// 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/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);
/* 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[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. */
/*
* 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;
}
/*
* 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_FREED ? &meta->free_track : &meta->alloc_track;
lockdep_assert_held(&meta->lock);
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. */
/*
* Pairs with READ_ONCE() in
* kfence_shutdown_cache(),
* kfence_handle_page_fault().
*/
WRITE_ONCE(meta->state, next);
}
/* Check canary byte at @addr. */
static inline 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 inline 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;
#if defined(CONFIG_SLUB)
slab->objects = 1;
#elif defined(CONFIG_SLAB)
slab->s_mem = addr;
#endif
/* 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 (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, 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 = (unsigned long)__kfence_pool;
struct page *pages;
int i;
if (!arch_kfence_init_pool())
return addr;
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->memcg_data = (unsigned long)&kfence_metadata[i / 2 - 1].objcg |
MEMCG_DATA_OBJCGS;
#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[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;
}
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->memcg_data = 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;
return false;
}
static bool kfence_init_pool_late(void)
{
unsigned long addr, free_size;
addr = kfence_init_pool();
if (!addr)
return true;
/* Same as above. */
free_size = KFENCE_POOL_SIZE - (addr - (unsigned long)__kfence_pool);
#ifdef CONFIG_CONTIG_ALLOC
free_contig_range(page_to_pfn(virt_to_page((void *)addr)), free_size / PAGE_SIZE);
#else
free_pages_exact((void *)addr, free_size);
#endif
__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 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 (meta->state == KFENCE_OBJECT_ALLOCATED)
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, 0);
#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));
/* 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(void)
{
if (!kfence_sample_interval)
return;
/* if the pool has already been initialized by arch, skip the below. */
if (__kfence_pool)
return;
__kfence_pool = memblock_alloc(KFENCE_POOL_SIZE, PAGE_SIZE);
if (!__kfence_pool)
pr_err("failed to allocate pool\n");
}
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 = KFENCE_POOL_SIZE / PAGE_SIZE;
#ifdef CONFIG_CONTIG_ALLOC
struct page *pages;
pages = alloc_contig_pages(nr_pages, GFP_KERNEL, first_online_node, NULL);
if (!pages)
return -ENOMEM;
__kfence_pool = page_to_virt(pages);
#else
if (nr_pages > 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;
#endif
if (!kfence_init_pool_late()) {
pr_err("%s failed\n", __func__);
return -EBUSY;
}
kfence_init_enable();
kfence_debugfs_init();
return 0;
}
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;
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)
{
unsigned long stack_entries[KFENCE_STACK_DEPTH];
size_t num_stack_entries;
u32 alloc_stack_hash;
/*
* 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) ||
(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;
if (atomic_inc_return(&kfence_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 (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->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.
*/
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, 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 && 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, 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. */
}
