kernel/kcsan/core.c

   1 // SPDX-License-Identifier: GPL-2.0
   2 /*
   3  * KCSAN core runtime.
   4  *
   5  * Copyright (C) 2019, Google LLC.
   6  */
   7
   8 #define pr_fmt(fmt) "kcsan: " fmt
   9
  10 #include <linux/atomic.h>
  11 #include <linux/bug.h>
  12 #include <linux/delay.h>
  13 #include <linux/export.h>
  14 #include <linux/init.h>
  15 #include <linux/kernel.h>
  16 #include <linux/list.h>
  17 #include <linux/moduleparam.h>
  18 #include <linux/percpu.h>
  19 #include <linux/preempt.h>
  20 #include <linux/sched.h>
  21 #include <linux/uaccess.h>
  22
  23 #include "encoding.h"
  24 #include "kcsan.h"
  25 #include "permissive.h"
  26
  27 static bool kcsan_early_enable = IS_ENABLED(CONFIG_KCSAN_EARLY_ENABLE);
  28 unsigned int kcsan_udelay_task = CONFIG_KCSAN_UDELAY_TASK;
  29 unsigned int kcsan_udelay_interrupt = CONFIG_KCSAN_UDELAY_INTERRUPT;
  30 static long kcsan_skip_watch = CONFIG_KCSAN_SKIP_WATCH;
  31 static bool kcsan_interrupt_watcher = IS_ENABLED(CONFIG_KCSAN_INTERRUPT_WATCHER);
  32
  33 #ifdef MODULE_PARAM_PREFIX
  34 #undef MODULE_PARAM_PREFIX
  35 #endif
  36 #define MODULE_PARAM_PREFIX "kcsan."
  37 module_param_named(early_enable, kcsan_early_enable, bool, 0);
  38 module_param_named(udelay_task, kcsan_udelay_task, uint, 0644);
  39 module_param_named(udelay_interrupt, kcsan_udelay_interrupt, uint, 0644);
  40 module_param_named(skip_watch, kcsan_skip_watch, long, 0644);
  41 module_param_named(interrupt_watcher, kcsan_interrupt_watcher, bool, 0444);
  42
  43 bool kcsan_enabled;
  44
  45 /* Per-CPU kcsan_ctx for interrupts */
  46 static DEFINE_PER_CPU(struct kcsan_ctx, kcsan_cpu_ctx) = {
  47         .disable_count          = 0,
  48         .atomic_next            = 0,
  49         .atomic_nest_count      = 0,
  50         .in_flat_atomic         = false,
  51         .access_mask            = 0,
  52         .scoped_accesses        = {LIST_POISON1, NULL},
  53 };
  54
  55 /*
  56  * Helper macros to index into adjacent slots, starting from address slot
  57  * itself, followed by the right and left slots.
  58  *
  59  * The purpose is 2-fold:
  60  *
  61  *      1. if during insertion the address slot is already occupied, check if
  62  *         any adjacent slots are free;
  63  *      2. accesses that straddle a slot boundary due to size that exceeds a
  64  *         slot's range may check adjacent slots if any watchpoint matches.
  65  *
  66  * Note that accesses with very large size may still miss a watchpoint; however,
  67  * given this should be rare, this is a reasonable trade-off to make, since this
  68  * will avoid:
  69  *
  70  *      1. excessive contention between watchpoint checks and setup;
  71  *      2. larger number of simultaneous watchpoints without sacrificing
  72  *         performance.
  73  *
  74  * Example: SLOT_IDX values for KCSAN_CHECK_ADJACENT=1, where i is [0, 1, 2]:
  75  *
  76  *   slot=0:  [ 1,  2,  0]
  77  *   slot=9:  [10, 11,  9]
  78  *   slot=63: [64, 65, 63]
  79  */
  80 #define SLOT_IDX(slot, i) (slot + ((i + KCSAN_CHECK_ADJACENT) % NUM_SLOTS))
  81
  82 /*
  83  * SLOT_IDX_FAST is used in the fast-path. Not first checking the address's primary
  84  * slot (middle) is fine if we assume that races occur rarely. The set of
  85  * indices {SLOT_IDX(slot, i) | i in [0, NUM_SLOTS)} is equivalent to
  86  * {SLOT_IDX_FAST(slot, i) | i in [0, NUM_SLOTS)}.
  87  */
  88 #define SLOT_IDX_FAST(slot, i) (slot + i)
  89
  90 /*
  91  * Watchpoints, with each entry encoded as defined in encoding.h: in order to be
  92  * able to safely update and access a watchpoint without introducing locking
  93  * overhead, we encode each watchpoint as a single atomic long. The initial
  94  * zero-initialized state matches INVALID_WATCHPOINT.
  95  *
  96  * Add NUM_SLOTS-1 entries to account for overflow; this helps avoid having to
  97  * use more complicated SLOT_IDX_FAST calculation with modulo in the fast-path.
  98  */
  99 static atomic_long_t watchpoints[CONFIG_KCSAN_NUM_WATCHPOINTS + NUM_SLOTS-1];
 100
 101 /*
 102  * Instructions to skip watching counter, used in should_watch(). We use a
 103  * per-CPU counter to avoid excessive contention.
