[linux-2.6-microblaze.git] / mm / kfence / core.c

// SPDX-License-Identifier: GPL-2.0
/*
 * KFENCE guarded object allocator and fault handling.
 *
 * Copyright (C) 2020, Google LLC.
 */

#define pr_fmt(fmt) "kfence: " fmt

#include <linux/atomic.h>
#include <linux/bug.h>
#include <linux/debugfs.h>
#include <linux/kcsan-checks.h>
#include <linux/kfence.h>
#include <linux/list.h>
#include <linux/lockdep.h>
#include <linux/memblock.h>
#include <linux/moduleparam.h>
#include <linux/random.h>
#include <linux/rcupdate.h>
#include <linux/seq_file.h>
#include <linux/slab.h>
#include <linux/spinlock.h>
#include <linux/string.h>

#include <asm/kfence.h>

#include "kfence.h"

/* Disables KFENCE on the first warning assuming an irrecoverable error. */
#define KFENCE_WARN_ON(cond)                                                   \
        ({                                                                     \
                const bool __cond = WARN_ON(cond);                             \
                if (unlikely(__cond))                                          \
                        WRITE_ONCE(kfence_enabled, false);                     \
                __cond;                                                        \
        })

/* === Data ================================================================= */

static bool kfence_enabled __read_mostly;

static unsigned long kfence_sample_interval __read_mostly = CONFIG_KFENCE_SAMPLE_INTERVAL;

#ifdef MODULE_PARAM_PREFIX
#undef MODULE_PARAM_PREFIX
#endif
#define MODULE_PARAM_PREFIX "kfence."

static int param_set_sample_interval(const char *val, const struct kernel_param *kp)
{
        unsigned long num;
        int ret = kstrtoul(val, 0, &num);

        if (ret < 0)
                return ret;

        if (!num) /* Using 0 to indicate KFENCE is disabled. */
                WRITE_ONCE(kfence_enabled, false);
        else if (!READ_ONCE(kfence_enabled) && system_state != SYSTEM_BOOTING)
                return -EINVAL; /* Cannot (re-)enable KFENCE on-the-fly. */

        *((unsigned long *)kp->arg) = num;
        return 0;
}

static int param_get_sample_interval(char *buffer, const struct kernel_param *kp)
{
        if (!READ_ONCE(kfence_enabled))
                return sprintf(buffer, "0\n");

        return param_get_ulong(buffer, kp);
}

static const struct kernel_param_ops sample_interval_param_ops = {
        .set = param_set_sample_interval,
        .get = param_get_sample_interval,
};
module_param_cb(sample_interval, &sample_interval_param_ops, &kfence_sample_interval, 0600);

/* The pool of pages used for guard pages and objects. */
char *__kfence_pool __ro_after_init;
EXPORT_SYMBOL(__kfence_pool); /* Export for test modules. */

/*
 * Per-object metadata, with one-to-one mapping of object metadata to
 * backing pages (in __kfence_pool).
 */
static_assert(CONFIG_KFENCE_NUM_OBJECTS > 0);
struct kfence_metadata kfence_metadata[CONFIG_KFENCE_NUM_OBJECTS];

/* Freelist with available objects. */
static struct list_head kfence_freelist = LIST_HEAD_INIT(kfence_freelist);
static DEFINE_RAW_SPINLOCK(kfence_freelist_lock); /* Lock protecting freelist. */

#ifdef CONFIG_KFENCE_STATIC_KEYS
/* The static key to set up a KFENCE allocation. */
DEFINE_STATIC_KEY_FALSE(kfence_allocation_key);
#endif

/* Gates the allocation, ensuring only one succeeds in a given period. */
atomic_t kfence_allocation_gate = ATOMIC_INIT(1);

/* Statistics counters for debugfs. */
enum kfence_counter_id {
        KFENCE_COUNTER_ALLOCATED,
        KFENCE_COUNTER_ALLOCS,
        KFENCE_COUNTER_FREES,
        KFENCE_COUNTER_ZOMBIES,
        KFENCE_COUNTER_BUGS,
        KFENCE_COUNTER_COUNT,
};
static atomic_long_t counters[KFENCE_COUNTER_COUNT];
static const char *const counter_names[] = {
        [KFENCE_COUNTER_ALLOCATED]      = "currently allocated",
        [KFENCE_COUNTER_ALLOCS]         = "total allocations",
        [KFENCE_COUNTER_FREES]          = "total frees",
        [KFENCE_COUNTER_ZOMBIES]        = "zombie allocations",
        [KFENCE_COUNTER_BUGS]           = "total bugs",
};
static_assert(ARRAY_SIZE(counter_names) == KFENCE_COUNTER_COUNT);

