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