xref: /openbmc/linux/mm/kfence/core.c (revision 90cb380f9ceb811059340d06ff5fd0c0e93ecbe1)
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/sched/sysctl.h>
30  #include <linux/seq_file.h>
31  #include <linux/slab.h>
32  #include <linux/spinlock.h>
33  #include <linux/string.h>
34  
35  #include <asm/kfence.h>
36  
37  #include "kfence.h"
38  
39  /* Disables KFENCE on the first warning assuming an irrecoverable error. */
40  #define KFENCE_WARN_ON(cond)                                                   \
41  	({                                                                     \
42  		const bool __cond = WARN_ON(cond);                             \
43  		if (unlikely(__cond)) {                                        \
44  			WRITE_ONCE(kfence_enabled, false);                     \
45  			disabled_by_warn = true;                               \
46  		}                                                              \
47  		__cond;                                                        \
48  	})
49  
50  /* === Data ================================================================= */
51  
52  static bool kfence_enabled __read_mostly;
53  static bool disabled_by_warn __read_mostly;
54  
55  unsigned long kfence_sample_interval __read_mostly = CONFIG_KFENCE_SAMPLE_INTERVAL;
56  EXPORT_SYMBOL_GPL(kfence_sample_interval); /* Export for test modules. */
57  
58  #ifdef MODULE_PARAM_PREFIX
59  #undef MODULE_PARAM_PREFIX
60  #endif
61  #define MODULE_PARAM_PREFIX "kfence."
62  
63  static int kfence_enable_late(void);
64  static int param_set_sample_interval(const char *val, const struct kernel_param *kp)
65  {
66  	unsigned long num;
67  	int ret = kstrtoul(val, 0, &num);
68  
69  	if (ret < 0)
70  		return ret;
71  
72  	/* Using 0 to indicate KFENCE is disabled. */
73  	if (!num && READ_ONCE(kfence_enabled)) {
74  		pr_info("disabled\n");
75  		WRITE_ONCE(kfence_enabled, false);
76  	}
77  
78  	*((unsigned long *)kp->arg) = num;
79  
80  	if (num && !READ_ONCE(kfence_enabled) && system_state != SYSTEM_BOOTING)
81  		return disabled_by_warn ? -EINVAL : kfence_enable_late();
82  	return 0;
83  }
84  
85  static int param_get_sample_interval(char *buffer, const struct kernel_param *kp)
86  {
87  	if (!READ_ONCE(kfence_enabled))
88  		return sprintf(buffer, "0\n");
89  
90  	return param_get_ulong(buffer, kp);
91  }
92  
93  static const struct kernel_param_ops sample_interval_param_ops = {
94  	.set = param_set_sample_interval,
95  	.get = param_get_sample_interval,
96  };
97  module_param_cb(sample_interval, &sample_interval_param_ops, &kfence_sample_interval, 0600);
98  
99  /* Pool usage% threshold when currently covered allocations are skipped. */
100  static unsigned long kfence_skip_covered_thresh __read_mostly = 75;
101  module_param_named(skip_covered_thresh, kfence_skip_covered_thresh, ulong, 0644);
102  
103  /* If true, use a deferrable timer. */
104  static bool kfence_deferrable __read_mostly = IS_ENABLED(CONFIG_KFENCE_DEFERRABLE);
105  module_param_named(deferrable, kfence_deferrable, bool, 0444);
106  
107  /* If true, check all canary bytes on panic. */
108  static bool kfence_check_on_panic __read_mostly;
109  module_param_named(check_on_panic, kfence_check_on_panic, bool, 0444);
110  
111  /* The pool of pages used for guard pages and objects. */
112  char *__kfence_pool __read_mostly;
113  EXPORT_SYMBOL(__kfence_pool); /* Export for test modules. */
114  
115  /*
116   * Per-object metadata, with one-to-one mapping of object metadata to
117   * backing pages (in __kfence_pool).
118   */
119  static_assert(CONFIG_KFENCE_NUM_OBJECTS > 0);
120  struct kfence_metadata kfence_metadata[CONFIG_KFENCE_NUM_OBJECTS];
121  
122  /* Freelist with available objects. */
123  static struct list_head kfence_freelist = LIST_HEAD_INIT(kfence_freelist);
124  static DEFINE_RAW_SPINLOCK(kfence_freelist_lock); /* Lock protecting freelist. */
125  
126  /*
127   * The static key to set up a KFENCE allocation; or if static keys are not used
128   * to gate allocations, to avoid a load and compare if KFENCE is disabled.
129   */
130  DEFINE_STATIC_KEY_FALSE(kfence_allocation_key);
131  
132  /* Gates the allocation, ensuring only one succeeds in a given period. */
133  atomic_t kfence_allocation_gate = ATOMIC_INIT(1);
134  
135  /*
136   * A Counting Bloom filter of allocation coverage: limits currently covered
137   * allocations of the same source filling up the pool.
138   *
139   * Assuming a range of 15%-85% unique allocations in the pool at any point in
140   * time, the below parameters provide a probablity of 0.02-0.33 for false
141   * positive hits respectively:
142   *
143   *	P(alloc_traces) = (1 - e^(-HNUM * (alloc_traces / SIZE)) ^ HNUM
144   */
145  #define ALLOC_COVERED_HNUM	2
146  #define ALLOC_COVERED_ORDER	(const_ilog2(CONFIG_KFENCE_NUM_OBJECTS) + 2)
147  #define ALLOC_COVERED_SIZE	(1 << ALLOC_COVERED_ORDER)
148  #define ALLOC_COVERED_HNEXT(h)	hash_32(h, ALLOC_COVERED_ORDER)
149  #define ALLOC_COVERED_MASK	(ALLOC_COVERED_SIZE - 1)
150  static atomic_t alloc_covered[ALLOC_COVERED_SIZE];
151  
152  /* Stack depth used to determine uniqueness of an allocation. */
153  #define UNIQUE_ALLOC_STACK_DEPTH ((size_t)8)
154  
155  /*
156   * Randomness for stack hashes, making the same collisions across reboots and
157   * different machines less likely.
