MEM44-CPP. Do not leak resources when handling exceptions

It is important that resources are reclaimed when exceptions are thrown. Throwing an exception may result in cleanup code being bypassed. As a result, it is the responsibility of the exception handler to properly clean up. This may be problematic if the exception is to be caught in a different function or module. Instead, it is preferable if resources are reclaimed automatically when objects go out of scope.

Non-Compliant Code Example

In this non-compliant code example, the resources associated with the object pointed to by pst are not recovered in the event that processItem throws an exception, thereby potentially causing a resource leak.

while (moreToDo) {
  SomeType *pst = new SomeType();
  try {
    pst->processItem();
  }
  catch (...) {
    // deal with exception
    throw;
  }
  delete pst;
}

Compliant Solution

In this code, the exception handler recovers the resources associated with the object pointed to by pst.

while (moreToDo) {
  SomeType *pst = new SomeType();
  try {
    pst->processItem();
  }
  catch (...) {
    // deal with exception
    delete pst;
    throw;
  }
  delete pst;
}

Compliant Solution

A better approach would be to employ RAII. This forces every object to 'clean up after itself' in the face of abnormal behavior, preventing the programmer from having to do so. A judicious unique_ptr would free the resource whether an error occurs or not.

while (moreToDo) {
  std::unique_ptr<SomeType> pst = new SomeType();
  try {
    pst->processItem();
  }
  catch (...) {
    // deal with exception
    throw; // pst automatically freed
  }
  // pst automatically freed
}

Risk Assessment

Memory and other resource leaks will eventually cause a program to crash. If an attacker can provoke repeated resource leaks by forcing an exception to be thrown through the submission of suitably crafted data, then the attacker can mount a denial-of-service attack.

References

[Meyers 96] Item 9: "Use destructors to prevent resource leaks".

VOID MEM43-CPP. Release resources that require paired acquire and release in the object's destructor      08. Memory Management (MEM)      09. Input Output (FIO)