summaryrefslogtreecommitdiffstats
path: root/mm/kfence/core.c
blob: 575c685aa642291bca3418828df953233f0ffe5e (plain)
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
61
62
63
64
65
66
67
68
69
70
71
72
73
74
75
76
77
78
79
80
81
82
83
84
85
86
87
88
89
90
91
92
93
94
95
96
97
98
99
100
101
102
103
104
105
106
107
108
109
110
111
112
113
114
115
116
117
118
119
120
121
122
123
124
125
126
127
128
129
130
131
132
133
134
135
136
137
138
139
140
141
142
143
144
145
146
147
148
149
150
151
152
153
154
155
156
157
158
159
160
161
162
163
164
165
166
167
168
169
170
171
172
173
174
175
176
177
178
179
180
181
182
183
184
185
186
187
188
189
190
191
192
193
194
195
196
197
198
199
200
201
202
203
204
205
206
207
208
209
210
211
212
213
214
215
216
217
218
219
220
221
222
223
224
225
226
227
228
229
230
231
232
233
234
235
236
237
238
239
240
241
242
243
244
245
246
247
248
249
250
251
252
253
254
255
256
257
258
259
260
261
262
263
264
265
266
267
268
269
270
271
272
273
274
275
276
277
278
279
280
281
282
283
284
285
286
287
288
289
290
291
292
293
294
295
296
297
298
299
300
301
302
303
304
305
306
307
308
309
310
311
312
313
314
315
316
317
318
319
320
321
322
323
324
325
326
327
328
329
330
331
332
333
334
335
336
337
338
339
340
341
342
343
344
345
346
347
348
349
350
351
352
353
354
355
356
357
358
359
360
361
362
363
364
365
366
367
368
369
370
371
372
373
374
375
376
377
378
379
380
381
382
383
384
385
386
387
388
389
390
391
392
393
394
395
396
397
398
399
400
401
402
403
404
405
406
407
408
409
410
411
412
413
414
415
416
417
418
419
420
421
422
423
424
425
426
427
428
429
430
431
432
433
434
435
436
437
438
439
440
441
442
443
444
445
446
447
448
449
450
451
452
453
454
455
456
457
458
459
460
461
462
463
464
465
466
467
468
469
470
471
472
473
474
475
476
477
478
479
480
481
482
483
484
485
486
487
488
489
490
491
492
493
494
495
496
497
498
499
500
501
502
503
504
505
506
507
508
509
510
511
512
513
514
515
516
517
518
519
520
521
522
523
524
525
526
527
528
529
530
531
532
533
534
535
536
537
538
539
540
541
542
543
544
545
546
547
548
549
550
551
552
553
554
555
556
557
558
559
560
561
562
563
564
565
566
567
568
569
570
571
572
573
574
575
576
577
578
579
580
581
582
583
584
585
586
587
588
589
590
591
592
593
594
595
596
597
598
599
600
601
602
603
604
605
606
607
608
609
610
611
612
613
614
615
616
617
618
619
620
621
622
623
624
625
626
627
628
629
630
631
632
633
634
635
636
637
638
639
640
641
642
643
644
645
646
647
648
649
650
651
652
653
654
655
656
657
658
659
660
661
662
663
664
665
666
667
668
669
670
671
672
673
674
675
676
677
678
679
680
681
682
683
684
685
686
687
688
689
690
691
692
693
694
695
696
697
698
699
700
701
702
703
704
705
706
707
708
709
710
711
712
713
714
715
716
717
718
719
720
721
722
723
724
725
726
727
728
729
730
731
732
733
734
735
736
737
738
739
740
741
742
743
744
745
746
747
748
749
750
751
752
753
754
755
756
757
758
759
760
761
762
763
764
765
766
767
768
769
770
771
772
773
774
775
776
777
778
779
780
781
782
783
784
785
786
787
788
789
790
791
792
793
794
795
796
797
798
799
800
801
802
803
804
805
806
807
808
809
810
811
812
813
814
815
816
817
818
819
820
821
822
823
824
825
826
827
828
829
830
831
832
833
834
835
836
837
838
839
840
841
842
843
844
845
846
847
848
849
850
851
852
853
854
855
856
857
858
859
860
861
862
863
864
865
866
867
868
869
870
871
872
873
874
875
876
877
878
879
880
881
882
883
884
885
886
887
888
// SPDX-License-Identifier: GPL-2.0
/*
 * KFENCE guarded object allocator and fault handling.
 *
 * Copyright (C) 2020, Google LLC.
 */

