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