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1// SPDX-License-Identifier: GPL-2.0
2/*
3 * KFENCE guarded object allocator and fault handling.
4 *
5 * Copyright (C) 2020, Google LLC.
6 */
7
8#define pr_fmt(fmt) "kfence: " fmt
9
10#include <linux/atomic.h>
11#include <linux/bug.h>
12#include <linux/debugfs.h>
13#include <linux/irq_work.h>
14#include <linux/kcsan-checks.h>
15#include <linux/kfence.h>
16#include <linux/kmemleak.h>
17#include <linux/list.h>
18#include <linux/lockdep.h>
19#include <linux/memblock.h>
20#include <linux/moduleparam.h>
21#include <linux/random.h>
22#include <linux/rcupdate.h>
23#include <linux/sched/sysctl.h>
24#include <linux/seq_file.h>
25#include <linux/slab.h>
26#include <linux/spinlock.h>
27#include <linux/string.h>
28
29#include <asm/kfence.h>
30
31#include "kfence.h"
32
33/* Disables KFENCE on the first warning assuming an irrecoverable error. */
34#define KFENCE_WARN_ON(cond) \
35 ({ \
36 const bool __cond = WARN_ON(cond); \
37 if (unlikely(__cond)) \
38 WRITE_ONCE(kfence_enabled, false); \
39 __cond; \
40 })
41
42/* === Data ================================================================= */
43
44static bool kfence_enabled __read_mostly;
45
46static unsigned long kfence_sample_interval __read_mostly = CONFIG_KFENCE_SAMPLE_INTERVAL;
47
48#ifdef MODULE_PARAM_PREFIX
49#undef MODULE_PARAM_PREFIX
50#endif
51#define MODULE_PARAM_PREFIX "kfence."
52
53static int param_set_sample_interval(const char *val, const struct kernel_param *kp)
54{
55 unsigned long num;
56 int ret = kstrtoul(val, 0, &num);
57
58 if (ret < 0)
59 return ret;
60
61 if (!num) /* Using 0 to indicate KFENCE is disabled. */
62 WRITE_ONCE(kfence_enabled, false);
63 else if (!READ_ONCE(kfence_enabled) && system_state != SYSTEM_BOOTING)
64 return -EINVAL; /* Cannot (re-)enable KFENCE on-the-fly. */
65
66 *((unsigned long *)kp->arg) = num;
67 return 0;
68}
69
70static int param_get_sample_interval(char *buffer, const struct kernel_param *kp)
71{
72 if (!READ_ONCE(kfence_enabled))
73 return sprintf(buffer, "0\n");
74
75 return param_get_ulong(buffer, kp);
76}
77
78static const struct kernel_param_ops sample_interval_param_ops = {
79 .set = param_set_sample_interval,
80 .get = param_get_sample_interval,
81};
82module_param_cb(sample_interval, &sample_interval_param_ops, &kfence_sample_interval, 0600);
83
84/* The pool of pages used for guard pages and objects. */
85char *__kfence_pool __ro_after_init;
86EXPORT_SYMBOL(__kfence_pool); /* Export for test modules. */
87
88/*
89 * Per-object metadata, with one-to-one mapping of object metadata to
90 * backing pages (in __kfence_pool).
