Linux 6.14-rc3

[linux.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/hash.h>
#include <linux/irq_work.h>
#include <linux/jhash.h>
#include <linux/kcsan-checks.h>
#include <linux/kfence.h>
#include <linux/kmemleak.h>
#include <linux/list.h>
#include <linux/lockdep.h>
#include <linux/log2.h>
#include <linux/memblock.h>
#include <linux/moduleparam.h>
#include <linux/nodemask.h>
#include <linux/notifier.h>
#include <linux/panic_notifier.h>
#include <linux/random.h>
#include <linux/rcupdate.h>
#include <linux/sched/clock.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);                     \
                        disabled_by_warn = true;                               \
                }                                                              \
                __cond;                                                        \
        })

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

static bool kfence_enabled __read_mostly;
static bool disabled_by_warn __read_mostly;

unsigned long kfence_sample_interval __read_mostly = CONFIG_KFENCE_SAMPLE_INTERVAL;
EXPORT_SYMBOL_GPL(kfence_sample_interval); /* Export for test modules. */

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

static int kfence_enable_late(void);
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;

        /* Using 0 to indicate KFENCE is disabled. */
        if (!num && READ_ONCE(kfence_enabled)) {
                pr_info("disabled\n");
                WRITE_ONCE(kfence_enabled, false);
        }

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

        if (num && !READ_ONCE(kfence_enabled) && system_state != SYSTEM_BOOTING)
                return disabled_by_warn ? -EINVAL : kfence_enable_late();
        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);

/* Pool usage% threshold when currently covered allocations are skipped. */
static unsigned long kfence_skip_covered_thresh __read_mostly = 75;
module_param_named(skip_covered_thresh, kfence_skip_covered_thresh, ulong, 0644);

/* Allocation burst count: number of excess KFENCE allocations per sample. */
static unsigned int kfence_burst __read_mostly;
module_param_named(burst, kfence_burst, uint, 0644);

/* If true, use a deferrable timer. */
static bool kfence_deferrable __read_mostly = IS_ENABLED(CONFIG_KFENCE_DEFERRABLE);
module_param_named(deferrable, kfence_deferrable, bool, 0444);

/* If true, check all canary bytes on panic. */
static bool kfence_check_on_panic __read_mostly;
module_param_named(check_on_panic, kfence_check_on_panic, bool, 0444);

/* The pool of pages used for guard pages and objects. */
char *__kfence_pool __read_mostly;
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 __read_mostly;

/*
 * If kfence_metadata is not NULL, it may be accessed by kfence_shutdown_cache().
 * So introduce kfence_metadata_init to initialize metadata, and then make
 * kfence_metadata visible after initialization is successful. This prevents
 * potential UAF or access to uninitialized metadata.
 */
static struct kfence_metadata *kfence_metadata_init __read_mostly;

/* 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. */

/*
 * The static key to set up a KFENCE allocation; or if static keys are not used
 * to gate allocations, to avoid a load and compare if KFENCE is disabled.
 */
DEFINE_STATIC_KEY_FALSE(kfence_allocation_key);

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

/*
 * A Counting Bloom filter of allocation coverage: limits currently covered
 * allocations of the same source filling up the pool.
 *
 * Assuming a range of 15%-85% unique allocations in the pool at any point in
 * time, the below parameters provide a probablity of 0.02-0.33 for false
 * positive hits respectively:
 *
 *      P(alloc_traces) = (1 - e^(-HNUM * (alloc_traces / SIZE)) ^ HNUM
 */
#define ALLOC_COVERED_HNUM      2
#define ALLOC_COVERED_ORDER     (const_ilog2(CONFIG_KFENCE_NUM_OBJECTS) + 2)
#define ALLOC_COVERED_SIZE      (1 << ALLOC_COVERED_ORDER)
#define ALLOC_COVERED_HNEXT(h)  hash_32(h, ALLOC_COVERED_ORDER)
#define ALLOC_COVERED_MASK      (ALLOC_COVERED_SIZE - 1)
static atomic_t alloc_covered[ALLOC_COVERED_SIZE];

/* Stack depth used to determine uniqueness of an allocation. */
#define UNIQUE_ALLOC_STACK_DEPTH ((size_t)8)

/*
 * Randomness for stack hashes, making the same collisions across reboots and
 * different machines less likely.
 */
static u32 stack_hash_seed __ro_after_init;