158   */
159  static u32 stack_hash_seed __ro_after_init;
160  
161  /* Statistics counters for debugfs. */
162  enum kfence_counter_id {
163  	KFENCE_COUNTER_ALLOCATED,
164  	KFENCE_COUNTER_ALLOCS,
165  	KFENCE_COUNTER_FREES,
166  	KFENCE_COUNTER_ZOMBIES,
167  	KFENCE_COUNTER_BUGS,
168  	KFENCE_COUNTER_SKIP_INCOMPAT,
169  	KFENCE_COUNTER_SKIP_CAPACITY,
170  	KFENCE_COUNTER_SKIP_COVERED,
171  	KFENCE_COUNTER_COUNT,
172  };
173  static atomic_long_t counters[KFENCE_COUNTER_COUNT];
174  static const char *const counter_names[] = {
175  	[KFENCE_COUNTER_ALLOCATED]	= "currently allocated",
176  	[KFENCE_COUNTER_ALLOCS]		= "total allocations",
177  	[KFENCE_COUNTER_FREES]		= "total frees",
178  	[KFENCE_COUNTER_ZOMBIES]	= "zombie allocations",
179  	[KFENCE_COUNTER_BUGS]		= "total bugs",
180  	[KFENCE_COUNTER_SKIP_INCOMPAT]	= "skipped allocations (incompatible)",
181  	[KFENCE_COUNTER_SKIP_CAPACITY]	= "skipped allocations (capacity)",
182  	[KFENCE_COUNTER_SKIP_COVERED]	= "skipped allocations (covered)",
183  };
184  static_assert(ARRAY_SIZE(counter_names) == KFENCE_COUNTER_COUNT);
185  
186  /* === Internals ============================================================ */
187  
188  static inline bool should_skip_covered(void)
189  {
190  	unsigned long thresh = (CONFIG_KFENCE_NUM_OBJECTS * kfence_skip_covered_thresh) / 100;
191  
192  	return atomic_long_read(&counters[KFENCE_COUNTER_ALLOCATED]) > thresh;
193  }
194  
195  static u32 get_alloc_stack_hash(unsigned long *stack_entries, size_t num_entries)
196  {
197  	num_entries = min(num_entries, UNIQUE_ALLOC_STACK_DEPTH);
198  	num_entries = filter_irq_stacks(stack_entries, num_entries);
199  	return jhash(stack_entries, num_entries * sizeof(stack_entries[0]), stack_hash_seed);
200  }
201  
202  /*
203   * Adds (or subtracts) count @val for allocation stack trace hash
204   * @alloc_stack_hash from Counting Bloom filter.
205   */
206  static void alloc_covered_add(u32 alloc_stack_hash, int val)
207  {
208  	int i;
209  
210  	for (i = 0; i < ALLOC_COVERED_HNUM; i++) {
211  		atomic_add(val, &alloc_covered[alloc_stack_hash & ALLOC_COVERED_MASK]);
212  		alloc_stack_hash = ALLOC_COVERED_HNEXT(alloc_stack_hash);
213  	}
214  }
215  
216  /*
217   * Returns true if the allocation stack trace hash @alloc_stack_hash is
218   * currently contained (non-zero count) in Counting Bloom filter.
219   */
220  static bool alloc_covered_contains(u32 alloc_stack_hash)
221  {
222  	int i;
223  
224  	for (i = 0; i < ALLOC_COVERED_HNUM; i++) {
225  		if (!atomic_read(&alloc_covered[alloc_stack_hash & ALLOC_COVERED_MASK]))
226  			return false;
227  		alloc_stack_hash = ALLOC_COVERED_HNEXT(alloc_stack_hash);
228  	}
229  
230  	return true;
231  }
232  
233  static bool kfence_protect(unsigned long addr)
234  {
235  	return !KFENCE_WARN_ON(!kfence_protect_page(ALIGN_DOWN(addr, PAGE_SIZE), true));
236  }
237  
238  static bool kfence_unprotect(unsigned long addr)
239  {
240  	return !KFENCE_WARN_ON(!kfence_protect_page(ALIGN_DOWN(addr, PAGE_SIZE), false));
241  }
242  
243  static inline unsigned long metadata_to_pageaddr(const struct kfence_metadata *meta)
244  {
245  	unsigned long offset = (meta - kfence_metadata + 1) * PAGE_SIZE * 2;
246  	unsigned long pageaddr = (unsigned long)&__kfence_pool[offset];
247  
248  	/* The checks do not affect performance; only called from slow-paths. */
249  
250  	/* Only call with a pointer into kfence_metadata. */
251  	if (KFENCE_WARN_ON(meta < kfence_metadata ||
252  			   meta >= kfence_metadata + CONFIG_KFENCE_NUM_OBJECTS))
253  		return 0;
254  
255  	/*
256  	 * This metadata object only ever maps to 1 page; verify that the stored
257  	 * address is in the expected range.
258  	 */
259  	if (KFENCE_WARN_ON(ALIGN_DOWN(meta->addr, PAGE_SIZE) != pageaddr))
260  		return 0;
261  
262  	return pageaddr;
263  }
264  
265  /*
266   * Update the object's metadata state, including updating the alloc/free stacks
267   * depending on the state transition.
268   */
269  static noinline void
270  metadata_update_state(struct kfence_metadata *meta, enum kfence_object_state next,
271  		      unsigned long *stack_entries, size_t num_stack_entries)
272  {
273  	struct kfence_track *track =
274  		next == KFENCE_OBJECT_FREED ? &meta->free_track : &meta->alloc_track;
275  
276  	lockdep_assert_held(&meta->lock);
277  
278  	if (stack_entries) {
279  		memcpy(track->stack_entries, stack_entries,
280  		       num_stack_entries * sizeof(stack_entries[0]));
281  	} else {
282  		/*
283  		 * Skip over 1 (this) functions; noinline ensures we do not
284  		 * accidentally skip over the caller by never inlining.
