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/reboot.h>
30 #include <linux/sched/clock.h>
31 #include <linux/seq_file.h>
32 #include <linux/slab.h>
33 #include <linux/spinlock.h>
34 #include <linux/string.h>
35
36 #include <asm/kfence.h>
37
38 #include "kfence.h"
39
40 /* Disables KFENCE on the first warning assuming an irrecoverable error. */
41 #define KFENCE_WARN_ON(cond) \
42 ({ \
43 const bool __cond = WARN_ON(cond); \
44 if (unlikely(__cond)) { \
45 WRITE_ONCE(kfence_enabled, false); \
46 disabled_by_warn = true; \
47 } \
48 __cond; \
49 })
50
51 /* === Data ================================================================= */
52
53 static bool kfence_enabled __read_mostly;
54 static bool disabled_by_warn __read_mostly;
55
56 unsigned long kfence_sample_interval __read_mostly = CONFIG_KFENCE_SAMPLE_INTERVAL;
57 EXPORT_SYMBOL_GPL(kfence_sample_interval); /* Export for test modules. */
58
59 #ifdef MODULE_PARAM_PREFIX
60 #undef MODULE_PARAM_PREFIX
61 #endif
62 #define MODULE_PARAM_PREFIX "kfence."
63
64 static int kfence_enable_late(void);
param_set_sample_interval(const char * val,const struct kernel_param * kp)65 static int param_set_sample_interval(const char *val, const struct kernel_param *kp)
66 {
67 unsigned long num;
68 int ret = kstrtoul(val, 0, &num);
69
70 if (ret < 0)
71 return ret;
72
73 /* Using 0 to indicate KFENCE is disabled. */
74 if (!num && READ_ONCE(kfence_enabled)) {
75 pr_info("disabled\n");
76 WRITE_ONCE(kfence_enabled, false);
77 }
78
79 *((unsigned long *)kp->arg) = num;
80
81 if (num && !READ_ONCE(kfence_enabled) && system_state != SYSTEM_BOOTING)
82 return disabled_by_warn ? -EINVAL : kfence_enable_late();
83 return 0;
84 }
85
param_get_sample_interval(char * buffer,const struct kernel_param * kp)86 static int param_get_sample_interval(char *buffer, const struct kernel_param *kp)
87 {
88 if (!READ_ONCE(kfence_enabled))
89 return sprintf(buffer, "0\n");
90
91 return param_get_ulong(buffer, kp);
92 }
93
94 static const struct kernel_param_ops sample_interval_param_ops = {
95 .set = param_set_sample_interval,
96 .get = param_get_sample_interval,
97 };
98 module_param_cb(sample_interval, &sample_interval_param_ops, &kfence_sample_interval, 0600);
99
100 /* Pool usage% threshold when currently covered allocations are skipped. */
101 static unsigned long kfence_skip_covered_thresh __read_mostly = 75;
102 module_param_named(skip_covered_thresh, kfence_skip_covered_thresh, ulong, 0644);
103
104 /* Allocation burst count: number of excess KFENCE allocations per sample. */
105 static unsigned int kfence_burst __read_mostly;
106 module_param_named(burst, kfence_burst, uint, 0644);
107
108 /* If true, use a deferrable timer. */
109 static bool kfence_deferrable __read_mostly = IS_ENABLED(CONFIG_KFENCE_DEFERRABLE);
110 module_param_named(deferrable, kfence_deferrable, bool, 0444);
111
112 /* If true, check all canary bytes on panic. */
113 static bool kfence_check_on_panic __read_mostly;
114 module_param_named(check_on_panic, kfence_check_on_panic, bool, 0444);
115
116 /* The pool of pages used for guard pages and objects. */
117 char *__kfence_pool __read_mostly;
118 EXPORT_SYMBOL(__kfence_pool); /* Export for test modules. */
119
120 /*
121 * Per-object metadata, with one-to-one mapping of object metadata to
122 * backing pages (in __kfence_pool).
123 */
124 static_assert(CONFIG_KFENCE_NUM_OBJECTS > 0);
125 struct kfence_metadata *kfence_metadata __read_mostly;
126
127 /*
128 * If kfence_metadata is not NULL, it may be accessed by kfence_shutdown_cache().
129 * So introduce kfence_metadata_init to initialize metadata, and then make
130 * kfence_metadata visible after initialization is successful. This prevents
131 * potential UAF or access to uninitialized metadata.
132 */
133 static struct kfence_metadata *kfence_metadata_init __read_mostly;
134
135 /* Freelist with available objects. */
136 static struct list_head kfence_freelist = LIST_HEAD_INIT(kfence_freelist);
137 static DEFINE_RAW_SPINLOCK(kfence_freelist_lock); /* Lock protecting freelist. */
138
139 /*
140 * The static key to set up a KFENCE allocation; or if static keys are not used
141 * to gate allocations, to avoid a load and compare if KFENCE is disabled.
142 */
143 DEFINE_STATIC_KEY_FALSE(kfence_allocation_key);
144
145 /* Gates the allocation, ensuring only one succeeds in a given period. */
146 atomic_t kfence_allocation_gate = ATOMIC_INIT(1);
147
148 /*
149 * A Counting Bloom filter of allocation coverage: limits currently covered
150 * allocations of the same source filling up the pool.
151 *
152 * Assuming a range of 15%-85% unique allocations in the pool at any point in
153 * time, the below parameters provide a probablity of 0.02-0.33 for false
154 * positive hits respectively:
155 *
156 * P(alloc_traces) = (1 - e^(-HNUM * (alloc_traces / SIZE)) ^ HNUM
157 */
158 #define ALLOC_COVERED_HNUM 2
159 #define ALLOC_COVERED_ORDER (const_ilog2(CONFIG_KFENCE_NUM_OBJECTS) + 2)
160 #define ALLOC_COVERED_SIZE (1 << ALLOC_COVERED_ORDER)
161 #define ALLOC_COVERED_HNEXT(h) hash_32(h, ALLOC_COVERED_ORDER)
162 #define ALLOC_COVERED_MASK (ALLOC_COVERED_SIZE - 1)
163 static atomic_t alloc_covered[ALLOC_COVERED_SIZE];
164
165 /* Stack depth used to determine uniqueness of an allocation. */
166 #define UNIQUE_ALLOC_STACK_DEPTH ((size_t)8)
167
168 /*
169 * Randomness for stack hashes, making the same collisions across reboots and
170 * different machines less likely.
