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/random.h> 25 #include <linux/rcupdate.h> 26 #include <linux/sched/clock.h> 27 #include <linux/sched/sysctl.h> 28 #include <linux/seq_file.h> 29 #include <linux/slab.h> 30 #include <linux/spinlock.h> 31 #include <linux/string.h> 32 33 #include <asm/kfence.h> 34 35 #include "kfence.h" 36 37 /* Disables KFENCE on the first warning assuming an irrecoverable error. */ 38 #define KFENCE_WARN_ON(cond) \ 39 ({ \ 40 const bool __cond = WARN_ON(cond); \ 41 if (unlikely(__cond)) \ 42 WRITE_ONCE(kfence_enabled, false); \ 43 __cond; \ 44 }) 45 46 /* === Data ================================================================= */ 47 48 static bool kfence_enabled __read_mostly; 49 50 unsigned long kfence_sample_interval __read_mostly = CONFIG_KFENCE_SAMPLE_INTERVAL; 51 EXPORT_SYMBOL_GPL(kfence_sample_interval); /* Export for test modules. */ 52 53 #ifdef MODULE_PARAM_PREFIX 54 #undef MODULE_PARAM_PREFIX 55 #endif 56 #define MODULE_PARAM_PREFIX "kfence." 57 58 static int param_set_sample_interval(const char *val, const struct kernel_param *kp) 59 { 60 unsigned long num; 61 int ret = kstrtoul(val, 0, &num); 62 63 if (ret < 0) 64 return ret; 65 66 if (!num) /* Using 0 to indicate KFENCE is disabled. */ 67 WRITE_ONCE(kfence_enabled, false); 68 else if (!READ_ONCE(kfence_enabled) && system_state != SYSTEM_BOOTING) 69 return -EINVAL; /* Cannot (re-)enable KFENCE on-the-fly. */ 70 71 *((unsigned long *)kp->arg) = num; 72 return 0; 73 } 74 75 static int param_get_sample_interval(char *buffer, const struct kernel_param *kp) 76 { 77 if (!READ_ONCE(kfence_enabled)) 78 return sprintf(buffer, "0\n"); 79 80 return param_get_ulong(buffer, kp); 81 } 82 83 static const struct kernel_param_ops sample_interval_param_ops = { 84 .set = param_set_sample_interval, 85 .get = param_get_sample_interval, 86 }; 87 module_param_cb(sample_interval, &sample_interval_param_ops, &kfence_sample_interval, 0600); 88 89 /* Pool usage% threshold when currently covered allocations are skipped. */ 90 static unsigned long kfence_skip_covered_thresh __read_mostly = 75; 91 module_param_named(skip_covered_thresh, kfence_skip_covered_thresh, ulong, 0644); 92 93 /* The pool of pages used for guard pages and objects. */ 94 char *__kfence_pool __ro_after_init; 95 EXPORT_SYMBOL(__kfence_pool); /* Export for test modules. */ 96 97 /* 98 * Per-object metadata, with one-to-one mapping of object metadata to 99 * backing pages (in __kfence_pool). 100 */ 101 static_assert(CONFIG_KFENCE_NUM_OBJECTS > 0); 102 struct kfence_metadata kfence_metadata[CONFIG_KFENCE_NUM_OBJECTS]; 103 104 /* Freelist with available objects. */ 105 static struct list_head kfence_freelist = LIST_HEAD_INIT(kfence_freelist); 106 static DEFINE_RAW_SPINLOCK(kfence_freelist_lock); /* Lock protecting freelist. */ 107 108 /* 109 * The static key to set up a KFENCE allocation; or if static keys are not used 110 * to gate allocations, to avoid a load and compare if KFENCE is disabled. 111 */ 112 DEFINE_STATIC_KEY_FALSE(kfence_allocation_key); 113 114 /* Gates the allocation, ensuring only one succeeds in a given period. */ 115 atomic_t kfence_allocation_gate = ATOMIC_INIT(1); 116 117 /* 118 * A Counting Bloom filter of allocation coverage: limits currently covered 119 * allocations of the same source filling up the pool. 120 * 121 * Assuming a range of 15%-85% unique allocations in the pool at any point in 122 * time, the below parameters provide a probablity of 0.02-0.33 for false 123 * positive hits respectively: 124 * 125 * P(alloc_traces) = (1 - e^(-HNUM * (alloc_traces / SIZE)) ^ HNUM 126 */ 127 #define ALLOC_COVERED_HNUM 2 128 #define ALLOC_COVERED_ORDER (const_ilog2(CONFIG_KFENCE_NUM_OBJECTS) + 2) 129 #define ALLOC_COVERED_SIZE (1 << ALLOC_COVERED_ORDER) 130 #define ALLOC_COVERED_HNEXT(h) hash_32(h, ALLOC_COVERED_ORDER) 131 #define ALLOC_COVERED_MASK (ALLOC_COVERED_SIZE - 1) 132 static atomic_t alloc_covered[ALLOC_COVERED_SIZE]; 133 134 /* Stack depth used to determine uniqueness of an allocation. */ 135 #define UNIQUE_ALLOC_STACK_DEPTH ((size_t)8) 136 137 /* 138 * Randomness for stack hashes, making the same collisions across reboots and 139 * different machines less likely. 