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