171 */
172 static u32 stack_hash_seed __ro_after_init;
173
174 /* Statistics counters for debugfs. */
175 enum kfence_counter_id {
176 KFENCE_COUNTER_ALLOCATED,
177 KFENCE_COUNTER_ALLOCS,
178 KFENCE_COUNTER_FREES,
179 KFENCE_COUNTER_ZOMBIES,
180 KFENCE_COUNTER_BUGS,
181 KFENCE_COUNTER_SKIP_INCOMPAT,
182 KFENCE_COUNTER_SKIP_CAPACITY,
183 KFENCE_COUNTER_SKIP_COVERED,
184 KFENCE_COUNTER_COUNT,
185 };
186 static atomic_long_t counters[KFENCE_COUNTER_COUNT];
187 static const char *const counter_names[] = {
188 [KFENCE_COUNTER_ALLOCATED] = "currently allocated",
189 [KFENCE_COUNTER_ALLOCS] = "total allocations",
190 [KFENCE_COUNTER_FREES] = "total frees",
191 [KFENCE_COUNTER_ZOMBIES] = "zombie allocations",
192 [KFENCE_COUNTER_BUGS] = "total bugs",
193 [KFENCE_COUNTER_SKIP_INCOMPAT] = "skipped allocations (incompatible)",
194 [KFENCE_COUNTER_SKIP_CAPACITY] = "skipped allocations (capacity)",
195 [KFENCE_COUNTER_SKIP_COVERED] = "skipped allocations (covered)",
196 };
197 static_assert(ARRAY_SIZE(counter_names) == KFENCE_COUNTER_COUNT);
198
199 /* === Internals ============================================================ */
200
should_skip_covered(void)201 static inline bool should_skip_covered(void)
202 {
203 unsigned long thresh = (CONFIG_KFENCE_NUM_OBJECTS * kfence_skip_covered_thresh) / 100;
204
205 return atomic_long_read(&counters[KFENCE_COUNTER_ALLOCATED]) > thresh;
206 }
207
get_alloc_stack_hash(unsigned long * stack_entries,size_t num_entries)208 static u32 get_alloc_stack_hash(unsigned long *stack_entries, size_t num_entries)
209 {
210 num_entries = min(num_entries, UNIQUE_ALLOC_STACK_DEPTH);
211 num_entries = filter_irq_stacks(stack_entries, num_entries);
212 return jhash(stack_entries, num_entries * sizeof(stack_entries[0]), stack_hash_seed);
213 }
214
215 /*
216 * Adds (or subtracts) count @val for allocation stack trace hash
217 * @alloc_stack_hash from Counting Bloom filter.
218 */
alloc_covered_add(u32 alloc_stack_hash,int val)219 static void alloc_covered_add(u32 alloc_stack_hash, int val)
220 {
221 int i;
222
223 for (i = 0; i < ALLOC_COVERED_HNUM; i++) {
224 atomic_add(val, &alloc_covered[alloc_stack_hash & ALLOC_COVERED_MASK]);
225 alloc_stack_hash = ALLOC_COVERED_HNEXT(alloc_stack_hash);
226 }
227 }
228
229 /*
230 * Returns true if the allocation stack trace hash @alloc_stack_hash is
231 * currently contained (non-zero count) in Counting Bloom filter.
232 */
alloc_covered_contains(u32 alloc_stack_hash)233 static bool alloc_covered_contains(u32 alloc_stack_hash)
234 {
235 int i;
236
237 for (i = 0; i < ALLOC_COVERED_HNUM; i++) {
238 if (!atomic_read(&alloc_covered[alloc_stack_hash & ALLOC_COVERED_MASK]))
239 return false;
240 alloc_stack_hash = ALLOC_COVERED_HNEXT(alloc_stack_hash);
241 }
242
243 return true;
244 }
245
kfence_protect(unsigned long addr)246 static bool kfence_protect(unsigned long addr)
247 {
248 return !KFENCE_WARN_ON(!kfence_protect_page(ALIGN_DOWN(addr, PAGE_SIZE), true));
249 }
250
kfence_unprotect(unsigned long addr)251 static bool kfence_unprotect(unsigned long addr)
252 {
253 return !KFENCE_WARN_ON(!kfence_protect_page(ALIGN_DOWN(addr, PAGE_SIZE), false));
254 }
255
metadata_to_pageaddr(const struct kfence_metadata * meta)256 static inline unsigned long metadata_to_pageaddr(const struct kfence_metadata *meta)
257 {
258 unsigned long offset = (meta - kfence_metadata + 1) * PAGE_SIZE * 2;
259 unsigned long pageaddr = (unsigned long)&__kfence_pool[offset];
260
261 /* The checks do not affect performance; only called from slow-paths. */
262
263 /* Only call with a pointer into kfence_metadata. */
264 if (KFENCE_WARN_ON(meta < kfence_metadata ||
265 meta >= kfence_metadata + CONFIG_KFENCE_NUM_OBJECTS))
266 return 0;
267
268 /*
269 * This metadata object only ever maps to 1 page; verify that the stored
270 * address is in the expected range.
271 */
272 if (KFENCE_WARN_ON(ALIGN_DOWN(meta->addr, PAGE_SIZE) != pageaddr))
273 return 0;
274
275 return pageaddr;
276 }
277
kfence_obj_allocated(const struct kfence_metadata * meta)278 static inline bool kfence_obj_allocated(const struct kfence_metadata *meta)
279 {
280 enum kfence_object_state state = READ_ONCE(meta->state);
281
282 return state == KFENCE_OBJECT_ALLOCATED || state == KFENCE_OBJECT_RCU_FREEING;
283 }
284
285 /*
286 * Update the object's metadata state, including updating the alloc/free stacks
287 * depending on the state transition.
288 */
289 static noinline void
metadata_update_state(struct kfence_metadata * meta,enum kfence_object_state next,unsigned long * stack_entries,size_t num_stack_entries)290 metadata_update_state(struct kfence_metadata *meta, enum kfence_object_state next,
291 unsigned long *stack_entries, size_t num_stack_entries)
292 {
293 struct kfence_track *track =
294 next == KFENCE_OBJECT_ALLOCATED ? &meta->alloc_track : &meta->free_track;
295
296 lockdep_assert_held(&meta->lock);
297
298 /* Stack has been saved when calling rcu, skip. */
299 if (READ_ONCE(meta->state) == KFENCE_OBJECT_RCU_FREEING)
300 goto out;
301
302 if (stack_entries) {
303 memcpy(track->stack_entries, stack_entries,
304 num_stack_entries * sizeof(stack_entries[0]));
305 } else {
306 /*
307 * Skip over 1 (this) functions; noinline ensures we do not
308 * accidentally skip over the caller by never inlining.