140 */ 141 static u32 stack_hash_seed __ro_after_init; 142 143 /* Statistics counters for debugfs. */ 144 enum kfence_counter_id { 145 KFENCE_COUNTER_ALLOCATED, 146 KFENCE_COUNTER_ALLOCS, 147 KFENCE_COUNTER_FREES, 148 KFENCE_COUNTER_ZOMBIES, 149 KFENCE_COUNTER_BUGS, 150 KFENCE_COUNTER_SKIP_INCOMPAT, 151 KFENCE_COUNTER_SKIP_CAPACITY, 152 KFENCE_COUNTER_SKIP_COVERED, 153 KFENCE_COUNTER_COUNT, 154 }; 155 static atomic_long_t counters[KFENCE_COUNTER_COUNT]; 156 static const char *const counter_names[] = { 157 [KFENCE_COUNTER_ALLOCATED] = "currently allocated", 158 [KFENCE_COUNTER_ALLOCS] = "total allocations", 159 [KFENCE_COUNTER_FREES] = "total frees", 160 [KFENCE_COUNTER_ZOMBIES] = "zombie allocations", 161 [KFENCE_COUNTER_BUGS] = "total bugs", 162 [KFENCE_COUNTER_SKIP_INCOMPAT] = "skipped allocations (incompatible)", 163 [KFENCE_COUNTER_SKIP_CAPACITY] = "skipped allocations (capacity)", 164 [KFENCE_COUNTER_SKIP_COVERED] = "skipped allocations (covered)", 165 }; 166 static_assert(ARRAY_SIZE(counter_names) == KFENCE_COUNTER_COUNT); 167 168 /* === Internals ============================================================ */ 169 170 static inline bool should_skip_covered(void) 171 { 172 unsigned long thresh = (CONFIG_KFENCE_NUM_OBJECTS * kfence_skip_covered_thresh) / 100; 173 174 return atomic_long_read(&counters[KFENCE_COUNTER_ALLOCATED]) > thresh; 175 } 176 177 static u32 get_alloc_stack_hash(unsigned long *stack_entries, size_t num_entries) 178 { 179 num_entries = min(num_entries, UNIQUE_ALLOC_STACK_DEPTH); 180 num_entries = filter_irq_stacks(stack_entries, num_entries); 181 return jhash(stack_entries, num_entries * sizeof(stack_entries[0]), stack_hash_seed); 182 } 183 184 /* 185 * Adds (or subtracts) count @val for allocation stack trace hash 186 * @alloc_stack_hash from Counting Bloom filter. 187 */ 188 static void alloc_covered_add(u32 alloc_stack_hash, int val) 189 { 190 int i; 191 192 for (i = 0; i < ALLOC_COVERED_HNUM; i++) { 193 atomic_add(val, &alloc_covered[alloc_stack_hash & ALLOC_COVERED_MASK]); 194 alloc_stack_hash = ALLOC_COVERED_HNEXT(alloc_stack_hash); 195 } 196 } 197 198 /* 199 * Returns true if the allocation stack trace hash @alloc_stack_hash is 200 * currently contained (non-zero count) in Counting Bloom filter. 201 */ 202 static bool alloc_covered_contains(u32 alloc_stack_hash) 203 { 204 int i; 205 206 for (i = 0; i < ALLOC_COVERED_HNUM; i++) { 207 if (!atomic_read(&alloc_covered[alloc_stack_hash & ALLOC_COVERED_MASK])) 208 return false; 209 alloc_stack_hash = ALLOC_COVERED_HNEXT(alloc_stack_hash); 210 } 211 212 return true; 213 } 214 215 static bool kfence_protect(unsigned long addr) 216 { 217 return !KFENCE_WARN_ON(!kfence_protect_page(ALIGN_DOWN(addr, PAGE_SIZE), true)); 218 } 219 220 static bool kfence_unprotect(unsigned long addr) 221 { 222 return !KFENCE_WARN_ON(!kfence_protect_page(ALIGN_DOWN(addr, PAGE_SIZE), false)); 223 } 224 225 static inline struct kfence_metadata *addr_to_metadata(unsigned long addr) 226 { 227 long index; 228 229 /* The checks do not affect performance; only called from slow-paths. */ 230 231 if (!is_kfence_address((void *)addr)) 232 return NULL; 233 234 /* 235 * May be an invalid index if called with an address at the edge of 236 * __kfence_pool, in which case we would report an "invalid access" 237 * error. 238 */ 239 index = (addr - (unsigned long)__kfence_pool) / (PAGE_SIZE * 2) - 1; 240 if (index < 0 || index >= CONFIG_KFENCE_NUM_OBJECTS) 241 return NULL; 242 243 return &kfence_metadata[index]; 244 } 245 246 static inline unsigned long metadata_to_pageaddr(const struct kfence_metadata *meta) 247 { 248 unsigned long offset = (meta - kfence_metadata + 1) * PAGE_SIZE * 2; 249 unsigned long pageaddr = (unsigned long)&__kfence_pool[offset]; 250 251 /* The checks do not affect performance; only called from slow-paths. */ 252 253 /* Only call with a pointer into kfence_metadata. */ 254 if (KFENCE_WARN_ON(meta < kfence_metadata || 255 meta >= kfence_metadata + CONFIG_KFENCE_NUM_OBJECTS)) 256 return 0; 257 258 /* 259 * This metadata object only ever maps to 1 page; verify that the stored 260 * address is in the expected range. 