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/kasan-enabled.h> 17 #include <linux/kcsan-checks.h> 18 #include <linux/kfence.h> 19 #include <linux/kmemleak.h> 20 #include <linux/list.h> 21 #include <linux/lockdep.h> 22 #include <linux/log2.h> 23 #include <linux/memblock.h> 24 #include <linux/moduleparam.h> 25 #include <linux/nodemask.h> 26 #include <linux/notifier.h> 27 #include <linux/panic_notifier.h> 28 #include <linux/random.h> 29 #include <linux/rcupdate.h> 30 #include <linux/reboot.h> 31 #include <linux/sched/clock.h> 32 #include <linux/seq_file.h> 33 #include <linux/slab.h> 34 #include <linux/spinlock.h> 35 #include <linux/string.h> 36 37 #include <asm/kfence.h> 38 39 #include "kfence.h" 40 41 /* Disables KFENCE on the first warning assuming an irrecoverable error. */ 42 #define KFENCE_WARN_ON(cond) \ 43 ({ \ 44 const bool __cond = WARN_ON(cond); \ 45 if (unlikely(__cond)) { \ 46 WRITE_ONCE(kfence_enabled, false); \ 47 disabled_by_warn = true; \ 48 } \ 49 __cond; \ 50 }) 51 52 /* === Data ================================================================= */ 53 54 static bool kfence_enabled __read_mostly; 55 static bool disabled_by_warn __read_mostly; 56 57 unsigned long kfence_sample_interval __read_mostly = CONFIG_KFENCE_SAMPLE_INTERVAL; 58 EXPORT_SYMBOL_GPL(kfence_sample_interval); /* Export for test modules. */ 59 60 #ifdef MODULE_PARAM_PREFIX 61 #undef MODULE_PARAM_PREFIX 62 #endif 63 #define MODULE_PARAM_PREFIX "kfence." 64 65 static int kfence_enable_late(void); 66 static int param_set_sample_interval(const char *val, const struct kernel_param *kp) 67 { 68 unsigned long num; 69 int ret = kstrtoul(val, 0, &num); 70 71 if (ret < 0) 72 return ret; 73 74 /* Using 0 to indicate KFENCE is disabled. */ 75 if (!num && READ_ONCE(kfence_enabled)) { 76 pr_info("disabled\n"); 77 WRITE_ONCE(kfence_enabled, false); 78 } 79 80 *((unsigned long *)kp->arg) = num; 81 82 if (num && !READ_ONCE(kfence_enabled) && system_state != SYSTEM_BOOTING) 83 return disabled_by_warn ? -EINVAL : kfence_enable_late(); 84 return 0; 85 } 86 87 static int param_get_sample_interval(char *buffer, const struct kernel_param *kp) 88 { 89 if (!READ_ONCE(kfence_enabled)) 90 return sprintf(buffer, "0\n"); 91 92 return param_get_ulong(buffer, kp); 93 } 94 95 static const struct kernel_param_ops sample_interval_param_ops = { 96 .set = param_set_sample_interval, 97 .get = param_get_sample_interval, 98 }; 99 module_param_cb(sample_interval, &sample_interval_param_ops, &kfence_sample_interval, 0600); 100 101 /* Pool usage% threshold when currently covered allocations are skipped. */ 102 static unsigned long kfence_skip_covered_thresh __read_mostly = 75; 103 module_param_named(skip_covered_thresh, kfence_skip_covered_thresh, ulong, 0644); 104 105 /* Allocation burst count: number of excess KFENCE allocations per sample. */ 106 static unsigned int kfence_burst __read_mostly; 107 module_param_named(burst, kfence_burst, uint, 0644); 108 109 /* If true, use a deferrable timer. */ 110 static bool kfence_deferrable __read_mostly = IS_ENABLED(CONFIG_KFENCE_DEFERRABLE); 111 module_param_named(deferrable, kfence_deferrable, bool, 0444); 112 113 /* If true, check all canary bytes on panic. */ 114 static bool kfence_check_on_panic __read_mostly; 115 module_param_named(check_on_panic, kfence_check_on_panic, bool, 0444); 116 117 /* The pool of pages used for guard pages and objects. */ 118 char *__kfence_pool __read_mostly; 119 EXPORT_SYMBOL(__kfence_pool); /* Export for test modules. */ 120 121 /* 122 * Per-object metadata, with one-to-one mapping of object metadata to 123 * backing pages (in __kfence_pool). 124 */ 125 static_assert(CONFIG_KFENCE_NUM_OBJECTS > 0); 126 struct kfence_metadata *kfence_metadata __read_mostly; 127 128 /* 129 * If kfence_metadata is not NULL, it may be accessed by kfence_shutdown_cache(). 130 * So introduce kfence_metadata_init to initialize metadata, and then make 131 * kfence_metadata visible after initialization is successful. This prevents 132 * potential UAF or access to uninitialized metadata. 133 */ 134 static struct kfence_metadata *kfence_metadata_init __read_mostly; 135 136 /* Freelist with available objects. */ 137 DEFINE_RAW_SPINLOCK(kfence_freelist_lock); /* Lock protecting freelist. */ 138 static struct list_head kfence_freelist __guarded_by(&kfence_freelist_lock) = LIST_HEAD_INIT(kfence_freelist); 139 140 /* 141 * The static key to set up a KFENCE allocation; or if static keys are not used 142 * to gate allocations, to avoid a load and compare if KFENCE is disabled. 143 */ 144 DEFINE_STATIC_KEY_FALSE(kfence_allocation_key); 145 146 /* Gates the allocation, ensuring only one succeeds in a given period. */ 147 atomic_t kfence_allocation_gate = ATOMIC_INIT(1); 148 149 /* 150 * A Counting Bloom filter of allocation coverage: limits currently covered 151 * allocations of the same source filling up the pool. 152 * 153 * Assuming a range of 15%-85% unique allocations in the pool at any point in 154 * time, the below parameters provide a probablity of 0.02-0.33 for false 155 * positive hits respectively: 156 * 157 * P(alloc_traces) = (1 - e^(-HNUM * (alloc_traces / SIZE)) ^ HNUM 158 */ 159 #define ALLOC_COVERED_HNUM 2 160 #define ALLOC_COVERED_ORDER (const_ilog2(CONFIG_KFENCE_NUM_OBJECTS) + 2) 161 #define ALLOC_COVERED_SIZE (1 << ALLOC_COVERED_ORDER) 162 #define ALLOC_COVERED_HNEXT(h) hash_32(h, ALLOC_COVERED_ORDER) 163 #define ALLOC_COVERED_MASK (ALLOC_COVERED_SIZE - 1) 164 static atomic_t alloc_covered[ALLOC_COVERED_SIZE]; 165 166 /* Stack depth used to determine uniqueness of an allocation. */ 167 #define UNIQUE_ALLOC_STACK_DEPTH ((size_t)8) 168 169 /* 170 * Randomness for stack hashes, making the same collisions across reboots and 171 * different machines less likely. 