309 */
310 num_stack_entries = stack_trace_save(track->stack_entries, KFENCE_STACK_DEPTH, 1);
311 }
312 track->num_stack_entries = num_stack_entries;
313 track->pid = task_pid_nr(current);
314 track->cpu = raw_smp_processor_id();
315 track->ts_nsec = local_clock(); /* Same source as printk timestamps. */
316
317 out:
318 /*
319 * Pairs with READ_ONCE() in
320 * kfence_shutdown_cache(),
321 * kfence_handle_page_fault().
322 */
323 WRITE_ONCE(meta->state, next);
324 }
325
326 #ifdef CONFIG_KMSAN
327 #define check_canary_attributes noinline __no_kmsan_checks
328 #else
329 #define check_canary_attributes inline
330 #endif
331
332 /* Check canary byte at @addr. */
check_canary_byte(u8 * addr)333 static check_canary_attributes bool check_canary_byte(u8 *addr)
334 {
335 struct kfence_metadata *meta;
336 unsigned long flags;
337
338 if (likely(*addr == KFENCE_CANARY_PATTERN_U8(addr)))
339 return true;
340
341 atomic_long_inc(&counters[KFENCE_COUNTER_BUGS]);
342
343 meta = addr_to_metadata((unsigned long)addr);
344 raw_spin_lock_irqsave(&meta->lock, flags);
345 kfence_report_error((unsigned long)addr, false, NULL, meta, KFENCE_ERROR_CORRUPTION);
346 raw_spin_unlock_irqrestore(&meta->lock, flags);
347
348 return false;
349 }
350
set_canary(const struct kfence_metadata * meta)351 static inline void set_canary(const struct kfence_metadata *meta)
352 {
353 const unsigned long pageaddr = ALIGN_DOWN(meta->addr, PAGE_SIZE);
354 unsigned long addr = pageaddr;
355
356 /*
357 * The canary may be written to part of the object memory, but it does
358 * not affect it. The user should initialize the object before using it.
359 */
360 for (; addr < meta->addr; addr += sizeof(u64))
361 *((u64 *)addr) = KFENCE_CANARY_PATTERN_U64;
362
363 addr = ALIGN_DOWN(meta->addr + meta->size, sizeof(u64));
364 for (; addr - pageaddr < PAGE_SIZE; addr += sizeof(u64))
365 *((u64 *)addr) = KFENCE_CANARY_PATTERN_U64;
366 }
367
368 static check_canary_attributes void
check_canary(const struct kfence_metadata * meta)369 check_canary(const struct kfence_metadata *meta)
370 {
371 const unsigned long pageaddr = ALIGN_DOWN(meta->addr, PAGE_SIZE);
372 unsigned long addr = pageaddr;
373
374 /*
375 * We'll iterate over each canary byte per-side until a corrupted byte
376 * is found. However, we'll still iterate over the canary bytes to the
377 * right of the object even if there was an error in the canary bytes to
378 * the left of the object. Specifically, if check_canary_byte()
379 * generates an error, showing both sides might give more clues as to
380 * what the error is about when displaying which bytes were corrupted.
381 */
382
383 /* Apply to left of object. */
384 for (; meta->addr - addr >= sizeof(u64); addr += sizeof(u64)) {
385 if (unlikely(*((u64 *)addr) != KFENCE_CANARY_PATTERN_U64))
386 break;
387 }
388
389 /*
390 * If the canary is corrupted in a certain 64 bytes, or the canary
391 * memory cannot be completely covered by multiple consecutive 64 bytes,
392 * it needs to be checked one by one.
393 */
394 for (; addr < meta->addr; addr++) {
395 if (unlikely(!check_canary_byte((u8 *)addr)))
396 break;
397 }
398
399 /* Apply to right of object. */
400 for (addr = meta->addr + meta->size; addr % sizeof(u64) != 0; addr++) {
401 if (unlikely(!check_canary_byte((u8 *)addr)))
402 return;
403 }
404 for (; addr - pageaddr < PAGE_SIZE; addr += sizeof(u64)) {
405 if (unlikely(*((u64 *)addr) != KFENCE_CANARY_PATTERN_U64)) {
406
407 for (; addr - pageaddr < PAGE_SIZE; addr++) {
408 if (!check_canary_byte((u8 *)addr))
409 return;
410 }
411 }
412 }
413 }
414
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)415 static void *kfence_guarded_alloc(struct kmem_cache *cache, size_t size, gfp_t gfp,
416 unsigned long *stack_entries, size_t num_stack_entries,
417 u32 alloc_stack_hash)
418 {
419 struct kfence_metadata *meta = NULL;
420 unsigned long flags;
421 struct slab *slab;
422 void *addr;
423 const bool random_right_allocate = get_random_u32_below(2);
424 const bool random_fault = CONFIG_KFENCE_STRESS_TEST_FAULTS &&
425 !get_random_u32_below(CONFIG_KFENCE_STRESS_TEST_FAULTS);
426
427 /* Try to obtain a free object. */
428 raw_spin_lock_irqsave(&kfence_freelist_lock, flags);
429 if (!list_empty(&kfence_freelist)) {
430 meta = list_entry(kfence_freelist.next, struct kfence_metadata, list);
431 list_del_init(&meta->list);
432 }
433 raw_spin_unlock_irqrestore(&kfence_freelist_lock, flags);
434 if (!meta) {
435 atomic_long_inc(&counters[KFENCE_COUNTER_SKIP_CAPACITY]);
436 return NULL;
437 }
438
439 if (unlikely(!raw_spin_trylock_irqsave(&meta->lock, flags))) {
440 /*
441 * This is extremely unlikely -- we are reporting on a
442 * use-after-free, which locked meta->lock, and the reporting
443 * code via printk calls kmalloc() which ends up in
444 * kfence_alloc() and tries to grab the same object that we're
445 * reporting on. While it has never been observed, lockdep does
446 * report that there is a possibility of deadlock. Fix it by
447 * using trylock and bailing out gracefully.
448 */
449 raw_spin_lock_irqsave(&kfence_freelist_lock, flags);
450 /* Put the object back on the freelist. */
451 list_add_tail(&meta->list, &kfence_freelist);
452 raw_spin_unlock_irqrestore(&kfence_freelist_lock, flags);
453
454 return NULL;
455 }
456
457 meta->addr = metadata_to_pageaddr(meta);
458 /* Unprotect if we're reusing this page. */
459 if (meta->state == KFENCE_OBJECT_FREED)
460 kfence_unprotect(meta->addr);
461
462 /*
463 * Note: for allocations made before RNG initialization, will always
464 * return zero. We still benefit from enabling KFENCE as early as
465 * possible, even when the RNG is not yet available, as this will allow
466 * KFENCE to detect bugs due to earlier allocations. The only downside
467 * is that the out-of-bounds accesses detected are deterministic for
468 * such allocations.