285  		 */
286  		num_stack_entries = stack_trace_save(track->stack_entries, KFENCE_STACK_DEPTH, 1);
287  	}
288  	track->num_stack_entries = num_stack_entries;
289  	track->pid = task_pid_nr(current);
290  	track->cpu = raw_smp_processor_id();
291  	track->ts_nsec = local_clock(); /* Same source as printk timestamps. */
292  
293  	/*
294  	 * Pairs with READ_ONCE() in
295  	 *	kfence_shutdown_cache(),
296  	 *	kfence_handle_page_fault().
297  	 */
298  	WRITE_ONCE(meta->state, next);
299  }
300  
301  /* Write canary byte to @addr. */
302  static inline bool set_canary_byte(u8 *addr)
303  {
304  	*addr = KFENCE_CANARY_PATTERN(addr);
305  	return true;
306  }
307  
308  /* Check canary byte at @addr. */
309  static inline bool check_canary_byte(u8 *addr)
310  {
311  	struct kfence_metadata *meta;
312  	unsigned long flags;
313  
314  	if (likely(*addr == KFENCE_CANARY_PATTERN(addr)))
315  		return true;
316  
317  	atomic_long_inc(&counters[KFENCE_COUNTER_BUGS]);
318  
319  	meta = addr_to_metadata((unsigned long)addr);
320  	raw_spin_lock_irqsave(&meta->lock, flags);
321  	kfence_report_error((unsigned long)addr, false, NULL, meta, KFENCE_ERROR_CORRUPTION);
322  	raw_spin_unlock_irqrestore(&meta->lock, flags);
323  
324  	return false;
325  }
326  
327  /* __always_inline this to ensure we won't do an indirect call to fn. */
328  static __always_inline void for_each_canary(const struct kfence_metadata *meta, bool (*fn)(u8 *))
329  {
330  	const unsigned long pageaddr = ALIGN_DOWN(meta->addr, PAGE_SIZE);
331  	unsigned long addr;
332  
333  	/*
334  	 * We'll iterate over each canary byte per-side until fn() returns
335  	 * false. However, we'll still iterate over the canary bytes to the
336  	 * right of the object even if there was an error in the canary bytes to
337  	 * the left of the object. Specifically, if check_canary_byte()
338  	 * generates an error, showing both sides might give more clues as to
339  	 * what the error is about when displaying which bytes were corrupted.
340  	 */
341  
342  	/* Apply to left of object. */
343  	for (addr = pageaddr; addr < meta->addr; addr++) {
344  		if (!fn((u8 *)addr))
345  			break;
346  	}
347  
348  	/* Apply to right of object. */
349  	for (addr = meta->addr + meta->size; addr < pageaddr + PAGE_SIZE; addr++) {
350  		if (!fn((u8 *)addr))
351  			break;
352  	}
353  }
354  
355  static void *kfence_guarded_alloc(struct kmem_cache *cache, size_t size, gfp_t gfp,
356  				  unsigned long *stack_entries, size_t num_stack_entries,
357  				  u32 alloc_stack_hash)
358  {
359  	struct kfence_metadata *meta = NULL;
360  	unsigned long flags;
361  	struct slab *slab;
362  	void *addr;
363  	const bool random_right_allocate = prandom_u32_max(2);
364  	const bool random_fault = CONFIG_KFENCE_STRESS_TEST_FAULTS &&
365  				  !prandom_u32_max(CONFIG_KFENCE_STRESS_TEST_FAULTS);
366  
367  	/* Try to obtain a free object. */
368  	raw_spin_lock_irqsave(&kfence_freelist_lock, flags);
369  	if (!list_empty(&kfence_freelist)) {
370  		meta = list_entry(kfence_freelist.next, struct kfence_metadata, list);
371  		list_del_init(&meta->list);
372  	}
373  	raw_spin_unlock_irqrestore(&kfence_freelist_lock, flags);
374  	if (!meta) {
375  		atomic_long_inc(&counters[KFENCE_COUNTER_SKIP_CAPACITY]);
376  		return NULL;
377  	}
378  
379  	if (unlikely(!raw_spin_trylock_irqsave(&meta->lock, flags))) {
380  		/*
381  		 * This is extremely unlikely -- we are reporting on a
382  		 * use-after-free, which locked meta->lock, and the reporting
383  		 * code via printk calls kmalloc() which ends up in
384  		 * kfence_alloc() and tries to grab the same object that we're
385  		 * reporting on. While it has never been observed, lockdep does
386  		 * report that there is a possibility of deadlock. Fix it by
387  		 * using trylock and bailing out gracefully.
388  		 */
389  		raw_spin_lock_irqsave(&kfence_freelist_lock, flags);
390  		/* Put the object back on the freelist. */
391  		list_add_tail(&meta->list, &kfence_freelist);
392  		raw_spin_unlock_irqrestore(&kfence_freelist_lock, flags);
393  
394  		return NULL;
395  	}
396  
397  	meta->addr = metadata_to_pageaddr(meta);
398  	/* Unprotect if we're reusing this page. */
399  	if (meta->state == KFENCE_OBJECT_FREED)
400  		kfence_unprotect(meta->addr);
401  
402  	/*
403  	 * Note: for allocations made before RNG initialization, will always
404  	 * return zero. We still benefit from enabling KFENCE as early as
405  	 * possible, even when the RNG is not yet available, as this will allow
406  	 * KFENCE to detect bugs due to earlier allocations. The only downside
407  	 * is that the out-of-bounds accesses detected are deterministic for
408  	 * such allocations.
