1 // SPDX-License-Identifier: GPL-2.0 2 /* 3 * KFENCE guarded object allocator and fault handling. 4 * 5 * Copyright (C) 2020, Google LLC. 6 */ 7 8 #define pr_fmt(fmt) "kfence: " fmt 9 10 #include <linux/atomic.h> 11 #include <linux/bug.h> 12 #include <linux/debugfs.h> 13 #include <linux/hash.h> 14 #include <linux/irq_work.h> 15 #include <linux/jhash.h> 16 #include <linux/kcsan-checks.h> 17 #include <linux/kfence.h> 18 #include <linux/kmemleak.h> 19 #include <linux/list.h> 20 #include <linux/lockdep.h> 21 #include <linux/log2.h> 22 #include <linux/memblock.h> 23 #include <linux/moduleparam.h> 24 #include <linux/notifier.h> 25 #include <linux/panic_notifier.h> 26 #include <linux/random.h> 27 #include <linux/rcupdate.h> 28 #include <linux/sched/clock.h> 29 #include <linux/seq_file.h> 30 #include <linux/slab.h> 31 #include <linux/spinlock.h> 32 #include <linux/string.h> 33 34 #include <asm/kfence.h> 35 36 #include "kfence.h" 37 38 /* Disables KFENCE on the first warning assuming an irrecoverable error. */ 39 #define KFENCE_WARN_ON(cond) \ 40 ({ \ 41 const bool __cond = WARN_ON(cond); \ 42 if (unlikely(__cond)) { \ 43 WRITE_ONCE(kfence_enabled, false); \ 44 disabled_by_warn = true; \ 45 } \ 46 __cond; \ 47 }) 48 49 /* === Data ================================================================= */ 50 51 static bool kfence_enabled __read_mostly; 52 static bool disabled_by_warn __read_mostly; 53 54 unsigned long kfence_sample_interval __read_mostly = CONFIG_KFENCE_SAMPLE_INTERVAL; 55 EXPORT_SYMBOL_GPL(kfence_sample_interval); /* Export for test modules. */ 56 57 #ifdef MODULE_PARAM_PREFIX 58 #undef MODULE_PARAM_PREFIX 59 #endif 60 #define MODULE_PARAM_PREFIX "kfence." 61 62 static int kfence_enable_late(void); 63 static int param_set_sample_interval(const char *val, const struct kernel_param *kp) 64 { 65 unsigned long num; 66 int ret = kstrtoul(val, 0, &num); 67 68 if (ret < 0) 69 return ret; 70 71 /* Using 0 to indicate KFENCE is disabled. */ 72 if (!num && READ_ONCE(kfence_enabled)) { 73 pr_info("disabled\n"); 74 WRITE_ONCE(kfence_enabled, false); 75 } 76 77 *((unsigned long *)kp->arg) = num; 78 79 if (num && !READ_ONCE(kfence_enabled) && system_state != SYSTEM_BOOTING) 80 return disabled_by_warn ? -EINVAL : kfence_enable_late(); 81 return 0; 82 } 83 84 static int param_get_sample_interval(char *buffer, const struct kernel_param *kp) 85 { 86 if (!READ_ONCE(kfence_enabled)) 87 return sprintf(buffer, "0\n"); 88 89 return param_get_ulong(buffer, kp); 90 } 91 92 static const struct kernel_param_ops sample_interval_param_ops = { 93 .set = param_set_sample_interval, 94 .get = param_get_sample_interval, 95 }; 96 module_param_cb(sample_interval, &sample_interval_param_ops, &kfence_sample_interval, 0600); 97 98 /* Pool usage% threshold when currently covered allocations are skipped. */ 99 static unsigned long kfence_skip_covered_thresh __read_mostly = 75; 100 module_param_named(skip_covered_thresh, kfence_skip_covered_thresh, ulong, 0644); 101 102 /* If true, use a deferrable timer. */ 103 static bool kfence_deferrable __read_mostly = IS_ENABLED(CONFIG_KFENCE_DEFERRABLE); 104 module_param_named(deferrable, kfence_deferrable, bool, 0444); 105 106 /* If true, check all canary bytes on panic. */ 107 static bool kfence_check_on_panic __read_mostly; 108 module_param_named(check_on_panic, kfence_check_on_panic, bool, 0444); 109 110 /* The pool of pages used for guard pages and objects. */ 111 char *__kfence_pool __read_mostly; 112 EXPORT_SYMBOL(__kfence_pool); /* Export for test modules. */ 113 114 /* 115 * Per-object metadata, with one-to-one mapping of object metadata to 116 * backing pages (in __kfence_pool). 117 */ 118 static_assert(CONFIG_KFENCE_NUM_OBJECTS > 0); 119 struct kfence_metadata *kfence_metadata __read_mostly; 120 121 /* 122 * If kfence_metadata is not NULL, it may be accessed by kfence_shutdown_cache(). 123 * So introduce kfence_metadata_init to initialize metadata, and then make 124 * kfence_metadata visible after initialization is successful. This prevents 125 * potential UAF or access to uninitialized metadata. 126 */ 127 static struct kfence_metadata *kfence_metadata_init __read_mostly; 128 129 /* Freelist with available objects. */ 130 static struct list_head kfence_freelist = LIST_HEAD_INIT(kfence_freelist); 131 static DEFINE_RAW_SPINLOCK(kfence_freelist_lock); /* Lock protecting freelist. */ 132 133 /* 134 * The static key to set up a KFENCE allocation; or if static keys are not used 135 * to gate allocations, to avoid a load and compare if KFENCE is disabled. 