir("kfence", NULL);
732
733 debugfs_create_file("stats", 0444, kfence_dir, NULL, &stats_fops);
734 debugfs_create_file("objects", 0400, kfence_dir, NULL, &objects_fops);
735 return 0;
736}
737
738late_initcall(kfence_debugfs_init);
739
740/* === Panic Notifier ====================================================== */
741
742static void kfence_check_all_canary(void)
743{
744 int i;
745
746 for (i = 0; i < CONFIG_KFENCE_NUM_OBJECTS; i++) {
747 struct kfence_metadata *meta = &kfence_metadata[i];
748
749 if (meta->state == KFENCE_OBJECT_ALLOCATED)
750 for_each_canary(meta, check_canary_byte);
751 }
752}
753
754static int kfence_check_canary_callback(struct notifier_block *nb,
755 unsigned long reason, void *arg)
756{
757 kfence_check_all_canary();
758 return NOTIFY_OK;
759}
760
761static struct notifier_block kfence_check_canary_notifier = {
762 .notifier_call = kfence_check_canary_callback,
763};
764
765/* === Allocation Gate Timer ================================================ */
766
767static struct delayed_work kfence_timer;
768
769#ifdef CONFIG_KFENCE_STATIC_KEYS
770/* Wait queue to wake up allocation-gate timer task. */
771static DECLARE_WAIT_QUEUE_HEAD(allocation_wait);
772
773static void wake_up_kfence_timer(struct irq_work *work)
774{
775 wake_up(&allocation_wait);
776}
777static DEFINE_IRQ_WORK(wake_up_kfence_timer_work, wake_up_kfence_timer);
778#endif
779
780/*
781 * Set up delayed work, which will enable and disable the static key. We need to
782 * use a work queue (rather than a simple timer), since enabling and disabling a
783 * static key cannot be done from an interrupt.
784 *
785 * Note: Toggling a static branch currently causes IPIs, and here we'll end up
786 * with a total of 2 IPIs to all CPUs. If this ends up a problem in future (with
787 * more aggressive sampling intervals), we could get away with a variant that
788 * avoids IPIs, at the cost of not immediately capturing allocations if the
789 * instructions remain cached.
790 */
791static void toggle_allocation_gate(struct work_struct *work)
792{
793 if (!READ_ONCE(kfence_enabled))
794 return;
795
796 atomic_set(&kfence_allocation_gate, 0);
797#ifdef CONFIG_KFENCE_STATIC_KEYS
798 /* Enable static key, and await allocation to happen. */
799 static_branch_enable(&kfence_allocation_key);
800
801 wait_event_idle(allocation_wait, atomic_read(&kfence_allocation_gate));
802
803 /* Disable static key and reset timer. */
804 static_branch_disable(&kfence_allocation_key);
805#endif
806 queue_delayed_work(system_unbound_wq, &kfence_timer,
807 msecs_to_jiffies(kfence_sample_interval));
808}
809
810/* === Public interface ===================================================== */
811
812void __init kfence_alloc_pool(void)
813{
814 if (!kfence_sample_interval)
815 return;
816
817 __kfence_pool = memblock_alloc(KFENCE_POOL_SIZE, PAGE_SIZE);
818
819 if (!__kfence_pool)
820 pr_err("failed to allocate pool\n");
821}
822
823static void kfence_init_enable(void)
824{
825 if (!IS_ENABLED(CONFIG_KFENCE_STATIC_KEYS))
826 static_branch_enable(&kfence_allocation_key);
827
828 if (kfence_deferrable)
829 INIT_DEFERRABLE_WORK(&kfence_timer, toggle_allocation_gate);
830 else
831 INIT_DELAYED_WORK(&kfence_timer, toggle_allocation_gate);
832