#define pr_fmt(fmt) "kfence: " fmt

#include <linux/atomic.h>
#include <linux/bug.h>
#include <linux/debugfs.h>
#include <linux/irq_work.h>
#include <linux/kcsan-checks.h>
#include <linux/kfence.h>
#include <linux/kmemleak.h>
#include <linux/list.h>
#include <linux/lockdep.h>
#include <linux/memblock.h>
#include <linux/moduleparam.h>
#include <linux/random.h>
#include <linux/rcupdate.h>
#include <linux/sched/sysctl.h>
#include <linux/seq_file.h>
#include <linux/slab.h>
#include <linux/spinlock.h>
#include <linux/string.h>

#include <asm/kfence.h>

#include "kfence.h"

/* Disables KFENCE on the first warning assuming an irrecoverable error. */
#define KFENCE_WARN_ON(cond)                                                   \
	({                                                                     \
		const bool __cond = WARN_ON(cond);                             \
		if (unlikely(__cond))                                          \
			WRITE_ONCE(kfence_enabled, false);                     \
		__cond;                                                        \
	})

/* === Data ================================================================= */

static bool kfence_enabled __read_mostly;

static unsigned long kfence_sample_interval __read_mostly = CONFIG_KFENCE_SAMPLE_INTERVAL;

#ifdef MODULE_PARAM_PREFIX
#undef MODULE_PARAM_PREFIX
#endif
#define MODULE_PARAM_PREFIX "kfence."

static int param_set_sample_interval(const char *val, const struct kernel_param *kp)
{
	unsigned long num;
	int ret = kstrtoul(val, 0, &num);

	if (ret < 0)
		return ret;

	if (!num) /* Using 0 to indicate KFENCE is disabled. */
		WRITE_ONCE(kfence_enabled, false);
	else if (!READ_ONCE(kfence_enabled) && system_state != SYSTEM_BOOTING)
		return -EINVAL; /* Cannot (re-)enable KFENCE on-the-fly. */

	*((unsigned long *)kp->arg) = num;
	return 0;
}

static int param_get_sample_interval(char *buffer, const struct kernel_param *kp)
{
	if (!READ_ONCE(kfence_enabled))
		return sprintf(buffer, "0\n");

	return param_get_ulong(buffer, kp);
}

static const struct kernel_param_ops sample_interval_param_ops = {
	.set = param_set_sample_interval,
	.get = param_get_sample_interval,
};
module_param_cb(sample_interval, &sample_interval_param_ops, &kfence_sample_interval, 0600);

/* The pool of pages used for guard pages and objects. */
char *__kfence_pool __ro_after_init;
EXPORT_SYMBOL(__kfence_pool); /* Export for test modules. */

/*
 * Per-object metadata, with one-to-one mapping of object metadata to
 * backing pages (in __kfence_pool).
 */
static_assert(CONFIG_KFENCE_NUM_OBJECTS > 0);
struct kfence_metadata kfence_metadata[CONFIG_KFENCE_NUM_OBJECTS];

/* Freelist with available objects. */
static struct list_head kfence_freelist = LIST_HEAD_INIT(kfence_freelist);
static DEFINE_RAW_SPINLOCK(kfence_freelist_lock); /* Lock protecting freelist. */

#ifdef CONFIG_KFENCE_STATIC_KEYS
/* The static key to set up a KFENCE allocation. */
DEFINE_STATIC_KEY_FALSE(kfence_allocation_key);
#endif

/* Gates the allocation, ensuring only one succeeds in a given period. */
atomic_t kfence_allocation_gate = ATOMIC_INIT(1);

/* Statistics counters for debugfs. */
enum kfence_counter_id {
	KFENCE_COUNTER_ALLOCATED,
	KFENCE_COUNTER_ALLOCS,
	KFENCE_COUNTER_FREES,
	KFENCE_COUNTER_ZOMBIES,
	KFENCE_COUNTER_BUGS,
	KFENCE_COUNTER_COUNT,
};
static atomic_long_t counters[KFENCE_COUNTER_COUNT];
static const char *const counter_names[] = {
	[KFENCE_COUNTER_ALLOCATED]	= "currently allocated",
	[KFENCE_COUNTER_ALLOCS]		= "total allocations",
	[KFENCE_COUNTER_FREES]		= "total frees",
	[KFENCE_COUNTER_ZOMBIES]	= "zombie allocations",
	[KFENCE_COUNTER_BUGS]		= "total bugs",
};
static_assert(ARRAY_SIZE(counter_names) == KFENCE_COUNTER_COUNT);

