kernel/kcsan/core.c

   1 // SPDX-License-Identifier: GPL-2.0
   2
   3 #define pr_fmt(fmt) "kcsan: " fmt
   4
   5 #include <linux/atomic.h>
   6 #include <linux/bug.h>
   7 #include <linux/delay.h>
   8 #include <linux/export.h>
   9 #include <linux/init.h>
  10 #include <linux/kernel.h>
  11 #include <linux/list.h>
  12 #include <linux/moduleparam.h>
  13 #include <linux/percpu.h>
  14 #include <linux/preempt.h>
  15 #include <linux/random.h>
  16 #include <linux/sched.h>
  17 #include <linux/uaccess.h>
  18
  19 #include "atomic.h"
  20 #include "encoding.h"
  21 #include "kcsan.h"
  22
  23 static bool kcsan_early_enable = IS_ENABLED(CONFIG_KCSAN_EARLY_ENABLE);
  24 unsigned int kcsan_udelay_task = CONFIG_KCSAN_UDELAY_TASK;
  25 unsigned int kcsan_udelay_interrupt = CONFIG_KCSAN_UDELAY_INTERRUPT;
  26 static long kcsan_skip_watch = CONFIG_KCSAN_SKIP_WATCH;
  27 static bool kcsan_interrupt_watcher = IS_ENABLED(CONFIG_KCSAN_INTERRUPT_WATCHER);
  28
  29 #ifdef MODULE_PARAM_PREFIX
  30 #undef MODULE_PARAM_PREFIX
  31 #endif
  32 #define MODULE_PARAM_PREFIX "kcsan."
  33 module_param_named(early_enable, kcsan_early_enable, bool, 0);
  34 module_param_named(udelay_task, kcsan_udelay_task, uint, 0644);
  35 module_param_named(udelay_interrupt, kcsan_udelay_interrupt, uint, 0644);
  36 module_param_named(skip_watch, kcsan_skip_watch, long, 0644);
  37 module_param_named(interrupt_watcher, kcsan_interrupt_watcher, bool, 0444);
  38
  39 bool kcsan_enabled;
  40
  41 /* Per-CPU kcsan_ctx for interrupts */
  42 static DEFINE_PER_CPU(struct kcsan_ctx, kcsan_cpu_ctx) = {
  43         .disable_count          = 0,
  44         .atomic_next            = 0,
  45         .atomic_nest_count      = 0,
  46         .in_flat_atomic         = false,
  47         .access_mask            = 0,
  48         .scoped_accesses        = {LIST_POISON1, NULL},
  49 };
  50
  51 /*
  52  * Helper macros to index into adjacent slots, starting from address slot
  53  * itself, followed by the right and left slots.
  54  *
  55  * The purpose is 2-fold:
  56  *
  57  *      1. if during insertion the address slot is already occupied, check if
  58  *         any adjacent slots are free;
  59  *      2. accesses that straddle a slot boundary due to size that exceeds a
  60  *         slot's range may check adjacent slots if any watchpoint matches.
  61  *
  62  * Note that accesses with very large size may still miss a watchpoint; however,
  63  * given this should be rare, this is a reasonable trade-off to make, since this
  64  * will avoid:
  65  *
  66  *      1. excessive contention between watchpoint checks and setup;
  67  *      2. larger number of simultaneous watchpoints without sacrificing
  68  *         performance.
  69  *
  70  * Example: SLOT_IDX values for KCSAN_CHECK_ADJACENT=1, where i is [0, 1, 2]:
  71  *
  72  *   slot=0:  [ 1,  2,  0]
  73  *   slot=9:  [10, 11,  9]
  74  *   slot=63: [64, 65, 63]
  75  */
  76 #define SLOT_IDX(slot, i) (slot + ((i + KCSAN_CHECK_ADJACENT) % NUM_SLOTS))
  77
  78 /*
  79  * SLOT_IDX_FAST is used in the fast-path. Not first checking the address's primary
  80  * slot (middle) is fine if we assume that races occur rarely. The set of
  81  * indices {SLOT_IDX(slot, i) | i in [0, NUM_SLOTS)} is equivalent to
  82  * {SLOT_IDX_FAST(slot, i) | i in [0, NUM_SLOTS)}.
  83  */
  84 #define SLOT_IDX_FAST(slot, i) (slot + i)
  85
  86 /*
  87  * Watchpoints, with each entry encoded as defined in encoding.h: in order to be
  88  * able to safely update and access a watchpoint without introducing locking
  89  * overhead, we encode each watchpoint as a single atomic long. The initial
  90  * zero-initialized state matches INVALID_WATCHPOINT.
  91  *
  92  * Add NUM_SLOTS-1 entries to account for overflow; this helps avoid having to
  93  * use more complicated SLOT_IDX_FAST calculation with modulo in the fast-path.
  94  */
  95 static atomic_long_t watchpoints[CONFIG_KCSAN_NUM_WATCHPOINTS + NUM_SLOTS-1];
  96
  97 /*
  98  * Instructions to skip watching counter, used in should_watch(). We use a
  99  * per-CPU counter to avoid excessive contention.