91 */
92static_assert(CONFIG_KFENCE_NUM_OBJECTS > 0);
93struct kfence_metadata kfence_metadata[CONFIG_KFENCE_NUM_OBJECTS];
94
95/* Freelist with available objects. */
96static struct list_head kfence_freelist = LIST_HEAD_INIT(kfence_freelist);
97static DEFINE_RAW_SPINLOCK(kfence_freelist_lock); /* Lock protecting freelist. */
98
99#ifdef CONFIG_KFENCE_STATIC_KEYS
100/* The static key to set up a KFENCE allocation. */
101DEFINE_STATIC_KEY_FALSE(kfence_allocation_key);
102#endif
103
104/* Gates the allocation, ensuring only one succeeds in a given period. */
105atomic_t kfence_allocation_gate = ATOMIC_INIT(1);
106
107/* Statistics counters for debugfs. */
108enum kfence_counter_id {
109 KFENCE_COUNTER_ALLOCATED,
110 KFENCE_COUNTER_ALLOCS,
111 KFENCE_COUNTER_FREES,
112 KFENCE_COUNTER_ZOMBIES,
113 KFENCE_COUNTER_BUGS,
114 KFENCE_COUNTER_COUNT,
115};
116static atomic_long_t counters[KFENCE_COUNTER_COUNT];
117static const char *const counter_names[] = {
118 [KFENCE_COUNTER_ALLOCATED] = "currently allocated",
119 [KFENCE_COUNTER_ALLOCS] = "total allocations",
120 [KFENCE_COUNTER_FREES] = "total frees",
121 [KFENCE_COUNTER_ZOMBIES] = "zombie allocations",
122 [KFENCE_COUNTER_BUGS] = "total bugs",
123};
124static_assert(ARRAY_SIZE(counter_names) == KFENCE_COUNTER_COUNT);
125
126/* === Internals ============================================================ */
127
128static bool kfence_protect(unsigned long addr)
129{
130 return !KFENCE_WARN_ON(!kfence_protect_page(ALIGN_DOWN(addr, PAGE_SIZE), true));
131}
132
133static bool kfence_unprotect(unsigned long addr)
134{
135 return !KFENCE_WARN_ON(!kfence_protect_page(ALIGN_DOWN(addr, PAGE_SIZE), false));
136}
137
138static inline struct kfence_metadata *addr_to_metadata(unsigned long addr)
139{
140 long index;
141
142 /* The checks do not affect performance; only called from slow-paths. */
143
144 if (!is_kfence_address((void *)addr))
145 return NULL;
146
147 /*
148 * May be an invalid index if called with an address at the edge of
149 * __kfence_pool, in which case we would report an "invalid access"
150 * error.
151 */
152 index = (addr - (unsigned long)__kfence_pool) / (PAGE_SIZE * 2) - 1;
153 if (index < 0 || index >= CONFIG_KFENCE_NUM_OBJECTS)
154 return NULL;
155
156 return &kfence_metadata[index];
157}
158
159static inline unsigned long metadata_to_pageaddr(const struct kfence_metadata *meta)
160{
161 unsigned long offset = (meta - kfence_metadata + 1) * PAGE_SIZE * 2;
162 unsigned long pageaddr = (unsigned long)&__kfence_pool[offset];
163
164 /* The checks do not affect performance; only called from slow-paths. */
165
166 /* Only call with a pointer into kfence_metadata. */
167 if (KFENCE_WARN_ON(meta < kfence_metadata ||
168 meta >= kfence_metadata + CONFIG_KFENCE_NUM_OBJECTS))
169 return 0;
170
171 /*
172 * This metadata object only ever maps to 1 page; verify that the stored
173 * address is in the expected range.
174 */
175 if (KFENCE_WARN_ON(ALIGN_DOWN(meta->addr, PAGE_SIZE) != pageaddr))
176 return 0;
177
178 return pageaddr;
179}
180
181/*
182 * Update the object's metadata state, including updating the alloc/free stacks
183 * depending on the state transition.
184 */
185static noinline void metadata_update_state(struct kfence_metadata *meta,
186 enum kfence_object_state next)
187{
188 struct kfence_track *track =
189 next == KFENCE_OBJECT_FREED ? &meta->free_track : &meta->alloc_track;
190
191 lockdep_assert_held(&meta->lock);
192
193 /*
194 * Skip over 1 (this) functions; noinline ensures we do not accidentally
195 * skip over the caller by never inlining.
196 */
197 track->num_stack_entries = stack_trace_save(track->stack_entries, KFENCE_STACK_DEPTH, 1);
198 track->pid = task_pid_nr(current);
199
200 /*
201 * Pairs with READ_ONCE() in
202 * kfence_shutdown_cache(),
203 * kfence_handle_page_fault().