409  	 */
410  	if (random_right_allocate) {
411  		/* Allocate on the "right" side, re-calculate address. */
412  		meta->addr += PAGE_SIZE - size;
413  		meta->addr = ALIGN_DOWN(meta->addr, cache->align);
414  	}
415  
416  	addr = (void *)meta->addr;
417  
418  	/* Update remaining metadata. */
419  	metadata_update_state(meta, KFENCE_OBJECT_ALLOCATED, stack_entries, num_stack_entries);
420  	/* Pairs with READ_ONCE() in kfence_shutdown_cache(). */
421  	WRITE_ONCE(meta->cache, cache);
422  	meta->size = size;
423  	meta->alloc_stack_hash = alloc_stack_hash;
424  	raw_spin_unlock_irqrestore(&meta->lock, flags);
425  
426  	alloc_covered_add(alloc_stack_hash, 1);
427  
428  	/* Set required slab fields. */
429  	slab = virt_to_slab((void *)meta->addr);
430  	slab->slab_cache = cache;
431  #if defined(CONFIG_SLUB)
432  	slab->objects = 1;
433  #elif defined(CONFIG_SLAB)
434  	slab->s_mem = addr;
435  #endif
436  
437  	/* Memory initialization. */
438  	for_each_canary(meta, set_canary_byte);
439  
440  	/*
441  	 * We check slab_want_init_on_alloc() ourselves, rather than letting
442  	 * SL*B do the initialization, as otherwise we might overwrite KFENCE's
443  	 * redzone.
444  	 */
445  	if (unlikely(slab_want_init_on_alloc(gfp, cache)))
446  		memzero_explicit(addr, size);
447  	if (cache->ctor)
448  		cache->ctor(addr);
449  
450  	if (random_fault)
451  		kfence_protect(meta->addr); /* Random "faults" by protecting the object. */
452  
453  	atomic_long_inc(&counters[KFENCE_COUNTER_ALLOCATED]);
454  	atomic_long_inc(&counters[KFENCE_COUNTER_ALLOCS]);
455  
456  	return addr;
457  }
458  
459  static void kfence_guarded_free(void *addr, struct kfence_metadata *meta, bool zombie)
460  {
461  	struct kcsan_scoped_access assert_page_exclusive;
462  	unsigned long flags;
463  	bool init;
464  
465  	raw_spin_lock_irqsave(&meta->lock, flags);
466  
467  	if (meta->state != KFENCE_OBJECT_ALLOCATED || meta->addr != (unsigned long)addr) {
468  		/* Invalid or double-free, bail out. */
469  		atomic_long_inc(&counters[KFENCE_COUNTER_BUGS]);
470  		kfence_report_error((unsigned long)addr, false, NULL, meta,
471  				    KFENCE_ERROR_INVALID_FREE);
472  		raw_spin_unlock_irqrestore(&meta->lock, flags);
473  		return;
474  	}
475  
476  	/* Detect racy use-after-free, or incorrect reallocation of this page by KFENCE. */
477  	kcsan_begin_scoped_access((void *)ALIGN_DOWN((unsigned long)addr, PAGE_SIZE), PAGE_SIZE,
478  				  KCSAN_ACCESS_SCOPED | KCSAN_ACCESS_WRITE | KCSAN_ACCESS_ASSERT,
479  				  &assert_page_exclusive);
480  
481  	if (CONFIG_KFENCE_STRESS_TEST_FAULTS)
482  		kfence_unprotect((unsigned long)addr); /* To check canary bytes. */
483  
484  	/* Restore page protection if there was an OOB access. */
485  	if (meta->unprotected_page) {
486  		memzero_explicit((void *)ALIGN_DOWN(meta->unprotected_page, PAGE_SIZE), PAGE_SIZE);
487  		kfence_protect(meta->unprotected_page);
488  		meta->unprotected_page = 0;
489  	}
490  
491  	/* Mark the object as freed. */
492  	metadata_update_state(meta, KFENCE_OBJECT_FREED, NULL, 0);
493  	init = slab_want_init_on_free(meta->cache);
494  	raw_spin_unlock_irqrestore(&meta->lock, flags);
495  
496  	alloc_covered_add(meta->alloc_stack_hash, -1);
497  
498  	/* Check canary bytes for memory corruption. */
499  	for_each_canary(meta, check_canary_byte);
500  
501  	/*
502  	 * Clear memory if init-on-free is set. While we protect the page, the
503  	 * data is still there, and after a use-after-free is detected, we
504  	 * unprotect the page, so the data is still accessible.
505  	 */
506  	if (!zombie && unlikely(init))
507  		memzero_explicit(addr, meta->size);
508  
509  	/* Protect to detect use-after-frees. */
510  	kfence_protect((unsigned long)addr);
511  
512  	kcsan_end_scoped_access(&assert_page_exclusive);
513  	if (!zombie) {
514  		/* Add it to the tail of the freelist for reuse. */
515  		raw_spin_lock_irqsave(&kfence_freelist_lock, flags);
516  		KFENCE_WARN_ON(!list_empty(&meta->list));
517  		list_add_tail(&meta->list, &kfence_freelist);
518  		raw_spin_unlock_irqrestore(&kfence_freelist_lock, flags);
519  
520  		atomic_long_dec(&counters[KFENCE_COUNTER_ALLOCATED]);
521  		atomic_long_inc(&counters[KFENCE_COUNTER_FREES]);
522  	} else {
523  		/* See kfence_shutdown_cache(). */
524  		atomic_long_inc(&counters[KFENCE_COUNTER_ZOMBIES]);
525  	}
526  }
527  
528  static void rcu_guarded_free(struct rcu_head *h)
529  {
530  	struct kfence_metadata *meta = container_of(h, struct kfence_metadata, rcu_head);
531  
532  	kfence_guarded_free((void *)meta->addr, meta, false);
533  }
534  
535  /*
536   * Initialization of the KFENCE pool after its allocation.
537   * Returns 0 on success; otherwise returns the address up to
538   * which partial initialization succeeded.
539   */
540  static unsigned long kfence_init_pool(void)
541  {
542  	unsigned long addr = (unsigned long)__kfence_pool;
543  	struct page *pages;
544  	int i;
545  
546  	if (!arch_kfence_init_pool())
547  		return addr;
548  
549  	pages = virt_to_page(__kfence_pool);
550  
551  	/*
552  	 * Set up object pages: they must have PG_slab set, to avoid freeing
553  	 * these as real pages.
554  	 *
555  	 * We also want to avoid inserting kfence_free() in the kfree()
556  	 * fast-path in SLUB, and therefore need to ensure kfree() correctly
557  	 * enters __slab_free() slow-path.