 100  */
 101 static DEFINE_PER_CPU(long, kcsan_skip);
 102
 103 /* For kcsan_prandom_u32_max(). */
 104 static DEFINE_PER_CPU(struct rnd_state, kcsan_rand_state);
 105
 106 static __always_inline atomic_long_t *find_watchpoint(unsigned long addr,
 107                                                       size_t size,
 108                                                       bool expect_write,
 109                                                       long *encoded_watchpoint)
 110 {
 111         const int slot = watchpoint_slot(addr);
 112         const unsigned long addr_masked = addr & WATCHPOINT_ADDR_MASK;
 113         atomic_long_t *watchpoint;
 114         unsigned long wp_addr_masked;
 115         size_t wp_size;
 116         bool is_write;
 117         int i;
 118
 119         BUILD_BUG_ON(CONFIG_KCSAN_NUM_WATCHPOINTS < NUM_SLOTS);
 120
 121         for (i = 0; i < NUM_SLOTS; ++i) {
 122                 watchpoint = &watchpoints[SLOT_IDX_FAST(slot, i)];
 123                 *encoded_watchpoint = atomic_long_read(watchpoint);
 124                 if (!decode_watchpoint(*encoded_watchpoint, &wp_addr_masked,
 125                                        &wp_size, &is_write))
 126                         continue;
 127
 128                 if (expect_write && !is_write)
 129                         continue;
 130
 131                 /* Check if the watchpoint matches the access. */
 132                 if (matching_access(wp_addr_masked, wp_size, addr_masked, size))
 133                         return watchpoint;
 134         }
 135
 136         return NULL;
 137 }
 138
 139 static inline atomic_long_t *
 140 insert_watchpoint(unsigned long addr, size_t size, bool is_write)
 141 {
 142         const int slot = watchpoint_slot(addr);
 143         const long encoded_watchpoint = encode_watchpoint(addr, size, is_write);
 144         atomic_long_t *watchpoint;
 145         int i;
 146
 147         /* Check slot index logic, ensuring we stay within array bounds. */
 148         BUILD_BUG_ON(SLOT_IDX(0, 0) != KCSAN_CHECK_ADJACENT);
 149         BUILD_BUG_ON(SLOT_IDX(0, KCSAN_CHECK_ADJACENT+1) != 0);
 150         BUILD_BUG_ON(SLOT_IDX(CONFIG_KCSAN_NUM_WATCHPOINTS-1, KCSAN_CHECK_ADJACENT) != ARRAY_SIZE(watchpoints)-1);
 151         BUILD_BUG_ON(SLOT_IDX(CONFIG_KCSAN_NUM_WATCHPOINTS-1, KCSAN_CHECK_ADJACENT+1) != ARRAY_SIZE(watchpoints) - NUM_SLOTS);
 152
 153         for (i = 0; i < NUM_SLOTS; ++i) {
 154                 long expect_val = INVALID_WATCHPOINT;
 155
 156                 /* Try to acquire this slot. */
 157                 watchpoint = &watchpoints[SLOT_IDX(slot, i)];
 158                 if (atomic_long_try_cmpxchg_relaxed(watchpoint, &expect_val, encoded_watchpoint))
 159                         return watchpoint;
 160         }
 161
 162         return NULL;
 163 }
 164
 165 /*
 166  * Return true if watchpoint was successfully consumed, false otherwise.
 167  *
 168  * This may return false if:
 169  *
 170  *      1. another thread already consumed the watchpoint;
 171  *      2. the thread that set up the watchpoint already removed it;
 172  *      3. the watchpoint was removed and then re-used.
 173  */
 174 static __always_inline bool
 175 try_consume_watchpoint(atomic_long_t *watchpoint, long encoded_watchpoint)
 176 {
 177         return atomic_long_try_cmpxchg_relaxed(watchpoint, &encoded_watchpoint, CONSUMED_WATCHPOINT);
 178 }
 179
 180 /* Return true if watchpoint was not touched, false if already consumed. */
 181 static inline bool consume_watchpoint(atomic_long_t *watchpoint)
 182 {
 183         return atomic_long_xchg_relaxed(watchpoint, CONSUMED_WATCHPOINT) != CONSUMED_WATCHPOINT;
 184 }
 185
 186 /* Remove the watchpoint -- its slot may be reused after. */
 187 static inline void remove_watchpoint(atomic_long_t *watchpoint)
 188 {
 189         atomic_long_set(watchpoint, INVALID_WATCHPOINT);
 190 }
 191
 192 static __always_inline struct kcsan_ctx *get_ctx(void)
 193 {
 194         /*
 195          * In interrupts, use raw_cpu_ptr to avoid unnecessary checks, that would
 196          * also result in calls that generate warnings in uaccess regions.
 197          */
 198         return in_task() ? &current->kcsan_ctx : raw_cpu_ptr(&kcsan_cpu_ctx);
 199 }
 200
 201 /* Check scoped accesses; never inline because this is a slow-path! */
 202 static noinline void kcsan_check_scoped_accesses(void)
 203 {
 204         struct kcsan_ctx *ctx = get_ctx();
 205         struct list_head *prev_save = ctx->scoped_accesses.prev;
 206         struct kcsan_scoped_access *scoped_access;
 207
 208         ctx->scoped_accesses.prev = NULL;  /* Avoid recursion. */
 209         list_for_each_entry(scoped_access, &ctx->scoped_accesses, list)
 210                 __kcsan_check_access(scoped_access->ptr, scoped_access->size, scoped_access->type);
 211         ctx->scoped_accesses.prev = prev_save;
 212 }
 213
 214 /* Rules for generic atomic accesses. Called from fast-path. */
 215 static __always_inline bool
 216 is_atomic(const volatile void *ptr, size_t size, int type, struct kcsan_ctx *ctx)
 217 {
 218         if (type & KCSAN_ACCESS_ATOMIC)
 219                 return true;
 220
 221         /*
 222          * Unless explicitly declared atomic, never consider an assertion access
 223          * as atomic. This allows using them also in atomic regions, such as
 224          * seqlocks, without implicitly changing their semantics.
 225          */
 226         if (type & KCSAN_ACCESS_ASSERT)
 227                 return false;
 228
 229         if (IS_ENABLED(CONFIG_KCSAN_ASSUME_PLAIN_WRITES_ATOMIC) &&
 230             (type & KCSAN_ACCESS_WRITE) && size <= sizeof(long) &&
 231             !(type & KCSAN_ACCESS_COMPOUND) && IS_ALIGNED((unsigned long)ptr, size))
 232                 return true; /* Assume aligned writes up to word size are atomic. */
 233
 234         if (ctx->atomic_next > 0) {
 235                 /*
 236                  * Because we do not have separate contexts for nested
 237                  * interrupts, in case atomic_next is set, we simply assume that
 238                  * the outer interrupt set atomic_next. In the worst case, we
 239                  * will conservatively consider operations as atomic. This is a
 240                  * reasonable trade-off to make, since this case should be
 241                  * extremely rare; however, even if extremely rare, it could
 242                  * lead to false positives otherwise.
 243                  */
 244                 if ((hardirq_count() >> HARDIRQ_SHIFT) < 2)
 245                         --ctx->atomic_next; /* in task, or outer interrupt */
 246                 return true;
 247         }
 248
 249         return ctx->atomic_nest_count > 0 || ctx->in_flat_atomic;
 250 }
 251
 252 static __always_inline bool
 253 should_watch(const volatile void *ptr, size_t size, int type, struct kcsan_ctx *ctx)
 254 {
 255         /*
 256          * Never set up watchpoints when memory operations are atomic.