136 */ 137 DEFINE_STATIC_KEY_FALSE(kfence_allocation_key); 138 139 /* Gates the allocation, ensuring only one succeeds in a given period. */ 140 atomic_t kfence_allocation_gate = ATOMIC_INIT(1); 141 142 /* 143 * A Counting Bloom filter of allocation coverage: limits currently covered 144 * allocations of the same source filling up the pool. 145 * 146 * Assuming a range of 15%-85% unique allocations in the pool at any point in 147 * time, the below parameters provide a probablity of 0.02-0.33 for false 148 * positive hits respectively: 149 * 150 * P(alloc_traces) = (1 - e^(-HNUM * (alloc_traces / SIZE)) ^ HNUM 151 */ 152 #define ALLOC_COVERED_HNUM 2 153 #define ALLOC_COVERED_ORDER (const_ilog2(CONFIG_KFENCE_NUM_OBJECTS) + 2) 154 #define ALLOC_COVERED_SIZE (1 << ALLOC_COVERED_ORDER) 155 #define ALLOC_COVERED_HNEXT(h) hash_32(h, ALLOC_COVERED_ORDER) 156 #define ALLOC_COVERED_MASK (ALLOC_COVERED_SIZE - 1) 157 static atomic_t alloc_covered[ALLOC_COVERED_SIZE]; 158 159 /* Stack depth used to determine uniqueness of an allocation. */ 160 #define UNIQUE_ALLOC_STACK_DEPTH ((size_t)8) 161 162 /* 163 * Randomness for stack hashes, making the same collisions across reboots and 164 * different machines less likely. 165 */ 166 static u32 stack_hash_seed __ro_after_init; 167 168 /* Statistics counters for debugfs. */ 169 enum kfence_counter_id { 170 KFENCE_COUNTER_ALLOCATED, 171 KFENCE_COUNTER_ALLOCS, 172 KFENCE_COUNTER_FREES, 173 KFENCE_COUNTER_ZOMBIES, 174 KFENCE_COUNTER_BUGS, 175 KFENCE_COUNTER_SKIP_INCOMPAT, 176 KFENCE_COUNTER_SKIP_CAPACITY, 177 KFENCE_COUNTER_SKIP_COVERED, 178 KFENCE_COUNTER_COUNT, 179 }; 180 static atomic_long_t counters[KFENCE_COUNTER_COUNT]; 181 static const char *const counter_names[] = { 182 [KFENCE_COUNTER_ALLOCATED] = "currently allocated", 183 [KFENCE_COUNTER_ALLOCS] = "total allocations", 184 [KFENCE_COUNTER_FREES] = "total frees", 185 [KFENCE_COUNTER_ZOMBIES] = "zombie allocations", 186 [KFENCE_COUNTER_BUGS] = "total bugs", 187 [KFENCE_COUNTER_SKIP_INCOMPAT] = "skipped allocations (incompatible)", 188 [KFENCE_COUNTER_SKIP_CAPACITY] = "skipped allocations (capacity)", 189 [KFENCE_COUNTER_SKIP_COVERED] = "skipped allocations (covered)", 190 }; 191 static_assert(ARRAY_SIZE(counter_names) == KFENCE_COUNTER_COUNT); 192 193 /* === Internals ============================================================ */ 194 195 static inline bool should_skip_covered(void) 196 { 197 unsigned long thresh = (CONFIG_KFENCE_NUM_OBJECTS * kfence_skip_covered_thresh) / 100; 198 199 return atomic_long_read(&counters[KFENCE_COUNTER_ALLOCATED]) > thresh; 200 } 201 202 static u32 get_alloc_stack_hash(unsigned long *stack_entries, size_t num_entries) 203 { 204 num_entries = min(num_entries, UNIQUE_ALLOC_STACK_DEPTH); 205 num_entries = filter_irq_stacks(stack_entries, num_entries); 206 return jhash(stack_entries, num_entries * sizeof(stack_entries[0]), stack_hash_seed); 207 } 208 209 /* 210 * Adds (or subtracts) count @val for allocation stack trace hash 211 * @alloc_stack_hash from Counting Bloom filter. 212 */ 213 static void alloc_covered_add(u32 alloc_stack_hash, int val) 214 { 215 int i; 216 217 for (i = 0; i < ALLOC_COVERED_HNUM; i++) { 218 atomic_add(val, &alloc_covered[alloc_stack_hash & ALLOC_COVERED_MASK]); 219 alloc_stack_hash = ALLOC_COVERED_HNEXT(alloc_stack_hash); 220 } 221 } 222 223 /* 224 * Returns true if the allocation stack trace hash @alloc_stack_hash is 225 * currently contained (non-zero count) in Counting Bloom filter. 226 */ 227 static bool alloc_covered_contains(u32 alloc_stack_hash) 228 { 229 int i; 230 231 for (i = 0; i < ALLOC_COVERED_HNUM; i++) { 232 if (!atomic_read(&alloc_covered[alloc_stack_hash & ALLOC_COVERED_MASK])) 233 return false; 234 alloc_stack_hash = ALLOC_COVERED_HNEXT(alloc_stack_hash); 235 } 236 237 return true; 238 } 239 240 static bool kfence_protect(unsigned long addr) 241 { 242 return !KFENCE_WARN_ON(!kfence_protect_page(ALIGN_DOWN(addr, PAGE_SIZE), true)); 243 } 244 245 static bool kfence_unprotect(unsigned long addr) 246 { 247 return !KFENCE_WARN_ON(!kfence_protect_page(ALIGN_DOWN(addr, PAGE_SIZE), false)); 248 } 249 250 static inline unsigned long metadata_to_pageaddr(const struct kfence_metadata *meta) 251 { 252 unsigned long offset = (meta - kfence_metadata + 1) * PAGE_SIZE * 2; 253 unsigned long pageaddr = (unsigned long)&__kfence_pool[offset]; 254 255 /* The checks do not affect performance; only called from slow-paths. */ 256 257 /* Only call with a pointer into kfence_metadata. */ 258 if (KFENCE_WARN_ON(meta < kfence_metadata || 259 meta >= kfence_metadata + CONFIG_KFENCE_NUM_OBJECTS)) 260 return 0; 261 262 /* 263 * This metadata object only ever maps to 1 page; verify that the stored 264 * address is in the expected range. 265 */ 266 if (KFENCE_WARN_ON(ALIGN_DOWN(meta->addr, PAGE_SIZE) != pageaddr)) 267 return 0; 268 269 return pageaddr; 270 } 271 272 /* 273 * Update the object's metadata state, including updating the alloc/free stacks 274 * depending on the state transition. 275 */ 276 static noinline void 277 metadata_update_state(struct kfence_metadata *meta, enum kfence_object_state next, 278 unsigned long *stack_entries, size_t num_stack_entries) 279 { 280 struct kfence_track *track = 281 next == KFENCE_OBJECT_FREED ? &meta->free_track : &meta->alloc_track; 282 283 lockdep_assert_held(&meta->lock); 284 285 if (stack_entries) { 286 memcpy(track->stack_entries, stack_entries, 287 num_stack_entries * sizeof(stack_entries[0])); 288 } else { 289 /* 290 * Skip over 1 (this) functions; noinline ensures we do not 291 * accidentally skip over the caller by never inlining. 