204 */
205 WRITE_ONCE(meta->state, next);
206}
207
208/* Write canary byte to @addr. */
209static inline bool set_canary_byte(u8 *addr)
210{
211 *addr = KFENCE_CANARY_PATTERN(addr);
212 return true;
213}
214
215/* Check canary byte at @addr. */
216static inline bool check_canary_byte(u8 *addr)
217{
218 if (likely(*addr == KFENCE_CANARY_PATTERN(addr)))
219 return true;
220
221 atomic_long_inc(&counters[KFENCE_COUNTER_BUGS]);
222 kfence_report_error((unsigned long)addr, false, NULL, addr_to_metadata((unsigned long)addr),
223 KFENCE_ERROR_CORRUPTION);
224 return false;
225}
226
227/* __always_inline this to ensure we won't do an indirect call to fn. */
228static __always_inline void for_each_canary(const struct kfence_metadata *meta, bool (*fn)(u8 *))
229{
230 const unsigned long pageaddr = ALIGN_DOWN(meta->addr, PAGE_SIZE);
231 unsigned long addr;
232
233 lockdep_assert_held(&meta->lock);
234
235 /*
236 * We'll iterate over each canary byte per-side until fn() returns
237 * false. However, we'll still iterate over the canary bytes to the
238 * right of the object even if there was an error in the canary bytes to
239 * the left of the object. Specifically, if check_canary_byte()
240 * generates an error, showing both sides might give more clues as to
241 * what the error is about when displaying which bytes were corrupted.
242 */
243
244 /* Apply to left of object. */
245 for (addr = pageaddr; addr < meta->addr; addr++) {
246 if (!fn((u8 *)addr))
247 break;
248 }
249
250 /* Apply to right of object. */
251 for (addr = meta->addr + meta->size; addr < pageaddr + PAGE_SIZE; addr++) {
252 if (!fn((u8 *)addr))
253 break;
254 }
255}
256
257static void *kfence_guarded_alloc(struct kmem_cache *cache, size_t size, gfp_t gfp)
258{
259 struct kfence_metadata *meta = NULL;
260 unsigned long flags;
261 struct page *page;
262 void *addr;
263
264 /* Try to obtain a free object. */
265 raw_spin_lock_irqsave(&kfence_freelist_lock, flags);
266 if (!list_empty(&kfence_freelist)) {
267 meta = list_entry(kfence_freelist.next, struct kfence_metadata, list);
268 list_del_init(&meta->list);
269 }
270 raw_spin_unlock_irqrestore(&kfence_freelist_lock, flags);
271 if (!meta)
272 return NULL;
273
274 if (unlikely(!raw_spin_trylock_irqsave(&meta->lock, flags))) {
275 /*
276 * This is extremely unlikely -- we are reporting on a
277 * use-after-free, which locked meta->lock, and the reporting
278 * code via printk calls kmalloc() which ends up in
279 * kfence_alloc() and tries to grab the same object that we're
280 * reporting on. While it has never been observed, lockdep does
281 * report that there is a possibility of deadlock. Fix it by
282 * using trylock and bailing out gracefully.
283 */
284 raw_spin_lock_irqsave(&kfence_freelist_lock, flags);
285 /* Put the object back on the freelist. */
286 list_add_tail(&meta->list, &kfence_freelist);
287 raw_spin_unlock_irqrestore(&kfence_freelist_lock, flags);
288
289 return NULL;
290 }
291
292 meta->addr = metadata_to_pageaddr(meta);
293 /* Unprotect if we're reusing this page. */
294 if (meta->state == KFENCE_OBJECT_FREED)
295 kfence_unprotect(meta->addr);
296
297 /*
298 * Note: for allocations made before RNG initialization, will always
299 * return zero. We still benefit from enabling KFENCE as early as
300 * possible, even when the RNG is not yet available, as this will allow
301 * KFENCE to detect bugs due to earlier allocations. The only downside
302 * is that the out-of-bounds accesses detected are deterministic for
303 * such allocations.
304 */
305 if (prandom_u32_max(2)) {
306 /* Allocate on the "right" side, re-calculate address. */
307 meta->addr += PAGE_SIZE - size;
308 meta->addr = ALIGN_DOWN(meta->addr, cache->align);
309 }
310
311 addr = (void *)meta->addr;
312
313 /* Update remaining metadata. */
314 metadata_update_state(meta, KFENCE_OBJECT_ALLOCATED);
315 /* Pairs with READ_ONCE() in kfence_shutdown_cache(). */
316 WRITE_ONCE(meta->cache, cache);
317 meta->size = size;
318 for_each_canary(meta, set_canary_byte);
319
320 /* Set required struct page fields. */
321 page = virt_to_page(meta->addr);
322 page->slab_cache = cache;
323 if (IS_ENABLED(CONFIG_SLUB))
324 page->objects = 1;
325 if (IS_ENABLED(CONFIG_SLAB))
326 page->s_mem = addr;
327
328 raw_spin_unlock_irqrestore(&meta->lock, flags);
329
330 /* Memory initialization. */
331
332 /*
333 * We check slab_want_init_on_alloc() ourselves, rather than letting
334 * SL*B do the initialization, as otherwise we might overwrite KFENCE's
335 * redzone.
