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/notifier.h>
  25 #include <linux/panic_notifier.h>
  26 #include <linux/random.h>
  27 #include <linux/rcupdate.h>
  28 #include <linux/sched/clock.h>
  29 #include <linux/seq_file.h>
  30 #include <linux/slab.h>
  31 #include <linux/spinlock.h>
  32 #include <linux/string.h>
  33
  34 #include <asm/kfence.h>
  35
  36 #include "kfence.h"
  37
  38 /* Disables KFENCE on the first warning assuming an irrecoverable error. */
  39 #define KFENCE_WARN_ON(cond)                                                   \
  40         ({                                                                     \
  41                 const bool __cond = WARN_ON(cond);                             \
  42                 if (unlikely(__cond)) {                                        \
  43                         WRITE_ONCE(kfence_enabled, false);                     \
  44                         disabled_by_warn = true;                               \
  45                 }                                                              \
  46                 __cond;                                                        \
  47         })
  48
  49 /* === Data ================================================================= */
  50
  51 static bool kfence_enabled __read_mostly;
  52 static bool disabled_by_warn __read_mostly;
  53
  54 unsigned long kfence_sample_interval __read_mostly = CONFIG_KFENCE_SAMPLE_INTERVAL;
  55 EXPORT_SYMBOL_GPL(kfence_sample_interval); /* Export for test modules. */
  56
  57 #ifdef MODULE_PARAM_PREFIX
  58 #undef MODULE_PARAM_PREFIX
  59 #endif
  60 #define MODULE_PARAM_PREFIX "kfence."
  61
  62 static int kfence_enable_late(void);
  63 static int param_set_sample_interval(const char *val, const struct kernel_param *kp)
  64 {
  65         unsigned long num;
  66         int ret = kstrtoul(val, 0, &num);
  67
  68         if (ret < 0)
  69                 return ret;
  70
  71         /* Using 0 to indicate KFENCE is disabled. */
  72         if (!num && READ_ONCE(kfence_enabled)) {
  73                 pr_info("disabled\n");
  74                 WRITE_ONCE(kfence_enabled, false);
  75         }
  76
  77         *((unsigned long *)kp->arg) = num;
  78
  79         if (num && !READ_ONCE(kfence_enabled) && system_state != SYSTEM_BOOTING)
  80                 return disabled_by_warn ? -EINVAL : kfence_enable_late();
  81         return 0;
  82 }
  83
  84 static int param_get_sample_interval(char *buffer, const struct kernel_param *kp)
  85 {
  86         if (!READ_ONCE(kfence_enabled))
  87                 return sprintf(buffer, "0\n");
  88
  89         return param_get_ulong(buffer, kp);
  90 }
  91
  92 static const struct kernel_param_ops sample_interval_param_ops = {
  93         .set = param_set_sample_interval,
  94         .get = param_get_sample_interval,
  95 };
  96 module_param_cb(sample_interval, &sample_interval_param_ops, &kfence_sample_interval, 0600);
  97
  98 /* Pool usage% threshold when currently covered allocations are skipped. */
  99 static unsigned long kfence_skip_covered_thresh __read_mostly = 75;
 100 module_param_named(skip_covered_thresh, kfence_skip_covered_thresh, ulong, 0644);
 101
 102 /* If true, use a deferrable timer. */
 103 static bool kfence_deferrable __read_mostly = IS_ENABLED(CONFIG_KFENCE_DEFERRABLE);
 104 module_param_named(deferrable, kfence_deferrable, bool, 0444);
 105
 106 /* If true, check all canary bytes on panic. */
 107 static bool kfence_check_on_panic __read_mostly;
 108 module_param_named(check_on_panic, kfence_check_on_panic, bool, 0444);
 109
 110 /* The pool of pages used for guard pages and objects. */
 111 char *__kfence_pool __read_mostly;
 112 EXPORT_SYMBOL(__kfence_pool); /* Export for test modules. */
 113
 114 /*
 115  * Per-object metadata, with one-to-one mapping of object metadata to
 116  * backing pages (in __kfence_pool).
 117  */
 118 static_assert(CONFIG_KFENCE_NUM_OBJECTS > 0);
 119 struct kfence_metadata *kfence_metadata __read_mostly;
 120
 121 /*
 122  * If kfence_metadata is not NULL, it may be accessed by kfence_shutdown_cache().
 123  * So introduce kfence_metadata_init to initialize metadata, and then make
 124  * kfence_metadata visible after initialization is successful. This prevents
 125  * potential UAF or access to uninitialized metadata.
 126  */
 127 static struct kfence_metadata *kfence_metadata_init __read_mostly;
 128
 129 /* Freelist with available objects. */
 130 static struct list_head kfence_freelist = LIST_HEAD_INIT(kfence_freelist);
 131 static DEFINE_RAW_SPINLOCK(kfence_freelist_lock); /* Lock protecting freelist. */
 132
 133 /*
 134  * The static key to set up a KFENCE allocation; or if static keys are not used
 135  * to gate allocations, to avoid a load and compare if KFENCE is disabled.
 136  */
 137 DEFINE_STATIC_KEY_FALSE(kfence_allocation_key);
 138
 139 /* Gates the allocation, ensuring only one succeeds in a given period. */
 140 atomic_t kfence_allocation_gate = ATOMIC_INIT(1);
 141
 142 /*
 143  * A Counting Bloom filter of allocation coverage: limits currently covered
 144  * allocations of the same source filling up the pool.
 145  *
 146  * Assuming a range of 15%-85% unique allocations in the pool at any point in
 147  * time, the below parameters provide a probablity of 0.02-0.33 for false
 148  * positive hits respectively:
 149  *
 150  *      P(alloc_traces) = (1 - e^(-HNUM * (alloc_traces / SIZE)) ^ HNUM
 151  */
 152 #define ALLOC_COVERED_HNUM      2
 153 #define ALLOC_COVERED_ORDER     (const_ilog2(CONFIG_KFENCE_NUM_OBJECTS) + 2)
 154 #define ALLOC_COVERED_SIZE      (1 << ALLOC_COVERED_ORDER)
 155 #define ALLOC_COVERED_HNEXT(h)  hash_32(h, ALLOC_COVERED_ORDER)
 156 #define ALLOC_COVERED_MASK      (ALLOC_COVERED_SIZE - 1)
 157 static atomic_t alloc_covered[ALLOC_COVERED_SIZE];
 158
 159 /* Stack depth used to determine uniqueness of an allocation. */
 160 #define UNIQUE_ALLOC_STACK_DEPTH ((size_t)8)
 161
 162 /*
 163  * Randomness for stack hashes, making the same collisions across reboots and
 164  * different machines less likely.
