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/hash.h>
14 #include <linux/irq_work.h>
15 #include <linux/jhash.h>
16 #include <linux/kcsan-checks.h>
17 #include <linux/kfence.h>
18 #include <linux/kmemleak.h>
19 #include <linux/list.h>
20 #include <linux/lockdep.h>
21 #include <linux/log2.h>
22 #include <linux/memblock.h>
23 #include <linux/moduleparam.h>
24 #include <linux/random.h>
25 #include <linux/rcupdate.h>
26 #include <linux/sched/clock.h>
27 #include <linux/sched/sysctl.h>
28 #include <linux/seq_file.h>
29 #include <linux/slab.h>
30 #include <linux/spinlock.h>
31 #include <linux/string.h>
32
33 #include <asm/kfence.h>
34
35 #include "kfence.h"
36
37 /* Disables KFENCE on the first warning assuming an irrecoverable error. */
38 #define KFENCE_WARN_ON(cond) \
39 ({ \
40 const bool __cond = WARN_ON(cond); \
41 if (unlikely(__cond)) \
42 WRITE_ONCE(kfence_enabled, false); \
43 __cond; \
44 })
45
46 /* === Data ================================================================= */
47
48 static bool kfence_enabled __read_mostly;
49
50 static unsigned long kfence_sample_interval __read_mostly = CONFIG_KFENCE_SAMPLE_INTERVAL;
51
52 #ifdef MODULE_PARAM_PREFIX
53 #undef MODULE_PARAM_PREFIX
54 #endif
55 #define MODULE_PARAM_PREFIX "kfence."
56
param_set_sample_interval(const char * val,const struct kernel_param * kp)57 static int param_set_sample_interval(const char *val, const struct kernel_param *kp)
58 {
59 unsigned long num;
60 int ret = kstrtoul(val, 0, &num);
61
62 if (ret < 0)
63 return ret;
64
65 if (!num) /* Using 0 to indicate KFENCE is disabled. */
66 WRITE_ONCE(kfence_enabled, false);
67 else if (!READ_ONCE(kfence_enabled) && system_state != SYSTEM_BOOTING)
68 return -EINVAL; /* Cannot (re-)enable KFENCE on-the-fly. */
69
70 *((unsigned long *)kp->arg) = num;
71 return 0;
72 }
73
param_get_sample_interval(char * buffer,const struct kernel_param * kp)74 static int param_get_sample_interval(char *buffer, const struct kernel_param *kp)
75 {
76 if (!READ_ONCE(kfence_enabled))
77 return sprintf(buffer, "0\n");
78
79 return param_get_ulong(buffer, kp);
80 }
81
82 static const struct kernel_param_ops sample_interval_param_ops = {
83 .set = param_set_sample_interval,
84 .get = param_get_sample_interval,
85 };
86 module_param_cb(sample_interval, &sample_interval_param_ops, &kfence_sample_interval, 0600);
87
88 /* Pool usage% threshold when currently covered allocations are skipped. */
89 static unsigned long kfence_skip_covered_thresh __read_mostly = 75;
90 module_param_named(skip_covered_thresh, kfence_skip_covered_thresh, ulong, 0644);
91
92 /* The pool of pages used for guard pages and objects. */
93 char *__kfence_pool __ro_after_init;
94 EXPORT_SYMBOL(__kfence_pool); /* Export for test modules. */
95
96 /*
97 * Per-object metadata, with one-to-one mapping of object metadata to
98 * backing pages (in __kfence_pool).
99 */
100 static_assert(CONFIG_KFENCE_NUM_OBJECTS > 0);
101 struct kfence_metadata kfence_metadata[CONFIG_KFENCE_NUM_OBJECTS];
102
103 /* Freelist with available objects. */
104 static struct list_head kfence_freelist = LIST_HEAD_INIT(kfence_freelist);
105 static DEFINE_RAW_SPINLOCK(kfence_freelist_lock); /* Lock protecting freelist. */
106
107 /*
108 * The static key to set up a KFENCE allocation; or if static keys are not used
109 * to gate allocations, to avoid a load and compare if KFENCE is disabled.
110 */
111 DEFINE_STATIC_KEY_FALSE(kfence_allocation_key);
112
113 /* Gates the allocation, ensuring only one succeeds in a given period. */
114 atomic_t kfence_allocation_gate = ATOMIC_INIT(1);
115
116 /*
117 * A Counting Bloom filter of allocation coverage: limits currently covered
118 * allocations of the same source filling up the pool.
119 *
120 * Assuming a range of 15%-85% unique allocations in the pool at any point in
121 * time, the below parameters provide a probablity of 0.02-0.33 for false
122 * positive hits respectively:
123 *
124 * P(alloc_traces) = (1 - e^(-HNUM * (alloc_traces / SIZE)) ^ HNUM
125 */
126 #define ALLOC_COVERED_HNUM 2
127 #define ALLOC_COVERED_ORDER (const_ilog2(CONFIG_KFENCE_NUM_OBJECTS) + 2)
128 #define ALLOC_COVERED_SIZE (1 << ALLOC_COVERED_ORDER)
129 #define ALLOC_COVERED_HNEXT(h) hash_32(h, ALLOC_COVERED_ORDER)
130 #define ALLOC_COVERED_MASK (ALLOC_COVERED_SIZE - 1)
131 static atomic_t alloc_covered[ALLOC_COVERED_SIZE];
132
133 /* Stack depth used to determine uniqueness of an allocation. */
134 #define UNIQUE_ALLOC_STACK_DEPTH ((size_t)8)
135
136 /*
137 * Randomness for stack hashes, making the same collisions across reboots and
138 * different machines less likely.