558  	 */
559  	for (i = 0; i < KFENCE_POOL_SIZE / PAGE_SIZE; i++) {
560  		struct slab *slab = page_slab(&pages[i]);
561  
562  		if (!i || (i % 2))
563  			continue;
564  
565  		/* Verify we do not have a compound head page. */
566  		if (WARN_ON(compound_head(&pages[i]) != &pages[i]))
567  			return addr;
568  
569  		__folio_set_slab(slab_folio(slab));
570  #ifdef CONFIG_MEMCG
571  		slab->memcg_data = (unsigned long)&kfence_metadata[i / 2 - 1].objcg |
572  				   MEMCG_DATA_OBJCGS;
573  #endif
574  	}
575  
576  	/*
577  	 * Protect the first 2 pages. The first page is mostly unnecessary, and
578  	 * merely serves as an extended guard page. However, adding one
579  	 * additional page in the beginning gives us an even number of pages,
580  	 * which simplifies the mapping of address to metadata index.
581  	 */
582  	for (i = 0; i < 2; i++) {
583  		if (unlikely(!kfence_protect(addr)))
584  			return addr;
585  
586  		addr += PAGE_SIZE;
587  	}
588  
589  	for (i = 0; i < CONFIG_KFENCE_NUM_OBJECTS; i++) {
590  		struct kfence_metadata *meta = &kfence_metadata[i];
591  
592  		/* Initialize metadata. */
593  		INIT_LIST_HEAD(&meta->list);
594  		raw_spin_lock_init(&meta->lock);
595  		meta->state = KFENCE_OBJECT_UNUSED;
596  		meta->addr = addr; /* Initialize for validation in metadata_to_pageaddr(). */
597  		list_add_tail(&meta->list, &kfence_freelist);
598  
599  		/* Protect the right redzone. */
600  		if (unlikely(!kfence_protect(addr + PAGE_SIZE)))
601  			return addr;
602  
603  		addr += 2 * PAGE_SIZE;
604  	}
605  
606  	return 0;
607  }
608  
609  static bool __init kfence_init_pool_early(void)
610  {
611  	unsigned long addr;
612  
613  	if (!__kfence_pool)
614  		return false;
615  
616  	addr = kfence_init_pool();
617  
618  	if (!addr) {
619  		/*
620  		 * The pool is live and will never be deallocated from this point on.
621  		 * Ignore the pool object from the kmemleak phys object tree, as it would
622  		 * otherwise overlap with allocations returned by kfence_alloc(), which
623  		 * are registered with kmemleak through the slab post-alloc hook.
624  		 */
625  		kmemleak_ignore_phys(__pa(__kfence_pool));
626  		return true;
627  	}
628  
629  	/*
630  	 * Only release unprotected pages, and do not try to go back and change
631  	 * page attributes due to risk of failing to do so as well. If changing
632  	 * page attributes for some pages fails, it is very likely that it also
633  	 * fails for the first page, and therefore expect addr==__kfence_pool in
634  	 * most failure cases.
635  	 */
636  	for (char *p = (char *)addr; p < __kfence_pool + KFENCE_POOL_SIZE; p += PAGE_SIZE) {
637  		struct slab *slab = virt_to_slab(p);
638  
639  		if (!slab)
640  			continue;
641  #ifdef CONFIG_MEMCG
642  		slab->memcg_data = 0;
643  #endif
644  		__folio_clear_slab(slab_folio(slab));
645  	}
646  	memblock_free_late(__pa(addr), KFENCE_POOL_SIZE - (addr - (unsigned long)__kfence_pool));
647  	__kfence_pool = NULL;
648  	return false;
649  }
650  
651  static bool kfence_init_pool_late(void)
652  {
653  	unsigned long addr, free_size;
654  
655  	addr = kfence_init_pool();
656  
657  	if (!addr)
658  		return true;
659  
660  	/* Same as above. */
661  	free_size = KFENCE_POOL_SIZE - (addr - (unsigned long)__kfence_pool);
662  #ifdef CONFIG_CONTIG_ALLOC
663  	free_contig_range(page_to_pfn(virt_to_page((void *)addr)), free_size / PAGE_SIZE);
664  #else
665  	free_pages_exact((void *)addr, free_size);
666  #endif
667  	__kfence_pool = NULL;
668  	return false;
669  }
670  
671  /* === DebugFS Interface ==================================================== */
672  
673  static int stats_show(struct seq_file *seq, void *v)
674  {
675  	int i;
676  
677  	seq_printf(seq, "enabled: %i\n", READ_ONCE(kfence_enabled));
678  	for (i = 0; i < KFENCE_COUNTER_COUNT; i++)
679  		seq_printf(seq, "%s: %ld\n", counter_names[i], atomic_long_read(&counters[i]));
680  
681  	return 0;
682  }
683  DEFINE_SHOW_ATTRIBUTE(stats);
684  
685  /*
686   * debugfs seq_file operations for /sys/kernel/debug/kfence/objects.
687   * start_object() and next_object() return the object index + 1, because NULL is used
688   * to stop iteration.
