xref: /linux/mm/kfence/core.c (revision ba6ec09911b805778a2fed6d626bfe77b011a717)
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/nodemask.h>
25 #include <linux/notifier.h>
26 #include <linux/panic_notifier.h>
27 #include <linux/random.h>
28 #include <linux/rcupdate.h>
29 #include <linux/sched/clock.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);
param_set_sample_interval(const char * val,const struct kernel_param * kp)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 
param_get_sample_interval(char * buffer,const struct kernel_param * kp)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 /* Allocation burst count: number of excess KFENCE allocations per sample. */
104 static unsigned int kfence_burst __read_mostly;
105 module_param_named(burst, kfence_burst, uint, 0644);
106 
107 /* If true, use a deferrable timer. */
108 static bool kfence_deferrable __read_mostly = IS_ENABLED(CONFIG_KFENCE_DEFERRABLE);
109 module_param_named(deferrable, kfence_deferrable, bool, 0444);
110 
111 /* If true, check all canary bytes on panic. */
112 static bool kfence_check_on_panic __read_mostly;
113 module_param_named(check_on_panic, kfence_check_on_panic, bool, 0444);
114 
115 /* The pool of pages used for guard pages and objects. */
116 char *__kfence_pool __read_mostly;
117 EXPORT_SYMBOL(__kfence_pool); /* Export for test modules. */
118 
119 /*
120  * Per-object metadata, with one-to-one mapping of object metadata to
121  * backing pages (in __kfence_pool).
122  */
123 static_assert(CONFIG_KFENCE_NUM_OBJECTS > 0);
124 struct kfence_metadata *kfence_metadata __read_mostly;
125 
126 /*
127  * If kfence_metadata is not NULL, it may be accessed by kfence_shutdown_cache().
128  * So introduce kfence_metadata_init to initialize metadata, and then make
129  * kfence_metadata visible after initialization is successful. This prevents
130  * potential UAF or access to uninitialized metadata.
131  */
132 static struct kfence_metadata *kfence_metadata_init __read_mostly;
133 
134 /* Freelist with available objects. */
135 static struct list_head kfence_freelist = LIST_HEAD_INIT(kfence_freelist);
136 static DEFINE_RAW_SPINLOCK(kfence_freelist_lock); /* Lock protecting freelist. */
137 
138 /*
139  * The static key to set up a KFENCE allocation; or if static keys are not used
140  * to gate allocations, to avoid a load and compare if KFENCE is disabled.
141  */
142 DEFINE_STATIC_KEY_FALSE(kfence_allocation_key);
143 
144 /* Gates the allocation, ensuring only one succeeds in a given period. */
145 atomic_t kfence_allocation_gate = ATOMIC_INIT(1);
146 
147 /*
148  * A Counting Bloom filter of allocation coverage: limits currently covered
149  * allocations of the same source filling up the pool.
150  *
151  * Assuming a range of 15%-85% unique allocations in the pool at any point in
152  * time, the below parameters provide a probablity of 0.02-0.33 for false
153  * positive hits respectively:
154  *
155  *	P(alloc_traces) = (1 - e^(-HNUM * (alloc_traces / SIZE)) ^ HNUM
156  */
157 #define ALLOC_COVERED_HNUM	2
158 #define ALLOC_COVERED_ORDER	(const_ilog2(CONFIG_KFENCE_NUM_OBJECTS) + 2)
159 #define ALLOC_COVERED_SIZE	(1 << ALLOC_COVERED_ORDER)
160 #define ALLOC_COVERED_HNEXT(h)	hash_32(h, ALLOC_COVERED_ORDER)
161 #define ALLOC_COVERED_MASK	(ALLOC_COVERED_SIZE - 1)
162 static atomic_t alloc_covered[ALLOC_COVERED_SIZE];
163 
164 /* Stack depth used to determine uniqueness of an allocation. */
165 #define UNIQUE_ALLOC_STACK_DEPTH ((size_t)8)
166 
167 /*
168  * Randomness for stack hashes, making the same collisions across reboots and
169  * different machines less likely.
170  */
171 static u32 stack_hash_seed __ro_after_init;
172 
173 /* Statistics counters for debugfs. */
174 enum kfence_counter_id {
175 	KFENCE_COUNTER_ALLOCATED,
176 	KFENCE_COUNTER_ALLOCS,
177 	KFENCE_COUNTER_FREES,
178 	KFENCE_COUNTER_ZOMBIES,
179 	KFENCE_COUNTER_BUGS,
180 	KFENCE_COUNTER_SKIP_INCOMPAT,
181 	KFENCE_COUNTER_SKIP_CAPACITY,
182 	KFENCE_COUNTER_SKIP_COVERED,
183 	KFENCE_COUNTER_COUNT,
184 };
185 static atomic_long_t counters[KFENCE_COUNTER_COUNT];
186 static const char *const counter_names[] = {
187 	[KFENCE_COUNTER_ALLOCATED]	= "currently allocated",
188 	[KFENCE_COUNTER_ALLOCS]		= "total allocations",
189 	[KFENCE_COUNTER_FREES]		= "total frees",
190 	[KFENCE_COUNTER_ZOMBIES]	= "zombie allocations",
191 	[KFENCE_COUNTER_BUGS]		= "total bugs",
192 	[KFENCE_COUNTER_SKIP_INCOMPAT]	= "skipped allocations (incompatible)",
193 	[KFENCE_COUNTER_SKIP_CAPACITY]	= "skipped allocations (capacity)",
194 	[KFENCE_COUNTER_SKIP_COVERED]	= "skipped allocations (covered)",
195 };
196 static_assert(ARRAY_SIZE(counter_names) == KFENCE_COUNTER_COUNT);
197 
198 /* === Internals ============================================================ */
199 
should_skip_covered(void)200 static inline bool should_skip_covered(void)
201 {
202 	unsigned long thresh = (CONFIG_KFENCE_NUM_OBJECTS * kfence_skip_covered_thresh) / 100;
203 
204 	return atomic_long_read(&counters[KFENCE_COUNTER_ALLOCATED]) > thresh;
205 }
206 
get_alloc_stack_hash(unsigned long * stack_entries,size_t num_entries)207 static u32 get_alloc_stack_hash(unsigned long *stack_entries, size_t num_entries)
208 {
209 	num_entries = min(num_entries, UNIQUE_ALLOC_STACK_DEPTH);
210 	num_entries = filter_irq_stacks(stack_entries, num_entries);
211 	return jhash(stack_entries, num_entries * sizeof(stack_entries[0]), stack_hash_seed);
212 }
213 
214 /*
215  * Adds (or subtracts) count @val for allocation stack trace hash
216  * @alloc_stack_hash from Counting Bloom filter.