292 */ 293 num_stack_entries = stack_trace_save(track->stack_entries, KFENCE_STACK_DEPTH, 1); 294 } 295 track->num_stack_entries = num_stack_entries; 296 track->pid = task_pid_nr(current); 297 track->cpu = raw_smp_processor_id(); 298 track->ts_nsec = local_clock(); /* Same source as printk timestamps. */ 299 300 /* 301 * Pairs with READ_ONCE() in 302 * kfence_shutdown_cache(), 303 * kfence_handle_page_fault(). 304 */ 305 WRITE_ONCE(meta->state, next); 306 } 307 308 /* Check canary byte at @addr. */ 309 static inline bool check_canary_byte(u8 *addr) 310 { 311 struct kfence_metadata *meta; 312 unsigned long flags; 313 314 if (likely(*addr == KFENCE_CANARY_PATTERN_U8(addr))) 315 return true; 316 317 atomic_long_inc(&counters[KFENCE_COUNTER_BUGS]); 318 319 meta = addr_to_metadata((unsigned long)addr); 320 raw_spin_lock_irqsave(&meta->lock, flags); 321 kfence_report_error((unsigned long)addr, false, NULL, meta, KFENCE_ERROR_CORRUPTION); 322 raw_spin_unlock_irqrestore(&meta->lock, flags); 323 324 return false; 325 } 326 327 static inline void set_canary(const struct kfence_metadata *meta) 328 { 329 const unsigned long pageaddr = ALIGN_DOWN(meta->addr, PAGE_SIZE); 330 unsigned long addr = pageaddr; 331 332 /* 333 * The canary may be written to part of the object memory, but it does 334 * not affect it. The user should initialize the object before using it. 335 */ 336 for (; addr < meta->addr; addr += sizeof(u64)) 337 *((u64 *)addr) = KFENCE_CANARY_PATTERN_U64; 338 339 addr = ALIGN_DOWN(meta->addr + meta->size, sizeof(u64)); 340 for (; addr - pageaddr < PAGE_SIZE; addr += sizeof(u64)) 341 *((u64 *)addr) = KFENCE_CANARY_PATTERN_U64; 342 } 343 344 static inline void check_canary(const struct kfence_metadata *meta) 345 { 346 const unsigned long pageaddr = ALIGN_DOWN(meta->addr, PAGE_SIZE); 347 unsigned long addr = pageaddr; 348 349 /* 350 * We'll iterate over each canary byte per-side until a corrupted byte 351 * is found. However, we'll still iterate over the canary bytes to the 352 * right of the object even if there was an error in the canary bytes to 353 * the left of the object. Specifically, if check_canary_byte() 354 * generates an error, showing both sides might give more clues as to 355 * what the error is about when displaying which bytes were corrupted. 356 */ 357 358 /* Apply to left of object. */ 359 for (; meta->addr - addr >= sizeof(u64); addr += sizeof(u64)) { 360 if (unlikely(*((u64 *)addr) != KFENCE_CANARY_PATTERN_U64)) 361 break; 362 } 363 364 /* 365 * If the canary is corrupted in a certain 64 bytes, or the canary 366 * memory cannot be completely covered by multiple consecutive 64 bytes, 367 * it needs to be checked one by one. 368 */ 369 for (; addr < meta->addr; addr++) { 370 if (unlikely(!check_canary_byte((u8 *)addr))) 371 break; 372 } 373 374 /* Apply to right of object. */ 375 for (addr = meta->addr + meta->size; addr % sizeof(u64) != 0; addr++) { 376 if (unlikely(!check_canary_byte((u8 *)addr))) 377 return; 378 } 379 for (; addr - pageaddr < PAGE_SIZE; addr += sizeof(u64)) { 380 if (unlikely(*((u64 *)addr) != KFENCE_CANARY_PATTERN_U64)) { 381 382 for (; addr - pageaddr < PAGE_SIZE; addr++) { 383 if (!check_canary_byte((u8 *)addr)) 384 return; 385 } 386 } 387 } 388 } 389 390 static void *kfence_guarded_alloc(struct kmem_cache *cache, size_t size, gfp_t gfp, 391 unsigned long *stack_entries, size_t num_stack_entries, 392 u32 alloc_stack_hash) 393 { 394 struct kfence_metadata *meta = NULL; 395 unsigned long flags; 396 struct slab *slab; 397 void *addr; 398 const bool random_right_allocate = get_random_u32_below(2); 399 const bool random_fault = CONFIG_KFENCE_STRESS_TEST_FAULTS && 400 !get_random_u32_below(CONFIG_KFENCE_STRESS_TEST_FAULTS); 401 402 /* Try to obtain a free object. */ 403 raw_spin_lock_irqsave(&kfence_freelist_lock, flags); 404 if (!list_empty(&kfence_freelist)) { 405 meta = list_entry(kfence_freelist.next, struct kfence_metadata, list); 406 list_del_init(&meta->list); 407 } 408 raw_spin_unlock_irqrestore(&kfence_freelist_lock, flags); 409 if (!meta) { 410 atomic_long_inc(&counters[KFENCE_COUNTER_SKIP_CAPACITY]); 411 return NULL; 412 } 413 414 if (unlikely(!raw_spin_trylock_irqsave(&meta->lock, flags))) { 415 /* 416 * This is extremely unlikely -- we are reporting on a 417 * use-after-free, which locked meta->lock, and the reporting 418 * code via printk calls kmalloc() which ends up in 419 * kfence_alloc() and tries to grab the same object that we're 420 * reporting on. While it has never been observed, lockdep does 421 * report that there is a possibility of deadlock. Fix it by 422 * using trylock and bailing out gracefully. 423 */ 424 raw_spin_lock_irqsave(&kfence_freelist_lock, flags); 425 /* Put the object back on the freelist. */ 426 list_add_tail(&meta->list, &kfence_freelist); 427 raw_spin_unlock_irqrestore(&kfence_freelist_lock, flags); 428 429 return NULL; 430 } 431 432 meta->addr = metadata_to_pageaddr(meta); 433 /* Unprotect if we're reusing this page. */ 434 if (meta->state == KFENCE_OBJECT_FREED) 435 kfence_unprotect(meta->addr); 436 437 /* 438 * Note: for allocations made before RNG initialization, will always 439 * return zero. We still benefit from enabling KFENCE as early as 440 * possible, even when the RNG is not yet available, as this will allow 441 * KFENCE to detect bugs due to earlier allocations. The only downside 442 * is that the out-of-bounds accesses detected are deterministic for 443 * such allocations. 