833 if (kfence_check_on_panic)
834 atomic_notifier_chain_register(&panic_notifier_list, &kfence_check_canary_notifier);
835
836 WRITE_ONCE(kfence_enabled, true);
837 queue_delayed_work(system_unbound_wq, &kfence_timer, 0);
838
839 pr_info("initialized - using %lu bytes for %d objects at 0x%p-0x%p\n", KFENCE_POOL_SIZE,
840 CONFIG_KFENCE_NUM_OBJECTS, (void *)__kfence_pool,
841 (void *)(__kfence_pool + KFENCE_POOL_SIZE));
842}
843
844void __init kfence_init(void)
845{
846 stack_hash_seed = get_random_u32();
847
848 /* Setting kfence_sample_interval to 0 on boot disables KFENCE. */
849 if (!kfence_sample_interval)
850 return;
851
852 if (!kfence_init_pool_early()) {
853 pr_err("%s failed\n", __func__);
854 return;
855 }
856
857 kfence_init_enable();
858}
859
860static int kfence_init_late(void)
861{
862 const unsigned long nr_pages = KFENCE_POOL_SIZE / PAGE_SIZE;
863#ifdef CONFIG_CONTIG_ALLOC
864 struct page *pages;
865
866 pages = alloc_contig_pages(nr_pages, GFP_KERNEL, first_online_node, NULL);
867 if (!pages)
868 return -ENOMEM;
869 __kfence_pool = page_to_virt(pages);
870#else
871 if (nr_pages > MAX_ORDER_NR_PAGES) {
872 pr_warn("KFENCE_NUM_OBJECTS too large for buddy allocator\n");
873 return -EINVAL;
874 }
875 __kfence_pool = alloc_pages_exact(KFENCE_POOL_SIZE, GFP_KERNEL);
876 if (!__kfence_pool)
877 return -ENOMEM;
878#endif
879
880 if (!kfence_init_pool_late()) {
881 pr_err("%s failed\n", __func__);
882 return -EBUSY;
883 }
884
885 kfence_init_enable();
886 return 0;
887}
888
889static int kfence_enable_late(void)
890{
891 if (!__kfence_pool)
892 return kfence_init_late();
893
894 WRITE_ONCE(kfence_enabled, true);
895 queue_delayed_work(system_unbound_wq, &kfence_timer, 0);
896 pr_info("re-enabled\n");
897 return 0;
898}
899
900void kfence_shutdown_cache(struct kmem_cache *s)
901{
902 unsigned long flags;
903 struct kfence_metadata *meta;
904 int i;
905
906 for (i = 0; i < CONFIG_KFENCE_NUM_OBJECTS; i++) {
907 bool in_use;
908
909 meta = &kfence_metadata[i];
910
911 /*
912 * If we observe some inconsistent cache and state pair where we
913 * should have returned false here, cache destruction is racing
914 * with either kmem_cache_alloc() or kmem_cache_free(). Taking
915 * the lock will not help, as different critical section
916 * serialization will have the same outcome.
917 */
918 if (READ_ONCE(meta->cache) != s ||
919 READ_ONCE(meta->state) != KFENCE_OBJECT_ALLOCATED)
920 continue;
921
922 raw_spin_lock_irqsave(&meta->lock, flags);
923 in_use = meta->cache == s && meta->state == KFENCE_OBJECT_ALLOCATED;
924 raw_spin_unlock_irqrestore(&meta->lock, flags);
925
926 if (in_use) {
927 /*
928 * This cache still has allocations, and we should not
929 * release them back into the freelist so they can still
930 * safely be used and retain the kernel's default
931 * behaviour of keeping the allocations alive (leak the
932 * cache); however, they effectively become "zombie
933 * allocations" as the KFENCE objects are the only ones
934 * still in use and the owning cache is being destroyed.
935 *
936 * We mark them freed, so that any subsequent use shows
937 * more useful error messages that will include stack
938 * traces of the user of the object, the original
939 * allocation, and caller to shutdown_cache().