217  */
alloc_covered_add(u32 alloc_stack_hash,int val)218 static void alloc_covered_add(u32 alloc_stack_hash, int val)
219 {
220 	int i;
221 
222 	for (i = 0; i < ALLOC_COVERED_HNUM; i++) {
223 		atomic_add(val, &alloc_covered[alloc_stack_hash & ALLOC_COVERED_MASK]);
224 		alloc_stack_hash = ALLOC_COVERED_HNEXT(alloc_stack_hash);
225 	}
226 }
227 
228 /*
229  * Returns true if the allocation stack trace hash @alloc_stack_hash is
230  * currently contained (non-zero count) in Counting Bloom filter.
231  */
alloc_covered_contains(u32 alloc_stack_hash)232 static bool alloc_covered_contains(u32 alloc_stack_hash)
233 {
234 	int i;
235 
236 	for (i = 0; i < ALLOC_COVERED_HNUM; i++) {
237 		if (!atomic_read(&alloc_covered[alloc_stack_hash & ALLOC_COVERED_MASK]))
238 			return false;
239 		alloc_stack_hash = ALLOC_COVERED_HNEXT(alloc_stack_hash);
240 	}
241 
242 	return true;
243 }
244 
kfence_protect(unsigned long addr)245 static bool kfence_protect(unsigned long addr)
246 {
247 	return !KFENCE_WARN_ON(!kfence_protect_page(ALIGN_DOWN(addr, PAGE_SIZE), true));
248 }
249 
kfence_unprotect(unsigned long addr)250 static bool kfence_unprotect(unsigned long addr)
251 {
252 	return !KFENCE_WARN_ON(!kfence_protect_page(ALIGN_DOWN(addr, PAGE_SIZE), false));
253 }
254 
metadata_to_pageaddr(const struct kfence_metadata * meta)255 static inline unsigned long metadata_to_pageaddr(const struct kfence_metadata *meta)
256 {
257 	unsigned long offset = (meta - kfence_metadata + 1) * PAGE_SIZE * 2;
258 	unsigned long pageaddr = (unsigned long)&__kfence_pool[offset];
259 
260 	/* The checks do not affect performance; only called from slow-paths. */
261 
262 	/* Only call with a pointer into kfence_metadata. */
263 	if (KFENCE_WARN_ON(meta < kfence_metadata ||
264 			   meta >= kfence_metadata + CONFIG_KFENCE_NUM_OBJECTS))
265 		return 0;
266 
267 	/*
268 	 * This metadata object only ever maps to 1 page; verify that the stored
269 	 * address is in the expected range.
270 	 */
271 	if (KFENCE_WARN_ON(ALIGN_DOWN(meta->addr, PAGE_SIZE) != pageaddr))
272 		return 0;
273 
274 	return pageaddr;
275 }
276 
kfence_obj_allocated(const struct kfence_metadata * meta)277 static inline bool kfence_obj_allocated(const struct kfence_metadata *meta)
278 {
279 	enum kfence_object_state state = READ_ONCE(meta->state);
280 
281 	return state == KFENCE_OBJECT_ALLOCATED || state == KFENCE_OBJECT_RCU_FREEING;
282 }
283 
284 /*
285  * Update the object's metadata state, including updating the alloc/free stacks
286  * depending on the state transition.
287  */
288 static noinline void
metadata_update_state(struct kfence_metadata * meta,enum kfence_object_state next,unsigned long * stack_entries,size_t num_stack_entries)289 metadata_update_state(struct kfence_metadata *meta, enum kfence_object_state next,
290 		      unsigned long *stack_entries, size_t num_stack_entries)
291 {
292 	struct kfence_track *track =
293 		next == KFENCE_OBJECT_ALLOCATED ? &meta->alloc_track : &meta->free_track;
294 
295 	lockdep_assert_held(&meta->lock);
296 
297 	/* Stack has been saved when calling rcu, skip. */
298 	if (READ_ONCE(meta->state) == KFENCE_OBJECT_RCU_FREEING)
299 		goto out;
300 
301 	if (stack_entries) {
302 		memcpy(track->stack_entries, stack_entries,
303 		       num_stack_entries * sizeof(stack_entries[0]));
304 	} else {
305 		/*
306 		 * Skip over 1 (this) functions; noinline ensures we do not
307 		 * accidentally skip over the caller by never inlining.
308 		 */
309 		num_stack_entries = stack_trace_save(track->stack_entries, KFENCE_STACK_DEPTH, 1);
310 	}
311 	track->num_stack_entries = num_stack_entries;
312 	track->pid = task_pid_nr(current);
313 	track->cpu = raw_smp_processor_id();
314 	track->ts_nsec = local_clock(); /* Same source as printk timestamps. */
315 
316 out:
317 	/*
318 	 * Pairs with READ_ONCE() in
319 	 *	kfence_shutdown_cache(),
320 	 *	kfence_handle_page_fault().
321 	 */
322 	WRITE_ONCE(meta->state, next);
323 }
324 
325 #ifdef CONFIG_KMSAN
326 #define check_canary_attributes noinline __no_kmsan_checks
327 #else
328 #define check_canary_attributes inline
329 #endif
330 
331 /* Check canary byte at @addr. */
check_canary_byte(u8 * addr)332 static check_canary_attributes bool check_canary_byte(u8 *addr)
333 {
334 	struct kfence_metadata *meta;
335 	unsigned long flags;
336 
337 	if (likely(*addr == KFENCE_CANARY_PATTERN_U8(addr)))
338 		return true;
339 
340 	atomic_long_inc(&counters[KFENCE_COUNTER_BUGS]);
341 
342 	meta = addr_to_metadata((unsigned long)addr);
343 	raw_spin_lock_irqsave(&meta->lock, flags);
344 	kfence_report_error((unsigned long)addr, false, NULL, meta, KFENCE_ERROR_CORRUPTION);
345 	raw_spin_unlock_irqrestore(&meta->lock, flags);
346 
347 	return false;
348 }
349 
set_canary(const struct kfence_metadata * meta)350 static inline void set_canary(const struct kfence_metadata *meta)
351 {
352 	const unsigned long pageaddr = ALIGN_DOWN(meta->addr, PAGE_SIZE);
353 	unsigned long addr = pageaddr;
354 
355 	/*
356 	 * The canary may be written to part of the object memory, but it does
357 	 * not affect it. The user should initialize the object before using it.