444 */ 445 if (random_right_allocate) { 446 /* Allocate on the "right" side, re-calculate address. */ 447 meta->addr += PAGE_SIZE - size; 448 meta->addr = ALIGN_DOWN(meta->addr, cache->align); 449 } 450 451 addr = (void *)meta->addr; 452 453 /* Update remaining metadata. */ 454 metadata_update_state(meta, KFENCE_OBJECT_ALLOCATED, stack_entries, num_stack_entries); 455 /* Pairs with READ_ONCE() in kfence_shutdown_cache(). */ 456 WRITE_ONCE(meta->cache, cache); 457 meta->size = size; 458 meta->alloc_stack_hash = alloc_stack_hash; 459 raw_spin_unlock_irqrestore(&meta->lock, flags); 460 461 alloc_covered_add(alloc_stack_hash, 1); 462 463 /* Set required slab fields. */ 464 slab = virt_to_slab((void *)meta->addr); 465 slab->slab_cache = cache; 466 #if defined(CONFIG_SLUB) 467 slab->objects = 1; 468 #elif defined(CONFIG_SLAB) 469 slab->s_mem = addr; 470 #endif 471 472 /* Memory initialization. */ 473 set_canary(meta); 474 475 /* 476 * We check slab_want_init_on_alloc() ourselves, rather than letting 477 * SL*B do the initialization, as otherwise we might overwrite KFENCE's 478 * redzone. 479 */ 480 if (unlikely(slab_want_init_on_alloc(gfp, cache))) 481 memzero_explicit(addr, size); 482 if (cache->ctor) 483 cache->ctor(addr); 484 485 if (random_fault) 486 kfence_protect(meta->addr); /* Random "faults" by protecting the object. */ 487 488 atomic_long_inc(&counters[KFENCE_COUNTER_ALLOCATED]); 489 atomic_long_inc(&counters[KFENCE_COUNTER_ALLOCS]); 490 491 return addr; 492 } 493 494 static void kfence_guarded_free(void *addr, struct kfence_metadata *meta, bool zombie) 495 { 496 struct kcsan_scoped_access assert_page_exclusive; 497 unsigned long flags; 498 bool init; 499 500 raw_spin_lock_irqsave(&meta->lock, flags); 501 502 if (meta->state != KFENCE_OBJECT_ALLOCATED || meta->addr != (unsigned long)addr) { 503 /* Invalid or double-free, bail out. */ 504 atomic_long_inc(&counters[KFENCE_COUNTER_BUGS]); 505 kfence_report_error((unsigned long)addr, false, NULL, meta, 506 KFENCE_ERROR_INVALID_FREE); 507 raw_spin_unlock_irqrestore(&meta->lock, flags); 508 return; 509 } 510 511 /* Detect racy use-after-free, or incorrect reallocation of this page by KFENCE. */ 512 kcsan_begin_scoped_access((void *)ALIGN_DOWN((unsigned long)addr, PAGE_SIZE), PAGE_SIZE, 513 KCSAN_ACCESS_SCOPED | KCSAN_ACCESS_WRITE | KCSAN_ACCESS_ASSERT, 514 &assert_page_exclusive); 515 516 if (CONFIG_KFENCE_STRESS_TEST_FAULTS) 517 kfence_unprotect((unsigned long)addr); /* To check canary bytes. */ 518 519 /* Restore page protection if there was an OOB access. */ 520 if (meta->unprotected_page) { 521 memzero_explicit((void *)ALIGN_DOWN(meta->unprotected_page, PAGE_SIZE), PAGE_SIZE); 522 kfence_protect(meta->unprotected_page); 523 meta->unprotected_page = 0; 524 } 525 526 /* Mark the object as freed. */ 527 metadata_update_state(meta, KFENCE_OBJECT_FREED, NULL, 0); 528 init = slab_want_init_on_free(meta->cache); 529 raw_spin_unlock_irqrestore(&meta->lock, flags); 530 531 alloc_covered_add(meta->alloc_stack_hash, -1); 532 533 /* Check canary bytes for memory corruption. */ 534 check_canary(meta); 535 536 /* 537 * Clear memory if init-on-free is set. While we protect the page, the 538 * data is still there, and after a use-after-free is detected, we 539 * unprotect the page, so the data is still accessible. 540 */ 541 if (!zombie && unlikely(init)) 542 memzero_explicit(addr, meta->size); 543 544 /* Protect to detect use-after-frees. */ 545 kfence_protect((unsigned long)addr); 546 547 kcsan_end_scoped_access(&assert_page_exclusive); 548 if (!zombie) { 549 /* Add it to the tail of the freelist for reuse. */ 550 raw_spin_lock_irqsave(&kfence_freelist_lock, flags); 551 KFENCE_WARN_ON(!list_empty(&meta->list)); 552 list_add_tail(&meta->list, &kfence_freelist); 553 raw_spin_unlock_irqrestore(&kfence_freelist_lock, flags); 554 555 atomic_long_dec(&counters[KFENCE_COUNTER_ALLOCATED]); 556 atomic_long_inc(&counters[KFENCE_COUNTER_FREES]); 557 } else { 558 /* See kfence_shutdown_cache(). */ 559 atomic_long_inc(&counters[KFENCE_COUNTER_ZOMBIES]); 560 } 561 } 562 563 static void rcu_guarded_free(struct rcu_head *h) 564 { 565 struct kfence_metadata *meta = container_of(h, struct kfence_metadata, rcu_head); 566 567 kfence_guarded_free((void *)meta->addr, meta, false); 568 } 569 570 /* 571 * Initialization of the KFENCE pool after its allocation. 572 * Returns 0 on success; otherwise returns the address up to 573 * which partial initialization succeeded. 574 */ 575 static unsigned long kfence_init_pool(void) 576 { 577 unsigned long addr; 578 struct page *pages; 579 int i; 580 581 if (!arch_kfence_init_pool()) 582 return (unsigned long)__kfence_pool; 583 584 addr = (unsigned long)__kfence_pool; 585 pages = virt_to_page(__kfence_pool); 586 587 /* 588 * Set up object pages: they must have PG_slab set, to avoid freeing 589 * these as real pages. 