 104  */
 105 static DEFINE_PER_CPU(long, kcsan_skip);
 106
 107 /* For kcsan_prandom_u32_max(). */
 108 static DEFINE_PER_CPU(u32, kcsan_rand_state);
 109
 110 static __always_inline atomic_long_t *find_watchpoint(unsigned long addr,
 111                                                       size_t size,
 112                                                       bool expect_write,
 113                                                       long *encoded_watchpoint)
 114 {
 115         const int slot = watchpoint_slot(addr);
 116         const unsigned long addr_masked = addr & WATCHPOINT_ADDR_MASK;
 117         atomic_long_t *watchpoint;
 118         unsigned long wp_addr_masked;
 119         size_t wp_size;
 120         bool is_write;
 121         int i;
 122
 123         BUILD_BUG_ON(CONFIG_KCSAN_NUM_WATCHPOINTS < NUM_SLOTS);
 124
 125         for (i = 0; i < NUM_SLOTS; ++i) {
 126                 watchpoint = &watchpoints[SLOT_IDX_FAST(slot, i)];
 127                 *encoded_watchpoint = atomic_long_read(watchpoint);
 128                 if (!decode_watchpoint(*encoded_watchpoint, &wp_addr_masked,
 129                                        &wp_size, &is_write))
 130                         continue;
 131
 132                 if (expect_write && !is_write)
 133                         continue;
 134
 135                 /* Check if the watchpoint matches the access. */
 136                 if (matching_access(wp_addr_masked, wp_size, addr_masked, size))
 137                         return watchpoint;
 138         }
 139
 140         return NULL;
 141 }
 142
 143 static inline atomic_long_t *
 144 insert_watchpoint(unsigned long addr, size_t size, bool is_write)
 145 {
 146         const int slot = watchpoint_slot(addr);
 147         const long encoded_watchpoint = encode_watchpoint(addr, size, is_write);
 148         atomic_long_t *watchpoint;
 149         int i;
 150
 151         /* Check slot index logic, ensuring we stay within array bounds. */
 152         BUILD_BUG_ON(SLOT_IDX(0, 0) != KCSAN_CHECK_ADJACENT);
 153         BUILD_BUG_ON(SLOT_IDX(0, KCSAN_CHECK_ADJACENT+1) != 0);
 154         BUILD_BUG_ON(SLOT_IDX(CONFIG_KCSAN_NUM_WATCHPOINTS-1, KCSAN_CHECK_ADJACENT) != ARRAY_SIZE(watchpoints)-1);
 155         BUILD_BUG_ON(SLOT_IDX(CONFIG_KCSAN_NUM_WATCHPOINTS-1, KCSAN_CHECK_ADJACENT+1) != ARRAY_SIZE(watchpoints) - NUM_SLOTS);
 156
 157         for (i = 0; i < NUM_SLOTS; ++i) {
 158                 long expect_val = INVALID_WATCHPOINT;
 159
 160                 /* Try to acquire this slot. */
 161                 watchpoint = &watchpoints[SLOT_IDX(slot, i)];
 162                 if (atomic_long_try_cmpxchg_relaxed(watchpoint, &expect_val, encoded_watchpoint))
 163                         return watchpoint;
 164         }
 165
 166         return NULL;
 167 }
 168
 169 /*
 170  * Return true if watchpoint was successfully consumed, false otherwise.
 171  *
 172  * This may return false if:
 173  *
 174  *      1. another thread already consumed the watchpoint;
 175  *      2. the thread that set up the watchpoint already removed it;
 176  *      3. the watchpoint was removed and then re-used.
 177  */
 178 static __always_inline bool
 179 try_consume_watchpoint(atomic_long_t *watchpoint, long encoded_watchpoint)
 180 {
 181         return atomic_long_try_cmpxchg_relaxed(watchpoint, &encoded_watchpoint, CONSUMED_WATCHPOINT);
 182 }
 183
 184 /* Return true if watchpoint was not touched, false if already consumed. */
 185 static inline bool consume_watchpoint(atomic_long_t *watchpoint)
 186 {
 187         return atomic_long_xchg_relaxed(watchpoint, CONSUMED_WATCHPOINT) != CONSUMED_WATCHPOINT;
 188 }
 189
 190 /* Remove the watchpoint -- its slot may be reused after. */
 191 static inline void remove_watchpoint(atomic_long_t *watchpoint)
 192 {
 193         atomic_long_set(watchpoint, INVALID_WATCHPOINT);
 194 }
 195
 196 static __always_inline struct kcsan_ctx *get_ctx(void)
 197 {
 198         /*
 199          * In interrupts, use raw_cpu_ptr to avoid unnecessary checks, that would
 200          * also result in calls that generate warnings in uaccess regions.
 201          */
 202         return in_task() ? &current->kcsan_ctx : raw_cpu_ptr(&kcsan_cpu_ctx);
 203 }
 204
 205 static __always_inline void
 206 check_access(const volatile void *ptr, size_t size, int type, unsigned long ip);
 207
 208 /* Check scoped accesses; never inline because this is a slow-path! */
 209 static noinline void kcsan_check_scoped_accesses(void)
 210 {
 211         struct kcsan_ctx *ctx = get_ctx();
 212         struct list_head *prev_save = ctx->scoped_accesses.prev;
 213         struct kcsan_scoped_access *scoped_access;
 214
 215         ctx->scoped_accesses.prev = NULL;  /* Avoid recursion. */
 216         list_for_each_entry(scoped_access, &ctx->scoped_accesses, list) {
 217                 check_access(scoped_access->ptr, scoped_access->size,
 218                              scoped_access->type, scoped_access->ip);
 219         }
 220         ctx->scoped_accesses.prev = prev_save;
 221 }
 222
 223 /* Rules for generic atomic accesses. Called from fast-path. */
 224 static __always_inline bool
 225 is_atomic(struct kcsan_ctx *ctx, const volatile void *ptr, size_t size, int type)
 226 {
 227         if (type & KCSAN_ACCESS_ATOMIC)
 228                 return true;
 229
 230         /*
 231          * Unless explicitly declared atomic, never consider an assertion access
 232          * as atomic. This allows using them also in atomic regions, such as
 233          * seqlocks, without implicitly changing their semantics.
 234          */
 235         if (type & KCSAN_ACCESS_ASSERT)
 236                 return false;
 237
 238         if (IS_ENABLED(CONFIG_KCSAN_ASSUME_PLAIN_WRITES_ATOMIC) &&
 239             (type & KCSAN_ACCESS_WRITE) && size <= sizeof(long) &&
 240             !(type & KCSAN_ACCESS_COMPOUND) && IS_ALIGNED((unsigned long)ptr, size))
 241                 return true; /* Assume aligned writes up to word size are atomic. */
 242
 243         if (ctx->atomic_next > 0) {
 244                 /*
 245                  * Because we do not have separate contexts for nested
 246                  * interrupts, in case atomic_next is set, we simply assume that
 247                  * the outer interrupt set atomic_next. In the worst case, we
 248                  * will conservatively consider operations as atomic. This is a
 249                  * reasonable trade-off to make, since this case should be
 250                  * extremely rare; however, even if extremely rare, it could
 251                  * lead to false positives otherwise.
 252                  */
 253                 if ((hardirq_count() >> HARDIRQ_SHIFT) < 2)
 254                         --ctx->atomic_next; /* in task, or outer interrupt */
 255                 return true;
 256         }
 257
 258         return ctx->atomic_nest_count > 0 || ctx->in_flat_atomic;
 259 }
 260
 261 static __always_inline bool
 262 should_watch(struct kcsan_ctx *ctx, const volatile void *ptr, size_t size, int type)
 263 {
 264         /*
 265          * Never set up watchpoints when memory operations are atomic.