358 	 */
359 	for (; addr < meta->addr; addr += sizeof(u64))
360 		*((u64 *)addr) = KFENCE_CANARY_PATTERN_U64;
361 
362 	addr = ALIGN_DOWN(meta->addr + meta->size, sizeof(u64));
363 	for (; addr - pageaddr < PAGE_SIZE; addr += sizeof(u64))
364 		*((u64 *)addr) = KFENCE_CANARY_PATTERN_U64;
365 }
366 
367 static check_canary_attributes void
check_canary(const struct kfence_metadata * meta)368 check_canary(const struct kfence_metadata *meta)
369 {
370 	const unsigned long pageaddr = ALIGN_DOWN(meta->addr, PAGE_SIZE);
371 	unsigned long addr = pageaddr;
372 
373 	/*
374 	 * We'll iterate over each canary byte per-side until a corrupted byte
375 	 * is found. However, we'll still iterate over the canary bytes to the
376 	 * right of the object even if there was an error in the canary bytes to
377 	 * the left of the object. Specifically, if check_canary_byte()
378 	 * generates an error, showing both sides might give more clues as to
379 	 * what the error is about when displaying which bytes were corrupted.
380 	 */
381 
382 	/* Apply to left of object. */
383 	for (; meta->addr - addr >= sizeof(u64); addr += sizeof(u64)) {
384 		if (unlikely(*((u64 *)addr) != KFENCE_CANARY_PATTERN_U64))
385 			break;
386 	}
387 
388 	/*
389 	 * If the canary is corrupted in a certain 64 bytes, or the canary
390 	 * memory cannot be completely covered by multiple consecutive 64 bytes,
391 	 * it needs to be checked one by one.
392 	 */
393 	for (; addr < meta->addr; addr++) {
394 		if (unlikely(!check_canary_byte((u8 *)addr)))
395 			break;
396 	}
397 
398 	/* Apply to right of object. */
399 	for (addr = meta->addr + meta->size; addr % sizeof(u64) != 0; addr++) {
400 		if (unlikely(!check_canary_byte((u8 *)addr)))
401 			return;
402 	}
403 	for (; addr - pageaddr < PAGE_SIZE; addr += sizeof(u64)) {
404 		if (unlikely(*((u64 *)addr) != KFENCE_CANARY_PATTERN_U64)) {
405 
406 			for (; addr - pageaddr < PAGE_SIZE; addr++) {
407 				if (!check_canary_byte((u8 *)addr))
408 					return;
409 			}
410 		}
411 	}
412 }
413 
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)414 static void *kfence_guarded_alloc(struct kmem_cache *cache, size_t size, gfp_t gfp,
415 				  unsigned long *stack_entries, size_t num_stack_entries,
416 				  u32 alloc_stack_hash)
417 {
418 	struct kfence_metadata *meta = NULL;
419 	unsigned long flags;
420 	struct slab *slab;
421 	void *addr;
422 	const bool random_right_allocate = get_random_u32_below(2);
423 	const bool random_fault = CONFIG_KFENCE_STRESS_TEST_FAULTS &&
424 				  !get_random_u32_below(CONFIG_KFENCE_STRESS_TEST_FAULTS);
425 
426 	/* Try to obtain a free object. */
427 	raw_spin_lock_irqsave(&kfence_freelist_lock, flags);
428 	if (!list_empty(&kfence_freelist)) {
429 		meta = list_entry(kfence_freelist.next, struct kfence_metadata, list);
430 		list_del_init(&meta->list);
431 	}
432 	raw_spin_unlock_irqrestore(&kfence_freelist_lock, flags);
433 	if (!meta) {
434 		atomic_long_inc(&counters[KFENCE_COUNTER_SKIP_CAPACITY]);
435 		return NULL;
436 	}
437 
438 	if (unlikely(!raw_spin_trylock_irqsave(&meta->lock, flags))) {
439 		/*
440 		 * This is extremely unlikely -- we are reporting on a
441 		 * use-after-free, which locked meta->lock, and the reporting
442 		 * code via printk calls kmalloc() which ends up in
443 		 * kfence_alloc() and tries to grab the same object that we're
444 		 * reporting on. While it has never been observed, lockdep does
445 		 * report that there is a possibility of deadlock. Fix it by
446 		 * using trylock and bailing out gracefully.
447 		 */
448 		raw_spin_lock_irqsave(&kfence_freelist_lock, flags);
449 		/* Put the object back on the freelist. */
450 		list_add_tail(&meta->list, &kfence_freelist);
451 		raw_spin_unlock_irqrestore(&kfence_freelist_lock, flags);
452 
453 		return NULL;
454 	}
455 
456 	meta->addr = metadata_to_pageaddr(meta);
457 	/* Unprotect if we're reusing this page. */
458 	if (meta->state == KFENCE_OBJECT_FREED)
459 		kfence_unprotect(meta->addr);
460 
461 	/*
462 	 * Note: for allocations made before RNG initialization, will always
463 	 * return zero. We still benefit from enabling KFENCE as early as
464 	 * possible, even when the RNG is not yet available, as this will allow
465 	 * KFENCE to detect bugs due to earlier allocations. The only downside
466 	 * is that the out-of-bounds accesses detected are deterministic for
467 	 * such allocations.
468 	 */
469 	if (random_right_allocate) {
470 		/* Allocate on the "right" side, re-calculate address. */
471 		meta->addr += PAGE_SIZE - size;
472 		meta->addr = ALIGN_DOWN(meta->addr, cache->align);
473 	}
474 
475 	addr = (void *)meta->addr;
476 
477 	/* Update remaining metadata. */
478 	metadata_update_state(meta, KFENCE_OBJECT_ALLOCATED, stack_entries, num_stack_entries);
479 	/* Pairs with READ_ONCE() in kfence_shutdown_cache(). */
480 	WRITE_ONCE(meta->cache, cache);
481 	meta->size = size;
482 	meta->alloc_stack_hash = alloc_stack_hash;
483 	raw_spin_unlock_irqrestore(&meta->lock, flags);
484 
485 	alloc_covered_add(alloc_stack_hash, 1);
486 
487 	/* Set required slab fields. */
488 	slab = virt_to_slab((void *)meta->addr);
489 	slab->slab_cache = cache;
490 	slab->objects = 1;
491 
492 	/* Memory initialization. */
493 	set_canary(meta);
494 
495 	/*
496 	 * We check slab_want_init_on_alloc() ourselves, rather than letting
497 	 * SL*B do the initialization, as otherwise we might overwrite KFENCE's
498 	 * redzone.
