xref: /linux/mm/kfence/core.c (revision 3a39d672e7f48b8d6b91a09afa4b55352773b4b5)
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);
param_set_sample_interval(const char * val,const struct kernel_param * kp)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 
param_get_sample_interval(char * buffer,const struct kernel_param * kp)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 /* Allocation burst count: number of excess KFENCE allocations per sample. */
103 static unsigned int kfence_burst __read_mostly;
104 module_param_named(burst, kfence_burst, uint, 0644);
105 
106 /* If true, use a deferrable timer. */
107 static bool kfence_deferrable __read_mostly = IS_ENABLED(CONFIG_KFENCE_DEFERRABLE);
108 module_param_named(deferrable, kfence_deferrable, bool, 0444);
109 
110 /* If true, check all canary bytes on panic. */
111 static bool kfence_check_on_panic __read_mostly;
112 module_param_named(check_on_panic, kfence_check_on_panic, bool, 0444);
113 
114 /* The pool of pages used for guard pages and objects. */
115 char *__kfence_pool __read_mostly;
116 EXPORT_SYMBOL(__kfence_pool); /* Export for test modules. */
117 
118 /*
119  * Per-object metadata, with one-to-one mapping of object metadata to
120  * backing pages (in __kfence_pool).
121  */
122 static_assert(CONFIG_KFENCE_NUM_OBJECTS > 0);
123 struct kfence_metadata *kfence_metadata __read_mostly;
124 
125 /*
126  * If kfence_metadata is not NULL, it may be accessed by kfence_shutdown_cache().
127  * So introduce kfence_metadata_init to initialize metadata, and then make
128  * kfence_metadata visible after initialization is successful. This prevents
129  * potential UAF or access to uninitialized metadata.
130  */
131 static struct kfence_metadata *kfence_metadata_init __read_mostly;
132 
133 /* Freelist with available objects. */
134 static struct list_head kfence_freelist = LIST_HEAD_INIT(kfence_freelist);
135 static DEFINE_RAW_SPINLOCK(kfence_freelist_lock); /* Lock protecting freelist. */
136 
137 /*
138  * The static key to set up a KFENCE allocation; or if static keys are not used
139  * to gate allocations, to avoid a load and compare if KFENCE is disabled.
140  */
141 DEFINE_STATIC_KEY_FALSE(kfence_allocation_key);
142 
143 /* Gates the allocation, ensuring only one succeeds in a given period. */
144 atomic_t kfence_allocation_gate = ATOMIC_INIT(1);
145 
146 /*
147  * A Counting Bloom filter of allocation coverage: limits currently covered
148  * allocations of the same source filling up the pool.
149  *
150  * Assuming a range of 15%-85% unique allocations in the pool at any point in
151  * time, the below parameters provide a probablity of 0.02-0.33 for false
152  * positive hits respectively:
153  *
154  *	P(alloc_traces) = (1 - e^(-HNUM * (alloc_traces / SIZE)) ^ HNUM
155  */
156 #define ALLOC_COVERED_HNUM	2
157 #define ALLOC_COVERED_ORDER	(const_ilog2(CONFIG_KFENCE_NUM_OBJECTS) + 2)
158 #define ALLOC_COVERED_SIZE	(1 << ALLOC_COVERED_ORDER)
159 #define ALLOC_COVERED_HNEXT(h)	hash_32(h, ALLOC_COVERED_ORDER)
160 #define ALLOC_COVERED_MASK	(ALLOC_COVERED_SIZE - 1)
161 static atomic_t alloc_covered[ALLOC_COVERED_SIZE];
162 
163 /* Stack depth used to determine uniqueness of an allocation. */
164 #define UNIQUE_ALLOC_STACK_DEPTH ((size_t)8)
165 
166 /*
167  * Randomness for stack hashes, making the same collisions across reboots and
168  * different machines less likely.
169  */
170 static u32 stack_hash_seed __ro_after_init;
171 
172 /* Statistics counters for debugfs. */
173 enum kfence_counter_id {
174 	KFENCE_COUNTER_ALLOCATED,
175 	KFENCE_COUNTER_ALLOCS,
176 	KFENCE_COUNTER_FREES,
177 	KFENCE_COUNTER_ZOMBIES,
178 	KFENCE_COUNTER_BUGS,
179 	KFENCE_COUNTER_SKIP_INCOMPAT,
180 	KFENCE_COUNTER_SKIP_CAPACITY,
181 	KFENCE_COUNTER_SKIP_COVERED,
182 	KFENCE_COUNTER_COUNT,
183 };
184 static atomic_long_t counters[KFENCE_COUNTER_COUNT];
185 static const char *const counter_names[] = {
186 	[KFENCE_COUNTER_ALLOCATED]	= "currently allocated",
187 	[KFENCE_COUNTER_ALLOCS]		= "total allocations",
188 	[KFENCE_COUNTER_FREES]		= "total frees",
189 	[KFENCE_COUNTER_ZOMBIES]	= "zombie allocations",
190 	[KFENCE_COUNTER_BUGS]		= "total bugs",
191 	[KFENCE_COUNTER_SKIP_INCOMPAT]	= "skipped allocations (incompatible)",
192 	[KFENCE_COUNTER_SKIP_CAPACITY]	= "skipped allocations (capacity)",
193 	[KFENCE_COUNTER_SKIP_COVERED]	= "skipped allocations (covered)",
194 };
195 static_assert(ARRAY_SIZE(counter_names) == KFENCE_COUNTER_COUNT);
196 
197 /* === Internals ============================================================ */
198 
should_skip_covered(void)199 static inline bool should_skip_covered(void)
200 {
201 	unsigned long thresh = (CONFIG_KFENCE_NUM_OBJECTS * kfence_skip_covered_thresh) / 100;
202 
203 	return atomic_long_read(&counters[KFENCE_COUNTER_ALLOCATED]) > thresh;
204 }
205 
get_alloc_stack_hash(unsigned long * stack_entries,size_t num_entries)206 static u32 get_alloc_stack_hash(unsigned long *stack_entries, size_t num_entries)
207 {
208 	num_entries = min(num_entries, UNIQUE_ALLOC_STACK_DEPTH);
209 	num_entries = filter_irq_stacks(stack_entries, num_entries);
210 	return jhash(stack_entries, num_entries * sizeof(stack_entries[0]), stack_hash_seed);
211 }
212 
213 /*
214  * Adds (or subtracts) count @val for allocation stack trace hash
215  * @alloc_stack_hash from Counting Bloom filter.