/* === Internals ============================================================ */

static bool kfence_protect(unsigned long addr)
{
	return !KFENCE_WARN_ON(!kfence_protect_page(ALIGN_DOWN(addr, PAGE_SIZE), true));
}

static bool kfence_unprotect(unsigned long addr)
{
	return !KFENCE_WARN_ON(!kfence_protect_page(ALIGN_DOWN(addr, PAGE_SIZE), false));
}

static inline struct kfence_metadata *addr_to_metadata(unsigned long addr)
{
	long index;

	/* The checks do not affect performance; only called from slow-paths. */

	if (!is_kfence_address((void *)addr))
		return NULL;

	/*
	 * May be an invalid index if called with an address at the edge of
	 * __kfence_pool, in which case we would report an "invalid access"
	 * error.
	 */
	index = (addr - (unsigned long)__kfence_pool) / (PAGE_SIZE * 2) - 1;
	if (index < 0 || index >= CONFIG_KFENCE_NUM_OBJECTS)
		return NULL;

	return &kfence_metadata[index];
}

static inline unsigned long metadata_to_pageaddr(const struct kfence_metadata *meta)
{
	unsigned long offset = (meta - kfence_metadata + 1) * PAGE_SIZE * 2;
	unsigned long pageaddr = (unsigned long)&__kfence_pool[offset];

	/* The checks do not affect performance; only called from slow-paths. */

	/* Only call with a pointer into kfence_metadata. */
	if (KFENCE_WARN_ON(meta < kfence_metadata ||
			   meta >= kfence_metadata + CONFIG_KFENCE_NUM_OBJECTS))
		return 0;

	/*
	 * This metadata object only ever maps to 1 page; verify that the stored
	 * address is in the expected range.
	 */
	if (KFENCE_WARN_ON(ALIGN_DOWN(meta->addr, PAGE_SIZE) != pageaddr))
		return 0;

	return pageaddr;
}

/*
 * Update the object's metadata state, including updating the alloc/free stacks
 * depending on the state transition.
 */
static noinline void metadata_update_state(struct kfence_metadata *meta,
					   enum kfence_object_state next)
{
	struct kfence_track *track =
		next == KFENCE_OBJECT_FREED ? &meta->free_track : &meta->alloc_track;

	lockdep_assert_held(&meta->lock);

	/*
	 * Skip over 1 (this) functions; noinline ensures we do not accidentally
	 * skip over the caller by never inlining.
	 */
	track->num_stack_entries = stack_trace_save(track->stack_entries, KFENCE_STACK_DEPTH, 1);
	track->pid = task_pid_nr(current);

	/*
	 * Pairs with READ_ONCE() in
	 *	kfence_shutdown_cache(),
	 *	kfence_handle_page_fault().
	 */
	WRITE_ONCE(meta->state, next);
}

/* Write canary byte to @addr. */
static inline bool set_canary_byte(u8 *addr)
{
	*addr = KFENCE_CANARY_PATTERN(addr);
	return true;
}

/* Check canary byte at @addr. */
static inline bool check_canary_byte(u8 *addr)
{
	if (likely(*addr == KFENCE_CANARY_PATTERN(addr)))
		return true;

	atomic_long_inc(&counters[KFENCE_COUNTER_BUGS]);
	kfence_report_error((unsigned long)addr, false, NULL, addr_to_metadata((unsigned long)addr),
			    KFENCE_ERROR_CORRUPTION);
	return false;
}

/* __always_inline this to ensure we won't do an indirect call to fn. */
static __always_inline void for_each_canary(const struct kfence_metadata *meta, bool (*fn)(u8 *))
{
	const unsigned long pageaddr = ALIGN_DOWN(meta->addr, PAGE_SIZE);
	unsigned long addr;

	lockdep_assert_held(&meta->lock);

	/*
	 * We'll iterate over each canary byte per-side until fn() returns
	 * false. However, we'll still iterate over the canary bytes to the
	 * right of the object even if there was an error in the canary bytes to
	 * the left of the object. Specifically, if check_canary_byte()
	 * generates an error, showing both sides might give more clues as to
	 * what the error is about when displaying which bytes were corrupted.
	 */

	/* Apply to left of object. */
	for (addr = pageaddr; addr < meta->addr; addr++) {
		if (!fn((u8 *)addr))
			break;
	}

	/* Apply to right of object. */
	for (addr = meta->addr + meta->size; addr < pageaddr + PAGE_SIZE; addr++) {
		if (!fn((u8 *)addr))
			break;
	}
}

static void *kfence_guarded_alloc(struct kmem_cache *cache, size_t size, gfp_t gfp)
{
	struct kfence_metadata *meta = NULL;
	unsigned long flags;
	struct page *page;
	void *addr;