139 */
140 static u32 stack_hash_seed __ro_after_init;
141
142 /* Statistics counters for debugfs. */
143 enum kfence_counter_id {
144 KFENCE_COUNTER_ALLOCATED,
145 KFENCE_COUNTER_ALLOCS,
146 KFENCE_COUNTER_FREES,
147 KFENCE_COUNTER_ZOMBIES,
148 KFENCE_COUNTER_BUGS,
149 KFENCE_COUNTER_SKIP_INCOMPAT,
150 KFENCE_COUNTER_SKIP_CAPACITY,
151 KFENCE_COUNTER_SKIP_COVERED,
152 KFENCE_COUNTER_COUNT,
153 };
154 static atomic_long_t counters[KFENCE_COUNTER_COUNT];
155 static const char *const counter_names[] = {
156 [KFENCE_COUNTER_ALLOCATED] = "currently allocated",
157 [KFENCE_COUNTER_ALLOCS] = "total allocations",
158 [KFENCE_COUNTER_FREES] = "total frees",
159 [KFENCE_COUNTER_ZOMBIES] = "zombie allocations",
160 [KFENCE_COUNTER_BUGS] = "total bugs",
161 [KFENCE_COUNTER_SKIP_INCOMPAT] = "skipped allocations (incompatible)",
162 [KFENCE_COUNTER_SKIP_CAPACITY] = "skipped allocations (capacity)",
163 [KFENCE_COUNTER_SKIP_COVERED] = "skipped allocations (covered)",
164 };
165 static_assert(ARRAY_SIZE(counter_names) == KFENCE_COUNTER_COUNT);
166
167 /* === Internals ============================================================ */
168
should_skip_covered(void)169 static inline bool should_skip_covered(void)
170 {
171 unsigned long thresh = (CONFIG_KFENCE_NUM_OBJECTS * kfence_skip_covered_thresh) / 100;
172
173 return atomic_long_read(&counters[KFENCE_COUNTER_ALLOCATED]) > thresh;
174 }
175
get_alloc_stack_hash(unsigned long * stack_entries,size_t num_entries)176 static u32 get_alloc_stack_hash(unsigned long *stack_entries, size_t num_entries)
177 {
178 num_entries = min(num_entries, UNIQUE_ALLOC_STACK_DEPTH);
179 num_entries = filter_irq_stacks(stack_entries, num_entries);
180 return jhash(stack_entries, num_entries * sizeof(stack_entries[0]), stack_hash_seed);
181 }
182
183 /*
184 * Adds (or subtracts) count @val for allocation stack trace hash
185 * @alloc_stack_hash from Counting Bloom filter.
186 */
alloc_covered_add(u32 alloc_stack_hash,int val)187 static void alloc_covered_add(u32 alloc_stack_hash, int val)
188 {
189 int i;
190
191 for (i = 0; i < ALLOC_COVERED_HNUM; i++) {
192 atomic_add(val, &alloc_covered[alloc_stack_hash & ALLOC_COVERED_MASK]);
193 alloc_stack_hash = ALLOC_COVERED_HNEXT(alloc_stack_hash);
194 }
195 }
196
197 /*
198 * Returns true if the allocation stack trace hash @alloc_stack_hash is
199 * currently contained (non-zero count) in Counting Bloom filter.
200 */
alloc_covered_contains(u32 alloc_stack_hash)201 static bool alloc_covered_contains(u32 alloc_stack_hash)
202 {
203 int i;
204
205 for (i = 0; i < ALLOC_COVERED_HNUM; i++) {
206 if (!atomic_read(&alloc_covered[alloc_stack_hash & ALLOC_COVERED_MASK]))
207 return false;
208 alloc_stack_hash = ALLOC_COVERED_HNEXT(alloc_stack_hash);
209 }
210
211 return true;
212 }
213
kfence_protect(unsigned long addr)214 static bool kfence_protect(unsigned long addr)
215 {
216 return !KFENCE_WARN_ON(!kfence_protect_page(ALIGN_DOWN(addr, PAGE_SIZE), true));
217 }
218
kfence_unprotect(unsigned long addr)219 static bool kfence_unprotect(unsigned long addr)
220 {
221 return !KFENCE_WARN_ON(!kfence_protect_page(ALIGN_DOWN(addr, PAGE_SIZE), false));
222 }
223
addr_to_metadata(unsigned long addr)224 static inline struct kfence_metadata *addr_to_metadata(unsigned long addr)
225 {
226 long index;
227
228 /* The checks do not affect performance; only called from slow-paths. */
229
230 if (!is_kfence_address((void *)addr))
231 return NULL;
232
233 /*
234 * May be an invalid index if called with an address at the edge of
235 * __kfence_pool, in which case we would report an "invalid access"
236 * error.
