// SPDX-License-Identifier: GPL-2.0 #include #include #include #include #include #include #include #include #include #include #include "atomic.h" #include "encoding.h" #include "kcsan.h" bool kcsan_enabled; /* Per-CPU kcsan_ctx for interrupts */ static DEFINE_PER_CPU(struct kcsan_ctx, kcsan_cpu_ctx) = { .disable_count = 0, .atomic_next = 0, .atomic_nest_count = 0, .in_flat_atomic = false, }; /* * Helper macros to index into adjacent slots slots, starting from address slot * itself, followed by the right and left slots. * * The purpose is 2-fold: * * 1. if during insertion the address slot is already occupied, check if * any adjacent slots are free; * 2. accesses that straddle a slot boundary due to size that exceeds a * slot's range may check adjacent slots if any watchpoint matches. * * Note that accesses with very large size may still miss a watchpoint; however, * given this should be rare, this is a reasonable trade-off to make, since this * will avoid: * * 1. excessive contention between watchpoint checks and setup; * 2. larger number of simultaneous watchpoints without sacrificing * performance. * * Example: SLOT_IDX values for KCSAN_CHECK_ADJACENT=1, where i is [0, 1, 2]: * * slot=0: [ 1, 2, 0] * slot=9: [10, 11, 9] * slot=63: [64, 65, 63] */ #define NUM_SLOTS (1 + 2*KCSAN_CHECK_ADJACENT) #define SLOT_IDX(slot, i) (slot + ((i + KCSAN_CHECK_ADJACENT) % NUM_SLOTS)) /* * SLOT_IDX_FAST is used in the fast-path. Not first checking the address's primary * slot (middle) is fine if we assume that data races occur rarely. The set of * indices {SLOT_IDX(slot, i) | i in [0, NUM_SLOTS)} is equivalent to * {SLOT_IDX_FAST(slot, i) | i in [0, NUM_SLOTS)}. */ #define SLOT_IDX_FAST(slot, i) (slot + i) /* * Watchpoints, with each entry encoded as defined in encoding.h: in order to be * able to safely update and access a watchpoint without introducing locking * overhead, we encode each watchpoint as a single atomic long. The initial * zero-initialized state matches INVALID_WATCHPOINT. * * Add NUM_SLOTS-1 entries to account for overflow; this helps avoid having to * use more complicated SLOT_IDX_FAST calculation with modulo in the fast-path. */ static atomic_long_t watchpoints[CONFIG_KCSAN_NUM_WATCHPOINTS + NUM_SLOTS-1]; /* * Instructions to skip watching counter, used in should_watch(). We use a * per-CPU counter to avoid excessive contention. */ static DEFINE_PER_CPU(long, kcsan_skip); static __always_inline atomic_long_t *find_watchpoint(unsigned long addr, size_t size, bool expect_write, long *encoded_watchpoint) { const int slot = watchpoint_slot(addr); const unsigned long addr_masked = addr & WATCHPOINT_ADDR_MASK; atomic_long_t *watchpoint; unsigned long wp_addr_masked; size_t wp_size; bool is_write; int i; BUILD_BUG_ON(CONFIG_KCSAN_NUM_WATCHPOINTS < NUM_SLOTS); for (i = 0; i < NUM_SLOTS; ++i) { watchpoint = &watchpoints[SLOT_IDX_FAST(slot, i)]; *encoded_watchpoint = atomic_long_read(watchpoint); if (!decode_watchpoint(*encoded_watchpoint, &wp_addr_masked, &wp_size, &is_write)) continue; if (expect_write && !is_write) continue; /* Check if the watchpoint matches the access. */ if (matching_access(wp_addr_masked, wp_size, addr_masked, size)) return watchpoint; } return NULL; } static inline atomic_long_t * insert_watchpoint(unsigned long addr, size_t size, bool is_write) { const int slot = watchpoint_slot(addr); const long encoded_watchpoint = encode_watchpoint(addr, size, is_write); atomic_long_t *watchpoint; int i; /* Check slot index logic, ensuring we stay within array