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0ce20dd8 AP |
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/kcsan-checks.h> | |
14 | #include <linux/kfence.h> | |
95511580 | 15 | #include <linux/kmemleak.h> |
0ce20dd8 AP |
16 | #include <linux/list.h> |
17 | #include <linux/lockdep.h> | |
18 | #include <linux/memblock.h> | |
19 | #include <linux/moduleparam.h> | |
20 | #include <linux/random.h> | |
21 | #include <linux/rcupdate.h> | |
22 | #include <linux/seq_file.h> | |
23 | #include <linux/slab.h> | |
24 | #include <linux/spinlock.h> | |
25 | #include <linux/string.h> | |
26 | ||
27 | #include <asm/kfence.h> | |
28 | ||
29 | #include "kfence.h" | |
30 | ||
31 | /* Disables KFENCE on the first warning assuming an irrecoverable error. */ | |
32 | #define KFENCE_WARN_ON(cond) \ | |
33 | ({ \ | |
34 | const bool __cond = WARN_ON(cond); \ | |
35 | if (unlikely(__cond)) \ | |
36 | WRITE_ONCE(kfence_enabled, false); \ | |
37 | __cond; \ | |
38 | }) | |
39 | ||
40 | /* === Data ================================================================= */ | |
41 | ||
42 | static bool kfence_enabled __read_mostly; | |
43 | ||
44 | static unsigned long kfence_sample_interval __read_mostly = CONFIG_KFENCE_SAMPLE_INTERVAL; | |
45 | ||
46 | #ifdef MODULE_PARAM_PREFIX | |
47 | #undef MODULE_PARAM_PREFIX | |
48 | #endif | |
49 | #define MODULE_PARAM_PREFIX "kfence." | |
50 | ||
51 | static int param_set_sample_interval(const char *val, const struct kernel_param *kp) | |
52 | { | |
53 | unsigned long num; | |
54 | int ret = kstrtoul(val, 0, &num); | |
55 | ||
56 | if (ret < 0) | |
57 | return ret; | |
58 | ||
59 | if (!num) /* Using 0 to indicate KFENCE is disabled. */ | |
60 | WRITE_ONCE(kfence_enabled, false); | |
61 | else if (!READ_ONCE(kfence_enabled) && system_state != SYSTEM_BOOTING) | |
62 | return -EINVAL; /* Cannot (re-)enable KFENCE on-the-fly. */ | |
63 | ||
64 | *((unsigned long *)kp->arg) = num; | |
65 | return 0; | |
66 | } | |
67 | ||
68 | static int param_get_sample_interval(char *buffer, const struct kernel_param *kp) | |
69 | { | |
70 | if (!READ_ONCE(kfence_enabled)) | |
71 | return sprintf(buffer, "0\n"); | |
72 | ||
73 | return param_get_ulong(buffer, kp); | |
74 | } | |
75 | ||
76 | static const struct kernel_param_ops sample_interval_param_ops = { | |
77 | .set = param_set_sample_interval, | |
78 | .get = param_get_sample_interval, | |
79 | }; | |
80 | module_param_cb(sample_interval, &sample_interval_param_ops, &kfence_sample_interval, 0600); | |
81 | ||
82 | /* The pool of pages used for guard pages and objects. */ | |
83 | char *__kfence_pool __ro_after_init; | |
84 | EXPORT_SYMBOL(__kfence_pool); /* Export for test modules. */ | |
85 | ||
86 | /* | |
87 | * Per-object metadata, with one-to-one mapping of object metadata to | |
88 | * backing pages (in __kfence_pool). | |
89 | */ | |
90 | static_assert(CONFIG_KFENCE_NUM_OBJECTS > 0); | |
91 | struct kfence_metadata kfence_metadata[CONFIG_KFENCE_NUM_OBJECTS]; | |
92 | ||
93 | /* Freelist with available objects. */ | |
94 | static struct list_head kfence_freelist = LIST_HEAD_INIT(kfence_freelist); | |
95 | static DEFINE_RAW_SPINLOCK(kfence_freelist_lock); /* Lock protecting freelist. */ | |
96 | ||
97 | #ifdef CONFIG_KFENCE_STATIC_KEYS | |
98 | /* The static key to set up a KFENCE allocation. */ | |
99 | DEFINE_STATIC_KEY_FALSE(kfence_allocation_key); | |
100 | #endif | |
101 | ||
102 | /* Gates the allocation, ensuring only one succeeds in a given period. */ | |
103 | atomic_t kfence_allocation_gate = ATOMIC_INIT(1); | |
104 | ||
105 | /* Statistics counters for debugfs. */ | |
106 | enum kfence_counter_id { | |
107 | KFENCE_COUNTER_ALLOCATED, | |
108 | KFENCE_COUNTER_ALLOCS, | |
109 | KFENCE_COUNTER_FREES, | |
110 | KFENCE_COUNTER_ZOMBIES, | |
111 | KFENCE_COUNTER_BUGS, | |
112 | KFENCE_COUNTER_COUNT, | |
113 | }; | |
114 | static atomic_long_t counters[KFENCE_COUNTER_COUNT]; | |
115 | static const char *const counter_names[] = { | |
116 | [KFENCE_COUNTER_ALLOCATED] = "currently allocated", | |
117 | [KFENCE_COUNTER_ALLOCS] = "total allocations", | |