/* === Internals ============================================================ */

static bool kfence_protect(unsigned long addr)
{
        return !KFENCE_WARN_ON(!kfence_protect_page(ALIGN_DOWN(addr, PAGE_SIZE), true));
}

static bool kfence_unprotect(unsigned long addr)
{
        return !KFENCE_WARN_ON(!kfence_protect_page(ALIGN_DOWN(addr, PAGE_SIZE), false));
}

static inline struct kfence_metadata *addr_to_metadata(unsigned long addr)
{
        long index;

        /* The checks do not affect performance; only called from slow-paths. */

        if (!is_kfence_address((void *)addr))
                return NULL;

        /*
         * May be an invalid index if called with an address at the edge of
         * __kfence_pool, in which case we would report an "invalid access"
         * error.
         */
        index = (addr - (unsigned long)__kfence_pool) / (PAGE_SIZE * 2) - 1;
        if (index < 0 || index >= CONFIG_KFENCE_NUM_OBJECTS)
                return NULL;

        return &kfence_metadata[index];
}

static inline unsigned long metadata_to_pageaddr(const struct kfence_metadata *meta)
{
        unsigned long offset = (meta - kfence_metadata + 1) * PAGE_SIZE * 2;
        unsigned long pageaddr = (unsigned long)&__kfence_pool[offset];

        /* The checks do not affect performance; only called from slow-paths. */

        /* Only call with a pointer into kfence_metadata. */
        if (KFENCE_WARN_ON(meta < kfence_metadata ||
                           meta >= kfence_metadata + CONFIG_KFENCE_NUM_OBJECTS))
                return 0;

        /*
         * This metadata object only ever maps to 1 page; verify that the stored
         * address is in the expected range.
         */
        if (KFENCE_WARN_ON(ALIGN_DOWN(meta->addr, PAGE_SIZE) != pageaddr))
                return 0;

        return pageaddr;
}

/*
 * Update the object's metadata state, including updating the alloc/free stacks
 * depending on the state transition.
 */
static noinline void metadata_update_state(struct kfence_metadata *meta,
                                           enum kfence_object_state next)
{
        struct kfence_track *track =
                next == KFENCE_OBJECT_FREED ? &meta->free_track : &meta->alloc_track;

        lockdep_assert_held(&meta->lock);

        /*
         * Skip over 1 (this) functions; noinline ensures we do not accidentally
         * skip over the caller by never inlining.
         */
        track->num_stack_entries = stack_trace_save(track->stack_entries, KFENCE_STACK_DEPTH, 1);
        track->pid = task_pid_nr(current);

        /*
         * Pairs with READ_ONCE() in
         *      kfence_shutdown_cache(),
         *      kfence_handle_page_fault().
         */
        WRITE_ONCE(meta->state, next);
}

/* Write canary byte to @addr. */
static inline bool set_canary_byte(u8 *addr)
{
        *addr = KFENCE_CANARY_PATTERN(addr);
        return true;
}

/* Check canary byte at @addr. */
static inline bool check_canary_byte(u8 *addr)
{
        if (likely(*addr == KFENCE_CANARY_PATTERN(addr)))
                return true;

        atomic_long_inc(&counters[KFENCE_COUNTER_BUGS]);
        kfence_report_error((unsigned long)addr, NULL, addr_to_metadata((unsigned long)addr),
                            KFENCE_ERROR_CORRUPTION);
        return false;
}

/* __always_inline this to ensure we won't do an indirect call to fn. */
static __always_inline void for_each_canary(const struct kfence_metadata *meta, bool (*fn)(u8 *))
{
        const unsigned long pageaddr = ALIGN_DOWN(meta->addr, PAGE_SIZE);
        unsigned long addr;

        lockdep_assert_held(&meta->lock);

        /*
         * We'll iterate over each canary byte per-side until fn() returns
         * false. However, we'll still iterate over the canary bytes to the
         * right of the object even if there was an error in the canary bytes to
         * the left of the object. Specifically, if check_canary_byte()
         * generates an error, showing both sides might give more clues as to
         * what the error is about when displaying which bytes were corrupted.
         */

