atomic/refcount

Linux v6.6: include/linux/atomic/atomic-instrumented.h
Linux v6.6: include/linux/refcount.h
Linux v6.6: lib/refcount.c

	REFCOUNT_ADD_UAF,
	REFCOUNT_SUB_UAF,
	REFCOUNT_DEC_LEAK,
};

void refcount_warn_saturate(refcount_t *r, enum refcount_saturation_type t);

/**
 * refcount_set - set a refcount's value
 * @r: the refcount
 * @n: value to which the refcount will be set
 */
static inline void refcount_set(refcount_t *r, int n)
{
	atomic_set(&r->refs, n);
}

/**
 * refcount_read - get a refcount's value
 * @r: the refcount
 *
 * Return: the refcount's value
 */
static inline unsigned int refcount_read(const refcount_t *r)
{
	return atomic_read(&r->refs);
}

static inline __must_check bool __refcount_add_not_zero(int i, refcount_t *r, int *oldp)
{
	int old = refcount_read(r);

	do {
		if (!old)
			break;
	} while (!atomic_try_cmpxchg_relaxed(&r->refs, &old, old + i));

	if (oldp)
		*oldp = old;

 *                                     <---------- bad value! ---------->
 *
 * (in a signed view of the world, the "bad value" range corresponds to
 * a negative counter value).
 *
 * As an example, consider a refcount_inc() operation that causes the counter
 * to overflow:
 *
 * 	int old = atomic_fetch_add_relaxed(r);
 *	// old is INT_MAX, refcount now INT_MIN (0x8000_0000)
 *	if (old < 0)
 *		atomic_set(r, REFCOUNT_SATURATED);
 *
 * If another thread also performs a refcount_inc() operation between the two
 * atomic operations, then the count will continue to edge closer to 0. If it
 * reaches a value of 1 before /any/ of the threads reset it to the saturated
 * value, then a concurrent refcount_dec_and_test() may erroneously free the
 * underlying object.
 * Linux limits the maximum number of tasks to PID_MAX_LIMIT, which is currently
 * 0x400000 (and can't easily be raised in the future beyond FUTEX_TID_MASK).
 * With the current PID limit, if no batched refcounting operations are used and
 * the attacker can't repeatedly trigger kernel oopses in the middle of refcount
 * operations, this makes it impossible for a saturated refcount to leave the
 * saturation range, even if it is possible for multiple uses of the same
 * refcount to nest in the context of a single task:
 *
 *     (UINT_MAX+1-REFCOUNT_SATURATED) / PID_MAX_LIMIT =
 *     0x40000000 / 0x400000 = 0x100 = 256
 *
 * If hundreds of references are added/removed with a single refcounting
 * operation, it may potentially be possible to leave the saturation range; but
 * given the precise timing details involved with the round-robin scheduling of
 * each thread manipulating the refcount and the need to hit the race multiple
 * times in succession, there doesn't appear to be a practical avenue of attack
 * even if using refcount_add() operations with larger increments.
 *
 * Memory ordering
 * ===============
 *
 * Memory ordering rules are slightly relaxed wrt regular atomic_t functions