// SPDX-License-Identifier: GPL-2.0 /* * KCSAN reporting. * * Copyright (C) 2019, Google LLC. */ #include #include #include #include #include #include #include #include #include #include #include "kcsan.h" #include "encoding.h" /* * Max. number of stack entries to show in the report. */ #define NUM_STACK_ENTRIES 64 /* Common access info. */ struct access_info { const volatile void *ptr; size_t size; int access_type; int task_pid; int cpu_id; }; /* * Other thread info: communicated from other racing thread to thread that set * up the watchpoint, which then prints the complete report atomically. */ struct other_info { struct access_info ai; unsigned long stack_entries[NUM_STACK_ENTRIES]; int num_stack_entries; /* * Optionally pass @current. Typically we do not need to pass @current * via @other_info since just @task_pid is sufficient. Passing @current * has additional overhead. * * To safely pass @current, we must either use get_task_struct/ * put_task_struct, or stall the thread that populated @other_info. * * We cannot rely on get_task_struct/put_task_struct in case * release_report() races with a task being released, and would have to * free it in release_report(). This may result in deadlock if we want * to use KCSAN on the allocators. * * Since we also want to reliably print held locks for * CONFIG_KCSAN_VERBOSE, the current implementation stalls the thread * that populated @other_info until it has been consumed. */ struct task_struct *task; }; /* * To never block any producers of struct other_info, we need as many elements * as we have watchpoints (upper bound on concurrent races to report). */ static struct other_info other_infos[CONFIG_KCSAN_NUM_WATCHPOINTS + NUM_SLOTS-1]; /* * Information about reported races; used to rate limit reporting. */ struct report_time { /* * The last time the race was reported. */ unsigned long time; /* * The frames of the 2 threads; if only 1 thread is known, one frame * will be 0. */ unsigned long frame1; unsigned long frame2; }; /* * Since we also want to be able to debug allocators with KCSAN, to avoid * deadlock, report_times cannot be dynamically resized with krealloc in * rate_limit_report. * * Therefore, we use a fixed-size array, which at most will occupy a page. This * still adequately rate limits reports, assuming that a) number of unique data * races is not excessive, and b) occurrence of unique races within the * same time window is limited. */ #define REPORT_TIMES_MAX (PAGE_SIZE / sizeof(struct report_time)) #define REPORT_TIMES_SIZE \ (CONFIG_KCSAN_REPORT_ONCE_IN_MS > REPORT_TIMES_MAX ? \ REPORT_TIMES_MAX : \ CONFIG_KCSAN_REPORT_ONCE_IN_MS) static struct report_time report_times[REPORT_TIMES_SIZE]; /* * Spinlock serializing report generation, and access to @other_infos. Although * it could make sense to have a finer-grained locking story for @other_infos, * report generation needs to be serialized either way, so not much is gained. */ static DEFINE_RAW_SPINLOCK(report_lock); /* * Checks if the race identified by thread frames frame1 and frame2 has * been reported since (now - KCSAN_REPORT_ONCE_IN_MS). */ static bool rate_limit_report(unsigned long frame1, unsigned long frame2) { struct report_time *use_entry = &report_times[0]; unsigned long invalid_before; int i; BUILD_BUG_ON(CONFIG_KCSAN_REPORT_ONCE_IN_MS != 0 && REPORT_TIMES_SIZE == 0); if (CONFIG_KCSAN_REPORT_ONCE_IN_MS == 0) return false; invalid_before = jiffies - msecs_to_jiffies(CONFIG_KCSAN_REPORT_ONCE_IN_MS); /* Check if a matching race report exists. */ for (i = 0; i < REPORT_TIMES_SIZE; ++i) { struct report_time *rt = &report_times[i]; /* * Must always select an entry