 257          *
 258          * Need to check this first, before kcsan_skip check below: (1) atomics
 259          * should not count towards skipped instructions, and (2) to actually
 260          * decrement kcsan_atomic_next for consecutive instruction stream.
 261          */
 262         if (is_atomic(ptr, size, type, ctx))
 263                 return false;
 264
 265         if (this_cpu_dec_return(kcsan_skip) >= 0)
 266                 return false;
 267
 268         /*
 269          * NOTE: If we get here, kcsan_skip must always be reset in slow path
 270          * via reset_kcsan_skip() to avoid underflow.
 271          */
 272
 273         /* this operation should be watched */
 274         return true;
 275 }
 276
 277 /*
 278  * Returns a pseudo-random number in interval [0, ep_ro). See prandom_u32_max()
 279  * for more details.
 280  *
 281  * The open-coded version here is using only safe primitives for all contexts
 282  * where we can have KCSAN instrumentation. In particular, we cannot use
 283  * prandom_u32() directly, as its tracepoint could cause recursion.
 284  */
 285 static u32 kcsan_prandom_u32_max(u32 ep_ro)
 286 {
 287         struct rnd_state *state = &get_cpu_var(kcsan_rand_state);
 288         const u32 res = prandom_u32_state(state);
 289
 290         put_cpu_var(kcsan_rand_state);
 291         return (u32)(((u64) res * ep_ro) >> 32);
 292 }
 293
 294 static inline void reset_kcsan_skip(void)
 295 {
 296         long skip_count = kcsan_skip_watch -
 297                           (IS_ENABLED(CONFIG_KCSAN_SKIP_WATCH_RANDOMIZE) ?
 298                                    kcsan_prandom_u32_max(kcsan_skip_watch) :
 299                                    0);
 300         this_cpu_write(kcsan_skip, skip_count);
 301 }
 302
 303 static __always_inline bool kcsan_is_enabled(void)
 304 {
 305         return READ_ONCE(kcsan_enabled) && get_ctx()->disable_count == 0;
 306 }
 307
 308 /* Introduce delay depending on context and configuration. */
 309 static void delay_access(int type)
 310 {
 311         unsigned int delay = in_task() ? kcsan_udelay_task : kcsan_udelay_interrupt;
 312         /* For certain access types, skew the random delay to be longer. */
 313         unsigned int skew_delay_order =
 314                 (type & (KCSAN_ACCESS_COMPOUND | KCSAN_ACCESS_ASSERT)) ? 1 : 0;
 315
 316         delay -= IS_ENABLED(CONFIG_KCSAN_DELAY_RANDOMIZE) ?
 317                                kcsan_prandom_u32_max(delay >> skew_delay_order) :
 318                                0;
 319         udelay(delay);
 320 }
 321
 322 void kcsan_save_irqtrace(struct task_struct *task)
 323 {
 324 #ifdef CONFIG_TRACE_IRQFLAGS
 325         task->kcsan_save_irqtrace = task->irqtrace;
 326 #endif
 327 }
 328
 329 void kcsan_restore_irqtrace(struct task_struct *task)
 330 {
 331 #ifdef CONFIG_TRACE_IRQFLAGS
 332         task->irqtrace = task->kcsan_save_irqtrace;
 333 #endif
 334 }
 335
 336 /*
 337  * Pull everything together: check_access() below contains the performance
 338  * critical operations; the fast-path (including check_access) functions should
 339  * all be inlinable by the instrumentation functions.
 340  *
 341  * The slow-path (kcsan_found_watchpoint, kcsan_setup_watchpoint) are
 342  * non-inlinable -- note that, we prefix these with "kcsan_" to ensure they can
 343  * be filtered from the stacktrace, as well as give them unique names for the
 344  * UACCESS whitelist of objtool. Each function uses user_access_save/restore(),
 345  * since they do not access any user memory, but instrumentation is still
 346  * emitted in UACCESS regions.
 347  */
 348
 349 static noinline void kcsan_found_watchpoint(const volatile void *ptr,
 350                                             size_t size,
 351                                             int type,
 352                                             atomic_long_t *watchpoint,
 353                                             long encoded_watchpoint)
 354 {
 355         unsigned long flags;
 356         bool consumed;
 357
 358         if (!kcsan_is_enabled())
 359                 return;
 360
 361         /*
 362          * The access_mask check relies on value-change comparison. To avoid
 363          * reporting a race where e.g. the writer set up the watchpoint, but the
 364          * reader has access_mask!=0, we have to ignore the found watchpoint.
 365          */
 366         if (get_ctx()->access_mask != 0)
 367                 return;
 368
 369         /*
 370          * Consume the watchpoint as soon as possible, to minimize the chances
 371          * of !consumed. Consuming the watchpoint must always be guarded by
 372          * kcsan_is_enabled() check, as otherwise we might erroneously
 373          * triggering reports when disabled.
 374          */
 375         consumed = try_consume_watchpoint(watchpoint, encoded_watchpoint);
 376
 377         /* keep this after try_consume_watchpoint */
 378         flags = user_access_save();
 379
 380         if (consumed) {
 381                 kcsan_save_irqtrace(current);
 382                 kcsan_report(ptr, size, type, KCSAN_VALUE_CHANGE_MAYBE,
 383                              KCSAN_REPORT_CONSUMED_WATCHPOINT,
 384                              watchpoint - watchpoints);
 385                 kcsan_restore_irqtrace(current);
 386         } else {
 387                 /*
 388                  * The other thread may not print any diagnostics, as it has
 389                  * already removed the watchpoint, or another thread consumed
 390                  * the watchpoint before this thread.