499 	 */
500 	if (unlikely(slab_want_init_on_alloc(gfp, cache)))
501 		memzero_explicit(addr, size);
502 	if (cache->ctor)
503 		cache->ctor(addr);
504 
505 	if (random_fault)
506 		kfence_protect(meta->addr); /* Random "faults" by protecting the object. */
507 
508 	atomic_long_inc(&counters[KFENCE_COUNTER_ALLOCATED]);
509 	atomic_long_inc(&counters[KFENCE_COUNTER_ALLOCS]);
510 
511 	return addr;
512 }
513 
kfence_guarded_free(void * addr,struct kfence_metadata * meta,bool zombie)514 static void kfence_guarded_free(void *addr, struct kfence_metadata *meta, bool zombie)
515 {
516 	struct kcsan_scoped_access assert_page_exclusive;
517 	unsigned long flags;
518 	bool init;
519 
520 	raw_spin_lock_irqsave(&meta->lock, flags);
521 
522 	if (!kfence_obj_allocated(meta) || meta->addr != (unsigned long)addr) {
523 		/* Invalid or double-free, bail out. */
524 		atomic_long_inc(&counters[KFENCE_COUNTER_BUGS]);
525 		kfence_report_error((unsigned long)addr, false, NULL, meta,
526 				    KFENCE_ERROR_INVALID_FREE);
527 		raw_spin_unlock_irqrestore(&meta->lock, flags);
528 		return;
529 	}
530 
531 	/* Detect racy use-after-free, or incorrect reallocation of this page by KFENCE. */
532 	kcsan_begin_scoped_access((void *)ALIGN_DOWN((unsigned long)addr, PAGE_SIZE), PAGE_SIZE,
533 				  KCSAN_ACCESS_SCOPED | KCSAN_ACCESS_WRITE | KCSAN_ACCESS_ASSERT,
534 				  &assert_page_exclusive);
535 
536 	if (CONFIG_KFENCE_STRESS_TEST_FAULTS)
537 		kfence_unprotect((unsigned long)addr); /* To check canary bytes. */
538 
539 	/* Restore page protection if there was an OOB access. */
540 	if (meta->unprotected_page) {
541 		memzero_explicit((void *)ALIGN_DOWN(meta->unprotected_page, PAGE_SIZE), PAGE_SIZE);
542 		kfence_protect(meta->unprotected_page);
543 		meta->unprotected_page = 0;
544 	}
545 
546 	/* Mark the object as freed. */
547 	metadata_update_state(meta, KFENCE_OBJECT_FREED, NULL, 0);
548 	init = slab_want_init_on_free(meta->cache);
549 	raw_spin_unlock_irqrestore(&meta->lock, flags);
550 
551 	alloc_covered_add(meta->alloc_stack_hash, -1);
552 
553 	/* Check canary bytes for memory corruption. */
554 	check_canary(meta);
555 
556 	/*
557 	 * Clear memory if init-on-free is set. While we protect the page, the
558 	 * data is still there, and after a use-after-free is detected, we
559 	 * unprotect the page, so the data is still accessible.
560 	 */
561 	if (!zombie && unlikely(init))
562 		memzero_explicit(addr, meta->size);
563 
564 	/* Protect to detect use-after-frees. */
565 	kfence_protect((unsigned long)addr);
566 
567 	kcsan_end_scoped_access(&assert_page_exclusive);
568 	if (!zombie) {
569 		/* Add it to the tail of the freelist for reuse. */
570 		raw_spin_lock_irqsave(&kfence_freelist_lock, flags);
571 		KFENCE_WARN_ON(!list_empty(&meta->list));
572 		list_add_tail(&meta->list, &kfence_freelist);
573 		raw_spin_unlock_irqrestore(&kfence_freelist_lock, flags);
574 
575 		atomic_long_dec(&counters[KFENCE_COUNTER_ALLOCATED]);
576 		atomic_long_inc(&counters[KFENCE_COUNTER_FREES]);
577 	} else {
578 		/* See kfence_shutdown_cache(). */
579 		atomic_long_inc(&counters[KFENCE_COUNTER_ZOMBIES]);
580 	}
581 }
582 
rcu_guarded_free(struct rcu_head * h)583 static void rcu_guarded_free(struct rcu_head *h)
584 {
585 	struct kfence_metadata *meta = container_of(h, struct kfence_metadata, rcu_head);
586 
587 	kfence_guarded_free((void *)meta->addr, meta, false);
588 }
589 
590 /*
591  * Initialization of the KFENCE pool after its allocation.
592  * Returns 0 on success; otherwise returns the address up to
593  * which partial initialization succeeded.
594  */
kfence_init_pool(void)595 static unsigned long kfence_init_pool(void)
596 {
597 	unsigned long addr;
598 	struct page *pages;
599 	int i;
600 
601 	if (!arch_kfence_init_pool())
602 		return (unsigned long)__kfence_pool;
603 
604 	addr = (unsigned long)__kfence_pool;
605 	pages = virt_to_page(__kfence_pool);
606 
607 	/*
608 	 * Set up object pages: they must have PG_slab set, to avoid freeing
609 	 * these as real pages.
610 	 *
611 	 * We also want to avoid inserting kfence_free() in the kfree()
612 	 * fast-path in SLUB, and therefore need to ensure kfree() correctly
613 	 * enters __slab_free() slow-path.
614 	 */
615 	for (i = 0; i < KFENCE_POOL_SIZE / PAGE_SIZE; i++) {
616 		struct slab *slab = page_slab(nth_page(pages, i));
617 
618 		if (!i || (i % 2))
619 			continue;
620 
621 		__folio_set_slab(slab_folio(slab));
622 #ifdef CONFIG_MEMCG
623 		slab->obj_exts = (unsigned long)&kfence_metadata_init[i / 2 - 1].obj_exts |
624 				 MEMCG_DATA_OBJEXTS;
625 #endif
626 	}
627 
628 	/*
629 	 * Protect the first 2 pages. The first page is mostly unnecessary, and
630 	 * merely serves as an extended guard page. However, adding one
631 	 * additional page in the beginning gives us an even number of pages,
632 	 * which simplifies the mapping of address to metadata index.