 266          *
 267          * Need to check this first, before kcsan_skip check below: (1) atomics
 268          * should not count towards skipped instructions, and (2) to actually
 269          * decrement kcsan_atomic_next for consecutive instruction stream.
 270          */
 271         if (is_atomic(ctx, ptr, size, type))
 272                 return false;
 273
 274         if (this_cpu_dec_return(kcsan_skip) >= 0)
 275                 return false;
 276
 277         /*
 278          * NOTE: If we get here, kcsan_skip must always be reset in slow path
 279          * via reset_kcsan_skip() to avoid underflow.
 280          */
 281
 282         /* this operation should be watched */
 283         return true;
 284 }
 285
 286 /*
 287  * Returns a pseudo-random number in interval [0, ep_ro). Simple linear
 288  * congruential generator, using constants from "Numerical Recipes".
 289  */
 290 static u32 kcsan_prandom_u32_max(u32 ep_ro)
 291 {
 292         u32 state = this_cpu_read(kcsan_rand_state);
 293
 294         state = 1664525 * state + 1013904223;
 295         this_cpu_write(kcsan_rand_state, state);
 296
 297         return state % ep_ro;
 298 }
 299
 300 static inline void reset_kcsan_skip(void)
 301 {
 302         long skip_count = kcsan_skip_watch -
 303                           (IS_ENABLED(CONFIG_KCSAN_SKIP_WATCH_RANDOMIZE) ?
 304                                    kcsan_prandom_u32_max(kcsan_skip_watch) :
 305                                    0);
 306         this_cpu_write(kcsan_skip, skip_count);
 307 }
 308
 309 static __always_inline bool kcsan_is_enabled(struct kcsan_ctx *ctx)
 310 {
 311         return READ_ONCE(kcsan_enabled) && !ctx->disable_count;
 312 }
 313
 314 /* Introduce delay depending on context and configuration. */
 315 static void delay_access(int type)
 316 {
 317         unsigned int delay = in_task() ? kcsan_udelay_task : kcsan_udelay_interrupt;
 318         /* For certain access types, skew the random delay to be longer. */
 319         unsigned int skew_delay_order =
 320                 (type & (KCSAN_ACCESS_COMPOUND | KCSAN_ACCESS_ASSERT)) ? 1 : 0;
 321
 322         delay -= IS_ENABLED(CONFIG_KCSAN_DELAY_RANDOMIZE) ?
 323                                kcsan_prandom_u32_max(delay >> skew_delay_order) :
 324                                0;
 325         udelay(delay);
 326 }
 327
 328 void kcsan_save_irqtrace(struct task_struct *task)
 329 {
 330 #ifdef CONFIG_TRACE_IRQFLAGS
 331         task->kcsan_save_irqtrace = task->irqtrace;
 332 #endif
 333 }
 334
 335 void kcsan_restore_irqtrace(struct task_struct *task)
 336 {
 337 #ifdef CONFIG_TRACE_IRQFLAGS
 338         task->irqtrace = task->kcsan_save_irqtrace;
 339 #endif
 340 }
 341
 342 /*
 343  * Pull everything together: check_access() below contains the performance
 344  * critical operations; the fast-path (including check_access) functions should
 345  * all be inlinable by the instrumentation functions.
 346  *
 347  * The slow-path (kcsan_found_watchpoint, kcsan_setup_watchpoint) are
 348  * non-inlinable -- note that, we prefix these with "kcsan_" to ensure they can
 349  * be filtered from the stacktrace, as well as give them unique names for the
 350  * UACCESS whitelist of objtool. Each function uses user_access_save/restore(),
 351  * since they do not access any user memory, but instrumentation is still
 352  * emitted in UACCESS regions.
 353  */
 354
 355 static noinline void kcsan_found_watchpoint(const volatile void *ptr,
 356                                             size_t size,
 357                                             int type,
 358                                             unsigned long ip,
 359                                             atomic_long_t *watchpoint,
 360                                             long encoded_watchpoint)
 361 {
 362         const bool is_assert = (type & KCSAN_ACCESS_ASSERT) != 0;
 363         struct kcsan_ctx *ctx = get_ctx();
 364         unsigned long flags;
 365         bool consumed;
 366
 367         /*
 368          * We know a watchpoint exists. Let's try to keep the race-window
 369          * between here and finally consuming the watchpoint below as small as
 370          * possible -- avoid unneccessarily complex code until consumed.
 371          */
 372
 373         if (!kcsan_is_enabled(ctx))
 374                 return;
 375
 376         /*
 377          * The access_mask check relies on value-change comparison. To avoid
 378          * reporting a race where e.g. the writer set up the watchpoint, but the
 379          * reader has access_mask!=0, we have to ignore the found watchpoint.
 380          */
 381         if (ctx->access_mask)
 382                 return;
 383
 384         /*
 385          * If the other thread does not want to ignore the access, and there was
 386          * a value change as a result of this thread's operation, we will still
 387          * generate a report of unknown origin.
 388          *
 389          * Use CONFIG_KCSAN_REPORT_RACE_UNKNOWN_ORIGIN=n to filter.
 390          */
 391         if (!is_assert && kcsan_ignore_address(ptr))
 392                 return;
 393
 394         /*
 395          * Consuming the watchpoint must be guarded by kcsan_is_enabled() to
 396          * avoid erroneously triggering reports if the context is disabled.
 397          */
 398         consumed = try_consume_watchpoint(watchpoint, encoded_watchpoint);
 399
 400         /* keep this after try_consume_watchpoint */
 401         flags = user_access_save();
 402
 403         if (consumed) {
 404                 kcsan_save_irqtrace(current);
 405                 kcsan_report_set_info(ptr, size, type, ip, watchpoint - watchpoints);
 406                 kcsan_restore_irqtrace(current);
 407         } else {
 408                 /*
 409                  * The other thread may not print any diagnostics, as it has
 410                  * already removed the watchpoint, or another thread consumed
 411                  * the watchpoint before this thread.