 391                  */
 392                 atomic_long_inc(&kcsan_counters[KCSAN_COUNTER_REPORT_RACES]);
 393         }
 394
 395         if ((type & KCSAN_ACCESS_ASSERT) != 0)
 396                 atomic_long_inc(&kcsan_counters[KCSAN_COUNTER_ASSERT_FAILURES]);
 397         else
 398                 atomic_long_inc(&kcsan_counters[KCSAN_COUNTER_DATA_RACES]);
 399
 400         user_access_restore(flags);
 401 }
 402
 403 static noinline void
 404 kcsan_setup_watchpoint(const volatile void *ptr, size_t size, int type)
 405 {
 406         const bool is_write = (type & KCSAN_ACCESS_WRITE) != 0;
 407         const bool is_assert = (type & KCSAN_ACCESS_ASSERT) != 0;
 408         atomic_long_t *watchpoint;
 409         union {
 410                 u8 _1;
 411                 u16 _2;
 412                 u32 _4;
 413                 u64 _8;
 414         } expect_value;
 415         unsigned long access_mask;
 416         enum kcsan_value_change value_change = KCSAN_VALUE_CHANGE_MAYBE;
 417         unsigned long ua_flags = user_access_save();
 418         unsigned long irq_flags = 0;
 419
 420         /*
 421          * Always reset kcsan_skip counter in slow-path to avoid underflow; see
 422          * should_watch().
 423          */
 424         reset_kcsan_skip();
 425
 426         if (!kcsan_is_enabled())
 427                 goto out;
 428
 429         /*
 430          * Special atomic rules: unlikely to be true, so we check them here in
 431          * the slow-path, and not in the fast-path in is_atomic(). Call after
 432          * kcsan_is_enabled(), as we may access memory that is not yet
 433          * initialized during early boot.
 434          */
 435         if (!is_assert && kcsan_is_atomic_special(ptr))
 436                 goto out;
 437
 438         if (!check_encodable((unsigned long)ptr, size)) {
 439                 atomic_long_inc(&kcsan_counters[KCSAN_COUNTER_UNENCODABLE_ACCESSES]);
 440                 goto out;
 441         }
 442
 443         /*
 444          * Save and restore the IRQ state trace touched by KCSAN, since KCSAN's
 445          * runtime is entered for every memory access, and potentially useful
 446          * information is lost if dirtied by KCSAN.
 447          */
 448         kcsan_save_irqtrace(current);
 449         if (!kcsan_interrupt_watcher)
 450                 local_irq_save(irq_flags);
 451
 452         watchpoint = insert_watchpoint((unsigned long)ptr, size, is_write);
 453         if (watchpoint == NULL) {
 454                 /*
 455                  * Out of capacity: the size of 'watchpoints', and the frequency
 456                  * with which should_watch() returns true should be tweaked so
 457                  * that this case happens very rarely.
 458                  */
 459                 atomic_long_inc(&kcsan_counters[KCSAN_COUNTER_NO_CAPACITY]);
 460                 goto out_unlock;
 461         }
 462
 463         atomic_long_inc(&kcsan_counters[KCSAN_COUNTER_SETUP_WATCHPOINTS]);
 464         atomic_long_inc(&kcsan_counters[KCSAN_COUNTER_USED_WATCHPOINTS]);
 465
 466         /*
 467          * Read the current value, to later check and infer a race if the data
 468          * was modified via a non-instrumented access, e.g. from a device.
 469          */
 470         expect_value._8 = 0;
 471         switch (size) {
 472         case 1:
 473                 expect_value._1 = READ_ONCE(*(const u8 *)ptr);
 474                 break;
 475         case 2:
 476                 expect_value._2 = READ_ONCE(*(const u16 *)ptr);
 477                 break;
 478         case 4:
 479                 expect_value._4 = READ_ONCE(*(const u32 *)ptr);
 480                 break;
 481         case 8:
 482                 expect_value._8 = READ_ONCE(*(const u64 *)ptr);
 483                 break;
 484         default:
 485                 break; /* ignore; we do not diff the values */
 486         }
 487
 488         if (IS_ENABLED(CONFIG_KCSAN_DEBUG)) {
 489                 kcsan_disable_current();
 490                 pr_err("watching %s, size: %zu, addr: %px [slot: %d, encoded: %lx]\n",
 491                        is_write ? "write" : "read", size, ptr,
 492                        watchpoint_slot((unsigned long)ptr),
 493                        encode_watchpoint((unsigned long)ptr, size, is_write));
 494                 kcsan_enable_current();
 495         }
 496
 497         /*
 498          * Delay this thread, to increase probability of observing a racy
 499          * conflicting access.
 500          */
 501         delay_access(type);
 502
 503         /*
 504          * Re-read value, and check if it is as expected; if not, we infer a
 505          * racy access.
 506          */
 507         access_mask = get_ctx()->access_mask;
 508         switch (size) {
 509         case 1:
 510                 expect_value._1 ^= READ_ONCE(*(const u8 *)ptr);
 511                 if (access_mask)
 512                         expect_value._1 &= (u8)access_mask;
 513                 break;
 514         case 2:
 515                 expect_value._2 ^= READ_ONCE(*(const u16 *)ptr);
 516                 if (access_mask)
 517                         expect_value._2 &= (u16)access_mask;
 518                 break;
 519         case 4:
 520                 expect_value._4 ^= READ_ONCE(*(const u32 *)ptr);
 521                 if (access_mask)
 522                         expect_value._4 &= (u32)access_mask;
 523                 break;
 524         case 8:
 525                 expect_value._8 ^= READ_ONCE(*(const u64 *)ptr);
 526                 if (access_mask)
 527                         expect_value._8 &= (u64)access_mask;
 528                 break;
 529         default:
 530                 break; /* ignore; we do not diff the values */
 531         }
 532
 533         /* Were we able to observe a value-change? */
 534         if (expect_value._8 != 0)
 535                 value_change = KCSAN_VALUE_CHANGE_TRUE;
 536
 537         /* Check if this access raced with another. */
 538         if (!consume_watchpoint(watchpoint)) {
 539                 /*
 540                  * Depending on the access type, map a value_change of MAYBE to
 541                  * TRUE (always report) or FALSE (never report).
 542                  */
 543                 if (value_change == KCSAN_VALUE_CHANGE_MAYBE) {
 544                         if (access_mask != 0) {
 545                                 /*
 546                                  * For access with access_mask, we require a
 547                                  * value-change, as it is likely that races on
 548                                  * ~access_mask bits are expected.
 549                                  */
 550                                 value_change = KCSAN_VALUE_CHANGE_FALSE;
 551                         } else if (size > 8 || is_assert) {
 552                                 /* Always assume a value-change. */
 553                                 value_change = KCSAN_VALUE_CHANGE_TRUE;
 554                         }
 555                 }
 556
 557                 /*
 558                  * No need to increment 'data_races' counter, as the racing
 559                  * thread already did.