	/* Try to obtain a free object. */
	raw_spin_lock_irqsave(&kfence_freelist_lock, flags);
	if (!list_empty(&kfence_freelist)) {
		meta = list_entry(kfence_freelist.next, struct kfence_metadata, list);
		list_del_init(&meta->list);
	}
	raw_spin_unlock_irqrestore(&kfence_freelist_lock, flags);
	if (!meta)
		return NULL;

	if (unlikely(!raw_spin_trylock_irqsave(&meta->lock, flags))) {
		/*
		 * This is extremely unlikely -- we are reporting on a
		 * use-after-free, which locked meta->lock, and the reporting
		 * code via printk calls kmalloc() which ends up in
		 * kfence_alloc() and tries to grab the same object that we're
		 * reporting on. While it has never been observed, lockdep does
		 * report that there is a possibility of deadlock. Fix it by
		 * using trylock and bailing out gracefully.
		 */
		raw_spin_lock_irqsave(&kfence_freelist_lock, flags);
		/* Put the object back on the freelist. */
		list_add_tail(&meta->list, &kfence_freelist);
		raw_spin_unlock_irqrestore(&kfence_freelist_lock, flags);

		return NULL;
	}

	meta->addr = metadata_to_pageaddr(meta);
	/* Unprotect if we're reusing this page. */
	if (meta->state == KFENCE_OBJECT_FREED)
		kfence_unprotect(meta->addr);

	/*
	 * Note: for allocations made before RNG initialization, will always
	 * return zero. We still benefit from enabling KFENCE as early as
	 * possible, even when the RNG is not yet available, as this will allow
	 * KFENCE to detect bugs due to earlier allocations. The only downside
	 * is that the out-of-bounds accesses detected are deterministic for
	 * such allocations.
	 */
	if (prandom_u32_max(2)) {
		/* Allocate on the "right" side, re-calculate address. */
		meta->addr += PAGE_SIZE - size;
		meta->addr = ALIGN_DOWN(meta->addr, cache->align);
	}

	addr = (void *)meta->addr;

	/* Update remaining metadata. */
	metadata_update_state(meta, KFENCE_OBJECT_ALLOCATED);
	/* Pairs with READ_ONCE() in kfence_shutdown_cache(). */
	WRITE_ONCE(meta->cache, cache);
	meta->size = size;
	for_each_canary(meta, set_canary_byte);

	/* Set required struct page fields. */
	page = virt_to_page(meta->addr);
	page->slab_cache = cache;
	if (IS_ENABLED(CONFIG_SLUB))
		page->objects = 1;
	if (IS_ENABLED(CONFIG_SLAB))
		page->s_mem = addr;

	raw_spin_unlock_irqrestore(&meta->lock, flags);

	/* Memory initialization. */

	/*
	 * We check slab_want_init_on_alloc() ourselves, rather than letting
	 * SL*B do the initialization, as otherwise we might overwrite KFENCE's
	 * redzone.
	 */
	if (unlikely(slab_want_init_on_alloc(gfp, cache)))
		memzero_explicit(addr, size);
	if (cache->ctor)
		cache->ctor(addr);

	if (CONFIG_KFENCE_STRESS_TEST_FAULTS && !prandom_u32_max(CONFIG_KFENCE_STRESS_TEST_FAULTS))
		kfence_protect(meta->addr); /* Random "faults" by protecting the object. */

	atomic_long_inc(&counters[KFENCE_COUNTER_ALLOCATED]);
	atomic_long_inc(&counters[KFENCE_COUNTER_ALLOCS]);

	return addr;
}

static void kfence_guarded_free(void *addr, struct kfence_metadata *meta, bool zombie)
{
	struct kcsan_scoped_access assert_page_exclusive;
	unsigned long flags;

	raw_spin_lock_irqsave(&meta->lock, flags);

	if (meta->state != KFENCE_OBJECT_ALLOCATED || meta->addr != (unsigned long)addr) {
		/* Invalid or double-free, bail out. */
		atomic_long_inc(&counters[KFENCE_COUNTER_BUGS]);
		kfence_report_error((unsigned long)addr, false, NULL, meta,
				    KFENCE_ERROR_INVALID_FREE);
		raw_spin_unlock_irqrestore(&meta->lock, flags);
		return;
	}

	/* Detect racy use-after-free, or incorrect reallocation of this page by KFENCE. */
	kcsan_begin_scoped_access((void *)ALIGN_DOWN((unsigned long)addr, PAGE_SIZE), PAGE_SIZE,
				  KCSAN_ACCESS_SCOPED | KCSAN_ACCESS_WRITE | KCSAN_ACCESS_ASSERT,
				  &assert_page_exclusive);

	if (CONFIG_KFENCE_STRESS_TEST_FAULTS)
		kfence_unprotect((unsigned long)addr); /* To check canary bytes. */