237 */
238 index = (addr - (unsigned long)__kfence_pool) / (PAGE_SIZE * 2) - 1;
239 if (index < 0 || index >= CONFIG_KFENCE_NUM_OBJECTS)
240 return NULL;
241
242 return &kfence_metadata[index];
243 }
244
metadata_to_pageaddr(const struct kfence_metadata * meta)245 static inline unsigned long metadata_to_pageaddr(const struct kfence_metadata *meta)
246 {
247 unsigned long offset = (meta - kfence_metadata + 1) * PAGE_SIZE * 2;
248 unsigned long pageaddr = (unsigned long)&__kfence_pool[offset];
249
250 /* The checks do not affect performance; only called from slow-paths. */
251
252 /* Only call with a pointer into kfence_metadata. */
253 if (KFENCE_WARN_ON(meta < kfence_metadata ||
254 meta >= kfence_metadata + CONFIG_KFENCE_NUM_OBJECTS))
255 return 0;
256
257 /*
258 * This metadata object only ever maps to 1 page; verify that the stored
259 * address is in the expected range.
260 */
261 if (KFENCE_WARN_ON(ALIGN_DOWN(meta->addr, PAGE_SIZE) != pageaddr))
262 return 0;
263
264 return pageaddr;
265 }
266
267 /*
268 * Update the object's metadata state, including updating the alloc/free stacks
269 * depending on the state transition.
270 */
271 static noinline void
metadata_update_state(struct kfence_metadata * meta,enum kfence_object_state next,unsigned long * stack_entries,size_t num_stack_entries)272 metadata_update_state(struct kfence_metadata *meta, enum kfence_object_state next,
273 unsigned long *stack_entries, size_t num_stack_entries)
274 {
275 struct kfence_track *track =
276 next == KFENCE_OBJECT_FREED ? &meta->free_track : &meta->alloc_track;
277
278 lockdep_assert_held(&meta->lock);
279
280 if (stack_entries) {
281 memcpy(track->stack_entries, stack_entries,
282 num_stack_entries * sizeof(stack_entries[0]));
283 } else {
284 /*
285 * Skip over 1 (this) functions; noinline ensures we do not
286 * accidentally skip over the caller by never inlining.
287 */
288 num_stack_entries = stack_trace_save(track->stack_entries, KFENCE_STACK_DEPTH, 1);
289 }
290 track->num_stack_entries = num_stack_entries;
291 track->pid = task_pid_nr(current);
292 track->cpu = raw_smp_processor_id();
293 track->ts_nsec = local_clock(); /* Same source as printk timestamps. */
294
295 /*
296 * Pairs with READ_ONCE() in
297 * kfence_shutdown_cache(),
298 * kfence_handle_page_fault().
299 */
300 WRITE_ONCE(meta->state, next);
301 }
302
303 /* Write canary byte to @addr. */
set_canary_byte(u8 * addr)304 static inline bool set_canary_byte(u8 *addr)
305 {
306 *addr = KFENCE_CANARY_PATTERN(addr);
307 return true;
308 }
309
310 /* Check canary byte at @addr. */
check_canary_byte(u8 * addr)311 static inline bool check_canary_byte(u8 *addr)
312 {
313 struct kfence_metadata *meta;
314 unsigned long flags;
315
316 if (likely(*addr == KFENCE_CANARY_PATTERN(addr)))
317 return true;
318
319 atomic_long_inc(&counters[KFENCE_COUNTER_BUGS]);
320
321 meta = addr_to_metadata((unsigned long)addr);
322 raw_spin_lock_irqsave(&meta->lock, flags);
323 kfence_report_error((unsigned long)addr, false, NULL, meta, KFENCE_ERROR_CORRUPTION);
324 raw_spin_unlock_irqrestore(&meta->lock, flags);
325
326 return false;
327 }
328
329 /* __always_inline this to ensure we won't do an indirect call to fn. */
for_each_canary(const struct kfence_metadata * meta,bool (* fn)(u8 *))330 static __always_inline void for_each_canary(const struct kfence_metadata *meta, bool (*fn)(u8 *))
331 {
332 const unsigned long pageaddr = ALIGN_DOWN(meta->addr, PAGE_SIZE);
333 unsigned long addr;
334
335 /*
336 * We'll iterate over each canary byte per-side until fn() returns
337 * false. However, we'll still iterate over the canary bytes to the
338 * right of the object even if there was an error in the canary bytes to
339 * the left of the object. Specifically, if check_canary_byte()
340 * generates an error, showing both sides might give more clues as to
341 * what the error is about when displaying which bytes were corrupted.
342 */
343
344 /* Apply to left of object. */
345 for (addr = pageaddr; addr < meta->addr; addr++) {
346 if (!fn((u8 *)addr))
347 break;
348 }
349
350 /* Apply to right of object. */
351 for (addr = meta->addr + meta->size; addr < pageaddr + PAGE_SIZE; addr++) {
352 if (!fn((u8 *)addr))
353 break;
354 }
355 }
356
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)357 static void *kfence_guarded_alloc(struct kmem_cache *cache, size_t size, gfp_t gfp,
358 unsigned long *stack_entries, size_t num_stack_entries,
359 u32 alloc_stack_hash)
360 {
361 struct kfence_metadata *meta = NULL;
362 unsigned long flags;
363 struct page *page;
364 void *addr;
365
366 /* Try to obtain a free object. */
367 raw_spin_lock_irqsave(&kfence_freelist_lock, flags);
368 if (!list_empty(&kfence_freelist)) {
369 meta = list_entry(kfence_freelist.next, struct kfence_metadata, list);
370 list_del_init(&meta->list);
371 }
372 raw_spin_unlock_irqrestore(&kfence_freelist_lock, flags);
373 if (!meta) {
374 atomic_long_inc(&counters[KFENCE_COUNTER_SKIP_CAPACITY]);
375 return NULL;
376 }
377
378 if (unlikely(!raw_spin_trylock_irqsave(&meta->lock, flags))) {
379 /*
380 * This is extremely unlikely -- we are reporting on a
381 * use-after-free, which locked meta->lock, and the reporting
382 * code via printk calls kmalloc() which ends up in
383 * kfence_alloc() and tries to grab the same object that we're
384 * reporting on. While it has never been observed, lockdep does
385 * report that there is a possibility of deadlock. Fix it by
386 * using trylock and bailing out gracefully.
