include/linux/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  */
  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         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         return atomic_read(&r->refs);
 148 }
 149
 150 static inline __must_check bool __refcount_add_not_zero(int i, refcount_t *r, int *oldp)
 151 {
 152         int old = refcount_read(r);
 153
 154         do {
 155                 if (!old)
 156                         break;
 157         } while (!atomic_try_cmpxchg_relaxed(&r->refs, &old, old + i));
 158
 159         if (oldp)
 160                 *oldp = old;
 161
 162         if (unlikely(old < 0 || old + i < 0))
 163                 refcount_warn_saturate(r, REFCOUNT_ADD_NOT_ZERO_OVF);
 164
 165         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 static inline void __refcount_add(int i, refcount_t *r, int *oldp)
 192 {
 193         int old = atomic_fetch_add_relaxed(i, &r->refs);
 194
 195         if (oldp)
 196                 *oldp = old;
 197
 198         if (unlikely(!old))
 199                 refcount_warn_saturate(r, REFCOUNT_ADD_UAF);
 200         else if (unlikely(old < 0 || old + i < 0))
 201                 refcount_warn_saturate(r, REFCOUNT_ADD_OVF);
 202 }
 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         __refcount_add(i, r, NULL);
 223 }
 224
 225 static inline __must_check bool __refcount_inc_not_zero(refcount_t *r, int *oldp)
 226 {
 227         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         return __refcount_inc_not_zero(r, NULL);
 246 }
 247
 248 static inline void __refcount_inc(refcount_t *r, int *oldp)
 249 {
 250         __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         __refcount_inc(r, NULL);
 268 }
 269
 270 static inline __must_check bool __refcount_sub_and_test(int i, refcount_t *r, int *oldp)
 271 {
 272         int old = atomic_fetch_sub_release(i, &r->refs);
 273
 274         if (oldp)
 275                 *oldp = old;
 276
 277         if (old == i) {
 278                 smp_acquire__after_ctrl_dep();
 279                 return true;
 280         }
 281
 282         if (unlikely(old < 0 || old - i < 0))
 283                 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         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         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         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);
 365 extern __must_check bool refcount_dec_and_lock(refcount_t *r, spinlock_t *lock);
 366 extern __must_check bool refcount_dec_and_lock_irqsave(refcount_t *r,
 367                                                        spinlock_t *lock,
 368                                                        unsigned long *flags);
 369 #endif /* _LINUX_REFCOUNT_H */