 165  */
 166 static u32 stack_hash_seed __ro_after_init;
 167
 168 /* Statistics counters for debugfs. */
 169 enum kfence_counter_id {
 170         KFENCE_COUNTER_ALLOCATED,
 171         KFENCE_COUNTER_ALLOCS,
 172         KFENCE_COUNTER_FREES,
 173         KFENCE_COUNTER_ZOMBIES,
 174         KFENCE_COUNTER_BUGS,
 175         KFENCE_COUNTER_SKIP_INCOMPAT,
 176         KFENCE_COUNTER_SKIP_CAPACITY,
 177         KFENCE_COUNTER_SKIP_COVERED,
 178         KFENCE_COUNTER_COUNT,
 179 };
 180 static atomic_long_t counters[KFENCE_COUNTER_COUNT];
 181 static const char *const counter_names[] = {
 182         [KFENCE_COUNTER_ALLOCATED]      = "currently allocated",
 183         [KFENCE_COUNTER_ALLOCS]         = "total allocations",
 184         [KFENCE_COUNTER_FREES]          = "total frees",
 185         [KFENCE_COUNTER_ZOMBIES]        = "zombie allocations",
 186         [KFENCE_COUNTER_BUGS]           = "total bugs",
 187         [KFENCE_COUNTER_SKIP_INCOMPAT]  = "skipped allocations (incompatible)",
 188         [KFENCE_COUNTER_SKIP_CAPACITY]  = "skipped allocations (capacity)",
 189         [KFENCE_COUNTER_SKIP_COVERED]   = "skipped allocations (covered)",
 190 };
 191 static_assert(ARRAY_SIZE(counter_names) == KFENCE_COUNTER_COUNT);
 192
 193 /* === Internals ============================================================ */
 194
 195 static inline bool should_skip_covered(void)
 196 {
 197         unsigned long thresh = (CONFIG_KFENCE_NUM_OBJECTS * kfence_skip_covered_thresh) / 100;
 198
 199         return atomic_long_read(&counters[KFENCE_COUNTER_ALLOCATED]) > thresh;
 200 }
 201
 202 static u32 get_alloc_stack_hash(unsigned long *stack_entries, size_t num_entries)
 203 {
 204         num_entries = min(num_entries, UNIQUE_ALLOC_STACK_DEPTH);
 205         num_entries = filter_irq_stacks(stack_entries, num_entries);
 206         return jhash(stack_entries, num_entries * sizeof(stack_entries[0]), stack_hash_seed);
 207 }
 208
 209 /*
 210  * Adds (or subtracts) count @val for allocation stack trace hash
 211  * @alloc_stack_hash from Counting Bloom filter.
 212  */
 213 static void alloc_covered_add(u32 alloc_stack_hash, int val)
 214 {
 215         int i;
 216
 217         for (i = 0; i < ALLOC_COVERED_HNUM; i++) {
 218                 atomic_add(val, &alloc_covered[alloc_stack_hash & ALLOC_COVERED_MASK]);
 219                 alloc_stack_hash = ALLOC_COVERED_HNEXT(alloc_stack_hash);
 220         }
 221 }
 222
 223 /*
 224  * Returns true if the allocation stack trace hash @alloc_stack_hash is
 225  * currently contained (non-zero count) in Counting Bloom filter.
 226  */
 227 static bool alloc_covered_contains(u32 alloc_stack_hash)
 228 {
 229         int i;
 230
 231         for (i = 0; i < ALLOC_COVERED_HNUM; i++) {
 232                 if (!atomic_read(&alloc_covered[alloc_stack_hash & ALLOC_COVERED_MASK]))
 233                         return false;
 234                 alloc_stack_hash = ALLOC_COVERED_HNEXT(alloc_stack_hash);
 235         }
 236
 237         return true;
 238 }
 239
 240 static bool kfence_protect(unsigned long addr)
 241 {
 242         return !KFENCE_WARN_ON(!kfence_protect_page(ALIGN_DOWN(addr, PAGE_SIZE), true));
 243 }
 244
 245 static bool kfence_unprotect(unsigned long addr)
 246 {
 247         return !KFENCE_WARN_ON(!kfence_protect_page(ALIGN_DOWN(addr, PAGE_SIZE), false));
 248 }
 249
 250 static inline unsigned long metadata_to_pageaddr(const struct kfence_metadata *meta)
 251 {
 252         unsigned long offset = (meta - kfence_metadata + 1) * PAGE_SIZE * 2;
 253         unsigned long pageaddr = (unsigned long)&__kfence_pool[offset];
 254
 255         /* The checks do not affect performance; only called from slow-paths. */
 256
 257         /* Only call with a pointer into kfence_metadata. */
 258         if (KFENCE_WARN_ON(meta < kfence_metadata ||
 259                            meta >= kfence_metadata + CONFIG_KFENCE_NUM_OBJECTS))
 260                 return 0;
 261
 262         /*
 263          * This metadata object only ever maps to 1 page; verify that the stored
 264          * address is in the expected range.
 265          */
 266         if (KFENCE_WARN_ON(ALIGN_DOWN(meta->addr, PAGE_SIZE) != pageaddr))
 267                 return 0;
 268
 269         return pageaddr;
 270 }
 271
 272 /*
 273  * Update the object's metadata state, including updating the alloc/free stacks
 274  * depending on the state transition.
 275  */
 276 static noinline void
 277 metadata_update_state(struct kfence_metadata *meta, enum kfence_object_state next,
 278                       unsigned long *stack_entries, size_t num_stack_entries)
 279 {
 280         struct kfence_track *track =
 281                 next == KFENCE_OBJECT_FREED ? &meta->free_track : &meta->alloc_track;
 282
 283         lockdep_assert_held(&meta->lock);
 284
 285         if (stack_entries) {
 286                 memcpy(track->stack_entries, stack_entries,
 287                        num_stack_entries * sizeof(stack_entries[0]));
 288         } else {
 289                 /*
 290                  * Skip over 1 (this) functions; noinline ensures we do not
 291                  * accidentally skip over the caller by never inlining.
 292                  */
 293                 num_stack_entries = stack_trace_save(track->stack_entries, KFENCE_STACK_DEPTH, 1);
 294         }
 295         track->num_stack_entries = num_stack_entries;
 296         track->pid = task_pid_nr(current);
 297         track->cpu = raw_smp_processor_id();
 298         track->ts_nsec = local_clock(); /* Same source as printk timestamps. */
 299
 300         /*
 301          * Pairs with READ_ONCE() in
 302          *      kfence_shutdown_cache(),
 303          *      kfence_handle_page_fault().