261 */ 262 if (KFENCE_WARN_ON(ALIGN_DOWN(meta->addr, PAGE_SIZE) != pageaddr)) 263 return 0; 264 265 return pageaddr; 266 } 267 268 /* 269 * Update the object's metadata state, including updating the alloc/free stacks 270 * depending on the state transition. 271 */ 272 static noinline void 273 metadata_update_state(struct kfence_metadata *meta, enum kfence_object_state next, 274 unsigned long *stack_entries, size_t num_stack_entries) 275 { 276 struct kfence_track *track = 277 next == KFENCE_OBJECT_FREED ? &meta->free_track : &meta->alloc_track; 278 279 lockdep_assert_held(&meta->lock); 280 281 if (stack_entries) { 282 memcpy(track->stack_entries, stack_entries, 283 num_stack_entries * sizeof(stack_entries[0])); 284 } else { 285 /* 286 * Skip over 1 (this) functions; noinline ensures we do not 287 * accidentally skip over the caller by never inlining. 288 */ 289 num_stack_entries = stack_trace_save(track->stack_entries, KFENCE_STACK_DEPTH, 1); 290 } 291 track->num_stack_entries = num_stack_entries; 292 track->pid = task_pid_nr(current); 293 track->cpu = raw_smp_processor_id(); 294 track->ts_nsec = local_clock(); /* Same source as printk timestamps. */ 295 296 /* 297 * Pairs with READ_ONCE() in 298 * kfence_shutdown_cache(), 299 * kfence_handle_page_fault(). 300 */ 301 WRITE_ONCE(meta->state, next); 302 } 303 304 /* Write canary byte to @addr. */ 305 static inline bool set_canary_byte(u8 *addr) 306 { 307 *addr = KFENCE_CANARY_PATTERN(addr); 308 return true; 309 } 310 311 /* Check canary byte at @addr. */ 312 static inline bool check_canary_byte(u8 *addr) 313 { 314 struct kfence_metadata *meta; 315 unsigned long flags; 316 317 if (likely(*addr == KFENCE_CANARY_PATTERN(addr))) 318 return true; 319 320 atomic_long_inc(&counters[KFENCE_COUNTER_BUGS]); 321 322 meta = addr_to_metadata((unsigned long)addr); 323 raw_spin_lock_irqsave(&meta->lock, flags); 324 kfence_report_error((unsigned long)addr, false, NULL, meta, KFENCE_ERROR_CORRUPTION); 325 raw_spin_unlock_irqrestore(&meta->lock, flags); 326 327 return false; 328 } 329 330 /* __always_inline this to ensure we won't do an indirect call to fn. */ 331 static __always_inline void for_each_canary(const struct kfence_metadata *meta, bool (*fn)(u8 *)) 332 { 333 const unsigned long pageaddr = ALIGN_DOWN(meta->addr, PAGE_SIZE); 334 unsigned long addr; 335 336 /* 337 * We'll iterate over each canary byte per-side until fn() returns 338 * false. However, we'll still iterate over the canary bytes to the 339 * right of the object even if there was an error in the canary bytes to 340 * the left of the object. Specifically, if check_canary_byte() 341 * generates an error, showing both sides might give more clues as to 342 * what the error is about when displaying which bytes were corrupted. 343 */ 344 345 /* Apply to left of object. */ 346 for (addr = pageaddr; addr < meta->addr; addr++) { 347 if (!fn((u8 *)addr)) 348 break; 349 } 350 351 /* Apply to right of object. */ 352 for (addr = meta->addr + meta->size; addr < pageaddr + PAGE_SIZE; addr++) { 353 if (!fn((u8 *)addr)) 354 break; 355 } 356 } 357 358 static void *kfence_guarded_alloc(struct kmem_cache *cache, size_t size, gfp_t gfp, 359 unsigned long *stack_entries, size_t num_stack_entries, 360 u32 alloc_stack_hash) 361 { 362 struct kfence_metadata *meta = NULL; 363 unsigned long flags; 364 struct slab *slab; 365 void *addr; 366 367 /* Try to obtain a free object. */ 368 raw_spin_lock_irqsave(&kfence_freelist_lock, flags); 369 if (!list_empty(&kfence_freelist)) { 370 meta = list_entry(kfence_freelist.next, struct kfence_metadata, list); 371 list_del_init(&meta->list); 372 } 373 raw_spin_unlock_irqrestore(&kfence_freelist_lock, flags); 374 if (!meta) { 375 atomic_long_inc(&counters[KFENCE_COUNTER_SKIP_CAPACITY]); 376 return NULL; 377 } 378 379 if (unlikely(!raw_spin_trylock_irqsave(&meta->lock, flags))) { 380 /* 381 * This is extremely unlikely -- we are reporting on a 382 * use-after-free, which locked meta->lock, and the reporting 383 * code via printk calls kmalloc() which ends up in 384 * kfence_alloc() and tries to grab the same object that we're 385 * reporting on. While it has never been observed, lockdep does 386 * report that there is a possibility of deadlock. Fix it by 387 * using trylock and bailing out gracefully. 