216  */
alloc_covered_add(u32 alloc_stack_hash,int val)217 static void alloc_covered_add(u32 alloc_stack_hash, int val)
218 {
219 	int i;
220 
221 	for (i = 0; i < ALLOC_COVERED_HNUM; i++) {
222 		atomic_add(val, &alloc_covered[alloc_stack_hash & ALLOC_COVERED_MASK]);
223 		alloc_stack_hash = ALLOC_COVERED_HNEXT(alloc_stack_hash);
224 	}
225 }
226 
227 /*
228  * Returns true if the allocation stack trace hash @alloc_stack_hash is
229  * currently contained (non-zero count) in Counting Bloom filter.
230  */
alloc_covered_contains(u32 alloc_stack_hash)231 static bool alloc_covered_contains(u32 alloc_stack_hash)
232 {
233 	int i;
234 
235 	for (i = 0; i < ALLOC_COVERED_HNUM; i++) {
236 		if (!atomic_read(&alloc_covered[alloc_stack_hash & ALLOC_COVERED_MASK]))
237 			return false;
238 		alloc_stack_hash = ALLOC_COVERED_HNEXT(alloc_stack_hash);
239 	}
240 
241 	return true;
242 }
243 
kfence_protect(unsigned long addr)244 static bool kfence_protect(unsigned long addr)
245 {
246 	return !KFENCE_WARN_ON(!kfence_protect_page(ALIGN_DOWN(addr, PAGE_SIZE), true));
247 }
248 
kfence_unprotect(unsigned long addr)249 static bool kfence_unprotect(unsigned long addr)
250 {
251 	return !KFENCE_WARN_ON(!kfence_protect_page(ALIGN_DOWN(addr, PAGE_SIZE), false));
252 }
253 
metadata_to_pageaddr(const struct kfence_metadata * meta)254 static inline unsigned long metadata_to_pageaddr(const struct kfence_metadata *meta)
255 {
256 	unsigned long offset = (meta - kfence_metadata + 1) * PAGE_SIZE * 2;
257 	unsigned long pageaddr = (unsigned long)&__kfence_pool[offset];
258 
259 	/* The checks do not affect performance; only called from slow-paths. */
260 
261 	/* Only call with a pointer into kfence_metadata. */
262 	if (KFENCE_WARN_ON(meta < kfence_metadata ||
263 			   meta >= kfence_metadata + CONFIG_KFENCE_NUM_OBJECTS))
264 		return 0;
265 
266 	/*
267 	 * This metadata object only ever maps to 1 page; verify that the stored
268 	 * address is in the expected range.
269 	 */
270 	if (KFENCE_WARN_ON(ALIGN_DOWN(meta->addr, PAGE_SIZE) != pageaddr))
271 		return 0;
272 
273 	return pageaddr;
274 }
275 
kfence_obj_allocated(const struct kfence_metadata * meta)276 static inline bool kfence_obj_allocated(const struct kfence_metadata *meta)
277 {
278 	enum kfence_object_state state = READ_ONCE(meta->state);
279 
280 	return state == KFENCE_OBJECT_ALLOCATED || state == KFENCE_OBJECT_RCU_FREEING;
281 }
282 
283 /*
284  * Update the object's metadata state, including updating the alloc/free stacks
285  * depending on the state transition.
286  */
287 static noinline void
metadata_update_state(struct kfence_metadata * meta,enum kfence_object_state next,unsigned long * stack_entries,size_t num_stack_entries)288 metadata_update_state(struct kfence_metadata *meta, enum kfence_object_state next,
289 		      unsigned long *stack_entries, size_t num_stack_entries)
290 {
291 	struct kfence_track *track =
292 		next == KFENCE_OBJECT_ALLOCATED ? &meta->alloc_track : &meta->free_track;
293 
294 	lockdep_assert_held(&meta->lock);
295 
296 	/* Stack has been saved when calling rcu, skip. */
297 	if (READ_ONCE(meta->state) == KFENCE_OBJECT_RCU_FREEING)
298 		goto out;
299 
300 	if (stack_entries) {
301 		memcpy(track->stack_entries, stack_entries,
302 		       num_stack_entries * sizeof(stack_entries[0]));
303 	} else {
304 		/*
305 		 * Skip over 1 (this) functions; noinline ensures we do not
306 		 * accidentally skip over the caller by never inlining.
307 		 */
308 		num_stack_entries = stack_trace_save(track->stack_entries, KFENCE_STACK_DEPTH, 1);
309 	}
310 	track->num_stack_entries = num_stack_entries;
311 	track->pid = task_pid_nr(current);
312 	track->cpu = raw_smp_processor_id();
313 	track->ts_nsec = local_clock(); /* Same source as printk timestamps. */
314 
315 out:
316 	/*
317 	 * Pairs with READ_ONCE() in
318 	 *	kfence_shutdown_cache(),
319 	 *	kfence_handle_page_fault().
320 	 */
321 	WRITE_ONCE(meta->state, next);
322 }
323 
324 #ifdef CONFIG_KMSAN
325 #define check_canary_attributes noinline __no_kmsan_checks
326 #else
327 #define check_canary_attributes inline
328 #endif
329 
330 /* Check canary byte at @addr. */
check_canary_byte(u8 * addr)331 static check_canary_attributes bool check_canary_byte(u8 *addr)
332 {
333 	struct kfence_metadata *meta;
334 	unsigned long flags;
335 
336 	if (likely(*addr == KFENCE_CANARY_PATTERN_U8(addr)))
337 		return true;
338 
339 	atomic_long_inc(&counters[KFENCE_COUNTER_BUGS]);
340 
341 	meta = addr_to_metadata((unsigned long)addr);
342 	raw_spin_lock_irqsave(&meta->lock, flags);
343 	kfence_report_error((unsigned long)addr, false, NULL, meta, KFENCE_ERROR_CORRUPTION);
344 	raw_spin_unlock_irqrestore(&meta->lock, flags);
345 
346 	return false;
347 }
348 
set_canary(const struct kfence_metadata * meta)349 static inline void set_canary(const struct kfence_metadata *meta)
350 {
351 	const unsigned long pageaddr = ALIGN_DOWN(meta->addr, PAGE_SIZE);
352 	unsigned long addr = pageaddr;
353 
354 	/*
355 	 * The canary may be written to part of the object memory, but it does
356 	 * not affect it. The user should initialize the object before using it.