689   */
690  static void *start_object(struct seq_file *seq, loff_t *pos)
691  {
692  	if (*pos < CONFIG_KFENCE_NUM_OBJECTS)
693  		return (void *)((long)*pos + 1);
694  	return NULL;
695  }
696  
697  static void stop_object(struct seq_file *seq, void *v)
698  {
699  }
700  
701  static void *next_object(struct seq_file *seq, void *v, loff_t *pos)
702  {
703  	++*pos;
704  	if (*pos < CONFIG_KFENCE_NUM_OBJECTS)
705  		return (void *)((long)*pos + 1);
706  	return NULL;
707  }
708  
709  static int show_object(struct seq_file *seq, void *v)
710  {
711  	struct kfence_metadata *meta = &kfence_metadata[(long)v - 1];
712  	unsigned long flags;
713  
714  	raw_spin_lock_irqsave(&meta->lock, flags);
715  	kfence_print_object(seq, meta);
716  	raw_spin_unlock_irqrestore(&meta->lock, flags);
717  	seq_puts(seq, "---------------------------------\n");
718  
719  	return 0;
720  }
721  
722  static const struct seq_operations object_seqops = {
723  	.start = start_object,
724  	.next = next_object,
725  	.stop = stop_object,
726  	.show = show_object,
727  };
728  
729  static int open_objects(struct inode *inode, struct file *file)
730  {
731  	return seq_open(file, &object_seqops);
732  }
733  
734  static const struct file_operations objects_fops = {
735  	.open = open_objects,
736  	.read = seq_read,
737  	.llseek = seq_lseek,
738  	.release = seq_release,
739  };
740  
741  static int __init kfence_debugfs_init(void)
742  {
743  	struct dentry *kfence_dir = debugfs_create_dir("kfence", NULL);
744  
745  	debugfs_create_file("stats", 0444, kfence_dir, NULL, &stats_fops);
746  	debugfs_create_file("objects", 0400, kfence_dir, NULL, &objects_fops);
747  	return 0;
748  }
749  
750  late_initcall(kfence_debugfs_init);
751  
752  /* === Panic Notifier ====================================================== */
753  
754  static void kfence_check_all_canary(void)
755  {
756  	int i;
757  
758  	for (i = 0; i < CONFIG_KFENCE_NUM_OBJECTS; i++) {
759  		struct kfence_metadata *meta = &kfence_metadata[i];
760  
761  		if (meta->state == KFENCE_OBJECT_ALLOCATED)
762  			for_each_canary(meta, check_canary_byte);
763  	}
764  }
765  
766  static int kfence_check_canary_callback(struct notifier_block *nb,
767  					unsigned long reason, void *arg)
768  {
769  	kfence_check_all_canary();
770  	return NOTIFY_OK;
771  }
772  
773  static struct notifier_block kfence_check_canary_notifier = {
774  	.notifier_call = kfence_check_canary_callback,
775  };
776  
777  /* === Allocation Gate Timer ================================================ */
778  
779  static struct delayed_work kfence_timer;
780  
781  #ifdef CONFIG_KFENCE_STATIC_KEYS
782  /* Wait queue to wake up allocation-gate timer task. */
783  static DECLARE_WAIT_QUEUE_HEAD(allocation_wait);
784  
785  static void wake_up_kfence_timer(struct irq_work *work)
786  {
787  	wake_up(&allocation_wait);
788  }
789  static DEFINE_IRQ_WORK(wake_up_kfence_timer_work, wake_up_kfence_timer);
790  #endif
791  
792  /*
793   * Set up delayed work, which will enable and disable the static key. We need to
794   * use a work queue (rather than a simple timer), since enabling and disabling a
795   * static key cannot be done from an interrupt.
796   *
797   * Note: Toggling a static branch currently causes IPIs, and here we'll end up
798   * with a total of 2 IPIs to all CPUs. If this ends up a problem in future (with
799   * more aggressive sampling intervals), we could get away with a variant that
800   * avoids IPIs, at the cost of not immediately capturing allocations if the
801   * instructions remain cached.
802   */
803  static void toggle_allocation_gate(struct work_struct *work)
804  {
805  	if (!READ_ONCE(kfence_enabled))
806  		return;
807  
808  	atomic_set(&kfence_allocation_gate, 0);
809  #ifdef CONFIG_KFENCE_STATIC_KEYS
810  	/* Enable static key, and await allocation to happen. */
811  	static_branch_enable(&kfence_allocation_key);
812  
813  	if (sysctl_hung_task_timeout_secs) {
814  		/*
815  		 * During low activity with no allocations we might wait a
816  		 * while; let's avoid the hung task warning.
817  		 */
818  		wait_event_idle_timeout(allocation_wait, atomic_read(&kfence_allocation_gate),
819  					sysctl_hung_task_timeout_secs * HZ / 2);
820  	} else {
821  		wait_event_idle(allocation_wait, atomic_read(&kfence_allocation_gate));
822  	}
823  
824  	/* Disable static key and reset timer. */
825  	static_branch_disable(&kfence_allocation_key);
826  #endif
827  	queue_delayed_work(system_unbound_wq, &kfence_timer,
828  			   msecs_to_jiffies(kfence_sample_interval));
829  }
830  
831  /* === Public interface ===================================================== */
832  
833  void __init kfence_alloc_pool(void)
834  {
835  	if (!kfence_sample_interval)
836  		return;
837  
838  	__kfence_pool = memblock_alloc(KFENCE_POOL_SIZE, PAGE_SIZE);
839  
840  	if (!__kfence_pool)
841  		pr_err("failed to allocate pool\n");
842  }
843  
844  static void kfence_init_enable(void)
845  {
846  	if (!IS_ENABLED(CONFIG_KFENCE_STATIC_KEYS))
847  		static_branch_enable(&kfence_allocation_key);
848  
849  	if (kfence_deferrable)
850  		INIT_DEFERRABLE_WORK(&kfence_timer, toggle_allocation_gate);
851  	else
852  		INIT_DELAYED_WORK(&kfence_timer, toggle_allocation_gate);
853  
854  	if (kfence_check_on_panic)
855  		atomic_notifier_chain_register(&panic_notifier_list, &kfence_check_canary_notifier);