/* 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_SKIP_INCOMPAT,
        KFENCE_COUNTER_SKIP_CAPACITY,
        KFENCE_COUNTER_SKIP_COVERED,
        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",
        [KFENCE_COUNTER_SKIP_INCOMPAT]  = "skipped allocations (incompatible)",
        [KFENCE_COUNTER_SKIP_CAPACITY]  = "skipped allocations (capacity)",
        [KFENCE_COUNTER_SKIP_COVERED]   = "skipped allocations (covered)",
};
static_assert(ARRAY_SIZE(counter_names) == KFENCE_COUNTER_COUNT);

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

static inline bool should_skip_covered(void)
{
        unsigned long thresh = (CONFIG_KFENCE_NUM_OBJECTS * kfence_skip_covered_thresh) / 100;

        return atomic_long_read(&counters[KFENCE_COUNTER_ALLOCATED]) > thresh;
}

static u32 get_alloc_stack_hash(unsigned long *stack_entries, size_t num_entries)
{
        num_entries = min(num_entries, UNIQUE_ALLOC_STACK_DEPTH);
        num_entries = filter_irq_stacks(stack_entries, num_entries);
        return jhash(stack_entries, num_entries * sizeof(stack_entries[0]), stack_hash_seed);
}

/*
 * Adds (or subtracts) count @val for allocation stack trace hash
 * @alloc_stack_hash from Counting Bloom filter.
 */
static void alloc_covered_add(u32 alloc_stack_hash, int val)
{
        int i;

        for (i = 0; i < ALLOC_COVERED_HNUM; i++) {
                atomic_add(val, &alloc_covered[alloc_stack_hash & ALLOC_COVERED_MASK]);
                alloc_stack_hash = ALLOC_COVERED_HNEXT(alloc_stack_hash);
        }
}

/*
 * Returns true if the allocation stack trace hash @alloc_stack_hash is
 * currently contained (non-zero count) in Counting Bloom filter.
 */
static bool alloc_covered_contains(u32 alloc_stack_hash)
{
        int i;

        for (i = 0; i < ALLOC_COVERED_HNUM; i++) {
                if (!atomic_read(&alloc_covered[alloc_stack_hash & ALLOC_COVERED_MASK]))
                        return false;
                alloc_stack_hash = ALLOC_COVERED_HNEXT(alloc_stack_hash);
        }

        return true;
}

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 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;
}

static inline bool kfence_obj_allocated(const struct kfence_metadata *meta)
{
        enum kfence_object_state state = READ_ONCE(meta->state);

        return state == KFENCE_OBJECT_ALLOCATED || state == KFENCE_OBJECT_RCU_FREEING;
}

/*
 * 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,
                      unsigned long *stack_entries, size_t num_stack_entries)
{
        struct kfence_track *track =
                next == KFENCE_OBJECT_ALLOCATED ? &meta->alloc_track : &meta->free_track;

        lockdep_assert_held(&meta->lock);

        /* Stack has been saved when calling rcu, skip. */
        if (READ_ONCE(meta->state) == KFENCE_OBJECT_RCU_FREEING)
                goto out;

        if (stack_entries) {
                memcpy(track->stack_entries, stack_entries,
                       num_stack_entries * sizeof(stack_entries[0]));
        } else {
                /*
                 * Skip over 1 (this) functions; noinline ensures we do not
                 * accidentally skip over the caller by never inlining.
                 */
                num_stack_entries = stack_trace_save(track->stack_entries, KFENCE_STACK_DEPTH, 1);
        }
        track->num_stack_entries = num_stack_entries;
        track->pid = task_pid_nr(current);
        track->cpu = raw_smp_processor_id();
        track->ts_nsec = local_clock(); /* Same source as printk timestamps. */

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

#ifdef CONFIG_KMSAN
#define check_canary_attributes noinline __no_kmsan_checks
#else
#define check_canary_attributes inline
#endif

/* Check canary byte at @addr. */
static check_canary_attributes bool check_canary_byte(u8 *addr)
{
        struct kfence_metadata *meta;
        unsigned long flags;

        if (likely(*addr == KFENCE_CANARY_PATTERN_U8(addr)))
                return true;

        atomic_long_inc(&counters[KFENCE_COUNTER_BUGS]);

        meta = addr_to_metadata((unsigned long)addr);
        raw_spin_lock_irqsave(&meta->lock, flags);
        kfence_report_error((unsigned long)addr, false, NULL, meta, KFENCE_ERROR_CORRUPTION);
        raw_spin_unlock_irqrestore(&meta->lock, flags);

        return false;
}

static inline void set_canary(const struct kfence_metadata *meta)
{
        const unsigned long pageaddr = ALIGN_DOWN(meta->addr, PAGE_SIZE);
        unsigned long addr = pageaddr;