bounds. */ BUILD_BUG_ON(SLOT_IDX(0, 0) != KCSAN_CHECK_ADJACENT); BUILD_BUG_ON(SLOT_IDX(0, KCSAN_CHECK_ADJACENT+1) != 0); BUILD_BUG_ON(SLOT_IDX(CONFIG_KCSAN_NUM_WATCHPOINTS-1, KCSAN_CHECK_ADJACENT) != ARRAY_SIZE(watchpoints)-1); BUILD_BUG_ON(SLOT_IDX(CONFIG_KCSAN_NUM_WATCHPOINTS-1, KCSAN_CHECK_ADJACENT+1) != ARRAY_SIZE(watchpoints) - NUM_SLOTS); for (i = 0; i < NUM_SLOTS; ++i) { long expect_val = INVALID_WATCHPOINT; /* Try to acquire this slot. */ watchpoint = &watchpoints[SLOT_IDX(slot, i)]; if (atomic_long_try_cmpxchg_relaxed(watchpoint, &expect_val, encoded_watchpoint)) return watchpoint; } return NULL; } /* * Return true if watchpoint was successfully consumed, false otherwise. * * This may return false if: * * 1. another thread already consumed the watchpoint; * 2. the thread that set up the watchpoint already removed it; * 3. the watchpoint was removed and then re-used. */ static __always_inline bool try_consume_watchpoint(atomic_long_t *watchpoint, long encoded_watchpoint) { return atomic_long_try_cmpxchg_relaxed(watchpoint, &encoded_watchpoint, CONSUMED_WATCHPOINT); } /* * Return true if watchpoint was not touched, false if consumed. */ static inline bool remove_watchpoint(atomic_long_t *watchpoint) { return atomic_long_xchg_relaxed(watchpoint, INVALID_WATCHPOINT) != CONSUMED_WATCHPOINT; } static __always_inline struct kcsan_ctx *get_ctx(void) { /* * In interrupts, use raw_cpu_ptr to avoid unnecessary checks, that would * also result in calls that generate warnings in uaccess regions. */ return in_task() ? ¤t->kcsan_ctx : raw_cpu_ptr(&kcsan_cpu_ctx); } static __always_inline bool is_atomic(const volatile void *ptr) { struct kcsan_ctx *ctx = get_ctx(); if (unlikely(ctx->atomic_next > 0)) { /* * Because we do not have separate contexts for nested * interrupts, in case atomic_next is set, we simply assume that * the outer interrupt set atomic_next. In the worst case, we * will conservatively consider operations as atomic. This is a * reasonable trade-off to make, since this case should be * extremely rare; however, even if extremely rare, it could * lead to false positives otherwise. */ if ((hardirq_count() >> HARDIRQ_SHIFT) < 2) --ctx->atomic_next; /* in task, or outer interrupt */ return true; } if (unlikely(ctx->atomic_nest_count > 0 || ctx->in_flat_atomic)) return true; return kcsan_is_atomic(ptr); } static __always_inline bool should_watch(const volatile void *ptr, int type) { /* * Never set up watchpoints when memory operations are atomic. * * Need to check this first, before kcsan_skip check below: (1) atomics * should not count towards skipped instructions, and (2) to actually * decrement kcsan_atomic_next for consecutive instruction stream. */ if ((type & KCSAN_ACCESS_ATOMIC) != 0 || is_atomic(ptr)) return false; if (this_cpu_dec_return(kcsan_skip) >= 0) return false; /* * NOTE: If we get here, kcsan_skip must always be reset in slow path * via reset_kcsan_skip() to avoid underflow. */ /* this operation should be watched */ return true; } static inline void reset_kcsan_skip(void) { long skip_count = CONFIG_KCSAN_SKIP_WATCH - (IS_ENABLED(CONFIG_KCSAN_SKIP_WATCH_RANDOMIZE) ? prandom_u32_max(CONFIG_KCSAN_SKIP_WATCH) : 0); this_cpu_write(kcsan_skip, skip_count); } static __always_inline bool kcsan_is_enabled(void) { return READ_ONCE(kcsan_enabled) && get_ctx()->disable_count == 0; } static inline unsigned int get_delay(void) { unsigned int delay = in_task() ? CONFIG_KCSAN_UDELAY_TASK : CONFIG_KCSAN_UDELAY_INTERRUPT; return delay - (IS_ENABLED(CONFIG_KCSAN_DELAY_RANDOMIZE) ? prandom_u32_max(delay) : 0); } /* * Pull everything together: check_access() below contains the performance * critical operations; the fast-path (including check_access) functions should * all be inlinable by the instrumentation functions. * * The slow-path (kcsan_found_watchpoint, kcsan_setup_watchpoint) are * non-inlinable -- note that, we prefix these with "kcsan_" to ensure they can * be filtered from the stacktrace, as well as give them unique names for the * UACCESS whitelist of objtool. Each function uses user_access_save/restore(), * since they do not access any user memory, but instrumentation is still * emitted in UACCESS regions. */ static noinline void kcsan_found_watchpoint(const volatile void *ptr, size_t size, int type, atomic_long_t *watchpoint, long encoded_watchpoint) { unsigned long flags; bool consumed; if (!kcsan_is_enabled()) return; /* * Consume the watchpoint as soon as possible, to minimize the chances * of !consumed. Consuming the watchpoint must always be guarded by * kcsan_is_enabled() check, as otherwise we might erroneously * triggering reports when disabled. */ consumed = try_consume_watchpoint(watchpoint, encoded_watchpoint); /* keep this after try_consume_watchpoint */ flags = user_access_save(); if (consumed) { kcsan_report(ptr, size, type, true, raw_smp_processor_id(), KCSAN_REPORT_CONSUMED_WATCHPOINT); } else { /* * The other thread may not print any diagnostics, as it has * already removed the watchpoint, or another thread consumed * the watchpoint before this thread. */ kcsan_counter_inc(KCSAN_COUNTER_REPORT_RACES); } kcsan_counter_inc(KCSAN_COUNTER_DATA_RACES); user_access_restore(flags); } static noinline void kcsan_setup_watchpoint(const volatile void *ptr, size_t size, int type) { const bool is_write = (type & KCSAN_ACCESS_WRITE) != 0; atomic_long_t *watchpoint; union { u8 _1; u16 _2; u32 _4; u64 _8; } expect_value; bool value_change = false; unsigned long ua_flags = user_access_save(); unsigned long irq_flags; /* * Always reset kcsan_skip counter in slow-path to avoid underflow; see * should_watch(). */ reset_kcsan_skip(); if (!kcsan_is_enabled()) goto out; if (!check_encodable((unsigned long)ptr, size)) { kcsan_counter_inc(KCSAN_COUNTER_UNENCODABLE_ACCESSES); goto out; } /* * Disable interrupts & preemptions to avoid another thread on the same * CPU accessing memory locations for the set up watchpoint; this is to * avoid reporting races to e.g. CPU-local data. * * An alternative would be adding the source CPU to the watchpoint * encoding, and checking that watchpoint-CPU != this-CPU. There are * several problems with this: * 1. we should avoid stealing more bits from the watchpoint encoding * as it would affect accuracy, as well as increase performance * overhead in the fast-path; * 2. if we are preempted, but there *is* a genuine data race, we * would *not* report it -- since this is the common case (vs. * CPU-local data accesses), it makes more sense (from a data race * detection point of view) to simply disable preemptions to ensure * as many tasks as possible run on other CPUs. * * Use raw versions, to avoid lockdep recursion via IRQ flags tracing. */ raw_local_irq_save(irq_flags); watchpoint = insert_watchpoint((unsigned long)ptr, size, is_write); if (watchpoint == NULL) { /* * Out of capacity: the size of 'watchpoints', and the frequency * with which should_watch() returns true should be tweaked so * that this case happens very rarely. */ kcsan_counter_inc(KCSAN_COUNTER_NO_CAPACITY); goto