940 */
941 kfence_guarded_free((void *)meta->addr, meta, /*zombie=*/true);
942 }
943 }
944
945 for (i = 0; i < CONFIG_KFENCE_NUM_OBJECTS; i++) {
946 meta = &kfence_metadata[i];
947
948 /* See above. */
949 if (READ_ONCE(meta->cache) != s || READ_ONCE(meta->state) != KFENCE_OBJECT_FREED)
950 continue;
951
952 raw_spin_lock_irqsave(&meta->lock, flags);
953 if (meta->cache == s && meta->state == KFENCE_OBJECT_FREED)
954 meta->cache = NULL;
955 raw_spin_unlock_irqrestore(&meta->lock, flags);
956 }
957}
958
959void *__kfence_alloc(struct kmem_cache *s, size_t size, gfp_t flags)
960{
961 unsigned long stack_entries[KFENCE_STACK_DEPTH];
962 size_t num_stack_entries;
963 u32 alloc_stack_hash;
964
965 /*
966 * Perform size check before switching kfence_allocation_gate, so that
967 * we don't disable KFENCE without making an allocation.
968 */
969 if (size > PAGE_SIZE) {
970 atomic_long_inc(&counters[KFENCE_COUNTER_SKIP_INCOMPAT]);
971 return NULL;
972 }
973
974 /*
975 * Skip allocations from non-default zones, including DMA. We cannot
976 * guarantee that pages in the KFENCE pool will have the requested
977 * properties (e.g. reside in DMAable memory).
978 */
979 if ((flags & GFP_ZONEMASK) ||
980 (s->flags & (SLAB_CACHE_DMA | SLAB_CACHE_DMA32))) {
981 atomic_long_inc(&counters[KFENCE_COUNTER_SKIP_INCOMPAT]);
982 return NULL;
983 }
984
985 /*
986 * Skip allocations for this slab, if KFENCE has been disabled for
987 * this slab.
988 */
989 if (s->flags & SLAB_SKIP_KFENCE)
990 return NULL;
991
992 if (atomic_inc_return(&kfence_allocation_gate) > 1)
993 return NULL;
994#ifdef CONFIG_KFENCE_STATIC_KEYS
995 /*
996 * waitqueue_active() is fully ordered after the update of
997 * kfence_allocation_gate per atomic_inc_return().
998 */
999 if (waitqueue_active(&allocation_wait)) {
1000 /*
1001 * Calling wake_up() here may deadlock when allocations happen
1002 * from within timer code. Use an irq_work to defer it.
1003 */
1004 irq_work_queue(&wake_up_kfence_timer_work);
1005 }
1006#endif
1007
1008 if (!READ_ONCE(kfence_enabled))
1009 return NULL;
1010
1011 num_stack_entries = stack_trace_save(stack_entries, KFENCE_STACK_DEPTH, 0);
1012
1013 /*
1014 * Do expensive check for coverage of allocation in slow-path after
1015 * allocation_gate has already become non-zero, even though it might
1016 * mean not making any allocation within a given sample interval.
1017 *
1018 * This ensures reasonable allocation coverage when the pool is almost
1019 * full, including avoiding long-lived allocations of the same source
1020 * filling up the pool (e.g. pagecache allocations).
1021 */
1022 alloc_stack_hash = get_alloc_stack_hash(stack_entries, num_stack_entries);
1023 if (should_skip_covered() && alloc_covered_contains(alloc_stack_hash)) {
1024 atomic_long_inc(&counters[KFENCE_COUNTER_SKIP_COVERED]);
1025 return NULL;
1026 }
1027
1028 return kfence_guarded_alloc(s, size, flags, stack_entries, num_stack_entries,
1029 alloc_stack_hash);
1030}
1031
1032size_t kfence_ksize(const void *addr)
1033{
1034 const struct kfence_metadata *meta = addr_to_metadata((unsigned long)addr);
1035
1036 /*
1037 * Read locklessly -- if there is a race with __kfence_alloc(), this is
1038 * either a use-after-free or invalid access.
1039 */
1040 return meta ? meta->size : 0;
1041}
1042
1043void *kfence_object_start(const void *addr)
1044{
1045 const struct kfence_metadata *meta = addr_to_metadata((unsigned long)addr);
1046
1047 /*
1048 * Read locklessly -- if there is a race with __kfence_alloc(), this is
1049 * either a use-after-free or invalid access.