172 */ 173 static u32 stack_hash_seed __ro_after_init; 174 175 /* Statistics counters for debugfs. */ 176 enum kfence_counter_id { 177 KFENCE_COUNTER_ALLOCATED, 178 KFENCE_COUNTER_ALLOCS, 179 KFENCE_COUNTER_FREES, 180 KFENCE_COUNTER_ZOMBIES, 181 KFENCE_COUNTER_BUGS, 182 KFENCE_COUNTER_SKIP_INCOMPAT, 183 KFENCE_COUNTER_SKIP_CAPACITY, 184 KFENCE_COUNTER_SKIP_COVERED, 185 KFENCE_COUNTER_COUNT, 186 }; 187 static atomic_long_t counters[KFENCE_COUNTER_COUNT]; 188 static const char *const counter_names[] = { 189 [KFENCE_COUNTER_ALLOCATED] = "currently allocated", 190 [KFENCE_COUNTER_ALLOCS] = "total allocations", 191 [KFENCE_COUNTER_FREES] = "total frees", 192 [KFENCE_COUNTER_ZOMBIES] = "zombie allocations", 193 [KFENCE_COUNTER_BUGS] = "total bugs", 194 [KFENCE_COUNTER_SKIP_INCOMPAT] = "skipped allocations (incompatible)", 195 [KFENCE_COUNTER_SKIP_CAPACITY] = "skipped allocations (capacity)", 196 [KFENCE_COUNTER_SKIP_COVERED] = "skipped allocations (covered)", 197 }; 198 static_assert(ARRAY_SIZE(counter_names) == KFENCE_COUNTER_COUNT); 199 200 /* === Internals ============================================================ */ 201 202 static inline bool should_skip_covered(void) 203 { 204 unsigned long thresh = (CONFIG_KFENCE_NUM_OBJECTS * kfence_skip_covered_thresh) / 100; 205 206 return atomic_long_read(&counters[KFENCE_COUNTER_ALLOCATED]) > thresh; 207 } 208 209 static u32 get_alloc_stack_hash(unsigned long *stack_entries, size_t num_entries) 210 { 211 num_entries = min(num_entries, UNIQUE_ALLOC_STACK_DEPTH); 212 num_entries = filter_irq_stacks(stack_entries, num_entries); 213 return jhash(stack_entries, num_entries * sizeof(stack_entries[0]), stack_hash_seed); 214 } 215 216 /* 217 * Adds (or subtracts) count @val for allocation stack trace hash 218 * @alloc_stack_hash from Counting Bloom filter. 219 */ 220 static void alloc_covered_add(u32 alloc_stack_hash, int val) 221 { 222 int i; 223 224 for (i = 0; i < ALLOC_COVERED_HNUM; i++) { 225 atomic_add(val, &alloc_covered[alloc_stack_hash & ALLOC_COVERED_MASK]); 226 alloc_stack_hash = ALLOC_COVERED_HNEXT(alloc_stack_hash); 227 } 228 } 229 230 /* 231 * Returns true if the allocation stack trace hash @alloc_stack_hash is 232 * currently contained (non-zero count) in Counting Bloom filter. 233 */ 234 static bool alloc_covered_contains(u32 alloc_stack_hash) 235 { 236 int i; 237 238 for (i = 0; i < ALLOC_COVERED_HNUM; i++) { 239 if (!atomic_read(&alloc_covered[alloc_stack_hash & ALLOC_COVERED_MASK])) 240 return false; 241 alloc_stack_hash = ALLOC_COVERED_HNEXT(alloc_stack_hash); 242 } 243 244 return true; 245 } 246 247 static bool kfence_protect(unsigned long addr) 248 { 249 return !KFENCE_WARN_ON(!kfence_protect_page(ALIGN_DOWN(addr, PAGE_SIZE), true)); 250 } 251 252 static bool kfence_unprotect(unsigned long addr) 253 { 254 return !KFENCE_WARN_ON(!kfence_protect_page(ALIGN_DOWN(addr, PAGE_SIZE), false)); 255 } 256 257 static inline unsigned long metadata_to_pageaddr(const struct kfence_metadata *meta) 258 __must_hold(&meta->lock) 259 { 260 unsigned long offset = (meta - kfence_metadata + 1) * PAGE_SIZE * 2; 261 unsigned long pageaddr = (unsigned long)&__kfence_pool[offset]; 262 263 /* The checks do not affect performance; only called from slow-paths. */ 264 265 /* Only call with a pointer into kfence_metadata. */ 266 if (KFENCE_WARN_ON(meta < kfence_metadata || 267 meta >= kfence_metadata + CONFIG_KFENCE_NUM_OBJECTS)) 268 return 0; 269 270 /* 271 * This metadata object only ever maps to 1 page; verify that the stored 272 * address is in the expected range. 273 */ 274 if (KFENCE_WARN_ON(ALIGN_DOWN(meta->addr, PAGE_SIZE) != pageaddr)) 275 return 0; 276 277 return pageaddr; 278 } 279 280 static inline bool kfence_obj_allocated(const struct kfence_metadata *meta) 281 { 282 enum kfence_object_state state = READ_ONCE(meta->state); 283 284 return state == KFENCE_OBJECT_ALLOCATED || state == KFENCE_OBJECT_RCU_FREEING; 285 } 286 287 /* 288 * Update the object's metadata state, including updating the alloc/free stacks 289 * depending on the state transition. 290 */ 291 static noinline void 292 metadata_update_state(struct kfence_metadata *meta, enum kfence_object_state next, 293 unsigned long *stack_entries, size_t num_stack_entries) 294 __must_hold(&meta->lock) 295 { 296 struct kfence_track *track = 297 next == KFENCE_OBJECT_ALLOCATED ? &meta->alloc_track : &meta->free_track; 298 299 lockdep_assert_held(&meta->lock); 300 301 /* Stack has been saved when calling rcu, skip. */ 302 if (READ_ONCE(meta->state) == KFENCE_OBJECT_RCU_FREEING) 303 goto out; 304 305 if (stack_entries) { 306 memcpy(track->stack_entries, stack_entries, 307 num_stack_entries * sizeof(stack_entries[0])); 308 } else { 309 /* 310 * Skip over 1 (this) functions; noinline ensures we do not 311 * accidentally skip over the caller by never inlining. 312 */ 313 num_stack_entries = stack_trace_save(track->stack_entries, KFENCE_STACK_DEPTH, 1); 314 } 315 track->num_stack_entries = num_stack_entries; 316 track->pid = task_pid_nr(current); 317 track->cpu = raw_smp_processor_id(); 318 track->ts_nsec = local_clock(); /* Same source as printk timestamps. */ 319 320 out: 321 /* 322 * Pairs with READ_ONCE() in 323 * kfence_shutdown_cache(), 324 * kfence_handle_page_fault(). 