387 */
388 raw_spin_lock_irqsave(&kfence_freelist_lock, flags);
389 /* Put the object back on the freelist. */
390 list_add_tail(&meta->list, &kfence_freelist);
391 raw_spin_unlock_irqrestore(&kfence_freelist_lock, flags);
392
393 return NULL;
394 }
395
396 meta->addr = metadata_to_pageaddr(meta);
397 /* Unprotect if we're reusing this page. */
398 if (meta->state == KFENCE_OBJECT_FREED)
399 kfence_unprotect(meta->addr);
400
401 /*
402 * Note: for allocations made before RNG initialization, will always
403 * return zero. We still benefit from enabling KFENCE as early as
404 * possible, even when the RNG is not yet available, as this will allow
405 * KFENCE to detect bugs due to earlier allocations. The only downside
406 * is that the out-of-bounds accesses detected are deterministic for
407 * such allocations.
408 */
409 if (prandom_u32_max(2)) {
410 /* Allocate on the "right" side, re-calculate address. */
411 meta->addr += PAGE_SIZE - size;
412 meta->addr = ALIGN_DOWN(meta->addr, cache->align);
413 }
414
415 addr = (void *)meta->addr;
416
417 /* Update remaining metadata. */
418 metadata_update_state(meta, KFENCE_OBJECT_ALLOCATED, stack_entries, num_stack_entries);
419 /* Pairs with READ_ONCE() in kfence_shutdown_cache(). */
420 WRITE_ONCE(meta->cache, cache);
421 meta->size = size;
422 meta->alloc_stack_hash = alloc_stack_hash;
423 raw_spin_unlock_irqrestore(&meta->lock, flags);
424
425 alloc_covered_add(alloc_stack_hash, 1);
426
427 /* Set required struct page fields. */
428 page = virt_to_page(meta->addr);
429 page->slab_cache = cache;
430 if (IS_ENABLED(CONFIG_SLUB))
431 page->objects = 1;
432 if (IS_ENABLED(CONFIG_SLAB))
433 page->s_mem = addr;
434
435 /* Memory initialization. */
436 for_each_canary(meta, set_canary_byte);
437
438 /*
439 * We check slab_want_init_on_alloc() ourselves, rather than letting
440 * SL*B do the initialization, as otherwise we might overwrite KFENCE's
441 * redzone.
442 */
443 if (unlikely(slab_want_init_on_alloc(gfp, cache)))
444 memzero_explicit(addr, size);
445 if (cache->ctor)
446 cache->ctor(addr);
447
448 if (CONFIG_KFENCE_STRESS_TEST_FAULTS && !prandom_u32_max(CONFIG_KFENCE_STRESS_TEST_FAULTS))
449 kfence_protect(meta->addr); /* Random "faults" by protecting the object. */
450
451 atomic_long_inc(&counters[KFENCE_COUNTER_ALLOCATED]);
452 atomic_long_inc(&counters[KFENCE_COUNTER_ALLOCS]);
453
454 return addr;
455 }
456
kfence_guarded_free(void * addr,struct kfence_metadata * meta,bool zombie)457 static void kfence_guarded_free(void *addr, struct kfence_metadata *meta, bool zombie)
458 {
459 struct kcsan_scoped_access assert_page_exclusive;
460 unsigned long flags;
461 bool init;
462
463 raw_spin_lock_irqsave(&meta->lock, flags);
464
465 if (meta->state != KFENCE_OBJECT_ALLOCATED || meta->addr != (unsigned long)addr) {
466 /* Invalid or double-free, bail out. */
467 atomic_long_inc(&counters[KFENCE_COUNTER_BUGS]);
468 kfence_report_error((unsigned long)addr, false, NULL, meta,
469 KFENCE_ERROR_INVALID_FREE);
470 raw_spin_unlock_irqrestore(&meta->lock, flags);
471 return;
472 }
473
474 /* Detect racy use-after-free, or incorrect reallocation of this page by KFENCE. */
475 kcsan_begin_scoped_access((void *)ALIGN_DOWN((unsigned long)addr, PAGE_SIZE), PAGE_SIZE,
476 KCSAN_ACCESS_SCOPED | KCSAN_ACCESS_WRITE | KCSAN_ACCESS_ASSERT,
477 &assert_page_exclusive);
478
479 if (CONFIG_KFENCE_STRESS_TEST_FAULTS)
480 kfence_unprotect((unsigned long)addr); /* To check canary bytes. */
481
482 /* Restore page protection if there was an OOB access. */
483 if (meta->unprotected_page) {
484 memzero_explicit((void *)ALIGN_DOWN(meta->unprotected_page, PAGE_SIZE), PAGE_SIZE);
485 kfence_protect(meta->unprotected_page);
486 meta->unprotected_page = 0;
487 }
488
489 /* Mark the object as freed. */
490 metadata_update_state(meta, KFENCE_OBJECT_FREED, NULL, 0);
491 init = slab_want_init_on_free(meta->cache);
492 raw_spin_unlock_irqrestore(&meta->lock, flags);
493
494 alloc_covered_add(meta->alloc_stack_hash, -1);
495
496 /* Check canary bytes for memory corruption. */
497 for_each_canary(meta, check_canary_byte);
498
499 /*
500 * Clear memory if init-on-free is set. While we protect the page, the
501 * data is still there, and after a use-after-free is detected, we
502 * unprotect the page, so the data is still accessible.