118 | [KFENCE_COUNTER_FREES] = "total frees", | |
119 | [KFENCE_COUNTER_ZOMBIES] = "zombie allocations", | |
120 | [KFENCE_COUNTER_BUGS] = "total bugs", | |
121 | }; | |
122 | static_assert(ARRAY_SIZE(counter_names) == KFENCE_COUNTER_COUNT); | |
123 | ||
124 | /* === Internals ============================================================ */ | |
125 | ||
126 | static bool kfence_protect(unsigned long addr) | |
127 | { | |
128 | return !KFENCE_WARN_ON(!kfence_protect_page(ALIGN_DOWN(addr, PAGE_SIZE), true)); | |
129 | } | |
130 | ||
131 | static bool kfence_unprotect(unsigned long addr) | |
132 | { | |
133 | return !KFENCE_WARN_ON(!kfence_protect_page(ALIGN_DOWN(addr, PAGE_SIZE), false)); | |
134 | } | |
135 | ||
136 | static inline struct kfence_metadata *addr_to_metadata(unsigned long addr) | |
137 | { | |
138 | long index; | |
139 | ||
140 | /* The checks do not affect performance; only called from slow-paths. */ | |
141 | ||
142 | if (!is_kfence_address((void *)addr)) | |
143 | return NULL; | |
144 | ||
145 | /* | |
146 | * May be an invalid index if called with an address at the edge of | |
147 | * __kfence_pool, in which case we would report an "invalid access" | |
148 | * error. | |
149 | */ | |
150 | index = (addr - (unsigned long)__kfence_pool) / (PAGE_SIZE * 2) - 1; | |
151 | if (index < 0 || index >= CONFIG_KFENCE_NUM_OBJECTS) | |
152 | return NULL; | |
153 | ||
154 | return &kfence_metadata[index]; | |
155 | } | |
156 | ||
157 | static inline unsigned long metadata_to_pageaddr(const struct kfence_metadata *meta) | |
158 | { | |
159 | unsigned long offset = (meta - kfence_metadata + 1) * PAGE_SIZE * 2; | |
160 | unsigned long pageaddr = (unsigned long)&__kfence_pool[offset]; | |
161 | ||
162 | /* The checks do not affect performance; only called from slow-paths. */ | |
163 | ||
164 | /* Only call with a pointer into kfence_metadata. */ | |
165 | if (KFENCE_WARN_ON(meta < kfence_metadata || | |
166 | meta >= kfence_metadata + CONFIG_KFENCE_NUM_OBJECTS)) | |
167 | return 0; | |
168 | ||
169 | /* | |
170 | * This metadata object only ever maps to 1 page; verify that the stored | |
171 | * address is in the expected range. | |
172 | */ | |
173 | if (KFENCE_WARN_ON(ALIGN_DOWN(meta->addr, PAGE_SIZE) != pageaddr)) | |
174 | return 0; | |
175 | ||
176 | return pageaddr; | |
177 | } | |
178 | ||
179 | /* | |
180 | * Update the object's metadata state, including updating the alloc/free stacks | |
181 | * depending on the state transition. | |
182 | */ | |
183 | static noinline void metadata_update_state(struct kfence_metadata *meta, | |
184 | enum kfence_object_state next) | |
185 | { | |
186 | struct kfence_track *track = | |
187 | next == KFENCE_OBJECT_FREED ? &meta->free_track : &meta->alloc_track; | |
188 | ||
189 | lockdep_assert_held(&meta->lock); | |
190 | ||
191 | /* | |
192 | * Skip over 1 (this) functions; noinline ensures we do not accidentally | |
193 | * skip over the caller by never inlining. | |
194 | */ | |
195 | track->num_stack_entries = stack_trace_save(track->stack_entries, KFENCE_STACK_DEPTH, 1); | |
196 | track->pid = task_pid_nr(current); | |
197 | ||
198 | /* | |
199 | * Pairs with READ_ONCE() in | |
200 | * kfence_shutdown_cache(), | |
201 | * kfence_handle_page_fault(). | |
202 | */ | |
203 | WRITE_ONCE(meta->state, next); | |
204 | } | |
205 | ||
206 | /* Write canary byte to @addr. */ | |
207 | static inline bool set_canary_byte(u8 *addr) | |
208 | { | |
209 | *addr = KFENCE_CANARY_PATTERN(addr); | |
210 | return true; | |
211 | } | |
212 | ||
213 | /* Check canary byte at @addr. */ | |
214 | static inline bool check_canary_byte(u8 *addr) | |
215 | { | |
216 | if (likely(*addr == KFENCE_CANARY_PATTERN(addr))) | |
217 | return true; | |
218 | ||
219 | atomic_long_inc(&counters[KFENCE_COUNTER_BUGS]); | |
bc8fbc5f | 220 | kfence_report_error((unsigned long)addr, false, NULL, addr_to_metadata((unsigned long)addr), |
0ce20dd8 AP |
221 | KFENCE_ERROR_CORRUPTION); |
222 | return false; | |
223 | } | |
224 | ||
225 | /* __always_inline this to ensure we won't do an indirect call to fn. */ | |