for use to store info as we * cannot resize report_times; at the end of the scan, use_entry * will be the oldest entry, which ideally also happened before * KCSAN_REPORT_ONCE_IN_MS ago. */ if (time_before(rt->time, use_entry->time)) use_entry = rt; /* * Initially, no need to check any further as this entry as well * as following entries have never been used. */ if (rt->time == 0) break; /* Check if entry expired. */ if (time_before(rt->time, invalid_before)) continue; /* before KCSAN_REPORT_ONCE_IN_MS ago */ /* Reported recently, check if race matches. */ if ((rt->frame1 == frame1 && rt->frame2 == frame2) || (rt->frame1 == frame2 && rt->frame2 == frame1)) return true; } use_entry->time = jiffies; use_entry->frame1 = frame1; use_entry->frame2 = frame2; return false; } /* * Special rules to skip reporting. */ static bool skip_report(enum kcsan_value_change value_change, unsigned long top_frame) { /* Should never get here if value_change==FALSE. */ WARN_ON_ONCE(value_change == KCSAN_VALUE_CHANGE_FALSE); /* * The first call to skip_report always has value_change==TRUE, since we * cannot know the value written of an instrumented access. For the 2nd * call there are 6 cases with CONFIG_KCSAN_REPORT_VALUE_CHANGE_ONLY: * * 1. read watchpoint, conflicting write (value_change==TRUE): report; * 2. read watchpoint, conflicting write (value_change==MAYBE): skip; * 3. write watchpoint, conflicting write (value_change==TRUE): report; * 4. write watchpoint, conflicting write (value_change==MAYBE): skip; * 5. write watchpoint, conflicting read (value_change==MAYBE): skip; * 6. write watchpoint, conflicting read (value_change==TRUE): report; * * Cases 1-4 are intuitive and expected; case 5 ensures we do not report * data races where the write may have rewritten the same value; case 6 * is possible either if the size is larger than what we check value * changes for or the access type is KCSAN_ACCESS_ASSERT. */ if (IS_ENABLED(CONFIG_KCSAN_REPORT_VALUE_CHANGE_ONLY) && value_change == KCSAN_VALUE_CHANGE_MAYBE) { /* * The access is a write, but the data value did not change. * * We opt-out of this filter for certain functions at request of * maintainers. */ char buf[64]; int len = scnprintf(buf, sizeof(buf), "%ps", (void *)top_frame); if (!strnstr(buf, "rcu_", len) && !strnstr(buf, "_rcu", len) && !strnstr(buf, "_srcu", len)) return true; } return kcsan_skip_report_debugfs(top_frame); } static const char *get_access_type(int type) { if (type & KCSAN_ACCESS_ASSERT) { if (type & KCSAN_ACCESS_SCOPED) { if (type & KCSAN_ACCESS_WRITE) return "assert no accesses (scoped)"; else return "assert no writes (scoped)"; } else { if (type & KCSAN_ACCESS_WRITE) return "assert no accesses"; else return "assert no writes"; } } switch (type) { case 0: return "read"; case KCSAN_ACCESS_ATOMIC: return "read (marked)"; case KCSAN_ACCESS_WRITE: return "write"; case KCSAN_ACCESS_WRITE | KCSAN_ACCESS_ATOMIC: return "write (marked)"; case KCSAN_ACCESS_COMPOUND | KCSAN_ACCESS_WRITE: return "read-write"; case KCSAN_ACCESS_COMPOUND | KCSAN_ACCESS_WRITE | KCSAN_ACCESS_ATOMIC: return "read-write (marked)"; case KCSAN_ACCESS_SCOPED: return "read (scoped)"; case KCSAN_ACCESS_SCOPED | KCSAN_ACCESS_ATOMIC: return "read (marked, scoped)"; case KCSAN_ACCESS_SCOPED | KCSAN_ACCESS_WRITE: return "write (scoped)"; case KCSAN_ACCESS_SCOPED | KCSAN_ACCESS_WRITE | KCSAN_ACCESS_ATOMIC: return "write (marked, scoped)"; default: BUG(); } } static const char *get_bug_type(int type) { return (type & KCSAN_ACCESS_ASSERT) != 0 ? "assert: race" : "data-race"; } /* Return thread description: in task or interrupt. */ static const char *get_thread_desc(int