469 */
470 if (random_right_allocate) {
471 /* Allocate on the "right" side, re-calculate address. */
472 meta->addr += PAGE_SIZE - size;
473 meta->addr = ALIGN_DOWN(meta->addr, cache->align);
474 }
475
476 addr = (void *)meta->addr;
477
478 /* Update remaining metadata. */
479 metadata_update_state(meta, KFENCE_OBJECT_ALLOCATED, stack_entries, num_stack_entries);
480 /* Pairs with READ_ONCE() in kfence_shutdown_cache(). */
481 WRITE_ONCE(meta->cache, cache);
482 meta->size = size;
483 meta->alloc_stack_hash = alloc_stack_hash;
484 raw_spin_unlock_irqrestore(&meta->lock, flags);
485
486 alloc_covered_add(alloc_stack_hash, 1);
487
488 /* Set required slab fields. */
489 slab = virt_to_slab((void *)meta->addr);
490 slab->slab_cache = cache;
491 slab->objects = 1;
492
493 /* Memory initialization. */
494 set_canary(meta);
495
496 /*
497 * We check slab_want_init_on_alloc() ourselves, rather than letting
498 * SL*B do the initialization, as otherwise we might overwrite KFENCE's
499 * redzone.
500 */
501 if (unlikely(slab_want_init_on_alloc(gfp, cache)))
502 memzero_explicit(addr, size);
503 if (cache->ctor)
504 cache->ctor(addr);
505
506 if (random_fault)
507 kfence_protect(meta->addr); /* Random "faults" by protecting the object. */
508
509 atomic_long_inc(&counters[KFENCE_COUNTER_ALLOCATED]);
510 atomic_long_inc(&counters[KFENCE_COUNTER_ALLOCS]);
511
512 return addr;
513 }
514
kfence_guarded_free(void * addr,struct kfence_metadata * meta,bool zombie)515 static void kfence_guarded_free(void *addr, struct kfence_metadata *meta, bool zombie)
516 {
517 struct kcsan_scoped_access assert_page_exclusive;
518 unsigned long flags;
519 bool init;
520
521 raw_spin_lock_irqsave(&meta->lock, flags);
522
523 if (!kfence_obj_allocated(meta) || meta->addr != (unsigned long)addr) {
524 /* Invalid or double-free, bail out. */
525 atomic_long_inc(&counters[KFENCE_COUNTER_BUGS]);
526 kfence_report_error((unsigned long)addr, false, NULL, meta,
527 KFENCE_ERROR_INVALID_FREE);
528 raw_spin_unlock_irqrestore(&meta->lock, flags);
529 return;
530 }
531
532 /* Detect racy use-after-free, or incorrect reallocation of this page by KFENCE. */
533 kcsan_begin_scoped_access((void *)ALIGN_DOWN((unsigned long)addr, PAGE_SIZE), PAGE_SIZE,
534 KCSAN_ACCESS_SCOPED | KCSAN_ACCESS_WRITE | KCSAN_ACCESS_ASSERT,
535 &assert_page_exclusive);
536
537 if (CONFIG_KFENCE_STRESS_TEST_FAULTS)
538 kfence_unprotect((unsigned long)addr); /* To check canary bytes. */
539
540 /* Restore page protection if there was an OOB access. */
541 if (meta->unprotected_page) {
542 memzero_explicit((void *)ALIGN_DOWN(meta->unprotected_page, PAGE_SIZE), PAGE_SIZE);
543 kfence_protect(meta->unprotected_page);
544 meta->unprotected_page = 0;
545 }
546
547 /* Mark the object as freed. */
548 metadata_update_state(meta, KFENCE_OBJECT_FREED, NULL, 0);
549 init = slab_want_init_on_free(meta->cache);
550 raw_spin_unlock_irqrestore(&meta->lock, flags);
551
552 alloc_covered_add(meta->alloc_stack_hash, -1);
553
554 /* Check canary bytes for memory corruption. */
555 check_canary(meta);
556
557 /*
558 * Clear memory if init-on-free is set. While we protect the page, the
559 * data is still there, and after a use-after-free is detected, we
560 * unprotect the page, so the data is still accessible.
561 */
562 if (!zombie && unlikely(init))
563 memzero_explicit(addr, meta->size);
564
565 /* Protect to detect use-after-frees. */
566 kfence_protect((unsigned long)addr);
567
568 kcsan_end_scoped_access(&assert_page_exclusive);
569 if (!zombie) {
570 /* Add it to the tail of the freelist for reuse. */
571 raw_spin_lock_irqsave(&kfence_freelist_lock, flags);
572 KFENCE_WARN_ON(!list_empty(&meta->list));
573 list_add_tail(&meta->list, &kfence_freelist);
574 raw_spin_unlock_irqrestore(&kfence_freelist_lock, flags);
575
576 atomic_long_dec(&counters[KFENCE_COUNTER_ALLOCATED]);
577 atomic_long_inc(&counters[KFENCE_COUNTER_FREES]);
578 } else {
579 /* See kfence_shutdown_cache(). */
580 atomic_long_inc(&counters[KFENCE_COUNTER_ZOMBIES]);
581 }
582 }
583
rcu_guarded_free(struct rcu_head * h)584 static void rcu_guarded_free(struct rcu_head *h)
585 {
586 struct kfence_metadata *meta = container_of(h, struct kfence_metadata, rcu_head);
587
588 kfence_guarded_free((void *)meta->addr, meta, false);
589 }
590
591 /*
592 * Initialization of the KFENCE pool after its allocation.
593 * Returns 0 on success; otherwise returns the address up to
594 * which partial initialization succeeded.
595 */
kfence_init_pool(void)596 static unsigned long kfence_init_pool(void)
597 {
598 unsigned long addr, start_pfn;
599 int i;
600
601 if (!arch_kfence_init_pool())
602 return (unsigned long)__kfence_pool;
603
604 addr = (unsigned long)__kfence_pool;
605 start_pfn = PHYS_PFN(virt_to_phys(__kfence_pool));
606
607 /*
608 * Set up object pages: they must have PGTY_slab set to avoid freeing
609 * them 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 page *page;
617
618 if (!i || (i % 2))
619 continue;
620
621 page = pfn_to_page(start_pfn + i);
622 __SetPageSlab(page);
623 #ifdef CONFIG_MEMCG
624 struct slab *slab = page_slab(page);
625 slab->obj_exts = (unsigned long)&kfence_metadata_init[i / 2 - 1].obj_exts |
626 MEMCG_DATA_OBJEXTS;
627 #endif
628 }
629
630 /*
631 * Protect the first 2 pages. The first page is mostly unnecessary, and
632 * merely serves as an extended guard page. However, adding one
633 * additional page in the beginning gives us an even number of pages,
634 * which simplifies the mapping of address to metadata index.