325 */ 326 WRITE_ONCE(meta->state, next); 327 } 328 329 #ifdef CONFIG_KMSAN 330 #define check_canary_attributes noinline __no_kmsan_checks 331 #else 332 #define check_canary_attributes inline 333 #endif 334 335 /* Check canary byte at @addr. */ 336 static check_canary_attributes bool check_canary_byte(u8 *addr) 337 { 338 struct kfence_metadata *meta; 339 unsigned long flags; 340 341 if (likely(*addr == KFENCE_CANARY_PATTERN_U8(addr))) 342 return true; 343 344 atomic_long_inc(&counters[KFENCE_COUNTER_BUGS]); 345 346 meta = addr_to_metadata((unsigned long)addr); 347 raw_spin_lock_irqsave(&meta->lock, flags); 348 kfence_report_error((unsigned long)addr, false, NULL, meta, KFENCE_ERROR_CORRUPTION); 349 raw_spin_unlock_irqrestore(&meta->lock, flags); 350 351 return false; 352 } 353 354 static inline void set_canary(const struct kfence_metadata *meta) 355 { 356 const unsigned long pageaddr = ALIGN_DOWN(meta->addr, PAGE_SIZE); 357 unsigned long addr = pageaddr; 358 359 /* 360 * The canary may be written to part of the object memory, but it does 361 * not affect it. The user should initialize the object before using it. 362 */ 363 for (; addr < meta->addr; addr += sizeof(u64)) 364 *((u64 *)addr) = KFENCE_CANARY_PATTERN_U64; 365 366 addr = ALIGN_DOWN(meta->addr + meta->size, sizeof(u64)); 367 for (; addr - pageaddr < PAGE_SIZE; addr += sizeof(u64)) 368 *((u64 *)addr) = KFENCE_CANARY_PATTERN_U64; 369 } 370 371 static check_canary_attributes void 372 check_canary(const struct kfence_metadata *meta) 373 { 374 const unsigned long pageaddr = ALIGN_DOWN(meta->addr, PAGE_SIZE); 375 unsigned long addr = pageaddr; 376 377 /* 378 * We'll iterate over each canary byte per-side until a corrupted byte 379 * is found. However, we'll still iterate over the canary bytes to the 380 * right of the object even if there was an error in the canary bytes to 381 * the left of the object. Specifically, if check_canary_byte() 382 * generates an error, showing both sides might give more clues as to 383 * what the error is about when displaying which bytes were corrupted. 384 */ 385 386 /* Apply to left of object. */ 387 for (; meta->addr - addr >= sizeof(u64); addr += sizeof(u64)) { 388 if (unlikely(*((u64 *)addr) != KFENCE_CANARY_PATTERN_U64)) 389 break; 390 } 391 392 /* 393 * If the canary is corrupted in a certain 64 bytes, or the canary 394 * memory cannot be completely covered by multiple consecutive 64 bytes, 395 * it needs to be checked one by one. 396 */ 397 for (; addr < meta->addr; addr++) { 398 if (unlikely(!check_canary_byte((u8 *)addr))) 399 break; 400 } 401 402 /* Apply to right of object. */ 403 for (addr = meta->addr + meta->size; addr % sizeof(u64) != 0; addr++) { 404 if (unlikely(!check_canary_byte((u8 *)addr))) 405 return; 406 } 407 for (; addr - pageaddr < PAGE_SIZE; addr += sizeof(u64)) { 408 if (unlikely(*((u64 *)addr) != KFENCE_CANARY_PATTERN_U64)) { 409 410 for (; addr - pageaddr < PAGE_SIZE; addr++) { 411 if (!check_canary_byte((u8 *)addr)) 412 return; 413 } 414 } 415 } 416 } 417 418 static void *kfence_guarded_alloc(struct kmem_cache *cache, size_t size, gfp_t gfp, 419 unsigned long *stack_entries, size_t num_stack_entries, 420 u32 alloc_stack_hash) 421 { 422 struct kfence_metadata *meta = NULL; 423 unsigned long flags; 424 struct slab *slab; 425 void *addr; 426 const bool random_right_allocate = get_random_u32_below(2); 427 const bool random_fault = CONFIG_KFENCE_STRESS_TEST_FAULTS && 428 !get_random_u32_below(CONFIG_KFENCE_STRESS_TEST_FAULTS); 429 430 /* Try to obtain a free object. */ 431 raw_spin_lock_irqsave(&kfence_freelist_lock, flags); 432 if (!list_empty(&kfence_freelist)) { 433 meta = list_entry(kfence_freelist.next, struct kfence_metadata, list); 434 list_del_init(&meta->list); 435 } 436 raw_spin_unlock_irqrestore(&kfence_freelist_lock, flags); 437 if (!meta) { 438 atomic_long_inc(&counters[KFENCE_COUNTER_SKIP_CAPACITY]); 439 return NULL; 440 } 441 442 if (unlikely(!raw_spin_trylock_irqsave(&meta->lock, flags))) { 443 /* 444 * This is extremely unlikely -- we are reporting on a 445 * use-after-free, which locked meta->lock, and the reporting 446 * code via printk calls kmalloc() which ends up in 447 * kfence_alloc() and tries to grab the same object that we're 448 * reporting on. While it has never been observed, lockdep does 449 * report that there is a possibility of deadlock. Fix it by 450 * using trylock and bailing out gracefully. 451 */ 452 raw_spin_lock_irqsave(&kfence_freelist_lock, flags); 453 /* Put the object back on the freelist. */ 454 list_add_tail(&meta->list, &kfence_freelist); 455 raw_spin_unlock_irqrestore(&kfence_freelist_lock, flags); 456 457 return NULL; 458 } 459 460 meta->addr = metadata_to_pageaddr(meta); 461 /* Unprotect if we're reusing this page. */ 462 if (meta->state == KFENCE_OBJECT_FREED) 463 kfence_unprotect(meta->addr); 464 465 /* 466 * Note: for allocations made before RNG initialization, will always 467 * return zero. We still benefit from enabling KFENCE as early as 468 * possible, even when the RNG is not yet available, as this will allow 469 * KFENCE to detect bugs due to earlier allocations. The only downside 470 * is that the out-of-bounds accesses detected are deterministic for 471 * such allocations. 