503 */
504 if (!zombie && unlikely(init))
505 memzero_explicit(addr, meta->size);
506
507 /* Protect to detect use-after-frees. */
508 kfence_protect((unsigned long)addr);
509
510 kcsan_end_scoped_access(&assert_page_exclusive);
511 if (!zombie) {
512 /* Add it to the tail of the freelist for reuse. */
513 raw_spin_lock_irqsave(&kfence_freelist_lock, flags);
514 KFENCE_WARN_ON(!list_empty(&meta->list));
515 list_add_tail(&meta->list, &kfence_freelist);
516 raw_spin_unlock_irqrestore(&kfence_freelist_lock, flags);
517
518 atomic_long_dec(&counters[KFENCE_COUNTER_ALLOCATED]);
519 atomic_long_inc(&counters[KFENCE_COUNTER_FREES]);
520 } else {
521 /* See kfence_shutdown_cache(). */
522 atomic_long_inc(&counters[KFENCE_COUNTER_ZOMBIES]);
523 }
524 }
525
rcu_guarded_free(struct rcu_head * h)526 static void rcu_guarded_free(struct rcu_head *h)
527 {
528 struct kfence_metadata *meta = container_of(h, struct kfence_metadata, rcu_head);
529
530 kfence_guarded_free((void *)meta->addr, meta, false);
531 }
532
kfence_init_pool(void)533 static bool __init kfence_init_pool(void)
534 {
535 unsigned long addr = (unsigned long)__kfence_pool;
536 struct page *pages;
537 int i;
538
539 if (!__kfence_pool)
540 return false;
541
542 if (!arch_kfence_init_pool())
543 goto err;
544
545 pages = virt_to_page(addr);
546
547 /*
548 * Set up object pages: they must have PG_slab set, to avoid freeing
549 * these as real pages.
550 *
551 * We also want to avoid inserting kfence_free() in the kfree()
552 * fast-path in SLUB, and therefore need to ensure kfree() correctly
553 * enters __slab_free() slow-path.
554 */
555 for (i = 0; i < KFENCE_POOL_SIZE / PAGE_SIZE; i++) {
556 if (!i || (i % 2))
557 continue;
558
559 /* Verify we do not have a compound head page. */
560 if (WARN_ON(compound_head(&pages[i]) != &pages[i]))
561 goto err;
562
563 __SetPageSlab(&pages[i]);
564 }
565
566 /*
567 * Protect the first 2 pages. The first page is mostly unnecessary, and
568 * merely serves as an extended guard page. However, adding one
569 * additional page in the beginning gives us an even number of pages,
570 * which simplifies the mapping of address to metadata index.
571 */
572 for (i = 0; i < 2; i++) {
573 if (unlikely(!kfence_protect(addr)))
574 goto err;
575
576 addr += PAGE_SIZE;
577 }
578
579 for (i = 0; i < CONFIG_KFENCE_NUM_OBJECTS; i++) {
580 struct kfence_metadata *meta = &kfence_metadata[i];
581
582 /* Initialize metadata. */
583 INIT_LIST_HEAD(&meta->list);
584 raw_spin_lock_init(&meta->lock);
585 meta->state = KFENCE_OBJECT_UNUSED;
586 meta->addr = addr; /* Initialize for validation in metadata_to_pageaddr(). */
587 list_add_tail(&meta->list, &kfence_freelist);
588
589 /* Protect the right redzone. */
590 if (unlikely(!kfence_protect(addr + PAGE_SIZE)))
591 goto err;
592
593 addr += 2 * PAGE_SIZE;
594 }
595
596 /*
597 * The pool is live and will never be deallocated from this point on.
598 * Remove the pool object from the kmemleak object tree, as it would
599 * otherwise overlap with allocations returned by kfence_alloc(), which
600 * are registered with kmemleak through the slab post-alloc hook.
601 */
602 kmemleak_free(__kfence_pool);
603
604 return true;
605
606 err:
607 /*
608 * Only release unprotected pages, and do not try to go back and change
609 * page attributes due to risk of failing to do so as well. If changing
610 * page attributes for some pages fails, it is very likely that it also
611 * fails for the first page, and therefore expect addr==__kfence_pool in
612 * most failure cases.
613 */
614 memblock_free_late(__pa(addr), KFENCE_POOL_SIZE - (addr - (unsigned long)__kfence_pool));
615 __kfence_pool = NULL;
616 return false;
617 }
618
619 /* === DebugFS Interface ==================================================== */
620
stats_show(struct seq_file * seq,void * v)621 static int stats_show(struct seq_file *seq, void *v)
622 {
623 int i;
624
625 seq_printf(seq, "enabled: %i\n", READ_ONCE(kfence_enabled));
626 for (i = 0; i < KFENCE_COUNTER_COUNT; i++)
627 seq_printf(seq, "%s: %ld\n", counter_names[i], atomic_long_read(&counters[i]));
628
629 return 0;
630 }
631 DEFINE_SHOW_ATTRIBUTE(stats);
632
633 /*
634 * debugfs seq_file operations for /sys/kernel/debug/kfence/objects.