	/* Restore page protection if there was an OOB access. */
	if (meta->unprotected_page) {
		memzero_explicit((void *)ALIGN_DOWN(meta->unprotected_page, PAGE_SIZE), PAGE_SIZE);
		kfence_protect(meta->unprotected_page);
		meta->unprotected_page = 0;
	}

	/* Check canary bytes for memory corruption. */
	for_each_canary(meta, check_canary_byte);

	/*
	 * Clear memory if init-on-free is set. While we protect the page, the
	 * data is still there, and after a use-after-free is detected, we
	 * unprotect the page, so the data is still accessible.
	 */
	if (!zombie && unlikely(slab_want_init_on_free(meta->cache)))
		memzero_explicit(addr, meta->size);

	/* Mark the object as freed. */
	metadata_update_state(meta, KFENCE_OBJECT_FREED);

	raw_spin_unlock_irqrestore(&meta->lock, flags);

	/* Protect to detect use-after-frees. */
	kfence_protect((unsigned long)addr);

	kcsan_end_scoped_access(&assert_page_exclusive);
	if (!zombie) {
		/* Add it to the tail of the freelist for reuse. */
		raw_spin_lock_irqsave(&kfence_freelist_lock, flags);
		KFENCE_WARN_ON(!list_empty(&meta->list));
		list_add_tail(&meta->list, &kfence_freelist);
		raw_spin_unlock_irqrestore(&kfence_freelist_lock, flags);

		atomic_long_dec(&counters[KFENCE_COUNTER_ALLOCATED]);
		atomic_long_inc(&counters[KFENCE_COUNTER_FREES]);
	} else {
		/* See kfence_shutdown_cache(). */
		atomic_long_inc(&counters[KFENCE_COUNTER_ZOMBIES]);
	}
}

static void rcu_guarded_free(struct rcu_head *h)
{
	struct kfence_metadata *meta = container_of(h, struct kfence_metadata, rcu_head);

	kfence_guarded_free((void *)meta->addr, meta, false);
}

static bool __init kfence_init_pool(void)
{
	unsigned long addr = (unsigned long)__kfence_pool;
	struct page *pages;
	int i;

	if (!__kfence_pool)
		return false;

	if (!arch_kfence_init_pool())
		goto err;

	pages = virt_to_page(addr);

	/*
	 * Set up object pages: they must have PG_slab set, to avoid freeing
	 * these as real pages.
	 *
	 * We also want to avoid inserting kfence_free() in the kfree()
	 * fast-path in SLUB, and therefore need to ensure kfree() correctly
	 * enters __slab_free() slow-path.
	 */
	for (i = 0; i < KFENCE_POOL_SIZE / PAGE_SIZE; i++) {
		if (!i || (i % 2))
			continue;

		/* Verify we do not have a compound head page. */
		if (WARN_ON(compound_head(&pages[i]) != &pages[i]))
			goto err;

		__SetPageSlab(&pages[i]);
	}

	/*
	 * Protect the first 2 pages. The first page is mostly unnecessary, and
	 * merely serves as an extended guard page. However, adding one
	 * additional page in the beginning gives us an even number of pages,
	 * which simplifies the mapping of address to metadata index.
	 */
	for (i = 0; i < 2; i++) {
		if (unlikely(!kfence_protect(addr)))
			goto err;

		addr += PAGE_SIZE;
	}

	for (i = 0; i < CONFIG_KFENCE_NUM_OBJECTS; i++) {
		struct kfence_metadata *meta = &kfence_metadata[i];

		/* Initialize metadata. */
		INIT_LIST_HEAD(&meta->list);
		raw_spin_lock_init(&meta->lock);
		meta->state = KFENCE_OBJECT_UNUSED;
		meta->addr = addr; /* Initialize for validation in metadata_to_pageaddr(). */
		list_add_tail(&meta->list, &kfence_freelist);