472 */ 473 if (random_right_allocate) { 474 /* Allocate on the "right" side, re-calculate address. */ 475 meta->addr += PAGE_SIZE - size; 476 meta->addr = ALIGN_DOWN(meta->addr, cache->align); 477 } 478 479 addr = (void *)meta->addr; 480 481 /* Update remaining metadata. */ 482 metadata_update_state(meta, KFENCE_OBJECT_ALLOCATED, stack_entries, num_stack_entries); 483 /* Pairs with READ_ONCE() in kfence_shutdown_cache(). */ 484 WRITE_ONCE(meta->cache, cache); 485 meta->size = size; 486 meta->alloc_stack_hash = alloc_stack_hash; 487 raw_spin_unlock_irqrestore(&meta->lock, flags); 488 489 alloc_covered_add(alloc_stack_hash, 1); 490 491 /* Set required slab fields. */ 492 slab = virt_to_slab(addr); 493 slab->slab_cache = cache; 494 slab->objects = 1; 495 496 /* Memory initialization. */ 497 set_canary(meta); 498 499 /* 500 * We check slab_want_init_on_alloc() ourselves, rather than letting 501 * SL*B do the initialization, as otherwise we might overwrite KFENCE's 502 * redzone. 503 */ 504 if (unlikely(slab_want_init_on_alloc(gfp, cache))) 505 memzero_explicit(addr, size); 506 if (cache->ctor) 507 cache->ctor(addr); 508 509 if (random_fault) 510 kfence_protect(meta->addr); /* Random "faults" by protecting the object. */ 511 512 atomic_long_inc(&counters[KFENCE_COUNTER_ALLOCATED]); 513 atomic_long_inc(&counters[KFENCE_COUNTER_ALLOCS]); 514 515 return addr; 516 } 517 518 static void kfence_guarded_free(void *addr, struct kfence_metadata *meta, bool zombie) 519 { 520 struct kcsan_scoped_access assert_page_exclusive; 521 u32 alloc_stack_hash; 522 unsigned long flags; 523 bool init; 524 525 raw_spin_lock_irqsave(&meta->lock, flags); 526 527 if (!kfence_obj_allocated(meta) || meta->addr != (unsigned long)addr) { 528 /* Invalid or double-free, bail out. */ 529 atomic_long_inc(&counters[KFENCE_COUNTER_BUGS]); 530 kfence_report_error((unsigned long)addr, false, NULL, meta, 531 KFENCE_ERROR_INVALID_FREE); 532 raw_spin_unlock_irqrestore(&meta->lock, flags); 533 return; 534 } 535 536 /* Detect racy use-after-free, or incorrect reallocation of this page by KFENCE. */ 537 kcsan_begin_scoped_access((void *)ALIGN_DOWN((unsigned long)addr, PAGE_SIZE), PAGE_SIZE, 538 KCSAN_ACCESS_SCOPED | KCSAN_ACCESS_WRITE | KCSAN_ACCESS_ASSERT, 539 &assert_page_exclusive); 540 541 if (CONFIG_KFENCE_STRESS_TEST_FAULTS) 542 kfence_unprotect((unsigned long)addr); /* To check canary bytes. */ 543 544 /* Restore page protection if there was an OOB access. */ 545 if (meta->unprotected_page) { 546 memzero_explicit((void *)ALIGN_DOWN(meta->unprotected_page, PAGE_SIZE), PAGE_SIZE); 547 kfence_protect(meta->unprotected_page); 548 meta->unprotected_page = 0; 549 } 550 551 /* Mark the object as freed. */ 552 metadata_update_state(meta, KFENCE_OBJECT_FREED, NULL, 0); 553 init = slab_want_init_on_free(meta->cache); 554 alloc_stack_hash = meta->alloc_stack_hash; 555 raw_spin_unlock_irqrestore(&meta->lock, flags); 556 557 alloc_covered_add(alloc_stack_hash, -1); 558 559 /* Check canary bytes for memory corruption. */ 560 check_canary(meta); 561 562 /* 563 * Clear memory if init-on-free is set. While we protect the page, the 564 * data is still there, and after a use-after-free is detected, we 565 * unprotect the page, so the data is still accessible. 566 */ 567 if (!zombie && unlikely(init)) 568 memzero_explicit(addr, meta->size); 569 570 /* Protect to detect use-after-frees. */ 571 kfence_protect((unsigned long)addr); 572 573 kcsan_end_scoped_access(&assert_page_exclusive); 574 if (!zombie) { 575 /* Add it to the tail of the freelist for reuse. */ 576 raw_spin_lock_irqsave(&kfence_freelist_lock, flags); 577 KFENCE_WARN_ON(!list_empty(&meta->list)); 578 list_add_tail(&meta->list, &kfence_freelist); 579 raw_spin_unlock_irqrestore(&kfence_freelist_lock, flags); 580 581 atomic_long_dec(&counters[KFENCE_COUNTER_ALLOCATED]); 582 atomic_long_inc(&counters[KFENCE_COUNTER_FREES]); 583 } else { 584 /* See kfence_shutdown_cache(). */ 585 atomic_long_inc(&counters[KFENCE_COUNTER_ZOMBIES]); 586 } 587 } 588 589 static void rcu_guarded_free(struct rcu_head *h) 590 { 591 struct kfence_metadata *meta = container_of(h, struct kfence_metadata, rcu_head); 592 593 kfence_guarded_free((void *)meta->addr, meta, false); 594 } 595 596 /* 597 * Initialization of the KFENCE pool after its allocation. 598 * Returns 0 on success; otherwise returns the address up to 599 * which partial initialization succeeded. 600 */ 601 static unsigned long kfence_init_pool(void) 602 __context_unsafe(/* constructor */) 603 { 604 unsigned long addr, start_pfn; 605 int i, rand; 606 607 if (!arch_kfence_init_pool()) 608 return (unsigned long)__kfence_pool; 609 610 addr = (unsigned long)__kfence_pool; 611 start_pfn = PHYS_PFN(virt_to_phys(__kfence_pool)); 612 613 /* 614 * Set up object pages: they must have PGTY_slab set to avoid freeing 615 * them as real pages. 616 * 617 * We also want to avoid inserting kfence_free() in the kfree() 618 * fast-path in SLUB, and therefore need to ensure kfree() correctly 619 * enters __slab_free() slow-path. 620 */ 621 for (i = 0; i < KFENCE_POOL_SIZE / PAGE_SIZE; i++) { 622 struct page *page; 623 624 if (!i || (i % 2)) 625 continue; 626 627 page = pfn_to_page(start_pfn + i); 628 __SetPageSlab(page); 629 #ifdef CONFIG_MEMCG 630 struct slab *slab = page_slab(page); 631 slab->obj_exts = (unsigned long)&kfence_metadata_init[i / 2 - 1].obj_exts | 632 MEMCG_DATA_OBJEXTS; 633 #endif 634 } 635 636 /* 637 * Protect the first 2 pages. The first page is mostly unnecessary, and 638 * merely serves as an extended guard page. However, adding one 639 * additional page in the beginning gives us an even number of pages, 640 * which simplifies the mapping of address to metadata index. 