Line data Source code
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 : */
91 :
92 : #ifndef _LINUX_REFCOUNT_H
93 : #define _LINUX_REFCOUNT_H
94 :
95 : #include <linux/atomic.h>
96 : #include <linux/bug.h>
97 : #include <linux/compiler.h>
98 : #include <linux/limits.h>
99 : #include <linux/spinlock_types.h>
100 :
101 : struct mutex;
102 :
103 : /**
104 : * typedef refcount_t - variant of atomic_t specialized for reference counts
105 : * @refs: atomic_t counter field
106 : *
107 : * The counter saturates at REFCOUNT_SATURATED and will not move once
108 : * there. This avoids wrapping the counter and causing 'spurious'
109 : * use-after-free bugs.
110 : */
111 : typedef struct refcount_struct {
112 : atomic_t refs;
113 : } refcount_t;
114 :
115 : #define REFCOUNT_INIT(n) { .refs = ATOMIC_INIT(n), }
116 : #define REFCOUNT_MAX INT_MAX
117 : #define REFCOUNT_SATURATED (INT_MIN / 2)
118 :
119 : enum refcount_saturation_type {
120 : REFCOUNT_ADD_NOT_ZERO_OVF,
121 : REFCOUNT_ADD_OVF,
122 : REFCOUNT_ADD_UAF,
123 : REFCOUNT_SUB_UAF,
124 : REFCOUNT_DEC_LEAK,
125 : };
126 :
127 : void refcount_warn_saturate(refcount_t *r, enum refcount_saturation_type t);
128 :
129 : /**
130 : * refcount_set - set a refcount's value
131 : * @r: the refcount
132 : * @n: value to which the refcount will be set
133 : */
134 : static inline void refcount_set(refcount_t *r, int n)
135 : {
136 7332 : atomic_set(&r->refs, n);
137 : }
138 :
139 : /**
140 : * refcount_read - get a refcount's value
141 : * @r: the refcount
142 : *
143 : * Return: the refcount's value
144 : */
145 : static inline unsigned int refcount_read(const refcount_t *r)
146 : {
147 724 : return atomic_read(&r->refs);
148 : }
149 :
150 28 : static inline __must_check bool __refcount_add_not_zero(int i, refcount_t *r, int *oldp)
151 : {
152 28 : int old = refcount_read(r);
153 :
154 : do {
155 28 : if (!old)
156 : break;
157 56 : } while (!atomic_try_cmpxchg_relaxed(&r->refs, &old, old + i));
158 :
159 28 : if (oldp)
160 0 : *oldp = old;
161 :
162 28 : if (unlikely(old < 0 || old + i < 0))
163 0 : refcount_warn_saturate(r, REFCOUNT_ADD_NOT_ZERO_OVF);
164 :
165 28 : return old;
166 : }
167 :
168 : /**
169 : * refcount_add_not_zero - add a value to a refcount unless it is 0
170 : * @i: the value to add to the refcount
171 : * @r: the refcount
172 : *
173 : * Will saturate at REFCOUNT_SATURATED and WARN.
174 : *
175 : * Provides no memory ordering, it is assumed the caller has guaranteed the
176 : * object memory to be stable (RCU, etc.). It does provide a control dependency
177 : * and thereby orders future stores. See the comment on top.
178 : *
179 : * Use of this function is not recommended for the normal reference counting
180 : * use case in which references are taken and released one at a time. In these
181 : * cases, refcount_inc(), or one of its variants, should instead be used to
182 : * increment a reference count.
183 : *
184 : * Return: false if the passed refcount is 0, true otherwise
185 : */
186 : static inline __must_check bool refcount_add_not_zero(int i, refcount_t *r)
187 : {
188 : return __refcount_add_not_zero(i, r, NULL);
189 : }
190 :
191 5222 : static inline void __refcount_add(int i, refcount_t *r, int *oldp)
192 : {
193 10444 : int old = atomic_fetch_add_relaxed(i, &r->refs);
194 :
195 5222 : if (oldp)
196 0 : *oldp = old;
197 :
198 5222 : if (unlikely(!old))
199 0 : refcount_warn_saturate(r, REFCOUNT_ADD_UAF);
200 5222 : else if (unlikely(old < 0 || old + i < 0))
201 0 : refcount_warn_saturate(r, REFCOUNT_ADD_OVF);
202 5222 : }
203 :
204 : /**
205 : * refcount_add - add a value to a refcount
206 : * @i: the value to add to the refcount
207 : * @r: the refcount
208 : *
209 : * Similar to atomic_add(), but will saturate at REFCOUNT_SATURATED and WARN.
210 : *
211 : * Provides no memory ordering, it is assumed the caller has guaranteed the
212 : * object memory to be stable (RCU, etc.). It does provide a control dependency
213 : * and thereby orders future stores. See the comment on top.
214 : *
215 : * Use of this function is not recommended for the normal reference counting
216 : * use case in which references are taken and released one at a time. In these
217 : * cases, refcount_inc(), or one of its variants, should instead be used to
218 : * increment a reference count.
219 : */
220 : static inline void refcount_add(int i, refcount_t *r)
221 : {
222 0 : __refcount_add(i, r, NULL);
223 : }
224 :
225 : static inline __must_check bool __refcount_inc_not_zero(refcount_t *r, int *oldp)
226 : {
227 28 : return __refcount_add_not_zero(1, r, oldp);
228 : }
229 :
230 : /**
231 : * refcount_inc_not_zero - increment a refcount unless it is 0
232 : * @r: the refcount to increment
233 : *
234 : * Similar to atomic_inc_not_zero(), but will saturate at REFCOUNT_SATURATED
235 : * and WARN.