388 */ 389 raw_spin_lock_irqsave(&kfence_freelist_lock, flags); 390 /* Put the object back on the freelist. */ 391 list_add_tail(&meta->list, &kfence_freelist); 392 raw_spin_unlock_irqrestore(&kfence_freelist_lock, flags); 393 394 return NULL; 395 } 396 397 meta->addr = metadata_to_pageaddr(meta); 398 /* Unprotect if we're reusing this page. */ 399 if (meta->state == KFENCE_OBJECT_FREED) 400 kfence_unprotect(meta->addr); 401 402 /* 403 * Note: for allocations made before RNG initialization, will always 404 * return zero. We still benefit from enabling KFENCE as early as 405 * possible, even when the RNG is not yet available, as this will allow 406 * KFENCE to detect bugs due to earlier allocations. The only downside 407 * is that the out-of-bounds accesses detected are deterministic for 408 * such allocations. 409 */ 410 if (prandom_u32_max(2)) { 411 /* Allocate on the "right" side, re-calculate address. */ 412 meta->addr += PAGE_SIZE - size; 413 meta->addr = ALIGN_DOWN(meta->addr, cache->align); 414 } 415 416 addr = (void *)meta->addr; 417 418 /* Update remaining metadata. */ 419 metadata_update_state(meta, KFENCE_OBJECT_ALLOCATED, stack_entries, num_stack_entries); 420 /* Pairs with READ_ONCE() in kfence_shutdown_cache(). */ 421 WRITE_ONCE(meta->cache, cache); 422 meta->size = size; 423 meta->alloc_stack_hash = alloc_stack_hash; 424 raw_spin_unlock_irqrestore(&meta->lock, flags); 425 426 alloc_covered_add(alloc_stack_hash, 1); 427 428 /* Set required slab fields. */ 429 slab = virt_to_slab((void *)meta->addr); 430 slab->slab_cache = cache; 431 #if defined(CONFIG_SLUB) 432 slab->objects = 1; 433 #elif defined(CONFIG_SLAB) 434 slab->s_mem = addr; 435 #endif 436 437 /* Memory initialization. */ 438 for_each_canary(meta, set_canary_byte); 439 440 /* 441 * We check slab_want_init_on_alloc() ourselves, rather than letting 442 * SL*B do the initialization, as otherwise we might overwrite KFENCE's 443 * redzone. 444 */ 445 if (unlikely(slab_want_init_on_alloc(gfp, cache))) 446 memzero_explicit(addr, size); 447 if (cache->ctor) 448 cache->ctor(addr); 449 450 if (CONFIG_KFENCE_STRESS_TEST_FAULTS && !prandom_u32_max(CONFIG_KFENCE_STRESS_TEST_FAULTS)) 451 kfence_protect(meta->addr); /* Random "faults" by protecting the object. */ 452 453 atomic_long_inc(&counters[KFENCE_COUNTER_ALLOCATED]); 454 atomic_long_inc(&counters[KFENCE_COUNTER_ALLOCS]); 455 456 return addr; 457 } 458 459 static void kfence_guarded_free(void *addr, struct kfence_metadata *meta, bool zombie) 460 { 461 struct kcsan_scoped_access assert_page_exclusive; 462 unsigned long flags; 463 bool init; 464 465 raw_spin_lock_irqsave(&meta->lock, flags); 466 467 if (meta->state != KFENCE_OBJECT_ALLOCATED || meta->addr != (unsigned long)addr) { 468 /* Invalid or double-free, bail out. */ 469 atomic_long_inc(&counters[KFENCE_COUNTER_BUGS]); 470 kfence_report_error((unsigned long)addr, false, NULL, meta, 471 KFENCE_ERROR_INVALID_FREE); 472 raw_spin_unlock_irqrestore(&meta->lock, flags); 473 return; 474 } 475 476 /* Detect racy use-after-free, or incorrect reallocation of this page by KFENCE. */ 477 kcsan_begin_scoped_access((void *)ALIGN_DOWN((unsigned long)addr, PAGE_SIZE), PAGE_SIZE, 478 KCSAN_ACCESS_SCOPED | KCSAN_ACCESS_WRITE | KCSAN_ACCESS_ASSERT, 479 &assert_page_exclusive); 480 481 if (CONFIG_KFENCE_STRESS_TEST_FAULTS) 482 kfence_unprotect((unsigned long)addr); /* To check canary bytes. */ 483 484 /* Restore page protection if there was an OOB access. */ 485 if (meta->unprotected_page) { 486 memzero_explicit((void *)ALIGN_DOWN(meta->unprotected_page, PAGE_SIZE), PAGE_SIZE); 487 kfence_protect(meta->unprotected_page); 488 meta->unprotected_page = 0; 489 } 490 491 /* Mark the object as freed. */ 492 metadata_update_state(meta, KFENCE_OBJECT_FREED, NULL, 0); 493 init = slab_want_init_on_free(meta->cache); 494 raw_spin_unlock_irqrestore(&meta->lock, flags); 495 496 alloc_covered_add(meta->alloc_stack_hash, -1); 497 498 /* Check canary bytes for memory corruption. */ 499 for_each_canary(meta, check_canary_byte); 500 501 /* 502 * Clear memory if init-on-free is set. While we protect the page, the 503 * data is still there, and after a use-after-free is detected, we 504 * unprotect the page, so the data is still accessible. 