226 | static __always_inline void for_each_canary(const struct kfence_metadata *meta, bool (*fn)(u8 *)) | |
227 | { | |
228 | const unsigned long pageaddr = ALIGN_DOWN(meta->addr, PAGE_SIZE); | |
229 | unsigned long addr; | |
230 | ||
231 | lockdep_assert_held(&meta->lock); | |
232 | ||
233 | /* | |
234 | * We'll iterate over each canary byte per-side until fn() returns | |
235 | * false. However, we'll still iterate over the canary bytes to the | |
236 | * right of the object even if there was an error in the canary bytes to | |
237 | * the left of the object. Specifically, if check_canary_byte() | |
238 | * generates an error, showing both sides might give more clues as to | |
239 | * what the error is about when displaying which bytes were corrupted. | |
240 | */ | |
241 | ||
242 | /* Apply to left of object. */ | |
243 | for (addr = pageaddr; addr < meta->addr; addr++) { | |
244 | if (!fn((u8 *)addr)) | |
245 | break; | |
246 | } | |
247 | ||
248 | /* Apply to right of object. */ | |
249 | for (addr = meta->addr + meta->size; addr < pageaddr + PAGE_SIZE; addr++) { | |
250 | if (!fn((u8 *)addr)) | |
251 | break; | |
252 | } | |
253 | } | |
254 | ||
255 | static void *kfence_guarded_alloc(struct kmem_cache *cache, size_t size, gfp_t gfp) | |
256 | { | |
257 | struct kfence_metadata *meta = NULL; | |
258 | unsigned long flags; | |
259 | struct page *page; | |
260 | void *addr; | |
261 | ||
262 | /* Try to obtain a free object. */ | |
263 | raw_spin_lock_irqsave(&kfence_freelist_lock, flags); | |
264 | if (!list_empty(&kfence_freelist)) { | |
265 | meta = list_entry(kfence_freelist.next, struct kfence_metadata, list); | |
266 | list_del_init(&meta->list); | |
267 | } | |
268 | raw_spin_unlock_irqrestore(&kfence_freelist_lock, flags); | |
269 | if (!meta) | |
270 | return NULL; | |
271 | ||
272 | if (unlikely(!raw_spin_trylock_irqsave(&meta->lock, flags))) { | |
273 | /* | |
274 | * This is extremely unlikely -- we are reporting on a | |
275 | * use-after-free, which locked meta->lock, and the reporting | |
276 | * code via printk calls kmalloc() which ends up in | |
277 | * kfence_alloc() and tries to grab the same object that we're | |
278 | * reporting on. While it has never been observed, lockdep does | |
279 | * report that there is a possibility of deadlock. Fix it by | |
280 | * using trylock and bailing out gracefully. | |
281 | */ | |
282 | raw_spin_lock_irqsave(&kfence_freelist_lock, flags); | |
283 | /* Put the object back on the freelist. */ | |
284 | list_add_tail(&meta->list, &kfence_freelist); | |
285 | raw_spin_unlock_irqrestore(&kfence_freelist_lock, flags); | |
286 | ||
287 | return NULL; | |
288 | } | |
289 | ||
290 | meta->addr = metadata_to_pageaddr(meta); | |
291 | /* Unprotect if we're reusing this page. */ | |
292 | if (meta->state == KFENCE_OBJECT_FREED) | |
293 | kfence_unprotect(meta->addr); | |
294 | ||
295 | /* | |
296 | * Note: for allocations made before RNG initialization, will always | |
297 | * return zero. We still benefit from enabling KFENCE as early as | |
298 | * possible, even when the RNG is not yet available, as this will allow | |
299 | * KFENCE to detect bugs due to earlier allocations. The only downside | |
300 | * is that the out-of-bounds accesses detected are deterministic for | |
301 | * such allocations. | |
302 | */ | |
303 | if (prandom_u32_max(2)) { | |
304 | /* Allocate on the "right" side, re-calculate address. */ | |
305 | meta->addr += PAGE_SIZE - size; | |
306 | meta->addr = ALIGN_DOWN(meta->addr, cache->align); | |
307 | } | |
308 | ||
309 | addr = (void *)meta->addr; | |
310 | ||
311 | /* Update remaining metadata. */ | |
312 | metadata_update_state(meta, KFENCE_OBJECT_ALLOCATED); | |
313 | /* Pairs with READ_ONCE() in kfence_shutdown_cache(). */ | |
314 | WRITE_ONCE(meta->cache, cache); | |
315 | meta->size = size; | |
316 | for_each_canary(meta, set_canary_byte); | |
317 | ||
318 | /* Set required struct page fields. */ | |
319 | page = virt_to_page(meta->addr); | |
320 | page->slab_cache = cache; | |
b89fb5ef AP |
321 | if (IS_ENABLED(CONFIG_SLUB)) |
