0b21d89499
Attempt to mitigate the security risks around refcount overflows by introducing a "saturated" state for the counter. Once a counter reaches INT_MAX+1, subsequent acquire and release operations will blindly set the counter value to INT_MAX + INT_MAX/2, ensuring that the protected resource will not be freed; instead, it will merely be leaked. The approach introduces a small race: if a refcount value reaches INT_MAX+1, a subsequent release will cause the releasing thread to set the counter to the saturation value after performing the decrement. If in the intervening window INT_MAX refcount releases are performed by a different thread, a use-after-free is possible. This is very difficult to trigger in practice, and any situation where it could be triggered would likely be vulnerable to reference count wraparound problems to begin with. An alternative would be to use atomic_cmpset to acquire and release references, but this would introduce a larger performance penalty, particularly when the counter is contended. Note that refcount_acquire_checked(9) maintains its previous behaviour; code which must accurately track references should use it instead of refcount_acquire(9). Reviewed by: kib, mjg MFC after: 3 weeks Sponsored by: The FreeBSD Foundation Differential Revision: https://reviews.freebsd.org/D21089
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