 304          */
 305         WRITE_ONCE(meta->state, next);
 306 }
 307
 308 /* Check canary byte at @addr. */
 309 static inline bool check_canary_byte(u8 *addr)
 310 {
 311         struct kfence_metadata *meta;
 312         unsigned long flags;
 313
 314         if (likely(*addr == KFENCE_CANARY_PATTERN_U8(addr)))
 315                 return true;
 316
 317         atomic_long_inc(&counters[KFENCE_COUNTER_BUGS]);
 318
 319         meta = addr_to_metadata((unsigned long)addr);
 320         raw_spin_lock_irqsave(&meta->lock, flags);
 321         kfence_report_error((unsigned long)addr, false, NULL, meta, KFENCE_ERROR_CORRUPTION);
 322         raw_spin_unlock_irqrestore(&meta->lock, flags);
 323
 324         return false;
 325 }
 326
 327 static inline void set_canary(const struct kfence_metadata *meta)
 328 {
 329         const unsigned long pageaddr = ALIGN_DOWN(meta->addr, PAGE_SIZE);
 330         unsigned long addr = pageaddr;
 331
 332         /*
 333          * The canary may be written to part of the object memory, but it does
 334          * not affect it. The user should initialize the object before using it.
 335          */
 336         for (; addr < meta->addr; addr += sizeof(u64))
 337                 *((u64 *)addr) = KFENCE_CANARY_PATTERN_U64;
 338
 339         addr = ALIGN_DOWN(meta->addr + meta->size, sizeof(u64));
 340         for (; addr - pageaddr < PAGE_SIZE; addr += sizeof(u64))
 341                 *((u64 *)addr) = KFENCE_CANARY_PATTERN_U64;
 342 }
 343
 344 static inline void check_canary(const struct kfence_metadata *meta)
 345 {
 346         const unsigned long pageaddr = ALIGN_DOWN(meta->addr, PAGE_SIZE);
 347         unsigned long addr = pageaddr;
 348
 349         /*
 350          * We'll iterate over each canary byte per-side until a corrupted byte
 351          * is found. However, we'll still iterate over the canary bytes to the
 352          * right of the object even if there was an error in the canary bytes to
 353          * the left of the object. Specifically, if check_canary_byte()
 354          * generates an error, showing both sides might give more clues as to
 355          * what the error is about when displaying which bytes were corrupted.
 356          */
 357
 358         /* Apply to left of object. */
 359         for (; meta->addr - addr >= sizeof(u64); addr += sizeof(u64)) {
 360                 if (unlikely(*((u64 *)addr) != KFENCE_CANARY_PATTERN_U64))
 361                         break;
 362         }
 363
 364         /*
 365          * If the canary is corrupted in a certain 64 bytes, or the canary
 366          * memory cannot be completely covered by multiple consecutive 64 bytes,
 367          * it needs to be checked one by one.
 368          */
 369         for (; addr < meta->addr; addr++) {
 370                 if (unlikely(!check_canary_byte((u8 *)addr)))
 371                         break;
 372         }
 373
 374         /* Apply to right of object. */
 375         for (addr = meta->addr + meta->size; addr % sizeof(u64) != 0; addr++) {
 376                 if (unlikely(!check_canary_byte((u8 *)addr)))
 377                         return;
 378         }
 379         for (; addr - pageaddr < PAGE_SIZE; addr += sizeof(u64)) {
 380                 if (unlikely(*((u64 *)addr) != KFENCE_CANARY_PATTERN_U64)) {
 381
 382                         for (; addr - pageaddr < PAGE_SIZE; addr++) {
 383                                 if (!check_canary_byte((u8 *)addr))
 384                                         return;
 385                         }
 386                 }
 387         }
 388 }
 389
 390 static void *kfence_guarded_alloc(struct kmem_cache *cache, size_t size, gfp_t gfp,
 391                                   unsigned long *stack_entries, size_t num_stack_entries,
 392                                   u32 alloc_stack_hash)
 393 {
 394         struct kfence_metadata *meta = NULL;
 395         unsigned long flags;
 396         struct slab *slab;
 397         void *addr;
 398         const bool random_right_allocate = get_random_u32_below(2);
 399         const bool random_fault = CONFIG_KFENCE_STRESS_TEST_FAULTS &&
 400                                   !get_random_u32_below(CONFIG_KFENCE_STRESS_TEST_FAULTS);
 401
 402         /* Try to obtain a free object. */
 403         raw_spin_lock_irqsave(&kfence_freelist_lock, flags);
 404         if (!list_empty(&kfence_freelist)) {
 405                 meta = list_entry(kfence_freelist.next, struct kfence_metadata, list);
 406                 list_del_init(&meta->list);
 407         }
 408         raw_spin_unlock_irqrestore(&kfence_freelist_lock, flags);
 409         if (!meta) {
 410                 atomic_long_inc(&counters[KFENCE_COUNTER_SKIP_CAPACITY]);
 411                 return NULL;
 412         }
 413
 414         if (unlikely(!raw_spin_trylock_irqsave(&meta->lock, flags))) {
 415                 /*
 416                  * This is extremely unlikely -- we are reporting on a
 417                  * use-after-free, which locked meta->lock, and the reporting
 418                  * code via printk calls kmalloc() which ends up in
 419                  * kfence_alloc() and tries to grab the same object that we're
 420                  * reporting on. While it has never been observed, lockdep does
 421                  * report that there is a possibility of deadlock. Fix it by
 422                  * using trylock and bailing out gracefully.
 423                  */
 424                 raw_spin_lock_irqsave(&kfence_freelist_lock, flags);
 425                 /* Put the object back on the freelist. */
 426                 list_add_tail(&meta->list, &kfence_freelist);
 427                 raw_spin_unlock_irqrestore(&kfence_freelist_lock, flags);
 428
 429                 return NULL;
 430         }
 431
 432         meta->addr = metadata_to_pageaddr(meta);
 433         /* Unprotect if we're reusing this page. */
 434         if (meta->state == KFENCE_OBJECT_FREED)
 435                 kfence_unprotect(meta->addr);
 436
 437         /*
 438          * Note: for allocations made before RNG initialization, will always
 439          * return zero. We still benefit from enabling KFENCE as early as
 440          * possible, even when the RNG is not yet available, as this will allow
 441          * KFENCE to detect bugs due to earlier allocations. The only downside
 442          * is that the out-of-bounds accesses detected are deterministic for
 443          * such allocations.