 560                  *
 561                  * Count 'assert_failures' for each failed ASSERT access,
 562                  * therefore both this thread and the racing thread may
 563                  * increment this counter.
 564                  */
 565                 if (is_assert && value_change == KCSAN_VALUE_CHANGE_TRUE)
 566                         atomic_long_inc(&kcsan_counters[KCSAN_COUNTER_ASSERT_FAILURES]);
 567
 568                 kcsan_report(ptr, size, type, value_change, KCSAN_REPORT_RACE_SIGNAL,
 569                              watchpoint - watchpoints);
 570         } else if (value_change == KCSAN_VALUE_CHANGE_TRUE) {
 571                 /* Inferring a race, since the value should not have changed. */
 572
 573                 atomic_long_inc(&kcsan_counters[KCSAN_COUNTER_RACES_UNKNOWN_ORIGIN]);
 574                 if (is_assert)
 575                         atomic_long_inc(&kcsan_counters[KCSAN_COUNTER_ASSERT_FAILURES]);
 576
 577                 if (IS_ENABLED(CONFIG_KCSAN_REPORT_RACE_UNKNOWN_ORIGIN) || is_assert)
 578                         kcsan_report(ptr, size, type, KCSAN_VALUE_CHANGE_TRUE,
 579                                      KCSAN_REPORT_RACE_UNKNOWN_ORIGIN,
 580                                      watchpoint - watchpoints);
 581         }
 582
 583         /*
 584          * Remove watchpoint; must be after reporting, since the slot may be
 585          * reused after this point.
 586          */
 587         remove_watchpoint(watchpoint);
 588         atomic_long_dec(&kcsan_counters[KCSAN_COUNTER_USED_WATCHPOINTS]);
 589 out_unlock:
 590         if (!kcsan_interrupt_watcher)
 591                 local_irq_restore(irq_flags);
 592         kcsan_restore_irqtrace(current);
 593 out:
 594         user_access_restore(ua_flags);
 595 }
 596
 597 static __always_inline void check_access(const volatile void *ptr, size_t size,
 598                                          int type)
 599 {
 600         const bool is_write = (type & KCSAN_ACCESS_WRITE) != 0;
 601         atomic_long_t *watchpoint;
 602         long encoded_watchpoint;
 603
 604         /*
 605          * Do nothing for 0 sized check; this comparison will be optimized out
 606          * for constant sized instrumentation (__tsan_{read,write}N).
 607          */
 608         if (unlikely(size == 0))
 609                 return;
 610
 611         /*
 612          * Avoid user_access_save in fast-path: find_watchpoint is safe without
 613          * user_access_save, as the address that ptr points to is only used to
 614          * check if a watchpoint exists; ptr is never dereferenced.
 615          */
 616         watchpoint = find_watchpoint((unsigned long)ptr, size, !is_write,
 617                                      &encoded_watchpoint);
 618         /*
 619          * It is safe to check kcsan_is_enabled() after find_watchpoint in the
 620          * slow-path, as long as no state changes that cause a race to be
 621          * detected and reported have occurred until kcsan_is_enabled() is
 622          * checked.
 623          */
 624
 625         if (unlikely(watchpoint != NULL))
 626                 kcsan_found_watchpoint(ptr, size, type, watchpoint,
 627                                        encoded_watchpoint);
 628         else {
 629                 struct kcsan_ctx *ctx = get_ctx(); /* Call only once in fast-path. */
 630
 631                 if (unlikely(should_watch(ptr, size, type, ctx)))
 632                         kcsan_setup_watchpoint(ptr, size, type);
 633                 else if (unlikely(ctx->scoped_accesses.prev))
 634                         kcsan_check_scoped_accesses();
 635         }
 636 }
 637
 638 /* === Public interface ===================================================== */
 639
 640 void __init kcsan_init(void)
 641 {
 642         BUG_ON(!in_task());
 643
 644         kcsan_debugfs_init();
 645         prandom_seed_full_state(&kcsan_rand_state);
 646
 647         /*
 648          * We are in the init task, and no other tasks should be running;
 649          * WRITE_ONCE without memory barrier is sufficient.
 650          */
 651         if (kcsan_early_enable) {
 652                 pr_info("enabled early\n");
 653                 WRITE_ONCE(kcsan_enabled, true);
 654         }
 655 }
 656
 657 /* === Exported interface =================================================== */
 658
 659 void kcsan_disable_current(void)
 660 {
 661         ++get_ctx()->disable_count;
 662 }
 663 EXPORT_SYMBOL(kcsan_disable_current);
 664
 665 void kcsan_enable_current(void)
 666 {
 667         if (get_ctx()->disable_count-- == 0) {
 668                 /*
 669                  * Warn if kcsan_enable_current() calls are unbalanced with
 670                  * kcsan_disable_current() calls, which causes disable_count to
 671                  * become negative and should not happen.
 672                  */
 673                 kcsan_disable_current(); /* restore to 0, KCSAN still enabled */
 674                 kcsan_disable_current(); /* disable to generate warning */
 675                 WARN(1, "Unbalanced %s()", __func__);
 676                 kcsan_enable_current();
 677         }
 678 }
 679 EXPORT_SYMBOL(kcsan_enable_current);
 680
 681 void kcsan_enable_current_nowarn(void)
 682 {
 683         if (get_ctx()->disable_count-- == 0)
 684                 kcsan_disable_current();
 685 }
 686 EXPORT_SYMBOL(kcsan_enable_current_nowarn);
 687
 688 void kcsan_nestable_atomic_begin(void)
 689 {
 690         /*
 691          * Do *not* check and warn if we are in a flat atomic region: nestable
 692          * and flat atomic regions are independent from each other.
 693          * See include/linux/kcsan.h: struct kcsan_ctx comments for more
 694          * comments.
 695          */
 696
 697         ++get_ctx()->atomic_nest_count;
 698 }
 699 EXPORT_SYMBOL(kcsan_nestable_atomic_begin);
 700
 701 void kcsan_nestable_atomic_end(void)
 702 {
 703         if (get_ctx()->atomic_nest_count-- == 0) {
 704                 /*
 705                  * Warn if kcsan_nestable_atomic_end() calls are unbalanced with
 706                  * kcsan_nestable_atomic_begin() calls, which causes
 707                  * atomic_nest_count to become negative and should not happen.