322 | page->objects = 1; | |
d3fb45f3 AP |
323 | if (IS_ENABLED(CONFIG_SLAB)) |
324 | page->s_mem = addr; | |
0ce20dd8 AP |
325 | |
326 | raw_spin_unlock_irqrestore(&meta->lock, flags); | |
327 | ||
328 | /* Memory initialization. */ | |
329 | ||
330 | /* | |
331 | * We check slab_want_init_on_alloc() ourselves, rather than letting | |
332 | * SL*B do the initialization, as otherwise we might overwrite KFENCE's | |
333 | * redzone. | |
334 | */ | |
335 | if (unlikely(slab_want_init_on_alloc(gfp, cache))) | |
336 | memzero_explicit(addr, size); | |
337 | if (cache->ctor) | |
338 | cache->ctor(addr); | |
339 | ||
340 | if (CONFIG_KFENCE_STRESS_TEST_FAULTS && !prandom_u32_max(CONFIG_KFENCE_STRESS_TEST_FAULTS)) | |
341 | kfence_protect(meta->addr); /* Random "faults" by protecting the object. */ | |
342 | ||
343 | atomic_long_inc(&counters[KFENCE_COUNTER_ALLOCATED]); | |
344 | atomic_long_inc(&counters[KFENCE_COUNTER_ALLOCS]); | |
345 | ||
346 | return addr; | |
347 | } | |
348 | ||
349 | static void kfence_guarded_free(void *addr, struct kfence_metadata *meta, bool zombie) | |
350 | { | |
351 | struct kcsan_scoped_access assert_page_exclusive; | |
352 | unsigned long flags; | |
353 | ||
354 | raw_spin_lock_irqsave(&meta->lock, flags); | |
355 | ||
356 | if (meta->state != KFENCE_OBJECT_ALLOCATED || meta->addr != (unsigned long)addr) { | |
357 | /* Invalid or double-free, bail out. */ | |
358 | atomic_long_inc(&counters[KFENCE_COUNTER_BUGS]); | |
bc8fbc5f ME |
359 | kfence_report_error((unsigned long)addr, false, NULL, meta, |
360 | KFENCE_ERROR_INVALID_FREE); | |
0ce20dd8 AP |
361 | raw_spin_unlock_irqrestore(&meta->lock, flags); |
362 | return; | |
363 | } | |
364 | ||
365 | /* Detect racy use-after-free, or incorrect reallocation of this page by KFENCE. */ | |
366 | kcsan_begin_scoped_access((void *)ALIGN_DOWN((unsigned long)addr, PAGE_SIZE), PAGE_SIZE, | |
367 | KCSAN_ACCESS_SCOPED | KCSAN_ACCESS_WRITE | KCSAN_ACCESS_ASSERT, | |
368 | &assert_page_exclusive); | |
369 | ||
370 | if (CONFIG_KFENCE_STRESS_TEST_FAULTS) | |
371 | kfence_unprotect((unsigned long)addr); /* To check canary bytes. */ | |
372 | ||
373 | /* Restore page protection if there was an OOB access. */ | |
374 | if (meta->unprotected_page) { | |
375 | kfence_protect(meta->unprotected_page); | |
376 | meta->unprotected_page = 0; | |
377 | } | |
378 | ||
379 | /* Check canary bytes for memory corruption. */ | |
380 | for_each_canary(meta, check_canary_byte); | |
381 | ||
382 | /* | |
383 | * Clear memory if init-on-free is set. While we protect the page, the | |
384 | * data is still there, and after a use-after-free is detected, we | |
385 | * unprotect the page, so the data is still accessible. | |
386 | */ | |
387 | if (!zombie && unlikely(slab_want_init_on_free(meta->cache))) | |
388 | memzero_explicit(addr, meta->size); | |
389 | ||
390 | /* Mark the object as freed. */ | |
391 | metadata_update_state(meta, KFENCE_OBJECT_FREED); | |
392 | ||
393 | raw_spin_unlock_irqrestore(&meta->lock, flags); | |
394 | ||
395 | /* Protect to detect use-after-frees. */ | |
396 | kfence_protect((unsigned long)addr); | |
397 | ||
398 | kcsan_end_scoped_access(&assert_page_exclusive); | |
399 | if (!zombie) { | |
400 | /* Add it to the tail of the freelist for reuse. */ | |
401 | raw_spin_lock_irqsave(&kfence_freelist_lock, flags); | |
402 | KFENCE_WARN_ON(!list_empty(&meta->list)); | |
403 | list_add_tail(&meta->list, &kfence_freelist); | |
404 | raw_spin_unlock_irqrestore(&kfence_freelist_lock, flags); | |
405 | ||
406 | atomic_long_dec(&counters[KFENCE_COUNTER_ALLOCATED]); | |
407 | atomic_long_inc(&counters[KFENCE_COUNTER_FREES]); | |
408 | } else { | |
409 | /* See kfence_shutdown_cache(). */ | |
410 | atomic_long_inc(&counters[KFENCE_COUNTER_ZOMBIES]); | |
411 | } | |
412 | } | |
413 | ||
414 | static void rcu_guarded_free(struct rcu_head *h) | |
415 | { | |
416 | struct kfence_metadata *meta = container_of(h, struct kfence_metadata, rcu_head); | |
417 | ||
418 | kfence_guarded_free((void *)meta->addr, meta, false); | |
419 | } | |
420 | ||
