1/* SPDX-License-Identifier: GPL-2.0 */
2/*
3 * Variant of atomic_t specialized for reference counts.
4 *
5 * The interface matches the atomic_t interface (to aid in porting) but only
6 * provides the few functions one should use for reference counting.
7 *
8 * Saturation semantics
9 * ====================
10 *
11 * refcount_t differs from atomic_t in that the counter saturates at
12 * REFCOUNT_SATURATED and will not move once there. This avoids wrapping the
13 * counter and causing 'spurious' use-after-free issues. In order to avoid the
14 * cost associated with introducing cmpxchg() loops into all of the saturating
15 * operations, we temporarily allow the counter to take on an unchecked value
16 * and then explicitly set it to REFCOUNT_SATURATED on detecting that underflow
17 * or overflow has occurred. Although this is racy when multiple threads
18 * access the refcount concurrently, by placing REFCOUNT_SATURATED roughly
19 * equidistant from 0 and INT_MAX we minimise the scope for error:
20 *
21 * INT_MAX REFCOUNT_SATURATED UINT_MAX
22 * 0 (0x7fff_ffff) (0xc000_0000) (0xffff_ffff)
23 * +--------------------------------+----------------+----------------+
24 * <---------- bad value! ---------->
25 *
26 * (in a signed view of the world, the "bad value" range corresponds to
27 * a negative counter value).
28 *
29 * As an example, consider a refcount_inc() operation that causes the counter
30 * to overflow:
31 *
32 * int old = atomic_fetch_add_relaxed(r);
33 * // old is INT_MAX, refcount now INT_MIN (0x8000_0000)
34 * if (old < 0)
35 * atomic_set(r, REFCOUNT_SATURATED);
36 *
37 * If another thread also performs a refcount_inc() operation between the two
38 * atomic operations, then the count will continue to edge closer to 0. If it
39 * reaches a value of 1 before /any/ of the threads reset it to the saturated
40 * value, then a concurrent refcount_dec_and_test() may erroneously free the
41 * underlying object.
42 * Linux limits the maximum number of tasks to PID_MAX_LIMIT, which is currently
43 * 0x400000 (and can't easily be raised in the future beyond FUTEX_TID_MASK).
44 * With the current PID limit, if no batched refcounting operations are used and
45 * the attacker can't repeatedly trigger kernel oopses in the middle of refcount
46 * operations, this makes it impossible for a saturated refcount to leave the
47 * saturation range, even if it is possible for multiple uses of the same
48 * refcount to nest in the context of a single task:
49 *
50 * (UINT_MAX+1-REFCOUNT_SATURATED) / PID_MAX_LIMIT =
51 * 0x40000000 / 0x400000 = 0x100 = 256
52 *
53 * If hundreds of references are added/removed with a single refcounting
54 * operation, it may potentially be possible to leave the saturation range; but
55 * given the precise timing details involved with the round-robin scheduling of
56 * each thread manipulating the refcount and the need to hit the race multiple
57 * times in succession, there doesn't appear to be a practical avenue of attack
58 * even if using refcount_add() operations with larger increments.
59 *
60 * Memory ordering
61 * ===============
62 *
63 * Memory ordering rules are slightly relaxed wrt regular atomic_t functions
64 * and provide only what is strictly required for refcounts.
65 *
66 * The increments are fully relaxed; these will not provide ordering. The
67 * rationale is that whatever is used to obtain the object we're increasing the
68 * reference count on will provide the ordering. For locked data structures,
69 * its the lock acquire, for RCU/lockless data structures its the dependent
70 * load.
71 *
72 * Do note that inc_not_zero() provides a control dependency which will order
73 * future stores against the inc, this ensures we'll never modify the object
74 * if we did not in fact acquire a reference.
75 *
76 * The decrements will provide release order, such that all the prior loads and
77 * stores will be issued before, it also provides a control dependency, which
78 * will order us against the subsequent free().
79 *
80 * The control dependency is against the load of the cmpxchg (ll/sc) that
81 * succeeded. This means the stores aren't fully ordered, but this is fine
82 * because the 1->0 transition indicates no concurrency.
83 *
84 * Note that the allocator is responsible for ordering things between free()
85 * and alloc().
86 *
87 * The decrements dec_and_test() and sub_and_test() also provide acquire
88 * ordering on success.
89 *
90 * refcount_{add|inc}_not_zero_acquire() and refcount_set_release() provide
91 * acquire and release ordering for cases when the memory occupied by the
92 * object might be reused to store another object. This is important for the
93 * cases where secondary validation is required to detect such reuse, e.g.