635 * start_object() and next_object() return the object index + 1, because NULL is used
636 * to stop iteration.
637 */
start_object(struct seq_file * seq,loff_t * pos)638 static void *start_object(struct seq_file *seq, loff_t *pos)
639 {
640 if (*pos < CONFIG_KFENCE_NUM_OBJECTS)
641 return (void *)((long)*pos + 1);
642 return NULL;
643 }
644
stop_object(struct seq_file * seq,void * v)645 static void stop_object(struct seq_file *seq, void *v)
646 {
647 }
648
next_object(struct seq_file * seq,void * v,loff_t * pos)649 static void *next_object(struct seq_file *seq, void *v, loff_t *pos)
650 {
651 ++*pos;
652 if (*pos < CONFIG_KFENCE_NUM_OBJECTS)
653 return (void *)((long)*pos + 1);
654 return NULL;
655 }
656
show_object(struct seq_file * seq,void * v)657 static int show_object(struct seq_file *seq, void *v)
658 {
659 struct kfence_metadata *meta = &kfence_metadata[(long)v - 1];
660 unsigned long flags;
661
662 raw_spin_lock_irqsave(&meta->lock, flags);
663 kfence_print_object(seq, meta);
664 raw_spin_unlock_irqrestore(&meta->lock, flags);
665 seq_puts(seq, "---------------------------------\n");
666
667 return 0;
668 }
669
670 static const struct seq_operations object_seqops = {
671 .start = start_object,
672 .next = next_object,
673 .stop = stop_object,
674 .show = show_object,
675 };
676
open_objects(struct inode * inode,struct file * file)677 static int open_objects(struct inode *inode, struct file *file)
678 {
679 return seq_open(file, &object_seqops);
680 }
681
682 static const struct file_operations objects_fops = {
683 .open = open_objects,
684 .read = seq_read,
685 .llseek = seq_lseek,
686 .release = seq_release,
687 };
688
kfence_debugfs_init(void)689 static int __init kfence_debugfs_init(void)
690 {
691 struct dentry *kfence_dir = debugfs_create_dir("kfence", NULL);
692
693 debugfs_create_file("stats", 0444, kfence_dir, NULL, &stats_fops);
694 debugfs_create_file("objects", 0400, kfence_dir, NULL, &objects_fops);
695 return 0;
696 }
697
698 late_initcall(kfence_debugfs_init);
699
700 /* === Allocation Gate Timer ================================================ */
701
702 #ifdef CONFIG_KFENCE_STATIC_KEYS
703 /* Wait queue to wake up allocation-gate timer task. */
704 static DECLARE_WAIT_QUEUE_HEAD(allocation_wait);
705
wake_up_kfence_timer(struct irq_work * work)706 static void wake_up_kfence_timer(struct irq_work *work)
707 {
708 wake_up(&allocation_wait);
709 }
710 static DEFINE_IRQ_WORK(wake_up_kfence_timer_work, wake_up_kfence_timer);
711 #endif
712
713 /*
714 * Set up delayed work, which will enable and disable the static key. We need to
715 * use a work queue (rather than a simple timer), since enabling and disabling a
716 * static key cannot be done from an interrupt.
717 *
718 * Note: Toggling a static branch currently causes IPIs, and here we'll end up
719 * with a total of 2 IPIs to all CPUs. If this ends up a problem in future (with
720 * more aggressive sampling intervals), we could get away with a variant that
721 * avoids IPIs, at the cost of not immediately capturing allocations if the
722 * instructions remain cached.
723 */
724 static struct delayed_work kfence_timer;
toggle_allocation_gate(struct work_struct * work)725 static void toggle_allocation_gate(struct work_struct *work)
726 {
727 if (!READ_ONCE(kfence_enabled))
728 return;
729
730 atomic_set(&kfence_allocation_gate, 0);
731 #ifdef CONFIG_KFENCE_STATIC_KEYS
732 /* Enable static key, and await allocation to happen. */
733 static_branch_enable(&kfence_allocation_key);
734
735 if (sysctl_hung_task_timeout_secs) {
736 /*
737 * During low activity with no allocations we might wait a
738 * while; let's avoid the hung task warning.
