mm/kfence/core.c

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