357 	 */
358 	for (; addr < meta->addr; addr += sizeof(u64))
359 		*((u64 *)addr) = KFENCE_CANARY_PATTERN_U64;
360 
361 	addr = ALIGN_DOWN(meta->addr + meta->size, sizeof(u64));
362 	for (; addr - pageaddr < PAGE_SIZE; addr += sizeof(u64))
363 		*((u64 *)addr) = KFENCE_CANARY_PATTERN_U64;
364 }
365 
366 static check_canary_attributes void
check_canary(const struct kfence_metadata * meta)367 check_canary(const struct kfence_metadata *meta)
368 {
369 	const unsigned long pageaddr = ALIGN_DOWN(meta->addr, PAGE_SIZE);
370 	unsigned long addr = pageaddr;
371 
372 	/*
373 	 * We'll iterate over each canary byte per-side until a corrupted byte
374 	 * is found. However, we'll still iterate over the canary bytes to the
375 	 * right of the object even if there was an error in the canary bytes to
376 	 * the left of the object. Specifically, if check_canary_byte()
377 	 * generates an error, showing both sides might give more clues as to
378 	 * what the error is about when displaying which bytes were corrupted.
379 	 */
380 
381 	/* Apply to left of object. */
382 	for (; meta->addr - addr >= sizeof(u64); addr += sizeof(u64)) {
383 		if (unlikely(*((u64 *)addr) != KFENCE_CANARY_PATTERN_U64))
384 			break;
385 	}
386 
387 	/*
388 	 * If the canary is corrupted in a certain 64 bytes, or the canary
389 	 * memory cannot be completely covered by multiple consecutive 64 bytes,
390 	 * it needs to be checked one by one.
391 	 */
392 	for (; addr < meta->addr; addr++) {
393 		if (unlikely(!check_canary_byte((u8 *)addr)))
394 			break;
395 	}
396 
397 	/* Apply to right of object. */
398 	for (addr = meta->addr + meta->size; addr % sizeof(u64) != 0; addr++) {
399 		if (unlikely(!check_canary_byte((u8 *)addr)))
400 			return;
401 	}
402 	for (; addr - pageaddr < PAGE_SIZE; addr += sizeof(u64)) {
403 		if (unlikely(*((u64 *)addr) != KFENCE_CANARY_PATTERN_U64)) {
404 
405 			for (; addr - pageaddr < PAGE_SIZE; addr++) {
406 				if (!check_canary_byte((u8 *)addr))
407 					return;
408 			}
409 		}
410 	}
411 }
412 
kfence_guarded_alloc(struct kmem_cache * cache,size_t size,gfp_t gfp,unsigned long * stack_entries,size_t num_stack_entries,u32 alloc_stack_hash)413 static void *kfence_guarded_alloc(struct kmem_cache *cache, size_t size, gfp_t gfp,
414 				  unsigned long *stack_entries, size_t num_stack_entries,
415 				  u32 alloc_stack_hash)
416 {
417 	struct kfence_metadata *meta = NULL;
418 	unsigned long flags;
419 	struct slab *slab;
420 	void *addr;
421 	const bool random_right_allocate = get_random_u32_below(2);
422 	const bool random_fault = CONFIG_KFENCE_STRESS_TEST_FAULTS &&
423 				  !get_random_u32_below(CONFIG_KFENCE_STRESS_TEST_FAULTS);
424 
425 	/* Try to obtain a free object. */
426 	raw_spin_lock_irqsave(&kfence_freelist_lock, flags);
427 	if (!list_empty(&kfence_freelist)) {
428 		meta = list_entry(kfence_freelist.next, struct kfence_metadata, list);
429 		list_del_init(&meta->list);
430 	}
431 	raw_spin_unlock_irqrestore(&kfence_freelist_lock, flags);
432 	if (!meta) {
433 		atomic_long_inc(&counters[KFENCE_COUNTER_SKIP_CAPACITY]);
434 		return NULL;
435 	}
436 
437 	if (unlikely(!raw_spin_trylock_irqsave(&meta->lock, flags))) {
438 		/*
439 		 * This is extremely unlikely -- we are reporting on a
440 		 * use-after-free, which locked meta->lock, and the reporting
441 		 * code via printk calls kmalloc() which ends up in
442 		 * kfence_alloc() and tries to grab the same object that we're
443 		 * reporting on. While it has never been observed, lockdep does
444 		 * report that there is a possibility of deadlock. Fix it by
445 		 * using trylock and bailing out gracefully.
446 		 */
447 		raw_spin_lock_irqsave(&kfence_freelist_lock, flags);
448 		/* Put the object back on the freelist. */
449 		list_add_tail(&meta->list, &kfence_freelist);
450 		raw_spin_unlock_irqrestore(&kfence_freelist_lock, flags);
451 
452 		return NULL;
453 	}
454 
455 	meta->addr = metadata_to_pageaddr(meta);
456 	/* Unprotect if we're reusing this page. */
457 	if (meta->state == KFENCE_OBJECT_FREED)
458 		kfence_unprotect(meta->addr);
459 
460 	/*
461 	 * Note: for allocations made before RNG initialization, will always
462 	 * return zero. We still benefit from enabling KFENCE as early as
463 	 * possible, even when the RNG is not yet available, as this will allow
464 	 * KFENCE to detect bugs due to earlier allocations. The only downside
465 	 * is that the out-of-bounds accesses detected are deterministic for
466 	 * such allocations.
467 	 */
468 	if (random_right_allocate) {
469 		/* Allocate on the "right" side, re-calculate address. */
470 		meta->addr += PAGE_SIZE - size;
471 		meta->addr = ALIGN_DOWN(meta->addr, cache->align);
472 	}
473 
474 	addr = (void *)meta->addr;
475 
476 	/* Update remaining metadata. */
477 	metadata_update_state(meta, KFENCE_OBJECT_ALLOCATED, stack_entries, num_stack_entries);
478 	/* Pairs with READ_ONCE() in kfence_shutdown_cache(). */
479 	WRITE_ONCE(meta->cache, cache);
480 	meta->size = size;
481 	meta->alloc_stack_hash = alloc_stack_hash;
482 	raw_spin_unlock_irqrestore(&meta->lock, flags);
483 
484 	alloc_covered_add(alloc_stack_hash, 1);
485 
486 	/* Set required slab fields. */
487 	slab = virt_to_slab((void *)meta->addr);
488 	slab->slab_cache = cache;
489 	slab->objects = 1;
490 
491 	/* Memory initialization. */
492 	set_canary(meta);
493 
494 	/*
495 	 * We check slab_want_init_on_alloc() ourselves, rather than letting
496 	 * SL*B do the initialization, as otherwise we might overwrite KFENCE's
497 	 * redzone.