task_id) { if (task_id != -1) { static char buf[32]; /* safe: protected by report_lock */ snprintf(buf, sizeof(buf), "task %i", task_id); return buf; } return "interrupt"; } /* Helper to skip KCSAN-related functions in stack-trace. */ static int get_stack_skipnr(const unsigned long stack_entries[], int num_entries) { char buf[64]; char *cur; int len, skip; for (skip = 0; skip < num_entries; ++skip) { len = scnprintf(buf, sizeof(buf), "%ps", (void *)stack_entries[skip]); /* Never show tsan_* or {read,write}_once_size. */ if (strnstr(buf, "tsan_", len) || strnstr(buf, "_once_size", len)) continue; cur = strnstr(buf, "kcsan_", len); if (cur) { cur += strlen("kcsan_"); if (!str_has_prefix(cur, "test")) continue; /* KCSAN runtime function. */ /* KCSAN related test. */ } /* * No match for runtime functions -- @skip entries to skip to * get to first frame of interest. */ break; } return skip; } /* Compares symbolized strings of addr1 and addr2. */ static int sym_strcmp(void *addr1, void *addr2) { char buf1[64]; char buf2[64]; snprintf(buf1, sizeof(buf1), "%pS", addr1); snprintf(buf2, sizeof(buf2), "%pS", addr2); return strncmp(buf1, buf2, sizeof(buf1)); } static void print_verbose_info(struct task_struct *task) { if (!task) return; /* Restore IRQ state trace for printing. */ kcsan_restore_irqtrace(task); pr_err("\n"); debug_show_held_locks(task); print_irqtrace_events(task); } /* * Returns true if a report was generated, false otherwise. */ static bool print_report(enum kcsan_value_change value_change, enum kcsan_report_type type, const struct access_info *ai, const struct other_info *other_info) { unsigned long stack_entries[NUM_STACK_ENTRIES] = { 0 }; int num_stack_entries = stack_trace_save(stack_entries, NUM_STACK_ENTRIES, 1); int skipnr = get_stack_skipnr(stack_entries, num_stack_entries); unsigned long this_frame = stack_entries[skipnr]; unsigned long other_frame = 0; int other_skipnr = 0; /* silence uninit warnings */ /* * Must check report filter rules before starting to print. */ if (skip_report(KCSAN_VALUE_CHANGE_TRUE, stack_entries[skipnr])) return false; if (type == KCSAN_REPORT_RACE_SIGNAL) { other_skipnr = get_stack_skipnr(other_info->stack_entries, other_info->num_stack_entries); other_frame = other_info->stack_entries[other_skipnr]; /* @value_change is only known for the other thread */ if (skip_report(value_change, other_frame)) return false; } if (rate_limit_report(this_frame, other_frame)) return false; /* Print report header. */ pr_err("==================================================================\n"); switch (type) { case KCSAN_REPORT_RACE_SIGNAL: { int cmp; /* * Order functions lexographically for consistent bug titles. * Do not print offset of functions to keep title short. */ cmp = sym_strcmp((void *)other_frame, (void *)this_frame); pr_err("BUG: KCSAN: %s in %ps / %ps\n", get_bug_type(ai->access_type | other_info->ai.access_type), (void *)(cmp < 0 ? other_frame : this_frame), (void *)(cmp < 0 ? this_frame : other_frame)); } break; case KCSAN_REPORT_RACE_UNKNOWN_ORIGIN: pr_err("BUG: KCSAN: %s in %pS\n", get_bug_type(ai->access_type), (void *)this_frame); break; default: BUG(); } pr_err("\n"); /* Print information about the racing accesses. */ switch (type) { case KCSAN_REPORT_RACE_SIGNAL: pr_err("%s to 0x%px of %zu bytes by %s on cpu %i:\n", get_access_type(other_info->ai.access_type), other_info->ai.ptr, other_info->ai.size, get_thread_desc(other_info->ai.task_pid), other_info->ai.cpu_id); /* Print the other thread's stack trace. */ stack_trace_print(other_info->stack_entries + other_skipnr, other_info->num_stack_entries - other_skipnr, 0); if (IS_ENABLED(CONFIG_KCSAN_VERBOSE)) print_verbose_info(other_info->task); pr_err("\n"); pr_err("%s to 0x%px of %zu bytes by %s on cpu %i:\n", get_access_type(ai->access_type), ai->ptr, ai->size, get_thread_desc(ai->task_pid), ai->cpu_id); break; case KCSAN_REPORT_RACE_UNKNOWN_ORIGIN: pr_err("race at unknown origin, with %s to 0x%px of %zu bytes by %s on cpu %i:\n", get_access_type(ai->access_type), ai->ptr, ai->size, get_thread_desc(ai->task_pid), ai->cpu_id); break; default: BUG(); } /* Print stack trace of this thread. */ stack_trace_print(stack_entries + skipnr, num_stack_entries - skipnr, 0); if (IS_ENABLED(CONFIG_KCSAN_VERBOSE)) print_verbose_info(current); /* Print report footer. */ pr_err("\n"); pr_err("Reported by Kernel Concurrency Sanitizer on:\n"); dump_stack_print_info(KERN_DEFAULT); pr_err("==================================================================\n"); return true; } static void release_report(unsigned long *flags, struct other_info *other_info) { if (other_info) /* * Use size to denote valid/invalid, since KCSAN entirely * ignores 0-sized accesses. */ other_info->ai.size = 0; raw_spin_unlock_irqrestore(&report_lock, *flags); } /* * Sets @other_info->task and awaits consumption of @other_info. * * Precondition: report_lock is held. * Postcondition: report_lock is held. */ static void set_other_info_task_blocking(unsigned long *flags, const struct access_info *ai, struct other_info *other_info) { /* * We may be instrumenting a code-path where current->state is already * something other than TASK_RUNNING. */ const bool is_running = task_is_running(current); /* * To avoid deadlock in case we are in an interrupt here and this is a * race with a task on the same CPU (KCSAN_INTERRUPT_WATCHER), provide a * timeout to ensure this works in all contexts. * * Await approximately the worst case delay of the reporting thread (if * we are not interrupted). */ int timeout = max(kcsan_udelay_task, kcsan_udelay_interrupt); other_info->task = current; do { if (is_running) { /* * Let lockdep know the real task is sleeping, to print * the held locks (recall we turned lockdep off, so * locking/unlocking @report_lock won't be recorded). */ set_current_state(TASK_UNINTERRUPTIBLE); } raw_spin_unlock_irqrestore(&report_lock, *flags); /* * We cannot call schedule() since we also cannot reliably * determine if sleeping here is permitted -- see in_atomic(). */ udelay(1); raw_spin_lock_irqsave(&report_lock, *flags); if (timeout-- < 0) { /* * Abort. Reset @other_info->task to NULL, since it * appears the other thread is still going to consume * it. It will result in no verbose info printed for * this task. */ other_info->task = NULL; break; } /* * If invalid, or @ptr nor @current matches, then @other_info * has been consumed and we may continue. If not, retry. */ } while (other_info->ai.size && other_info->ai.ptr == ai->ptr && other_info->task == current); if (is_running) set_current_state(TASK_RUNNING); } /* Populate @other_info; requires that the provided @other_info not in use. */ static void prepare_report_producer(unsigned long *flags, const struct access_info *ai, struct other_info *other_info) { raw_spin_lock_irqsave(&report_lock, *flags); /* * The same @other_infos entry cannot be used concurrently, because * there is a one-to-one mapping to watchpoint slots (@watchpoints in * core.c), and a watchpoint is only released for reuse after reporting * is done by the consumer of @other_info. Therefore, it is impossible * for another concurrent prepare_report_producer() to set the same * @other_info, and are guaranteed exclusivity for the @other_infos * entry pointed to by @other_info. * * To