        /* Apply to left of object. */
        for (addr = pageaddr; addr < meta->addr; addr++) {
                if (!fn((u8 *)addr))
                        break;
        }

        /* Apply to right of object. */
        for (addr = meta->addr + meta->size; addr < pageaddr + PAGE_SIZE; addr++) {
                if (!fn((u8 *)addr))
                        break;
        }
}

static void *kfence_guarded_alloc(struct kmem_cache *cache, size_t size, gfp_t gfp)
{
        struct kfence_metadata *meta = NULL;
        unsigned long flags;
        struct page *page;
        void *addr;

        /* Try to obtain a free object. */
        raw_spin_lock_irqsave(&kfence_freelist_lock, flags);
        if (!list_empty(&kfence_freelist)) {
                meta = list_entry(kfence_freelist.next, struct kfence_metadata, list);
                list_del_init(&meta->list);
        }
        raw_spin_unlock_irqrestore(&kfence_freelist_lock, flags);
        if (!meta)
                return NULL;

        if (unlikely(!raw_spin_trylock_irqsave(&meta->lock, flags))) {
                /*
                 * This is extremely unlikely -- we are reporting on a
                 * use-after-free, which locked meta->lock, and the reporting
                 * code via printk calls kmalloc() which ends up in
                 * kfence_alloc() and tries to grab the same object that we're
                 * reporting on. While it has never been observed, lockdep does
                 * report that there is a possibility of deadlock. Fix it by
                 * using trylock and bailing out gracefully.
                 */
                raw_spin_lock_irqsave(&kfence_freelist_lock, flags);
                /* Put the object back on the freelist. */
                list_add_tail(&meta->list, &kfence_freelist);
                raw_spin_unlock_irqrestore(&kfence_freelist_lock, flags);

                return NULL;
        }

        meta->addr = metadata_to_pageaddr(meta);
        /* Unprotect if we're reusing this page. */
        if (meta->state == KFENCE_OBJECT_FREED)
                kfence_unprotect(meta->addr);

        /*
         * Note: for allocations made before RNG initialization, will always
         * return zero. We still benefit from enabling KFENCE as early as
         * possible, even when the RNG is not yet available, as this will allow
         * KFENCE to detect bugs due to earlier allocations. The only downside
         * is that the out-of-bounds accesses detected are deterministic for
         * such allocations.
         */
        if (prandom_u32_max(2)) {
                /* Allocate on the "right" side, re-calculate address. */
                meta->addr += PAGE_SIZE - size;
                meta->addr = ALIGN_DOWN(meta->addr, cache->align);
        }

        addr = (void *)meta->addr;

        /* Update remaining metadata. */
        metadata_update_state(meta, KFENCE_OBJECT_ALLOCATED);
        /* Pairs with READ_ONCE() in kfence_shutdown_cache(). */
        WRITE_ONCE(meta->cache, cache);
        meta->size = size;
        for_each_canary(meta, set_canary_byte);

        /* Set required struct page fields. */
        page = virt_to_page(meta->addr);
        page->slab_cache = cache;

        raw_spin_unlock_irqrestore(&meta->lock, flags);

        /* Memory initialization. */

        /*
         * We check slab_want_init_on_alloc() ourselves, rather than letting
         * SL*B do the initialization, as otherwise we might overwrite KFENCE's
         * redzone.
         */
        if (unlikely(slab_want_init_on_alloc(gfp, cache)))
                memzero_explicit(addr, size);
        if (cache->ctor)
                cache->ctor(addr);

        if (CONFIG_KFENCE_STRESS_TEST_FAULTS && !prandom_u32_max(CONFIG_KFENCE_STRESS_TEST_FAULTS))
                kfence_protect(meta->addr); /* Random "faults" by protecting the object. */

        atomic_long_inc(&counters[KFENCE_COUNTER_ALLOCATED]);
        atomic_long_inc(&counters[KFENCE_COUNTER_ALLOCS]);

        return addr;
}

static void kfence_guarded_free(void *addr, struct kfence_metadata *meta, bool zombie)
{
        struct kcsan_scoped_access assert_page_exclusive;
        unsigned long flags;

        raw_spin_lock_irqsave(&meta->lock, flags);

        if (meta->state != KFENCE_OBJECT_ALLOCATED || meta->addr != (unsigned long)addr) {
                /* Invalid or double-free, bail out. */
                atomic_long_inc(&counters[KFENCE_COUNTER_BUGS]);
                kfence_report_error((unsigned long)addr, NULL, meta, KFENCE_ERROR_INVALID_FREE);
                raw_spin_unlock_irqrestore(&meta->lock, flags);
                return;
        }