498 	 */
499 	if (unlikely(slab_want_init_on_alloc(gfp, cache)))
500 		memzero_explicit(addr, size);
501 	if (cache->ctor)
502 		cache->ctor(addr);
503 
504 	if (random_fault)
505 		kfence_protect(meta->addr); /* Random "faults" by protecting the object. */
506 
507 	atomic_long_inc(&counters[KFENCE_COUNTER_ALLOCATED]);
508 	atomic_long_inc(&counters[KFENCE_COUNTER_ALLOCS]);
509 
510 	return addr;
511 }
512 
kfence_guarded_free(void * addr,struct kfence_metadata * meta,bool zombie)513 static void kfence_guarded_free(void *addr, struct kfence_metadata *meta, bool zombie)
514 {
515 	struct kcsan_scoped_access assert_page_exclusive;
516 	unsigned long flags;
517 	bool init;
518 
519 	raw_spin_lock_irqsave(&meta->lock, flags);
520 
521 	if (!kfence_obj_allocated(meta) || meta->addr != (unsigned long)addr) {
522 		/* Invalid or double-free, bail out. */
523 		atomic_long_inc(&counters[KFENCE_COUNTER_BUGS]);
524 		kfence_report_error((unsigned long)addr, false, NULL, meta,
525 				    KFENCE_ERROR_INVALID_FREE);
526 		raw_spin_unlock_irqrestore(&meta->lock, flags);
527 		return;
528 	}
529 
530 	/* Detect racy use-after-free, or incorrect reallocation of this page by KFENCE. */
531 	kcsan_begin_scoped_access((void *)ALIGN_DOWN((unsigned long)addr, PAGE_SIZE), PAGE_SIZE,
532 				  KCSAN_ACCESS_SCOPED | KCSAN_ACCESS_WRITE | KCSAN_ACCESS_ASSERT,
533 				  &assert_page_exclusive);
534 
535 	if (CONFIG_KFENCE_STRESS_TEST_FAULTS)
536 		kfence_unprotect((unsigned long)addr); /* To check canary bytes. */
537 
538 	/* Restore page protection if there was an OOB access. */
539 	if (meta->unprotected_page) {
540 		memzero_explicit((void *)ALIGN_DOWN(meta->unprotected_page, PAGE_SIZE), PAGE_SIZE);
541 		kfence_protect(meta->unprotected_page);
542 		meta->unprotected_page = 0;
543 	}
544 
545 	/* Mark the object as freed. */
546 	metadata_update_state(meta, KFENCE_OBJECT_FREED, NULL, 0);
547 	init = slab_want_init_on_free(meta->cache);
548 	raw_spin_unlock_irqrestore(&meta->lock, flags);
549 
550 	alloc_covered_add(meta->alloc_stack_hash, -1);
551 
552 	/* Check canary bytes for memory corruption. */
553 	check_canary(meta);
554 
555 	/*
556 	 * Clear memory if init-on-free is set. While we protect the page, the
557 	 * data is still there, and after a use-after-free is detected, we
558 	 * unprotect the page, so the data is still accessible.
559 	 */
560 	if (!zombie && unlikely(init))
561 		memzero_explicit(addr, meta->size);
562 
563 	/* Protect to detect use-after-frees. */
564 	kfence_protect((unsigned long)addr);
565 
566 	kcsan_end_scoped_access(&assert_page_exclusive);
567 	if (!zombie) {
568 		/* Add it to the tail of the freelist for reuse. */
569 		raw_spin_lock_irqsave(&kfence_freelist_lock, flags);
570 		KFENCE_WARN_ON(!list_empty(&meta->list));
571 		list_add_tail(&meta->list, &kfence_freelist);
572 		raw_spin_unlock_irqrestore(&kfence_freelist_lock, flags);
573 
574 		atomic_long_dec(&counters[KFENCE_COUNTER_ALLOCATED]);
575 		atomic_long_inc(&counters[KFENCE_COUNTER_FREES]);
576 	} else {
577 		/* See kfence_shutdown_cache(). */
578 		atomic_long_inc(&counters[KFENCE_COUNTER_ZOMBIES]);
579 	}
580 }
581 
rcu_guarded_free(struct rcu_head * h)582 static void rcu_guarded_free(struct rcu_head *h)
583 {
584 	struct kfence_metadata *meta = container_of(h, struct kfence_metadata, rcu_head);
585 
586 	kfence_guarded_free((void *)meta->addr, meta, false);
587 }
588 
589 /*
590  * Initialization of the KFENCE pool after its allocation.
591  * Returns 0 on success; otherwise returns the address up to
592  * which partial initialization succeeded.
593  */
kfence_init_pool(void)594 static unsigned long kfence_init_pool(void)
595 {
596 	unsigned long addr;
597 	struct page *pages;
598 	int i;
599 
600 	if (!arch_kfence_init_pool())
601 		return (unsigned long)__kfence_pool;
602 
603 	addr = (unsigned long)__kfence_pool;
604 	pages = virt_to_page(__kfence_pool);
605 
606 	/*
607 	 * Set up object pages: they must have PG_slab set, to avoid freeing
608 	 * these as real pages.
609 	 *
610 	 * We also want to avoid inserting kfence_free() in the kfree()
611 	 * fast-path in SLUB, and therefore need to ensure kfree() correctly
612 	 * enters __slab_free() slow-path.
613 	 */
614 	for (i = 0; i < KFENCE_POOL_SIZE / PAGE_SIZE; i++) {
615 		struct slab *slab = page_slab(nth_page(pages, i));
616 
617 		if (!i || (i % 2))
618 			continue;
619 
620 		__folio_set_slab(slab_folio(slab));
621 #ifdef CONFIG_MEMCG
622 		slab->obj_exts = (unsigned long)&kfence_metadata_init[i / 2 - 1].obj_exts |
623 				 MEMCG_DATA_OBJEXTS;
624 #endif
625 	}
626 
627 	/*
628 	 * Protect the first 2 pages. The first page is mostly unnecessary, and
629 	 * merely serves as an extended guard page. However, adding one
630 	 * additional page in the beginning gives us an even number of pages,
631 	 * which simplifies the mapping of address to metadata index.
