xref: /openbmc/linux/mm/kfence/core.c (revision 7ac3945d)
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  
364  	/* Try to obtain a free object. */
365  	raw_spin_lock_irqsave(&kfence_freelist_lock, flags);
366  	if (!list_empty(&kfence_freelist)) {
367  		meta = list_entry(kfence_freelist.next, struct kfence_metadata, list);
368  		list_del_init(&meta->list);
369  	}
370  	raw_spin_unlock_irqrestore(&kfence_freelist_lock, flags);
371  	if (!meta) {
372  		atomic_long_inc(&counters[KFENCE_COUNTER_SKIP_CAPACITY]);
373  		return NULL;
374  	}
375  
376  	if (unlikely(!raw_spin_trylock_irqsave(&meta->lock, flags))) {
377  		/*
378  		 * This is extremely unlikely -- we are reporting on a
379  		 * use-after-free, which locked meta->lock, and the reporting
380  		 * code via printk calls kmalloc() which ends up in
381  		 * kfence_alloc() and tries to grab the same object that we're
382  		 * reporting on. While it has never been observed, lockdep does
383  		 * report that there is a possibility of deadlock. Fix it by
384  		 * using trylock and bailing out gracefully.
385  		 */
386  		raw_spin_lock_irqsave(&kfence_freelist_lock, flags);
387  		/* Put the object back on the freelist. */
388  		list_add_tail(&meta->list, &kfence_freelist);
389  		raw_spin_unlock_irqrestore(&kfence_freelist_lock, flags);
390  
391  		return NULL;
392  	}
393  
394  	meta->addr = metadata_to_pageaddr(meta);
395  	/* Unprotect if we're reusing this page. */
396  	if (meta->state == KFENCE_OBJECT_FREED)
397  		kfence_unprotect(meta->addr);
398  
399  	/*
400  	 * Note: for allocations made before RNG initialization, will always
401  	 * return zero. We still benefit from enabling KFENCE as early as
402  	 * possible, even when the RNG is not yet available, as this will allow
403  	 * KFENCE to detect bugs due to earlier allocations. The only downside
404  	 * is that the out-of-bounds accesses detected are deterministic for
405  	 * such allocations.
406  	 */
407  	if (prandom_u32_max(2)) {
408  		/* Allocate on the "right" side, re-calculate address. */
409  		meta->addr += PAGE_SIZE - size;
410  		meta->addr = ALIGN_DOWN(meta->addr, cache->align);
411  	}
412  
413  	addr = (void *)meta->addr;
414  
415  	/* Update remaining metadata. */
416  	metadata_update_state(meta, KFENCE_OBJECT_ALLOCATED, stack_entries, num_stack_entries);
417  	/* Pairs with READ_ONCE() in kfence_shutdown_cache(). */
418  	WRITE_ONCE(meta->cache, cache);
419  	meta->size = size;
420  	meta->alloc_stack_hash = alloc_stack_hash;
421  	raw_spin_unlock_irqrestore(&meta->lock, flags);
422  
423  	alloc_covered_add(alloc_stack_hash, 1);
424  
425  	/* Set required slab fields. */
426  	slab = virt_to_slab((void *)meta->addr);
427  	slab->slab_cache = cache;
428  #if defined(CONFIG_SLUB)
429  	slab->objects = 1;
430  #elif defined(CONFIG_SLAB)
431  	slab->s_mem = addr;
432  #endif
433  
434  	/* Memory initialization. */
435  	for_each_canary(meta, set_canary_byte);
436  
437  	/*
438  	 * We check slab_want_init_on_alloc() ourselves, rather than letting
439  	 * SL*B do the initialization, as otherwise we might overwrite KFENCE's
440  	 * redzone.