421 | static bool __init kfence_init_pool(void) | |
422 | { | |
423 | unsigned long addr = (unsigned long)__kfence_pool; | |
424 | struct page *pages; | |
425 | int i; | |
426 | ||
427 | if (!__kfence_pool) | |
428 | return false; | |
429 | ||
430 | if (!arch_kfence_init_pool()) | |
431 | goto err; | |
432 | ||
433 | pages = virt_to_page(addr); | |
434 | ||
435 | /* | |
436 | * Set up object pages: they must have PG_slab set, to avoid freeing | |
437 | * these as real pages. | |
438 | * | |
439 | * We also want to avoid inserting kfence_free() in the kfree() | |
440 | * fast-path in SLUB, and therefore need to ensure kfree() correctly | |
441 | * enters __slab_free() slow-path. | |
442 | */ | |
443 | for (i = 0; i < KFENCE_POOL_SIZE / PAGE_SIZE; i++) { | |
444 | if (!i || (i % 2)) | |
445 | continue; | |
446 | ||
447 | /* Verify we do not have a compound head page. */ | |
448 | if (WARN_ON(compound_head(&pages[i]) != &pages[i])) | |
449 | goto err; | |
450 | ||
451 | __SetPageSlab(&pages[i]); | |
452 | } | |
453 | ||
454 | /* | |
455 | * Protect the first 2 pages. The first page is mostly unnecessary, and | |
456 | * merely serves as an extended guard page. However, adding one | |
457 | * additional page in the beginning gives us an even number of pages, | |
458 | * which simplifies the mapping of address to metadata index. | |
459 | */ | |
460 | for (i = 0; i < 2; i++) { | |
461 | if (unlikely(!kfence_protect(addr))) | |
462 | goto err; | |
463 | ||
464 | addr += PAGE_SIZE; | |
465 | } | |
466 | ||
467 | for (i = 0; i < CONFIG_KFENCE_NUM_OBJECTS; i++) { | |
468 | struct kfence_metadata *meta = &kfence_metadata[i]; | |
469 | ||
470 | /* Initialize metadata. */ | |
471 | INIT_LIST_HEAD(&meta->list); | |
472 | raw_spin_lock_init(&meta->lock); | |
473 | meta->state = KFENCE_OBJECT_UNUSED; | |
474 | meta->addr = addr; /* Initialize for validation in metadata_to_pageaddr(). */ | |
475 | list_add_tail(&meta->list, &kfence_freelist); | |
476 | ||
477 | /* Protect the right redzone. */ | |
478 | if (unlikely(!kfence_protect(addr + PAGE_SIZE))) | |
479 | goto err; | |
480 | ||
481 | addr += 2 * PAGE_SIZE; | |
482 | } | |
483 | ||
95511580 ME |
484 | /* |
485 | * The pool is live and will never be deallocated from this point on. | |
486 | * Remove the pool object from the kmemleak object tree, as it would | |
487 | * otherwise overlap with allocations returned by kfence_alloc(), which | |
488 | * are registered with kmemleak through the slab post-alloc hook. | |
489 | */ | |
490 | kmemleak_free(__kfence_pool); | |
491 | ||
0ce20dd8 AP |
492 | return true; |
493 | ||
494 | err: | |
495 | /* | |
496 | * Only release unprotected pages, and do not try to go back and change | |
497 | * page attributes due to risk of failing to do so as well. If changing | |
498 | * page attributes for some pages fails, it is very likely that it also | |
499 | * fails for the first page, and therefore expect addr==__kfence_pool in | |
500 | * most failure cases. | |
501 | */ | |
502 | memblock_free_late(__pa(addr), KFENCE_POOL_SIZE - (addr - (unsigned long)__kfence_pool)); | |
503 | __kfence_pool = NULL; | |
504 | return false; | |
505 | } | |
506 | ||
507 | /* === DebugFS Interface ==================================================== */ | |
508 | ||
509 | static int stats_show(struct seq_file *seq, void *v) | |
510 | { | |
511 | int i; | |
512 | ||
513 | seq_printf(seq, "enabled: %i\n", READ_ONCE(kfence_enabled)); | |
514 | for (i = 0; i < KFENCE_COUNTER_COUNT; i++) | |
515 | seq_printf(seq, "%s: %ld\n", counter_names[i], atomic_long_read(&counters[i])); | |
516 | ||
517 | return 0; | |
518 | } | |
519 | DEFINE_SHOW_ATTRIBUTE(stats); | |
520 | ||
521 | /* | |
522 | * debugfs seq_file operations for /sys/kernel/debug/kfence/objects. | |
523 | * start_object() and next_object() return the object index + 1, because NULL is used | |
524 | * to stop iteration. | |
525 | */ | |
526 | static void *start_object(struct seq_file *seq, loff_t *pos) | |
527 | { | |
528 | if (*pos < CONFIG_KFENCE_NUM_OBJECTS) | |
529 | return (void *)((long)*pos + 1); | |
530 | return NULL; | |