590 * 591 * We also want to avoid inserting kfence_free() in the kfree() 592 * fast-path in SLUB, and therefore need to ensure kfree() correctly 593 * enters __slab_free() slow-path. 594 */ 595 for (i = 0; i < KFENCE_POOL_SIZE / PAGE_SIZE; i++) { 596 struct slab *slab = page_slab(nth_page(pages, i)); 597 598 if (!i || (i % 2)) 599 continue; 600 601 __folio_set_slab(slab_folio(slab)); 602 #ifdef CONFIG_MEMCG 603 slab->memcg_data = (unsigned long)&kfence_metadata_init[i / 2 - 1].objcg | 604 MEMCG_DATA_OBJCGS; 605 #endif 606 } 607 608 /* 609 * Protect the first 2 pages. The first page is mostly unnecessary, and 610 * merely serves as an extended guard page. However, adding one 611 * additional page in the beginning gives us an even number of pages, 612 * which simplifies the mapping of address to metadata index. 613 */ 614 for (i = 0; i < 2; i++) { 615 if (unlikely(!kfence_protect(addr))) 616 return addr; 617 618 addr += PAGE_SIZE; 619 } 620 621 for (i = 0; i < CONFIG_KFENCE_NUM_OBJECTS; i++) { 622 struct kfence_metadata *meta = &kfence_metadata_init[i]; 623 624 /* Initialize metadata. */ 625 INIT_LIST_HEAD(&meta->list); 626 raw_spin_lock_init(&meta->lock); 627 meta->state = KFENCE_OBJECT_UNUSED; 628 meta->addr = addr; /* Initialize for validation in metadata_to_pageaddr(). */ 629 list_add_tail(&meta->list, &kfence_freelist); 630 631 /* Protect the right redzone. */ 632 if (unlikely(!kfence_protect(addr + PAGE_SIZE))) 633 goto reset_slab; 634 635 addr += 2 * PAGE_SIZE; 636 } 637 638 /* 639 * Make kfence_metadata visible only when initialization is successful. 640 * Otherwise, if the initialization fails and kfence_metadata is freed, 641 * it may cause UAF in kfence_shutdown_cache(). 642 */ 643 smp_store_release(&kfence_metadata, kfence_metadata_init); 644 return 0; 645 646 reset_slab: 647 for (i = 0; i < KFENCE_POOL_SIZE / PAGE_SIZE; i++) { 648 struct slab *slab = page_slab(nth_page(pages, i)); 649 650 if (!i || (i % 2)) 651 continue; 652 #ifdef CONFIG_MEMCG 653 slab->memcg_data = 0; 654 #endif 655 __folio_clear_slab(slab_folio(slab)); 656 } 657 658 return addr; 659 } 660 661 static bool __init kfence_init_pool_early(void) 662 { 663 unsigned long addr; 664 665 if (!__kfence_pool) 666 return false; 667 668 addr = kfence_init_pool(); 669 670 if (!addr) { 671 /* 672 * The pool is live and will never be deallocated from this point on. 673 * Ignore the pool object from the kmemleak phys object tree, as it would 674 * otherwise overlap with allocations returned by kfence_alloc(), which 675 * are registered with kmemleak through the slab post-alloc hook. 676 */ 677 kmemleak_ignore_phys(__pa(__kfence_pool)); 678 return true; 679 } 680 681 /* 682 * Only release unprotected pages, and do not try to go back and change 683 * page attributes due to risk of failing to do so as well. If changing 684 * page attributes for some pages fails, it is very likely that it also 685 * fails for the first page, and therefore expect addr==__kfence_pool in 686 * most failure cases. 687 */ 688 memblock_free_late(__pa(addr), KFENCE_POOL_SIZE - (addr - (unsigned long)__kfence_pool)); 689 __kfence_pool = NULL; 690 691 memblock_free_late(__pa(kfence_metadata_init), KFENCE_METADATA_SIZE); 692 kfence_metadata_init = NULL; 693 694 return false; 695 } 696 697 /* === DebugFS Interface ==================================================== */ 698 699 static int stats_show(struct seq_file *seq, void *v) 700 { 701 int i; 702 703 seq_printf(seq, "enabled: %i\n", READ_ONCE(kfence_enabled)); 704 for (i = 0; i < KFENCE_COUNTER_COUNT; i++) 705 seq_printf(seq, "%s: %ld\n", counter_names[i], atomic_long_read(&counters[i])); 706 707 return 0; 708 } 709 DEFINE_SHOW_ATTRIBUTE(stats); 710 711 /* 712 * debugfs seq_file operations for /sys/kernel/debug/kfence/objects. 713 * start_object() and next_object() return the object index + 1, because NULL is used 714 * to stop iteration. 715 */ 716 static void *start_object(struct seq_file *seq, loff_t *pos) 717 { 718 if (*pos < CONFIG_KFENCE_NUM_OBJECTS) 719 return (void *)((long)*pos + 1); 720 return NULL; 721 } 722 723 static void stop_object(struct seq_file *seq, void *v) 724 { 725 } 726 727 static void *next_object(struct seq_file *seq, void *v, loff_t *pos) 728 { 729 ++*pos; 730 if (*pos < CONFIG_KFENCE_NUM_OBJECTS) 731 return (void *)((long)*pos + 1); 732 return NULL; 733 } 734 735 static int show_object(struct seq_file *seq, void *v) 736 { 737 struct kfence_metadata *meta = &kfence_metadata[(long)v - 1]; 738 unsigned long flags; 739 740 raw_spin_lock_irqsave(&meta->lock, flags); 741 kfence_print_object(seq, meta); 742 raw_spin_unlock_irqrestore(&meta->lock, flags); 743 seq_puts(seq, "---------------------------------\n"); 744 745 return 0; 746 } 747 748 static const struct seq_operations objects_sops = { 749 .start = start_object, 750 .next = next_object, 751 .stop = stop_object, 752 .show = show_object, 753 }; 754 DEFINE_SEQ_ATTRIBUTE(objects); 755 756 static int kfence_debugfs_init(void) 757 { 758 struct dentry *kfence_dir; 759 