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  * struct 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 /**
 151  * refcount_add_not_zero - add a value to a refcount unless it is 0
 152  * @i: the value to add to the refcount
 153  * @r: the refcount
 154  *
 155  * Will saturate at REFCOUNT_SATURATED and WARN.
 156  *
 157  * Provides no memory ordering, it is assumed the caller has guaranteed the
 158  * object memory to be stable (RCU, etc.). It does provide a control dependency
 159  * and thereby orders future stores. See the comment on top.
 160  *
 161  * Use of this function is not recommended for the normal reference counting
 162  * use case in which references are taken and released one at a time.  In these
 163  * cases, refcount_inc(), or one of its variants, should instead be used to
 164  * increment a reference count.
 165  *
 166  * Return: false if the passed refcount is 0, true otherwise
 167  */
 168 static inline __must_check bool refcount_add_not_zero(int i, refcount_t *r)
 169 {
 170         int old = refcount_read(r);
 171
 172         do {
 173                 if (!old)
 174                         break;
 175         } while (!atomic_try_cmpxchg_relaxed(&r->refs, &old, old + i));
 176
 177         if (unlikely(old < 0 || old + i < 0))
 178                 refcount_warn_saturate(r, REFCOUNT_ADD_NOT_ZERO_OVF);
 179
 180         return old;
 181 }
 182
 183 /**
 184  * refcount_add - add a value to a refcount
 185  * @i: the value to add to the refcount
 186  * @r: the refcount
 187  *
 188  * Similar to atomic_add(), but will saturate at REFCOUNT_SATURATED and WARN.
 189  *
 190  * Provides no memory ordering, it is assumed the caller has guaranteed the
 191  * object memory to be stable (RCU, etc.). It does provide a control dependency
 192  * and thereby orders future stores. See the comment on top.
 193  *
 194  * Use of this function is not recommended for the normal reference counting
 195  * use case in which references are taken and released one at a time.  In these
 196  * cases, refcount_inc(), or one of its variants, should instead be used to
 197  * increment a reference count.
 198  */
 199 static inline void refcount_add(int i, refcount_t *r)
 200 {
 201         int old = atomic_fetch_add_relaxed(i, &r->refs);
 202
 203         if (unlikely(!old))
 204                 refcount_warn_saturate(r, REFCOUNT_ADD_UAF);
 205         else if (unlikely(old < 0 || old + i < 0))
 206                 refcount_warn_saturate(r, REFCOUNT_ADD_OVF);
 207 }
 208
 209 /**
 210  * refcount_inc_not_zero - increment a refcount unless it is 0
 211  * @r: the refcount to increment
 212  *
 213  * Similar to atomic_inc_not_zero(), but will saturate at REFCOUNT_SATURATED
 214  * and WARN.
 215  *
 216  * Provides no memory ordering, it is assumed the caller has guaranteed the
 217  * object memory to be stable (RCU, etc.). It does provide a control dependency
 218  * and thereby orders future stores. See the comment on top.
 219  *
 220  * Return: true if the increment was successful, false otherwise
 221  */
 222 static inline __must_check bool refcount_inc_not_zero(refcount_t *r)
 223 {
 224         return refcount_add_not_zero(1, r);
 225 }
 226
 227 /**
 228  * refcount_inc - increment a refcount
 229  * @r: the refcount to increment
 230  *
 231  * Similar to atomic_inc(), but will saturate at REFCOUNT_SATURATED and WARN.
 232  *
 233  * Provides no memory ordering, it is assumed the caller already has a
 234  * reference on the object.
 235  *
 236  * Will WARN if the refcount is 0, as this represents a possible use-after-free
 237  * condition.
 238  */
 239 static inline void refcount_inc(refcount_t *r)
 240 {
 241         refcount_add(1, r);
 242 }
 243
 244 /**
 245  * refcount_sub_and_test - subtract from a refcount and test if it is 0
 246  * @i: amount to subtract from the refcount
 247  * @r: the refcount
 248  *
 249  * Similar to atomic_dec_and_test(), but it will WARN, return false and
 250  * ultimately leak on underflow and will fail to decrement when saturated
 251  * at REFCOUNT_SATURATED.
 252  *
 253  * Provides release memory ordering, such that prior loads and stores are done
 254  * before, and provides an acquire ordering on success such that free()
 255  * must come after.
 256  *
 257  * Use of this function is not recommended for the normal reference counting
 258  * use case in which references are taken and released one at a time.  In these
 259  * cases, refcount_dec(), or one of its variants, should instead be used to
 260  * decrement a reference count.
 261  *
 262  * Return: true if the resulting refcount is 0, false otherwise
 263  */
 264 static inline __must_check bool refcount_sub_and_test(int i, refcount_t *r)
 265 {
 266         int old = atomic_fetch_sub_release(i, &r->refs);
 267
 268         if (old == i) {
 269                 smp_acquire__after_ctrl_dep();
 270                 return true;
 271         }
 272
 273         if (unlikely(old < 0 || old - i < 0))
 274                 refcount_warn_saturate(r, REFCOUNT_SUB_UAF);
 275
 276         return false;
 277 }
 278
 279 /**
 280  * refcount_dec_and_test - decrement a refcount and test if it is 0
 281  * @r: the refcount
 282  *
 283  * Similar to atomic_dec_and_test(), it will WARN on underflow and fail to
 284  * decrement when saturated at REFCOUNT_SATURATED.
 285  *
 286  * Provides release memory ordering, such that prior loads and stores are done
 287  * before, and provides an acquire ordering on success such that free()
 288  * must come after.
 289  *
 290  * Return: true if the resulting refcount is 0, false otherwise
 291  */
 292 static inline __must_check bool refcount_dec_and_test(refcount_t *r)
 293 {
 294         return refcount_sub_and_test(1, r);
 295 }
 296
 297 /**
 298  * refcount_dec - decrement a refcount
 299  * @r: the refcount
 300  *
 301  * Similar to atomic_dec(), it will WARN on underflow and fail to decrement
 302  * when saturated at REFCOUNT_SATURATED.
 303  *
 304  * Provides release memory ordering, such that prior loads and stores are done
 305  * before.
 306  */
 307 static inline void refcount_dec(refcount_t *r)
 308 {
 309         if (unlikely(atomic_fetch_sub_release(1, &r->refs) <= 1))
 310                 refcount_warn_saturate(r, REFCOUNT_DEC_LEAK);
 311 }
 312
 313 extern __must_check bool refcount_dec_if_one(refcount_t *r);
 314 extern __must_check bool refcount_dec_not_one(refcount_t *r);
 315 extern __must_check bool refcount_dec_and_mutex_lock(refcount_t *r, struct mutex *lock);
 316 extern __must_check bool refcount_dec_and_lock(refcount_t *r, spinlock_t *lock);
 317 extern __must_check bool refcount_dec_and_lock_irqsave(refcount_t *r,
 318                                                        spinlock_t *lock,
 319                                                        unsigned long *flags);
 320 #endif /* _LINUX_REFCOUNT_H */