        /* Detect racy use-after-free, or incorrect reallocation of this page by KFENCE. */
        kcsan_begin_scoped_access((void *)ALIGN_DOWN((unsigned long)addr, PAGE_SIZE), PAGE_SIZE,
                                  KCSAN_ACCESS_SCOPED | KCSAN_ACCESS_WRITE | KCSAN_ACCESS_ASSERT,
                                  &assert_page_exclusive);

        if (CONFIG_KFENCE_STRESS_TEST_FAULTS)
                kfence_unprotect((unsigned long)addr); /* To check canary bytes. */

        /* Restore page protection if there was an OOB access. */
        if (meta->unprotected_page) {
                kfence_protect(meta->unprotected_page);
                meta->unprotected_page = 0;
        }

        /* Check canary bytes for memory corruption. */
        for_each_canary(meta, check_canary_byte);

        /*
         * Clear memory if init-on-free is set. While we protect the page, the
         * data is still there, and after a use-after-free is detected, we
         * unprotect the page, so the data is still accessible.
         */
        if (!zombie && unlikely(slab_want_init_on_free(meta->cache)))
                memzero_explicit(addr, meta->size);

        /* Mark the object as freed. */
        metadata_update_state(meta, KFENCE_OBJECT_FREED);

        raw_spin_unlock_irqrestore(&meta->lock, flags);

        /* Protect to detect use-after-frees. */
        kfence_protect((unsigned long)addr);

        kcsan_end_scoped_access(&assert_page_exclusive);
        if (!zombie) {
                /* Add it to the tail of the freelist for reuse. */
                raw_spin_lock_irqsave(&kfence_freelist_lock, flags);
                KFENCE_WARN_ON(!list_empty(&meta->list));
                list_add_tail(&meta->list, &kfence_freelist);
                raw_spin_unlock_irqrestore(&kfence_freelist_lock, flags);

                atomic_long_dec(&counters[KFENCE_COUNTER_ALLOCATED]);
                atomic_long_inc(&counters[KFENCE_COUNTER_FREES]);
        } else {
                /* See kfence_shutdown_cache(). */
                atomic_long_inc(&counters[KFENCE_COUNTER_ZOMBIES]);
        }
}

static void rcu_guarded_free(struct rcu_head *h)
{
        struct kfence_metadata *meta = container_of(h, struct kfence_metadata, rcu_head);

        kfence_guarded_free((void *)meta->addr, meta, false);
}

static bool __init kfence_init_pool(void)
{
        unsigned long addr = (unsigned long)__kfence_pool;
        struct page *pages;
        int i;

        if (!__kfence_pool)
                return false;

        if (!arch_kfence_init_pool())
                goto err;

        pages = virt_to_page(addr);

        /*
         * Set up object pages: they must have PG_slab set, to avoid freeing
         * these as real pages.
         *
         * We also want to avoid inserting kfence_free() in the kfree()
         * fast-path in SLUB, and therefore need to ensure kfree() correctly
         * enters __slab_free() slow-path.
         */
        for (i = 0; i < KFENCE_POOL_SIZE / PAGE_SIZE; i++) {
                if (!i || (i % 2))
                        continue;

                /* Verify we do not have a compound head page. */
                if (WARN_ON(compound_head(&pages[i]) != &pages[i]))
                        goto err;

                __SetPageSlab(&pages[i]);
        }

        /*
         * Protect the first 2 pages. The first page is mostly unnecessary, and
         * merely serves as an extended guard page. However, adding one
         * additional page in the beginning gives us an even number of pages,
         * which simplifies the mapping of address to metadata index.
         */
        for (i = 0; i < 2; i++) {
                if (unlikely(!kfence_protect(addr)))
                        goto err;

                addr += PAGE_SIZE;
        }

        for (i = 0; i < CONFIG_KFENCE_NUM_OBJECTS; i++) {
                struct kfence_metadata *meta = &kfence_metadata[i];