760 if (!READ_ONCE(kfence_enabled)) 761 return 0; 762 763 kfence_dir = debugfs_create_dir("kfence", NULL); 764 debugfs_create_file("stats", 0444, kfence_dir, NULL, &stats_fops); 765 debugfs_create_file("objects", 0400, kfence_dir, NULL, &objects_fops); 766 return 0; 767 } 768 769 late_initcall(kfence_debugfs_init); 770 771 /* === Panic Notifier ====================================================== */ 772 773 static void kfence_check_all_canary(void) 774 { 775 int i; 776 777 for (i = 0; i < CONFIG_KFENCE_NUM_OBJECTS; i++) { 778 struct kfence_metadata *meta = &kfence_metadata[i]; 779 780 if (meta->state == KFENCE_OBJECT_ALLOCATED) 781 check_canary(meta); 782 } 783 } 784 785 static int kfence_check_canary_callback(struct notifier_block *nb, 786 unsigned long reason, void *arg) 787 { 788 kfence_check_all_canary(); 789 return NOTIFY_OK; 790 } 791 792 static struct notifier_block kfence_check_canary_notifier = { 793 .notifier_call = kfence_check_canary_callback, 794 }; 795 796 /* === Allocation Gate Timer ================================================ */ 797 798 static struct delayed_work kfence_timer; 799 800 #ifdef CONFIG_KFENCE_STATIC_KEYS 801 /* Wait queue to wake up allocation-gate timer task. */ 802 static DECLARE_WAIT_QUEUE_HEAD(allocation_wait); 803 804 static void wake_up_kfence_timer(struct irq_work *work) 805 { 806 wake_up(&allocation_wait); 807 } 808 static DEFINE_IRQ_WORK(wake_up_kfence_timer_work, wake_up_kfence_timer); 809 #endif 810 811 /* 812 * Set up delayed work, which will enable and disable the static key. We need to 813 * use a work queue (rather than a simple timer), since enabling and disabling a 814 * static key cannot be done from an interrupt. 815 * 816 * Note: Toggling a static branch currently causes IPIs, and here we'll end up 817 * with a total of 2 IPIs to all CPUs. If this ends up a problem in future (with 818 * more aggressive sampling intervals), we could get away with a variant that 819 * avoids IPIs, at the cost of not immediately capturing allocations if the 820 * instructions remain cached. 821 */ 822 static void toggle_allocation_gate(struct work_struct *work) 823 { 824 if (!READ_ONCE(kfence_enabled)) 825 return; 826 827 atomic_set(&kfence_allocation_gate, 0); 828 #ifdef CONFIG_KFENCE_STATIC_KEYS 829 /* Enable static key, and await allocation to happen. */ 830 static_branch_enable(&kfence_allocation_key); 831 832 wait_event_idle(allocation_wait, atomic_read(&kfence_allocation_gate)); 833 834 /* Disable static key and reset timer. */ 835 static_branch_disable(&kfence_allocation_key); 836 #endif 837 queue_delayed_work(system_unbound_wq, &kfence_timer, 838 msecs_to_jiffies(kfence_sample_interval)); 839 } 840 841 /* === Public interface ===================================================== */ 842 843 void __init kfence_alloc_pool_and_metadata(void) 844 { 845 if (!kfence_sample_interval) 846 return; 847 848 /* 849 * If the pool has already been initialized by arch, there is no need to 850 * re-allocate the memory pool. 851 */ 852 if (!__kfence_pool) 853 __kfence_pool = memblock_alloc(KFENCE_POOL_SIZE, PAGE_SIZE); 854 855 if (!__kfence_pool) { 856 pr_err("failed to allocate pool\n"); 857 return; 858 } 859 860 /* The memory allocated by memblock has been zeroed out. */ 861 kfence_metadata_init = memblock_alloc(KFENCE_METADATA_SIZE, PAGE_SIZE); 862 if (!kfence_metadata_init) { 863 pr_err("failed to allocate metadata\n"); 864 memblock_free(__kfence_pool, KFENCE_POOL_SIZE); 865 __kfence_pool = NULL; 866 } 867 } 868 869 static void kfence_init_enable(void) 870 { 871 if (!IS_ENABLED(CONFIG_KFENCE_STATIC_KEYS)) 872 static_branch_enable(&kfence_allocation_key); 873 874 if (kfence_deferrable) 875 INIT_DEFERRABLE_WORK(&kfence_timer, toggle_allocation_gate); 876 else 877 INIT_DELAYED_WORK(&kfence_timer, toggle_allocation_gate); 878 879 if (kfence_check_on_panic) 880 atomic_notifier_chain_register(&panic_notifier_list, &kfence_check_canary_notifier); 881 882 WRITE_ONCE(kfence_enabled, true); 883 queue_delayed_work(system_unbound_wq, &kfence_timer, 0); 884 885 pr_info("initialized - using %lu bytes for %d objects at 0x%p-0x%p\n", KFENCE_POOL_SIZE, 886 CONFIG_KFENCE_NUM_OBJECTS, (void *)__kfence_pool, 887 (void *)(__kfence_pool + KFENCE_POOL_SIZE)); 888 } 889 890 void __init kfence_init(void) 891 { 892 stack_hash_seed = get_random_u32(); 893 894 /* Setting kfence_sample_interval to 0 on boot disables KFENCE. */ 895 if (!kfence_sample_interval) 896 return; 897 898 if (!kfence_init_pool_early()) { 899 pr_err("%s failed\n", __func__); 900 return; 901 } 902 903 kfence_init_enable(); 904 } 905 906 static int kfence_init_late(void) 907 { 908 const unsigned long nr_pages_pool = KFENCE_POOL_SIZE / PAGE_SIZE; 909 const unsigned long nr_pages_meta = KFENCE_METADATA_SIZE / PAGE_SIZE; 910 unsigned long addr = (unsigned long)__kfence_pool; 911 unsigned long free_size = KFENCE_POOL_SIZE; 912 int err = -ENOMEM; 913 914 #ifdef CONFIG_CONTIG_ALLOC 915 struct page *pages; 916 917 pages = alloc_contig_pages(nr_pages_pool, GFP_KERNEL, first_online_node, 918 NULL); 919 if (!pages) 920 return -ENOMEM; 921 922 __kfence_pool = page_to_virt(pages); 923 pages = alloc_contig_pages(nr_pages_meta, GFP_KERNEL, first_online_node, 924 NULL); 925 if (pages) 926 kfence_metadata_init = page_to_virt(pages); 927 #else 928 if (nr_pages_pool > MAX_ORDER_NR_PAGES || 929 nr_pages_meta > MAX_ORDER_NR_PAGES) { 930 pr_warn("KFENCE_NUM_OBJECTS too large for buddy allocator\n"); 931 return -EINVAL; 932 } 933 934 __kfence_pool = alloc_pages_exact(KFENCE_POOL_SIZE, GFP_KERNEL); 935 if (!