632 	 */
633 	for (i = 0; i < 2; i++) {
634 		if (unlikely(!kfence_protect(addr)))
635 			return addr;
636 
637 		addr += PAGE_SIZE;
638 	}
639 
640 	for (i = 0; i < CONFIG_KFENCE_NUM_OBJECTS; i++) {
641 		struct kfence_metadata *meta = &kfence_metadata_init[i];
642 
643 		/* Initialize metadata. */
644 		INIT_LIST_HEAD(&meta->list);
645 		raw_spin_lock_init(&meta->lock);
646 		meta->state = KFENCE_OBJECT_UNUSED;
647 		meta->addr = addr; /* Initialize for validation in metadata_to_pageaddr(). */
648 		list_add_tail(&meta->list, &kfence_freelist);
649 
650 		/* Protect the right redzone. */
651 		if (unlikely(!kfence_protect(addr + PAGE_SIZE)))
652 			goto reset_slab;
653 
654 		addr += 2 * PAGE_SIZE;
655 	}
656 
657 	/*
658 	 * Make kfence_metadata visible only when initialization is successful.
659 	 * Otherwise, if the initialization fails and kfence_metadata is freed,
660 	 * it may cause UAF in kfence_shutdown_cache().
661 	 */
662 	smp_store_release(&kfence_metadata, kfence_metadata_init);
663 	return 0;
664 
665 reset_slab:
666 	for (i = 0; i < KFENCE_POOL_SIZE / PAGE_SIZE; i++) {
667 		struct slab *slab = page_slab(nth_page(pages, i));
668 
669 		if (!i || (i % 2))
670 			continue;
671 #ifdef CONFIG_MEMCG
672 		slab->obj_exts = 0;
673 #endif
674 		__folio_clear_slab(slab_folio(slab));
675 	}
676 
677 	return addr;
678 }
679 
kfence_init_pool_early(void)680 static bool __init kfence_init_pool_early(void)
681 {
682 	unsigned long addr;
683 
684 	if (!__kfence_pool)
685 		return false;
686 
687 	addr = kfence_init_pool();
688 
689 	if (!addr) {
690 		/*
691 		 * The pool is live and will never be deallocated from this point on.
692 		 * Ignore the pool object from the kmemleak phys object tree, as it would
693 		 * otherwise overlap with allocations returned by kfence_alloc(), which
694 		 * are registered with kmemleak through the slab post-alloc hook.
695 		 */
696 		kmemleak_ignore_phys(__pa(__kfence_pool));
697 		return true;
698 	}
699 
700 	/*
701 	 * Only release unprotected pages, and do not try to go back and change
702 	 * page attributes due to risk of failing to do so as well. If changing
703 	 * page attributes for some pages fails, it is very likely that it also
704 	 * fails for the first page, and therefore expect addr==__kfence_pool in
705 	 * most failure cases.
706 	 */
707 	memblock_free_late(__pa(addr), KFENCE_POOL_SIZE - (addr - (unsigned long)__kfence_pool));
708 	__kfence_pool = NULL;
709 
710 	memblock_free_late(__pa(kfence_metadata_init), KFENCE_METADATA_SIZE);
711 	kfence_metadata_init = NULL;
712 
713 	return false;
714 }
715 
716 /* === DebugFS Interface ==================================================== */
717 
stats_show(struct seq_file * seq,void * v)718 static int stats_show(struct seq_file *seq, void *v)
719 {
720 	int i;
721 
722 	seq_printf(seq, "enabled: %i\n", READ_ONCE(kfence_enabled));
723 	for (i = 0; i < KFENCE_COUNTER_COUNT; i++)
724 		seq_printf(seq, "%s: %ld\n", counter_names[i], atomic_long_read(&counters[i]));
725 
726 	return 0;
727 }
728 DEFINE_SHOW_ATTRIBUTE(stats);
729 
730 /*
731  * debugfs seq_file operations for /sys/kernel/debug/kfence/objects.
732  * start_object() and next_object() return the object index + 1, because NULL is used
733  * to stop iteration.
734  */
start_object(struct seq_file * seq,loff_t * pos)735 static void *start_object(struct seq_file *seq, loff_t *pos)
736 {
737 	if (*pos < CONFIG_KFENCE_NUM_OBJECTS)
738 		return (void *)((long)*pos + 1);
739 	return NULL;
740 }
741 
stop_object(struct seq_file * seq,void * v)742 static void stop_object(struct seq_file *seq, void *v)
743 {
744 }
745 
next_object(struct seq_file * seq,void * v,loff_t * pos)746 static void *next_object(struct seq_file *seq, void *v, loff_t *pos)
747 {
748 	++*pos;
749 	if (*pos < CONFIG_KFENCE_NUM_OBJECTS)
750 		return (void *)((long)*pos + 1);
751 	return NULL;
752 }
753 
show_object(struct seq_file * seq,void * v)754 static int show_object(struct seq_file *seq, void *v)
755 {
756 	struct kfence_metadata *meta = &kfence_metadata[(long)v - 1];
757 	unsigned long flags;
758 
759 	raw_spin_lock_irqsave(&meta->lock, flags);
760 	kfence_print_object(seq, meta);
761 	raw_spin_unlock_irqrestore(&meta->lock, flags);
762 	seq_puts(seq, "---------------------------------\n");
763 
764 	return 0;
765 }
766 
767 static const struct seq_operations objects_sops = {
768 	.start = start_object,
769 	.next = next_object,
770 	.stop = stop_object,
771 	.show = show_object,
772 };
773 DEFINE_SEQ_ATTRIBUTE(objects);
774 
kfence_debugfs_init(void)775 static int kfence_debugfs_init(void)
776 {
777 	struct dentry *kfence_dir;
778 
779 	if (!READ_ONCE(kfence_enabled))
780 		return 0;
781 
782 	kfence_dir = debugfs_create_dir("kfence", NULL);
783 	debugfs_create_file("stats", 0444, kfence_dir, NULL, &stats_fops);
784 	debugfs_create_file("objects", 0400, kfence_dir, NULL, &objects_fops);
785 	return 0;
786 }
787 
788 late_initcall(kfence_debugfs_init);
789 
790 /* === Panic Notifier ====================================================== */
791 
kfence_check_all_canary(void)792 static void kfence_check_all_canary(void)
793 {
794 	int i;
795 
796 	for (i = 0; i < CONFIG_KFENCE_NUM_OBJECTS; i++) {
797 		struct kfence_metadata *meta = &kfence_metadata[i];
798 
799 		if (kfence_obj_allocated(meta))
800 			check_canary(meta);
801 	}
802 }
803 
kfence_check_canary_callback(struct notifier_block * nb,unsigned long reason,void * arg)804 static int kfence_check_canary_callback(struct notifier_block *nb,
805 					unsigned long reason, void *arg)
806 {
807 	kfence_check_all_canary();
808 	return NOTIFY_OK;
809 }
810 
811 static struct notifier_block kfence_check_canary_notifier = {
812 	.notifier_call = kfence_check_canary_callback,
813 };
814 
815 /* === Allocation Gate Timer ================================================ */
816 
817 static struct delayed_work kfence_timer;
818 
819 #ifdef CONFIG_KFENCE_STATIC_KEYS
820 /* Wait queue to wake up allocation-gate timer task. */
821 static DECLARE_WAIT_QUEUE_HEAD(allocation_wait);
822 
wake_up_kfence_timer(struct irq_work * work)823 static void wake_up_kfence_timer(struct irq_work *work)
824 {
825 	wake_up(&allocation_wait);
826 }
827 static DEFINE_IRQ_WORK(wake_up_kfence_timer_work, wake_up_kfence_timer);
828 #endif
829 
830 /*
831  * Set up delayed work, which will enable and disable the static key. We need to
832  * use a work queue (rather than a simple timer), since enabling and disabling a
833  * static key cannot be done from an interrupt.