94 * SLAB_TYPESAFE_BY_RCU. The secondary validation checks have to happen after
95 * the refcount is taken, hence acquire order is necessary. Similarly, when the
96 * object is initialized, all stores to its attributes should be visible before
97 * the refcount is set, otherwise a stale attribute value might be used by
98 * another task which succeeds in taking a refcount to the new object.
99 */
100
101#ifndef _LINUX_REFCOUNT_H
102#define _LINUX_REFCOUNT_H
103
104#include <linux/atomic.h>
105#include <linux/bug.h>
106#include <linux/compiler.h>
107#include <linux/limits.h>
108#include <linux/refcount_types.h>
109#include <linux/spinlock_types.h>
110
111struct mutex;
112
113#define REFCOUNT_INIT(n) { .refs = ATOMIC_INIT(n), }
114#define REFCOUNT_MAX INT_MAX
115#define REFCOUNT_SATURATED (INT_MIN / 2)
116
117enum refcount_saturation_type {
118 REFCOUNT_ADD_NOT_ZERO_OVF,
119 REFCOUNT_ADD_OVF,
120 REFCOUNT_ADD_UAF,
121 REFCOUNT_SUB_UAF,
122 REFCOUNT_DEC_LEAK,
123};
124
125void refcount_warn_saturate(refcount_t *r, enum refcount_saturation_type t);
126
127/**
128 * refcount_set - set a refcount's value
129 * @r: the refcount
130 * @n: value to which the refcount will be set
131 */
132static inline void refcount_set(refcount_t *r, int n)
133{
134 atomic_set(v: &r->refs, i: n);
135}
136
137/**
138 * refcount_set_release - set a refcount's value with release ordering
139 * @r: the refcount
140 * @n: value to which the refcount will be set
141 *
142 * This function should be used when memory occupied by the object might be
143 * reused to store another object -- consider SLAB_TYPESAFE_BY_RCU.
144 *
145 * Provides release memory ordering which will order previous memory operations
146 * against this store. This ensures all updates to this object are visible
147 * once the refcount is set and stale values from the object previously
148 * occupying this memory are overwritten with new ones.
149 *
150 * This function should be called only after new object is fully initialized.
151 * After this call the object should be considered visible to other tasks even
152 * if it was not yet added into an object collection normally used to discover
153 * it. This is because other tasks might have discovered the object previously
154 * occupying the same memory and after memory reuse they can succeed in taking
155 * refcount to the new object and start using it.
156 */
157static inline void refcount_set_release(refcount_t *r, int n)
158{
159 atomic_set_release(v: &r->refs, i: n);
160}
161
162/**
163 * refcount_read - get a refcount's value
164 * @r: the refcount
165 *
166 * Return: the refcount's value
167 */
168static inline unsigned int refcount_read(const refcount_t *r)
169{
170 return atomic_read(v: &r->refs);
171}
172
173static inline __must_check __signed_wrap
174bool __refcount_add_not_zero(int i, refcount_t *r, int *oldp)
175{
176 int old = refcount_read(r);
177
178 do {
179 if (!old)
180 break;
181 } while (!atomic_try_cmpxchg_relaxed(v: &r->refs, old: &old, new: old + i));
182
183 if (oldp)
184 *oldp = old;
185
186 if (unlikely(old < 0 || old + i < 0))
187 refcount_warn_saturate(r, t: REFCOUNT_ADD_NOT_ZERO_OVF);
188
189 return old;
190}
191
192/**
193 * refcount_add_not_zero - add a value to a refcount unless it is 0
194 * @i: the value to add to the refcount
195 * @r: the refcount
196 *
197 * Will saturate at REFCOUNT_SATURATED and WARN.
198 *
199 * Provides no memory ordering, it is assumed the caller has guaranteed the
200 * object memory to be stable (RCU, etc.). It does provide a control dependency
201 * and thereby orders future stores. See the comment on top.
202 *
203 * Use of this function is not recommended for the normal reference counting
204 * use case in which references are taken and released one at a time. In these
205 * cases, refcount_inc(), or one of its variants, should instead be used to
206 * increment a reference count.
