xref: /openbmc/linux/mm/kfence/core.c (revision f412eef0)
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  
51  static bool kfence_enabled __read_mostly;
52  static bool disabled_by_warn __read_mostly;
53  
54  unsigned long kfence_sample_interval __read_mostly = CONFIG_KFENCE_SAMPLE_INTERVAL;
55  EXPORT_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  
62  static int kfence_enable_late(void);
63  static 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  
84  static 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  
92  static const struct kernel_param_ops sample_interval_param_ops = {
93  	.set = param_set_sample_interval,
94  	.get = param_get_sample_interval,
95  };
96  module_param_cb(sample_interval, &sample_interval_param_ops, &kfence_sample_interval, 0600);
97  
98  /* Pool usage% threshold when currently covered allocations are skipped. */
99  static unsigned long kfence_skip_covered_thresh __read_mostly = 75;
100  module_param_named(skip_covered_thresh, kfence_skip_covered_thresh, ulong, 0644);
101  
102  /* If true, use a deferrable timer. */
103  static bool kfence_deferrable __read_mostly = IS_ENABLED(CONFIG_KFENCE_DEFERRABLE);
104  module_param_named(deferrable, kfence_deferrable, bool, 0444);
105  
106  /* If true, check all canary bytes on panic. */
107  static bool kfence_check_on_panic __read_mostly;
108  module_param_named(check_on_panic, kfence_check_on_panic, bool, 0444);
109  
110  /* The pool of pages used for guard pages and objects. */
111  char *__kfence_pool __read_mostly;
112  EXPORT_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   */
118  static_assert(CONFIG_KFENCE_NUM_OBJECTS > 0);
119  struct kfence_metadata kfence_metadata[CONFIG_KFENCE_NUM_OBJECTS];
120  
121  /* Freelist with available objects. */
122  static struct list_head kfence_freelist = LIST_HEAD_INIT(kfence_freelist);
123  static 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   */
129  DEFINE_STATIC_KEY_FALSE(kfence_allocation_key);
130  
131  /* Gates the allocation, ensuring only one succeeds in a given period. */
132  atomic_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)
149  static 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   */
158  static u32 stack_hash_seed __ro_after_init;
159  
160  /* Statistics counters for debugfs. */
161  enum 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  };
172  static atomic_long_t counters[KFENCE_COUNTER_COUNT];
173  static 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  };
183  static_assert(ARRAY_SIZE(counter_names) == KFENCE_COUNTER_COUNT);
184  
185  /* === Internals ============================================================ */
186  
187  static 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  
194  static 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   */
205  static 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   */
219  static 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  
232  static bool kfence_protect(unsigned long addr)
233  {
234  	return !KFENCE_WARN_ON(!kfence_protect_page(ALIGN_DOWN(addr, PAGE_SIZE), true));
235  }
236  
237  static bool kfence_unprotect(unsigned long addr)
238  {
239  	return !KFENCE_WARN_ON(!kfence_protect_page(ALIGN_DOWN(addr, PAGE_SIZE), false));
240  }
241  
242  static 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   */
268  static noinline void
269  metadata_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. */
301  static 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. */
308  static 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. */
327  static __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  
354  static 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  
458  static 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  
527  static 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   */
539  static 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  
608  static 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  
650  static 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  
672  static 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  }
682  DEFINE_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   */
689  static 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  
696  static void stop_object(struct seq_file *seq, void *v)
697  {
698  }
699  
700  static 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  
708  static 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  
721  static const struct seq_operations objects_sops = {
722  	.start = start_object,
723  	.next = next_object,
724  	.stop = stop_object,
725  	.show = show_object,
726  };
727  DEFINE_SEQ_ATTRIBUTE(objects);
728  
729  static 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  
738  late_initcall(kfence_debugfs_init);
739  
740  /* === Panic Notifier ====================================================== */
741  
742  static 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  
754  static 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  
761  static struct notifier_block kfence_check_canary_notifier = {
762  	.notifier_call = kfence_check_canary_callback,
763  };
764  
765  /* === Allocation Gate Timer ================================================ */
766  
767  static struct delayed_work kfence_timer;
768  
769  #ifdef CONFIG_KFENCE_STATIC_KEYS
770  /* Wait queue to wake up allocation-gate timer task. */
771  static DECLARE_WAIT_QUEUE_HEAD(allocation_wait);
772  
773  static void wake_up_kfence_timer(struct irq_work *work)
774  {
775  	wake_up(&allocation_wait);
776  }
777  static 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   */
791  static 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  
812  void __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  
823  static 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  
844  void __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  
860  static 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  
889  static 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  
900  void 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  
959  void *__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  
1032  size_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  
1043  void *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  
1054  void __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  
1073  bool 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  
1134  out:
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  }
1145