                /* Initialize metadata. */
                INIT_LIST_HEAD(&meta->list);
                raw_spin_lock_init(&meta->lock);
                meta->state = KFENCE_OBJECT_UNUSED;
                meta->addr = addr; /* Initialize for validation in metadata_to_pageaddr(). */
                list_add_tail(&meta->list, &kfence_freelist);

                /* Protect the right redzone. */
                if (unlikely(!kfence_protect(addr + PAGE_SIZE)))
                        goto err;

                addr += 2 * PAGE_SIZE;
        }

        return true;

err:
        /*
         * Only release unprotected pages, and do not try to go back and change
         * page attributes due to risk of failing to do so as well. If changing
         * page attributes for some pages fails, it is very likely that it also
         * fails for the first page, and therefore expect addr==__kfence_pool in
         * most failure cases.
         */
        memblock_free_late(__pa(addr), KFENCE_POOL_SIZE - (addr - (unsigned long)__kfence_pool));
        __kfence_pool = NULL;
        return false;
}

/* === DebugFS Interface ==================================================== */

static int stats_show(struct seq_file *seq, void *v)
{
        int i;

        seq_printf(seq, "enabled: %i\n", READ_ONCE(kfence_enabled));
        for (i = 0; i < KFENCE_COUNTER_COUNT; i++)
                seq_printf(seq, "%s: %ld\n", counter_names[i], atomic_long_read(&counters[i]));

        return 0;
}
DEFINE_SHOW_ATTRIBUTE(stats);

/*
 * debugfs seq_file operations for /sys/kernel/debug/kfence/objects.
 * start_object() and next_object() return the object index + 1, because NULL is used
 * to stop iteration.
 */
static void *start_object(struct seq_file *seq, loff_t *pos)
{
        if (*pos < CONFIG_KFENCE_NUM_OBJECTS)
                return (void *)((long)*pos + 1);
        return NULL;
}

static void stop_object(struct seq_file *seq, void *v)
{
}

static void *next_object(struct seq_file *seq, void *v, loff_t *pos)
{
        ++*pos;
        if (*pos < CONFIG_KFENCE_NUM_OBJECTS)
                return (void *)((long)*pos + 1);
        return NULL;
}

static int show_object(struct seq_file *seq, void *v)
{
        struct kfence_metadata *meta = &kfence_metadata[(long)v - 1];
        unsigned long flags;

        raw_spin_lock_irqsave(&meta->lock, flags);
        kfence_print_object(seq, meta);
        raw_spin_unlock_irqrestore(&meta->lock, flags);
        seq_puts(seq, "---------------------------------\n");

        return 0;
}

static const struct seq_operations object_seqops = {
        .start = start_object,
        .next = next_object,
        .stop = stop_object,
        .show = show_object,
};

static int open_objects(struct inode *inode, struct file *file)
{
        return seq_open(file, &object_seqops);
}

static const struct file_operations objects_fops = {
        .open = open_objects,
        .read = seq_read,
        .llseek = seq_lseek,
};

static int __init kfence_debugfs_init(void)
{
        struct dentry *kfence_dir = debugfs_create_dir("kfence", NULL);

        debugfs_create_file("stats", 0444, kfence_dir, NULL, &stats_fops);
        debugfs_create_file("objects", 0400, kfence_dir, NULL, &objects_fops);
        return 0;
}

late_initcall(kfence_debugfs_init);

/* === Allocation Gate Timer ================================================ */

/*
 * Set up delayed work, which will enable and disable the static key. We need to
 * use a work queue (rather than a simple timer), since enabling and disabling a
 * static key cannot be done from an interrupt.
 *
 * Note: Toggling a static branch currently causes IPIs, and here we'll end up
 * with a total of 2 IPIs to all CPUs. If this ends up a problem in future (with
 * more aggressive sampling intervals), we could get away with a variant that
 * avoids IPIs, at the cost of not immediately capturing allocations if the
 * instructions remain cached.
 */
static struct delayed_work kfence_timer;
static void toggle_allocation_gate(struct work_struct *work)
{
        if (!READ_ONCE(kfence_enabled))
                return;