 412                  */
 413                 atomic_long_inc(&kcsan_counters[KCSAN_COUNTER_REPORT_RACES]);
 414         }
 415
 416         if (is_assert)
 417                 atomic_long_inc(&kcsan_counters[KCSAN_COUNTER_ASSERT_FAILURES]);
 418         else
 419                 atomic_long_inc(&kcsan_counters[KCSAN_COUNTER_DATA_RACES]);
 420
 421         user_access_restore(flags);
 422 }
 423
 424 static noinline void
 425 kcsan_setup_watchpoint(const volatile void *ptr, size_t size, int type, unsigned long ip)
 426 {
 427         const bool is_write = (type & KCSAN_ACCESS_WRITE) != 0;
 428         const bool is_assert = (type & KCSAN_ACCESS_ASSERT) != 0;
 429         atomic_long_t *watchpoint;
 430         u64 old, new, diff;
 431         unsigned long access_mask;
 432         enum kcsan_value_change value_change = KCSAN_VALUE_CHANGE_MAYBE;
 433         unsigned long ua_flags = user_access_save();
 434         struct kcsan_ctx *ctx = get_ctx();
 435         unsigned long irq_flags = 0;
 436
 437         /*
 438          * Always reset kcsan_skip counter in slow-path to avoid underflow; see
 439          * should_watch().
 440          */
 441         reset_kcsan_skip();
 442
 443         if (!kcsan_is_enabled(ctx))
 444                 goto out;
 445
 446         /*
 447          * Check to-ignore addresses after kcsan_is_enabled(), as we may access
 448          * memory that is not yet initialized during early boot.
 449          */
 450         if (!is_assert && kcsan_ignore_address(ptr))
 451                 goto out;
 452
 453         if (!check_encodable((unsigned long)ptr, size)) {
 454                 atomic_long_inc(&kcsan_counters[KCSAN_COUNTER_UNENCODABLE_ACCESSES]);
 455                 goto out;
 456         }
 457
 458         /*
 459          * Save and restore the IRQ state trace touched by KCSAN, since KCSAN's
 460          * runtime is entered for every memory access, and potentially useful
 461          * information is lost if dirtied by KCSAN.
 462          */
 463         kcsan_save_irqtrace(current);
 464         if (!kcsan_interrupt_watcher)
 465                 local_irq_save(irq_flags);
 466
 467         watchpoint = insert_watchpoint((unsigned long)ptr, size, is_write);
 468         if (watchpoint == NULL) {
 469                 /*
 470                  * Out of capacity: the size of 'watchpoints', and the frequency
 471                  * with which should_watch() returns true should be tweaked so
 472                  * that this case happens very rarely.
 473                  */
 474                 atomic_long_inc(&kcsan_counters[KCSAN_COUNTER_NO_CAPACITY]);
 475                 goto out_unlock;
 476         }
 477
 478         atomic_long_inc(&kcsan_counters[KCSAN_COUNTER_SETUP_WATCHPOINTS]);
 479         atomic_long_inc(&kcsan_counters[KCSAN_COUNTER_USED_WATCHPOINTS]);
 480
 481         /*
 482          * Read the current value, to later check and infer a race if the data
 483          * was modified via a non-instrumented access, e.g. from a device.
 484          */
 485         old = 0;
 486         switch (size) {
 487         case 1:
 488                 old = READ_ONCE(*(const u8 *)ptr);
 489                 break;
 490         case 2:
 491                 old = READ_ONCE(*(const u16 *)ptr);
 492                 break;
 493         case 4:
 494                 old = READ_ONCE(*(const u32 *)ptr);
 495                 break;
 496         case 8:
 497                 old = READ_ONCE(*(const u64 *)ptr);
 498                 break;
 499         default:
 500                 break; /* ignore; we do not diff the values */
 501         }
 502
 503         /*
 504          * Delay this thread, to increase probability of observing a racy
 505          * conflicting access.
 506          */
 507         delay_access(type);
 508
 509         /*
 510          * Re-read value, and check if it is as expected; if not, we infer a
 511          * racy access.
 512          */
 513         access_mask = ctx->access_mask;
 514         new = 0;
 515         switch (size) {
 516         case 1:
 517                 new = READ_ONCE(*(const u8 *)ptr);
 518                 break;
 519         case 2:
 520                 new = READ_ONCE(*(const u16 *)ptr);
 521                 break;
 522         case 4:
 523                 new = READ_ONCE(*(const u32 *)ptr);
 524                 break;
 525         case 8:
 526                 new = READ_ONCE(*(const u64 *)ptr);
 527                 break;
 528         default:
 529                 break; /* ignore; we do not diff the values */
 530         }
 531
 532         diff = old ^ new;
 533         if (access_mask)
 534                 diff &= access_mask;
 535
 536         /*
 537          * Check if we observed a value change.
 538          *
 539          * Also check if the data race should be ignored (the rules depend on
 540          * non-zero diff); if it is to be ignored, the below rules for
 541          * KCSAN_VALUE_CHANGE_MAYBE apply.
 542          */
 543         if (diff && !kcsan_ignore_data_race(size, type, old, new, diff))
 544                 value_change = KCSAN_VALUE_CHANGE_TRUE;
 545
 546         /* Check if this access raced with another. */
 547         if (!consume_watchpoint(watchpoint)) {
 548                 /*
 549                  * Depending on the access type, map a value_change of MAYBE to
 550                  * TRUE (always report) or FALSE (never report).
 551                  */
 552                 if (value_change == KCSAN_VALUE_CHANGE_MAYBE) {
 553                         if (access_mask != 0) {
 554                                 /*
 555                                  * For access with access_mask, we require a
 556                                  * value-change, as it is likely that races on
 557                                  * ~access_mask bits are expected.
 558                                  */
 559                                 value_change = KCSAN_VALUE_CHANGE_FALSE;
 560                         } else if (size > 8 || is_assert) {
 561                                 /* Always assume a value-change. */
 562                                 value_change = KCSAN_VALUE_CHANGE_TRUE;
 563                         }
 564                 }
 565
 566                 /*
 567                  * No need to increment 'data_races' counter, as the racing
 568                  * thread already did.
 569                  *
 570                  * Count 'assert_failures' for each failed ASSERT access,
 571                  * therefore both this thread and the racing thread may
 572                  * increment this counter.