__kfence_pool) 936 return -ENOMEM; 937 938 kfence_metadata_init = alloc_pages_exact(KFENCE_METADATA_SIZE, GFP_KERNEL); 939 #endif 940 941 if (!kfence_metadata_init) 942 goto free_pool; 943 944 memzero_explicit(kfence_metadata_init, KFENCE_METADATA_SIZE); 945 addr = kfence_init_pool(); 946 if (!addr) { 947 kfence_init_enable(); 948 kfence_debugfs_init(); 949 return 0; 950 } 951 952 pr_err("%s failed\n", __func__); 953 free_size = KFENCE_POOL_SIZE - (addr - (unsigned long)__kfence_pool); 954 err = -EBUSY; 955 956 #ifdef CONFIG_CONTIG_ALLOC 957 free_contig_range(page_to_pfn(virt_to_page((void *)kfence_metadata_init)), 958 nr_pages_meta); 959 free_pool: 960 free_contig_range(page_to_pfn(virt_to_page((void *)addr)), 961 free_size / PAGE_SIZE); 962 #else 963 free_pages_exact((void *)kfence_metadata_init, KFENCE_METADATA_SIZE); 964 free_pool: 965 free_pages_exact((void *)addr, free_size); 966 #endif 967 968 kfence_metadata_init = NULL; 969 __kfence_pool = NULL; 970 return err; 971 } 972 973 static int kfence_enable_late(void) 974 { 975 if (!__kfence_pool) 976 return kfence_init_late(); 977 978 WRITE_ONCE(kfence_enabled, true); 979 queue_delayed_work(system_unbound_wq, &kfence_timer, 0); 980 pr_info("re-enabled\n"); 981 return 0; 982 } 983 984 void kfence_shutdown_cache(struct kmem_cache *s) 985 { 986 unsigned long flags; 987 struct kfence_metadata *meta; 988 int i; 989 990 /* Pairs with release in kfence_init_pool(). */ 991 if (!smp_load_acquire(&kfence_metadata)) 992 return; 993 994 for (i = 0; i < CONFIG_KFENCE_NUM_OBJECTS; i++) { 995 bool in_use; 996 997 meta = &kfence_metadata[i]; 998 999 /* 1000 * If we observe some inconsistent cache and state pair where we 1001 * should have returned false here, cache destruction is racing 1002 * with either kmem_cache_alloc() or kmem_cache_free(). Taking 1003 * the lock will not help, as different critical section 1004 * serialization will have the same outcome. 1005 */ 1006 if (READ_ONCE(meta->cache) != s || 1007 READ_ONCE(meta->state) != KFENCE_OBJECT_ALLOCATED) 1008 continue; 1009 1010 raw_spin_lock_irqsave(&meta->lock, flags); 1011 in_use = meta->cache == s && meta->state == KFENCE_OBJECT_ALLOCATED; 1012 raw_spin_unlock_irqrestore(&meta->lock, flags); 1013 1014 if (in_use) { 1015 /* 1016 * This cache still has allocations, and we should not 1017 * release them back into the freelist so they can still 1018 * safely be used and retain the kernel's default 1019 * behaviour of keeping the allocations alive (leak the 1020 * cache); however, they effectively become "zombie 1021 * allocations" as the KFENCE objects are the only ones 1022 * still in use and the owning cache is being destroyed. 1023 * 1024 * We mark them freed, so that any subsequent use shows 1025 * more useful error messages that will include stack 1026 * traces of the user of the object, the original 1027 * allocation, and caller to shutdown_cache(). 1028 */ 1029 kfence_guarded_free((void *)meta->addr, meta, /*zombie=*/true); 1030 } 1031 } 1032 1033 for (i = 0; i < CONFIG_KFENCE_NUM_OBJECTS; i++) { 1034 meta = &kfence_metadata[i]; 1035 1036 /* See above. */ 1037 if (READ_ONCE(meta->cache) != s || READ_ONCE(meta->state) != KFENCE_OBJECT_FREED) 1038 continue; 1039 1040 raw_spin_lock_irqsave(&meta->lock, flags); 1041 if (meta->cache == s && meta->state == KFENCE_OBJECT_FREED) 1042 meta->cache = NULL; 1043 raw_spin_unlock_irqrestore(&meta->lock, flags); 1044 } 1045 } 1046 1047 void *__kfence_alloc(struct kmem_cache *s, size_t size, gfp_t flags) 1048 { 1049 unsigned long stack_entries[KFENCE_STACK_DEPTH]; 1050 size_t num_stack_entries; 1051 u32 alloc_stack_hash; 1052 1053 /* 1054 * Perform size check before switching kfence_allocation_gate, so that 1055 * we don't disable KFENCE without making an allocation. 1056 */ 1057 if (size > PAGE_SIZE) { 1058 atomic_long_inc(&counters[KFENCE_COUNTER_SKIP_INCOMPAT]); 1059 return NULL; 1060 } 1061 1062 /* 1063 * Skip allocations from non-default zones, including DMA. We cannot 1064 * guarantee that pages in the KFENCE pool will have the requested 1065 * properties (e.g. reside in DMAable memory). 1066 */ 1067 if ((flags & GFP_ZONEMASK) || 1068 (s->flags & (SLAB_CACHE_DMA | SLAB_CACHE_DMA32))) { 1069 atomic_long_inc(&counters[KFENCE_COUNTER_SKIP_INCOMPAT]); 1070 return NULL; 1071 } 1072 1073 /* 1074 * Skip allocations for this slab, if KFENCE has been disabled for 1075 * this slab. 1076 */ 1077 if (s->flags & SLAB_SKIP_KFENCE) 1078 return NULL; 1079 1080 if (atomic_inc_return(&kfence_allocation_gate) > 1) 1081 return NULL; 1082 #ifdef CONFIG_KFENCE_STATIC_KEYS 1083 /* 1084 * waitqueue_active() is fully ordered after the update of 1085 * kfence_allocation_gate per atomic_inc_return(). 