633 	 */
634 	for (i = 0; i < 2; i++) {
635 		if (unlikely(!kfence_protect(addr)))
636 			return addr;
637 
638 		addr += PAGE_SIZE;
639 	}
640 
641 	for (i = 0; i < CONFIG_KFENCE_NUM_OBJECTS; i++) {
642 		struct kfence_metadata *meta = &kfence_metadata_init[i];
643 
644 		/* Initialize metadata. */
645 		INIT_LIST_HEAD(&meta->list);
646 		raw_spin_lock_init(&meta->lock);
647 		meta->state = KFENCE_OBJECT_UNUSED;
648 		meta->addr = addr; /* Initialize for validation in metadata_to_pageaddr(). */
649 		list_add_tail(&meta->list, &kfence_freelist);
650 
651 		/* Protect the right redzone. */
652 		if (unlikely(!kfence_protect(addr + PAGE_SIZE)))
653 			goto reset_slab;
654 
655 		addr += 2 * PAGE_SIZE;
656 	}
657 
658 	/*
659 	 * Make kfence_metadata visible only when initialization is successful.
660 	 * Otherwise, if the initialization fails and kfence_metadata is freed,
661 	 * it may cause UAF in kfence_shutdown_cache().
662 	 */
663 	smp_store_release(&kfence_metadata, kfence_metadata_init);
664 	return 0;
665 
666 reset_slab:
667 	for (i = 0; i < KFENCE_POOL_SIZE / PAGE_SIZE; i++) {
668 		struct slab *slab = page_slab(nth_page(pages, i));
669 
670 		if (!i || (i % 2))
671 			continue;
672 #ifdef CONFIG_MEMCG
673 		slab->obj_exts = 0;
674 #endif
675 		__folio_clear_slab(slab_folio(slab));
676 	}
677 
678 	return addr;
679 }
680 
kfence_init_pool_early(void)681 static bool __init kfence_init_pool_early(void)
682 {
683 	unsigned long addr;
684 
685 	if (!__kfence_pool)
686 		return false;
687 
688 	addr = kfence_init_pool();
689 
690 	if (!addr) {
691 		/*
692 		 * The pool is live and will never be deallocated from this point on.
693 		 * Ignore the pool object from the kmemleak phys object tree, as it would
694 		 * otherwise overlap with allocations returned by kfence_alloc(), which
695 		 * are registered with kmemleak through the slab post-alloc hook.
696 		 */
697 		kmemleak_ignore_phys(__pa(__kfence_pool));
698 		return true;
699 	}
700 
701 	/*
702 	 * Only release unprotected pages, and do not try to go back and change
703 	 * page attributes due to risk of failing to do so as well. If changing
704 	 * page attributes for some pages fails, it is very likely that it also
705 	 * fails for the first page, and therefore expect addr==__kfence_pool in
706 	 * most failure cases.
707 	 */
708 	memblock_free_late(__pa(addr), KFENCE_POOL_SIZE - (addr - (unsigned long)__kfence_pool));
709 	__kfence_pool = NULL;
710 
711 	memblock_free_late(__pa(kfence_metadata_init), KFENCE_METADATA_SIZE);
712 	kfence_metadata_init = NULL;
713 
714 	return false;
715 }
716 
717 /* === DebugFS Interface ==================================================== */
718 
stats_show(struct seq_file * seq,void * v)719 static int stats_show(struct seq_file *seq, void *v)
720 {
721 	int i;
722 
723 	seq_printf(seq, "enabled: %i\n", READ_ONCE(kfence_enabled));
724 	for (i = 0; i < KFENCE_COUNTER_COUNT; i++)
725 		seq_printf(seq, "%s: %ld\n", counter_names[i], atomic_long_read(&counters[i]));
726 
727 	return 0;
728 }
729 DEFINE_SHOW_ATTRIBUTE(stats);
730 
731 /*
732  * debugfs seq_file operations for /sys/kernel/debug/kfence/objects.
733  * start_object() and next_object() return the object index + 1, because NULL is used
734  * to stop iteration.
735  */
start_object(struct seq_file * seq,loff_t * pos)736 static void *start_object(struct seq_file *seq, loff_t *pos)
737 {
738 	if (*pos < CONFIG_KFENCE_NUM_OBJECTS)
739 		return (void *)((long)*pos + 1);
740 	return NULL;
741 }
742 
stop_object(struct seq_file * seq,void * v)743 static void stop_object(struct seq_file *seq, void *v)
744 {
745 }
746 
next_object(struct seq_file * seq,void * v,loff_t * pos)747 static void *next_object(struct seq_file *seq, void *v, loff_t *pos)
748 {
749 	++*pos;
750 	if (*pos < CONFIG_KFENCE_NUM_OBJECTS)
751 		return (void *)((long)*pos + 1);
752 	return NULL;
753 }
754 
show_object(struct seq_file * seq,void * v)755 static int show_object(struct seq_file *seq, void *v)
756 {
757 	struct kfence_metadata *meta = &kfence_metadata[(long)v - 1];
758 	unsigned long flags;
759 
760 	raw_spin_lock_irqsave(&meta->lock, flags);
761 	kfence_print_object(seq, meta);
762 	raw_spin_unlock_irqrestore(&meta->lock, flags);
763 	seq_puts(seq, "---------------------------------\n");
764 
765 	return 0;
766 }
767 
768 static const struct seq_operations objects_sops = {
769 	.start = start_object,
770 	.next = next_object,
771 	.stop = stop_object,
772 	.show = show_object,
773 };
774 DEFINE_SEQ_ATTRIBUTE(objects);
775 
kfence_debugfs_init(void)776 static int kfence_debugfs_init(void)
777 {
778 	struct dentry *kfence_dir;
779 
780 	if (!READ_ONCE(kfence_enabled))
781 		return 0;
782 
783 	kfence_dir = debugfs_create_dir("kfence", NULL);
784 	debugfs_create_file("stats", 0444, kfence_dir, NULL, &stats_fops);
785 	debugfs_create_file("objects", 0400, kfence_dir, NULL, &objects_fops);
786 	return 0;
787 }
788 
789 late_initcall(kfence_debugfs_init);
790 
791 /* === Panic Notifier ====================================================== */
792 
kfence_check_all_canary(void)793 static void kfence_check_all_canary(void)
794 {
795 	int i;
796 
797 	for (i = 0; i < CONFIG_KFENCE_NUM_OBJECTS; i++) {
798 		struct kfence_metadata *meta = &kfence_metadata[i];
799 
800 		if (kfence_obj_allocated(meta))
801 			check_canary(meta);
802 	}
803 }
804 
kfence_check_canary_callback(struct notifier_block * nb,unsigned long reason,void * arg)805 static int kfence_check_canary_callback(struct notifier_block *nb,
806 					unsigned long reason, void *arg)
807 {
808 	kfence_check_all_canary();
809 	return NOTIFY_OK;
810 }
811 
812 static struct notifier_block kfence_check_canary_notifier = {
813 	.notifier_call = kfence_check_canary_callback,
814 };
815 
816 /* === Allocation Gate Timer ================================================ */
817 
818 static struct delayed_work kfence_timer;
819 
820 #ifdef CONFIG_KFENCE_STATIC_KEYS
821 /* Wait queue to wake up allocation-gate timer task. */
822 static DECLARE_WAIT_QUEUE_HEAD(allocation_wait);
823 
wake_up_kfence_timer(struct irq_work * work)824 static void wake_up_kfence_timer(struct irq_work *work)
825 {
826 	wake_up(&allocation_wait);
827 }
828 static DEFINE_IRQ_WORK(wake_up_kfence_timer_work, wake_up_kfence_timer);
829 #endif
830 
831 /*
832  * Set up delayed work, which will enable and disable the static key. We need to
833  * use a work queue (rather than a simple timer), since enabling and disabling a
834  * static key cannot be done from an interrupt.