 573                  */
 574                 if (is_assert && value_change == KCSAN_VALUE_CHANGE_TRUE)
 575                         atomic_long_inc(&kcsan_counters[KCSAN_COUNTER_ASSERT_FAILURES]);
 576
 577                 kcsan_report_known_origin(ptr, size, type, ip,
 578                                           value_change, watchpoint - watchpoints,
 579                                           old, new, access_mask);
 580         } else if (value_change == KCSAN_VALUE_CHANGE_TRUE) {
 581                 /* Inferring a race, since the value should not have changed. */
 582
 583                 atomic_long_inc(&kcsan_counters[KCSAN_COUNTER_RACES_UNKNOWN_ORIGIN]);
 584                 if (is_assert)
 585                         atomic_long_inc(&kcsan_counters[KCSAN_COUNTER_ASSERT_FAILURES]);
 586
 587                 if (IS_ENABLED(CONFIG_KCSAN_REPORT_RACE_UNKNOWN_ORIGIN) || is_assert) {
 588                         kcsan_report_unknown_origin(ptr, size, type, ip,
 589                                                     old, new, access_mask);
 590                 }
 591         }
 592
 593         /*
 594          * Remove watchpoint; must be after reporting, since the slot may be
 595          * reused after this point.
 596          */
 597         remove_watchpoint(watchpoint);
 598         atomic_long_dec(&kcsan_counters[KCSAN_COUNTER_USED_WATCHPOINTS]);
 599 out_unlock:
 600         if (!kcsan_interrupt_watcher)
 601                 local_irq_restore(irq_flags);
 602         kcsan_restore_irqtrace(current);
 603 out:
 604         user_access_restore(ua_flags);
 605 }
 606
 607 static __always_inline void
 608 check_access(const volatile void *ptr, size_t size, int type, unsigned long ip)
 609 {
 610         const bool is_write = (type & KCSAN_ACCESS_WRITE) != 0;
 611         atomic_long_t *watchpoint;
 612         long encoded_watchpoint;
 613
 614         /*
 615          * Do nothing for 0 sized check; this comparison will be optimized out
 616          * for constant sized instrumentation (__tsan_{read,write}N).
 617          */
 618         if (unlikely(size == 0))
 619                 return;
 620
 621         /*
 622          * Avoid user_access_save in fast-path: find_watchpoint is safe without
 623          * user_access_save, as the address that ptr points to is only used to
 624          * check if a watchpoint exists; ptr is never dereferenced.
 625          */
 626         watchpoint = find_watchpoint((unsigned long)ptr, size, !is_write,
 627                                      &encoded_watchpoint);
 628         /*
 629          * It is safe to check kcsan_is_enabled() after find_watchpoint in the
 630          * slow-path, as long as no state changes that cause a race to be
 631          * detected and reported have occurred until kcsan_is_enabled() is
 632          * checked.
 633          */
 634
 635         if (unlikely(watchpoint != NULL))
 636                 kcsan_found_watchpoint(ptr, size, type, ip, watchpoint, encoded_watchpoint);
 637         else {
 638                 struct kcsan_ctx *ctx = get_ctx(); /* Call only once in fast-path. */
 639
 640                 if (unlikely(should_watch(ctx, ptr, size, type)))
 641                         kcsan_setup_watchpoint(ptr, size, type, ip);
 642                 else if (unlikely(ctx->scoped_accesses.prev))
 643                         kcsan_check_scoped_accesses();
 644         }
 645 }
 646
 647 /* === Public interface ===================================================== */
 648
 649 void __init kcsan_init(void)
 650 {
 651         int cpu;
 652
 653         BUG_ON(!in_task());
 654
 655         for_each_possible_cpu(cpu)
 656                 per_cpu(kcsan_rand_state, cpu) = (u32)get_cycles();
 657
 658         /*
 659          * We are in the init task, and no other tasks should be running;
 660          * WRITE_ONCE without memory barrier is sufficient.
 661          */
 662         if (kcsan_early_enable) {
 663                 pr_info("enabled early\n");
 664                 WRITE_ONCE(kcsan_enabled, true);
 665         }
 666
 667         if (IS_ENABLED(CONFIG_KCSAN_REPORT_VALUE_CHANGE_ONLY) ||
 668             IS_ENABLED(CONFIG_KCSAN_ASSUME_PLAIN_WRITES_ATOMIC) ||
 669             IS_ENABLED(CONFIG_KCSAN_PERMISSIVE) ||
 670             IS_ENABLED(CONFIG_KCSAN_IGNORE_ATOMICS)) {
 671                 pr_warn("non-strict mode configured - use CONFIG_KCSAN_STRICT=y to see all data races\n");
 672         } else {
 673                 pr_info("strict mode configured\n");
 674         }
 675 }
 676
 677 /* === Exported interface =================================================== */
 678
 679 void kcsan_disable_current(void)
 680 {
 681         ++get_ctx()->disable_count;
 682 }
 683 EXPORT_SYMBOL(kcsan_disable_current);
 684
 685 void kcsan_enable_current(void)
 686 {
 687         if (get_ctx()->disable_count-- == 0) {
 688                 /*
 689                  * Warn if kcsan_enable_current() calls are unbalanced with
 690                  * kcsan_disable_current() calls, which causes disable_count to
 691                  * become negative and should not happen.
 692                  */
 693                 kcsan_disable_current(); /* restore to 0, KCSAN still enabled */
 694                 kcsan_disable_current(); /* disable to generate warning */
 695                 WARN(1, "Unbalanced %s()", __func__);
 696                 kcsan_enable_current();
 697         }
 698 }
 699 EXPORT_SYMBOL(kcsan_enable_current);
 700
 701 void kcsan_enable_current_nowarn(void)
 702 {
 703         if (get_ctx()->disable_count-- == 0)
 704                 kcsan_disable_current();
 705 }
 706 EXPORT_SYMBOL(kcsan_enable_current_nowarn);
 707
 708 void kcsan_nestable_atomic_begin(void)
 709 {
 710         /*
 711          * Do *not* check and warn if we are in a flat atomic region: nestable
 712          * and flat atomic regions are independent from each other.
 713          * See include/linux/kcsan.h: struct kcsan_ctx comments for more
 714          * comments.
 715          */
 716
 717         ++get_ctx()->atomic_nest_count;
 718 }
 719 EXPORT_SYMBOL(kcsan_nestable_atomic_begin);
 720
 721 void kcsan_nestable_atomic_end(void)
 722 {
 723         if (get_ctx()->atomic_nest_count-- == 0) {
 724                 /*
 725                  * Warn if kcsan_nestable_atomic_end() calls are unbalanced with
 726                  * kcsan_nestable_atomic_begin() calls, which causes
 727                  * atomic_nest_count to become negative and should not happen.