 444          */
 445         if (random_right_allocate) {
 446                 /* Allocate on the "right" side, re-calculate address. */
 447                 meta->addr += PAGE_SIZE - size;
 448                 meta->addr = ALIGN_DOWN(meta->addr, cache->align);
 449         }
 450
 451         addr = (void *)meta->addr;
 452
 453         /* Update remaining metadata. */
 454         metadata_update_state(meta, KFENCE_OBJECT_ALLOCATED, stack_entries, num_stack_entries);
 455         /* Pairs with READ_ONCE() in kfence_shutdown_cache(). */
 456         WRITE_ONCE(meta->cache, cache);
 457         meta->size = size;
 458         meta->alloc_stack_hash = alloc_stack_hash;
 459         raw_spin_unlock_irqrestore(&meta->lock, flags);
 460
 461         alloc_covered_add(alloc_stack_hash, 1);
 462
 463         /* Set required slab fields. */
 464         slab = virt_to_slab((void *)meta->addr);
 465         slab->slab_cache = cache;
 466         slab->objects = 1;
 467
 468         /* Memory initialization. */
 469         set_canary(meta);
 470
 471         /*
 472          * We check slab_want_init_on_alloc() ourselves, rather than letting
 473          * SL*B do the initialization, as otherwise we might overwrite KFENCE's
 474          * redzone.
 475          */
 476         if (unlikely(slab_want_init_on_alloc(gfp, cache)))
 477                 memzero_explicit(addr, size);
 478         if (cache->ctor)
 479                 cache->ctor(addr);
 480
 481         if (random_fault)
 482                 kfence_protect(meta->addr); /* Random "faults" by protecting the object. */
 483
 484         atomic_long_inc(&counters[KFENCE_COUNTER_ALLOCATED]);
 485         atomic_long_inc(&counters[KFENCE_COUNTER_ALLOCS]);
 486
 487         return addr;
 488 }
 489
 490 static void kfence_guarded_free(void *addr, struct kfence_metadata *meta, bool zombie)
 491 {
 492         struct kcsan_scoped_access assert_page_exclusive;
 493         unsigned long flags;
 494         bool init;
 495
 496         raw_spin_lock_irqsave(&meta->lock, flags);
 497
 498         if (meta->state != KFENCE_OBJECT_ALLOCATED || meta->addr != (unsigned long)addr) {
 499                 /* Invalid or double-free, bail out. */
 500                 atomic_long_inc(&counters[KFENCE_COUNTER_BUGS]);
 501                 kfence_report_error((unsigned long)addr, false, NULL, meta,
 502                                     KFENCE_ERROR_INVALID_FREE);
 503                 raw_spin_unlock_irqrestore(&meta->lock, flags);
 504                 return;
 505         }
 506
 507         /* Detect racy use-after-free, or incorrect reallocation of this page by KFENCE. */
 508         kcsan_begin_scoped_access((void *)ALIGN_DOWN((unsigned long)addr, PAGE_SIZE), PAGE_SIZE,
 509                                   KCSAN_ACCESS_SCOPED | KCSAN_ACCESS_WRITE | KCSAN_ACCESS_ASSERT,
 510                                   &assert_page_exclusive);
 511
 512         if (CONFIG_KFENCE_STRESS_TEST_FAULTS)
 513                 kfence_unprotect((unsigned long)addr); /* To check canary bytes. */
 514
 515         /* Restore page protection if there was an OOB access. */
 516         if (meta->unprotected_page) {
 517                 memzero_explicit((void *)ALIGN_DOWN(meta->unprotected_page, PAGE_SIZE), PAGE_SIZE);
 518                 kfence_protect(meta->unprotected_page);
 519                 meta->unprotected_page = 0;
 520         }
 521
 522         /* Mark the object as freed. */
 523         metadata_update_state(meta, KFENCE_OBJECT_FREED, NULL, 0);
 524         init = slab_want_init_on_free(meta->cache);
 525         raw_spin_unlock_irqrestore(&meta->lock, flags);
 526
 527         alloc_covered_add(meta->alloc_stack_hash, -1);
 528
 529         /* Check canary bytes for memory corruption. */
 530         check_canary(meta);
 531
 532         /*
 533          * Clear memory if init-on-free is set. While we protect the page, the
 534          * data is still there, and after a use-after-free is detected, we
 535          * unprotect the page, so the data is still accessible.
 536          */
 537         if (!zombie && unlikely(init))
 538                 memzero_explicit(addr, meta->size);
 539
 540         /* Protect to detect use-after-frees. */
 541         kfence_protect((unsigned long)addr);
 542
 543         kcsan_end_scoped_access(&assert_page_exclusive);
 544         if (!zombie) {
 545                 /* Add it to the tail of the freelist for reuse. */
 546                 raw_spin_lock_irqsave(&kfence_freelist_lock, flags);
 547                 KFENCE_WARN_ON(!list_empty(&meta->list));
 548                 list_add_tail(&meta->list, &kfence_freelist);
 549                 raw_spin_unlock_irqrestore(&kfence_freelist_lock, flags);
 550
 551                 atomic_long_dec(&counters[KFENCE_COUNTER_ALLOCATED]);
 552                 atomic_long_inc(&counters[KFENCE_COUNTER_FREES]);
 553         } else {
 554                 /* See kfence_shutdown_cache(). */
 555                 atomic_long_inc(&counters[KFENCE_COUNTER_ZOMBIES]);
 556         }
 557 }
 558
 559 static void rcu_guarded_free(struct rcu_head *h)
 560 {
 561         struct kfence_metadata *meta = container_of(h, struct kfence_metadata, rcu_head);
 562
 563         kfence_guarded_free((void *)meta->addr, meta, false);
 564 }
 565
 566 /*
 567  * Initialization of the KFENCE pool after its allocation.
 568  * Returns 0 on success; otherwise returns the address up to
 569  * which partial initialization succeeded.
 570  */
 571 static unsigned long kfence_init_pool(void)
 572 {
 573         unsigned long addr;
 574         struct page *pages;
 575         int i;
 576
 577         if (!arch_kfence_init_pool())
 578                 return (unsigned long)__kfence_pool;
 579
 580         addr = (unsigned long)__kfence_pool;
 581         pages = virt_to_page(__kfence_pool);
 582
 583         /*
 584          * Set up object pages: they must have PG_slab set, to avoid freeing
 585          * these as real pages.