834  *
835  * Note: Toggling a static branch currently causes IPIs, and here we'll end up
836  * with a total of 2 IPIs to all CPUs. If this ends up a problem in future (with
837  * more aggressive sampling intervals), we could get away with a variant that
838  * avoids IPIs, at the cost of not immediately capturing allocations if the
839  * instructions remain cached.
840  */
toggle_allocation_gate(struct work_struct * work)841 static void toggle_allocation_gate(struct work_struct *work)
842 {
843 	if (!READ_ONCE(kfence_enabled))
844 		return;
845 
846 	atomic_set(&kfence_allocation_gate, -kfence_burst);
847 #ifdef CONFIG_KFENCE_STATIC_KEYS
848 	/* Enable static key, and await allocation to happen. */
849 	static_branch_enable(&kfence_allocation_key);
850 
851 	wait_event_idle(allocation_wait, atomic_read(&kfence_allocation_gate) > 0);
852 
853 	/* Disable static key and reset timer. */
854 	static_branch_disable(&kfence_allocation_key);
855 #endif
856 	queue_delayed_work(system_unbound_wq, &kfence_timer,
857 			   msecs_to_jiffies(kfence_sample_interval));
858 }
859 
860 /* === Public interface ===================================================== */
861 
kfence_alloc_pool_and_metadata(void)862 void __init kfence_alloc_pool_and_metadata(void)
863 {
864 	if (!kfence_sample_interval)
865 		return;
866 
867 	/*
868 	 * If the pool has already been initialized by arch, there is no need to
869 	 * re-allocate the memory pool.
870 	 */
871 	if (!__kfence_pool)
872 		__kfence_pool = memblock_alloc(KFENCE_POOL_SIZE, PAGE_SIZE);
873 
874 	if (!__kfence_pool) {
875 		pr_err("failed to allocate pool\n");
876 		return;
877 	}
878 
879 	/* The memory allocated by memblock has been zeroed out. */
880 	kfence_metadata_init = memblock_alloc(KFENCE_METADATA_SIZE, PAGE_SIZE);
881 	if (!kfence_metadata_init) {
882 		pr_err("failed to allocate metadata\n");
883 		memblock_free(__kfence_pool, KFENCE_POOL_SIZE);
884 		__kfence_pool = NULL;
885 	}
886 }
887 
kfence_init_enable(void)888 static void kfence_init_enable(void)
889 {
890 	if (!IS_ENABLED(CONFIG_KFENCE_STATIC_KEYS))
891 		static_branch_enable(&kfence_allocation_key);
892 
893 	if (kfence_deferrable)
894 		INIT_DEFERRABLE_WORK(&kfence_timer, toggle_allocation_gate);
895 	else
896 		INIT_DELAYED_WORK(&kfence_timer, toggle_allocation_gate);
897 
898 	if (kfence_check_on_panic)
899 		atomic_notifier_chain_register(&panic_notifier_list, &kfence_check_canary_notifier);
900 
901 	WRITE_ONCE(kfence_enabled, true);
902 	queue_delayed_work(system_unbound_wq, &kfence_timer, 0);
903 
904 	pr_info("initialized - using %lu bytes for %d objects at 0x%p-0x%p\n", KFENCE_POOL_SIZE,
905 		CONFIG_KFENCE_NUM_OBJECTS, (void *)__kfence_pool,
906 		(void *)(__kfence_pool + KFENCE_POOL_SIZE));
907 }
908 
kfence_init(void)909 void __init kfence_init(void)
910 {
911 	stack_hash_seed = get_random_u32();
912 
913 	/* Setting kfence_sample_interval to 0 on boot disables KFENCE. */
914 	if (!kfence_sample_interval)
915 		return;
916 
917 	if (!kfence_init_pool_early()) {
918 		pr_err("%s failed\n", __func__);
919 		return;
920 	}
921 
922 	kfence_init_enable();
923 }
924 
kfence_init_late(void)925 static int kfence_init_late(void)
926 {
927 	const unsigned long nr_pages_pool = KFENCE_POOL_SIZE / PAGE_SIZE;
928 	const unsigned long nr_pages_meta = KFENCE_METADATA_SIZE / PAGE_SIZE;
929 	unsigned long addr = (unsigned long)__kfence_pool;
930 	unsigned long free_size = KFENCE_POOL_SIZE;
931 	int err = -ENOMEM;
932 
933 #ifdef CONFIG_CONTIG_ALLOC
934 	struct page *pages;
935 
936 	pages = alloc_contig_pages(nr_pages_pool, GFP_KERNEL, first_online_node,
937 				   NULL);
938 	if (!pages)
939 		return -ENOMEM;
940 
941 	__kfence_pool = page_to_virt(pages);
942 	pages = alloc_contig_pages(nr_pages_meta, GFP_KERNEL, first_online_node,
943 				   NULL);
944 	if (pages)
945 		kfence_metadata_init = page_to_virt(pages);
946 #else
947 	if (nr_pages_pool > MAX_ORDER_NR_PAGES ||
948 	    nr_pages_meta > MAX_ORDER_NR_PAGES) {
949 		pr_warn("KFENCE_NUM_OBJECTS too large for buddy allocator\n");
950 		return -EINVAL;
951 	}
952 
953 	__kfence_pool = alloc_pages_exact(KFENCE_POOL_SIZE, GFP_KERNEL);
954 	if (!__kfence_pool)
955 		return -ENOMEM;
956 
957 	kfence_metadata_init = alloc_pages_exact(KFENCE_METADATA_SIZE, GFP_KERNEL);
958 #endif
959 
960 	if (!kfence_metadata_init)
961 		goto free_pool;
962 
963 	memzero_explicit(kfence_metadata_init, KFENCE_METADATA_SIZE);
964 	addr = kfence_init_pool();
965 	if (!addr) {
966 		kfence_init_enable();
967 		kfence_debugfs_init();
968 		return 0;
969 	}
970 
971 	pr_err("%s failed\n", __func__);
972 	free_size = KFENCE_POOL_SIZE - (addr - (unsigned long)__kfence_pool);
973 	err = -EBUSY;
974 
975 #ifdef CONFIG_CONTIG_ALLOC
