refcount.h

  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(&r->refs, 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(&r->refs, 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(&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(&r->refs, &old, old + i));
182
183	if (oldp)
184		*oldp = old;
185
186	if (unlikely(old < 0 || old + i < 0))
187		refcount_warn_saturate(r, 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(&r->refs, &old, old + i));
231
232	if (oldp)
233		*oldp = old;
234
235	if (unlikely(old < 0 || old + i < 0))
236		refcount_warn_saturate(r, 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(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, &r->refs);
284
285	if (oldp)
286		*oldp = old;
287
288	if (unlikely(!old))
289		refcount_warn_saturate(r, REFCOUNT_ADD_UAF);
290	else if (unlikely(old < 0 || old + i < 0))
291		refcount_warn_saturate(r, 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(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(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(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, &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, 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(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(1, &r->refs);
456
457	if (oldp)
458		*oldp = old;
459
460	if (unlikely(old <= 1))
461		refcount_warn_saturate(r, 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 */