505 */ 506 if (!zombie && unlikely(init)) 507 memzero_explicit(addr, meta->size); 508 509 /* Protect to detect use-after-frees. */ 510 kfence_protect((unsigned long)addr); 511 512 kcsan_end_scoped_access(&assert_page_exclusive); 513 if (!zombie) { 514 /* Add it to the tail of the freelist for reuse. */ 515 raw_spin_lock_irqsave(&kfence_freelist_lock, flags); 516 KFENCE_WARN_ON(!list_empty(&meta->list)); 517 list_add_tail(&meta->list, &kfence_freelist); 518 raw_spin_unlock_irqrestore(&kfence_freelist_lock, flags); 519 520 atomic_long_dec(&counters[KFENCE_COUNTER_ALLOCATED]); 521 atomic_long_inc(&counters[KFENCE_COUNTER_FREES]); 522 } else { 523 /* See kfence_shutdown_cache(). */ 524 atomic_long_inc(&counters[KFENCE_COUNTER_ZOMBIES]); 525 } 526 } 527 528 static void rcu_guarded_free(struct rcu_head *h) 529 { 530 struct kfence_metadata *meta = container_of(h, struct kfence_metadata, rcu_head); 531 532 kfence_guarded_free((void *)meta->addr, meta, false); 533 } 534 535 static bool __init kfence_init_pool(void) 536 { 537 unsigned long addr = (unsigned long)__kfence_pool; 538 struct page *pages; 539 int i; 540 541 if (!__kfence_pool) 542 return false; 543 544 if (!arch_kfence_init_pool()) 545 goto err; 546 547 pages = virt_to_page(addr); 548 549 /* 550 * Set up object pages: they must have PG_slab set, to avoid freeing 551 * these as real pages. 552 * 553 * We also want to avoid inserting kfence_free() in the kfree() 554 * fast-path in SLUB, and therefore need to ensure kfree() correctly 555 * enters __slab_free() slow-path. 556 */ 557 for (i = 0; i < KFENCE_POOL_SIZE / PAGE_SIZE; i++) { 558 if (!i || (i % 2)) 559 continue; 560 561 /* Verify we do not have a compound head page. */ 562 if (WARN_ON(compound_head(&pages[i]) != &pages[i])) 563 goto err; 564 565 __SetPageSlab(&pages[i]); 566 } 567 568 /* 569 * Protect the first 2 pages. The first page is mostly unnecessary, and 570 * merely serves as an extended guard page. However, adding one 571 * additional page in the beginning gives us an even number of pages, 572 * which simplifies the mapping of address to metadata index. 573 */ 574 for (i = 0; i < 2; i++) { 575 if (unlikely(!kfence_protect(addr))) 576 goto err; 577 578 addr += PAGE_SIZE; 579 } 580 581 for (i = 0; i < CONFIG_KFENCE_NUM_OBJECTS; i++) { 582 struct kfence_metadata *meta = &kfence_metadata[i]; 583 584 /* Initialize metadata. */ 585 INIT_LIST_HEAD(&meta->list); 586 raw_spin_lock_init(&meta->lock); 587 meta->state = KFENCE_OBJECT_UNUSED; 588 meta->addr = addr; /* Initialize for validation in metadata_to_pageaddr(). */ 589 list_add_tail(&meta->list, &kfence_freelist); 590 591 /* Protect the right redzone. */ 592 if (unlikely(!kfence_protect(addr + PAGE_SIZE))) 593 goto err; 594 595 addr += 2 * PAGE_SIZE; 596 } 597 598 /* 599 * The pool is live and will never be deallocated from this point on. 600 * Remove the pool object from the kmemleak object tree, as it would 601 * otherwise overlap with allocations returned by kfence_alloc(), which 602 * are registered with kmemleak through the slab post-alloc hook. 603 */ 604 kmemleak_free(__kfence_pool); 605 606 return true; 607 608 err: 609 /* 610 * Only release unprotected pages, and do not try to go back and change 611 * page attributes due to risk of failing to do so as well. If changing 612 * page attributes for some pages fails, it is very likely that it also 613 * fails for the first page, and therefore expect addr==__kfence_pool in 614 * most failure cases. 615 */ 616 memblock_free_late(__pa(addr), KFENCE_POOL_SIZE - (addr - (unsigned long)__kfence_pool)); 617 __kfence_pool = NULL; 618 return false; 619 } 620 621 /* === DebugFS Interface ==================================================== */ 622 623 static int stats_show(struct seq_file *seq, void *v) 624 { 625 int i; 626 627 seq_printf(seq, "enabled: %i\n", READ_ONCE(kfence_enabled)); 628 for (i = 0; i < KFENCE_COUNTER_COUNT; i++) 629 seq_printf(seq, "%s: %ld\n", counter_names[i], atomic_long_read(&counters[i])); 630 631 return 0; 632 } 633 DEFINE_SHOW_ATTRIBUTE(stats); 634 635 /* 636 * debugfs seq_file operations for /sys/kernel/debug/kfence/objects. 637 * start_object() and next_object() return the object index + 1, because NULL is used 638 * to stop iteration. 