 586          *
 587          * We also want to avoid inserting kfence_free() in the kfree()
 588          * fast-path in SLUB, and therefore need to ensure kfree() correctly
 589          * enters __slab_free() slow-path.
 590          */
 591         for (i = 0; i < KFENCE_POOL_SIZE / PAGE_SIZE; i++) {
 592                 struct slab *slab = page_slab(nth_page(pages, i));
 593
 594                 if (!i || (i % 2))
 595                         continue;
 596
 597                 __folio_set_slab(slab_folio(slab));
 598 #ifdef CONFIG_MEMCG
 599                 slab->memcg_data = (unsigned long)&kfence_metadata_init[i / 2 - 1].objcg |
 600                                    MEMCG_DATA_OBJCGS;
 601 #endif
 602         }
 603
 604         /*
 605          * Protect the first 2 pages. The first page is mostly unnecessary, and
 606          * merely serves as an extended guard page. However, adding one
 607          * additional page in the beginning gives us an even number of pages,
 608          * which simplifies the mapping of address to metadata index.
 609          */
 610         for (i = 0; i < 2; i++) {
 611                 if (unlikely(!kfence_protect(addr)))
 612                         return addr;
 613
 614                 addr += PAGE_SIZE;
 615         }
 616
 617         for (i = 0; i < CONFIG_KFENCE_NUM_OBJECTS; i++) {
 618                 struct kfence_metadata *meta = &kfence_metadata_init[i];
 619
 620                 /* Initialize metadata. */
 621                 INIT_LIST_HEAD(&meta->list);
 622                 raw_spin_lock_init(&meta->lock);
 623                 meta->state = KFENCE_OBJECT_UNUSED;
 624                 meta->addr = addr; /* Initialize for validation in metadata_to_pageaddr(). */
 625                 list_add_tail(&meta->list, &kfence_freelist);
 626
 627                 /* Protect the right redzone. */
 628                 if (unlikely(!kfence_protect(addr + PAGE_SIZE)))
 629                         goto reset_slab;
 630
 631                 addr += 2 * PAGE_SIZE;
 632         }
 633
 634         /*
 635          * Make kfence_metadata visible only when initialization is successful.
 636          * Otherwise, if the initialization fails and kfence_metadata is freed,
 637          * it may cause UAF in kfence_shutdown_cache().
 638          */
 639         smp_store_release(&kfence_metadata, kfence_metadata_init);
 640         return 0;
 641
 642 reset_slab:
 643         for (i = 0; i < KFENCE_POOL_SIZE / PAGE_SIZE; i++) {
 644                 struct slab *slab = page_slab(nth_page(pages, i));
 645
 646                 if (!i || (i % 2))
 647                         continue;
 648 #ifdef CONFIG_MEMCG
 649                 slab->memcg_data = 0;
 650 #endif
 651                 __folio_clear_slab(slab_folio(slab));
 652         }
 653
 654         return addr;
 655 }
 656
 657 static bool __init kfence_init_pool_early(void)
 658 {
 659         unsigned long addr;
 660
 661         if (!__kfence_pool)
 662                 return false;
 663
 664         addr = kfence_init_pool();
 665
 666         if (!addr) {
 667                 /*
 668                  * The pool is live and will never be deallocated from this point on.
 669                  * Ignore the pool object from the kmemleak phys object tree, as it would
 670                  * otherwise overlap with allocations returned by kfence_alloc(), which
 671                  * are registered with kmemleak through the slab post-alloc hook.
 672                  */
 673                 kmemleak_ignore_phys(__pa(__kfence_pool));
 674                 return true;
 675         }
 676
 677         /*
 678          * Only release unprotected pages, and do not try to go back and change
 679          * page attributes due to risk of failing to do so as well. If changing
 680          * page attributes for some pages fails, it is very likely that it also
 681          * fails for the first page, and therefore expect addr==__kfence_pool in
 682          * most failure cases.
 683          */
 684         memblock_free_late(__pa(addr), KFENCE_POOL_SIZE - (addr - (unsigned long)__kfence_pool));
 685         __kfence_pool = NULL;
 686
 687         memblock_free_late(__pa(kfence_metadata_init), KFENCE_METADATA_SIZE);
 688         kfence_metadata_init = NULL;
 689
 690         return false;
 691 }
 692
 693 /* === DebugFS Interface ==================================================== */
 694
 695 static int stats_show(struct seq_file *seq, void *v)
 696 {
 697         int i;
 698
 699         seq_printf(seq, "enabled: %i\n", READ_ONCE(kfence_enabled));
 700         for (i = 0; i < KFENCE_COUNTER_COUNT; i++)
 701                 seq_printf(seq, "%s: %ld\n", counter_names[i], atomic_long_read(&counters[i]));
 702
 703         return 0;
 704 }
 705 DEFINE_SHOW_ATTRIBUTE(stats);
 706
 707 /*
 708  * debugfs seq_file operations for /sys/kernel/debug/kfence/objects.
 709  * start_object() and next_object() return the object index + 1, because NULL is used
 710  * to stop iteration.