336 */
337 if (unlikely(slab_want_init_on_alloc(gfp, cache)))
338 memzero_explicit(addr, size);
339 if (cache->ctor)
340 cache->ctor(addr);
341
342 if (CONFIG_KFENCE_STRESS_TEST_FAULTS && !prandom_u32_max(CONFIG_KFENCE_STRESS_TEST_FAULTS))
343 kfence_protect(meta->addr); /* Random "faults" by protecting the object. */
344
345 atomic_long_inc(&counters[KFENCE_COUNTER_ALLOCATED]);
346 atomic_long_inc(&counters[KFENCE_COUNTER_ALLOCS]);
347
348 return addr;
349}
350
351static void kfence_guarded_free(void *addr, struct kfence_metadata *meta, bool zombie)
352{
353 struct kcsan_scoped_access assert_page_exclusive;
354 unsigned long flags;
355
356 raw_spin_lock_irqsave(&meta->lock, flags);
357
358 if (meta->state != KFENCE_OBJECT_ALLOCATED || meta->addr != (unsigned long)addr) {
359 /* Invalid or double-free, bail out. */
360 atomic_long_inc(&counters[KFENCE_COUNTER_BUGS]);
361 kfence_report_error((unsigned long)addr, false, NULL, meta,
362 KFENCE_ERROR_INVALID_FREE);
363 raw_spin_unlock_irqrestore(&meta->lock, flags);
364 return;
365 }
366
367 /* Detect racy use-after-free, or incorrect reallocation of this page by KFENCE. */
368 kcsan_begin_scoped_access((void *)ALIGN_DOWN((unsigned long)addr, PAGE_SIZE), PAGE_SIZE,
369 KCSAN_ACCESS_SCOPED | KCSAN_ACCESS_WRITE | KCSAN_ACCESS_ASSERT,
370 &assert_page_exclusive);
371
372 if (CONFIG_KFENCE_STRESS_TEST_FAULTS)
373 kfence_unprotect((unsigned long)addr); /* To check canary bytes. */
374
375 /* Restore page protection if there was an OOB access. */
376 if (meta->unprotected_page) {
377 memzero_explicit((void *)ALIGN_DOWN(meta->unprotected_page, PAGE_SIZE), PAGE_SIZE);
378 kfence_protect(meta->unprotected_page);
379 meta->unprotected_page = 0;
380 }
381
382 /* Check canary bytes for memory corruption. */
383 for_each_canary(meta, check_canary_byte);
384
385 /*
386 * Clear memory if init-on-free is set. While we protect the page, the
387 * data is still there, and after a use-after-free is detected, we
388 * unprotect the page, so the data is still accessible.
389 */
390 if (!zombie && unlikely(slab_want_init_on_free(meta->cache)))
391 memzero_explicit(addr, meta->size);
392
393 /* Mark the object as freed. */
394 metadata_update_state(meta, KFENCE_OBJECT_FREED);
395
396 raw_spin_unlock_irqrestore(&meta->lock, flags);
397
398 /* Protect to detect use-after-frees. */
399 kfence_protect((unsigned long)addr);
400
401 kcsan_end_scoped_access(&assert_page_exclusive);
402 if (!zombie) {
403 /* Add it to the tail of the freelist for reuse. */
404 raw_spin_lock_irqsave(&kfence_freelist_lock, flags);
405 KFENCE_WARN_ON(!list_empty(&meta->list));
406 list_add_tail(&meta->list, &kfence_freelist);
407 raw_spin_unlock_irqrestore(&kfence_freelist_lock, flags);
408
409 atomic_long_dec(&counters[KFENCE_COUNTER_ALLOCATED]);
410 atomic_long_inc(&counters[KFENCE_COUNTER_FREES]);
411 } else {
412 /* See kfence_shutdown_cache(). */
413 atomic_long_inc(&counters[KFENCE_COUNTER_ZOMBIES]);
414 }
415}
416
417static void rcu_guarded_free(struct rcu_head *h)
418{
419 struct kfence_metadata *meta = container_of(h, struct kfence_metadata, rcu_head);
420
421 kfence_guarded_free((void *)meta->addr, meta, false);
422}
423
424static bool __init kfence_init_pool(void)
425{
426 unsigned long addr = (unsigned long)__kfence_pool;
427 struct page *pages;
428 int i;
429
430 if (!__kfence_pool)
431 return false;
432
433 if (!arch_kfence_init_pool())
434 goto err;
435
436 pages = virt_to_page(addr);
437
438 /*
439 * Set up object pages: they must have PG_slab set, to avoid freeing
440 * these as real pages.