639 */ 640 static void *start_object(struct seq_file *seq, loff_t *pos) 641 { 642 if (*pos < CONFIG_KFENCE_NUM_OBJECTS) 643 return (void *)((long)*pos + 1); 644 return NULL; 645 } 646 647 static void stop_object(struct seq_file *seq, void *v) 648 { 649 } 650 651 static void *next_object(struct seq_file *seq, void *v, loff_t *pos) 652 { 653 ++*pos; 654 if (*pos < CONFIG_KFENCE_NUM_OBJECTS) 655 return (void *)((long)*pos + 1); 656 return NULL; 657 } 658 659 static int show_object(struct seq_file *seq, void *v) 660 { 661 struct kfence_metadata *meta = &kfence_metadata[(long)v - 1]; 662 unsigned long flags; 663 664 raw_spin_lock_irqsave(&meta->lock, flags); 665 kfence_print_object(seq, meta); 666 raw_spin_unlock_irqrestore(&meta->lock, flags); 667 seq_puts(seq, "---------------------------------\n"); 668 669 return 0; 670 } 671 672 static const struct seq_operations object_seqops = { 673 .start = start_object, 674 .next = next_object, 675 .stop = stop_object, 676 .show = show_object, 677 }; 678 679 static int open_objects(struct inode *inode, struct file *file) 680 { 681 return seq_open(file, &object_seqops); 682 } 683 684 static const struct file_operations objects_fops = { 685 .open = open_objects, 686 .read = seq_read, 687 .llseek = seq_lseek, 688 .release = seq_release, 689 }; 690 691 static int __init kfence_debugfs_init(void) 692 { 693 struct dentry *kfence_dir = debugfs_create_dir("kfence", NULL); 694 695 debugfs_create_file("stats", 0444, kfence_dir, NULL, &stats_fops); 696 debugfs_create_file("objects", 0400, kfence_dir, NULL, &objects_fops); 697 return 0; 698 } 699 700 late_initcall(kfence_debugfs_init); 701 702 /* === Allocation Gate Timer ================================================ */ 703 704 #ifdef CONFIG_KFENCE_STATIC_KEYS 705 /* Wait queue to wake up allocation-gate timer task. */ 706 static DECLARE_WAIT_QUEUE_HEAD(allocation_wait); 707 708 static void wake_up_kfence_timer(struct irq_work *work) 709 { 710 wake_up(&allocation_wait); 711 } 712 static DEFINE_IRQ_WORK(wake_up_kfence_timer_work, wake_up_kfence_timer); 713 #endif 714 715 /* 716 * Set up delayed work, which will enable and disable the static key. We need to 717 * use a work queue (rather than a simple timer), since enabling and disabling a 718 * static key cannot be done from an interrupt. 719 * 720 * Note: Toggling a static branch currently causes IPIs, and here we'll end up 721 * with a total of 2 IPIs to all CPUs. If this ends up a problem in future (with 722 * more aggressive sampling intervals), we could get away with a variant that 723 * avoids IPIs, at the cost of not immediately capturing allocations if the 724 * instructions remain cached. 725 */ 726 static struct delayed_work kfence_timer; 727 static void toggle_allocation_gate(struct work_struct *work) 728 { 729 if (!READ_ONCE(kfence_enabled)) 730 return; 731 732 atomic_set(&kfence_allocation_gate, 0); 733 #ifdef CONFIG_KFENCE_STATIC_KEYS 734 /* Enable static key, and await allocation to happen. */ 735 static_branch_enable(&kfence_allocation_key); 736 737 if (sysctl_hung_task_timeout_secs) { 738 /* 739 * During low activity with no allocations we might wait a 740 * while; let's avoid the hung task warning. 741 */ 742 wait_event_idle_timeout(allocation_wait, atomic_read(&kfence_allocation_gate), 743 sysctl_hung_task_timeout_secs * HZ / 2); 744 } else { 745 wait_event_idle(allocation_wait, atomic_read(&kfence_allocation_gate)); 746 } 747 748 /* Disable static key and reset timer. */ 749 static_branch_disable(&kfence_allocation_key); 750 #endif 751 queue_delayed_work(system_unbound_wq, &kfence_timer, 752 msecs_to_jiffies(kfence_sample_interval)); 753 } 754 static DECLARE_DELAYED_WORK(kfence_timer, toggle_allocation_gate); 755 756 /* === Public interface ===================================================== */ 757 758 void __init kfence_alloc_pool(void) 759 { 760 if (!kfence_sample_interval) 761 return; 762 763 __kfence_pool = memblock_alloc(KFENCE_POOL_SIZE, PAGE_SIZE); 764 765 if (!