out_unlock; } kcsan_counter_inc(KCSAN_COUNTER_SETUP_WATCHPOINTS); kcsan_counter_inc(KCSAN_COUNTER_USED_WATCHPOINTS); /* * Read the current value, to later check and infer a race if the data * was modified via a non-instrumented access, e.g. from a device. */ switch (size) { case 1: expect_value._1 = READ_ONCE(*(const u8 *)ptr); break; case 2: expect_value._2 = READ_ONCE(*(const u16 *)ptr); break; case 4: expect_value._4 = READ_ONCE(*(const u32 *)ptr); break; case 8: expect_value._8 = READ_ONCE(*(const u64 *)ptr); break; default: break; /* ignore; we do not diff the values */ } if (IS_ENABLED(CONFIG_KCSAN_DEBUG)) { kcsan_disable_current(); pr_err("KCSAN: watching %s, size: %zu, addr: %px [slot: %d, encoded: %lx]\n", is_write ? "write" : "read", size, ptr, watchpoint_slot((unsigned long)ptr), encode_watchpoint((unsigned long)ptr, size, is_write)); kcsan_enable_current(); } /* * Delay this thread, to increase probability of observing a racy * conflicting access. */ udelay(get_delay()); /* * Re-read value, and check if it is as expected; if not, we infer a * racy access. */ switch (size) { case 1: value_change = expect_value._1 != READ_ONCE(*(const u8 *)ptr); break; case 2: value_change = expect_value._2 != READ_ONCE(*(const u16 *)ptr); break; case 4: value_change = expect_value._4 != READ_ONCE(*(const u32 *)ptr); break; case 8: value_change = expect_value._8 != READ_ONCE(*(const u64 *)ptr); break; default: break; /* ignore; we do not diff the values */ } /* Check if this access raced with another. */ if (!remove_watchpoint(watchpoint)) { /* * No need to increment 'data_races' counter, as the racing * thread already did. */ kcsan_report(ptr, size, type, size > 8 || value_change, smp_processor_id(), KCSAN_REPORT_RACE_SIGNAL); } else if (value_change) { /* Inferring a race, since the value should not have changed. */ kcsan_counter_inc(KCSAN_COUNTER_RACES_UNKNOWN_ORIGIN); if (IS_ENABLED(CONFIG_KCSAN_REPORT_RACE_UNKNOWN_ORIGIN)) kcsan_report(ptr, size, type, true, smp_processor_id(), KCSAN_REPORT_RACE_UNKNOWN_ORIGIN); } kcsan_counter_dec(KCSAN_COUNTER_USED_WATCHPOINTS); out_unlock: raw_local_irq_restore(irq_flags); out: user_access_restore(ua_flags); } static __always_inline void check_access(const volatile void *ptr, size_t size, int type) { const bool is_write = (type & KCSAN_ACCESS_WRITE) != 0; atomic_long_t *watchpoint; long encoded_watchpoint; /* * Avoid user_access_save in fast-path: find_watchpoint is safe without * user_access_save, as the address that ptr points to is only used to * check if a watchpoint exists; ptr is never dereferenced. */ watchpoint = find_watchpoint((unsigned long)ptr, size, !is_write, &encoded_watchpoint); /* * It is safe to check kcsan_is_enabled() after find_watchpoint in the * slow-path, as long as no state changes that cause a data race to be * detected and reported have occurred until kcsan_is_enabled() is * checked. */ if (unlikely(watchpoint != NULL)) kcsan_found_watchpoint(ptr, size, type, watchpoint, encoded_watchpoint); else if (unlikely(should_watch(ptr, type))) kcsan_setup_watchpoint(ptr, size, type); } /* === Public interface ===================================================== */ void __init kcsan_init(void) { BUG_ON(!in_task()); kcsan_debugfs_init(); /* * We are in the init task, and no other tasks should be running; * WRITE_ONCE without memory barrier is sufficient. */ if (IS_ENABLED(CONFIG_KCSAN_EARLY_ENABLE)) WRITE_ONCE(kcsan_enabled, true); } /* === Exported interface =================================================== */ void