635 */
636 for (i = 0; i < 2; i++) {
637 if (unlikely(!kfence_protect(addr)))
638 return addr;
639
640 addr += PAGE_SIZE;
641 }
642
643 for (i = 0; i < CONFIG_KFENCE_NUM_OBJECTS; i++) {
644 struct kfence_metadata *meta = &kfence_metadata_init[i];
645
646 /* Initialize metadata. */
647 INIT_LIST_HEAD(&meta->list);
648 raw_spin_lock_init(&meta->lock);
649 meta->state = KFENCE_OBJECT_UNUSED;
650 meta->addr = addr; /* Initialize for validation in metadata_to_pageaddr(). */
651 list_add_tail(&meta->list, &kfence_freelist);
652
653 /* Protect the right redzone. */
654 if (unlikely(!kfence_protect(addr + PAGE_SIZE)))
655 goto reset_slab;
656
657 addr += 2 * PAGE_SIZE;
658 }
659
660 /*
661 * Make kfence_metadata visible only when initialization is successful.
662 * Otherwise, if the initialization fails and kfence_metadata is freed,
663 * it may cause UAF in kfence_shutdown_cache().
664 */
665 smp_store_release(&kfence_metadata, kfence_metadata_init);
666 return 0;
667
668 reset_slab:
669 for (i = 0; i < KFENCE_POOL_SIZE / PAGE_SIZE; i++) {
670 struct page *page;
671
672 if (!i || (i % 2))
673 continue;
674
675 page = pfn_to_page(start_pfn + i);
676 #ifdef CONFIG_MEMCG
677 struct slab *slab = page_slab(page);
678 slab->obj_exts = 0;
679 #endif
680 __ClearPageSlab(page);
681 }
682
683 return addr;
684 }
685
kfence_init_pool_early(void)686 static bool __init kfence_init_pool_early(void)
687 {
688 unsigned long addr;
689
690 if (!__kfence_pool)
691 return false;
692
693 addr = kfence_init_pool();
694
695 if (!addr) {
696 /*
697 * The pool is live and will never be deallocated from this point on.
698 * Ignore the pool object from the kmemleak phys object tree, as it would
699 * otherwise overlap with allocations returned by kfence_alloc(), which
700 * are registered with kmemleak through the slab post-alloc hook.
701 */
702 kmemleak_ignore_phys(__pa(__kfence_pool));
703 return true;
704 }
705
706 /*
707 * Only release unprotected pages, and do not try to go back and change
708 * page attributes due to risk of failing to do so as well. If changing
709 * page attributes for some pages fails, it is very likely that it also
710 * fails for the first page, and therefore expect addr==__kfence_pool in
711 * most failure cases.
712 */
713 memblock_free_late(__pa(addr), KFENCE_POOL_SIZE - (addr - (unsigned long)__kfence_pool));
714 __kfence_pool = NULL;
715
716 memblock_free_late(__pa(kfence_metadata_init), KFENCE_METADATA_SIZE);
717 kfence_metadata_init = NULL;
718
719 return false;
720 }
721
722 /* === DebugFS Interface ==================================================== */
723
stats_show(struct seq_file * seq,void * v)724 static int stats_show(struct seq_file *seq, void *v)
725 {
726 int i;
727
728 seq_printf(seq, "enabled: %i\n", READ_ONCE(kfence_enabled));
729 for (i = 0; i < KFENCE_COUNTER_COUNT; i++)
730 seq_printf(seq, "%s: %ld\n", counter_names[i], atomic_long_read(&counters[i]));
731
732 return 0;
733 }
734 DEFINE_SHOW_ATTRIBUTE(stats);
735
736 /*
737 * debugfs seq_file operations for /sys/kernel/debug/kfence/objects.
738 * start_object() and next_object() return the object index + 1, because NULL is used
739 * to stop iteration.
740 */
start_object(struct seq_file * seq,loff_t * pos)741 static void *start_object(struct seq_file *seq, loff_t *pos)
742 {
743 if (*pos < CONFIG_KFENCE_NUM_OBJECTS)
744 return (void *)((long)*pos + 1);
745 return NULL;
746 }
747
stop_object(struct seq_file * seq,void * v)748 static void stop_object(struct seq_file *seq, void *v)
749 {
750 }
751
next_object(struct seq_file * seq,void * v,loff_t * pos)752 static void *next_object(struct seq_file *seq, void *v, loff_t *pos)
753 {
754 ++*pos;
755 if (*pos < CONFIG_KFENCE_NUM_OBJECTS)
756 return (void *)((long)*pos + 1);
757 return NULL;
758 }
759
show_object(struct seq_file * seq,void * v)760 static int show_object(struct seq_file *seq, void *v)
761 {
762 struct kfence_metadata *meta = &kfence_metadata[(long)v - 1];
763 unsigned long flags;
764
765 raw_spin_lock_irqsave(&meta->lock, flags);
766 kfence_print_object(seq, meta);
767 raw_spin_unlock_irqrestore(&meta->lock, flags);
768 seq_puts(seq, "---------------------------------\n");
769
770 return 0;
771 }
772
773 static const struct seq_operations objects_sops = {
774 .start = start_object,
775 .next = next_object,
776 .stop = stop_object,
777 .show = show_object,
778 };
779 DEFINE_SEQ_ATTRIBUTE(objects);
780
kfence_debugfs_init(void)781 static int kfence_debugfs_init(void)
782 {
783 struct dentry *kfence_dir;
784
785 if (!READ_ONCE(kfence_enabled))
786 return 0;
787
788 kfence_dir = debugfs_create_dir("kfence", NULL);
789 debugfs_create_file("stats", 0444, kfence_dir, NULL, &stats_fops);
790 debugfs_create_file("objects", 0400, kfence_dir, NULL, &objects_fops);
791 return 0;
792 }
793
794 late_initcall(kfence_debugfs_init);
795
796 /* === Panic Notifier ====================================================== */
797
kfence_check_all_canary(void)798 static void kfence_check_all_canary(void)
799 {
800 int i;
801
802 for (i = 0; i < CONFIG_KFENCE_NUM_OBJECTS; i++) {
803 struct kfence_metadata *meta = &kfence_metadata[i];
804
805 if (kfence_obj_allocated(meta))
806 check_canary(meta);
807 }
808 }
809
kfence_check_canary_callback(struct notifier_block * nb,unsigned long reason,void * arg)810 static int kfence_check_canary_callback(struct notifier_block *nb,
811 unsigned long reason, void *arg)
812 {
813 kfence_check_all_canary();
814 return NOTIFY_OK;
815 }
816
817 static struct notifier_block kfence_check_canary_notifier = {
818 .notifier_call = kfence_check_canary_callback,
819 };
820
821 /* === Allocation Gate Timer ================================================ */
822
823 static struct delayed_work kfence_timer;
824
825 #ifdef CONFIG_KFENCE_STATIC_KEYS
kfence_reboot_callback(struct notifier_block * nb,unsigned long action,void * data)826 static int kfence_reboot_callback(struct notifier_block *nb,
827 unsigned long action, void *data)
828 {
829 /*
830 * Disable kfence to avoid static keys IPI synchronization during
831 * late shutdown/kexec
832 */
833 WRITE_ONCE(kfence_enabled, false);
834 /* Cancel any pending timer work */
835 cancel_delayed_work_sync(&kfence_timer);
836
837 return NOTIFY_OK;
838 }
839
840 static struct notifier_block kfence_reboot_notifier = {
841 .notifier_call = kfence_reboot_callback,
842 .priority = INT_MAX, /* Run early to stop timers ASAP */
843 };
844
845 /* Wait queue to wake up allocation-gate timer task. */
846 static DECLARE_WAIT_QUEUE_HEAD(allocation_wait);
847
wake_up_kfence_timer(struct irq_work * work)848 static void wake_up_kfence_timer(struct irq_work *work)
849 {
850 wake_up(&allocation_wait);
851 }
852 static DEFINE_IRQ_WORK(wake_up_kfence_timer_work, wake_up_kfence_timer);
853 #endif
854
855 /*
856 * Set up delayed work, which will enable and disable the static key. We need to
857 * use a work queue (rather than a simple timer), since enabling and disabling a
858 * static key cannot be done from an interrupt.