 711  */
 712 static void *start_object(struct seq_file *seq, loff_t *pos)
 713 {
 714         if (*pos < CONFIG_KFENCE_NUM_OBJECTS)
 715                 return (void *)((long)*pos + 1);
 716         return NULL;
 717 }
 718
 719 static void stop_object(struct seq_file *seq, void *v)
 720 {
 721 }
 722
 723 static void *next_object(struct seq_file *seq, void *v, loff_t *pos)
 724 {
 725         ++*pos;
 726         if (*pos < CONFIG_KFENCE_NUM_OBJECTS)
 727                 return (void *)((long)*pos + 1);
 728         return NULL;
 729 }
 730
 731 static int show_object(struct seq_file *seq, void *v)
 732 {
 733         struct kfence_metadata *meta = &kfence_metadata[(long)v - 1];
 734         unsigned long flags;
 735
 736         raw_spin_lock_irqsave(&meta->lock, flags);
 737         kfence_print_object(seq, meta);
 738         raw_spin_unlock_irqrestore(&meta->lock, flags);
 739         seq_puts(seq, "---------------------------------\n");
 740
 741         return 0;
 742 }
 743
 744 static const struct seq_operations objects_sops = {
 745         .start = start_object,
 746         .next = next_object,
 747         .stop = stop_object,
 748         .show = show_object,
 749 };
 750 DEFINE_SEQ_ATTRIBUTE(objects);
 751
 752 static int kfence_debugfs_init(void)
 753 {
 754         struct dentry *kfence_dir;
 755
 756         if (!READ_ONCE(kfence_enabled))
 757                 return 0;
 758
 759         kfence_dir = debugfs_create_dir("kfence", NULL);
 760         debugfs_create_file("stats", 0444, kfence_dir, NULL, &stats_fops);
 761         debugfs_create_file("objects", 0400, kfence_dir, NULL, &objects_fops);
 762         return 0;
 763 }
 764
 765 late_initcall(kfence_debugfs_init);
 766
 767 /* === Panic Notifier ====================================================== */
 768
 769 static void kfence_check_all_canary(void)
 770 {
 771         int i;
 772
 773         for (i = 0; i < CONFIG_KFENCE_NUM_OBJECTS; i++) {
 774                 struct kfence_metadata *meta = &kfence_metadata[i];
 775
 776                 if (meta->state == KFENCE_OBJECT_ALLOCATED)
 777                         check_canary(meta);
 778         }
 779 }
 780
 781 static int kfence_check_canary_callback(struct notifier_block *nb,
 782                                         unsigned long reason, void *arg)
 783 {
 784         kfence_check_all_canary();
 785         return NOTIFY_OK;
 786 }
 787
 788 static struct notifier_block kfence_check_canary_notifier = {
 789         .notifier_call = kfence_check_canary_callback,
 790 };
 791
 792 /* === Allocation Gate Timer ================================================ */
 793
 794 static struct delayed_work kfence_timer;
 795
 796 #ifdef CONFIG_KFENCE_STATIC_KEYS
 797 /* Wait queue to wake up allocation-gate timer task. */
 798 static DECLARE_WAIT_QUEUE_HEAD(allocation_wait);
 799
 800 static void wake_up_kfence_timer(struct irq_work *work)
 801 {
 802         wake_up(&allocation_wait);
 803 }
 804 static DEFINE_IRQ_WORK(wake_up_kfence_timer_work, wake_up_kfence_timer);
 805 #endif
 806
 807 /*
 808  * Set up delayed work, which will enable and disable the static key. We need to
 809  * use a work queue (rather than a simple timer), since enabling and disabling a
 810  * static key cannot be done from an interrupt.
 811  *
 812  * Note: Toggling a static branch currently causes IPIs, and here we'll end up
 813  * with a total of 2 IPIs to all CPUs. If this ends up a problem in future (with
 814  * more aggressive sampling intervals), we could get away with a variant that
 815  * avoids IPIs, at the cost of not immediately capturing allocations if the
 816  * instructions remain cached.
 817  */
 818 static void toggle_allocation_gate(struct work_struct *work)
 819 {
 820         if (!READ_ONCE(kfence_enabled))
 821                 return;
 822
 823         atomic_set(&kfence_allocation_gate, 0);
 824 #ifdef CONFIG_KFENCE_STATIC_KEYS
 825         /* Enable static key, and await allocation to happen. */
 826         static_branch_enable(&kfence_allocation_key);
 827
 828         wait_event_idle(allocation_wait, atomic_read(&kfence_allocation_gate));
 829
 830         /* Disable static key and reset timer. */
 831         static_branch_disable(&kfence_allocation_key);
 832 #endif
 833         queue_delayed_work(system_unbound_wq, &kfence_timer,
 834                            msecs_to_jiffies(kfence_sample_interval));
 835 }
 836
 837 /* === Public interface ===================================================== */
 838
 839 void __init kfence_alloc_pool_and_metadata(void)
 840 {
 841         if (!kfence_sample_interval)
 842                 return;
 843
 844         /*
 845          * If the pool has already been initialized by arch, there is no need to
 846          * re-allocate the memory pool.
 847          */
 848         if (!__kfence_pool)
 849                 __kfence_pool = memblock_alloc(KFENCE_POOL_SIZE, PAGE_SIZE);
 850
 851         if (!__kfence_pool) {
 852                 pr_err("failed to allocate pool\n");
 853                 return;
 854         }
 855
 856         /* The memory allocated by memblock has been zeroed out. */
 857         kfence_metadata_init = memblock_alloc(KFENCE_METADATA_SIZE, PAGE_SIZE);
 858         if (!kfence_metadata_init) {
 859                 pr_err("failed to allocate metadata\n");
 860                 memblock_free(__kfence_pool, KFENCE_POOL_SIZE);
 861                 __kfence_pool = NULL;
 862         }
 863 }
 864
 865 static void kfence_init_enable(void)
 866 {
 867         if (!IS_ENABLED(CONFIG_KFENCE_STATIC_KEYS))
 868                 static_branch_enable(&kfence_allocation_key);
 869
 870         if (kfence_deferrable)
 871                 INIT_DEFERRABLE_WORK(&kfence_timer, toggle_allocation_gate);
 872         else
 873                 INIT_DELAYED_WORK(&kfence_timer, toggle_allocation_gate);
 874
 875         if (kfence_check_on_panic)
 876                 atomic_notifier_chain_register(&panic_notifier_list, &kfence_check_canary_notifier);
 877
 878         WRITE_ONCE(kfence_enabled, true);
 879         queue_delayed_work(system_unbound_wq, &kfence_timer, 0);
 880
 881         pr_info("initialized - using %lu bytes for %d objects at 0x%p-0x%p\n", KFENCE_POOL_SIZE,
 882                 CONFIG_KFENCE_NUM_OBJECTS, (void *)__kfence_pool,
 883                 (void *)(__kfence_pool + KFENCE_POOL_SIZE));
 884 }
 885
 886 void __init kfence_init(void)
 887 {
 888         stack_hash_seed = get_random_u32();
 889
 890         /* Setting kfence_sample_interval to 0 on boot disables KFENCE. */
 891         if (!kfence_sample_interval)
 892                 return;
 893
 894         if (!kfence_init_pool_early()) {
 895                 pr_err("%s failed\n", __func__);
 896                 return;
 897         }
 898
 899         kfence_init_enable();
 900 }
 901
 902 static int kfence_init_late(void)
 903 {
 904         const unsigned long nr_pages_pool = KFENCE_POOL_SIZE / PAGE_SIZE;