		/* Protect the right redzone. */
		if (unlikely(!kfence_protect(addr + PAGE_SIZE)))
			goto err;

		addr += 2 * PAGE_SIZE;
	}

	/*
	 * The pool is live and will never be deallocated from this point on.
	 * Remove the pool object from the kmemleak object tree, as it would
	 * otherwise overlap with allocations returned by kfence_alloc(), which
	 * are registered with kmemleak through the slab post-alloc hook.
	 */
	kmemleak_free(__kfence_pool);

	return true;

err:
	/*
	 * Only release unprotected pages, and do not try to go back and change
	 * page attributes due to risk of failing to do so as well. If changing
	 * page attributes for some pages fails, it is very likely that it also
	 * fails for the first page, and therefore expect addr==__kfence_pool in
	 * most failure cases.
	 */
	memblock_free_late(__pa(addr), KFENCE_POOL_SIZE - (addr - (unsigned long)__kfence_pool));
	__kfence_pool = NULL;
	return false;
}

/* === DebugFS Interface ==================================================== */

static int stats_show(struct seq_file *seq, void *v)
{
	int i;

	seq_printf(seq, "enabled: %i\n", READ_ONCE(kfence_enabled));
	for (i = 0; i < KFENCE_COUNTER_COUNT; i++)
		seq_printf(seq, "%s: %ld\n", counter_names[i], atomic_long_read(&counters[i]));

	return 0;
}
DEFINE_SHOW_ATTRIBUTE(stats);

/*
 * debugfs seq_file operations for /sys/kernel/debug/kfence/objects.
 * start_object() and next_object() return the object index + 1, because NULL is used
 * to stop iteration.
 */
static void *start_object(struct seq_file *seq, loff_t *pos)
{
	if (*pos < CONFIG_KFENCE_NUM_OBJECTS)
		return (void *)((long)*pos + 1);
	return NULL;
}

static void stop_object(struct seq_file *seq, void *v)
{
}

static void *next_object(struct seq_file *seq, void *v, loff_t *pos)
{
	++*pos;
	if (*pos < CONFIG_KFENCE_NUM_OBJECTS)
		return (void *)((long)*pos + 1);
	return NULL;
}

static int show_object(struct seq_file *seq, void *v)
{
	struct kfence_metadata *meta = &kfence_metadata[(long)v - 1];
	unsigned long flags;

	raw_spin_lock_irqsave(&meta->lock, flags);
	kfence_print_object(seq, meta);
	raw_spin_unlock_irqrestore(&meta->lock, flags);
	seq_puts(seq, "---------------------------------\n");

	return 0;
}

static const struct seq_operations object_seqops = {
	.start = start_object,
	.next = next_object,
	.stop = stop_object,
	.show = show_object,
};

static int open_objects(struct inode *inode, struct file *file)
{
	return seq_open(file, &object_seqops);
}

static const struct file_operations objects_fops = {
	.open = open_objects,
	.read = seq_read,
	.llseek = seq_lseek,
};

static int __init kfence_debugfs_init(void)
{
	struct dentry *kfence_dir = debugfs_create_dir("kfence", NULL);

	debugfs_create_file("stats", 0444, kfence_dir, NULL, &stats_fops);
	debugfs_create_file("objects", 0400, kfence_dir, NULL, &objects_fops);
	return 0;
}

late_initcall(kfence_debugfs_init);

/* === Allocation Gate Timer ================================================ */

#ifdef CONFIG_KFENCE_STATIC_KEYS
/* Wait queue to wake up allocation-gate timer task. */
static DECLARE_WAIT_QUEUE_HEAD(allocation_wait);

static void wake_up_kfence_timer(struct irq_work *work)
{
	wake_up(&allocation_wait);
}
static DEFINE_IRQ_WORK(wake_up_kfence_timer_work, wake_up_kfence_timer);
#endif

/*
 * Set up delayed work, which will enable and disable the static key. We need to
 * use a work queue (rather than a simple timer), since enabling and disabling a
 * static key cannot be done from an interrupt.
 *
 * Note: Toggling a static branch currently causes IPIs, and here we'll end up
 * with a total of 2 IPIs to all CPUs. If this ends up a problem in future (with
 * more aggressive sampling intervals), we could get away with a variant that
 * avoids IPIs, at the cost of not immediately capturing allocations if the
 * instructions remain cached.
 */
static struct delayed_work kfence_timer;
static void toggle_allocation_gate(struct work_struct *work)
{
	if (!READ_ONCE(kfence_enabled))
		return;

	atomic_set(&kfence_allocation_gate, 0);
#ifdef CONFIG_KFENCE_STATIC_KEYS
	/* Enable static key, and await allocation to happen. */
	static_branch_enable(&kfence_allocation_key);

	if (sysctl_hung_task_timeout_secs) {
		/*
		 * During low activity with no allocations we might wait a
		 * while; let's avoid the hung task warning.
		 */
		wait_event_idle_timeout(allocation_wait, atomic_read(&kfence_allocation_gate),
					sysctl_hung_task_timeout_secs * HZ / 2);
	} else {
		wait_event_idle(allocation_wait, atomic_read(&kfence_allocation_gate));
	}

