Merge tag 'block-5.11-2021-01-16' of git://git.kernel.dk/linux-block

[linux/fpc-iii.git] / kernel / kcsan / report.c

// SPDX-License-Identifier: GPL-2.0

#include <linux/debug_locks.h>
#include <linux/delay.h>
#include <linux/jiffies.h>
#include <linux/kernel.h>
#include <linux/lockdep.h>
#include <linux/preempt.h>
#include <linux/printk.h>
#include <linux/sched.h>
#include <linux/spinlock.h>
#include <linux/stacktrace.h>

#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 = current->state == TASK_RUNNING;
        /*
         * 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();
}