check this property holds, size should never be non-zero here, * because every consumer of struct other_info resets size to 0 in * release_report(). */ WARN_ON(other_info->ai.size); other_info->ai = *ai; other_info->num_stack_entries = stack_trace_save(other_info->stack_entries, NUM_STACK_ENTRIES, 2); if (IS_ENABLED(CONFIG_KCSAN_VERBOSE)) set_other_info_task_blocking(flags, ai, other_info); raw_spin_unlock_irqrestore(&report_lock, *flags); } /* Awaits producer to fill @other_info and then returns. */ static bool prepare_report_consumer(unsigned long *flags, const struct access_info *ai, struct other_info *other_info) { raw_spin_lock_irqsave(&report_lock, *flags); while (!other_info->ai.size) { /* Await valid @other_info. */ raw_spin_unlock_irqrestore(&report_lock, *flags); cpu_relax(); raw_spin_lock_irqsave(&report_lock, *flags); } /* Should always have a matching access based on watchpoint encoding. */ if (WARN_ON(!matching_access((unsigned long)other_info->ai.ptr & WATCHPOINT_ADDR_MASK, other_info->ai.size, (unsigned long)ai->ptr & WATCHPOINT_ADDR_MASK, ai->size))) goto discard; if (!matching_access((unsigned long)other_info->ai.ptr, other_info->ai.size, (unsigned long)ai->ptr, ai->size)) { /* * If the actual accesses to not match, this was a false * positive due to watchpoint encoding. */ atomic_long_inc(&kcsan_counters[KCSAN_COUNTER_ENCODING_FALSE_POSITIVES]); goto discard; } return true; discard: release_report(flags, other_info); return false; } /* * Depending on the report type either sets @other_info and returns false, or * awaits @other_info and returns true. If @other_info is not required for the * report type, simply acquires @report_lock and returns true. */ static noinline bool prepare_report(unsigned long *flags, enum kcsan_report_type type, const struct access_info *ai, struct other_info *other_info) { switch (type) { case KCSAN_REPORT_CONSUMED_WATCHPOINT: prepare_report_producer(flags, ai, other_info); return false; case KCSAN_REPORT_RACE_SIGNAL: return prepare_report_consumer(flags, ai, other_info); default: /* @other_info not required; just acquire @report_lock. */ raw_spin_lock_irqsave(&report_lock, *flags); return true; } } void kcsan_report(const volatile void *ptr, size_t size, int access_type, enum kcsan_value_change value_change, enum kcsan_report_type type, int watchpoint_idx) { unsigned long flags = 0; const struct access_info ai = { .ptr = ptr, .size = size, .access_type = access_type, .task_pid = in_task() ? task_pid_nr(current) : -1, .cpu_id = raw_smp_processor_id() }; struct other_info *other_info = type == KCSAN_REPORT_RACE_UNKNOWN_ORIGIN ? NULL : &other_infos[watchpoint_idx]; kcsan_disable_current(); if (WARN_ON(watchpoint_idx < 0 || watchpoint_idx >= ARRAY_SIZE(other_infos))) goto out; /* * Because we may generate reports when we're in scheduler code, the use * of printk() could deadlock. Until such time that all printing code * called in print_report() is scheduler-safe, accept the risk, and just * get our message out. As such, also disable lockdep to hide the * warning, and avoid disabling lockdep for the rest of the kernel. */ lockdep_off(); if (prepare_report(&flags, type, &ai, other_info)) { /* * Never report if value_change is FALSE, only if we it is * either TRUE or MAYBE. In case of MAYBE, further filtering may * be done once we know the full stack trace in print_report(). */ bool reported = value_change != KCSAN_VALUE_CHANGE_FALSE && print_report(value_change, type, &ai, other_info); if (reported && panic_on_warn) panic("panic_on_warn set ...\n"); release_report(&flags, other_info); } lockdep_on(); out: kcsan_enable_current(); }