441  	 */
442  	if (unlikely(slab_want_init_on_alloc(gfp, cache)))
443  		memzero_explicit(addr, size);
444  	if (cache->ctor)
445  		cache->ctor(addr);
446  
447  	if (CONFIG_KFENCE_STRESS_TEST_FAULTS && !prandom_u32_max(CONFIG_KFENCE_STRESS_TEST_FAULTS))
448  		kfence_protect(meta->addr); /* Random "faults" by protecting the object. */
449  
450  	atomic_long_inc(&counters[KFENCE_COUNTER_ALLOCATED]);
451  	atomic_long_inc(&counters[KFENCE_COUNTER_ALLOCS]);
452  
453  	return addr;
454  }
455  
456  static void kfence_guarded_free(void *addr, struct kfence_metadata *meta, bool zombie)
457  {
458  	struct kcsan_scoped_access assert_page_exclusive;
459  	unsigned long flags;
460  	bool init;
461  
462  	raw_spin_lock_irqsave(&meta->lock, flags);
463  
464  	if (meta->state != KFENCE_OBJECT_ALLOCATED || meta->addr != (unsigned long)addr) {
465  		/* Invalid or double-free, bail out. */
466  		atomic_long_inc(&counters[KFENCE_COUNTER_BUGS]);
467  		kfence_report_error((unsigned long)addr, false, NULL, meta,
468  				    KFENCE_ERROR_INVALID_FREE);
469  		raw_spin_unlock_irqrestore(&meta->lock, flags);
470  		return;
471  	}
472  
473  	/* Detect racy use-after-free, or incorrect reallocation of this page by KFENCE. */
474  	kcsan_begin_scoped_access((void *)ALIGN_DOWN((unsigned long)addr, PAGE_SIZE), PAGE_SIZE,
475  				  KCSAN_ACCESS_SCOPED | KCSAN_ACCESS_WRITE | KCSAN_ACCESS_ASSERT,
476  				  &assert_page_exclusive);
477  
478  	if (CONFIG_KFENCE_STRESS_TEST_FAULTS)
479  		kfence_unprotect((unsigned long)addr); /* To check canary bytes. */
480  
481  	/* Restore page protection if there was an OOB access. */
482  	if (meta->unprotected_page) {
483  		memzero_explicit((void *)ALIGN_DOWN(meta->unprotected_page, PAGE_SIZE), PAGE_SIZE);
484  		kfence_protect(meta->unprotected_page);
485  		meta->unprotected_page = 0;
486  	}
487  
488  	/* Mark the object as freed. */
489  	metadata_update_state(meta, KFENCE_OBJECT_FREED, NULL, 0);
490  	init = slab_want_init_on_free(meta->cache);
491  	raw_spin_unlock_irqrestore(&meta->lock, flags);
492  
493  	alloc_covered_add(meta->alloc_stack_hash, -1);
494  
495  	/* Check canary bytes for memory corruption. */
496  	for_each_canary(meta, check_canary_byte);
497  
498  	/*
499  	 * Clear memory if init-on-free is set. While we protect the page, the
500  	 * data is still there, and after a use-after-free is detected, we
501  	 * unprotect the page, so the data is still accessible.
502  	 */
503  	if (!zombie && unlikely(init))
504  		memzero_explicit(addr, meta->size);
505  
506  	/* Protect to detect use-after-frees. */
507  	kfence_protect((unsigned long)addr);
508  
509  	kcsan_end_scoped_access(&assert_page_exclusive);
510  	if (!zombie) {
511  		/* Add it to the tail of the freelist for reuse. */
512  		raw_spin_lock_irqsave(&kfence_freelist_lock, flags);
513  		KFENCE_WARN_ON(!list_empty(&meta->list));
514  		list_add_tail(&meta->list, &kfence_freelist);
515  		raw_spin_unlock_irqrestore(&kfence_freelist_lock, flags);
516  
517  		atomic_long_dec(&counters[KFENCE_COUNTER_ALLOCATED]);
518  		atomic_long_inc(&counters[KFENCE_COUNTER_FREES]);
519  	} else {
520  		/* See kfence_shutdown_cache(). */
521  		atomic_long_inc(&counters[KFENCE_COUNTER_ZOMBIES]);
522  	}
523  }
524  
525  static void rcu_guarded_free(struct rcu_head *h)
526  {
527  	struct kfence_metadata *meta = container_of(h, struct kfence_metadata, rcu_head);
528  
529  	kfence_guarded_free((void *)meta->addr, meta, false);
530  }
531  
532  /*
533   * Initialization of the KFENCE pool after its allocation.