 708                  */
 709                 kcsan_nestable_atomic_begin(); /* restore to 0 */
 710                 kcsan_disable_current(); /* disable to generate warning */
 711                 WARN(1, "Unbalanced %s()", __func__);
 712                 kcsan_enable_current();
 713         }
 714 }
 715 EXPORT_SYMBOL(kcsan_nestable_atomic_end);
 716
 717 void kcsan_flat_atomic_begin(void)
 718 {
 719         get_ctx()->in_flat_atomic = true;
 720 }
 721 EXPORT_SYMBOL(kcsan_flat_atomic_begin);
 722
 723 void kcsan_flat_atomic_end(void)
 724 {
 725         get_ctx()->in_flat_atomic = false;
 726 }
 727 EXPORT_SYMBOL(kcsan_flat_atomic_end);
 728
 729 void kcsan_atomic_next(int n)
 730 {
 731         get_ctx()->atomic_next = n;
 732 }
 733 EXPORT_SYMBOL(kcsan_atomic_next);
 734
 735 void kcsan_set_access_mask(unsigned long mask)
 736 {
 737         get_ctx()->access_mask = mask;
 738 }
 739 EXPORT_SYMBOL(kcsan_set_access_mask);
 740
 741 struct kcsan_scoped_access *
 742 kcsan_begin_scoped_access(const volatile void *ptr, size_t size, int type,
 743                           struct kcsan_scoped_access *sa)
 744 {
 745         struct kcsan_ctx *ctx = get_ctx();
 746
 747         __kcsan_check_access(ptr, size, type);
 748
 749         ctx->disable_count++; /* Disable KCSAN, in case list debugging is on. */
 750
 751         INIT_LIST_HEAD(&sa->list);
 752         sa->ptr = ptr;
 753         sa->size = size;
 754         sa->type = type;
 755
 756         if (!ctx->scoped_accesses.prev) /* Lazy initialize list head. */
 757                 INIT_LIST_HEAD(&ctx->scoped_accesses);
 758         list_add(&sa->list, &ctx->scoped_accesses);
 759
 760         ctx->disable_count--;
 761         return sa;
 762 }
 763 EXPORT_SYMBOL(kcsan_begin_scoped_access);
 764
 765 void kcsan_end_scoped_access(struct kcsan_scoped_access *sa)
 766 {
 767         struct kcsan_ctx *ctx = get_ctx();
 768
 769         if (WARN(!ctx->scoped_accesses.prev, "Unbalanced %s()?", __func__))
 770                 return;
 771
 772         ctx->disable_count++; /* Disable KCSAN, in case list debugging is on. */
 773
 774         list_del(&sa->list);
 775         if (list_empty(&ctx->scoped_accesses))
 776                 /*
 777                  * Ensure we do not enter kcsan_check_scoped_accesses()
 778                  * slow-path if unnecessary, and avoids requiring list_empty()
 779                  * in the fast-path (to avoid a READ_ONCE() and potential
 780                  * uaccess warning).
 781                  */
 782                 ctx->scoped_accesses.prev = NULL;
 783
 784         ctx->disable_count--;
 785
 786         __kcsan_check_access(sa->ptr, sa->size, sa->type);
 787 }
 788 EXPORT_SYMBOL(kcsan_end_scoped_access);
 789
 790 void __kcsan_check_access(const volatile void *ptr, size_t size, int type)
 791 {
 792         check_access(ptr, size, type);
 793 }
 794 EXPORT_SYMBOL(__kcsan_check_access);
 795
 796 /*
 797  * KCSAN uses the same instrumentation that is emitted by supported compilers
 798  * for ThreadSanitizer (TSAN).
 799  *
 800  * When enabled, the compiler emits instrumentation calls (the functions
 801  * prefixed with "__tsan" below) for all loads and stores that it generated;
 802  * inline asm is not instrumented.
 803  *
 804  * Note that, not all supported compiler versions distinguish aligned/unaligned
 805  * accesses, but e.g. recent versions of Clang do. We simply alias the unaligned
 806  * version to the generic version, which can handle both.
 807  */
 808
 809 #define DEFINE_TSAN_READ_WRITE(size)                                           \
 810         void __tsan_read##size(void *ptr);                                     \
 811         void __tsan_read##size(void *ptr)                                      \
 812         {                                                                      \
 813                 check_access(ptr, size, 0);                                    \
 814         }                                                                      \
 815         EXPORT_SYMBOL(__tsan_read##size);                                      \
 816         void __tsan_unaligned_read##size(void *ptr)                            \
 817                 __alias(__tsan_read##size);                                    \
 818         EXPORT_SYMBOL(__tsan_unaligned_read##size);                            \
 819         void __tsan_write##size(void *ptr);                                    \
 820         void __tsan_write##size(void *ptr)                                     \
 821         {                                                                      \
 822                 check_access(ptr, size, KCSAN_ACCESS_WRITE);                   \
 823         }                                                                      \
 824         EXPORT_SYMBOL(__tsan_write##size);                                     \
 825         void __tsan_unaligned_write##size(void *ptr)                           \
 826                 __alias(__tsan_write##size);                                   \
 827         EXPORT_SYMBOL(__tsan_unaligned_write##size);                           \
 828         void __tsan_read_write##size(void *ptr);                               \
 829         void __tsan_read_write##size(void *ptr)                                \
 830         {                                                                      \
 831                 check_access(ptr, size,                                        \
 832                              KCSAN_ACCESS_COMPOUND | KCSAN_ACCESS_WRITE);      \
 833         }                                                                      \
 834         EXPORT_SYMBOL(__tsan_read_write##size);                                \
 835         void __tsan_unaligned_read_write##size(void *ptr)                      \
 836                 __alias(__tsan_read_write##size);                              \
 837         EXPORT_SYMBOL(__tsan_unaligned_read_write##size)
 838
 839 DEFINE_TSAN_READ_WRITE(1);
 840 DEFINE_TSAN_READ_WRITE(2);
 841 DEFINE_TSAN_READ_WRITE(4);
 842 DEFINE_TSAN_READ_WRITE(8);
 843 DEFINE_TSAN_READ_WRITE(16);
 844
 845 void __tsan_read_range(void *ptr, size_t size);
 846 void __tsan_read_range(void *ptr, size_t size)
 847 {
 848         check_access(ptr, size, 0);
 849 }
 850 EXPORT_SYMBOL(__tsan_read_range);
 851
 852 void __tsan_write_range(void *ptr, size_t size);
 853 void __tsan_write_range(void *ptr, size_t size)
 854 {
 855         check_access(ptr, size, KCSAN_ACCESS_WRITE);
 856 }
 857 EXPORT_SYMBOL(__tsan_write_range);
 858
 859 /*
 860  * Use of explicit volatile is generally disallowed [1], however, volatile is
 861  * still used in various concurrent context, whether in low-level
 862  * synchronization primitives or for legacy reasons.