1050 */
1051 return meta ? (void *)meta->addr : NULL;
1052}
1053
1054void __kfence_free(void *addr)
1055{
1056 struct kfence_metadata *meta = addr_to_metadata((unsigned long)addr);
1057
1058#ifdef CONFIG_MEMCG
1059 KFENCE_WARN_ON(meta->objcg);
1060#endif
1061 /*
1062 * If the objects of the cache are SLAB_TYPESAFE_BY_RCU, defer freeing
1063 * the object, as the object page may be recycled for other-typed
1064 * objects once it has been freed. meta->cache may be NULL if the cache
1065 * was destroyed.
1066 */
1067 if (unlikely(meta->cache && (meta->cache->flags & SLAB_TYPESAFE_BY_RCU)))
1068 call_rcu(&meta->rcu_head, rcu_guarded_free);
1069 else
1070 kfence_guarded_free(addr, meta, false);
1071}
1072
1073bool kfence_handle_page_fault(unsigned long addr, bool is_write, struct pt_regs *regs)
1074{
1075 const int page_index = (addr - (unsigned long)__kfence_pool) / PAGE_SIZE;
1076 struct kfence_metadata *to_report = NULL;
1077 enum kfence_error_type error_type;
1078 unsigned long flags;
1079
1080 if (!is_kfence_address((void *)addr))
1081 return false;
1082
1083 if (!READ_ONCE(kfence_enabled)) /* If disabled at runtime ... */
1084 return kfence_unprotect(addr); /* ... unprotect and proceed. */
1085
1086 atomic_long_inc(&counters[KFENCE_COUNTER_BUGS]);
1087
1088 if (page_index % 2) {
1089 /* This is a redzone, report a buffer overflow. */
1090 struct kfence_metadata *meta;
1091 int distance = 0;
1092
1093 meta = addr_to_metadata(addr - PAGE_SIZE);
1094 if (meta && READ_ONCE(meta->state) == KFENCE_OBJECT_ALLOCATED) {
1095 to_report = meta;
1096 /* Data race ok; distance calculation approximate. */
1097 distance = addr - data_race(meta->addr + meta->size);
1098 }
1099
1100 meta = addr_to_metadata(addr + PAGE_SIZE);
1101 if (meta && READ_ONCE(meta->state) == KFENCE_OBJECT_ALLOCATED) {
1102 /* Data race ok; distance calculation approximate. */
1103 if (!to_report || distance > data_race(meta->addr) - addr)
1104 to_report = meta;
1105 }
1106
1107 if (!to_report)
1108 goto out;
1109
1110 raw_spin_lock_irqsave(&to_report->lock, flags);
1111 to_report->unprotected_page = addr;
1112 error_type = KFENCE_ERROR_OOB;
1113
1114 /*
1115 * If the object was freed before we took the look we can still
1116 * report this as an OOB -- the report will simply show the
1117 * stacktrace of the free as well.
1118 */
1119 } else {
1120 to_report = addr_to_metadata(addr);
1121 if (!to_report)
1122 goto out;
1123
1124 raw_spin_lock_irqsave(&to_report->lock, flags);
1125 error_type = KFENCE_ERROR_UAF;
1126 /*
1127 * We may race with __kfence_alloc(), and it is possible that a
1128 * freed object may be reallocated. We simply report this as a
1129 * use-after-free, with the stack trace showing the place where
1130 * the object was re-allocated.
1131 */
1132 }
1133
1134out:
1135 if (to_report) {
1136 kfence_report_error(addr, is_write, regs, to_report, error_type);
1137 raw_spin_unlock_irqrestore(&to_report->lock, flags);
1138 } else {
1139 /* This may be a UAF or OOB access, but we can't be sure. */
1140 kfence_report_error(addr, is_write, regs, NULL, KFENCE_ERROR_INVALID);
1141 }
1142
1143 return kfence_unprotect(addr); /* Unprotect and let access proceed. */
1144}