	/* Disable static key and reset timer. */
	static_branch_disable(&kfence_allocation_key);
#endif
	queue_delayed_work(system_unbound_wq, &kfence_timer,
			   msecs_to_jiffies(kfence_sample_interval));
}
static DECLARE_DELAYED_WORK(kfence_timer, toggle_allocation_gate);

/* === Public interface ===================================================== */

void __init kfence_alloc_pool(void)
{
	if (!kfence_sample_interval)
		return;

	__kfence_pool = memblock_alloc(KFENCE_POOL_SIZE, PAGE_SIZE);

	if (!__kfence_pool)
		pr_err("failed to allocate pool\n");
}

void __init kfence_init(void)
{
	/* Setting kfence_sample_interval to 0 on boot disables KFENCE. */
	if (!kfence_sample_interval)
		return;

	if (!kfence_init_pool()) {
		pr_err("%s failed\n", __func__);
		return;
	}

	WRITE_ONCE(kfence_enabled, true);
	queue_delayed_work(system_unbound_wq, &kfence_timer, 0);
	pr_info("initialized - using %lu bytes for %d objects at 0x%p-0x%p\n", KFENCE_POOL_SIZE,
		CONFIG_KFENCE_NUM_OBJECTS, (void *)__kfence_pool,
		(void *)(__kfence_pool + KFENCE_POOL_SIZE));
}

void kfence_shutdown_cache(struct kmem_cache *s)
{
	unsigned long flags;
	struct kfence_metadata *meta;
	int i;

	for (i = 0; i < CONFIG_KFENCE_NUM_OBJECTS; i++) {
		bool in_use;

		meta = &kfence_metadata[i];

		/*
		 * If we observe some inconsistent cache and state pair where we
		 * should have returned false here, cache destruction is racing
		 * with either kmem_cache_alloc() or kmem_cache_free(). Taking
		 * the lock will not help, as different critical section
		 * serialization will have the same outcome.
		 */
		if (READ_ONCE(meta->cache) != s ||
		    READ_ONCE(meta->state) != KFENCE_OBJECT_ALLOCATED)
			continue;

		raw_spin_lock_irqsave(&meta->lock, flags);
		in_use = meta->cache == s && meta->state == KFENCE_OBJECT_ALLOCATED;
		raw_spin_unlock_irqrestore(&meta->lock, flags);

		if (in_use) {
			/*
			 * This cache still has allocations, and we should not
			 * release them back into the freelist so they can still
			 * safely be used and retain the kernel's default
			 * behaviour of keeping the allocations alive (leak the
			 * cache); however, they effectively become "zombie
			 * allocations" as the KFENCE objects are the only ones
			 * still in use and the owning cache is being destroyed.
			 *
			 * We mark them freed, so that any subsequent use shows
			 * more useful error messages that will include stack
			 * traces of the user of the object, the original
			 * allocation, and caller to shutdown_cache().
			 */
			kfence_guarded_free((void *)meta->addr, meta, /*zombie=*/true);
		}
	}

	for (i = 0; i < CONFIG_KFENCE_NUM_OBJECTS; i++) {
		meta = &kfence_metadata[i];

		/* See above. */
		if (READ_ONCE(meta->cache) != s || READ_ONCE(meta->state) != KFENCE_OBJECT_FREED)
			continue;

		raw_spin_lock_irqsave(&meta->lock, flags);
		if (meta->cache == s && meta->state == KFENCE_OBJECT_FREED)
			meta->cache = NULL;
		raw_spin_unlock_irqrestore(&meta->lock, flags);
	}
}

void *__kfence_alloc(struct kmem_cache *s, size_t size, gfp_t flags)
{
	/*
	 * Perform size check before switching kfence_allocation_gate, so that
	 * we don't disable KFENCE without making an allocation.
	 */
	if (size > PAGE_SIZE)
		return NULL;

	/*
	 * Skip allocations from non-default zones, including DMA. We cannot
	 * guarantee that pages in the KFENCE pool will have the requested
	 * properties (e.g. reside in DMAable memory).
	 */
	if ((flags & GFP_ZONEMASK) ||
	    (s->flags & (SLAB_CACHE_DMA | SLAB_CACHE_DMA32)))
		return NULL;