534   * Returns 0 on success; otherwise returns the address up to
535   * which partial initialization succeeded.
536   */
537  static unsigned long kfence_init_pool(void)
538  {
539  	unsigned long addr = (unsigned long)__kfence_pool;
540  	struct page *pages;
541  	int i;
542  
543  	if (!arch_kfence_init_pool())
544  		return addr;
545  
546  	pages = virt_to_page(addr);
547  
548  	/*
549  	 * Set up object pages: they must have PG_slab set, to avoid freeing
550  	 * these as real pages.
551  	 *
552  	 * We also want to avoid inserting kfence_free() in the kfree()
553  	 * fast-path in SLUB, and therefore need to ensure kfree() correctly
554  	 * enters __slab_free() slow-path.
555  	 */
556  	for (i = 0; i < KFENCE_POOL_SIZE / PAGE_SIZE; i++) {
557  		struct slab *slab = page_slab(&pages[i]);
558  
559  		if (!i || (i % 2))
560  			continue;
561  
562  		/* Verify we do not have a compound head page. */
563  		if (WARN_ON(compound_head(&pages[i]) != &pages[i]))
564  			return addr;
565  
566  		__folio_set_slab(slab_folio(slab));
567  #ifdef CONFIG_MEMCG
568  		slab->memcg_data = (unsigned long)&kfence_metadata[i / 2 - 1].objcg |
569  				   MEMCG_DATA_OBJCGS;
570  #endif
571  	}
572  
573  	/*
574  	 * Protect the first 2 pages. The first page is mostly unnecessary, and
575  	 * merely serves as an extended guard page. However, adding one
576  	 * additional page in the beginning gives us an even number of pages,
577  	 * which simplifies the mapping of address to metadata index.
578  	 */
579  	for (i = 0; i < 2; i++) {
580  		if (unlikely(!kfence_protect(addr)))
581  			return addr;
582  
583  		addr += PAGE_SIZE;
584  	}
585  
586  	for (i = 0; i < CONFIG_KFENCE_NUM_OBJECTS; i++) {
587  		struct kfence_metadata *meta = &kfence_metadata[i];
588  
589  		/* Initialize metadata. */
590  		INIT_LIST_HEAD(&meta->list);
591  		raw_spin_lock_init(&meta->lock);
592  		meta->state = KFENCE_OBJECT_UNUSED;
593  		meta->addr = addr; /* Initialize for validation in metadata_to_pageaddr(). */
594  		list_add_tail(&meta->list, &kfence_freelist);
595  
596  		/* Protect the right redzone. */
597  		if (unlikely(!kfence_protect(addr + PAGE_SIZE)))
598  			return addr;
599  
600  		addr += 2 * PAGE_SIZE;
601  	}
602  
603  	/*
604  	 * The pool is live and will never be deallocated from this point on.
605  	 * Remove the pool object from the kmemleak object tree, as it would
606  	 * otherwise overlap with allocations returned by kfence_alloc(), which
607  	 * are registered with kmemleak through the slab post-alloc hook.
608  	 */
609  	kmemleak_free(__kfence_pool);
610  
611  	return 0;
612  }
613  
614  static bool __init kfence_init_pool_early(void)
615  {
616  	unsigned long addr;
617  
618  	if (!__kfence_pool)
619  		return false;
620  
621  	addr = kfence_init_pool();
622  
623  	if (!addr)
624  		return true;
625  
626  	/*
627  	 * Only release unprotected pages, and do not try to go back and change
628  	 * page attributes due to risk of failing to do so as well. If changing
629  	 * page attributes for some pages fails, it is very likely that it also
630  	 * fails for the first page, and therefore expect addr==__kfence_pool in
631  	 * most failure cases.