641 */ 642 for (i = 0; i < 2; i++) { 643 if (unlikely(!kfence_protect(addr))) 644 return addr; 645 646 addr += PAGE_SIZE; 647 } 648 649 for (i = 0; i < CONFIG_KFENCE_NUM_OBJECTS; i++) { 650 struct kfence_metadata *meta = &kfence_metadata_init[i]; 651 652 /* Initialize metadata. */ 653 INIT_LIST_HEAD(&meta->list); 654 raw_spin_lock_init(&meta->lock); 655 meta->state = KFENCE_OBJECT_UNUSED; 656 /* Use addr to randomize the freelist. */ 657 meta->addr = i; 658 659 /* Protect the right redzone. */ 660 if (unlikely(!kfence_protect(addr + 2 * i * PAGE_SIZE + PAGE_SIZE))) 661 goto reset_slab; 662 } 663 664 for (i = CONFIG_KFENCE_NUM_OBJECTS; i > 0; i--) { 665 rand = get_random_u32_below(i); 666 swap(kfence_metadata_init[i - 1].addr, kfence_metadata_init[rand].addr); 667 } 668 669 for (i = 0; i < CONFIG_KFENCE_NUM_OBJECTS; i++) { 670 struct kfence_metadata *meta_1 = &kfence_metadata_init[i]; 671 struct kfence_metadata *meta_2 = &kfence_metadata_init[meta_1->addr]; 672 673 list_add_tail(&meta_2->list, &kfence_freelist); 674 } 675 for (i = 0; i < CONFIG_KFENCE_NUM_OBJECTS; i++) { 676 kfence_metadata_init[i].addr = addr; 677 addr += 2 * PAGE_SIZE; 678 } 679 680 /* 681 * Make kfence_metadata visible only when initialization is successful. 682 * Otherwise, if the initialization fails and kfence_metadata is freed, 683 * it may cause UAF in kfence_shutdown_cache(). 684 */ 685 smp_store_release(&kfence_metadata, kfence_metadata_init); 686 return 0; 687 688 reset_slab: 689 addr += 2 * i * PAGE_SIZE; 690 for (i = 0; i < KFENCE_POOL_SIZE / PAGE_SIZE; i++) { 691 struct page *page; 692 693 if (!i || (i % 2)) 694 continue; 695 696 page = pfn_to_page(start_pfn + i); 697 #ifdef CONFIG_MEMCG 698 struct slab *slab = page_slab(page); 699 slab->obj_exts = 0; 700 #endif 701 __ClearPageSlab(page); 702 } 703 704 return addr; 705 } 706 707 static bool __init kfence_init_pool_early(void) 708 { 709 unsigned long addr; 710 711 if (!__kfence_pool) 712 return false; 713 714 addr = kfence_init_pool(); 715 716 if (!addr) { 717 /* 718 * The pool is live and will never be deallocated from this point on. 719 * Ignore the pool object from the kmemleak phys object tree, as it would 720 * otherwise overlap with allocations returned by kfence_alloc(), which 721 * are registered with kmemleak through the slab post-alloc hook. 722 */ 723 kmemleak_ignore_phys(__pa(__kfence_pool)); 724 return true; 725 } 726 727 /* 728 * Only release unprotected pages, and do not try to go back and change 729 * page attributes due to risk of failing to do so as well. If changing 730 * page attributes for some pages fails, it is very likely that it also 731 * fails for the first page, and therefore expect addr==__kfence_pool in 732 * most failure cases. 733 */ 734 memblock_free_late(__pa(addr), KFENCE_POOL_SIZE - (addr - (unsigned long)__kfence_pool)); 735 __kfence_pool = NULL; 736 737 memblock_free_late(__pa(kfence_metadata_init), KFENCE_METADATA_SIZE); 738 kfence_metadata_init = NULL; 739 740 return false; 741 } 742 743 /* === DebugFS Interface ==================================================== */ 744 745 static int stats_show(struct seq_file *seq, void *v) 746 { 747 int i; 748 749 seq_printf(seq, "enabled: %i\n", READ_ONCE(kfence_enabled)); 750 for (i = 0; i < KFENCE_COUNTER_COUNT; i++) 751 seq_printf(seq, "%s: %ld\n", counter_names[i], atomic_long_read(&counters[i])); 752 753 return 0; 754 } 755 DEFINE_SHOW_ATTRIBUTE(stats); 756 757 /* 758 * debugfs seq_file operations for /sys/kernel/debug/kfence/objects. 759 * start_object() and next_object() return the object index + 1, because NULL is used 760 * to stop iteration. 761 */ 762 static void *start_object(struct seq_file *seq, loff_t *pos) 763 { 764 if (*pos < CONFIG_KFENCE_NUM_OBJECTS) 765 return (void *)((long)*pos + 1); 766 return NULL; 767 } 768 769 static void stop_object(struct seq_file *seq, void *v) 770 { 771 } 772 773 static void *next_object(struct seq_file *seq, void *v, loff_t *pos) 774 { 775 ++*pos; 776 if (*pos < CONFIG_KFENCE_NUM_OBJECTS) 777 return (void *)((long)*pos + 1); 778 return NULL; 779 } 780 781 static int show_object(struct seq_file *seq, void *v) 782 { 783 struct kfence_metadata *meta = &kfence_metadata[(long)v - 1]; 784 unsigned long flags; 785 786 raw_spin_lock_irqsave(&meta->lock, flags); 787 kfence_print_object(seq, meta); 788 raw_spin_unlock_irqrestore(&meta->lock, flags); 789 seq_puts(seq, "---------------------------------\n"); 790 791 return 0; 792 } 793 794 static const struct seq_operations objects_sops = { 795 .start = start_object, 796 .next = next_object, 797 .stop = stop_object, 798 .show = show_object, 799 }; 800 DEFINE_SEQ_ATTRIBUTE(objects); 801 802 static int kfence_debugfs_init(void) 803 { 804 struct dentry *kfence_dir; 805 806 if (!READ_ONCE(kfence_enabled)) 807 return 0; 808 809 kfence_dir = debugfs_create_dir("kfence", NULL); 810 debugfs_create_file("stats", 0444, kfence_dir, NULL, &stats_fops); 811 debugfs_create_file("objects", 0400, kfence_dir, NULL, &objects_fops); 812 return 0; 813 } 814 815 late_initcall(kfence_debugfs_init); 816 817 /* === Panic Notifier ====================================================== */ 818 819 static void kfence_check_all_canary(void) 820 { 821 int i; 822 823 for (i = 0; i < CONFIG_KFENCE_NUM_OBJECTS; i++) { 824 struct kfence_metadata *meta = &kfence_metadata[i]; 825 826 if (kfence_obj_allocated(meta)) 827 check_canary(meta); 828 } 829 } 830 831 static