kcsan_disable_current(void) { ++get_ctx()->disable_count; } EXPORT_SYMBOL(kcsan_disable_current); void kcsan_enable_current(void) { if (get_ctx()->disable_count-- == 0) { /* * Warn if kcsan_enable_current() calls are unbalanced with * kcsan_disable_current() calls, which causes disable_count to * become negative and should not happen. */ kcsan_disable_current(); /* restore to 0, KCSAN still enabled */ kcsan_disable_current(); /* disable to generate warning */ WARN(1, "Unbalanced %s()", __func__); kcsan_enable_current(); } } EXPORT_SYMBOL(kcsan_enable_current); void kcsan_nestable_atomic_begin(void) { /* * Do *not* check and warn if we are in a flat atomic region: nestable * and flat atomic regions are independent from each other. * See include/linux/kcsan.h: struct kcsan_ctx comments for more * comments. */ ++get_ctx()->atomic_nest_count; } EXPORT_SYMBOL(kcsan_nestable_atomic_begin); void kcsan_nestable_atomic_end(void) { if (get_ctx()->atomic_nest_count-- == 0) { /* * Warn if kcsan_nestable_atomic_end() calls are unbalanced with * kcsan_nestable_atomic_begin() calls, which causes * atomic_nest_count to become negative and should not happen. */ kcsan_nestable_atomic_begin(); /* restore to 0 */ kcsan_disable_current(); /* disable to generate warning */ WARN(1, "Unbalanced %s()", __func__); kcsan_enable_current(); } } EXPORT_SYMBOL(kcsan_nestable_atomic_end); void kcsan_flat_atomic_begin(void) { get_ctx()->in_flat_atomic = true; } EXPORT_SYMBOL(kcsan_flat_atomic_begin); void kcsan_flat_atomic_end(void) { get_ctx()->in_flat_atomic = false; } EXPORT_SYMBOL(kcsan_flat_atomic_end); void kcsan_atomic_next(int n) { get_ctx()->atomic_next = n; } EXPORT_SYMBOL(kcsan_atomic_next); void __kcsan_check_access(const volatile void *ptr, size_t size, int type) { check_access(ptr, size, type); } EXPORT_SYMBOL(__kcsan_check_access); /* * KCSAN uses the same instrumentation that is emitted by supported compilers * for ThreadSanitizer (TSAN). * * When enabled, the compiler emits instrumentation calls (the functions * prefixed with "__tsan" below) for all loads and stores that it generated; * inline asm is not instrumented. * * Note that, not all supported compiler versions distinguish aligned/unaligned * accesses, but e.g. recent versions of Clang do. We simply alias the unaligned * version to the generic version, which can handle both. */ #define DEFINE_TSAN_READ_WRITE(size) \ void __tsan_read##size(void *ptr) \ { \ check_access(ptr, size, 0); \ } \ EXPORT_SYMBOL(__tsan_read##size); \ void __tsan_unaligned_read##size(void *ptr) \ __alias(__tsan_read##size); \ EXPORT_SYMBOL(__tsan_unaligned_read##size); \ void __tsan_write##size(void *ptr) \ { \ check_access(ptr, size, KCSAN_ACCESS_WRITE); \ } \ EXPORT_SYMBOL(__tsan_write##size); \ void __tsan_unaligned_write##size(void *ptr) \ __alias(__tsan_write##size); \ EXPORT_SYMBOL(__tsan_unaligned_write##size) DEFINE_TSAN_READ_WRITE(1); DEFINE_TSAN_READ_WRITE(2); DEFINE_TSAN_READ_WRITE(4); DEFINE_TSAN_READ_WRITE(8); DEFINE_TSAN_READ_WRITE(16); void __tsan_read_range(void *ptr, size_t size) { check_access(ptr, size, 0); } EXPORT_SYMBOL(__tsan_read_range); void __tsan_write_range(void *ptr, size_t size) { check_access(ptr, size, KCSAN_ACCESS_WRITE); } EXPORT_SYMBOL(__tsan_write_range); /* * The below are not required by KCSAN, but can still be emitted by the * compiler. */ void __tsan_func_entry(void *call_pc) { } EXPORT_SYMBOL(__tsan_func_entry); void __tsan_func_exit(void) { } EXPORT_SYMBOL(__tsan_func_exit); void __tsan_init(void) { } EXPORT_SYMBOL(__tsan_init);