859 *
860 * Note: Toggling a static branch currently causes IPIs, and here we'll end up
861 * with a total of 2 IPIs to all CPUs. If this ends up a problem in future (with
862 * more aggressive sampling intervals), we could get away with a variant that
863 * avoids IPIs, at the cost of not immediately capturing allocations if the
864 * instructions remain cached.
865 */
toggle_allocation_gate(struct work_struct * work)866 static void toggle_allocation_gate(struct work_struct *work)
867 {
868 if (!READ_ONCE(kfence_enabled))
869 return;
870
871 atomic_set(&kfence_allocation_gate, -kfence_burst);
872 #ifdef CONFIG_KFENCE_STATIC_KEYS
873 /* Enable static key, and await allocation to happen. */
874 static_branch_enable(&kfence_allocation_key);
875
876 wait_event_idle(allocation_wait, atomic_read(&kfence_allocation_gate) > 0);
877
878 /* Disable static key and reset timer. */
879 static_branch_disable(&kfence_allocation_key);
880 #endif
881 queue_delayed_work(system_unbound_wq, &kfence_timer,
882 msecs_to_jiffies(kfence_sample_interval));
883 }
884
885 /* === Public interface ===================================================== */
886
kfence_alloc_pool_and_metadata(void)887 void __init kfence_alloc_pool_and_metadata(void)
888 {
889 if (!kfence_sample_interval)
890 return;
891
892 /*
893 * If the pool has already been initialized by arch, there is no need to
894 * re-allocate the memory pool.
895 */
896 if (!__kfence_pool)
897 __kfence_pool = memblock_alloc(KFENCE_POOL_SIZE, PAGE_SIZE);
898
899 if (!__kfence_pool) {
900 pr_err("failed to allocate pool\n");
901 return;
902 }
903
904 /* The memory allocated by memblock has been zeroed out. */
905 kfence_metadata_init = memblock_alloc(KFENCE_METADATA_SIZE, PAGE_SIZE);
906 if (!kfence_metadata_init) {
907 pr_err("failed to allocate metadata\n");
908 memblock_free(__kfence_pool, KFENCE_POOL_SIZE);
909 __kfence_pool = NULL;
910 }
911 }
912
kfence_init_enable(void)913 static void kfence_init_enable(void)
914 {
915 if (!IS_ENABLED(CONFIG_KFENCE_STATIC_KEYS))
916 static_branch_enable(&kfence_allocation_key);
917
918 if (kfence_deferrable)
919 INIT_DEFERRABLE_WORK(&kfence_timer, toggle_allocation_gate);
920 else
921 INIT_DELAYED_WORK(&kfence_timer, toggle_allocation_gate);
922
923 if (kfence_check_on_panic)
924 atomic_notifier_chain_register(&panic_notifier_list, &kfence_check_canary_notifier);
925
926 #ifdef CONFIG_KFENCE_STATIC_KEYS
927 register_reboot_notifier(&kfence_reboot_notifier);
928 #endif
929
930 WRITE_ONCE(kfence_enabled, true);
931 queue_delayed_work(system_unbound_wq, &kfence_timer, 0);
932
933 pr_info("initialized - using %lu bytes for %d objects at 0x%p-0x%p\n", KFENCE_POOL_SIZE,
934 CONFIG_KFENCE_NUM_OBJECTS, (void *)__kfence_pool,
935 (void *)(__kfence_pool + KFENCE_POOL_SIZE));
936 }
937
kfence_init(void)938 void __init kfence_init(void)
939 {
940 stack_hash_seed = get_random_u32();
941
942 /* Setting kfence_sample_interval to 0 on boot disables KFENCE. */
943 if (!kfence_sample_interval)
944 return;
945
946 if (!kfence_init_pool_early()) {
947 pr_err("%s failed\n", __func__);
948 return;
949 }
950
951 kfence_init_enable();
952 }
953
kfence_init_late(void)954 static int kfence_init_late(void)
955 {
956 const unsigned long nr_pages_pool = KFENCE_POOL_SIZE / PAGE_SIZE;
957 const unsigned long nr_pages_meta = KFENCE_METADATA_SIZE / PAGE_SIZE;
958 unsigned long addr = (unsigned long)__kfence_pool;
959 unsigned long free_size = KFENCE_POOL_SIZE;
960 int err = -ENOMEM;
961
962 #ifdef CONFIG_CONTIG_ALLOC
963 struct page *pages;
964
965 pages = alloc_contig_pages(nr_pages_pool, GFP_KERNEL, first_online_node,
966 NULL);
967 if (!pages)
968 return -ENOMEM;
969
970 __kfence_pool = page_to_virt(pages);
971 pages = alloc_contig_pages(nr_pages_meta, GFP_KERNEL, first_online_node,
972 NULL);
973 if (pages)
974 kfence_metadata_init = page_to_virt(pages);
975 #else
976 if (nr_pages_pool > MAX_ORDER_NR_PAGES ||
977 nr_pages_meta > MAX_ORDER_NR_PAGES) {
978 pr_warn("KFENCE_NUM_OBJECTS too large for buddy allocator\n");
979 return -EINVAL;
980 }
981
982 __kfence_pool = alloc_pages_exact(KFENCE_POOL_SIZE, GFP_KERNEL);
983 if (!__kfence_pool)
984 return -ENOMEM;
985
986 kfence_metadata_init = alloc_pages_exact(KFENCE_METADATA_SIZE, GFP_KERNEL);
987 #endif
988
989 if (!kfence_metadata_init)
990 goto free_pool;
991
992 memzero_explicit(kfence_metadata_init, KFENCE_METADATA_SIZE);
993 addr = kfence_init_pool();
994 if (!addr) {
995 kfence_init_enable();
996 kfence_debugfs_init();
997 return 0;
998 }
999
1000 pr_err("%s failed\n", __func__);