 863  * [1] https://lwn.net/Articles/233479/
 864  *
 865  * We only consider volatile accesses atomic if they are aligned and would pass
 866  * the size-check of compiletime_assert_rwonce_type().
 867  */
 868 #define DEFINE_TSAN_VOLATILE_READ_WRITE(size)                                  \
 869         void __tsan_volatile_read##size(void *ptr);                            \
 870         void __tsan_volatile_read##size(void *ptr)                             \
 871         {                                                                      \
 872                 const bool is_atomic = size <= sizeof(long long) &&            \
 873                                        IS_ALIGNED((unsigned long)ptr, size);   \
 874                 if (IS_ENABLED(CONFIG_KCSAN_IGNORE_ATOMICS) && is_atomic)      \
 875                         return;                                                \
 876                 check_access(ptr, size, is_atomic ? KCSAN_ACCESS_ATOMIC : 0);  \
 877         }                                                                      \
 878         EXPORT_SYMBOL(__tsan_volatile_read##size);                             \
 879         void __tsan_unaligned_volatile_read##size(void *ptr)                   \
 880                 __alias(__tsan_volatile_read##size);                           \
 881         EXPORT_SYMBOL(__tsan_unaligned_volatile_read##size);                   \
 882         void __tsan_volatile_write##size(void *ptr);                           \
 883         void __tsan_volatile_write##size(void *ptr)                            \
 884         {                                                                      \
 885                 const bool is_atomic = size <= sizeof(long long) &&            \
 886                                        IS_ALIGNED((unsigned long)ptr, size);   \
 887                 if (IS_ENABLED(CONFIG_KCSAN_IGNORE_ATOMICS) && is_atomic)      \
 888                         return;                                                \
 889                 check_access(ptr, size,                                        \
 890                              KCSAN_ACCESS_WRITE |                              \
 891                                      (is_atomic ? KCSAN_ACCESS_ATOMIC : 0));   \
 892         }                                                                      \
 893         EXPORT_SYMBOL(__tsan_volatile_write##size);                            \
 894         void __tsan_unaligned_volatile_write##size(void *ptr)                  \
 895                 __alias(__tsan_volatile_write##size);                          \
 896         EXPORT_SYMBOL(__tsan_unaligned_volatile_write##size)
 897
 898 DEFINE_TSAN_VOLATILE_READ_WRITE(1);
 899 DEFINE_TSAN_VOLATILE_READ_WRITE(2);
 900 DEFINE_TSAN_VOLATILE_READ_WRITE(4);
 901 DEFINE_TSAN_VOLATILE_READ_WRITE(8);
 902 DEFINE_TSAN_VOLATILE_READ_WRITE(16);
 903
 904 /*
 905  * The below are not required by KCSAN, but can still be emitted by the
 906  * compiler.
 907  */
 908 void __tsan_func_entry(void *call_pc);
 909 void __tsan_func_entry(void *call_pc)
 910 {
 911 }
 912 EXPORT_SYMBOL(__tsan_func_entry);
 913 void __tsan_func_exit(void);
 914 void __tsan_func_exit(void)
 915 {
 916 }
 917 EXPORT_SYMBOL(__tsan_func_exit);
 918 void __tsan_init(void);
 919 void __tsan_init(void)
 920 {
 921 }
 922 EXPORT_SYMBOL(__tsan_init);
 923
 924 /*
 925  * Instrumentation for atomic builtins (__atomic_*, __sync_*).
 926  *
 927  * Normal kernel code _should not_ be using them directly, but some
 928  * architectures may implement some or all atomics using the compilers'
 929  * builtins.
 930  *
 931  * Note: If an architecture decides to fully implement atomics using the
 932  * builtins, because they are implicitly instrumented by KCSAN (and KASAN,
 933  * etc.), implementing the ARCH_ATOMIC interface (to get instrumentation via
 934  * atomic-instrumented) is no longer necessary.
 935  *
 936  * TSAN instrumentation replaces atomic accesses with calls to any of the below
 937  * functions, whose job is to also execute the operation itself.
 938  */
 939
 940 #define DEFINE_TSAN_ATOMIC_LOAD_STORE(bits)                                                        \
 941         u##bits __tsan_atomic##bits##_load(const u##bits *ptr, int memorder);                      \
 942         u##bits __tsan_atomic##bits##_load(const u##bits *ptr, int memorder)                       \
 943         {                                                                                          \
 944                 if (!IS_ENABLED(CONFIG_KCSAN_IGNORE_ATOMICS)) {                                    \
 945                         check_access(ptr, bits / BITS_PER_BYTE, KCSAN_ACCESS_ATOMIC);              \
 946                 }                                                                                  \
 947                 return __atomic_load_n(ptr, memorder);                                             \
 948         }                                                                                          \
 949         EXPORT_SYMBOL(__tsan_atomic##bits##_load);                                                 \
 950         void __tsan_atomic##bits##_store(u##bits *ptr, u##bits v, int memorder);                   \
 951         void __tsan_atomic##bits##_store(u##bits *ptr, u##bits v, int memorder)                    \
 952         {                                                                                          \
 953                 if (!IS_ENABLED(CONFIG_KCSAN_IGNORE_ATOMICS)) {                                    \
 954                         check_access(ptr, bits / BITS_PER_BYTE,                                    \
 955                                      KCSAN_ACCESS_WRITE | KCSAN_ACCESS_ATOMIC);                    \
 956                 }                                                                                  \
 957                 __atomic_store_n(ptr, v, memorder);                                                \
 958         }                                                                                          \
 959         EXPORT_SYMBOL(__tsan_atomic##bits##_store)
 960
 961 #define DEFINE_TSAN_ATOMIC_RMW(op, bits, suffix)                                                   \
 962         u##bits __tsan_atomic##bits##_##op(u##bits *ptr, u##bits v, int memorder);                 \
 963         u##bits __tsan_atomic##bits##_##op(u##bits *ptr, u##bits v, int memorder)                  \
 964         {                                                                                          \
 965                 if (!IS_ENABLED(CONFIG_KCSAN_IGNORE_ATOMICS)) {                                    \
 966                         check_access(ptr, bits / BITS_PER_BYTE,                                    \
 967                                      KCSAN_ACCESS_COMPOUND | KCSAN_ACCESS_WRITE |                  \
 968                                              KCSAN_ACCESS_ATOMIC);                                 \
 969                 }                                                                                  \
 970                 return __atomic_##op##suffix(ptr, v, memorder);                                    \
 971         }                                                                                          \
 972         EXPORT_SYMBOL(__tsan_atomic##bits##_##op)
 973
 974 /*
 975  * Note: CAS operations are always classified as write, even in case they
 976  * fail. We cannot perform check_access() after a write, as it might lead to
 977  * false positives, in cases such as:
 978  *
 979  *      T0: __atomic_compare_exchange_n(&p->flag, &old, 1, ...)