236 : *
237 : * Provides no memory ordering, it is assumed the caller has guaranteed the
238 : * object memory to be stable (RCU, etc.). It does provide a control dependency
239 : * and thereby orders future stores. See the comment on top.
240 : *
241 : * Return: true if the increment was successful, false otherwise
242 : */
243 : static inline __must_check bool refcount_inc_not_zero(refcount_t *r)
244 : {
245 28 : return __refcount_inc_not_zero(r, NULL);
246 : }
247 :
248 : static inline void __refcount_inc(refcount_t *r, int *oldp)
249 : {
250 5222 : __refcount_add(1, r, oldp);
251 : }
252 :
253 : /**
254 : * refcount_inc - increment a refcount
255 : * @r: the refcount to increment
256 : *
257 : * Similar to atomic_inc(), but will saturate at REFCOUNT_SATURATED and WARN.
258 : *
259 : * Provides no memory ordering, it is assumed the caller already has a
260 : * reference on the object.
261 : *
262 : * Will WARN if the refcount is 0, as this represents a possible use-after-free
263 : * condition.
264 : */
265 : static inline void refcount_inc(refcount_t *r)
266 : {
267 5222 : __refcount_inc(r, NULL);
268 : }
269 :
270 4944 : static inline __must_check bool __refcount_sub_and_test(int i, refcount_t *r, int *oldp)
271 : {
272 9888 : int old = atomic_fetch_sub_release(i, &r->refs);
273 :
274 4944 : if (oldp)
275 0 : *oldp = old;
276 :
277 4944 : if (old == i) {
278 2115 : smp_acquire__after_ctrl_dep();
279 2115 : return true;
280 : }
281 :
282 2829 : if (unlikely(old < 0 || old - i < 0))
283 0 : refcount_warn_saturate(r, REFCOUNT_SUB_UAF);
284 :
285 : return false;
286 : }
287 :
288 : /**
289 : * refcount_sub_and_test - subtract from a refcount and test if it is 0
290 : * @i: amount to subtract from the refcount
291 : * @r: the refcount
292 : *
293 : * Similar to atomic_dec_and_test(), but it will WARN, return false and
294 : * ultimately leak on underflow and will fail to decrement when saturated
295 : * at REFCOUNT_SATURATED.
296 : *
297 : * Provides release memory ordering, such that prior loads and stores are done
298 : * before, and provides an acquire ordering on success such that free()
299 : * must come after.
300 : *
301 : * Use of this function is not recommended for the normal reference counting
302 : * use case in which references are taken and released one at a time. In these
303 : * cases, refcount_dec(), or one of its variants, should instead be used to
304 : * decrement a reference count.
305 : *
306 : * Return: true if the resulting refcount is 0, false otherwise
307 : */
308 : static inline __must_check bool refcount_sub_and_test(int i, refcount_t *r)
309 : {
310 0 : return __refcount_sub_and_test(i, r, NULL);
311 : }
312 :
313 : static inline __must_check bool __refcount_dec_and_test(refcount_t *r, int *oldp)
314 : {
315 4944 : return __refcount_sub_and_test(1, r, oldp);
316 : }
317 :
318 : /**
319 : * refcount_dec_and_test - decrement a refcount and test if it is 0
320 : * @r: the refcount
321 : *
322 : * Similar to atomic_dec_and_test(), it will WARN on underflow and fail to
323 : * decrement when saturated at REFCOUNT_SATURATED.
324 : *
325 : * Provides release memory ordering, such that prior loads and stores are done
326 : * before, and provides an acquire ordering on success such that free()
327 : * must come after.
328 : *
329 : * Return: true if the resulting refcount is 0, false otherwise
330 : */
331 : static inline __must_check bool refcount_dec_and_test(refcount_t *r)
332 : {
333 4944 : return __refcount_dec_and_test(r, NULL);
334 : }
335 :
336 : static inline void __refcount_dec(refcount_t *r, int *oldp)
337 : {
338 : int old = atomic_fetch_sub_release(1, &r->refs);
339 :
340 : if (oldp)
341 : *oldp = old;
342 :
343 : if (unlikely(old <= 1))
344 : refcount_warn_saturate(r, REFCOUNT_DEC_LEAK);
345 : }
346 :
347 : /**
348 : * refcount_dec - decrement a refcount
349 : * @r: the refcount
350 : *
351 : * Similar to atomic_dec(), it will WARN on underflow and fail to decrement
352 : * when saturated at REFCOUNT_SATURATED.
353 : *
354 : * Provides release memory ordering, such that prior loads and stores are done
355 : * before.
356 : */
357 : static inline void refcount_dec(refcount_t *r)
358 : {
359 : __refcount_dec(r, NULL);
360 : }
361 :
362 : extern __must_check bool refcount_dec_if_one(refcount_t *r);
363 : extern __must_check bool refcount_dec_not_one(refcount_t *r);
364 : extern __must_check bool refcount_dec_and_mutex_lock(refcount_t *r, struct mutex *lock) __cond_acquires(lock);
365 : extern __must_check bool refcount_dec_and_lock(refcount_t *r, spinlock_t *lock) __cond_acquires(lock);
366 : extern __must_check bool refcount_dec_and_lock_irqsave(refcount_t *r,
367 : spinlock_t *lock,
368 : unsigned long *flags) __cond_acquires(lock);
369 : #endif /* _LINUX_REFCOUNT_H */
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