441 *
442 * We also want to avoid inserting kfence_free() in the kfree()
443 * fast-path in SLUB, and therefore need to ensure kfree() correctly
444 * enters __slab_free() slow-path.
445 */
446 for (i = 0; i < KFENCE_POOL_SIZE / PAGE_SIZE; i++) {
447 if (!i || (i % 2))
448 continue;
449
450 /* Verify we do not have a compound head page. */
451 if (WARN_ON(compound_head(&pages[i]) != &pages[i]))
452 goto err;
453
454 __SetPageSlab(&pages[i]);
455 }
456
457 /*
458 * Protect the first 2 pages. The first page is mostly unnecessary, and
459 * merely serves as an extended guard page. However, adding one
460 * additional page in the beginning gives us an even number of pages,
461 * which simplifies the mapping of address to metadata index.
462 */
463 for (i = 0; i < 2; i++) {
464 if (unlikely(!kfence_protect(addr)))
465 goto err;
466
467 addr += PAGE_SIZE;
468 }
469
470 for (i = 0; i < CONFIG_KFENCE_NUM_OBJECTS; i++) {
471 struct kfence_metadata *meta = &kfence_metadata[i];
472
473 /* Initialize metadata. */
474 INIT_LIST_HEAD(&meta->list);
475 raw_spin_lock_init(&meta->lock);
476 meta->state = KFENCE_OBJECT_UNUSED;
477 meta->addr = addr; /* Initialize for validation in metadata_to_pageaddr(). */
478 list_add_tail(&meta->list, &kfence_freelist);
479
480 /* Protect the right redzone. */
481 if (unlikely(!kfence_protect(addr + PAGE_SIZE)))
482 goto err;
483
484 addr += 2 * PAGE_SIZE;
485 }
486
487 /*
488 * The pool is live and will never be deallocated from this point on.
489 * Remove the pool object from the kmemleak object tree, as it would
490 * otherwise overlap with allocations returned by kfence_alloc(), which
491 * are registered with kmemleak through the slab post-alloc hook.
492 */
493 kmemleak_free(__kfence_pool);
494
495 return true;
496
497err:
498 /*
499 * Only release unprotected pages, and do not try to go back and change
500 * page attributes due to risk of failing to do so as well. If changing
501 * page attributes for some pages fails, it is very likely that it also
502 * fails for the first page, and therefore expect addr==__kfence_pool in
503 * most failure cases.
504 */
505 memblock_free_late(__pa(addr), KFENCE_POOL_SIZE - (addr - (unsigned long)__kfence_pool));
506 __kfence_pool = NULL;
507 return false;
508}
509
510/* === DebugFS Interface ==================================================== */
511
512static int stats_show(struct seq_file *seq, void *v)
513{
514 int i;
515
516 seq_printf(seq, "enabled: %i\n", READ_ONCE(kfence_enabled));
517 for (i = 0; i < KFENCE_COUNTER_COUNT; i++)
518 seq_printf(seq, "%s: %ld\n", counter_names[i], atomic_long_read(&counters[i]));
519
520 return 0;
521}
522DEFINE_SHOW_ATTRIBUTE(stats);
523
524/*
525 * debugfs seq_file operations for /sys/kernel/debug/kfence/objects.
526 * start_object() and next_object() return the object index + 1, because NULL is used
527 * to stop iteration.