__kfence_pool) 766 pr_err("failed to allocate pool\n"); 767 } 768 769 void __init kfence_init(void) 770 { 771 /* Setting kfence_sample_interval to 0 on boot disables KFENCE. */ 772 if (!kfence_sample_interval) 773 return; 774 775 stack_hash_seed = (u32)random_get_entropy(); 776 if (!kfence_init_pool()) { 777 pr_err("%s failed\n", __func__); 778 return; 779 } 780 781 if (!IS_ENABLED(CONFIG_KFENCE_STATIC_KEYS)) 782 static_branch_enable(&kfence_allocation_key); 783 WRITE_ONCE(kfence_enabled, true); 784 queue_delayed_work(system_unbound_wq, &kfence_timer, 0); 785 pr_info("initialized - using %lu bytes for %d objects at 0x%p-0x%p\n", KFENCE_POOL_SIZE, 786 CONFIG_KFENCE_NUM_OBJECTS, (void *)__kfence_pool, 787 (void *)(__kfence_pool + KFENCE_POOL_SIZE)); 788 } 789 790 void kfence_shutdown_cache(struct kmem_cache *s) 791 { 792 unsigned long flags; 793 struct kfence_metadata *meta; 794 int i; 795 796 for (i = 0; i < CONFIG_KFENCE_NUM_OBJECTS; i++) { 797 bool in_use; 798 799 meta = &kfence_metadata[i]; 800 801 /* 802 * If we observe some inconsistent cache and state pair where we 803 * should have returned false here, cache destruction is racing 804 * with either kmem_cache_alloc() or kmem_cache_free(). Taking 805 * the lock will not help, as different critical section 806 * serialization will have the same outcome. 807 */ 808 if (READ_ONCE(meta->cache) != s || 809 READ_ONCE(meta->state) != KFENCE_OBJECT_ALLOCATED) 810 continue; 811 812 raw_spin_lock_irqsave(&meta->lock, flags); 813 in_use = meta->cache == s && meta->state == KFENCE_OBJECT_ALLOCATED; 814 raw_spin_unlock_irqrestore(&meta->lock, flags); 815 816 if (in_use) { 817 /* 818 * This cache still has allocations, and we should not 819 * release them back into the freelist so they can still 820 * safely be used and retain the kernel's default 821 * behaviour of keeping the allocations alive (leak the 822 * cache); however, they effectively become "zombie 823 * allocations" as the KFENCE objects are the only ones 824 * still in use and the owning cache is being destroyed. 825 * 826 * We mark them freed, so that any subsequent use shows 827 * more useful error messages that will include stack 828 * traces of the user of the object, the original 829 * allocation, and caller to shutdown_cache(). 830 */ 831 kfence_guarded_free((void *)meta->addr, meta, /*zombie=*/true); 832 } 833 } 834 835 for (i = 0; i < CONFIG_KFENCE_NUM_OBJECTS; i++) { 836 meta = &kfence_metadata[i]; 837 838 /* See above. */ 839 if (READ_ONCE(meta->cache) != s || READ_ONCE(meta->state) != KFENCE_OBJECT_FREED) 840 continue; 841 842 raw_spin_lock_irqsave(&meta->lock, flags); 843 if (meta->cache == s && meta->state == KFENCE_OBJECT_FREED) 844 meta->cache = NULL; 845 raw_spin_unlock_irqrestore(&meta->lock, flags); 846 } 847 } 848 849 void *__kfence_alloc(struct kmem_cache *s, size_t size, gfp_t flags) 850 { 851 unsigned long stack_entries[KFENCE_STACK_DEPTH]; 852 size_t num_stack_entries; 853 u32 alloc_stack_hash; 854 855 /* 856 * Perform size check before switching kfence_allocation_gate, so that 857 * we don't disable KFENCE without making an allocation. 858 */ 859 if (size > PAGE_SIZE) { 860 atomic_long_inc(&counters[KFENCE_COUNTER_SKIP_INCOMPAT]); 861 return NULL; 862 } 863 864 /* 865 * Skip allocations from non-default zones, including DMA. We cannot 866 * guarantee that pages in the KFENCE pool will have the requested 867 * properties (e.g. reside in DMAable memory). 868 */ 869 if ((flags & GFP_ZONEMASK) || 870 (s->flags & (SLAB_CACHE_DMA | SLAB_CACHE_DMA32))) { 871 atomic_long_inc(&counters[KFENCE_COUNTER_SKIP_INCOMPAT]); 872 return NULL; 873 } 874 875 if (atomic_inc_return(&kfence_allocation_gate) > 1) 876 return NULL; 877 #ifdef CONFIG_KFENCE_STATIC_KEYS 878 /* 879 * waitqueue_active() is fully ordered after the update of 880 * kfence_allocation_gate per atomic_inc_return(). 881 */ 882 if (waitqueue_active(&allocation_wait)) { 883 /* 884 * Calling wake_up() here may deadlock when allocations happen 885 * from within timer code. Use an irq_work to defer it. 886 */ 887 irq_work_queue(&wake_up_kfence_timer_work); 888 } 889 #endif 890 891 if (!READ_ONCE(kfence_enabled)) 892 return NULL; 893 894 num_stack_entries = stack_trace_save(stack_entries, KFENCE_STACK_DEPTH, 0); 895 896 /* 897 * Do expensive check for coverage of allocation in slow-path after 898 * allocation_gate has already become non-zero, even though it might 899 * mean not making any allocation within a given sample interval. 