856  
857  	WRITE_ONCE(kfence_enabled, true);
858  	queue_delayed_work(system_unbound_wq, &kfence_timer, 0);
859  
860  	pr_info("initialized - using %lu bytes for %d objects at 0x%p-0x%p\n", KFENCE_POOL_SIZE,
861  		CONFIG_KFENCE_NUM_OBJECTS, (void *)__kfence_pool,
862  		(void *)(__kfence_pool + KFENCE_POOL_SIZE));
863  }
864  
865  void __init kfence_init(void)
866  {
867  	stack_hash_seed = (u32)random_get_entropy();
868  
869  	/* Setting kfence_sample_interval to 0 on boot disables KFENCE. */
870  	if (!kfence_sample_interval)
871  		return;
872  
873  	if (!kfence_init_pool_early()) {
874  		pr_err("%s failed\n", __func__);
875  		return;
876  	}
877  
878  	kfence_init_enable();
879  }
880  
881  static int kfence_init_late(void)
882  {
883  	const unsigned long nr_pages = KFENCE_POOL_SIZE / PAGE_SIZE;
884  #ifdef CONFIG_CONTIG_ALLOC
885  	struct page *pages;
886  
887  	pages = alloc_contig_pages(nr_pages, GFP_KERNEL, first_online_node, NULL);
888  	if (!pages)
889  		return -ENOMEM;
890  	__kfence_pool = page_to_virt(pages);
891  #else
892  	if (nr_pages > MAX_ORDER_NR_PAGES) {
893  		pr_warn("KFENCE_NUM_OBJECTS too large for buddy allocator\n");
894  		return -EINVAL;
895  	}
896  	__kfence_pool = alloc_pages_exact(KFENCE_POOL_SIZE, GFP_KERNEL);
897  	if (!__kfence_pool)
898  		return -ENOMEM;
899  #endif
900  
901  	if (!kfence_init_pool_late()) {
902  		pr_err("%s failed\n", __func__);
903  		return -EBUSY;
904  	}
905  
906  	kfence_init_enable();
907  	return 0;
908  }
909  
910  static int kfence_enable_late(void)
911  {
912  	if (!__kfence_pool)
913  		return kfence_init_late();
914  
915  	WRITE_ONCE(kfence_enabled, true);
916  	queue_delayed_work(system_unbound_wq, &kfence_timer, 0);
917  	pr_info("re-enabled\n");
918  	return 0;
919  }
920  
921  void kfence_shutdown_cache(struct kmem_cache *s)
922  {
923  	unsigned long flags;
924  	struct kfence_metadata *meta;
925  	int i;
926  
927  	for (i = 0; i < CONFIG_KFENCE_NUM_OBJECTS; i++) {
928  		bool in_use;
929  
930  		meta = &kfence_metadata[i];
931  
932  		/*
933  		 * If we observe some inconsistent cache and state pair where we
934  		 * should have returned false here, cache destruction is racing
935  		 * with either kmem_cache_alloc() or kmem_cache_free(). Taking
936  		 * the lock will not help, as different critical section
937  		 * serialization will have the same outcome.
938  		 */
939  		if (READ_ONCE(meta->cache) != s ||
940  		    READ_ONCE(meta->state) != KFENCE_OBJECT_ALLOCATED)
941  			continue;
942  
943  		raw_spin_lock_irqsave(&meta->lock, flags);
944  		in_use = meta->cache == s && meta->state == KFENCE_OBJECT_ALLOCATED;
945  		raw_spin_unlock_irqrestore(&meta->lock, flags);
946  
947  		if (in_use) {
948  			/*
949  			 * This cache still has allocations, and we should not
950  			 * release them back into the freelist so they can still
951  			 * safely be used and retain the kernel's default
952  			 * behaviour of keeping the allocations alive (leak the
953  			 * cache); however, they effectively become "zombie
954  			 * allocations" as the KFENCE objects are the only ones
955  			 * still in use and the owning cache is being destroyed.
956  			 *
957  			 * We mark them freed, so that any subsequent use shows
958  			 * more useful error messages that will include stack
959  			 * traces of the user of the object, the original
960  			 * allocation, and caller to shutdown_cache().
961  			 */
962  			kfence_guarded_free((void *)meta->addr, meta, /*zombie=*/true);
963  		}
964  	}
965  
966  	for (i = 0; i < CONFIG_KFENCE_NUM_OBJECTS; i++) {
967  		meta = &kfence_metadata[i];
968  
969  		/* See above. */
970  		if (READ_ONCE(meta->cache) != s || READ_ONCE(meta->state) != KFENCE_OBJECT_FREED)
971  			continue;
972  
973  		raw_spin_lock_irqsave(&meta->lock, flags);
974  		if (meta->cache == s && meta->state == KFENCE_OBJECT_FREED)
975  			meta->cache = NULL;
976  		raw_spin_unlock_irqrestore(&meta->lock, flags);
977  	}
978  }
979  
980  void *__kfence_alloc(struct kmem_cache *s, size_t size, gfp_t flags)
981  {
982  	unsigned long stack_entries[KFENCE_STACK_DEPTH];
983  	size_t num_stack_entries;
984  	u32 alloc_stack_hash;
985  
986  	/*
987  	 * Perform size check before switching kfence_allocation_gate, so that
988  	 * we don't disable KFENCE without making an allocation.
989  	 */
990  	if (size > PAGE_SIZE) {
991  		atomic_long_inc(&counters[KFENCE_COUNTER_SKIP_INCOMPAT]);
992  		return NULL;
993  	}
994  
995  	/*
996  	 * Skip allocations from non-default zones, including DMA. We cannot
997  	 * guarantee that pages in the KFENCE pool will have the requested
998  	 * properties (e.g. reside in DMAable memory).
999  	 */
1000  	if ((flags & GFP_ZONEMASK) ||
1001  	    (s->flags & (SLAB_CACHE_DMA | SLAB_CACHE_DMA32))) {
1002  		atomic_long_inc(&counters[KFENCE_COUNTER_SKIP_INCOMPAT]);
1003  		return NULL;
1004  	}
1005  
1006  	if (atomic_inc_return(&kfence_allocation_gate) > 1)
1007  		return NULL;
1008  #ifdef CONFIG_KFENCE_STATIC_KEYS
1009  	/*
1010  	 * waitqueue_active() is fully ordered after the update of
1011  	 * kfence_allocation_gate per atomic_inc_return().
1012  	 */
1013  	if (waitqueue_active(&allocation_wait)) {
1014  		/*
1015  		 * Calling wake_up() here may deadlock when allocations happen
1016  		 * from within timer code. Use an irq_work to defer it.
