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