632  	 */
633  	for (char *p = (char *)addr; p < __kfence_pool + KFENCE_POOL_SIZE; p += PAGE_SIZE) {
634  		struct slab *slab = virt_to_slab(p);
635  
636  		if (!slab)
637  			continue;
638  #ifdef CONFIG_MEMCG
639  		slab->memcg_data = 0;
640  #endif
641  		__folio_clear_slab(slab_folio(slab));
642  	}
643  	memblock_free_late(__pa(addr), KFENCE_POOL_SIZE - (addr - (unsigned long)__kfence_pool));
644  	__kfence_pool = NULL;
645  	return false;
646  }
647  
648  static bool kfence_init_pool_late(void)
649  {
650  	unsigned long addr, free_size;
651  
652  	addr = kfence_init_pool();
653  
654  	if (!addr)
655  		return true;
656  
657  	/* Same as above. */
658  	free_size = KFENCE_POOL_SIZE - (addr - (unsigned long)__kfence_pool);
659  #ifdef CONFIG_CONTIG_ALLOC
660  	free_contig_range(page_to_pfn(virt_to_page(addr)), free_size / PAGE_SIZE);
661  #else
662  	free_pages_exact((void *)addr, free_size);
663  #endif
664  	__kfence_pool = NULL;
665  	return false;
666  }
667  
668  /* === DebugFS Interface ==================================================== */
669  
670  static int stats_show(struct seq_file *seq, void *v)
671  {
672  	int i;
673  
674  	seq_printf(seq, "enabled: %i\n", READ_ONCE(kfence_enabled));
675  	for (i = 0; i < KFENCE_COUNTER_COUNT; i++)
676  		seq_printf(seq, "%s: %ld\n", counter_names[i], atomic_long_read(&counters[i]));
677  
678  	return 0;
679  }
680  DEFINE_SHOW_ATTRIBUTE(stats);
681  
682  /*
683   * debugfs seq_file operations for /sys/kernel/debug/kfence/objects.
684   * start_object() and next_object() return the object index + 1, because NULL is used
685   * to stop iteration.
686   */
687  static void *start_object(struct seq_file *seq, loff_t *pos)
688  {
689  	if (*pos < CONFIG_KFENCE_NUM_OBJECTS)
690  		return (void *)((long)*pos + 1);
691  	return NULL;
692  }
693  
694  static void stop_object(struct seq_file *seq, void *v)
695  {
696  }
697  
698  static void *next_object(struct seq_file *seq, void *v, loff_t *pos)
699  {
700  	++*pos;
701  	if (*pos < CONFIG_KFENCE_NUM_OBJECTS)
702  		return (void *)((long)*pos + 1);
703  	return NULL;
704  }
705  
706  static int show_object(struct seq_file *seq, void *v)
707  {
708  	struct kfence_metadata *meta = &kfence_metadata[(long)v - 1];
709  	unsigned long flags;
710  
711  	raw_spin_lock_irqsave(&meta->lock, flags);
712  	kfence_print_object(seq, meta);
713  	raw_spin_unlock_irqrestore(&meta->lock, flags);
714  	seq_puts(seq, "---------------------------------\n");
715  
716  	return 0;
717  }
718  
719  static const struct seq_operations object_seqops = {
720  	.start = start_object,
721  	.next = next_object,
722  	.stop = stop_object,
723  	.show = show_object,
724  };
725  
726  static int open_objects(struct inode *inode, struct file *file)
727  {
728  	return seq_open(file, &object_seqops);
729  }
730  
731  static const struct file_operations objects_fops = {
732  	.open = open_objects,
733  	.read = seq_read,
734  	.llseek = seq_lseek,
735  	.release = seq_release,
736  };
737  
738  static int __init kfence_debugfs_init(void)
739  {
740  	struct dentry *kfence_dir = debugfs_create_dir("kfence", NULL);
741  
742  	debugfs_create_file("stats", 0444, kfence_dir, NULL, &stats_fops);
743  	debugfs_create_file("objects", 0400, kfence_dir, NULL, &objects_fops);
744  	return 0;
745  }
746  
747  late_initcall(kfence_debugfs_init);
748  
749  /* === Panic Notifier ====================================================== */
750  
751  static void kfence_check_all_canary(void)
