Memory is a limited resource shared between all the applications running on the same host machine.
C and C++ do not automatically reclaim unused memory. The developer has to release the memory claimed for their application that is no longer
needed. Unlike the stack that automatically allocates local variables on a function call and deallocates them on a function return, the heap offers no
automatic memory management. The developer has to make sure to deallocate the memory they allocate dynamically on the heap.
This rule raises an issue when memory is allocated dynamically and not freed within the same function.
What is the potential impact?
Neglecting to free the memory leads to a memory leak.
The application that leaks memory will consume more and more of it over time, eventually claiming all the memory available on the host machine.
When this happens and the system runs out of memory, it typically does one of the following:
- The operating system (if any) terminates the application.
- The operating system (if any) terminates some other application, and the problem reoccurs when the reclaimed memory gets used up by the leaking
application.
- The operating system (if any) starts offloading some of the memory pages to disk and slows down some memory accesses by orders of magnitude.
- The entire system crashes as a whole and reboots automatically or hangs waiting for a manual reboot.
Moreover, memory leaks can help an attacker to take over the system. An attacker could use a memory leak to fill the memory with malicious code.
This facilitates remote code execution through another chained vulnerability.
Even if the attacker cannot take over the system she can intentionally trigger the condition leading to a memory leak to make use of the issue
above and cause denial-of-service (DoS) of the system.
A memory leak can have a significant impact on the energy footprint of an application.
- If an application demands more memory than necessary, the user will have to install more memory banks than necessary. Each memory bank consumes
additional power.
- As the application continues to reserve more and more memory, it places an increased load on the memory management subsystem. This increased
load can lead to a larger computation demand, which in turn translates to higher power consumption by the CPU.
Finally, memory leaks degrade the user experience. The user often experiences a system slowdown caused by the uncontrolled memory use of an
application. Delayed response time, system freezes, and crashes degrade the user experience and discourage the further use of the application.
Exceptions
If a function returns a pointer to the caller or stores it in an external structure, this pointer is said to escape (it is
now accessible outside of function, and no longer local to it). This includes storing the pointer in a static or global variable, passing it to a
function that can potentially do that, or returning the pointer directly or as part of an aggregate object.
The memory pointed to by an escaping pointer might be used somewhere else in the program. For that reason, the analyzer cannot proclaim a leak for
an escaping pointer by only looking at a function scope.
While in some cases the leak might be detectable in the scope of a caller, in others, the analyzer would need to simulate the entire program to
verify that the memory is not used anywhere, which is not feasible.
For this technical reason, this rule often ignores escaping pointers.