	/*
	 * allocation_gate only needs to become non-zero, so it doesn't make
	 * sense to continue writing to it and pay the associated contention
	 * cost, in case we have a large number of concurrent allocations.
	 */
	if (atomic_read(&kfence_allocation_gate) || atomic_inc_return(&kfence_allocation_gate) > 1)
		return NULL;
#ifdef CONFIG_KFENCE_STATIC_KEYS
	/*
	 * waitqueue_active() is fully ordered after the update of
	 * kfence_allocation_gate per atomic_inc_return().
	 */
	if (waitqueue_active(&allocation_wait)) {
		/*
		 * Calling wake_up() here may deadlock when allocations happen
		 * from within timer code. Use an irq_work to defer it.
		 */
		irq_work_queue(&wake_up_kfence_timer_work);
	}
#endif

	if (!READ_ONCE(kfence_enabled))
		return NULL;

	return kfence_guarded_alloc(s, size, flags);
}

size_t kfence_ksize(const void *addr)
{
	const struct kfence_metadata *meta = addr_to_metadata((unsigned long)addr);

	/*
	 * Read locklessly -- if there is a race with __kfence_alloc(), this is
	 * either a use-after-free or invalid access.
	 */
	return meta ? meta->size : 0;
}

void *kfence_object_start(const void *addr)
{
	const struct kfence_metadata *meta = addr_to_metadata((unsigned long)addr);

	/*
	 * Read locklessly -- if there is a race with __kfence_alloc(), this is
	 * either a use-after-free or invalid access.
	 */
	return meta ? (void *)meta->addr : NULL;
}

void __kfence_free(void *addr)
{
	struct kfence_metadata *meta = addr_to_metadata((unsigned long)addr);

	/*
	 * If the objects of the cache are SLAB_TYPESAFE_BY_RCU, defer freeing
	 * the object, as the object page may be recycled for other-typed
	 * objects once it has been freed. meta->cache may be NULL if the cache
	 * was destroyed.
	 */
	if (unlikely(meta->cache && (meta->cache->flags & SLAB_TYPESAFE_BY_RCU)))
		call_rcu(&meta->rcu_head, rcu_guarded_free);
	else
		kfence_guarded_free(addr, meta, false);
}

bool kfence_handle_page_fault(unsigned long addr, bool is_write, struct pt_regs *regs)
{
	const int page_index = (addr - (unsigned long)__kfence_pool) / PAGE_SIZE;
	struct kfence_metadata *to_report = NULL;
	enum kfence_error_type error_type;
	unsigned long flags;

	if (!is_kfence_address((void *)addr))
		return false;

	if (!READ_ONCE(kfence_enabled)) /* If disabled at runtime ... */
		return kfence_unprotect(addr); /* ... unprotect and proceed. */

	atomic_long_inc(&counters[KFENCE_COUNTER_BUGS]);

	if (page_index % 2) {
		/* This is a redzone, report a buffer overflow. */
		struct kfence_metadata *meta;
		int distance = 0;

		meta = addr_to_metadata(addr - PAGE_SIZE);
		if (meta && READ_ONCE(meta->state) == KFENCE_OBJECT_ALLOCATED) {
			to_report = meta;
			/* Data race ok; distance calculation approximate. */
			distance = addr - data_race(meta->addr + meta->size);
		}

		meta = addr_to_metadata(addr + PAGE_SIZE);
		if (meta && READ_ONCE(meta->state) == KFENCE_OBJECT_ALLOCATED) {
			/* Data race ok; distance calculation approximate. */
			if (!to_report || distance > data_race(meta->addr) - addr)
				to_report = meta;
		}

		if (!to_report)
			goto out;

		raw_spin_lock_irqsave(&to_report->lock, flags);
		to_report->unprotected_page = addr;
		error_type = KFENCE_ERROR_OOB;

		/*
		 * If the object was freed before we took the look we can still
		 * report this as an OOB -- the report will simply show the
		 * stacktrace of the free as well.
		 */
	} else {
		to_report = addr_to_metadata(addr);
		if (!to_report)
			goto out;

		raw_spin_lock_irqsave(&to_report->lock, flags);
		error_type = KFENCE_ERROR_UAF;
		/*
		 * We may race with __kfence_alloc(), and it is possible that a
		 * freed object may be reallocated. We simply report this as a
		 * use-after-free, with the stack trace showing the place where
		 * the object was re-allocated.
		 */
	}

out:
	if (to_report) {
		kfence_report_error(addr, is_write, regs, to_report, error_type);
		raw_spin_unlock_irqrestore(&to_report->lock, flags);
	} else {
		/* This may be a UAF or OOB access, but we can't be sure. */
		kfence_report_error(addr, is_write, regs, NULL, KFENCE_ERROR_INVALID);
	}

	return kfence_unprotect(addr); /* Unprotect and let access proceed. */
}