752  {
753  	int i;
754  
755  	for (i = 0; i < CONFIG_KFENCE_NUM_OBJECTS; i++) {
756  		struct kfence_metadata *meta = &kfence_metadata[i];
757  
758  		if (meta->state == KFENCE_OBJECT_ALLOCATED)
759  			for_each_canary(meta, check_canary_byte);
760  	}
761  }
762  
763  static int kfence_check_canary_callback(struct notifier_block *nb,
764  					unsigned long reason, void *arg)
765  {
766  	kfence_check_all_canary();
767  	return NOTIFY_OK;
768  }
769  
770  static struct notifier_block kfence_check_canary_notifier = {
771  	.notifier_call = kfence_check_canary_callback,
772  };
773  
774  /* === Allocation Gate Timer ================================================ */
775  
776  static struct delayed_work kfence_timer;
777  
778  #ifdef CONFIG_KFENCE_STATIC_KEYS
779  /* Wait queue to wake up allocation-gate timer task. */
780  static DECLARE_WAIT_QUEUE_HEAD(allocation_wait);
781  
782  static void wake_up_kfence_timer(struct irq_work *work)
783  {
784  	wake_up(&allocation_wait);
785  }
786  static DEFINE_IRQ_WORK(wake_up_kfence_timer_work, wake_up_kfence_timer);
787  #endif
788  
789  /*
790   * Set up delayed work, which will enable and disable the static key. We need to
791   * use a work queue (rather than a simple timer), since enabling and disabling a
792   * static key cannot be done from an interrupt.
793   *
794   * Note: Toggling a static branch currently causes IPIs, and here we'll end up
795   * with a total of 2 IPIs to all CPUs. If this ends up a problem in future (with
796   * more aggressive sampling intervals), we could get away with a variant that
797   * avoids IPIs, at the cost of not immediately capturing allocations if the
798   * instructions remain cached.
799   */
800  static void toggle_allocation_gate(struct work_struct *work)
801  {
802  	if (!READ_ONCE(kfence_enabled))
803  		return;
804  
805  	atomic_set(&kfence_allocation_gate, 0);
806  #ifdef CONFIG_KFENCE_STATIC_KEYS
807  	/* Enable static key, and await allocation to happen. */
808  	static_branch_enable(&kfence_allocation_key);
809  
810  	if (sysctl_hung_task_timeout_secs) {
811  		/*
812  		 * During low activity with no allocations we might wait a
813  		 * while; let's avoid the hung task warning.
814  		 */
815  		wait_event_idle_timeout(allocation_wait, atomic_read(&kfence_allocation_gate),
816  					sysctl_hung_task_timeout_secs * HZ / 2);
817  	} else {
818  		wait_event_idle(allocation_wait, atomic_read(&kfence_allocation_gate));
819  	}
820  
821  	/* Disable static key and reset timer. */
822  	static_branch_disable(&kfence_allocation_key);
823  #endif
824  	queue_delayed_work(system_unbound_wq, &kfence_timer,
825  			   msecs_to_jiffies(kfence_sample_interval));
826  }
827  
828  /* === Public interface ===================================================== */
829  
830  void __init kfence_alloc_pool(void)
831  {
832  	if (!kfence_sample_interval)
833  		return;
834  
835  	__kfence_pool = memblock_alloc(KFENCE_POOL_SIZE, PAGE_SIZE);
836  
837  	if (!__kfence_pool)
838  		pr_err("failed to allocate pool\n");
839  }
840  
841  static void kfence_init_enable(void)
842  {
843  	if (!IS_ENABLED(CONFIG_KFENCE_STATIC_KEYS))
844  		static_branch_enable(&kfence_allocation_key);
845  
846  	if (kfence_deferrable)
847  		INIT_DEFERRABLE_WORK(&kfence_timer, toggle_allocation_gate);
848  	else
849  		INIT_DELAYED_WORK(&kfence_timer, toggle_allocation_gate);
850  
851  	if (kfence_check_on_panic)