1001 free_size = KFENCE_POOL_SIZE - (addr - (unsigned long)__kfence_pool);
1002 err = -EBUSY;
1003
1004 #ifdef CONFIG_CONTIG_ALLOC
1005 free_contig_range(page_to_pfn(virt_to_page((void *)kfence_metadata_init)),
1006 nr_pages_meta);
1007 free_pool:
1008 free_contig_range(page_to_pfn(virt_to_page((void *)addr)),
1009 free_size / PAGE_SIZE);
1010 #else
1011 free_pages_exact((void *)kfence_metadata_init, KFENCE_METADATA_SIZE);
1012 free_pool:
1013 free_pages_exact((void *)addr, free_size);
1014 #endif
1015
1016 kfence_metadata_init = NULL;
1017 __kfence_pool = NULL;
1018 return err;
1019 }
1020
kfence_enable_late(void)1021 static int kfence_enable_late(void)
1022 {
1023 if (!__kfence_pool)
1024 return kfence_init_late();
1025
1026 WRITE_ONCE(kfence_enabled, true);
1027 queue_delayed_work(system_unbound_wq, &kfence_timer, 0);
1028 pr_info("re-enabled\n");
1029 return 0;
1030 }
1031
kfence_shutdown_cache(struct kmem_cache * s)1032 void kfence_shutdown_cache(struct kmem_cache *s)
1033 {
1034 unsigned long flags;
1035 struct kfence_metadata *meta;
1036 int i;
1037
1038 /* Pairs with release in kfence_init_pool(). */
1039 if (!smp_load_acquire(&kfence_metadata))
1040 return;
1041
1042 for (i = 0; i < CONFIG_KFENCE_NUM_OBJECTS; i++) {
1043 bool in_use;
1044
1045 meta = &kfence_metadata[i];
1046
1047 /*
1048 * If we observe some inconsistent cache and state pair where we
1049 * should have returned false here, cache destruction is racing
1050 * with either kmem_cache_alloc() or kmem_cache_free(). Taking
1051 * the lock will not help, as different critical section
1052 * serialization will have the same outcome.
1053 */
1054 if (READ_ONCE(meta->cache) != s || !kfence_obj_allocated(meta))
1055 continue;
1056
1057 raw_spin_lock_irqsave(&meta->lock, flags);
1058 in_use = meta->cache == s && kfence_obj_allocated(meta);
1059 raw_spin_unlock_irqrestore(&meta->lock, flags);
1060
1061 if (in_use) {
1062 /*
1063 * This cache still has allocations, and we should not
1064 * release them back into the freelist so they can still
1065 * safely be used and retain the kernel's default
1066 * behaviour of keeping the allocations alive (leak the
1067 * cache); however, they effectively become "zombie
1068 * allocations" as the KFENCE objects are the only ones
1069 * still in use and the owning cache is being destroyed.
1070 *
1071 * We mark them freed, so that any subsequent use shows
1072 * more useful error messages that will include stack
1073 * traces of the user of the object, the original
1074 * allocation, and caller to shutdown_cache().
1075 */
1076 kfence_guarded_free((void *)meta->addr, meta, /*zombie=*/true);
1077 }
1078 }
1079
1080 for (i = 0; i < CONFIG_KFENCE_NUM_OBJECTS; i++) {
1081 meta = &kfence_metadata[i];
1082
1083 /* See above. */
1084 if (READ_ONCE(meta->cache) != s || READ_ONCE(meta->state) != KFENCE_OBJECT_FREED)
1085 continue;
1086
1087 raw_spin_lock_irqsave(&meta->lock, flags);
1088 if (meta->cache == s && meta->state == KFENCE_OBJECT_FREED)
1089 meta->cache = NULL;
1090 raw_spin_unlock_irqrestore(&meta->lock, flags);
1091 }
1092 }
1093
__kfence_alloc(struct kmem_cache * s,size_t size,gfp_t flags)1094 void *__kfence_alloc(struct kmem_cache *s, size_t size, gfp_t flags)
1095 {
1096 unsigned long stack_entries[KFENCE_STACK_DEPTH];
1097 size_t num_stack_entries;
1098 u32 alloc_stack_hash;
1099 int allocation_gate;
1100
1101 /*
1102 * Perform size check before switching kfence_allocation_gate, so that
1103 * we don't disable KFENCE without making an allocation.
1104 */
1105 if (size > PAGE_SIZE) {
1106 atomic_long_inc(&counters[KFENCE_COUNTER_SKIP_INCOMPAT]);
1107 return NULL;
1108 }
1109
1110 /*
1111 * Skip allocations from non-default zones, including DMA. We cannot
1112 * guarantee that pages in the KFENCE pool will have the requested
1113 * properties (e.g. reside in DMAable memory).
1114 */
1115 if ((flags & GFP_ZONEMASK) ||
1116 ((flags & __GFP_THISNODE) && num_online_nodes() > 1) ||
1117 (s->flags & (SLAB_CACHE_DMA | SLAB_CACHE_DMA32))) {
1118 atomic_long_inc(&counters[KFENCE_COUNTER_SKIP_INCOMPAT]);
1119 return NULL;
1120 }
1121
1122 /*
1123 * Skip allocations for this slab, if KFENCE has been disabled for
1124 * this slab.
1125 */
1126 if (s->flags & SLAB_SKIP_KFENCE)
1127 return NULL;
1128
1129 allocation_gate = atomic_inc_return(&kfence_allocation_gate);
1130 if (allocation_gate > 1)
1131 return NULL;
1132 #ifdef CONFIG_KFENCE_STATIC_KEYS
1133 /*
1134 * waitqueue_active() is fully ordered after the update of
1135 * kfence_allocation_gate per atomic_inc_return().