900 * 901 * This ensures reasonable allocation coverage when the pool is almost 902 * full, including avoiding long-lived allocations of the same source 903 * filling up the pool (e.g. pagecache allocations). 904 */ 905 alloc_stack_hash = get_alloc_stack_hash(stack_entries, num_stack_entries); 906 if (should_skip_covered() && alloc_covered_contains(alloc_stack_hash)) { 907 atomic_long_inc(&counters[KFENCE_COUNTER_SKIP_COVERED]); 908 return NULL; 909 } 910 911 return kfence_guarded_alloc(s, size, flags, stack_entries, num_stack_entries, 912 alloc_stack_hash); 913 } 914 915 size_t kfence_ksize(const void *addr) 916 { 917 const struct kfence_metadata *meta = addr_to_metadata((unsigned long)addr); 918 919 /* 920 * Read locklessly -- if there is a race with __kfence_alloc(), this is 921 * either a use-after-free or invalid access. 922 */ 923 return meta ? meta->size : 0; 924 } 925 926 void *kfence_object_start(const void *addr) 927 { 928 const struct kfence_metadata *meta = addr_to_metadata((unsigned long)addr); 929 930 /* 931 * Read locklessly -- if there is a race with __kfence_alloc(), this is 932 * either a use-after-free or invalid access. 933 */ 934 return meta ? (void *)meta->addr : NULL; 935 } 936 937 void __kfence_free(void *addr) 938 { 939 struct kfence_metadata *meta = addr_to_metadata((unsigned long)addr); 940 941 /* 942 * If the objects of the cache are SLAB_TYPESAFE_BY_RCU, defer freeing 943 * the object, as the object page may be recycled for other-typed 944 * objects once it has been freed. meta->cache may be NULL if the cache 945 * was destroyed. 946 */ 947 if (unlikely(meta->cache && (meta->cache->flags & SLAB_TYPESAFE_BY_RCU))) 948 call_rcu(&meta->rcu_head, rcu_guarded_free); 949 else 950 kfence_guarded_free(addr, meta, false); 951 } 952 953 bool kfence_handle_page_fault(unsigned long addr, bool is_write, struct pt_regs *regs) 954 { 955 const int page_index = (addr - (unsigned long)__kfence_pool) / PAGE_SIZE; 956 struct kfence_metadata *to_report = NULL; 957 enum kfence_error_type error_type; 958 unsigned long flags; 959 960 if (!is_kfence_address((void *)addr)) 961 return false; 962 963 if (!READ_ONCE(kfence_enabled)) /* If disabled at runtime ... */ 964 return kfence_unprotect(addr); /* ... unprotect and proceed. */ 965 966 atomic_long_inc(&counters[KFENCE_COUNTER_BUGS]); 967 968 if (page_index % 2) { 969 /* This is a redzone, report a buffer overflow. */ 970 struct kfence_metadata *meta; 971 int distance = 0; 972 973 meta = addr_to_metadata(addr - PAGE_SIZE); 974 if (meta && READ_ONCE(meta->state) == KFENCE_OBJECT_ALLOCATED) { 975 to_report = meta; 976 /* Data race ok; distance calculation approximate. */ 977 distance = addr - data_race(meta->addr + meta->size); 978 } 979 980 meta = addr_to_metadata(addr + PAGE_SIZE); 981 if (meta && READ_ONCE(meta->state) == KFENCE_OBJECT_ALLOCATED) { 982 /* Data race ok; distance calculation approximate. */ 983 if (!to_report || distance > data_race(meta->addr) - addr) 984 to_report = meta; 985 } 986 987 if (!to_report) 988 goto out; 989 990 raw_spin_lock_irqsave(&to_report->lock, flags); 991 to_report->unprotected_page = addr; 992 error_type = KFENCE_ERROR_OOB; 993 994 /* 995 * If the object was freed before we took the look we can still 996 * report this as an OOB -- the report will simply show the 997 * stacktrace of the free as well. 998 */ 999 } else { 1000 to_report = addr_to_metadata(addr); 1001 if (!to_report) 1002 goto out; 1003 1004 raw_spin_lock_irqsave(&to_report->lock, flags); 1005 error_type = KFENCE_ERROR_UAF; 1006 /* 1007 * We may race with __kfence_alloc(), and it is possible that a 1008 * freed object may be reallocated. We simply report this as a 1009 * use-after-free, with the stack trace showing the place where 1010 * the object was re-allocated. 1011 */ 1012 } 1013 1014 out: 1015 if (to_report) { 1016 kfence_report_error(addr, is_write, regs, to_report, error_type); 1017 raw_spin_unlock_irqrestore(&to_report->lock, flags); 1018 } else { 1019 /* This may be a UAF or OOB access, but we can't be sure. */ 1020 kfence_report_error(addr, is_write, regs, NULL, KFENCE_ERROR_INVALID); 1021 } 1022 1023 return kfence_unprotect(addr); /* Unprotect and let access proceed. */ 1024 } 1025