 728                  */
 729                 kcsan_nestable_atomic_begin(); /* restore to 0 */
 730                 kcsan_disable_current(); /* disable to generate warning */
 731                 WARN(1, "Unbalanced %s()", __func__);
 732                 kcsan_enable_current();
 733         }
 734 }
 735 EXPORT_SYMBOL(kcsan_nestable_atomic_end);
 736
 737 void kcsan_flat_atomic_begin(void)
 738 {
 739         get_ctx()->in_flat_atomic = true;
 740 }
 741 EXPORT_SYMBOL(kcsan_flat_atomic_begin);
 742
 743 void kcsan_flat_atomic_end(void)
 744 {
 745         get_ctx()->in_flat_atomic = false;
 746 }
 747 EXPORT_SYMBOL(kcsan_flat_atomic_end);
 748
 749 void kcsan_atomic_next(int n)
 750 {
 751         get_ctx()->atomic_next = n;
 752 }
 753 EXPORT_SYMBOL(kcsan_atomic_next);
 754
 755 void kcsan_set_access_mask(unsigned long mask)
 756 {
 757         get_ctx()->access_mask = mask;
 758 }
 759 EXPORT_SYMBOL(kcsan_set_access_mask);
 760
 761 struct kcsan_scoped_access *
 762 kcsan_begin_scoped_access(const volatile void *ptr, size_t size, int type,
 763                           struct kcsan_scoped_access *sa)
 764 {
 765         struct kcsan_ctx *ctx = get_ctx();
 766
 767         check_access(ptr, size, type, _RET_IP_);
 768
 769         ctx->disable_count++; /* Disable KCSAN, in case list debugging is on. */
 770
 771         INIT_LIST_HEAD(&sa->list);
 772         sa->ptr = ptr;
 773         sa->size = size;
 774         sa->type = type;
 775         sa->ip = _RET_IP_;
 776
 777         if (!ctx->scoped_accesses.prev) /* Lazy initialize list head. */
 778                 INIT_LIST_HEAD(&ctx->scoped_accesses);
 779         list_add(&sa->list, &ctx->scoped_accesses);
 780
 781         ctx->disable_count--;
 782         return sa;
 783 }
 784 EXPORT_SYMBOL(kcsan_begin_scoped_access);
 785
 786 void kcsan_end_scoped_access(struct kcsan_scoped_access *sa)
 787 {
 788         struct kcsan_ctx *ctx = get_ctx();
 789
 790         if (WARN(!ctx->scoped_accesses.prev, "Unbalanced %s()?", __func__))
 791                 return;
 792
 793         ctx->disable_count++; /* Disable KCSAN, in case list debugging is on. */
 794
 795         list_del(&sa->list);
 796         if (list_empty(&ctx->scoped_accesses))
 797                 /*
 798                  * Ensure we do not enter kcsan_check_scoped_accesses()
 799                  * slow-path if unnecessary, and avoids requiring list_empty()
 800                  * in the fast-path (to avoid a READ_ONCE() and potential
 801                  * uaccess warning).
 802                  */
 803                 ctx->scoped_accesses.prev = NULL;
 804
 805         ctx->disable_count--;
 806
 807         check_access(sa->ptr, sa->size, sa->type, sa->ip);
 808 }
 809 EXPORT_SYMBOL(kcsan_end_scoped_access);
 810
 811 void __kcsan_check_access(const volatile void *ptr, size_t size, int type)
 812 {
 813         check_access(ptr, size, type, _RET_IP_);
 814 }
 815 EXPORT_SYMBOL(__kcsan_check_access);
 816
 817 /*
 818  * KCSAN uses the same instrumentation that is emitted by supported compilers
 819  * for ThreadSanitizer (TSAN).
 820  *
 821  * When enabled, the compiler emits instrumentation calls (the functions
 822  * prefixed with "__tsan" below) for all loads and stores that it generated;
 823  * inline asm is not instrumented.
 824  *
 825  * Note that, not all supported compiler versions distinguish aligned/unaligned
 826  * accesses, but e.g. recent versions of Clang do. We simply alias the unaligned
 827  * version to the generic version, which can handle both.
 828  */
 829
 830 #define DEFINE_TSAN_READ_WRITE(size)                                           \
 831         void __tsan_read##size(void *ptr);                                     \
 832         void __tsan_read##size(void *ptr)                                      \
 833         {                                                                      \
 834                 check_access(ptr, size, 0, _RET_IP_);                          \
 835         }                                                                      \
 836         EXPORT_SYMBOL(__tsan_read##size);                                      \
 837         void __tsan_unaligned_read##size(void *ptr)                            \
 838                 __alias(__tsan_read##size);                                    \
 839         EXPORT_SYMBOL(__tsan_unaligned_read##size);                            \
 840         void __tsan_write##size(void *ptr);                                    \
 841         void __tsan_write##size(void *ptr)                                     \
 842         {                                                                      \
 843                 check_access(ptr, size, KCSAN_ACCESS_WRITE, _RET_IP_);         \
 844         }                                                                      \
 845         EXPORT_SYMBOL(__tsan_write##size);                                     \
 846         void __tsan_unaligned_write##size(void *ptr)                           \
 847                 __alias(__tsan_write##size);                                   \
 848         EXPORT_SYMBOL(__tsan_unaligned_write##size);                           \
 849         void __tsan_read_write##size(void *ptr);                               \
 850         void __tsan_read_write##size(void *ptr)                                \
 851         {                                                                      \
 852                 check_access(ptr, size,                                        \
 853                              KCSAN_ACCESS_COMPOUND | KCSAN_ACCESS_WRITE,       \
 854                              _RET_IP_);                                        \
 855         }                                                                      \
 856         EXPORT_SYMBOL(__tsan_read_write##size);                                \
 857         void __tsan_unaligned_read_write##size(void *ptr)                      \
 858                 __alias(__tsan_read_write##size);                              \
 859         EXPORT_SYMBOL(__tsan_unaligned_read_write##size)
 860
 861 DEFINE_TSAN_READ_WRITE(1);
 862 DEFINE_TSAN_READ_WRITE(2);
 863 DEFINE_TSAN_READ_WRITE(4);
 864 DEFINE_TSAN_READ_WRITE(8);
 865 DEFINE_TSAN_READ_WRITE(16);
 866
 867 void __tsan_read_range(void *ptr, size_t size);
 868 void __tsan_read_range(void *ptr, size_t size)
 869 {
 870         check_access(ptr, size, 0, _RET_IP_);
 871 }
 872 EXPORT_SYMBOL(__tsan_read_range);
 873
 874 void __tsan_write_range(void *ptr, size_t size);
 875 void __tsan_write_range(void *ptr, size_t size)
 876 {
 877         check_access(ptr, size, KCSAN_ACCESS_WRITE, _RET_IP_);
 878 }
 879 EXPORT_SYMBOL(__tsan_write_range);
 880
 881 /*
 882  * Use of explicit volatile is generally disallowed [1], however, volatile is
 883  * still used in various concurrent context, whether in low-level
 884  * synchronization primitives or for legacy reasons.