1136 */
1137 if (allocation_gate == 1 && waitqueue_active(&allocation_wait)) {
1138 /*
1139 * Calling wake_up() here may deadlock when allocations happen
1140 * from within timer code. Use an irq_work to defer it.
1141 */
1142 irq_work_queue(&wake_up_kfence_timer_work);
1143 }
1144 #endif
1145
1146 if (!READ_ONCE(kfence_enabled))
1147 return NULL;
1148
1149 num_stack_entries = stack_trace_save(stack_entries, KFENCE_STACK_DEPTH, 0);
1150
1151 /*
1152 * Do expensive check for coverage of allocation in slow-path after
1153 * allocation_gate has already become non-zero, even though it might
1154 * mean not making any allocation within a given sample interval.
1155 *
1156 * This ensures reasonable allocation coverage when the pool is almost
1157 * full, including avoiding long-lived allocations of the same source
1158 * filling up the pool (e.g. pagecache allocations).
1159 */
1160 alloc_stack_hash = get_alloc_stack_hash(stack_entries, num_stack_entries);
1161 if (should_skip_covered() && alloc_covered_contains(alloc_stack_hash)) {
1162 atomic_long_inc(&counters[KFENCE_COUNTER_SKIP_COVERED]);
1163 return NULL;
1164 }
1165
1166 return kfence_guarded_alloc(s, size, flags, stack_entries, num_stack_entries,
1167 alloc_stack_hash);
1168 }
1169
kfence_ksize(const void * addr)1170 size_t kfence_ksize(const void *addr)
1171 {
1172 const struct kfence_metadata *meta = addr_to_metadata((unsigned long)addr);
1173
1174 /*
1175 * Read locklessly -- if there is a race with __kfence_alloc(), this is
1176 * either a use-after-free or invalid access.
1177 */
1178 return meta ? meta->size : 0;
1179 }
1180
kfence_object_start(const void * addr)1181 void *kfence_object_start(const void *addr)
1182 {
1183 const struct kfence_metadata *meta = addr_to_metadata((unsigned long)addr);
1184
1185 /*
1186 * Read locklessly -- if there is a race with __kfence_alloc(), this is
1187 * either a use-after-free or invalid access.
1188 */
1189 return meta ? (void *)meta->addr : NULL;
1190 }
1191
__kfence_free(void * addr)1192 void __kfence_free(void *addr)
1193 {
1194 struct kfence_metadata *meta = addr_to_metadata((unsigned long)addr);
1195
1196 #ifdef CONFIG_MEMCG
1197 KFENCE_WARN_ON(meta->obj_exts.objcg);
1198 #endif
1199 /*
1200 * If the objects of the cache are SLAB_TYPESAFE_BY_RCU, defer freeing
1201 * the object, as the object page may be recycled for other-typed
1202 * objects once it has been freed. meta->cache may be NULL if the cache
1203 * was destroyed.
1204 * Save the stack trace here so that reports show where the user freed
1205 * the object.
1206 */
1207 if (unlikely(meta->cache && (meta->cache->flags & SLAB_TYPESAFE_BY_RCU))) {
1208 unsigned long flags;
1209
1210 raw_spin_lock_irqsave(&meta->lock, flags);
1211 metadata_update_state(meta, KFENCE_OBJECT_RCU_FREEING, NULL, 0);
1212 raw_spin_unlock_irqrestore(&meta->lock, flags);
1213 call_rcu(&meta->rcu_head, rcu_guarded_free);
1214 } else {
1215 kfence_guarded_free(addr, meta, false);
1216 }
1217 }
1218
kfence_handle_page_fault(unsigned long addr,bool is_write,struct pt_regs * regs)1219 bool kfence_handle_page_fault(unsigned long addr, bool is_write, struct pt_regs *regs)
1220 {
1221 const int page_index = (addr - (unsigned long)__kfence_pool) / PAGE_SIZE;
1222 struct kfence_metadata *to_report = NULL;
1223 enum kfence_error_type error_type;
1224 unsigned long flags;
1225
1226 if (!is_kfence_address((void *)addr))
1227 return false;
1228
1229 if (!READ_ONCE(kfence_enabled)) /* If disabled at runtime ... */
1230 return kfence_unprotect(addr); /* ... unprotect and proceed. */
1231
1232 atomic_long_inc(&counters[KFENCE_COUNTER_BUGS]);
1233
1234 if (page_index % 2) {
1235 /* This is a redzone, report a buffer overflow. */
1236 struct kfence_metadata *meta;
1237 int distance = 0;
1238
1239 meta = addr_to_metadata(addr - PAGE_SIZE);
1240 if (meta && kfence_obj_allocated(meta)) {
1241 to_report = meta;
1242 /* Data race ok; distance calculation approximate. */
1243 distance = addr - data_race(meta->addr + meta->size);
1244 }
1245
1246 meta = addr_to_metadata(addr + PAGE_SIZE);
1247 if (meta && kfence_obj_allocated(meta)) {
1248 /* Data race ok; distance calculation approximate. */
1249 if (!to_report || distance > data_race(meta->addr) - addr)
1250 to_report = meta;
1251 }
1252
1253 if (!to_report)
1254 goto out;
1255
1256 raw_spin_lock_irqsave(&to_report->lock, flags);
1257 to_report->unprotected_page = addr;
1258 error_type = KFENCE_ERROR_OOB;
1259
1260 /*
1261 * If the object was freed before we took the look we can still
1262 * report this as an OOB -- the report will simply show the
1263 * stacktrace of the free as well.
1264 */
1265 } else {
1266 to_report = addr_to_metadata(addr);
1267 if (!to_report)
1268 goto out;
1269
1270 raw_spin_lock_irqsave(&to_report->lock, flags);
1271 error_type = KFENCE_ERROR_UAF;
1272 /*
1273 * We may race with __kfence_alloc(), and it is possible that a
1274 * freed object may be reallocated. We simply report this as a
1275 * use-after-free, with the stack trace showing the place where
1276 * the object was re-allocated.
1277 */
1278 }
1279
1280 out:
1281 if (to_report) {
1282 kfence_report_error(addr, is_write, regs, to_report, error_type);
1283 raw_spin_unlock_irqrestore(&to_report->lock, flags);
1284 } else {
1285 /* This may be a UAF or OOB access, but we can't be sure. */
1286 kfence_report_error(addr, is_write, regs, NULL, KFENCE_ERROR_INVALID);
1287 }
1288
1289 return kfence_unprotect(addr); /* Unprotect and let access proceed. */
1290 }
1291