int kfence_check_canary_callback(struct notifier_block *nb, 832 unsigned long reason, void *arg) 833 { 834 kfence_check_all_canary(); 835 return NOTIFY_OK; 836 } 837 838 static struct notifier_block kfence_check_canary_notifier = { 839 .notifier_call = kfence_check_canary_callback, 840 }; 841 842 /* === Allocation Gate Timer ================================================ */ 843 844 static struct delayed_work kfence_timer; 845 846 #ifdef CONFIG_KFENCE_STATIC_KEYS 847 /* Wait queue to wake up allocation-gate timer task. */ 848 static DECLARE_WAIT_QUEUE_HEAD(allocation_wait); 849 850 static int kfence_reboot_callback(struct notifier_block *nb, 851 unsigned long action, void *data) 852 { 853 /* 854 * Disable kfence to avoid static keys IPI synchronization during 855 * late shutdown/kexec 856 */ 857 WRITE_ONCE(kfence_enabled, false); 858 /* Cancel any pending timer work */ 859 cancel_delayed_work(&kfence_timer); 860 /* 861 * Wake up any blocked toggle_allocation_gate() so it can complete 862 * early while the system is still able to handle IPIs. 863 */ 864 wake_up(&allocation_wait); 865 866 return NOTIFY_OK; 867 } 868 869 static struct notifier_block kfence_reboot_notifier = { 870 .notifier_call = kfence_reboot_callback, 871 .priority = INT_MAX, /* Run early to stop timers ASAP */ 872 }; 873 874 static void wake_up_kfence_timer(struct irq_work *work) 875 { 876 wake_up(&allocation_wait); 877 } 878 static DEFINE_IRQ_WORK(wake_up_kfence_timer_work, wake_up_kfence_timer); 879 #endif 880 881 /* 882 * Set up delayed work, which will enable and disable the static key. We need to 883 * use a work queue (rather than a simple timer), since enabling and disabling a 884 * static key cannot be done from an interrupt. 885 * 886 * Note: Toggling a static branch currently causes IPIs, and here we'll end up 887 * with a total of 2 IPIs to all CPUs. If this ends up a problem in future (with 888 * more aggressive sampling intervals), we could get away with a variant that 889 * avoids IPIs, at the cost of not immediately capturing allocations if the 890 * instructions remain cached. 891 */ 892 static void toggle_allocation_gate(struct work_struct *work) 893 { 894 if (!READ_ONCE(kfence_enabled)) 895 return; 896 897 atomic_set(&kfence_allocation_gate, -kfence_burst); 898 #ifdef CONFIG_KFENCE_STATIC_KEYS 899 /* Enable static key, and await allocation to happen. */ 900 static_branch_enable(&kfence_allocation_key); 901 902 wait_event_idle(allocation_wait, 903 atomic_read(&kfence_allocation_gate) > 0 || 904 !READ_ONCE(kfence_enabled)); 905 906 /* Disable static key and reset timer. */ 907 static_branch_disable(&kfence_allocation_key); 908 #endif 909 queue_delayed_work(system_dfl_wq, &kfence_timer, 910 msecs_to_jiffies(kfence_sample_interval)); 911 } 912 913 /* === Public interface ===================================================== */ 914 915 void __init kfence_alloc_pool_and_metadata(void) 916 { 917 if (!kfence_sample_interval) 918 return; 919 920 /* 921 * If KASAN hardware tags are enabled, disable KFENCE, because it 922 * does not support MTE yet. 923 */ 924 if (kasan_hw_tags_enabled()) { 925 pr_info("disabled as KASAN HW tags are enabled\n"); 926 if (__kfence_pool) { 927 memblock_free(__kfence_pool, KFENCE_POOL_SIZE); 928 __kfence_pool = NULL; 929 } 930 kfence_sample_interval = 0; 931 return; 932 } 933 934 /* 935 * If the pool has already been initialized by arch, there is no need to 936 * re-allocate the memory pool. 937 */ 938 if (!__kfence_pool) 939 __kfence_pool = memblock_alloc(KFENCE_POOL_SIZE, PAGE_SIZE); 940 941 if (!__kfence_pool) { 942 pr_err("failed to allocate pool\n"); 943 return; 944 } 945 946 /* The memory allocated by memblock has been zeroed out. */ 947 kfence_metadata_init = memblock_alloc(KFENCE_METADATA_SIZE, PAGE_SIZE); 948 if (!kfence_metadata_init) { 949 pr_err("failed to allocate metadata\n"); 950 memblock_free(__kfence_pool, KFENCE_POOL_SIZE); 951 __kfence_pool = NULL; 952 } 953 } 954 955 static void kfence_init_enable(void) 956 { 957 if (!IS_ENABLED(CONFIG_KFENCE_STATIC_KEYS)) 958 static_branch_enable(&kfence_allocation_key); 959 960 if (kfence_deferrable) 961 INIT_DEFERRABLE_WORK(&kfence_timer, toggle_allocation_gate); 962 else 963 INIT_DELAYED_WORK(&kfence_timer, toggle_allocation_gate); 964 965 if (kfence_check_on_panic) 966 atomic_notifier_chain_register(&panic_notifier_list, &kfence_check_canary_notifier); 967 968 #ifdef CONFIG_KFENCE_STATIC_KEYS 969 register_reboot_notifier(&kfence_reboot_notifier); 970 #endif 971 972 WRITE_ONCE(kfence_enabled, true); 973 queue_delayed_work(system_dfl_wq, &kfence_timer, 0); 974 975 pr_info("initialized - using %lu bytes for %d objects at 0x%p-0x%p\n", KFENCE_POOL_SIZE, 976 CONFIG_KFENCE_NUM_OBJECTS, (void *)__kfence_pool, 977 (void *)(__kfence_pool + KFENCE_POOL_SIZE)); 978 } 979 980 void __init kfence_init(void) 981 { 982 stack_hash_seed = get_random_u32(); 983 984 /* Setting kfence_sample_interval to 0 on boot disables KFENCE. */ 985 if (!kfence_sample_interval) 986 return; 987 988 if (!kfence_init_pool_early()) { 989 pr_err("%s failed\n", __func__); 990 return; 991 } 992 993 kfence_init_enable(); 994 } 995 996 static int kfence_init_late(void) 997 { 998 const unsigned long nr_pages_pool = KFENCE_POOL_SIZE / PAGE_SIZE; 999 const unsigned long nr_pages_meta = KFENCE_METADATA_SIZE / PAGE_SIZE; 1000 unsigned long addr = (unsigned long)__kfence_pool; 1001 unsigned long free_size = KFENCE_POOL_SIZE; 1002 int err = -ENOMEM; 1003 1004 #ifdef CONFIG_CONTIG_ALLOC 1005 struct page *pages; 1006 1007 pages = alloc_contig_pages(nr_pages_pool, GFP_KERNEL | __GFP_SKIP_KASAN, 1008 first_online_node, NULL); 1009 if (!pages) 1010 return -ENOMEM; 1011 1012 __kfence_pool = page_to_virt(pages); 1013 pages = alloc_contig_pages(nr_pages_meta, GFP_KERNEL | __GFP_SKIP_KASAN, 1014 first_online_node, NULL); 1015 if (pages) 1016 kfence_metadata_init = page_to_virt(pages); 1017 #else 1018 if (nr_pages_pool > MAX_ORDER_NR_PAGES || 1019 nr_pages_meta > MAX_ORDER_NR_PAGES) { 1020 pr_warn("KFENCE_NUM_OBJECTS too large for buddy allocator\n"); 1021 return -EINVAL; 1022 } 1023 1024 __kfence_pool = alloc_pages_exact(KFENCE_POOL_SIZE, 1025 GFP_KERNEL | __GFP_SKIP_KASAN); 1026 if (!