531 | } | |
532 | ||
533 | static void stop_object(struct seq_file *seq, void *v) | |
534 | { | |
535 | } | |
536 | ||
537 | static void *next_object(struct seq_file *seq, void *v, loff_t *pos) | |
538 | { | |
539 | ++*pos; | |
540 | if (*pos < CONFIG_KFENCE_NUM_OBJECTS) | |
541 | return (void *)((long)*pos + 1); | |
542 | return NULL; | |
543 | } | |
544 | ||
545 | static int show_object(struct seq_file *seq, void *v) | |
546 | { | |
547 | struct kfence_metadata *meta = &kfence_metadata[(long)v - 1]; | |
548 | unsigned long flags; | |
549 | ||
550 | raw_spin_lock_irqsave(&meta->lock, flags); | |
551 | kfence_print_object(seq, meta); | |
552 | raw_spin_unlock_irqrestore(&meta->lock, flags); | |
553 | seq_puts(seq, "---------------------------------\n"); | |
554 | ||
555 | return 0; | |
556 | } | |
557 | ||
558 | static const struct seq_operations object_seqops = { | |
559 | .start = start_object, | |
560 | .next = next_object, | |
561 | .stop = stop_object, | |
562 | .show = show_object, | |
563 | }; | |
564 | ||
565 | static int open_objects(struct inode *inode, struct file *file) | |
566 | { | |
567 | return seq_open(file, &object_seqops); | |
568 | } | |
569 | ||
570 | static const struct file_operations objects_fops = { | |
571 | .open = open_objects, | |
572 | .read = seq_read, | |
573 | .llseek = seq_lseek, | |
574 | }; | |
575 | ||
576 | static int __init kfence_debugfs_init(void) | |
577 | { | |
578 | struct dentry *kfence_dir = debugfs_create_dir("kfence", NULL); | |
579 | ||
580 | debugfs_create_file("stats", 0444, kfence_dir, NULL, &stats_fops); | |
581 | debugfs_create_file("objects", 0400, kfence_dir, NULL, &objects_fops); | |
582 | return 0; | |
583 | } | |
584 | ||
585 | late_initcall(kfence_debugfs_init); | |
586 | ||
587 | /* === Allocation Gate Timer ================================================ */ | |
588 | ||
589 | /* | |
590 | * Set up delayed work, which will enable and disable the static key. We need to | |
591 | * use a work queue (rather than a simple timer), since enabling and disabling a | |
592 | * static key cannot be done from an interrupt. | |
593 | * | |
594 | * Note: Toggling a static branch currently causes IPIs, and here we'll end up | |
595 | * with a total of 2 IPIs to all CPUs. If this ends up a problem in future (with | |
596 | * more aggressive sampling intervals), we could get away with a variant that | |
597 | * avoids IPIs, at the cost of not immediately capturing allocations if the | |
598 | * instructions remain cached. | |
599 | */ | |
600 | static struct delayed_work kfence_timer; | |
601 | static void toggle_allocation_gate(struct work_struct *work) | |
602 | { | |
603 | if (!READ_ONCE(kfence_enabled)) | |
604 | return; | |
605 | ||
606 | /* Enable static key, and await allocation to happen. */ | |
607 | atomic_set(&kfence_allocation_gate, 0); | |
608 | #ifdef CONFIG_KFENCE_STATIC_KEYS | |
609 | static_branch_enable(&kfence_allocation_key); | |
610 | /* | |
611 | * Await an allocation. Timeout after 1 second, in case the kernel stops | |
612 | * doing allocations, to avoid stalling this worker task for too long. | |
613 | */ | |
614 | { | |
615 | unsigned long end_wait = jiffies + HZ; | |
616 | ||
617 | do { | |
618 | set_current_state(TASK_UNINTERRUPTIBLE); | |
619 | if (atomic_read(&kfence_allocation_gate) != 0) | |
620 | break; | |
621 | schedule_timeout(1); | |
622 | } while (time_before(jiffies, end_wait)); | |
623 | __set_current_state(TASK_RUNNING); | |
624 | } | |
625 | /* Disable static key and reset timer. */ | |
626 | static_branch_disable(&kfence_allocation_key); | |
627 | #endif | |
628 | schedule_delayed_work(&kfence_timer, msecs_to_jiffies(kfence_sample_interval)); | |
629 | } | |
630 | static DECLARE_DELAYED_WORK(kfence_timer, toggle_allocation_gate); | |
631 | ||
632 | /* === Public interface ===================================================== */ | |
633 | ||
634 | void __init kfence_alloc_pool(void) | |
635 | { | |
636 | if (!kfence_sample_interval) | |
637 | return; | |
638 | ||
639 | __kfence_pool = memblock_alloc(KFENCE_POOL_SIZE, PAGE_SIZE); | |