        /* Enable static key, and await allocation to happen. */
        atomic_set(&kfence_allocation_gate, 0);
#ifdef CONFIG_KFENCE_STATIC_KEYS
        static_branch_enable(&kfence_allocation_key);
        /*
         * Await an allocation. Timeout after 1 second, in case the kernel stops
         * doing allocations, to avoid stalling this worker task for too long.
         */
        {
                unsigned long end_wait = jiffies + HZ;

                do {
                        set_current_state(TASK_UNINTERRUPTIBLE);
                        if (atomic_read(&kfence_allocation_gate) != 0)
                                break;
                        schedule_timeout(1);
                } while (time_before(jiffies, end_wait));
                __set_current_state(TASK_RUNNING);
        }
        /* Disable static key and reset timer. */
        static_branch_disable(&kfence_allocation_key);
#endif
        schedule_delayed_work(&kfence_timer, msecs_to_jiffies(kfence_sample_interval));
}
static DECLARE_DELAYED_WORK(kfence_timer, toggle_allocation_gate);

/* === Public interface ===================================================== */

void __init kfence_alloc_pool(void)
{
        if (!kfence_sample_interval)
                return;

        __kfence_pool = memblock_alloc(KFENCE_POOL_SIZE, PAGE_SIZE);

        if (!__kfence_pool)
                pr_err("failed to allocate pool\n");
}

void __init kfence_init(void)
{
        /* Setting kfence_sample_interval to 0 on boot disables KFENCE. */
        if (!kfence_sample_interval)
                return;

        if (!kfence_init_pool()) {
                pr_err("%s failed\n", __func__);
                return;
        }

        WRITE_ONCE(kfence_enabled, true);
        schedule_delayed_work(&kfence_timer, 0);
        pr_info("initialized - using %lu bytes for %d objects", KFENCE_POOL_SIZE,
                CONFIG_KFENCE_NUM_OBJECTS);
        if (IS_ENABLED(CONFIG_DEBUG_KERNEL))
                pr_cont(" at 0x%px-0x%px\n", (void *)__kfence_pool,
                        (void *)(__kfence_pool + KFENCE_POOL_SIZE));
        else
                pr_cont("\n");
}

void kfence_shutdown_cache(struct kmem_cache *s)
{
        unsigned long flags;
        struct kfence_metadata *meta;
        int i;

        for (i = 0; i < CONFIG_KFENCE_NUM_OBJECTS; i++) {
                bool in_use;

                meta = &kfence_metadata[i];

                /*
                 * If we observe some inconsistent cache and state pair where we
                 * should have returned false here, cache destruction is racing
                 * with either kmem_cache_alloc() or kmem_cache_free(). Taking
                 * the lock will not help, as different critical section
                 * serialization will have the same outcome.
                 */
                if (READ_ONCE(meta->cache) != s ||
                    READ_ONCE(meta->state) != KFENCE_OBJECT_ALLOCATED)
                        continue;

                raw_spin_lock_irqsave(&meta->lock, flags);
                in_use = meta->cache == s && meta->state == KFENCE_OBJECT_ALLOCATED;
                raw_spin_unlock_irqrestore(&meta->lock, flags);

                if (in_use) {
                        /*
                         * This cache still has allocations, and we should not
                         * release them back into the freelist so they can still
                         * safely be used and retain the kernel's default
                         * behaviour of keeping the allocations alive (leak the
                         * cache); however, they effectively become "zombie
                         * allocations" as the KFENCE objects are the only ones
                         * still in use and the owning cache is being destroyed.
                         *
                         * We mark them freed, so that any subsequent use shows
                         * more useful error messages that will include stack
                         * traces of the user of the object, the original
                         * allocation, and caller to shutdown_cache().
                         */
                        kfence_guarded_free((void *)meta->addr, meta, /*zombie=*/true);
                }
        }

        for (i = 0; i < CONFIG_KFENCE_NUM_OBJECTS; i++) {
                meta = &kfence_metadata[i];