1086 */ 1087 if (waitqueue_active(&allocation_wait)) { 1088 /* 1089 * Calling wake_up() here may deadlock when allocations happen 1090 * from within timer code. Use an irq_work to defer it. 1091 */ 1092 irq_work_queue(&wake_up_kfence_timer_work); 1093 } 1094 #endif 1095 1096 if (!READ_ONCE(kfence_enabled)) 1097 return NULL; 1098 1099 num_stack_entries = stack_trace_save(stack_entries, KFENCE_STACK_DEPTH, 0); 1100 1101 /* 1102 * Do expensive check for coverage of allocation in slow-path after 1103 * allocation_gate has already become non-zero, even though it might 1104 * mean not making any allocation within a given sample interval. 1105 * 1106 * This ensures reasonable allocation coverage when the pool is almost 1107 * full, including avoiding long-lived allocations of the same source 1108 * filling up the pool (e.g. pagecache allocations). 1109 */ 1110 alloc_stack_hash = get_alloc_stack_hash(stack_entries, num_stack_entries); 1111 if (should_skip_covered() && alloc_covered_contains(alloc_stack_hash)) { 1112 atomic_long_inc(&counters[KFENCE_COUNTER_SKIP_COVERED]); 1113 return NULL; 1114 } 1115 1116 return kfence_guarded_alloc(s, size, flags, stack_entries, num_stack_entries, 1117 alloc_stack_hash); 1118 } 1119 1120 size_t kfence_ksize(const void *addr) 1121 { 1122 const struct kfence_metadata *meta = addr_to_metadata((unsigned long)addr); 1123 1124 /* 1125 * Read locklessly -- if there is a race with __kfence_alloc(), this is 1126 * either a use-after-free or invalid access. 1127 */ 1128 return meta ? meta->size : 0; 1129 } 1130 1131 void *kfence_object_start(const void *addr) 1132 { 1133 const struct kfence_metadata *meta = addr_to_metadata((unsigned long)addr); 1134 1135 /* 1136 * Read locklessly -- if there is a race with __kfence_alloc(), this is 1137 * either a use-after-free or invalid access. 1138 */ 1139 return meta ? (void *)meta->addr : NULL; 1140 } 1141 1142 void __kfence_free(void *addr) 1143 { 1144 struct kfence_metadata *meta = addr_to_metadata((unsigned long)addr); 1145 1146 #ifdef CONFIG_MEMCG 1147 KFENCE_WARN_ON(meta->objcg); 1148 #endif 1149 /* 1150 * If the objects of the cache are SLAB_TYPESAFE_BY_RCU, defer freeing 1151 * the object, as the object page may be recycled for other-typed 1152 * objects once it has been freed. meta->cache may be NULL if the cache 1153 * was destroyed. 1154 */ 1155 if (unlikely(meta->cache && (meta->cache->flags & SLAB_TYPESAFE_BY_RCU))) 1156 call_rcu(&meta->rcu_head, rcu_guarded_free); 1157 else 1158 kfence_guarded_free(addr, meta, false); 1159 } 1160 1161 bool kfence_handle_page_fault(unsigned long addr, bool is_write, struct pt_regs *regs) 1162 { 1163 const int page_index = (addr - (unsigned long)__kfence_pool) / PAGE_SIZE; 1164 struct kfence_metadata *to_report = NULL; 1165 enum kfence_error_type error_type; 1166 unsigned long flags; 1167 1168 if (!is_kfence_address((void *)addr)) 1169 return false; 1170 1171 if (!READ_ONCE(kfence_enabled)) /* If disabled at runtime ... */ 1172 return kfence_unprotect(addr); /* ... unprotect and proceed. */ 1173 1174 atomic_long_inc(&counters[KFENCE_COUNTER_BUGS]); 1175 1176 if (page_index % 2) { 1177 /* This is a redzone, report a buffer overflow. */ 1178 struct kfence_metadata *meta; 1179 int distance = 0; 1180 1181 meta = addr_to_metadata(addr - PAGE_SIZE); 1182 if (meta && READ_ONCE(meta->state) == KFENCE_OBJECT_ALLOCATED) { 1183 to_report = meta; 1184 /* Data race ok; distance calculation approximate. */ 1185 distance = addr - data_race(meta->addr + meta->size); 1186 } 1187 1188 meta = addr_to_metadata(addr + PAGE_SIZE); 1189 if (meta && READ_ONCE(meta->state) == KFENCE_OBJECT_ALLOCATED) { 1190 /* Data race ok; distance calculation approximate. */ 1191 if (!to_report || distance > data_race(meta->addr) - addr) 1192 to_report = meta; 1193 } 1194 1195 if (!to_report) 1196 goto out; 1197 1198 raw_spin_lock_irqsave(&to_report->lock, flags); 1199 to_report->unprotected_page = addr; 1200 error_type = KFENCE_ERROR_OOB; 1201 1202 /* 1203 * If the object was freed before we took the look we can still 1204 * report this as an OOB -- the report will simply show the 1205 * stacktrace of the free as well. 1206 */ 1207 } else { 1208 to_report = addr_to_metadata(addr); 1209 if (!to_report) 1210 goto out; 1211 1212 raw_spin_lock_irqsave(&to_report->lock, flags); 1213 error_type = KFENCE_ERROR_UAF; 1214 /* 1215 * We may race with __kfence_alloc(), and it is possible that a 1216 * freed object may be reallocated. We simply report this as a 1217 * use-after-free, with the stack trace showing the place where 1218 * the object was re-allocated. 1219 */ 1220 } 1221 1222 out: 1223 if (to_report) { 1224 kfence_report_error(addr, is_write, regs, to_report, error_type); 1225 raw_spin_unlock_irqrestore(&to_report->lock, flags); 1226 } else { 1227 /* This may be a UAF or OOB access, but we can't be sure. */ 1228 kfence_report_error(addr, is_write, regs, NULL, KFENCE_ERROR_INVALID); 1229 } 1230 1231 return kfence_unprotect(addr); /* Unprotect and let access proceed. */ 1232 } 1233