__kfence_pool) 1027 return -ENOMEM; 1028 1029 kfence_metadata_init = alloc_pages_exact(KFENCE_METADATA_SIZE, 1030 GFP_KERNEL | __GFP_SKIP_KASAN); 1031 #endif 1032 1033 if (!kfence_metadata_init) 1034 goto free_pool; 1035 1036 memzero_explicit(kfence_metadata_init, KFENCE_METADATA_SIZE); 1037 addr = kfence_init_pool(); 1038 if (!addr) { 1039 kfence_init_enable(); 1040 kfence_debugfs_init(); 1041 return 0; 1042 } 1043 1044 pr_err("%s failed\n", __func__); 1045 free_size = KFENCE_POOL_SIZE - (addr - (unsigned long)__kfence_pool); 1046 err = -EBUSY; 1047 1048 #ifdef CONFIG_CONTIG_ALLOC 1049 free_contig_range(page_to_pfn(virt_to_page((void *)kfence_metadata_init)), 1050 nr_pages_meta); 1051 free_pool: 1052 free_contig_range(page_to_pfn(virt_to_page((void *)addr)), 1053 free_size / PAGE_SIZE); 1054 #else 1055 free_pages_exact((void *)kfence_metadata_init, KFENCE_METADATA_SIZE); 1056 free_pool: 1057 free_pages_exact((void *)addr, free_size); 1058 #endif 1059 1060 kfence_metadata_init = NULL; 1061 __kfence_pool = NULL; 1062 return err; 1063 } 1064 1065 static int kfence_enable_late(void) 1066 { 1067 if (!__kfence_pool) 1068 return kfence_init_late(); 1069 1070 WRITE_ONCE(kfence_enabled, true); 1071 queue_delayed_work(system_dfl_wq, &kfence_timer, 0); 1072 pr_info("re-enabled\n"); 1073 return 0; 1074 } 1075 1076 void kfence_shutdown_cache(struct kmem_cache *s) 1077 { 1078 unsigned long flags; 1079 struct kfence_metadata *meta; 1080 int i; 1081 1082 /* Pairs with release in kfence_init_pool(). */ 1083 if (!smp_load_acquire(&kfence_metadata)) 1084 return; 1085 1086 for (i = 0; i < CONFIG_KFENCE_NUM_OBJECTS; i++) { 1087 bool in_use; 1088 1089 meta = &kfence_metadata[i]; 1090 1091 /* 1092 * If we observe some inconsistent cache and state pair where we 1093 * should have returned false here, cache destruction is racing 1094 * with either kmem_cache_alloc() or kmem_cache_free(). Taking 1095 * the lock will not help, as different critical section 1096 * serialization will have the same outcome. 1097 */ 1098 if (READ_ONCE(meta->cache) != s || !kfence_obj_allocated(meta)) 1099 continue; 1100 1101 raw_spin_lock_irqsave(&meta->lock, flags); 1102 in_use = meta->cache == s && kfence_obj_allocated(meta); 1103 raw_spin_unlock_irqrestore(&meta->lock, flags); 1104 1105 if (in_use) { 1106 /* 1107 * This cache still has allocations, and we should not 1108 * release them back into the freelist so they can still 1109 * safely be used and retain the kernel's default 1110 * behaviour of keeping the allocations alive (leak the 1111 * cache); however, they effectively become "zombie 1112 * allocations" as the KFENCE objects are the only ones 1113 * still in use and the owning cache is being destroyed. 1114 * 1115 * We mark them freed, so that any subsequent use shows 1116 * more useful error messages that will include stack 1117 * traces of the user of the object, the original 1118 * allocation, and caller to shutdown_cache(). 1119 */ 1120 kfence_guarded_free((void *)meta->addr, meta, /*zombie=*/true); 1121 } 1122 } 1123 1124 for (i = 0; i < CONFIG_KFENCE_NUM_OBJECTS; i++) { 1125 meta = &kfence_metadata[i]; 1126 1127 /* See above. */ 1128 if (READ_ONCE(meta->cache) != s || READ_ONCE(meta->state) != KFENCE_OBJECT_FREED) 1129 continue; 1130 1131 raw_spin_lock_irqsave(&meta->lock, flags); 1132 if (meta->cache == s && meta->state == KFENCE_OBJECT_FREED) 1133 meta->cache = NULL; 1134 raw_spin_unlock_irqrestore(&meta->lock, flags); 1135 } 1136 } 1137 1138 void *__kfence_alloc(struct kmem_cache *s, size_t size, gfp_t flags) 1139 { 1140 unsigned long stack_entries[KFENCE_STACK_DEPTH]; 1141 size_t num_stack_entries; 1142 u32 alloc_stack_hash; 1143 int allocation_gate; 1144 1145 /* 1146 * Perform size check before switching kfence_allocation_gate, so that 1147 * we don't disable KFENCE without making an allocation. 1148 */ 1149 if (size > PAGE_SIZE) { 1150 atomic_long_inc(&counters[KFENCE_COUNTER_SKIP_INCOMPAT]); 1151 return NULL; 1152 } 1153 1154 /* 1155 * Skip allocations from non-default zones, including DMA. We cannot 1156 * guarantee that pages in the KFENCE pool will have the requested 1157 * properties (e.g. reside in DMAable memory). 1158 */ 1159 if ((flags & GFP_ZONEMASK) || 1160 ((flags & __GFP_THISNODE) && num_online_nodes() > 1) || 1161 (s->flags & (SLAB_CACHE_DMA | SLAB_CACHE_DMA32))) { 1162 atomic_long_inc(&counters[KFENCE_COUNTER_SKIP_INCOMPAT]); 1163 return NULL; 1164 } 1165 1166 /* 1167 * Skip allocations for this slab, if KFENCE has been disabled for 1168 * this slab. 1169 */ 1170 if (s->flags & SLAB_SKIP_KFENCE) 1171 return NULL; 1172 1173 allocation_gate = atomic_inc_return(&kfence_allocation_gate); 1174 if (allocation_gate > 1) 1175 return NULL; 1176 #ifdef CONFIG_KFENCE_STATIC_KEYS 1177 /* 1178 * waitqueue_active() is fully ordered after the update of 1179 * kfence_allocation_gate per atomic_inc_return(). 