                /* See above. */
                if (READ_ONCE(meta->cache) != s || READ_ONCE(meta->state) != KFENCE_OBJECT_FREED)
                        continue;

                raw_spin_lock_irqsave(&meta->lock, flags);
                if (meta->cache == s && meta->state == KFENCE_OBJECT_FREED)
                        meta->cache = NULL;
                raw_spin_unlock_irqrestore(&meta->lock, flags);
        }
}

void *__kfence_alloc(struct kmem_cache *s, size_t size, gfp_t flags)
{
        /*
         * allocation_gate only needs to become non-zero, so it doesn't make
         * sense to continue writing to it and pay the associated contention
         * cost, in case we have a large number of concurrent allocations.
         */
        if (atomic_read(&kfence_allocation_gate) || atomic_inc_return(&kfence_allocation_gate) > 1)
                return NULL;

        if (!READ_ONCE(kfence_enabled))
                return NULL;

        if (size > PAGE_SIZE)
                return NULL;

        return kfence_guarded_alloc(s, size, flags);
}

size_t kfence_ksize(const void *addr)
{
        const struct kfence_metadata *meta = addr_to_metadata((unsigned long)addr);

        /*
         * Read locklessly -- if there is a race with __kfence_alloc(), this is
         * either a use-after-free or invalid access.
         */
        return meta ? meta->size : 0;
}

void *kfence_object_start(const void *addr)
{
        const struct kfence_metadata *meta = addr_to_metadata((unsigned long)addr);

        /*
         * Read locklessly -- if there is a race with __kfence_alloc(), this is
         * either a use-after-free or invalid access.
         */
        return meta ? (void *)meta->addr : NULL;
}

void __kfence_free(void *addr)
{
        struct kfence_metadata *meta = addr_to_metadata((unsigned long)addr);

        /*
         * If the objects of the cache are SLAB_TYPESAFE_BY_RCU, defer freeing
         * the object, as the object page may be recycled for other-typed
         * objects once it has been freed. meta->cache may be NULL if the cache
         * was destroyed.
         */
        if (unlikely(meta->cache && (meta->cache->flags & SLAB_TYPESAFE_BY_RCU)))
                call_rcu(&meta->rcu_head, rcu_guarded_free);
        else
                kfence_guarded_free(addr, meta, false);
}

bool kfence_handle_page_fault(unsigned long addr, struct pt_regs *regs)
{
        const int page_index = (addr - (unsigned long)__kfence_pool) / PAGE_SIZE;
        struct kfence_metadata *to_report = NULL;
        enum kfence_error_type error_type;
        unsigned long flags;

        if (!is_kfence_address((void *)addr))
                return false;

        if (!READ_ONCE(kfence_enabled)) /* If disabled at runtime ... */
                return kfence_unprotect(addr); /* ... unprotect and proceed. */

        atomic_long_inc(&counters[KFENCE_COUNTER_BUGS]);

        if (page_index % 2) {
                /* This is a redzone, report a buffer overflow. */
                struct kfence_metadata *meta;
                int distance = 0;

                meta = addr_to_metadata(addr - PAGE_SIZE);
                if (meta && READ_ONCE(meta->state) == KFENCE_OBJECT_ALLOCATED) {
                        to_report = meta;
                        /* Data race ok; distance calculation approximate. */
                        distance = addr - data_race(meta->addr + meta->size);
                }

                meta = addr_to_metadata(addr + PAGE_SIZE);
                if (meta && READ_ONCE(meta->state) == KFENCE_OBJECT_ALLOCATED) {
                        /* Data race ok; distance calculation approximate. */
                        if (!to_report || distance > data_race(meta->addr) - addr)
                                to_report = meta;
                }

                if (!to_report)
                        goto out;

                raw_spin_lock_irqsave(&to_report->lock, flags);
                to_report->unprotected_page = addr;
                error_type = KFENCE_ERROR_OOB;

                /*
                 * If the object was freed before we took the look we can still
                 * report this as an OOB -- the report will simply show the
                 * stacktrace of the free as well.
                 */
        } else {
                to_report = addr_to_metadata(addr);
                if (!to_report)
                        goto out;

                raw_spin_lock_irqsave(&to_report->lock, flags);
                error_type = KFENCE_ERROR_UAF;
                /*
                 * We may race with __kfence_alloc(), and it is possible that a
                 * freed object may be reallocated. We simply report this as a
                 * use-after-free, with the stack trace showing the place where
                 * the object was re-allocated.
                 */
        }

out:
        if (to_report) {
                kfence_report_error(addr, regs, to_report, error_type);
                raw_spin_unlock_irqrestore(&to_report->lock, flags);
        } else {
                /* This may be a UAF or OOB access, but we can't be sure. */
                kfence_report_error(addr, regs, NULL, KFENCE_ERROR_INVALID);
        }

        return kfence_unprotect(addr); /* Unprotect and let access proceed. */
}