        /*
         * The canary may be written to part of the object memory, but it does
         * not affect it. The user should initialize the object before using it.
         */
        for (; addr < meta->addr; addr += sizeof(u64))
                *((u64 *)addr) = KFENCE_CANARY_PATTERN_U64;

        addr = ALIGN_DOWN(meta->addr + meta->size, sizeof(u64));
        for (; addr - pageaddr < PAGE_SIZE; addr += sizeof(u64))
                *((u64 *)addr) = KFENCE_CANARY_PATTERN_U64;
}

static check_canary_attributes void
check_canary(const struct kfence_metadata *meta)
{
        const unsigned long pageaddr = ALIGN_DOWN(meta->addr, PAGE_SIZE);
        unsigned long addr = pageaddr;

        /*
         * We'll iterate over each canary byte per-side until a corrupted byte
         * is found. 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 (; meta->addr - addr >= sizeof(u64); addr += sizeof(u64)) {
                if (unlikely(*((u64 *)addr) != KFENCE_CANARY_PATTERN_U64))
                        break;
        }

        /*
         * If the canary is corrupted in a certain 64 bytes, or the canary
         * memory cannot be completely covered by multiple consecutive 64 bytes,
         * it needs to be checked one by one.
         */
        for (; addr < meta->addr; addr++) {
                if (unlikely(!check_canary_byte((u8 *)addr)))
                        break;
        }

        /* Apply to right of object. */
        for (addr = meta->addr + meta->size; addr % sizeof(u64) != 0; addr++) {
                if (unlikely(!check_canary_byte((u8 *)addr)))
                        return;
        }
        for (; addr - pageaddr < PAGE_SIZE; addr += sizeof(u64)) {
                if (unlikely(*((u64 *)addr) != KFENCE_CANARY_PATTERN_U64)) {

                        for (; addr - pageaddr < PAGE_SIZE; addr++) {
                                if (!check_canary_byte((u8 *)addr))
                                        return;
                        }
                }
        }
}

static void *kfence_guarded_alloc(struct kmem_cache *cache, size_t size, gfp_t gfp,
                                  unsigned long *stack_entries, size_t num_stack_entries,
                                  u32 alloc_stack_hash)
{
        struct kfence_metadata *meta = NULL;
        unsigned long flags;
        struct slab *slab;
        void *addr;
        const bool random_right_allocate = get_random_u32_below(2);
        const bool random_fault = CONFIG_KFENCE_STRESS_TEST_FAULTS &&
                                  !get_random_u32_below(CONFIG_KFENCE_STRESS_TEST_FAULTS);

        /* 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) {
                atomic_long_inc(&counters[KFENCE_COUNTER_SKIP_CAPACITY]);
                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 (random_right_allocate) {
                /* 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, stack_entries, num_stack_entries);
        /* Pairs with READ_ONCE() in kfence_shutdown_cache(). */
        WRITE_ONCE(meta->cache, cache);
        meta->size = size;
        meta->alloc_stack_hash = alloc_stack_hash;
        raw_spin_unlock_irqrestore(&meta->lock, flags);

        alloc_covered_add(alloc_stack_hash, 1);

        /* Set required slab fields. */
        slab = virt_to_slab((void *)meta->addr);
        slab->slab_cache = cache;
        slab->objects = 1;

        /* Memory initialization. */
        set_canary(meta);

        /*
         * 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 (random_fault)
                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;
        bool init;

        raw_spin_lock_irqsave(&meta->lock, flags);

        if (!kfence_obj_allocated(meta) || meta->addr != (unsigned long)addr) {
                /* Invalid or double-free, bail out. */
                atomic_long_inc(&counters[KFENCE_COUNTER_BUGS]);
                kfence_report_error((unsigned long)addr, false, 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) {
                memzero_explicit((void *)ALIGN_DOWN(meta->unprotected_page, PAGE_SIZE), PAGE_SIZE);
                kfence_protect(meta->unprotected_page);
                meta->unprotected_page = 0;
        }

        /* Mark the object as freed. */
        metadata_update_state(meta, KFENCE_OBJECT_FREED, NULL, 0);
        init = slab_want_init_on_free(meta->cache);
        raw_spin_unlock_irqrestore(&meta->lock, flags);

        alloc_covered_add(meta->alloc_stack_hash, -1);