640 | ||
641 | if (!__kfence_pool) | |
642 | pr_err("failed to allocate pool\n"); | |
643 | } | |
644 | ||
645 | void __init kfence_init(void) | |
646 | { | |
647 | /* Setting kfence_sample_interval to 0 on boot disables KFENCE. */ | |
648 | if (!kfence_sample_interval) | |
649 | return; | |
650 | ||
651 | if (!kfence_init_pool()) { | |
652 | pr_err("%s failed\n", __func__); | |
653 | return; | |
654 | } | |
655 | ||
656 | WRITE_ONCE(kfence_enabled, true); | |
657 | schedule_delayed_work(&kfence_timer, 0); | |
35beccf0 ME |
658 | pr_info("initialized - using %lu bytes for %d objects at 0x%p-0x%p\n", KFENCE_POOL_SIZE, |
659 | CONFIG_KFENCE_NUM_OBJECTS, (void *)__kfence_pool, | |
660 | (void *)(__kfence_pool + KFENCE_POOL_SIZE)); | |
0ce20dd8 AP |
661 | } |
662 | ||
663 | void kfence_shutdown_cache(struct kmem_cache *s) | |
664 | { | |
665 | unsigned long flags; | |
666 | struct kfence_metadata *meta; | |
667 | int i; | |
668 | ||
669 | for (i = 0; i < CONFIG_KFENCE_NUM_OBJECTS; i++) { | |
670 | bool in_use; | |
671 | ||
672 | meta = &kfence_metadata[i]; | |
673 | ||
674 | /* | |
675 | * If we observe some inconsistent cache and state pair where we | |
676 | * should have returned false here, cache destruction is racing | |
677 | * with either kmem_cache_alloc() or kmem_cache_free(). Taking | |
678 | * the lock will not help, as different critical section | |
679 | * serialization will have the same outcome. | |
680 | */ | |
681 | if (READ_ONCE(meta->cache) != s || | |
682 | READ_ONCE(meta->state) != KFENCE_OBJECT_ALLOCATED) | |
683 | continue; | |
684 | ||
685 | raw_spin_lock_irqsave(&meta->lock, flags); | |
686 | in_use = meta->cache == s && meta->state == KFENCE_OBJECT_ALLOCATED; | |
687 | raw_spin_unlock_irqrestore(&meta->lock, flags); | |
688 | ||
689 | if (in_use) { | |
690 | /* | |
691 | * This cache still has allocations, and we should not | |
692 | * release them back into the freelist so they can still | |
693 | * safely be used and retain the kernel's default | |
694 | * behaviour of keeping the allocations alive (leak the | |
695 | * cache); however, they effectively become "zombie | |
696 | * allocations" as the KFENCE objects are the only ones | |
697 | * still in use and the owning cache is being destroyed. | |
698 | * | |
699 | * We mark them freed, so that any subsequent use shows | |
700 | * more useful error messages that will include stack | |
701 | * traces of the user of the object, the original | |
702 | * allocation, and caller to shutdown_cache(). | |
703 | */ | |
704 | kfence_guarded_free((void *)meta->addr, meta, /*zombie=*/true); | |
705 | } | |
706 | } | |
707 | ||
708 | for (i = 0; i < CONFIG_KFENCE_NUM_OBJECTS; i++) { | |
709 | meta = &kfence_metadata[i]; | |
710 | ||
711 | /* See above. */ | |
712 | if (READ_ONCE(meta->cache) != s || READ_ONCE(meta->state) != KFENCE_OBJECT_FREED) | |
713 | continue; | |
714 | ||
715 | raw_spin_lock_irqsave(&meta->lock, flags); | |
716 | if (meta->cache == s && meta->state == KFENCE_OBJECT_FREED) | |
717 | meta->cache = NULL; | |
718 | raw_spin_unlock_irqrestore(&meta->lock, flags); | |
719 | } | |
720 | } | |
721 | ||
722 | void *__kfence_alloc(struct kmem_cache *s, size_t size, gfp_t flags) | |
723 | { | |
724 | /* | |
725 | * allocation_gate only needs to become non-zero, so it doesn't make | |
726 | * sense to continue writing to it and pay the associated contention | |
727 | * cost, in case we have a large number of concurrent allocations. | |
728 | */ | |
729 | if (atomic_read(&kfence_allocation_gate) || atomic_inc_return(&kfence_allocation_gate) > 1) | |
730 | return NULL; | |
731 | ||
732 | if (!READ_ONCE(kfence_enabled)) | |
733 | return NULL; | |
734 | ||
735 | if (size > PAGE_SIZE) | |
736 | return NULL; | |
737 | ||
738 | return kfence_guarded_alloc(s, size, flags); | |
739 | } | |
740 | ||
741 | size_t kfence_ksize(const void *addr) | |
742 | { | |
743 | const struct kfence_metadata *meta = addr_to_metadata((unsigned long)addr); | |
744 | ||
745 | /* | |
746 | * Read locklessly -- if there is a race with __kfence_alloc(), this is | |