852  		atomic_notifier_chain_register(&panic_notifier_list, &kfence_check_canary_notifier);
853  
854  	WRITE_ONCE(kfence_enabled, true);
855  	queue_delayed_work(system_unbound_wq, &kfence_timer, 0);
856  
857  	pr_info("initialized - using %lu bytes for %d objects at 0x%p-0x%p\n", KFENCE_POOL_SIZE,
858  		CONFIG_KFENCE_NUM_OBJECTS, (void *)__kfence_pool,
859  		(void *)(__kfence_pool + KFENCE_POOL_SIZE));
860  }
861  
862  void __init kfence_init(void)
863  {
864  	stack_hash_seed = (u32)random_get_entropy();
865  
866  	/* Setting kfence_sample_interval to 0 on boot disables KFENCE. */
867  	if (!kfence_sample_interval)
868  		return;
869  
870  	if (!kfence_init_pool_early()) {
871  		pr_err("%s failed\n", __func__);
872  		return;
873  	}
874  
875  	kfence_init_enable();
876  }
877  
878  static int kfence_init_late(void)
879  {
880  	const unsigned long nr_pages = KFENCE_POOL_SIZE / PAGE_SIZE;
881  #ifdef CONFIG_CONTIG_ALLOC
882  	struct page *pages;
883  
884  	pages = alloc_contig_pages(nr_pages, GFP_KERNEL, first_online_node, NULL);
885  	if (!pages)
886  		return -ENOMEM;
887  	__kfence_pool = page_to_virt(pages);
888  #else
889  	if (nr_pages > MAX_ORDER_NR_PAGES) {
890  		pr_warn("KFENCE_NUM_OBJECTS too large for buddy allocator\n");
891  		return -EINVAL;
892  	}
893  	__kfence_pool = alloc_pages_exact(KFENCE_POOL_SIZE, GFP_KERNEL);
894  	if (!__kfence_pool)
895  		return -ENOMEM;
896  #endif
897  
898  	if (!kfence_init_pool_late()) {
899  		pr_err("%s failed\n", __func__);
900  		return -EBUSY;
901  	}
902  
903  	kfence_init_enable();
904  	return 0;
905  }
906  
907  static int kfence_enable_late(void)
908  {
909  	if (!__kfence_pool)
910  		return kfence_init_late();
911  
912  	WRITE_ONCE(kfence_enabled, true);
913  	queue_delayed_work(system_unbound_wq, &kfence_timer, 0);
914  	pr_info("re-enabled\n");
915  	return 0;
916  }
917  
918  void kfence_shutdown_cache(struct kmem_cache *s)
919  {
920  	unsigned long flags;
921  	struct kfence_metadata *meta;
922  	int i;
923  
924  	for (i = 0; i < CONFIG_KFENCE_NUM_OBJECTS; i++) {
925  		bool in_use;
926  
927  		meta = &kfence_metadata[i];
928  
929  		/*
930  		 * If we observe some inconsistent cache and state pair where we
931  		 * should have returned false here, cache destruction is racing
932  		 * with either kmem_cache_alloc() or kmem_cache_free(). Taking
933  		 * the lock will not help, as different critical section
934  		 * serialization will have the same outcome.
935  		 */
936  		if (READ_ONCE(meta->cache) != s ||
937  		    READ_ONCE(meta->state) != KFENCE_OBJECT_ALLOCATED)
938  			continue;
939  
940  		raw_spin_lock_irqsave(&meta->lock, flags);
941  		in_use = meta->cache == s && meta->state == KFENCE_OBJECT_ALLOCATED;
942  		raw_spin_unlock_irqrestore(&meta->lock, flags);
943  
944  		if (in_use) {
945  			/*
946  			 * This cache still has allocations, and we should not
947  			 * release them back into the freelist so they can still
948  			 * safely be used and retain the kernel's default
949  			 * behaviour of keeping the allocations alive (leak the
950  			 * cache); however, they effectively become "zombie
951  			 * allocations" as the KFENCE objects are the only ones
952  			 * still in use and the owning cache is being destroyed.
953  			 *
954  			 * We mark them freed, so that any subsequent use shows
955  			 * more useful error messages that will include stack
956  			 * traces of the user of the object, the original
957  			 * allocation, and caller to shutdown_cache().