739 */
740 wait_event_idle_timeout(allocation_wait, atomic_read(&kfence_allocation_gate),
741 sysctl_hung_task_timeout_secs * HZ / 2);
742 } else {
743 wait_event_idle(allocation_wait, atomic_read(&kfence_allocation_gate));
744 }
745
746 /* Disable static key and reset timer. */
747 static_branch_disable(&kfence_allocation_key);
748 #endif
749 queue_delayed_work(system_unbound_wq, &kfence_timer,
750 msecs_to_jiffies(kfence_sample_interval));
751 }
752 static DECLARE_DELAYED_WORK(kfence_timer, toggle_allocation_gate);
753
754 /* === Public interface ===================================================== */
755
kfence_alloc_pool(void)756 void __init kfence_alloc_pool(void)
757 {
758 if (!kfence_sample_interval)
759 return;
760
761 __kfence_pool = memblock_alloc(KFENCE_POOL_SIZE, PAGE_SIZE);
762
763 if (!__kfence_pool)
764 pr_err("failed to allocate pool\n");
765 }
766
kfence_init(void)767 void __init kfence_init(void)
768 {
769 /* Setting kfence_sample_interval to 0 on boot disables KFENCE. */
770 if (!kfence_sample_interval)
771 return;
772
773 stack_hash_seed = (u32)random_get_entropy();
774 if (!kfence_init_pool()) {
775 pr_err("%s failed\n", __func__);
776 return;
777 }
778
779 if (!IS_ENABLED(CONFIG_KFENCE_STATIC_KEYS))
780 static_branch_enable(&kfence_allocation_key);
781 WRITE_ONCE(kfence_enabled, true);
782 queue_delayed_work(system_unbound_wq, &kfence_timer, 0);
783 pr_info("initialized - using %lu bytes for %d objects at 0x%p-0x%p\n", KFENCE_POOL_SIZE,
784 CONFIG_KFENCE_NUM_OBJECTS, (void *)__kfence_pool,
785 (void *)(__kfence_pool + KFENCE_POOL_SIZE));
786 }
787
kfence_shutdown_cache(struct kmem_cache * s)788 void kfence_shutdown_cache(struct kmem_cache *s)
789 {
790 unsigned long flags;
791 struct kfence_metadata *meta;
792 int i;
793
794 for (i = 0; i < CONFIG_KFENCE_NUM_OBJECTS; i++) {
795 bool in_use;
796
797 meta = &kfence_metadata[i];
798
799 /*
800 * If we observe some inconsistent cache and state pair where we
801 * should have returned false here, cache destruction is racing
802 * with either kmem_cache_alloc() or kmem_cache_free(). Taking
803 * the lock will not help, as different critical section
804 * serialization will have the same outcome.
805 */
806 if (READ_ONCE(meta->cache) != s ||
807 READ_ONCE(meta->state) != KFENCE_OBJECT_ALLOCATED)
808 continue;
809
810 raw_spin_lock_irqsave(&meta->lock, flags);
811 in_use = meta->cache == s && meta->state == KFENCE_OBJECT_ALLOCATED;
812 raw_spin_unlock_irqrestore(&meta->lock, flags);
813
814 if (in_use) {
815 /*
816 * This cache still has allocations, and we should not
817 * release them back into the freelist so they can still
818 * safely be used and retain the kernel's default
819 * behaviour of keeping the allocations alive (leak the
820 * cache); however, they effectively become "zombie
821 * allocations" as the KFENCE objects are the only ones
822 * still in use and the owning cache is being destroyed.
823 *
824 * We mark them freed, so that any subsequent use shows
825 * more useful error messages that will include stack
826 * traces of the user of the object, the original
827 * allocation, and caller to shutdown_cache().
828 */
829 kfence_guarded_free((void *)meta->addr, meta, /*zombie=*/true);
830 }
831 }
832
833 for (i = 0; i < CONFIG_KFENCE_NUM_OBJECTS; i++) {
834 meta = &kfence_metadata[i];
835
836 /* See above. */
837 if (READ_ONCE(meta->cache) != s || READ_ONCE(meta->state) != KFENCE_OBJECT_FREED)
838 continue;
839
840 raw_spin_lock_irqsave(&meta->lock, flags);
841 if (meta->cache == s && meta->state == KFENCE_OBJECT_FREED)
842 meta->cache = NULL;
843 raw_spin_unlock_irqrestore(&meta->lock, flags);
844 }
845 }
846
__kfence_alloc(struct kmem_cache * s,size_t size,gfp_t flags)847 void *__kfence_alloc(struct kmem_cache *s, size_t size, gfp_t flags)
848 {
849 unsigned long stack_entries[KFENCE_STACK_DEPTH];
850 size_t num_stack_entries;
851 u32 alloc_stack_hash;
852
853 /*
854 * Perform size check before switching kfence_allocation_gate, so that
855 * we don't disable KFENCE without making an allocation.
856 */
857 if (size > PAGE_SIZE) {
858 atomic_long_inc(&counters[KFENCE_COUNTER_SKIP_INCOMPAT]);
859 return NULL;
860 }
861
862 /*
863 * Skip allocations from non-default zones, including DMA. We cannot
864 * guarantee that pages in the KFENCE pool will have the requested
865 * properties (e.g. reside in DMAable memory).
866 */
867 if ((flags & GFP_ZONEMASK) ||
868 (s->flags & (SLAB_CACHE_DMA | SLAB_CACHE_DMA32))) {
869 atomic_long_inc(&counters[KFENCE_COUNTER_SKIP_INCOMPAT]);
870 return NULL;
871 }
872
873 if (atomic_inc_return(&kfence_allocation_gate) > 1)
874 return NULL;
875 #ifdef CONFIG_KFENCE_STATIC_KEYS
876 /*
877 * waitqueue_active() is fully ordered after the update of
878 * kfence_allocation_gate per atomic_inc_return().
879 */
880 if (waitqueue_active(&allocation_wait)) {
881 /*
882 * Calling wake_up() here may deadlock when allocations happen
883 * from within timer code. Use an irq_work to defer it.