747 | * either a use-after-free or invalid access. | |
748 | */ | |
749 | return meta ? meta->size : 0; | |
750 | } | |
751 | ||
752 | void *kfence_object_start(const void *addr) | |
753 | { | |
754 | const struct kfence_metadata *meta = addr_to_metadata((unsigned long)addr); | |
755 | ||
756 | /* | |
757 | * Read locklessly -- if there is a race with __kfence_alloc(), this is | |
758 | * either a use-after-free or invalid access. | |
759 | */ | |
760 | return meta ? (void *)meta->addr : NULL; | |
761 | } | |
762 | ||
763 | void __kfence_free(void *addr) | |
764 | { | |
765 | struct kfence_metadata *meta = addr_to_metadata((unsigned long)addr); | |
766 | ||
767 | /* | |
768 | * If the objects of the cache are SLAB_TYPESAFE_BY_RCU, defer freeing | |
769 | * the object, as the object page may be recycled for other-typed | |
770 | * objects once it has been freed. meta->cache may be NULL if the cache | |
771 | * was destroyed. | |
772 | */ | |
773 | if (unlikely(meta->cache && (meta->cache->flags & SLAB_TYPESAFE_BY_RCU))) | |
774 | call_rcu(&meta->rcu_head, rcu_guarded_free); | |
775 | else | |
776 | kfence_guarded_free(addr, meta, false); | |
777 | } | |
778 | ||
bc8fbc5f | 779 | bool kfence_handle_page_fault(unsigned long addr, bool is_write, struct pt_regs *regs) |
0ce20dd8 AP |
780 | { |
781 | const int page_index = (addr - (unsigned long)__kfence_pool) / PAGE_SIZE; | |
782 | struct kfence_metadata *to_report = NULL; | |
783 | enum kfence_error_type error_type; | |
784 | unsigned long flags; | |
785 | ||
786 | if (!is_kfence_address((void *)addr)) | |
787 | return false; | |
788 | ||
789 | if (!READ_ONCE(kfence_enabled)) /* If disabled at runtime ... */ | |
790 | return kfence_unprotect(addr); /* ... unprotect and proceed. */ | |
791 | ||
792 | atomic_long_inc(&counters[KFENCE_COUNTER_BUGS]); | |
793 | ||
794 | if (page_index % 2) { | |
795 | /* This is a redzone, report a buffer overflow. */ | |
796 | struct kfence_metadata *meta; | |
797 | int distance = 0; | |
798 | ||
799 | meta = addr_to_metadata(addr - PAGE_SIZE); | |
800 | if (meta && READ_ONCE(meta->state) == KFENCE_OBJECT_ALLOCATED) { | |
801 | to_report = meta; | |
802 | /* Data race ok; distance calculation approximate. */ | |
803 | distance = addr - data_race(meta->addr + meta->size); | |
804 | } | |
805 | ||
806 | meta = addr_to_metadata(addr + PAGE_SIZE); | |
807 | if (meta && READ_ONCE(meta->state) == KFENCE_OBJECT_ALLOCATED) { | |
808 | /* Data race ok; distance calculation approximate. */ | |
809 | if (!to_report || distance > data_race(meta->addr) - addr) | |
810 | to_report = meta; | |
811 | } | |
812 | ||
813 | if (!to_report) | |
814 | goto out; | |
815 | ||
816 | raw_spin_lock_irqsave(&to_report->lock, flags); | |
817 | to_report->unprotected_page = addr; | |
818 | error_type = KFENCE_ERROR_OOB; | |
819 | ||
820 | /* | |
821 | * If the object was freed before we took the look we can still | |
822 | * report this as an OOB -- the report will simply show the | |
823 | * stacktrace of the free as well. | |
824 | */ | |
825 | } else { | |
826 | to_report = addr_to_metadata(addr); | |
827 | if (!to_report) | |
828 | goto out; | |
829 | ||
830 | raw_spin_lock_irqsave(&to_report->lock, flags); | |
831 | error_type = KFENCE_ERROR_UAF; | |
832 | /* | |
833 | * We may race with __kfence_alloc(), and it is possible that a | |
834 | * freed object may be reallocated. We simply report this as a | |
835 | * use-after-free, with the stack trace showing the place where | |
836 | * the object was re-allocated. | |
837 | */ | |
838 | } | |
839 | ||
840 | out: | |
841 | if (to_report) { | |
bc8fbc5f | 842 | kfence_report_error(addr, is_write, regs, to_report, error_type); |
0ce20dd8 AP |
843 | raw_spin_unlock_irqrestore(&to_report->lock, flags); |
844 | } else { | |
845 | /* This may be a UAF or OOB access, but we can't be sure. */ | |
bc8fbc5f | 846 | kfence_report_error(addr, is_write, regs, NULL, KFENCE_ERROR_INVALID); |
0ce20dd8 AP |
847 | } |
848 | ||
849 | return kfence_unprotect(addr); /* Unprotect and let access proceed. */ | |
850 | } |