835  *
836  * Note: Toggling a static branch currently causes IPIs, and here we'll end up
837  * with a total of 2 IPIs to all CPUs. If this ends up a problem in future (with
838  * more aggressive sampling intervals), we could get away with a variant that
839  * avoids IPIs, at the cost of not immediately capturing allocations if the
840  * instructions remain cached.
841  */
toggle_allocation_gate(struct work_struct * work)842 static void toggle_allocation_gate(struct work_struct *work)
843 {
844 	if (!READ_ONCE(kfence_enabled))
845 		return;
846 
847 	atomic_set(&kfence_allocation_gate, -kfence_burst);
848 #ifdef CONFIG_KFENCE_STATIC_KEYS
849 	/* Enable static key, and await allocation to happen. */
850 	static_branch_enable(&kfence_allocation_key);
851 
852 	wait_event_idle(allocation_wait, atomic_read(&kfence_allocation_gate) > 0);
853 
854 	/* Disable static key and reset timer. */
855 	static_branch_disable(&kfence_allocation_key);
856 #endif
857 	queue_delayed_work(system_unbound_wq, &kfence_timer,
858 			   msecs_to_jiffies(kfence_sample_interval));
859 }
860 
861 /* === Public interface ===================================================== */
862 
kfence_alloc_pool_and_metadata(void)863 void __init kfence_alloc_pool_and_metadata(void)
864 {
865 	if (!kfence_sample_interval)
866 		return;
867 
868 	/*
869 	 * If the pool has already been initialized by arch, there is no need to
870 	 * re-allocate the memory pool.
871 	 */
872 	if (!__kfence_pool)
873 		__kfence_pool = memblock_alloc(KFENCE_POOL_SIZE, PAGE_SIZE);
874 
875 	if (!__kfence_pool) {
876 		pr_err("failed to allocate pool\n");
877 		return;
878 	}
879 
880 	/* The memory allocated by memblock has been zeroed out. */
881 	kfence_metadata_init = memblock_alloc(KFENCE_METADATA_SIZE, PAGE_SIZE);
882 	if (!kfence_metadata_init) {
883 		pr_err("failed to allocate metadata\n");
884 		memblock_free(__kfence_pool, KFENCE_POOL_SIZE);
885 		__kfence_pool = NULL;
886 	}
887 }
888 
kfence_init_enable(void)889 static void kfence_init_enable(void)
890 {
891 	if (!IS_ENABLED(CONFIG_KFENCE_STATIC_KEYS))
892 		static_branch_enable(&kfence_allocation_key);
893 
894 	if (kfence_deferrable)
895 		INIT_DEFERRABLE_WORK(&kfence_timer, toggle_allocation_gate);
896 	else
897 		INIT_DELAYED_WORK(&kfence_timer, toggle_allocation_gate);
898 
899 	if (kfence_check_on_panic)
900 		atomic_notifier_chain_register(&panic_notifier_list, &kfence_check_canary_notifier);
901 
902 	WRITE_ONCE(kfence_enabled, true);
903 	queue_delayed_work(system_unbound_wq, &kfence_timer, 0);
904 
905 	pr_info("initialized - using %lu bytes for %d objects at 0x%p-0x%p\n", KFENCE_POOL_SIZE,
906 		CONFIG_KFENCE_NUM_OBJECTS, (void *)__kfence_pool,
907 		(void *)(__kfence_pool + KFENCE_POOL_SIZE));
908 }
909 
kfence_init(void)910 void __init kfence_init(void)
911 {
912 	stack_hash_seed = get_random_u32();
913 
914 	/* Setting kfence_sample_interval to 0 on boot disables KFENCE. */
915 	if (!kfence_sample_interval)
916 		return;
917 
918 	if (!kfence_init_pool_early()) {
919 		pr_err("%s failed\n", __func__);
920 		return;
921 	}
922 
923 	kfence_init_enable();
924 }
925 
kfence_init_late(void)926 static int kfence_init_late(void)
927 {
928 	const unsigned long nr_pages_pool = KFENCE_POOL_SIZE / PAGE_SIZE;
929 	const unsigned long nr_pages_meta = KFENCE_METADATA_SIZE / PAGE_SIZE;
930 	unsigned long addr = (unsigned long)__kfence_pool;
931 	unsigned long free_size = KFENCE_POOL_SIZE;
932 	int err = -ENOMEM;
933 
934 #ifdef CONFIG_CONTIG_ALLOC
935 	struct page *pages;
936 
937 	pages = alloc_contig_pages(nr_pages_pool, GFP_KERNEL, first_online_node,
938 				   NULL);
939 	if (!pages)
940 		return -ENOMEM;
941 
942 	__kfence_pool = page_to_virt(pages);
943 	pages = alloc_contig_pages(nr_pages_meta, GFP_KERNEL, first_online_node,
944 				   NULL);
945 	if (pages)
946 		kfence_metadata_init = page_to_virt(pages);
947 #else
948 	if (nr_pages_pool > MAX_ORDER_NR_PAGES ||
949 	    nr_pages_meta > MAX_ORDER_NR_PAGES) {
950 		pr_warn("KFENCE_NUM_OBJECTS too large for buddy allocator\n");
951 		return -EINVAL;
952 	}
953 
954 	__kfence_pool = alloc_pages_exact(KFENCE_POOL_SIZE, GFP_KERNEL);
955 	if (!__kfence_pool)
956 		return -ENOMEM;
957 
958 	kfence_metadata_init = alloc_pages_exact(KFENCE_METADATA_SIZE, GFP_KERNEL);
959 #endif
960 
961 	if (!kfence_metadata_init)
962 		goto free_pool;
963 
964 	memzero_explicit(kfence_metadata_init, KFENCE_METADATA_SIZE);
965 	addr = kfence_init_pool();
966 	if (!addr) {
967 		kfence_init_enable();
968 		kfence_debugfs_init();
969 		return 0;
970 	}
971 
972 	pr_err("%s failed\n", __func__);
973 	free_size = KFENCE_POOL_SIZE - (addr - (unsigned long)__kfence_pool);
974 	err = -EBUSY;
975 
976 #ifdef CONFIG_CONTIG_ALLOC
977 	free_contig_range(page_to_pfn(virt_to_page((void *)kfence_metadata_init)),