 885  * [1] https://lwn.net/Articles/233479/
 886  *
 887  * We only consider volatile accesses atomic if they are aligned and would pass
 888  * the size-check of compiletime_assert_rwonce_type().
 889  */
 890 #define DEFINE_TSAN_VOLATILE_READ_WRITE(size)                                  \
 891         void __tsan_volatile_read##size(void *ptr);                            \
 892         void __tsan_volatile_read##size(void *ptr)                             \
 893         {                                                                      \
 894                 const bool is_atomic = size <= sizeof(long long) &&            \
 895                                        IS_ALIGNED((unsigned long)ptr, size);   \
 896                 if (IS_ENABLED(CONFIG_KCSAN_IGNORE_ATOMICS) && is_atomic)      \
 897                         return;                                                \
 898                 check_access(ptr, size, is_atomic ? KCSAN_ACCESS_ATOMIC : 0,   \
 899                              _RET_IP_);                                        \
 900         }                                                                      \
 901         EXPORT_SYMBOL(__tsan_volatile_read##size);                             \
 902         void __tsan_unaligned_volatile_read##size(void *ptr)                   \
 903                 __alias(__tsan_volatile_read##size);                           \
 904         EXPORT_SYMBOL(__tsan_unaligned_volatile_read##size);                   \
 905         void __tsan_volatile_write##size(void *ptr);                           \
 906         void __tsan_volatile_write##size(void *ptr)                            \
 907         {                                                                      \
 908                 const bool is_atomic = size <= sizeof(long long) &&            \
 909                                        IS_ALIGNED((unsigned long)ptr, size);   \
 910                 if (IS_ENABLED(CONFIG_KCSAN_IGNORE_ATOMICS) && is_atomic)      \
 911                         return;                                                \
 912                 check_access(ptr, size,                                        \
 913                              KCSAN_ACCESS_WRITE |                              \
 914                                      (is_atomic ? KCSAN_ACCESS_ATOMIC : 0),    \
 915                              _RET_IP_);                                        \
 916         }                                                                      \
 917         EXPORT_SYMBOL(__tsan_volatile_write##size);                            \
 918         void __tsan_unaligned_volatile_write##size(void *ptr)                  \
 919                 __alias(__tsan_volatile_write##size);                          \
 920         EXPORT_SYMBOL(__tsan_unaligned_volatile_write##size)
 921
 922 DEFINE_TSAN_VOLATILE_READ_WRITE(1);
 923 DEFINE_TSAN_VOLATILE_READ_WRITE(2);
 924 DEFINE_TSAN_VOLATILE_READ_WRITE(4);
 925 DEFINE_TSAN_VOLATILE_READ_WRITE(8);
 926 DEFINE_TSAN_VOLATILE_READ_WRITE(16);
 927
 928 /*
 929  * The below are not required by KCSAN, but can still be emitted by the
 930  * compiler.
 931  */
 932 void __tsan_func_entry(void *call_pc);
 933 void __tsan_func_entry(void *call_pc)
 934 {
 935 }
 936 EXPORT_SYMBOL(__tsan_func_entry);
 937 void __tsan_func_exit(void);
 938 void __tsan_func_exit(void)
 939 {
 940 }
 941 EXPORT_SYMBOL(__tsan_func_exit);
 942 void __tsan_init(void);
 943 void __tsan_init(void)
 944 {
 945 }
 946 EXPORT_SYMBOL(__tsan_init);
 947
 948 /*
 949  * Instrumentation for atomic builtins (__atomic_*, __sync_*).
 950  *
 951  * Normal kernel code _should not_ be using them directly, but some
 952  * architectures may implement some or all atomics using the compilers'
 953  * builtins.
 954  *
 955  * Note: If an architecture decides to fully implement atomics using the
 956  * builtins, because they are implicitly instrumented by KCSAN (and KASAN,
 957  * etc.), implementing the ARCH_ATOMIC interface (to get instrumentation via
 958  * atomic-instrumented) is no longer necessary.
 959  *
 960  * TSAN instrumentation replaces atomic accesses with calls to any of the below
 961  * functions, whose job is to also execute the operation itself.