 905         const unsigned long nr_pages_meta = KFENCE_METADATA_SIZE / PAGE_SIZE;
 906         unsigned long addr = (unsigned long)__kfence_pool;
 907         unsigned long free_size = KFENCE_POOL_SIZE;
 908         int err = -ENOMEM;
 909
 910 #ifdef CONFIG_CONTIG_ALLOC
 911         struct page *pages;
 912
 913         pages = alloc_contig_pages(nr_pages_pool, GFP_KERNEL, first_online_node,
 914                                    NULL);
 915         if (!pages)
 916                 return -ENOMEM;
 917
 918         __kfence_pool = page_to_virt(pages);
 919         pages = alloc_contig_pages(nr_pages_meta, GFP_KERNEL, first_online_node,
 920                                    NULL);
 921         if (pages)
 922                 kfence_metadata_init = page_to_virt(pages);
 923 #else
 924         if (nr_pages_pool > MAX_ORDER_NR_PAGES ||
 925             nr_pages_meta > MAX_ORDER_NR_PAGES) {
 926                 pr_warn("KFENCE_NUM_OBJECTS too large for buddy allocator\n");
 927                 return -EINVAL;
 928         }
 929
 930         __kfence_pool = alloc_pages_exact(KFENCE_POOL_SIZE, GFP_KERNEL);
 931         if (!__kfence_pool)
 932                 return -ENOMEM;
 933
 934         kfence_metadata_init = alloc_pages_exact(KFENCE_METADATA_SIZE, GFP_KERNEL);
 935 #endif
 936
 937         if (!kfence_metadata_init)
 938                 goto free_pool;
 939
 940         memzero_explicit(kfence_metadata_init, KFENCE_METADATA_SIZE);
 941         addr = kfence_init_pool();
 942         if (!addr) {
 943                 kfence_init_enable();
 944                 kfence_debugfs_init();
 945                 return 0;
 946         }
 947
 948         pr_err("%s failed\n", __func__);
 949         free_size = KFENCE_POOL_SIZE - (addr - (unsigned long)__kfence_pool);
 950         err = -EBUSY;
 951
 952 #ifdef CONFIG_CONTIG_ALLOC
 953         free_contig_range(page_to_pfn(virt_to_page((void *)kfence_metadata_init)),
 954                           nr_pages_meta);
 955 free_pool:
 956         free_contig_range(page_to_pfn(virt_to_page((void *)addr)),
 957                           free_size / PAGE_SIZE);
 958 #else
 959         free_pages_exact((void *)kfence_metadata_init, KFENCE_METADATA_SIZE);
 960 free_pool:
 961         free_pages_exact((void *)addr, free_size);
 962 #endif
 963
 964         kfence_metadata_init = NULL;
 965         __kfence_pool = NULL;
 966         return err;
 967 }
 968
 969 static int kfence_enable_late(void)
 970 {
 971         if (!__kfence_pool)
 972                 return kfence_init_late();
 973
 974         WRITE_ONCE(kfence_enabled, true);
 975         queue_delayed_work(system_unbound_wq, &kfence_timer, 0);
 976         pr_info("re-enabled\n");
 977         return 0;
 978 }
 979
 980 void kfence_shutdown_cache(struct kmem_cache *s)
 981 {
 982         unsigned long flags;
 983         struct kfence_metadata *meta;
 984         int i;
 985
 986         /* Pairs with release in kfence_init_pool(). */
 987         if (!smp_load_acquire(&kfence_metadata))
 988                 return;
 989
 990         for (i = 0; i < CONFIG_KFENCE_NUM_OBJECTS; i++) {
 991                 bool in_use;
 992
 993                 meta = &kfence_metadata[i];
 994
 995                 /*
 996                  * If we observe some inconsistent cache and state pair where we
 997                  * should have returned false here, cache destruction is racing
 998                  * with either kmem_cache_alloc() or kmem_cache_free(). Taking
 999                  * the lock will not help, as different critical section
1000                  * serialization will have the same outcome.
1001                  */
1002                 if (READ_ONCE(meta->cache) != s ||
1003                     READ_ONCE(meta->state) != KFENCE_OBJECT_ALLOCATED)
1004                         continue;
1005
1006                 raw_spin_lock_irqsave(&meta->lock, flags);
1007                 in_use = meta->cache == s && meta->state == KFENCE_OBJECT_ALLOCATED;
1008                 raw_spin_unlock_irqrestore(&meta->lock, flags);
1009
1010                 if (in_use) {
1011                         /*
1012                          * This cache still has allocations, and we should not
1013                          * release them back into the freelist so they can still
1014                          * safely be used and retain the kernel's default
1015                          * behaviour of keeping the allocations alive (leak the
1016                          * cache); however, they effectively become "zombie
1017                          * allocations" as the KFENCE objects are the only ones
1018                          * still in use and the owning cache is being destroyed.
1019                          *
1020                          * We mark them freed, so that any subsequent use shows
1021                          * more useful error messages that will include stack
1022                          * traces of the user of the object, the original
1023                          * allocation, and caller to shutdown_cache().
1024                          */
1025                         kfence_guarded_free((void *)meta->addr, meta, /*zombie=*/true);
1026                 }
1027         }
1028
1029         for (i = 0; i < CONFIG_KFENCE_NUM_OBJECTS; i++) {
1030                 meta = &kfence_metadata[i];
1031
1032                 /* See above. */
1033                 if (READ_ONCE(meta->cache) != s || READ_ONCE(meta->state) != KFENCE_OBJECT_FREED)
1034                         continue;
1035
1036                 raw_spin_lock_irqsave(&meta->lock, flags);
1037                 if (meta->cache == s && meta->state == KFENCE_OBJECT_FREED)
1038                         meta->cache = NULL;
1039                 raw_spin_unlock_irqrestore(&meta->lock, flags);
1040         }
1041 }
1042
1043 void *__kfence_alloc(struct kmem_cache *s, size_t size, gfp_t flags)
1044 {
1045         unsigned long stack_entries[KFENCE_STACK_DEPTH];
1046         size_t num_stack_entries;
1047         u32 alloc_stack_hash;
1048
1049         /*
1050          * Perform size check before switching kfence_allocation_gate, so that
1051          * we don't disable KFENCE without making an allocation.
1052          */
1053         if (size > PAGE_SIZE) {
1054                 atomic_long_inc(&counters[KFENCE_COUNTER_SKIP_INCOMPAT]);
1055                 return NULL;
1056         }
1057
1058         /*
1059          * Skip allocations from non-default zones, including DMA. We cannot
1060          * guarantee that pages in the KFENCE pool will have the requested
1061          * properties (e.g. reside in DMAable memory).
1062          */
1063         if ((flags & GFP_ZONEMASK) ||
1064             (s->flags & (SLAB_CACHE_DMA | SLAB_CACHE_DMA32))) {
1065                 atomic_long_inc(&counters[KFENCE_COUNTER_SKIP_INCOMPAT]);
1066                 return NULL;
1067         }
1068
1069         /*
1070          * Skip allocations for this slab, if KFENCE has been disabled for
1071          * this slab.