978 			  nr_pages_meta);
979 free_pool:
980 	free_contig_range(page_to_pfn(virt_to_page((void *)addr)),
981 			  free_size / PAGE_SIZE);
982 #else
983 	free_pages_exact((void *)kfence_metadata_init, KFENCE_METADATA_SIZE);
984 free_pool:
985 	free_pages_exact((void *)addr, free_size);
986 #endif
987 
988 	kfence_metadata_init = NULL;
989 	__kfence_pool = NULL;
990 	return err;
991 }
992 
kfence_enable_late(void)993 static int kfence_enable_late(void)
994 {
995 	if (!__kfence_pool)
996 		return kfence_init_late();
997 
998 	WRITE_ONCE(kfence_enabled, true);
999 	queue_delayed_work(system_unbound_wq, &kfence_timer, 0);
1000 	pr_info("re-enabled\n");
1001 	return 0;
1002 }
1003 
kfence_shutdown_cache(struct kmem_cache * s)1004 void kfence_shutdown_cache(struct kmem_cache *s)
1005 {
1006 	unsigned long flags;
1007 	struct kfence_metadata *meta;
1008 	int i;
1009 
1010 	/* Pairs with release in kfence_init_pool(). */
1011 	if (!smp_load_acquire(&kfence_metadata))
1012 		return;
1013 
1014 	for (i = 0; i < CONFIG_KFENCE_NUM_OBJECTS; i++) {
1015 		bool in_use;
1016 
1017 		meta = &kfence_metadata[i];
1018 
1019 		/*
1020 		 * If we observe some inconsistent cache and state pair where we
1021 		 * should have returned false here, cache destruction is racing
1022 		 * with either kmem_cache_alloc() or kmem_cache_free(). Taking
1023 		 * the lock will not help, as different critical section
1024 		 * serialization will have the same outcome.
1025 		 */
1026 		if (READ_ONCE(meta->cache) != s || !kfence_obj_allocated(meta))
1027 			continue;
1028 
1029 		raw_spin_lock_irqsave(&meta->lock, flags);
1030 		in_use = meta->cache == s && kfence_obj_allocated(meta);
1031 		raw_spin_unlock_irqrestore(&meta->lock, flags);
1032 
1033 		if (in_use) {
1034 			/*
1035 			 * This cache still has allocations, and we should not
1036 			 * release them back into the freelist so they can still
1037 			 * safely be used and retain the kernel's default
1038 			 * behaviour of keeping the allocations alive (leak the
1039 			 * cache); however, they effectively become "zombie
1040 			 * allocations" as the KFENCE objects are the only ones
1041 			 * still in use and the owning cache is being destroyed.
1042 			 *
1043 			 * We mark them freed, so that any subsequent use shows
1044 			 * more useful error messages that will include stack
1045 			 * traces of the user of the object, the original
1046 			 * allocation, and caller to shutdown_cache().
1047 			 */
1048 			kfence_guarded_free((void *)meta->addr, meta, /*zombie=*/true);
1049 		}
1050 	}
1051 
1052 	for (i = 0; i < CONFIG_KFENCE_NUM_OBJECTS; i++) {
1053 		meta = &kfence_metadata[i];
1054 
1055 		/* See above. */
1056 		if (READ_ONCE(meta->cache) != s || READ_ONCE(meta->state) != KFENCE_OBJECT_FREED)
1057 			continue;
1058 
1059 		raw_spin_lock_irqsave(&meta->lock, flags);
1060 		if (meta->cache == s && meta->state == KFENCE_OBJECT_FREED)
1061 			meta->cache = NULL;
1062 		raw_spin_unlock_irqrestore(&meta->lock, flags);
1063 	}
1064 }
1065 
__kfence_alloc(struct kmem_cache * s,size_t size,gfp_t flags)1066 void *__kfence_alloc(struct kmem_cache *s, size_t size, gfp_t flags)
1067 {
1068 	unsigned long stack_entries[KFENCE_STACK_DEPTH];
1069 	size_t num_stack_entries;
1070 	u32 alloc_stack_hash;
1071 	int allocation_gate;
1072 
1073 	/*
1074 	 * Perform size check before switching kfence_allocation_gate, so that
1075 	 * we don't disable KFENCE without making an allocation.
1076 	 */
1077 	if (size > PAGE_SIZE) {
1078 		atomic_long_inc(&counters[KFENCE_COUNTER_SKIP_INCOMPAT]);
1079 		return NULL;
1080 	}
1081 
1082 	/*
1083 	 * Skip allocations from non-default zones, including DMA. We cannot
1084 	 * guarantee that pages in the KFENCE pool will have the requested
1085 	 * properties (e.g. reside in DMAable memory).
1086 	 */
1087 	if ((flags & GFP_ZONEMASK) ||
1088 	    ((flags & __GFP_THISNODE) && num_online_nodes() > 1) ||
1089 	    (s->flags & (SLAB_CACHE_DMA | SLAB_CACHE_DMA32))) {
1090 		atomic_long_inc(&counters[KFENCE_COUNTER_SKIP_INCOMPAT]);
1091 		return NULL;
1092 	}
1093 
1094 	/*
1095 	 * Skip allocations for this slab, if KFENCE has been disabled for
1096 	 * this slab.
1097 	 */
1098 	if (s->flags & SLAB_SKIP_KFENCE)
1099 		return NULL;
1100 
1101 	allocation_gate = atomic_inc_return(&kfence_allocation_gate);
1102 	if (allocation_gate > 1)
1103 		return NULL;
1104 #ifdef CONFIG_KFENCE_STATIC_KEYS
1105 	/*
1106 	 * waitqueue_active() is fully ordered after the update of
1107 	 * kfence_allocation_gate per atomic_inc_return().
1108 	 */
1109 	if (allocation_gate == 1 && waitqueue_active(&allocation_wait)) {
1110 		/*
1111 		 * Calling wake_up() here may deadlock when allocations happen
1112 		 * from within timer code. Use an irq_work to defer it.