1017  		 */
1018  		irq_work_queue(&wake_up_kfence_timer_work);
1019  	}
1020  #endif
1021  
1022  	if (!READ_ONCE(kfence_enabled))
1023  		return NULL;
1024  
1025  	num_stack_entries = stack_trace_save(stack_entries, KFENCE_STACK_DEPTH, 0);
1026  
1027  	/*
1028  	 * Do expensive check for coverage of allocation in slow-path after
1029  	 * allocation_gate has already become non-zero, even though it might
1030  	 * mean not making any allocation within a given sample interval.
1031  	 *
1032  	 * This ensures reasonable allocation coverage when the pool is almost
1033  	 * full, including avoiding long-lived allocations of the same source
1034  	 * filling up the pool (e.g. pagecache allocations).
1035  	 */
1036  	alloc_stack_hash = get_alloc_stack_hash(stack_entries, num_stack_entries);
1037  	if (should_skip_covered() && alloc_covered_contains(alloc_stack_hash)) {
1038  		atomic_long_inc(&counters[KFENCE_COUNTER_SKIP_COVERED]);
1039  		return NULL;
1040  	}
1041  
1042  	return kfence_guarded_alloc(s, size, flags, stack_entries, num_stack_entries,
1043  				    alloc_stack_hash);
1044  }
1045  
1046  size_t kfence_ksize(const void *addr)
1047  {
1048  	const struct kfence_metadata *meta = addr_to_metadata((unsigned long)addr);
1049  
1050  	/*
1051  	 * Read locklessly -- if there is a race with __kfence_alloc(), this is
1052  	 * either a use-after-free or invalid access.
1053  	 */
1054  	return meta ? meta->size : 0;
1055  }
1056  
1057  void *kfence_object_start(const void *addr)
1058  {
1059  	const struct kfence_metadata *meta = addr_to_metadata((unsigned long)addr);
1060  
1061  	/*
1062  	 * Read locklessly -- if there is a race with __kfence_alloc(), this is
1063  	 * either a use-after-free or invalid access.
1064  	 */
1065  	return meta ? (void *)meta->addr : NULL;
1066  }
1067  
1068  void __kfence_free(void *addr)
1069  {
1070  	struct kfence_metadata *meta = addr_to_metadata((unsigned long)addr);
1071  
1072  #ifdef CONFIG_MEMCG
1073  	KFENCE_WARN_ON(meta->objcg);
1074  #endif
1075  	/*
1076  	 * If the objects of the cache are SLAB_TYPESAFE_BY_RCU, defer freeing
1077  	 * the object, as the object page may be recycled for other-typed
1078  	 * objects once it has been freed. meta->cache may be NULL if the cache
1079  	 * was destroyed.
1080  	 */
1081  	if (unlikely(meta->cache && (meta->cache->flags & SLAB_TYPESAFE_BY_RCU)))
1082  		call_rcu(&meta->rcu_head, rcu_guarded_free);
1083  	else
1084  		kfence_guarded_free(addr, meta, false);
1085  }
1086  
1087  bool kfence_handle_page_fault(unsigned long addr, bool is_write, struct pt_regs *regs)
1088  {
1089  	const int page_index = (addr - (unsigned long)__kfence_pool) / PAGE_SIZE;
1090  	struct kfence_metadata *to_report = NULL;
1091  	enum kfence_error_type error_type;
1092  	unsigned long flags;
1093  
1094  	if (!is_kfence_address((void *)addr))
1095  		return false;
1096  
1097  	if (!READ_ONCE(kfence_enabled)) /* If disabled at runtime ... */
1098  		return kfence_unprotect(addr); /* ... unprotect and proceed. */
1099  
1100  	atomic_long_inc(&counters[KFENCE_COUNTER_BUGS]);
1101  
1102  	if (page_index % 2) {
1103  		/* This is a redzone, report a buffer overflow. */
1104  		struct kfence_metadata *meta;
1105  		int distance = 0;
1106  
1107  		meta = addr_to_metadata(addr - PAGE_SIZE);
1108  		if (meta && READ_ONCE(meta->state) == KFENCE_OBJECT_ALLOCATED) {
1109  			to_report = meta;
1110  			/* Data race ok; distance calculation approximate. */
1111  			distance = addr - data_race(meta->addr + meta->size);
1112  		}
1113  
1114  		meta = addr_to_metadata(addr + PAGE_SIZE);
1115  		if (meta && READ_ONCE(meta->state) == KFENCE_OBJECT_ALLOCATED) {
1116  			/* Data race ok; distance calculation approximate. */
1117  			if (!to_report || distance > data_race(meta->addr) - addr)
1118  				to_report = meta;
1119  		}
1120  
1121  		if (!to_report)
1122  			goto out;
1123  
1124  		raw_spin_lock_irqsave(&to_report->lock, flags);
1125  		to_report->unprotected_page = addr;
1126  		error_type = KFENCE_ERROR_OOB;
1127  
1128  		/*
1129  		 * If the object was freed before we took the look we can still
1130  		 * report this as an OOB -- the report will simply show the
1131  		 * stacktrace of the free as well.
1132  		 */
1133  	} else {
1134  		to_report = addr_to_metadata(addr);
1135  		if (!to_report)
1136  			goto out;
1137  
1138  		raw_spin_lock_irqsave(&to_report->lock, flags);
1139  		error_type = KFENCE_ERROR_UAF;
1140  		/*
1141  		 * We may race with __kfence_alloc(), and it is possible that a
1142  		 * freed object may be reallocated. We simply report this as a
1143  		 * use-after-free, with the stack trace showing the place where
1144  		 * the object was re-allocated.
1145  		 */
1146  	}
1147  
1148  out:
1149  	if (to_report) {
1150  		kfence_report_error(addr, is_write, regs, to_report, error_type);
1151  		raw_spin_unlock_irqrestore(&to_report->lock, flags);
1152  	} else {
1153  		/* This may be a UAF or OOB access, but we can't be sure. */
1154  		kfence_report_error(addr, is_write, regs, NULL, KFENCE_ERROR_INVALID);
1155  	}
1156  
1157  	return kfence_unprotect(addr); /* Unprotect and let access proceed. */
1158  }
1159