 980  *
 981  *      T1: if (__atomic_load_n(&p->flag, ...)) {
 982  *              modify *p;
 983  *              p->flag = 0;
 984  *          }
 985  *
 986  * The only downside is that, if there are 3 threads, with one CAS that
 987  * succeeds, another CAS that fails, and an unmarked racing operation, we may
 988  * point at the wrong CAS as the source of the race. However, if we assume that
 989  * all CAS can succeed in some other execution, the data race is still valid.
 990  */
 991 #define DEFINE_TSAN_ATOMIC_CMPXCHG(bits, strength, weak)                                           \
 992         int __tsan_atomic##bits##_compare_exchange_##strength(u##bits *ptr, u##bits *exp,          \
 993                                                               u##bits val, int mo, int fail_mo);   \
 994         int __tsan_atomic##bits##_compare_exchange_##strength(u##bits *ptr, u##bits *exp,          \
 995                                                               u##bits val, int mo, int fail_mo)    \
 996         {                                                                                          \
 997                 if (!IS_ENABLED(CONFIG_KCSAN_IGNORE_ATOMICS)) {                                    \
 998                         check_access(ptr, bits / BITS_PER_BYTE,                                    \
 999                                      KCSAN_ACCESS_COMPOUND | KCSAN_ACCESS_WRITE |                  \
1000                                              KCSAN_ACCESS_ATOMIC);                                 \
1001                 }                                                                                  \
1002                 return __atomic_compare_exchange_n(ptr, exp, val, weak, mo, fail_mo);              \
1003         }                                                                                          \
1004         EXPORT_SYMBOL(__tsan_atomic##bits##_compare_exchange_##strength)
1005
1006 #define DEFINE_TSAN_ATOMIC_CMPXCHG_VAL(bits)                                                       \
1007         u##bits __tsan_atomic##bits##_compare_exchange_val(u##bits *ptr, u##bits exp, u##bits val, \
1008                                                            int mo, int fail_mo);                   \
1009         u##bits __tsan_atomic##bits##_compare_exchange_val(u##bits *ptr, u##bits exp, u##bits val, \
1010                                                            int mo, int fail_mo)                    \
1011         {                                                                                          \
1012                 if (!IS_ENABLED(CONFIG_KCSAN_IGNORE_ATOMICS)) {                                    \
1013                         check_access(ptr, bits / BITS_PER_BYTE,                                    \
1014                                      KCSAN_ACCESS_COMPOUND | KCSAN_ACCESS_WRITE |                  \
1015                                              KCSAN_ACCESS_ATOMIC);                                 \
1016                 }                                                                                  \
1017                 __atomic_compare_exchange_n(ptr, &exp, val, 0, mo, fail_mo);                       \
1018                 return exp;                                                                        \
1019         }                                                                                          \
1020         EXPORT_SYMBOL(__tsan_atomic##bits##_compare_exchange_val)
1021
1022 #define DEFINE_TSAN_ATOMIC_OPS(bits)                                                               \
1023         DEFINE_TSAN_ATOMIC_LOAD_STORE(bits);                                                       \
1024         DEFINE_TSAN_ATOMIC_RMW(exchange, bits, _n);                                                \
1025         DEFINE_TSAN_ATOMIC_RMW(fetch_add, bits, );                                                 \
1026         DEFINE_TSAN_ATOMIC_RMW(fetch_sub, bits, );                                                 \
1027         DEFINE_TSAN_ATOMIC_RMW(fetch_and, bits, );                                                 \
1028         DEFINE_TSAN_ATOMIC_RMW(fetch_or, bits, );                                                  \
1029         DEFINE_TSAN_ATOMIC_RMW(fetch_xor, bits, );                                                 \
1030         DEFINE_TSAN_ATOMIC_RMW(fetch_nand, bits, );                                                \
1031         DEFINE_TSAN_ATOMIC_CMPXCHG(bits, strong, 0);                                               \
1032         DEFINE_TSAN_ATOMIC_CMPXCHG(bits, weak, 1);                                                 \
1033         DEFINE_TSAN_ATOMIC_CMPXCHG_VAL(bits)
1034
1035 DEFINE_TSAN_ATOMIC_OPS(8);
1036 DEFINE_TSAN_ATOMIC_OPS(16);
1037 DEFINE_TSAN_ATOMIC_OPS(32);
1038 DEFINE_TSAN_ATOMIC_OPS(64);
1039
1040 void __tsan_atomic_thread_fence(int memorder);
1041 void __tsan_atomic_thread_fence(int memorder)
1042 {
1043         __atomic_thread_fence(memorder);
1044 }
1045 EXPORT_SYMBOL(__tsan_atomic_thread_fence);
1046
1047 void __tsan_atomic_signal_fence(int memorder);
1048 void __tsan_atomic_signal_fence(int memorder) { }
1049 EXPORT_SYMBOL(__tsan_atomic_signal_fence);