976 	free_contig_range(page_to_pfn(virt_to_page((void *)kfence_metadata_init)),
977 			  nr_pages_meta);
978 free_pool:
979 	free_contig_range(page_to_pfn(virt_to_page((void *)addr)),
980 			  free_size / PAGE_SIZE);
981 #else
982 	free_pages_exact((void *)kfence_metadata_init, KFENCE_METADATA_SIZE);
983 free_pool:
984 	free_pages_exact((void *)addr, free_size);
985 #endif
986 
987 	kfence_metadata_init = NULL;
988 	__kfence_pool = NULL;
989 	return err;
990 }
991 
kfence_enable_late(void)992 static int kfence_enable_late(void)
993 {
994 	if (!__kfence_pool)
995 		return kfence_init_late();
996 
997 	WRITE_ONCE(kfence_enabled, true);
998 	queue_delayed_work(system_unbound_wq, &kfence_timer, 0);
999 	pr_info("re-enabled\n");
1000 	return 0;
1001 }
1002 
kfence_shutdown_cache(struct kmem_cache * s)1003 void kfence_shutdown_cache(struct kmem_cache *s)
1004 {
1005 	unsigned long flags;
1006 	struct kfence_metadata *meta;
1007 	int i;
1008 
1009 	/* Pairs with release in kfence_init_pool(). */
1010 	if (!smp_load_acquire(&kfence_metadata))
1011 		return;
1012 
1013 	for (i = 0; i < CONFIG_KFENCE_NUM_OBJECTS; i++) {
1014 		bool in_use;
1015 
1016 		meta = &kfence_metadata[i];
1017 
1018 		/*
1019 		 * If we observe some inconsistent cache and state pair where we
1020 		 * should have returned false here, cache destruction is racing
1021 		 * with either kmem_cache_alloc() or kmem_cache_free(). Taking
1022 		 * the lock will not help, as different critical section
1023 		 * serialization will have the same outcome.
1024 		 */
1025 		if (READ_ONCE(meta->cache) != s || !kfence_obj_allocated(meta))
1026 			continue;
1027 
1028 		raw_spin_lock_irqsave(&meta->lock, flags);
1029 		in_use = meta->cache == s && kfence_obj_allocated(meta);
1030 		raw_spin_unlock_irqrestore(&meta->lock, flags);
1031 
1032 		if (in_use) {
1033 			/*
1034 			 * This cache still has allocations, and we should not
1035 			 * release them back into the freelist so they can still
1036 			 * safely be used and retain the kernel's default
1037 			 * behaviour of keeping the allocations alive (leak the
1038 			 * cache); however, they effectively become "zombie
1039 			 * allocations" as the KFENCE objects are the only ones
1040 			 * still in use and the owning cache is being destroyed.
1041 			 *
1042 			 * We mark them freed, so that any subsequent use shows
1043 			 * more useful error messages that will include stack
1044 			 * traces of the user of the object, the original
1045 			 * allocation, and caller to shutdown_cache().
1046 			 */
1047 			kfence_guarded_free((void *)meta->addr, meta, /*zombie=*/true);
1048 		}
1049 	}
1050 
1051 	for (i = 0; i < CONFIG_KFENCE_NUM_OBJECTS; i++) {
1052 		meta = &kfence_metadata[i];
1053 
1054 		/* See above. */
1055 		if (READ_ONCE(meta->cache) != s || READ_ONCE(meta->state) != KFENCE_OBJECT_FREED)
1056 			continue;
1057 
1058 		raw_spin_lock_irqsave(&meta->lock, flags);
1059 		if (meta->cache == s && meta->state == KFENCE_OBJECT_FREED)
1060 			meta->cache = NULL;
1061 		raw_spin_unlock_irqrestore(&meta->lock, flags);
1062 	}
1063 }
1064 
__kfence_alloc(struct kmem_cache * s,size_t size,gfp_t flags)1065 void *__kfence_alloc(struct kmem_cache *s, size_t size, gfp_t flags)
1066 {
1067 	unsigned long stack_entries[KFENCE_STACK_DEPTH];
1068 	size_t num_stack_entries;
1069 	u32 alloc_stack_hash;
1070 	int allocation_gate;
1071 
1072 	/*
1073 	 * Perform size check before switching kfence_allocation_gate, so that
1074 	 * we don't disable KFENCE without making an allocation.
1075 	 */
1076 	if (size > PAGE_SIZE) {
1077 		atomic_long_inc(&counters[KFENCE_COUNTER_SKIP_INCOMPAT]);
1078 		return NULL;
1079 	}
1080 
1081 	/*
1082 	 * Skip allocations from non-default zones, including DMA. We cannot
1083 	 * guarantee that pages in the KFENCE pool will have the requested
1084 	 * properties (e.g. reside in DMAable memory).
1085 	 */
1086 	if ((flags & GFP_ZONEMASK) ||
1087 	    (s->flags & (SLAB_CACHE_DMA | SLAB_CACHE_DMA32))) {
1088 		atomic_long_inc(&counters[KFENCE_COUNTER_SKIP_INCOMPAT]);
1089 		return NULL;
1090 	}
1091 
1092 	/*
1093 	 * Skip allocations for this slab, if KFENCE has been disabled for
1094 	 * this slab.
1095 	 */
1096 	if (s->flags & SLAB_SKIP_KFENCE)
1097 		return NULL;
1098 
1099 	allocation_gate = atomic_inc_return(&kfence_allocation_gate);
1100 	if (allocation_gate > 1)
1101 		return NULL;
1102 #ifdef CONFIG_KFENCE_STATIC_KEYS
1103 	/*
1104 	 * waitqueue_active() is fully ordered after the update of
1105 	 * kfence_allocation_gate per atomic_inc_return().
1106 	 */
1107 	if (allocation_gate == 1 && waitqueue_active(&allocation_wait)) {
1108 		/*
1109 		 * Calling wake_up() here may deadlock when allocations happen
1110 		 * from within timer code. Use an irq_work to defer it.