884 */
885 irq_work_queue(&wake_up_kfence_timer_work);
886 }
887 #endif
888
889 if (!READ_ONCE(kfence_enabled))
890 return NULL;
891
892 num_stack_entries = stack_trace_save(stack_entries, KFENCE_STACK_DEPTH, 0);
893
894 /*
895 * Do expensive check for coverage of allocation in slow-path after
896 * allocation_gate has already become non-zero, even though it might
897 * mean not making any allocation within a given sample interval.
898 *
899 * This ensures reasonable allocation coverage when the pool is almost
900 * full, including avoiding long-lived allocations of the same source
901 * filling up the pool (e.g. pagecache allocations).
902 */
903 alloc_stack_hash = get_alloc_stack_hash(stack_entries, num_stack_entries);
904 if (should_skip_covered() && alloc_covered_contains(alloc_stack_hash)) {
905 atomic_long_inc(&counters[KFENCE_COUNTER_SKIP_COVERED]);
906 return NULL;
907 }
908
909 return kfence_guarded_alloc(s, size, flags, stack_entries, num_stack_entries,
910 alloc_stack_hash);
911 }
912
kfence_ksize(const void * addr)913 size_t kfence_ksize(const void *addr)
914 {
915 const struct kfence_metadata *meta = addr_to_metadata((unsigned long)addr);
916
917 /*
918 * Read locklessly -- if there is a race with __kfence_alloc(), this is
919 * either a use-after-free or invalid access.
920 */
921 return meta ? meta->size : 0;
922 }
923
kfence_object_start(const void * addr)924 void *kfence_object_start(const void *addr)
925 {
926 const struct kfence_metadata *meta = addr_to_metadata((unsigned long)addr);
927
928 /*
929 * Read locklessly -- if there is a race with __kfence_alloc(), this is
930 * either a use-after-free or invalid access.
931 */
932 return meta ? (void *)meta->addr : NULL;
933 }
934
__kfence_free(void * addr)935 void __kfence_free(void *addr)
936 {
937 struct kfence_metadata *meta = addr_to_metadata((unsigned long)addr);
938
939 /*
940 * If the objects of the cache are SLAB_TYPESAFE_BY_RCU, defer freeing
941 * the object, as the object page may be recycled for other-typed
942 * objects once it has been freed. meta->cache may be NULL if the cache
943 * was destroyed.
944 */
945 if (unlikely(meta->cache && (meta->cache->flags & SLAB_TYPESAFE_BY_RCU)))
946 call_rcu(&meta->rcu_head, rcu_guarded_free);
947 else
948 kfence_guarded_free(addr, meta, false);
949 }
950
kfence_handle_page_fault(unsigned long addr,bool is_write,struct pt_regs * regs)951 bool kfence_handle_page_fault(unsigned long addr, bool is_write, struct pt_regs *regs)
952 {
953 const int page_index = (addr - (unsigned long)__kfence_pool) / PAGE_SIZE;
954 struct kfence_metadata *to_report = NULL;
955 enum kfence_error_type error_type;
956 unsigned long flags;
957
958 if (!is_kfence_address((void *)addr))
959 return false;
960
961 if (!READ_ONCE(kfence_enabled)) /* If disabled at runtime ... */
962 return kfence_unprotect(addr); /* ... unprotect and proceed. */
963
964 atomic_long_inc(&counters[KFENCE_COUNTER_BUGS]);
965
966 if (page_index % 2) {
967 /* This is a redzone, report a buffer overflow. */
968 struct kfence_metadata *meta;
969 int distance = 0;
970
971 meta = addr_to_metadata(addr - PAGE_SIZE);
972 if (meta && READ_ONCE(meta->state) == KFENCE_OBJECT_ALLOCATED) {
973 to_report = meta;
974 /* Data race ok; distance calculation approximate. */
975 distance = addr - data_race(meta->addr + meta->size);
976 }
977
978 meta = addr_to_metadata(addr + PAGE_SIZE);
979 if (meta && READ_ONCE(meta->state) == KFENCE_OBJECT_ALLOCATED) {
980 /* Data race ok; distance calculation approximate. */
981 if (!to_report || distance > data_race(meta->addr) - addr)
982 to_report = meta;
983 }
984
985 if (!to_report)
986 goto out;
987
988 raw_spin_lock_irqsave(&to_report->lock, flags);
989 to_report->unprotected_page = addr;
990 error_type = KFENCE_ERROR_OOB;
991
992 /*
993 * If the object was freed before we took the look we can still
994 * report this as an OOB -- the report will simply show the
995 * stacktrace of the free as well.
996 */
997 } else {
998 to_report = addr_to_metadata(addr);
999 if (!to_report)
1000 goto out;
1001
1002 raw_spin_lock_irqsave(&to_report->lock, flags);
1003 error_type = KFENCE_ERROR_UAF;
1004 /*
1005 * We may race with __kfence_alloc(), and it is possible that a
1006 * freed object may be reallocated. We simply report this as a
1007 * use-after-free, with the stack trace showing the place where
1008 * the object was re-allocated.
1009 */
1010 }
1011
1012 out:
1013 if (to_report) {
1014 kfence_report_error(addr, is_write, regs, to_report, error_type);
1015 raw_spin_unlock_irqrestore(&to_report->lock, flags);
1016 } else {
1017 /* This may be a UAF or OOB access, but we can't be sure. */
1018 kfence_report_error(addr, is_write, regs, NULL, KFENCE_ERROR_INVALID);
1019 }
1020
1021 return kfence_unprotect(addr); /* Unprotect and let access proceed. */
1022 }
1023