207 *
208 * Return: false if the passed refcount is 0, true otherwise
209 */
210static inline __must_check bool refcount_add_not_zero(int i, refcount_t *r)
211{
212 return __refcount_add_not_zero(i, r, NULL);
213}
214
215static inline __must_check __signed_wrap
216bool __refcount_add_not_zero_limited_acquire(int i, refcount_t *r, int *oldp,
217 int limit)
218{
219 int old = refcount_read(r);
220
221 do {
222 if (!old)
223 break;
224
225 if (i > limit - old) {
226 if (oldp)
227 *oldp = old;
228 return false;
229 }
230 } while (!atomic_try_cmpxchg_acquire(v: &r->refs, old: &old, new: old + i));
231
232 if (oldp)
233 *oldp = old;
234
235 if (unlikely(old < 0 || old + i < 0))
236 refcount_warn_saturate(r, t: REFCOUNT_ADD_NOT_ZERO_OVF);
237
238 return old;
239}
240
241static inline __must_check bool
242__refcount_inc_not_zero_limited_acquire(refcount_t *r, int *oldp, int limit)
243{
244 return __refcount_add_not_zero_limited_acquire(i: 1, r, oldp, limit);
245}
246
247static inline __must_check __signed_wrap
248bool __refcount_add_not_zero_acquire(int i, refcount_t *r, int *oldp)
249{
250 return __refcount_add_not_zero_limited_acquire(i, r, oldp, INT_MAX);
251}
252
253/**
254 * refcount_add_not_zero_acquire - add a value to a refcount with acquire ordering unless it is 0
255 *
256 * @i: the value to add to the refcount
257 * @r: the refcount
258 *
259 * Will saturate at REFCOUNT_SATURATED and WARN.
260 *
261 * This function should be used when memory occupied by the object might be
262 * reused to store another object -- consider SLAB_TYPESAFE_BY_RCU.
263 *
264 * Provides acquire memory ordering on success, it is assumed the caller has
265 * guaranteed the object memory to be stable (RCU, etc.). It does provide a
266 * control dependency and thereby orders future stores. See the comment on top.
267 *
268 * Use of this function is not recommended for the normal reference counting
269 * use case in which references are taken and released one at a time. In these
270 * cases, refcount_inc_not_zero_acquire() should instead be used to increment a
271 * reference count.
272 *
273 * Return: false if the passed refcount is 0, true otherwise
274 */
275static inline __must_check bool refcount_add_not_zero_acquire(int i, refcount_t *r)
276{
277 return __refcount_add_not_zero_acquire(i, r, NULL);
278}
279
280static inline __signed_wrap
281void __refcount_add(int i, refcount_t *r, int *oldp)
282{
283 int old = atomic_fetch_add_relaxed(i, v: &r->refs);
284
285 if (oldp)
286 *oldp = old;
287
288 if (unlikely(!old))
289 refcount_warn_saturate(r, t: REFCOUNT_ADD_UAF);
290 else if (unlikely(old < 0 || old + i < 0))
291 refcount_warn_saturate(r, t: REFCOUNT_ADD_OVF);
292}
293
294/**
295 * refcount_add - add a value to a refcount
296 * @i: the value to add to the refcount
297 * @r: the refcount
298 *
299 * Similar to atomic_add(), but will saturate at REFCOUNT_SATURATED and WARN.
300 *
301 * Provides no memory ordering, it is assumed the caller has guaranteed the
302 * object memory to be stable (RCU, etc.). It does provide a control dependency
303 * and thereby orders future stores. See the comment on top.
304 *
305 * Use of this function is not recommended for the normal reference counting
306 * use case in which references are taken and released one at a time. In these
307 * cases, refcount_inc(), or one of its variants, should instead be used to
308 * increment a reference count.
309 */
310static inline void refcount_add(int i, refcount_t *r)
311{
312 __refcount_add(i, r, NULL);
313}
314
315static inline __must_check bool __refcount_inc_not_zero(refcount_t *r, int *oldp)
316{
317 return __refcount_add_not_zero(i: 1, r, oldp);
318}
319
320/**
321 * refcount_inc_not_zero - increment a refcount unless it is 0
322 * @r: the refcount to increment
323 *
324 * Similar to atomic_inc_not_zero(), but will saturate at REFCOUNT_SATURATED
325 * and WARN.
326 *
327 * Provides no memory ordering, it is assumed the caller has guaranteed the
328 * object memory to be stable (RCU, etc.). It does provide a control dependency
329 * and thereby orders future stores. See the comment on top.
330 *
331 * Return: true if the increment was successful, false otherwise
332 */
333static inline __must_check bool refcount_inc_not_zero(refcount_t *r)
334{
335 return __refcount_inc_not_zero(r, NULL);
336}
337
338static inline __must_check bool __refcount_inc_not_zero_acquire(refcount_t *r, int *oldp)
339{
340 return __refcount_add_not_zero_acquire(i: 1, r, oldp);
341}
342
343/**
344 * refcount_inc_not_zero_acquire - increment a refcount with acquire ordering unless it is 0
345 * @r: the refcount to increment
346 *
347 * Similar to refcount_inc_not_zero(), but provides acquire memory ordering on
348 * success.
349 *
350 * This function should be used when memory occupied by the object might be
351 * reused to store another object -- consider SLAB_TYPESAFE_BY_RCU.
352 *
353 * Provides acquire memory ordering on success, it is assumed the caller has
354 * guaranteed the object memory to be stable (RCU, etc.). It does provide a
355 * control dependency and thereby orders future stores. See the comment on top.