1111 		 */
1112 		irq_work_queue(&wake_up_kfence_timer_work);
1113 	}
1114 #endif
1115 
1116 	if (!READ_ONCE(kfence_enabled))
1117 		return NULL;
1118 
1119 	num_stack_entries = stack_trace_save(stack_entries, KFENCE_STACK_DEPTH, 0);
1120 
1121 	/*
1122 	 * Do expensive check for coverage of allocation in slow-path after
1123 	 * allocation_gate has already become non-zero, even though it might
1124 	 * mean not making any allocation within a given sample interval.
1125 	 *
1126 	 * This ensures reasonable allocation coverage when the pool is almost
1127 	 * full, including avoiding long-lived allocations of the same source
1128 	 * filling up the pool (e.g. pagecache allocations).
1129 	 */
1130 	alloc_stack_hash = get_alloc_stack_hash(stack_entries, num_stack_entries);
1131 	if (should_skip_covered() && alloc_covered_contains(alloc_stack_hash)) {
1132 		atomic_long_inc(&counters[KFENCE_COUNTER_SKIP_COVERED]);
1133 		return NULL;
1134 	}
1135 
1136 	return kfence_guarded_alloc(s, size, flags, stack_entries, num_stack_entries,
1137 				    alloc_stack_hash);
1138 }
1139 
kfence_ksize(const void * addr)1140 size_t kfence_ksize(const void *addr)
1141 {
1142 	const struct kfence_metadata *meta = addr_to_metadata((unsigned long)addr);
1143 
1144 	/*
1145 	 * Read locklessly -- if there is a race with __kfence_alloc(), this is
1146 	 * either a use-after-free or invalid access.
1147 	 */
1148 	return meta ? meta->size : 0;
1149 }
1150 
kfence_object_start(const void * addr)1151 void *kfence_object_start(const void *addr)
1152 {
1153 	const struct kfence_metadata *meta = addr_to_metadata((unsigned long)addr);
1154 
1155 	/*
1156 	 * Read locklessly -- if there is a race with __kfence_alloc(), this is
1157 	 * either a use-after-free or invalid access.
1158 	 */
1159 	return meta ? (void *)meta->addr : NULL;
1160 }
1161 
__kfence_free(void * addr)1162 void __kfence_free(void *addr)
1163 {
1164 	struct kfence_metadata *meta = addr_to_metadata((unsigned long)addr);
1165 
1166 #ifdef CONFIG_MEMCG
1167 	KFENCE_WARN_ON(meta->obj_exts.objcg);
1168 #endif
1169 	/*
1170 	 * If the objects of the cache are SLAB_TYPESAFE_BY_RCU, defer freeing
1171 	 * the object, as the object page may be recycled for other-typed
1172 	 * objects once it has been freed. meta->cache may be NULL if the cache
1173 	 * was destroyed.
1174 	 * Save the stack trace here so that reports show where the user freed
1175 	 * the object.
1176 	 */
1177 	if (unlikely(meta->cache && (meta->cache->flags & SLAB_TYPESAFE_BY_RCU))) {
1178 		unsigned long flags;
1179 
1180 		raw_spin_lock_irqsave(&meta->lock, flags);
1181 		metadata_update_state(meta, KFENCE_OBJECT_RCU_FREEING, NULL, 0);
1182 		raw_spin_unlock_irqrestore(&meta->lock, flags);
1183 		call_rcu(&meta->rcu_head, rcu_guarded_free);
1184 	} else {
1185 		kfence_guarded_free(addr, meta, false);
1186 	}
1187 }
1188 
kfence_handle_page_fault(unsigned long addr,bool is_write,struct pt_regs * regs)1189 bool kfence_handle_page_fault(unsigned long addr, bool is_write, struct pt_regs *regs)
1190 {
1191 	const int page_index = (addr - (unsigned long)__kfence_pool) / PAGE_SIZE;
1192 	struct kfence_metadata *to_report = NULL;
1193 	enum kfence_error_type error_type;
1194 	unsigned long flags;
1195 
1196 	if (!is_kfence_address((void *)addr))
1197 		return false;
1198 
1199 	if (!READ_ONCE(kfence_enabled)) /* If disabled at runtime ... */
1200 		return kfence_unprotect(addr); /* ... unprotect and proceed. */
1201 
1202 	atomic_long_inc(&counters[KFENCE_COUNTER_BUGS]);
1203 
1204 	if (page_index % 2) {
1205 		/* This is a redzone, report a buffer overflow. */
1206 		struct kfence_metadata *meta;
1207 		int distance = 0;
1208 
1209 		meta = addr_to_metadata(addr - PAGE_SIZE);
1210 		if (meta && kfence_obj_allocated(meta)) {
1211 			to_report = meta;
1212 			/* Data race ok; distance calculation approximate. */
1213 			distance = addr - data_race(meta->addr + meta->size);
1214 		}
1215 
1216 		meta = addr_to_metadata(addr + PAGE_SIZE);
1217 		if (meta && kfence_obj_allocated(meta)) {
1218 			/* Data race ok; distance calculation approximate. */
1219 			if (!to_report || distance > data_race(meta->addr) - addr)
1220 				to_report = meta;
1221 		}
1222 
1223 		if (!to_report)
1224 			goto out;
1225 
1226 		raw_spin_lock_irqsave(&to_report->lock, flags);
1227 		to_report->unprotected_page = addr;
1228 		error_type = KFENCE_ERROR_OOB;
1229 
1230 		/*
1231 		 * If the object was freed before we took the look we can still
1232 		 * report this as an OOB -- the report will simply show the
1233 		 * stacktrace of the free as well.
1234 		 */
1235 	} else {
1236 		to_report = addr_to_metadata(addr);
1237 		if (!to_report)
1238 			goto out;
1239 
1240 		raw_spin_lock_irqsave(&to_report->lock, flags);
1241 		error_type = KFENCE_ERROR_UAF;
1242 		/*
1243 		 * We may race with __kfence_alloc(), and it is possible that a
1244 		 * freed object may be reallocated. We simply report this as a
1245 		 * use-after-free, with the stack trace showing the place where
1246 		 * the object was re-allocated.
1247 		 */
1248 	}
1249 
1250 out:
1251 	if (to_report) {
1252 		kfence_report_error(addr, is_write, regs, to_report, error_type);
1253 		raw_spin_unlock_irqrestore(&to_report->lock, flags);
1254 	} else {
1255 		/* This may be a UAF or OOB access, but we can't be sure. */
1256 		kfence_report_error(addr, is_write, regs, NULL, KFENCE_ERROR_INVALID);
1257 	}
1258 
1259 	return kfence_unprotect(addr); /* Unprotect and let access proceed. */
1260 }
1261