        /* Check canary bytes for memory corruption. */
        check_canary(meta);

        /*
         * 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(init))
                memzero_explicit(addr, meta->size);

        /* 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);
}

/*
 * Initialization of the KFENCE pool after its allocation.
 * Returns 0 on success; otherwise returns the address up to
 * which partial initialization succeeded.
 */
static unsigned long kfence_init_pool(void)
{
        unsigned long addr;
        struct page *pages;
        int i;

        if (!arch_kfence_init_pool())
                return (unsigned long)__kfence_pool;

        addr = (unsigned long)__kfence_pool;
        pages = virt_to_page(__kfence_pool);

        /*
         * 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++) {
                struct slab *slab = page_slab(nth_page(pages, i));

                if (!i || (i % 2))
                        continue;

                __folio_set_slab(slab_folio(slab));
#ifdef CONFIG_MEMCG
                slab->obj_exts = (unsigned long)&kfence_metadata_init[i / 2 - 1].obj_exts |
                                 MEMCG_DATA_OBJEXTS;
#endif
        }

        /*
         * 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)))
                        return addr;

                addr += PAGE_SIZE;
        }

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

                addr += 2 * PAGE_SIZE;
        }

        /*
         * Make kfence_metadata visible only when initialization is successful.
         * Otherwise, if the initialization fails and kfence_metadata is freed,
         * it may cause UAF in kfence_shutdown_cache().
         */
        smp_store_release(&kfence_metadata, kfence_metadata_init);
        return 0;

reset_slab:
        for (i = 0; i < KFENCE_POOL_SIZE / PAGE_SIZE; i++) {
                struct slab *slab = page_slab(nth_page(pages, i));

                if (!i || (i % 2))
                        continue;
#ifdef CONFIG_MEMCG
                slab->obj_exts = 0;
#endif
                __folio_clear_slab(slab_folio(slab));
        }

        return addr;
}

static bool __init kfence_init_pool_early(void)
{
        unsigned long addr;

        if (!__kfence_pool)
                return false;

        addr = kfence_init_pool();

        if (!addr) {
                /*
                 * The pool is live and will never be deallocated from this point on.
                 * Ignore the pool object from the kmemleak phys object tree, as it would
                 * otherwise overlap with allocations returned by kfence_alloc(), which
                 * are registered with kmemleak through the slab post-alloc hook.
                 */
                kmemleak_ignore_phys(__pa(__kfence_pool));
                return true;
        }

        /*
         * 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;

        memblock_free_late(__pa(kfence_metadata_init), KFENCE_METADATA_SIZE);
        kfence_metadata_init = 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 objects_sops = {
        .start = start_object,
        .next = next_object,
        .stop = stop_object,
        .show = show_object,
};
DEFINE_SEQ_ATTRIBUTE(objects);

static int kfence_debugfs_init(void)
{
        struct dentry *kfence_dir;

        if (!READ_ONCE(kfence_enabled))
                return 0;

        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);

/* === Panic Notifier ====================================================== */

static void kfence_check_all_canary(void)
{
        int i;

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

                if (kfence_obj_allocated(meta))
                        check_canary(meta);
        }
}

static int kfence_check_canary_callback(struct notifier_block *nb,
                                        unsigned long reason, void *arg)
{
        kfence_check_all_canary();
        return NOTIFY_OK;
}

static struct notifier_block kfence_check_canary_notifier = {
        .notifier_call = kfence_check_canary_callback,
};

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

static struct delayed_work kfence_timer;

#ifdef CONFIG_KFENCE_STATIC_KEYS
/* Wait queue to wake up allocation-gate timer task. */
static DECLARE_WAIT_QUEUE_HEAD(allocation_wait);

static void wake_up_kfence_timer(struct irq_work *work)
{
        wake_up(&allocation_wait);
}
static DEFINE_IRQ_WORK(wake_up_kfence_timer_work, wake_up_kfence_timer);
#endif

/*
 * 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 void toggle_allocation_gate(struct work_struct *work)
{
        if (!READ_ONCE(kfence_enabled))
                return;

        atomic_set(&kfence_allocation_gate, -kfence_burst);
#ifdef CONFIG_KFENCE_STATIC_KEYS
        /* Enable static key, and await allocation to happen. */
        static_branch_enable(&kfence_allocation_key);

        wait_event_idle(allocation_wait, atomic_read(&kfence_allocation_gate) > 0);