528 */
529static void *start_object(struct seq_file *seq, loff_t *pos)
530{
531 if (*pos < CONFIG_KFENCE_NUM_OBJECTS)
532 return (void *)((long)*pos + 1);
533 return NULL;
534}
535
536static void stop_object(struct seq_file *seq, void *v)
537{
538}
539
540static void *next_object(struct seq_file *seq, void *v, loff_t *pos)
541{
542 ++*pos;
543 if (*pos < CONFIG_KFENCE_NUM_OBJECTS)
544 return (void *)((long)*pos + 1);
545 return NULL;
546}
547
548static int show_object(struct seq_file *seq, void *v)
549{
550 struct kfence_metadata *meta = &kfence_metadata[(long)v - 1];
551 unsigned long flags;
552
553 raw_spin_lock_irqsave(&meta->lock, flags);
554 kfence_print_object(seq, meta);
555 raw_spin_unlock_irqrestore(&meta->lock, flags);
556 seq_puts(seq, "---------------------------------\n");
557
558 return 0;
559}
560
561static const struct seq_operations object_seqops = {
562 .start = start_object,
563 .next = next_object,
564 .stop = stop_object,
565 .show = show_object,
566};
567
568static int open_objects(struct inode *inode, struct file *file)
569{
570 return seq_open(file, &object_seqops);
571}
572
573static const struct file_operations objects_fops = {
574 .open = open_objects,
575 .read = seq_read,
576 .llseek = seq_lseek,
577};
578
579static int __init kfence_debugfs_init(void)
580{
581 struct dentry *kfence_dir = debugfs_create_dir("kfence", NULL);
582
583 debugfs_create_file("stats", 0444, kfence_dir, NULL, &stats_fops);
584 debugfs_create_file("objects", 0400, kfence_dir, NULL, &objects_fops);
585 return 0;
586}
587
588late_initcall(kfence_debugfs_init);
589
590/* === Allocation Gate Timer ================================================ */
591
592#ifdef CONFIG_KFENCE_STATIC_KEYS
593/* Wait queue to wake up allocation-gate timer task. */
594static DECLARE_WAIT_QUEUE_HEAD(allocation_wait);
595
596static void wake_up_kfence_timer(struct irq_work *work)
597{
598 wake_up(&allocation_wait);
599}
600static DEFINE_IRQ_WORK(wake_up_kfence_timer_work, wake_up_kfence_timer);
601#endif
602
603/*
604 * Set up delayed work, which will enable and disable the static key. We need to
605 * use a work queue (rather than a simple timer), since enabling and disabling a
606 * static key cannot be done from an interrupt.
607 *
608 * Note: Toggling a static branch currently causes IPIs, and here we'll end up
609 * with a total of 2 IPIs to all CPUs. If this ends up a problem in future (with
610 * more aggressive sampling intervals), we could get away with a variant that
611 * avoids IPIs, at the cost of not immediately capturing allocations if the
612 * instructions remain cached.
613 */
614static struct delayed_work kfence_timer;
615static void toggle_allocation_gate(struct work_struct *work)
616{
617 if (!READ_ONCE(kfence_enabled))
618 return;
619
620 atomic_set(&kfence_allocation_gate, 0);
621#ifdef CONFIG_KFENCE_STATIC_KEYS
622 /* Enable static key, and await allocation to happen. */
623 static_branch_enable(&kfence_allocation_key);
624
625 if (sysctl_hung_task_timeout_secs) {
626 /*
627 * During low activity with no allocations we might wait a
628 * while; let's avoid the hung task warning.
629 */
630 wait_event_idle_timeout(allocation_wait, atomic_read(&kfence_allocation_gate),
631 sysctl_hung_task_timeout_secs * HZ / 2);
632 } else {
633 wait_event_idle(allocation_wait, atomic_read(&kfence_allocation_gate));
634 }
635
636 /* Disable static key and reset timer. */
637 static_branch_disable(&kfence_allocation_key);
638#endif
639 queue_delayed_work(system_unbound_wq, &kfence_timer,
640 msecs_to_jiffies(kfence_sample_interval));
641}
642static DECLARE_DELAYED_WORK(kfence_timer, toggle_allocation_gate);
643
644/* === Public interface ===================================================== */
645
646void __init kfence_alloc_pool(void)
647{
648 if (!kfence_sample_interval)
649 return;
650
651 __kfence_pool = memblock_alloc(KFENCE_POOL_SIZE, PAGE_SIZE);
652
653 if (!__kfence_pool)
654 pr_err("failed to allocate pool\n");
655}
656
657void __init kfence_init(void)
658{
659 /* Setting kfence_sample_interval to 0 on boot disables KFENCE. */
660 if (!kfence_sample_interval)
661 return;
662
663 if (!kfence_init_pool()) {
664 pr_err("%s failed\n", __func__);
665 return;
666 }
667
668 WRITE_ONCE(kfence_enabled, true);
669 queue_delayed_work(system_unbound_wq, &kfence_timer, 0);
670 pr_info("initialized - using %lu bytes for %d objects at 0x%p-0x%p\n", KFENCE_POOL_SIZE,
671 CONFIG_KFENCE_NUM_OBJECTS, (void *)__kfence_pool,
672 (void *)(__kfence_pool + KFENCE_POOL_SIZE));
673}
674
675void kfence_shutdown_cache(struct kmem_cache *s)
676{
677 unsigned long flags;
678 struct kfence_metadata *meta;
679 int i;
680
681 for (i = 0; i < CONFIG_KFENCE_NUM_OBJECTS; i++) {
682 bool in_use;
683
684 meta = &kfence_metadata[i];
685
686 /*
687 * If we observe some inconsistent cache and state pair where we
688 * should have returned false here, cache destruction is racing
689 * with either kmem_cache_alloc() or kmem_cache_free(). Taking
690 * the lock will not help, as different critical section
691 * serialization will have the same outcome.