356 *
357 * Return: true if the increment was successful, false otherwise
358 */
359static inline __must_check bool refcount_inc_not_zero_acquire(refcount_t *r)
360{
361 return __refcount_inc_not_zero_acquire(r, NULL);
362}
363
364static inline void __refcount_inc(refcount_t *r, int *oldp)
365{
366 __refcount_add(i: 1, r, oldp);
367}
368
369/**
370 * refcount_inc - increment a refcount
371 * @r: the refcount to increment
372 *
373 * Similar to atomic_inc(), but will saturate at REFCOUNT_SATURATED and WARN.
374 *
375 * Provides no memory ordering, it is assumed the caller already has a
376 * reference on the object.
377 *
378 * Will WARN if the refcount is 0, as this represents a possible use-after-free
379 * condition.
380 */
381static inline void refcount_inc(refcount_t *r)
382{
383 __refcount_inc(r, NULL);
384}
385
386static inline __must_check __signed_wrap
387bool __refcount_sub_and_test(int i, refcount_t *r, int *oldp)
388{
389 int old = atomic_fetch_sub_release(i, v: &r->refs);
390
391 if (oldp)
392 *oldp = old;
393
394 if (old > 0 && old == i) {
395 smp_acquire__after_ctrl_dep();
396 return true;
397 }
398
399 if (unlikely(old <= 0 || old - i < 0))
400 refcount_warn_saturate(r, t: REFCOUNT_SUB_UAF);
401
402 return false;
403}
404
405/**
406 * refcount_sub_and_test - subtract from a refcount and test if it is 0
407 * @i: amount to subtract from the refcount
408 * @r: the refcount
409 *
410 * Similar to atomic_dec_and_test(), but it will WARN, return false and
411 * ultimately leak on underflow and will fail to decrement when saturated
412 * at REFCOUNT_SATURATED.
413 *
414 * Provides release memory ordering, such that prior loads and stores are done
415 * before, and provides an acquire ordering on success such that free()
416 * must come after.
417 *
418 * Use of this function is not recommended for the normal reference counting
419 * use case in which references are taken and released one at a time. In these
420 * cases, refcount_dec(), or one of its variants, should instead be used to
421 * decrement a reference count.
422 *
423 * Return: true if the resulting refcount is 0, false otherwise
424 */
425static inline __must_check bool refcount_sub_and_test(int i, refcount_t *r)
426{
427 return __refcount_sub_and_test(i, r, NULL);
428}
429
430static inline __must_check bool __refcount_dec_and_test(refcount_t *r, int *oldp)
431{
432 return __refcount_sub_and_test(i: 1, r, oldp);
433}
434
435/**
436 * refcount_dec_and_test - decrement a refcount and test if it is 0
437 * @r: the refcount
438 *
439 * Similar to atomic_dec_and_test(), it will WARN on underflow and fail to
440 * decrement when saturated at REFCOUNT_SATURATED.
441 *
442 * Provides release memory ordering, such that prior loads and stores are done
443 * before, and provides an acquire ordering on success such that free()
444 * must come after.
445 *
446 * Return: true if the resulting refcount is 0, false otherwise
447 */
448static inline __must_check bool refcount_dec_and_test(refcount_t *r)
449{
450 return __refcount_dec_and_test(r, NULL);
451}
452
453static inline void __refcount_dec(refcount_t *r, int *oldp)
454{
455 int old = atomic_fetch_sub_release(i: 1, v: &r->refs);
456
457 if (oldp)
458 *oldp = old;
459
460 if (unlikely(old <= 1))
461 refcount_warn_saturate(r, t: REFCOUNT_DEC_LEAK);
462}
463
464/**
465 * refcount_dec - decrement a refcount
466 * @r: the refcount
467 *
468 * Similar to atomic_dec(), it will WARN on underflow and fail to decrement
469 * when saturated at REFCOUNT_SATURATED.
470 *
471 * Provides release memory ordering, such that prior loads and stores are done
472 * before.
473 */
474static inline void refcount_dec(refcount_t *r)
475{
476 __refcount_dec(r, NULL);
477}
478
479extern __must_check bool refcount_dec_if_one(refcount_t *r);
480extern __must_check bool refcount_dec_not_one(refcount_t *r);
481extern __must_check bool refcount_dec_and_mutex_lock(refcount_t *r, struct mutex *lock) __cond_acquires(lock);
482extern __must_check bool refcount_dec_and_lock(refcount_t *r, spinlock_t *lock) __cond_acquires(lock);
483extern __must_check bool refcount_dec_and_lock_irqsave(refcount_t *r,
484 spinlock_t *lock,
485 unsigned long *flags) __cond_acquires(lock);
486#endif /* _LINUX_REFCOUNT_H */
487