958  			 */
959  			kfence_guarded_free((void *)meta->addr, meta, /*zombie=*/true);
960  		}
961  	}
962  
963  	for (i = 0; i < CONFIG_KFENCE_NUM_OBJECTS; i++) {
964  		meta = &kfence_metadata[i];
965  
966  		/* See above. */
967  		if (READ_ONCE(meta->cache) != s || READ_ONCE(meta->state) != KFENCE_OBJECT_FREED)
968  			continue;
969  
970  		raw_spin_lock_irqsave(&meta->lock, flags);
971  		if (meta->cache == s && meta->state == KFENCE_OBJECT_FREED)
972  			meta->cache = NULL;
973  		raw_spin_unlock_irqrestore(&meta->lock, flags);
974  	}
975  }
976  
977  void *__kfence_alloc(struct kmem_cache *s, size_t size, gfp_t flags)
978  {
979  	unsigned long stack_entries[KFENCE_STACK_DEPTH];
980  	size_t num_stack_entries;
981  	u32 alloc_stack_hash;
982  
983  	/*
984  	 * Perform size check before switching kfence_allocation_gate, so that
985  	 * we don't disable KFENCE without making an allocation.
986  	 */
987  	if (size > PAGE_SIZE) {
988  		atomic_long_inc(&counters[KFENCE_COUNTER_SKIP_INCOMPAT]);
989  		return NULL;
990  	}
991  
992  	/*
993  	 * Skip allocations from non-default zones, including DMA. We cannot
994  	 * guarantee that pages in the KFENCE pool will have the requested
995  	 * properties (e.g. reside in DMAable memory).
996  	 */
997  	if ((flags & GFP_ZONEMASK) ||
998  	    (s->flags & (SLAB_CACHE_DMA | SLAB_CACHE_DMA32))) {
999  		atomic_long_inc(&counters[KFENCE_COUNTER_SKIP_INCOMPAT]);
1000  		return NULL;
1001  	}
1002  
1003  	if (atomic_inc_return(&kfence_allocation_gate) > 1)
1004  		return NULL;
1005  #ifdef CONFIG_KFENCE_STATIC_KEYS
1006  	/*
1007  	 * waitqueue_active() is fully ordered after the update of
1008  	 * kfence_allocation_gate per atomic_inc_return().
1009  	 */
1010  	if (waitqueue_active(&allocation_wait)) {
1011  		/*
1012  		 * Calling wake_up() here may deadlock when allocations happen
1013  		 * from within timer code. Use an irq_work to defer it.
1014  		 */
1015  		irq_work_queue(&wake_up_kfence_timer_work);
1016  	}
1017  #endif
1018  
1019  	if (!READ_ONCE(kfence_enabled))
1020  		return NULL;
1021  
1022  	num_stack_entries = stack_trace_save(stack_entries, KFENCE_STACK_DEPTH, 0);
1023  
1024  	/*
1025  	 * Do expensive check for coverage of allocation in slow-path after
1026  	 * allocation_gate has already become non-zero, even though it might
1027  	 * mean not making any allocation within a given sample interval.
1028  	 *
1029  	 * This ensures reasonable allocation coverage when the pool is almost
1030  	 * full, including avoiding long-lived allocations of the same source
1031  	 * filling up the pool (e.g. pagecache allocations).
1032  	 */
1033  	alloc_stack_hash = get_alloc_stack_hash(stack_entries, num_stack_entries);
1034  	if (should_skip_covered() && alloc_covered_contains(alloc_stack_hash)) {
1035  		atomic_long_inc(&counters[KFENCE_COUNTER_SKIP_COVERED]);
1036  		return NULL;
1037  	}
1038  
1039  	return kfence_guarded_alloc(s, size, flags, stack_entries, num_stack_entries,
1040  				    alloc_stack_hash);
1041  }
1042  
1043  size_t kfence_ksize(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 ? meta->size : 0;
1052  }
1053  
1054  void *kfence_object_start(const void *addr)
1055  {
1056  	const struct kfence_metadata *meta = addr_to_metadata((unsigned long)addr);
1057  
1058  	/*
1059  	 * Read locklessly -- if there is a race with __kfence_alloc(), this is
1060  	 * either a use-after-free or invalid access.