        /* Disable static key and reset timer. */
        static_branch_disable(&kfence_allocation_key);
#endif
        queue_delayed_work(system_unbound_wq, &kfence_timer,
                           msecs_to_jiffies(kfence_sample_interval));
}

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

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

        /*
         * If the pool has already been initialized by arch, there is no need to
         * re-allocate the memory pool.
         */
        if (!__kfence_pool)
                __kfence_pool = memblock_alloc(KFENCE_POOL_SIZE, PAGE_SIZE);

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

        /* The memory allocated by memblock has been zeroed out. */
        kfence_metadata_init = memblock_alloc(KFENCE_METADATA_SIZE, PAGE_SIZE);
        if (!kfence_metadata_init) {
                pr_err("failed to allocate metadata\n");
                memblock_free(__kfence_pool, KFENCE_POOL_SIZE);
                __kfence_pool = NULL;
        }
}

static void kfence_init_enable(void)
{
        if (!IS_ENABLED(CONFIG_KFENCE_STATIC_KEYS))
                static_branch_enable(&kfence_allocation_key);

        if (kfence_deferrable)
                INIT_DEFERRABLE_WORK(&kfence_timer, toggle_allocation_gate);
        else
                INIT_DELAYED_WORK(&kfence_timer, toggle_allocation_gate);

        if (kfence_check_on_panic)
                atomic_notifier_chain_register(&panic_notifier_list, &kfence_check_canary_notifier);

        WRITE_ONCE(kfence_enabled, true);
        queue_delayed_work(system_unbound_wq, &kfence_timer, 0);

        pr_info("initialized - using %lu bytes for %d objects at 0x%p-0x%p\n", KFENCE_POOL_SIZE,
                CONFIG_KFENCE_NUM_OBJECTS, (void *)__kfence_pool,
                (void *)(__kfence_pool + KFENCE_POOL_SIZE));
}

void __init kfence_init(void)
{
        stack_hash_seed = get_random_u32();

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

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

        kfence_init_enable();
}

static int kfence_init_late(void)
{
        const unsigned long nr_pages_pool = KFENCE_POOL_SIZE / PAGE_SIZE;
        const unsigned long nr_pages_meta = KFENCE_METADATA_SIZE / PAGE_SIZE;
        unsigned long addr = (unsigned long)__kfence_pool;
        unsigned long free_size = KFENCE_POOL_SIZE;
        int err = -ENOMEM;

#ifdef CONFIG_CONTIG_ALLOC
        struct page *pages;

        pages = alloc_contig_pages(nr_pages_pool, GFP_KERNEL, first_online_node,
                                   NULL);
        if (!pages)
                return -ENOMEM;

        __kfence_pool = page_to_virt(pages);
        pages = alloc_contig_pages(nr_pages_meta, GFP_KERNEL, first_online_node,
                                   NULL);
        if (pages)
                kfence_metadata_init = page_to_virt(pages);
#else
        if (nr_pages_pool > MAX_ORDER_NR_PAGES ||
            nr_pages_meta > MAX_ORDER_NR_PAGES) {
                pr_warn("KFENCE_NUM_OBJECTS too large for buddy allocator\n");
                return -EINVAL;
        }

        __kfence_pool = alloc_pages_exact(KFENCE_POOL_SIZE, GFP_KERNEL);
        if (!__kfence_pool)
                return -ENOMEM;

        kfence_metadata_init = alloc_pages_exact(KFENCE_METADATA_SIZE, GFP_KERNEL);
#endif

        if (!kfence_metadata_init)
                goto free_pool;

        memzero_explicit(kfence_metadata_init, KFENCE_METADATA_SIZE);
        addr = kfence_init_pool();
        if (!addr) {
                kfence_init_enable();
                kfence_debugfs_init();
                return 0;
        }

        pr_err("%s failed\n", __func__);
        free_size = KFENCE_POOL_SIZE - (addr - (unsigned long)__kfence_pool);
        err = -EBUSY;

#ifdef CONFIG_CONTIG_ALLOC
        free_contig_range(page_to_pfn(virt_to_page((void *)kfence_metadata_init)),
                          nr_pages_meta);
free_pool:
        free_contig_range(page_to_pfn(virt_to_page((void *)addr)),
                          free_size / PAGE_SIZE);
#else
        free_pages_exact((void *)kfence_metadata_init, KFENCE_METADATA_SIZE);
free_pool:
        free_pages_exact((void *)addr, free_size);
#endif

        kfence_metadata_init = NULL;
        __kfence_pool = NULL;
        return err;
}

static int kfence_enable_late(void)
{
        if (!__kfence_pool)
                return kfence_init_late();