1180 */ 1181 if (allocation_gate == 1 && waitqueue_active(&allocation_wait)) { 1182 /* 1183 * Calling wake_up() here may deadlock when allocations happen 1184 * from within timer code. Use an irq_work to defer it. 1185 */ 1186 irq_work_queue(&wake_up_kfence_timer_work); 1187 } 1188 #endif 1189 1190 if (!READ_ONCE(kfence_enabled)) 1191 return NULL; 1192 1193 num_stack_entries = stack_trace_save(stack_entries, KFENCE_STACK_DEPTH, 0); 1194 1195 /* 1196 * Do expensive check for coverage of allocation in slow-path after 1197 * allocation_gate has already become non-zero, even though it might 1198 * mean not making any allocation within a given sample interval. 1199 * 1200 * This ensures reasonable allocation coverage when the pool is almost 1201 * full, including avoiding long-lived allocations of the same source 1202 * filling up the pool (e.g. pagecache allocations). 1203 */ 1204 alloc_stack_hash = get_alloc_stack_hash(stack_entries, num_stack_entries); 1205 if (should_skip_covered() && alloc_covered_contains(alloc_stack_hash)) { 1206 atomic_long_inc(&counters[KFENCE_COUNTER_SKIP_COVERED]); 1207 return NULL; 1208 } 1209 1210 return kfence_guarded_alloc(s, size, flags, stack_entries, num_stack_entries, 1211 alloc_stack_hash); 1212 } 1213 1214 size_t kfence_ksize(const void *addr) 1215 { 1216 const struct kfence_metadata *meta = addr_to_metadata((unsigned long)addr); 1217 1218 /* 1219 * Read locklessly -- if there is a race with __kfence_alloc(), this is 1220 * either a use-after-free or invalid access. 1221 */ 1222 return meta ? meta->size : 0; 1223 } 1224 1225 void *kfence_object_start(const void *addr) 1226 { 1227 const struct kfence_metadata *meta = addr_to_metadata((unsigned long)addr); 1228 1229 /* 1230 * Read locklessly -- if there is a race with __kfence_alloc(), this is 1231 * either a use-after-free or invalid access. 1232 */ 1233 return meta ? (void *)meta->addr : NULL; 1234 } 1235 1236 void __kfence_free(void *addr) 1237 { 1238 struct kfence_metadata *meta = addr_to_metadata((unsigned long)addr); 1239 1240 #ifdef CONFIG_MEMCG 1241 KFENCE_WARN_ON(meta->obj_exts.objcg); 1242 #endif 1243 /* 1244 * If the objects of the cache are SLAB_TYPESAFE_BY_RCU, defer freeing 1245 * the object, as the object page may be recycled for other-typed 1246 * objects once it has been freed. meta->cache may be NULL if the cache 1247 * was destroyed. 1248 * Save the stack trace here so that reports show where the user freed 1249 * the object. 1250 */ 1251 if (unlikely(meta->cache && (meta->cache->flags & SLAB_TYPESAFE_BY_RCU))) { 1252 unsigned long flags; 1253 1254 raw_spin_lock_irqsave(&meta->lock, flags); 1255 metadata_update_state(meta, KFENCE_OBJECT_RCU_FREEING, NULL, 0); 1256 raw_spin_unlock_irqrestore(&meta->lock, flags); 1257 call_rcu(&meta->rcu_head, rcu_guarded_free); 1258 } else { 1259 kfence_guarded_free(addr, meta, false); 1260 } 1261 } 1262 1263 bool kfence_handle_page_fault(unsigned long addr, bool is_write, struct pt_regs *regs) 1264 { 1265 const int page_index = (addr - (unsigned long)__kfence_pool) / PAGE_SIZE; 1266 struct kfence_metadata *to_report = NULL; 1267 unsigned long unprotected_page = 0; 1268 enum kfence_error_type error_type; 1269 unsigned long flags; 1270 1271 if (!is_kfence_address((void *)addr)) 1272 return false; 1273 1274 if (!READ_ONCE(kfence_enabled)) /* If disabled at runtime ... */ 1275 return kfence_unprotect(addr); /* ... unprotect and proceed. */ 1276 1277 atomic_long_inc(&counters[KFENCE_COUNTER_BUGS]); 1278 1279 if (page_index % 2) { 1280 /* This is a redzone, report a buffer overflow. */ 1281 struct kfence_metadata *meta; 1282 int distance = 0; 1283 1284 meta = addr_to_metadata(addr - PAGE_SIZE); 1285 if (meta && kfence_obj_allocated(meta)) { 1286 to_report = meta; 1287 /* Data race ok; distance calculation approximate. */ 1288 distance = addr - data_race(meta->addr + meta->size); 1289 } 1290 1291 meta = addr_to_metadata(addr + PAGE_SIZE); 1292 if (meta && kfence_obj_allocated(meta)) { 1293 /* Data race ok; distance calculation approximate. */ 1294 if (!to_report || distance > data_race(meta->addr) - addr) 1295 to_report = meta; 1296 } 1297 1298 if (!to_report) 1299 goto out; 1300 1301 error_type = KFENCE_ERROR_OOB; 1302 unprotected_page = addr; 1303 1304 /* 1305 * If the object was freed before we took the look we can still 1306 * report this as an OOB -- the report will simply show the 1307 * stacktrace of the free as well. 1308 */ 1309 } else { 1310 to_report = addr_to_metadata(addr); 1311 if (!to_report) 1312 goto out; 1313 1314 error_type = KFENCE_ERROR_UAF; 1315 /* 1316 * We may race with __kfence_alloc(), and it is possible that a 1317 * freed object may be reallocated. We simply report this as a 1318 * use-after-free, with the stack trace showing the place where 1319 * the object was re-allocated. 1320 */ 1321 } 1322 1323 out: 1324 if (to_report) { 1325 raw_spin_lock_irqsave(&to_report->lock, flags); 1326 to_report->unprotected_page = unprotected_page; 1327 kfence_report_error(addr, is_write, regs, to_report, error_type); 1328 raw_spin_unlock_irqrestore(&to_report->lock, flags); 1329 } else { 1330 /* This may be a UAF or OOB access, but we can't be sure. */ 1331 kfence_report_error(addr, is_write, regs, NULL, KFENCE_ERROR_INVALID); 1332 } 1333 1334 return kfence_unprotect(addr); /* Unprotect and let access proceed. */ 1335 } 1336