692 */
693 if (READ_ONCE(meta->cache) != s ||
694 READ_ONCE(meta->state) != KFENCE_OBJECT_ALLOCATED)
695 continue;
696
697 raw_spin_lock_irqsave(&meta->lock, flags);
698 in_use = meta->cache == s && meta->state == KFENCE_OBJECT_ALLOCATED;
699 raw_spin_unlock_irqrestore(&meta->lock, flags);
700
701 if (in_use) {
702 /*
703 * This cache still has allocations, and we should not
704 * release them back into the freelist so they can still
705 * safely be used and retain the kernel's default
706 * behaviour of keeping the allocations alive (leak the
707 * cache); however, they effectively become "zombie
708 * allocations" as the KFENCE objects are the only ones
709 * still in use and the owning cache is being destroyed.
710 *
711 * We mark them freed, so that any subsequent use shows
712 * more useful error messages that will include stack
713 * traces of the user of the object, the original
714 * allocation, and caller to shutdown_cache().
715 */
716 kfence_guarded_free((void *)meta->addr, meta, /*zombie=*/true);
717 }
718 }
719
720 for (i = 0; i < CONFIG_KFENCE_NUM_OBJECTS; i++) {
721 meta = &kfence_metadata[i];
722
723 /* See above. */
724 if (READ_ONCE(meta->cache) != s || READ_ONCE(meta->state) != KFENCE_OBJECT_FREED)
725 continue;
726
727 raw_spin_lock_irqsave(&meta->lock, flags);
728 if (meta->cache == s && meta->state == KFENCE_OBJECT_FREED)
729 meta->cache = NULL;
730 raw_spin_unlock_irqrestore(&meta->lock, flags);
731 }
732}
733
734void *__kfence_alloc(struct kmem_cache *s, size_t size, gfp_t flags)
735{
736 /*
737 * Perform size check before switching kfence_allocation_gate, so that
738 * we don't disable KFENCE without making an allocation.
739 */
740 if (size > PAGE_SIZE)
741 return NULL;
742
743 /*
744 * Skip allocations from non-default zones, including DMA. We cannot
745 * guarantee that pages in the KFENCE pool will have the requested
746 * properties (e.g. reside in DMAable memory).
747 */
748 if ((flags & GFP_ZONEMASK) ||
749 (s->flags & (SLAB_CACHE_DMA | SLAB_CACHE_DMA32)))
750 return NULL;
751
752 /*
753 * allocation_gate only needs to become non-zero, so it doesn't make
754 * sense to continue writing to it and pay the associated contention
755 * cost, in case we have a large number of concurrent allocations.
756 */
757 if (atomic_read(&kfence_allocation_gate) || atomic_inc_return(&kfence_allocation_gate) > 1)
758 return NULL;
759#ifdef CONFIG_KFENCE_STATIC_KEYS
760 /*
761 * waitqueue_active() is fully ordered after the update of
762 * kfence_allocation_gate per atomic_inc_return().
763 */
764 if (waitqueue_active(&allocation_wait)) {
765 /*
766 * Calling wake_up() here may deadlock when allocations happen
767 * from within timer code. Use an irq_work to defer it.