 962  */
 963
 964 #define DEFINE_TSAN_ATOMIC_LOAD_STORE(bits)                                                        \
 965         u##bits __tsan_atomic##bits##_load(const u##bits *ptr, int memorder);                      \
 966         u##bits __tsan_atomic##bits##_load(const u##bits *ptr, int memorder)                       \
 967         {                                                                                          \
 968                 if (!IS_ENABLED(CONFIG_KCSAN_IGNORE_ATOMICS)) {                                    \
 969                         check_access(ptr, bits / BITS_PER_BYTE, KCSAN_ACCESS_ATOMIC, _RET_IP_);    \
 970                 }                                                                                  \
 971                 return __atomic_load_n(ptr, memorder);                                             \
 972         }                                                                                          \
 973         EXPORT_SYMBOL(__tsan_atomic##bits##_load);                                                 \
 974         void __tsan_atomic##bits##_store(u##bits *ptr, u##bits v, int memorder);                   \
 975         void __tsan_atomic##bits##_store(u##bits *ptr, u##bits v, int memorder)                    \
 976         {                                                                                          \
 977                 if (!IS_ENABLED(CONFIG_KCSAN_IGNORE_ATOMICS)) {                                    \
 978                         check_access(ptr, bits / BITS_PER_BYTE,                                    \
 979                                      KCSAN_ACCESS_WRITE | KCSAN_ACCESS_ATOMIC, _RET_IP_);          \
 980                 }                                                                                  \
 981                 __atomic_store_n(ptr, v, memorder);                                                \
 982         }                                                                                          \
 983         EXPORT_SYMBOL(__tsan_atomic##bits##_store)
 984
 985 #define DEFINE_TSAN_ATOMIC_RMW(op, bits, suffix)                                                   \
 986         u##bits __tsan_atomic##bits##_##op(u##bits *ptr, u##bits v, int memorder);                 \
 987         u##bits __tsan_atomic##bits##_##op(u##bits *ptr, u##bits v, int memorder)                  \
 988         {                                                                                          \
 989                 if (!IS_ENABLED(CONFIG_KCSAN_IGNORE_ATOMICS)) {                                    \
 990                         check_access(ptr, bits / BITS_PER_BYTE,                                    \
 991                                      KCSAN_ACCESS_COMPOUND | KCSAN_ACCESS_WRITE |                  \
 992                                              KCSAN_ACCESS_ATOMIC, _RET_IP_);                       \
 993                 }                                                                                  \
 994                 return __atomic_##op##suffix(ptr, v, memorder);                                    \
 995         }                                                                                          \
 996         EXPORT_SYMBOL(__tsan_atomic##bits##_##op)
 997
 998 /*
 999  * Note: CAS operations are always classified as write, even in case they
1000  * fail. We cannot perform check_access() after a write, as it might lead to
1001  * false positives, in cases such as:
1002  *
1003  *      T0: __atomic_compare_exchange_n(&p->flag, &old, 1, ...)
1004  *
1005  *      T1: if (__atomic_load_n(&p->flag, ...)) {
1006  *              modify *p;
1007  *              p->flag = 0;
1008  *          }
1009  *
1010  * The only downside is that, if there are 3 threads, with one CAS that
1011  * succeeds, another CAS that fails, and an unmarked racing operation, we may
1012  * point at the wrong CAS as the source of the race. However, if we assume that
1013  * all CAS can succeed in some other execution, the data race is still valid.
1014  */
1015 #define DEFINE_TSAN_ATOMIC_CMPXCHG(bits, strength, weak)                                           \
1016         int __tsan_atomic##bits##_compare_exchange_##strength(u##bits *ptr, u##bits *exp,          \
1017                                                               u##bits val, int mo, int fail_mo);   \
1018         int __tsan_atomic##bits##_compare_exchange_##strength(u##bits *ptr, u##bits *exp,          \
1019                                                               u##bits val, int mo, int fail_mo)    \
1020         {                                                                                          \
1021                 if (!IS_ENABLED(CONFIG_KCSAN_IGNORE_ATOMICS)) {                                    \
1022                         check_access(ptr, bits / BITS_PER_BYTE,                                    \
1023                                      KCSAN_ACCESS_COMPOUND | KCSAN_ACCESS_WRITE |                  \
1024                                              KCSAN_ACCESS_ATOMIC, _RET_IP_);                       \
1025                 }                                                                                  \
1026                 return __atomic_compare_exchange_n(ptr, exp, val, weak, mo, fail_mo);              \
1027         }                                                                                          \
1028         EXPORT_SYMBOL(__tsan_atomic##bits##_compare_exchange_##strength)
1029
1030 #define DEFINE_TSAN_ATOMIC_CMPXCHG_VAL(bits)                                                       \
1031         u##bits __tsan_atomic##bits##_compare_exchange_val(u##bits *ptr, u##bits exp, u##bits val, \
1032                                                            int mo, int fail_mo);                   \
1033         u##bits __tsan_atomic##bits##_compare_exchange_val(u##bits *ptr, u##bits exp, u##bits val, \
1034                                                            int mo, int fail_mo)                    \
1035         {                                                                                          \
1036                 if (!IS_ENABLED(CONFIG_KCSAN_IGNORE_ATOMICS)) {                                    \
1037                         check_access(ptr, bits / BITS_PER_BYTE,                                    \
1038                                      KCSAN_ACCESS_COMPOUND | KCSAN_ACCESS_WRITE |                  \
1039                                              KCSAN_ACCESS_ATOMIC, _RET_IP_);                       \
1040                 }                                                                                  \
1041                 __atomic_compare_exchange_n(ptr, &exp, val, 0, mo, fail_mo);                       \
1042                 return exp;                                                                        \
1043         }                                                                                          \
1044         EXPORT_SYMBOL(__tsan_atomic##bits##_compare_exchange_val)
1045
1046 #define DEFINE_TSAN_ATOMIC_OPS(bits)                                                               \
1047         DEFINE_TSAN_ATOMIC_LOAD_STORE(bits);                                                       \
1048         DEFINE_TSAN_ATOMIC_RMW(exchange, bits, _n);                                                \
1049         DEFINE_TSAN_ATOMIC_RMW(fetch_add, bits, );                                                 \
1050         DEFINE_TSAN_ATOMIC_RMW(fetch_sub, bits, );                                                 \
1051         DEFINE_TSAN_ATOMIC_RMW(fetch_and, bits, );                                                 \
1052         DEFINE_TSAN_ATOMIC_RMW(fetch_or, bits, );                                                  \
1053         DEFINE_TSAN_ATOMIC_RMW(fetch_xor, bits, );                                                 \
1054         DEFINE_TSAN_ATOMIC_RMW(fetch_nand, bits, );                                                \
1055         DEFINE_TSAN_ATOMIC_CMPXCHG(bits, strong, 0);                                               \
1056         DEFINE_TSAN_ATOMIC_CMPXCHG(bits, weak, 1);                                                 \
1057         DEFINE_TSAN_ATOMIC_CMPXCHG_VAL(bits)
1058
1059 DEFINE_TSAN_ATOMIC_OPS(8);
1060 DEFINE_TSAN_ATOMIC_OPS(16);
1061 DEFINE_TSAN_ATOMIC_OPS(32);
1062 DEFINE_TSAN_ATOMIC_OPS(64);
1063
1064 void __tsan_atomic_thread_fence(int memorder);
1065 void __tsan_atomic_thread_fence(int memorder)
1066 {
1067         __atomic_thread_fence(memorder);
1068 }
1069 EXPORT_SYMBOL(__tsan_atomic_thread_fence);
1070
1071 void __tsan_atomic_signal_fence(int memorder);
1072 void __tsan_atomic_signal_fence(int memorder) { }
1073 EXPORT_SYMBOL(__tsan_atomic_signal_fence);