1061  	 */
1062  	return meta ? (void *)meta->addr : NULL;
1063  }
1064  
1065  void __kfence_free(void *addr)
1066  {
1067  	struct kfence_metadata *meta = addr_to_metadata((unsigned long)addr);
1068  
1069  #ifdef CONFIG_MEMCG
1070  	KFENCE_WARN_ON(meta->objcg);
1071  #endif
1072  	/*
1073  	 * If the objects of the cache are SLAB_TYPESAFE_BY_RCU, defer freeing
1074  	 * the object, as the object page may be recycled for other-typed
1075  	 * objects once it has been freed. meta->cache may be NULL if the cache
1076  	 * was destroyed.
1077  	 */
1078  	if (unlikely(meta->cache && (meta->cache->flags & SLAB_TYPESAFE_BY_RCU)))
1079  		call_rcu(&meta->rcu_head, rcu_guarded_free);
1080  	else
1081  		kfence_guarded_free(addr, meta, false);
1082  }
1083  
1084  bool kfence_handle_page_fault(unsigned long addr, bool is_write, struct pt_regs *regs)
1085  {
1086  	const int page_index = (addr - (unsigned long)__kfence_pool) / PAGE_SIZE;
1087  	struct kfence_metadata *to_report = NULL;
1088  	enum kfence_error_type error_type;
1089  	unsigned long flags;
1090  
1091  	if (!is_kfence_address((void *)addr))
1092  		return false;
1093  
1094  	if (!READ_ONCE(kfence_enabled)) /* If disabled at runtime ... */
1095  		return kfence_unprotect(addr); /* ... unprotect and proceed. */
1096  
1097  	atomic_long_inc(&counters[KFENCE_COUNTER_BUGS]);
1098  
1099  	if (page_index % 2) {
1100  		/* This is a redzone, report a buffer overflow. */
1101  		struct kfence_metadata *meta;
1102  		int distance = 0;
1103  
1104  		meta = addr_to_metadata(addr - PAGE_SIZE);
1105  		if (meta && READ_ONCE(meta->state) == KFENCE_OBJECT_ALLOCATED) {
1106  			to_report = meta;
1107  			/* Data race ok; distance calculation approximate. */
1108  			distance = addr - data_race(meta->addr + meta->size);
1109  		}
1110  
1111  		meta = addr_to_metadata(addr + PAGE_SIZE);
1112  		if (meta && READ_ONCE(meta->state) == KFENCE_OBJECT_ALLOCATED) {
1113  			/* Data race ok; distance calculation approximate. */
1114  			if (!to_report || distance > data_race(meta->addr) - addr)
1115  				to_report = meta;
1116  		}
1117  
1118  		if (!to_report)
1119  			goto out;
1120  
1121  		raw_spin_lock_irqsave(&to_report->lock, flags);
1122  		to_report->unprotected_page = addr;
1123  		error_type = KFENCE_ERROR_OOB;
1124  
1125  		/*
1126  		 * If the object was freed before we took the look we can still
1127  		 * report this as an OOB -- the report will simply show the
1128  		 * stacktrace of the free as well.
1129  		 */
1130  	} else {
1131  		to_report = addr_to_metadata(addr);
1132  		if (!to_report)
1133  			goto out;
1134  
1135  		raw_spin_lock_irqsave(&to_report->lock, flags);
1136  		error_type = KFENCE_ERROR_UAF;
1137  		/*
1138  		 * We may race with __kfence_alloc(), and it is possible that a
1139  		 * freed object may be reallocated. We simply report this as a
1140  		 * use-after-free, with the stack trace showing the place where
1141  		 * the object was re-allocated.
1142  		 */
1143  	}
1144  
1145  out:
1146  	if (to_report) {
1147  		kfence_report_error(addr, is_write, regs, to_report, error_type);
1148  		raw_spin_unlock_irqrestore(&to_report->lock, flags);
1149  	} else {
1150  		/* This may be a UAF or OOB access, but we can't be sure. */
1151  		kfence_report_error(addr, is_write, regs, NULL, KFENCE_ERROR_INVALID);
1152  	}
1153  
1154  	return kfence_unprotect(addr); /* Unprotect and let access proceed. */
1155  }
1156