768 */
769 irq_work_queue(&wake_up_kfence_timer_work);
770 }
771#endif
772
773 if (!READ_ONCE(kfence_enabled))
774 return NULL;
775
776 return kfence_guarded_alloc(s, size, flags);
777}
778
779size_t kfence_ksize(const void *addr)
780{
781 const struct kfence_metadata *meta = addr_to_metadata((unsigned long)addr);
782
783 /*
784 * Read locklessly -- if there is a race with __kfence_alloc(), this is
785 * either a use-after-free or invalid access.
786 */
787 return meta ? meta->size : 0;
788}
789
790void *kfence_object_start(const void *addr)
791{
792 const struct kfence_metadata *meta = addr_to_metadata((unsigned long)addr);
793
794 /*
795 * Read locklessly -- if there is a race with __kfence_alloc(), this is
796 * either a use-after-free or invalid access.
797 */
798 return meta ? (void *)meta->addr : NULL;
799}
800
801void __kfence_free(void *addr)
802{
803 struct kfence_metadata *meta = addr_to_metadata((unsigned long)addr);
804
805 /*
806 * If the objects of the cache are SLAB_TYPESAFE_BY_RCU, defer freeing
807 * the object, as the object page may be recycled for other-typed
808 * objects once it has been freed. meta->cache may be NULL if the cache
809 * was destroyed.
810 */
811 if (unlikely(meta->cache && (meta->cache->flags & SLAB_TYPESAFE_BY_RCU)))
812 call_rcu(&meta->rcu_head, rcu_guarded_free);
813 else
814 kfence_guarded_free(addr, meta, false);
815}
816
817bool kfence_handle_page_fault(unsigned long addr, bool is_write, struct pt_regs *regs)
818{
819 const int page_index = (addr - (unsigned long)__kfence_pool) / PAGE_SIZE;
820 struct kfence_metadata *to_report = NULL;
821 enum kfence_error_type error_type;
822 unsigned long flags;
823
824 if (!is_kfence_address((void *)addr))
825 return false;
826
827 if (!READ_ONCE(kfence_enabled)) /* If disabled at runtime ... */
828 return kfence_unprotect(addr); /* ... unprotect and proceed. */
829
830 atomic_long_inc(&counters[KFENCE_COUNTER_BUGS]);
831
832 if (page_index % 2) {
833 /* This is a redzone, report a buffer overflow. */
834 struct kfence_metadata *meta;
835 int distance = 0;
836
837 meta = addr_to_metadata(addr - PAGE_SIZE);
838 if (meta && READ_ONCE(meta->state) == KFENCE_OBJECT_ALLOCATED) {
839 to_report = meta;
840 /* Data race ok; distance calculation approximate. */
841 distance = addr - data_race(meta->addr + meta->size);
842 }
843
844 meta = addr_to_metadata(addr + PAGE_SIZE);
845 if (meta && READ_ONCE(meta->state) == KFENCE_OBJECT_ALLOCATED) {
846 /* Data race ok; distance calculation approximate. */
847 if (!to_report || distance > data_race(meta->addr) - addr)
848 to_report = meta;
849 }
850
851 if (!to_report)
852 goto out;
853
854 raw_spin_lock_irqsave(&to_report->lock, flags);
855 to_report->unprotected_page = addr;
856 error_type = KFENCE_ERROR_OOB;
857
858 /*
859 * If the object was freed before we took the look we can still
860 * report this as an OOB -- the report will simply show the
861 * stacktrace of the free as well.
862 */
863 } else {
864 to_report = addr_to_metadata(addr);
865 if (!to_report)
866 goto out;
867
868 raw_spin_lock_irqsave(&to_report->lock, flags);
869 error_type = KFENCE_ERROR_UAF;
870 /*
871 * We may race with __kfence_alloc(), and it is possible that a
872 * freed object may be reallocated. We simply report this as a
873 * use-after-free, with the stack trace showing the place where
874 * the object was re-allocated.
875 */
876 }
877
878out:
879 if (to_report) {
880 kfence_report_error(addr, is_write, regs, to_report, error_type);
881 raw_spin_unlock_irqrestore(&to_report->lock, flags);
882 } else {
883 /* This may be a UAF or OOB access, but we can't be sure. */
884 kfence_report_error(addr, is_write, regs, NULL, KFENCE_ERROR_INVALID);
885 }
886
887 return kfence_unprotect(addr); /* Unprotect and let access proceed. */
888}