1072          */
1073         if (s->flags & SLAB_SKIP_KFENCE)
1074                 return NULL;
1075
1076         if (atomic_inc_return(&kfence_allocation_gate) > 1)
1077                 return NULL;
1078 #ifdef CONFIG_KFENCE_STATIC_KEYS
1079         /*
1080          * waitqueue_active() is fully ordered after the update of
1081          * kfence_allocation_gate per atomic_inc_return().
1082          */
1083         if (waitqueue_active(&allocation_wait)) {
1084                 /*
1085                  * Calling wake_up() here may deadlock when allocations happen
1086                  * from within timer code. Use an irq_work to defer it.
1087                  */
1088                 irq_work_queue(&wake_up_kfence_timer_work);
1089         }
1090 #endif
1091
1092         if (!READ_ONCE(kfence_enabled))
1093                 return NULL;
1094
1095         num_stack_entries = stack_trace_save(stack_entries, KFENCE_STACK_DEPTH, 0);
1096
1097         /*
1098          * Do expensive check for coverage of allocation in slow-path after
1099          * allocation_gate has already become non-zero, even though it might
1100          * mean not making any allocation within a given sample interval.
1101          *
1102          * This ensures reasonable allocation coverage when the pool is almost
1103          * full, including avoiding long-lived allocations of the same source
1104          * filling up the pool (e.g. pagecache allocations).
1105          */
1106         alloc_stack_hash = get_alloc_stack_hash(stack_entries, num_stack_entries);
1107         if (should_skip_covered() && alloc_covered_contains(alloc_stack_hash)) {
1108                 atomic_long_inc(&counters[KFENCE_COUNTER_SKIP_COVERED]);
1109                 return NULL;
1110         }
1111
1112         return kfence_guarded_alloc(s, size, flags, stack_entries, num_stack_entries,
1113                                     alloc_stack_hash);
1114 }
1115
1116 size_t kfence_ksize(const void *addr)
1117 {
1118         const struct kfence_metadata *meta = addr_to_metadata((unsigned long)addr);
1119
1120         /*
1121          * Read locklessly -- if there is a race with __kfence_alloc(), this is
1122          * either a use-after-free or invalid access.
1123          */
1124         return meta ? meta->size : 0;
1125 }
1126
1127 void *kfence_object_start(const void *addr)
1128 {
1129         const struct kfence_metadata *meta = addr_to_metadata((unsigned long)addr);
1130
1131         /*
1132          * Read locklessly -- if there is a race with __kfence_alloc(), this is
1133          * either a use-after-free or invalid access.
1134          */
1135         return meta ? (void *)meta->addr : NULL;
1136 }
1137
1138 void __kfence_free(void *addr)
1139 {
1140         struct kfence_metadata *meta = addr_to_metadata((unsigned long)addr);
1141
1142 #ifdef CONFIG_MEMCG
1143         KFENCE_WARN_ON(meta->objcg);
1144 #endif
1145         /*
1146          * If the objects of the cache are SLAB_TYPESAFE_BY_RCU, defer freeing
1147          * the object, as the object page may be recycled for other-typed
1148          * objects once it has been freed. meta->cache may be NULL if the cache
1149          * was destroyed.
1150          */
1151         if (unlikely(meta->cache && (meta->cache->flags & SLAB_TYPESAFE_BY_RCU)))
1152                 call_rcu(&meta->rcu_head, rcu_guarded_free);
1153         else
1154                 kfence_guarded_free(addr, meta, false);
1155 }
1156
1157 bool kfence_handle_page_fault(unsigned long addr, bool is_write, struct pt_regs *regs)
1158 {
1159         const int page_index = (addr - (unsigned long)__kfence_pool) / PAGE_SIZE;
1160         struct kfence_metadata *to_report = NULL;
1161         enum kfence_error_type error_type;
1162         unsigned long flags;
1163
1164         if (!is_kfence_address((void *)addr))
1165                 return false;
1166
1167         if (!READ_ONCE(kfence_enabled)) /* If disabled at runtime ... */
1168                 return kfence_unprotect(addr); /* ... unprotect and proceed. */
1169
1170         atomic_long_inc(&counters[KFENCE_COUNTER_BUGS]);
1171
1172         if (page_index % 2) {
1173                 /* This is a redzone, report a buffer overflow. */
1174                 struct kfence_metadata *meta;
1175                 int distance = 0;
1176
1177                 meta = addr_to_metadata(addr - PAGE_SIZE);
1178                 if (meta && READ_ONCE(meta->state) == KFENCE_OBJECT_ALLOCATED) {
1179                         to_report = meta;
1180                         /* Data race ok; distance calculation approximate. */
1181                         distance = addr - data_race(meta->addr + meta->size);
1182                 }
1183
1184                 meta = addr_to_metadata(addr + PAGE_SIZE);
1185                 if (meta && READ_ONCE(meta->state) == KFENCE_OBJECT_ALLOCATED) {
1186                         /* Data race ok; distance calculation approximate. */
1187                         if (!to_report || distance > data_race(meta->addr) - addr)
1188                                 to_report = meta;
1189                 }
1190
1191                 if (!to_report)
1192                         goto out;
1193
1194                 raw_spin_lock_irqsave(&to_report->lock, flags);
1195                 to_report->unprotected_page = addr;
1196                 error_type = KFENCE_ERROR_OOB;
1197
1198                 /*
1199                  * If the object was freed before we took the look we can still
1200                  * report this as an OOB -- the report will simply show the
1201                  * stacktrace of the free as well.
1202                  */
1203         } else {
1204                 to_report = addr_to_metadata(addr);
1205                 if (!to_report)
1206                         goto out;
1207
1208                 raw_spin_lock_irqsave(&to_report->lock, flags);
1209                 error_type = KFENCE_ERROR_UAF;
1210                 /*
1211                  * We may race with __kfence_alloc(), and it is possible that a
1212                  * freed object may be reallocated. We simply report this as a
1213                  * use-after-free, with the stack trace showing the place where
1214                  * the object was re-allocated.
1215                  */
1216         }
1217
1218 out:
1219         if (to_report) {
1220                 kfence_report_error(addr, is_write, regs, to_report, error_type);
1221                 raw_spin_unlock_irqrestore(&to_report->lock, flags);
1222         } else {
1223                 /* This may be a UAF or OOB access, but we can't be sure. */
1224                 kfence_report_error(addr, is_write, regs, NULL, KFENCE_ERROR_INVALID);
1225         }
1226
1227         return kfence_unprotect(addr); /* Unprotect and let access proceed. */
1228 }