1113 		 */
1114 		irq_work_queue(&wake_up_kfence_timer_work);
1115 	}
1116 #endif
1117 
1118 	if (!READ_ONCE(kfence_enabled))
1119 		return NULL;
1120 
1121 	num_stack_entries = stack_trace_save(stack_entries, KFENCE_STACK_DEPTH, 0);
1122 
1123 	/*
1124 	 * Do expensive check for coverage of allocation in slow-path after
1125 	 * allocation_gate has already become non-zero, even though it might
1126 	 * mean not making any allocation within a given sample interval.
1127 	 *
1128 	 * This ensures reasonable allocation coverage when the pool is almost
1129 	 * full, including avoiding long-lived allocations of the same source
1130 	 * filling up the pool (e.g. pagecache allocations).
1131 	 */
1132 	alloc_stack_hash = get_alloc_stack_hash(stack_entries, num_stack_entries);
1133 	if (should_skip_covered() && alloc_covered_contains(alloc_stack_hash)) {
1134 		atomic_long_inc(&counters[KFENCE_COUNTER_SKIP_COVERED]);
1135 		return NULL;
1136 	}
1137 
1138 	return kfence_guarded_alloc(s, size, flags, stack_entries, num_stack_entries,
1139 				    alloc_stack_hash);
1140 }
1141 
kfence_ksize(const void * addr)1142 size_t kfence_ksize(const void *addr)
1143 {
1144 	const struct kfence_metadata *meta = addr_to_metadata((unsigned long)addr);
1145 
1146 	/*
1147 	 * Read locklessly -- if there is a race with __kfence_alloc(), this is
1148 	 * either a use-after-free or invalid access.
1149 	 */
1150 	return meta ? meta->size : 0;
1151 }
1152 
kfence_object_start(const void * addr)1153 void *kfence_object_start(const void *addr)
1154 {
1155 	const struct kfence_metadata *meta = addr_to_metadata((unsigned long)addr);
1156 
1157 	/*
1158 	 * Read locklessly -- if there is a race with __kfence_alloc(), this is
1159 	 * either a use-after-free or invalid access.
1160 	 */
1161 	return meta ? (void *)meta->addr : NULL;
1162 }
1163 
__kfence_free(void * addr)1164 void __kfence_free(void *addr)
1165 {
1166 	struct kfence_metadata *meta = addr_to_metadata((unsigned long)addr);
1167 
1168 #ifdef CONFIG_MEMCG
1169 	KFENCE_WARN_ON(meta->obj_exts.objcg);
1170 #endif
1171 	/*
1172 	 * If the objects of the cache are SLAB_TYPESAFE_BY_RCU, defer freeing
1173 	 * the object, as the object page may be recycled for other-typed
1174 	 * objects once it has been freed. meta->cache may be NULL if the cache
1175 	 * was destroyed.
1176 	 * Save the stack trace here so that reports show where the user freed
1177 	 * the object.
1178 	 */
1179 	if (unlikely(meta->cache && (meta->cache->flags & SLAB_TYPESAFE_BY_RCU))) {
1180 		unsigned long flags;
1181 
1182 		raw_spin_lock_irqsave(&meta->lock, flags);
1183 		metadata_update_state(meta, KFENCE_OBJECT_RCU_FREEING, NULL, 0);
1184 		raw_spin_unlock_irqrestore(&meta->lock, flags);
1185 		call_rcu(&meta->rcu_head, rcu_guarded_free);
1186 	} else {
1187 		kfence_guarded_free(addr, meta, false);
1188 	}
1189 }
1190 
kfence_handle_page_fault(unsigned long addr,bool is_write,struct pt_regs * regs)1191 bool kfence_handle_page_fault(unsigned long addr, bool is_write, struct pt_regs *regs)
1192 {
1193 	const int page_index = (addr - (unsigned long)__kfence_pool) / PAGE_SIZE;
1194 	struct kfence_metadata *to_report = NULL;
1195 	enum kfence_error_type error_type;
1196 	unsigned long flags;
1197 
1198 	if (!is_kfence_address((void *)addr))
1199 		return false;
1200 
1201 	if (!READ_ONCE(kfence_enabled)) /* If disabled at runtime ... */
1202 		return kfence_unprotect(addr); /* ... unprotect and proceed. */
1203 
1204 	atomic_long_inc(&counters[KFENCE_COUNTER_BUGS]);
1205 
1206 	if (page_index % 2) {
1207 		/* This is a redzone, report a buffer overflow. */
1208 		struct kfence_metadata *meta;
1209 		int distance = 0;
1210 
1211 		meta = addr_to_metadata(addr - PAGE_SIZE);
1212 		if (meta && kfence_obj_allocated(meta)) {
1213 			to_report = meta;
1214 			/* Data race ok; distance calculation approximate. */
1215 			distance = addr - data_race(meta->addr + meta->size);
1216 		}
1217 
1218 		meta = addr_to_metadata(addr + PAGE_SIZE);
1219 		if (meta && kfence_obj_allocated(meta)) {
1220 			/* Data race ok; distance calculation approximate. */
1221 			if (!to_report || distance > data_race(meta->addr) - addr)
1222 				to_report = meta;
1223 		}
1224 
1225 		if (!to_report)
1226 			goto out;
1227 
1228 		raw_spin_lock_irqsave(&to_report->lock, flags);
1229 		to_report->unprotected_page = addr;
1230 		error_type = KFENCE_ERROR_OOB;
1231 
1232 		/*
1233 		 * If the object was freed before we took the look we can still
1234 		 * report this as an OOB -- the report will simply show the
1235 		 * stacktrace of the free as well.
1236 		 */
1237 	} else {
1238 		to_report = addr_to_metadata(addr);
1239 		if (!to_report)
1240 			goto out;
1241 
1242 		raw_spin_lock_irqsave(&to_report->lock, flags);
1243 		error_type = KFENCE_ERROR_UAF;
1244 		/*
1245 		 * We may race with __kfence_alloc(), and it is possible that a
1246 		 * freed object may be reallocated. We simply report this as a
1247 		 * use-after-free, with the stack trace showing the place where
1248 		 * the object was re-allocated.
1249 		 */
1250 	}
1251 
1252 out:
1253 	if (to_report) {
1254 		kfence_report_error(addr, is_write, regs, to_report, error_type);
1255 		raw_spin_unlock_irqrestore(&to_report->lock, flags);
1256 	} else {
1257 		/* This may be a UAF or OOB access, but we can't be sure. */
1258 		kfence_report_error(addr, is_write, regs, NULL, KFENCE_ERROR_INVALID);
1259 	}
1260 
1261 	return kfence_unprotect(addr); /* Unprotect and let access proceed. */
1262 }
1263