        WRITE_ONCE(kfence_enabled, true);
        queue_delayed_work(system_unbound_wq, &kfence_timer, 0);
        pr_info("re-enabled\n");
        return 0;
}

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

        /* Pairs with release in kfence_init_pool(). */
        if (!smp_load_acquire(&kfence_metadata))
                return;

        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 || !kfence_obj_allocated(meta))
                        continue;

                raw_spin_lock_irqsave(&meta->lock, flags);
                in_use = meta->cache == s && kfence_obj_allocated(meta);
                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)
{
        unsigned long stack_entries[KFENCE_STACK_DEPTH];
        size_t num_stack_entries;
        u32 alloc_stack_hash;
        int allocation_gate;

        /*
         * Perform size check before switching kfence_allocation_gate, so that
         * we don't disable KFENCE without making an allocation.
         */
        if (size > PAGE_SIZE) {
                atomic_long_inc(&counters[KFENCE_COUNTER_SKIP_INCOMPAT]);
                return NULL;
        }

        /*
         * Skip allocations from non-default zones, including DMA. We cannot
         * guarantee that pages in the KFENCE pool will have the requested
         * properties (e.g. reside in DMAable memory).
         */
        if ((flags & GFP_ZONEMASK) ||
            ((flags & __GFP_THISNODE) && num_online_nodes() > 1) ||
            (s->flags & (SLAB_CACHE_DMA | SLAB_CACHE_DMA32))) {
                atomic_long_inc(&counters[KFENCE_COUNTER_SKIP_INCOMPAT]);
                return NULL;
        }

        /*
         * Skip allocations for this slab, if KFENCE has been disabled for
         * this slab.
         */
        if (s->flags & SLAB_SKIP_KFENCE)
                return NULL;

        allocation_gate = atomic_inc_return(&kfence_allocation_gate);
        if (allocation_gate > 1)
                return NULL;
#ifdef CONFIG_KFENCE_STATIC_KEYS
        /*
         * waitqueue_active() is fully ordered after the update of
         * kfence_allocation_gate per atomic_inc_return().
         */
        if (allocation_gate == 1 && waitqueue_active(&allocation_wait)) {
                /*
                 * Calling wake_up() here may deadlock when allocations happen
                 * from within timer code. Use an irq_work to defer it.
                 */
                irq_work_queue(&wake_up_kfence_timer_work);
        }
#endif

        if (!READ_ONCE(kfence_enabled))
                return NULL;

        num_stack_entries = stack_trace_save(stack_entries, KFENCE_STACK_DEPTH, 0);

        /*
         * Do expensive check for coverage of allocation in slow-path after
         * allocation_gate has already become non-zero, even though it might
         * mean not making any allocation within a given sample interval.
         *
         * This ensures reasonable allocation coverage when the pool is almost
         * full, including avoiding long-lived allocations of the same source
         * filling up the pool (e.g. pagecache allocations).
         */
        alloc_stack_hash = get_alloc_stack_hash(stack_entries, num_stack_entries);
        if (should_skip_covered() && alloc_covered_contains(alloc_stack_hash)) {
                atomic_long_inc(&counters[KFENCE_COUNTER_SKIP_COVERED]);
                return NULL;
        }

        return kfence_guarded_alloc(s, size, flags, stack_entries, num_stack_entries,
                                    alloc_stack_hash);
}

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);

#ifdef CONFIG_MEMCG
        KFENCE_WARN_ON(meta->obj_exts.objcg);
#endif
        /*
         * 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.
         * Save the stack trace here so that reports show where the user freed
         * the object.
         */
        if (unlikely(meta->cache && (meta->cache->flags & SLAB_TYPESAFE_BY_RCU))) {
                unsigned long flags;

                raw_spin_lock_irqsave(&meta->lock, flags);
                metadata_update_state(meta, KFENCE_OBJECT_RCU_FREEING, NULL, 0);
                raw_spin_unlock_irqrestore(&meta->lock, flags);
                call_rcu(&meta->rcu_head, rcu_guarded_free);
        } else {
                kfence_guarded_free(addr, meta, false);
        }
}

bool kfence_handle_page_fault(unsigned long addr, bool is_write, 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 && kfence_obj_allocated(meta)) {
                        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 && kfence_obj_allocated(meta)) {
                        /* 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, is_write, 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, is_write, regs, NULL, KFENCE_ERROR_INVALID);
        }

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