Well shared memory is a non-adversarial and limited way to effectively read and write the memory of an other process. It doesn't have any obvious implications

Well shared memory is a non-adversarial and limited way to effectively read and write the memory of an other process. It doesn't have any obvious implications security-wise since obviously if a process can read and write an other processe's memory then obviously you have some amount of trust that it's not going to do that. That's why you generally need certain privileges to do that. The main problem is that messing with a process memory is inherently non-portable and very likely to break if the program changes even slightly. Actually if the system uses address space randomization it may even change between invocations of the program. As such the only programs that usually bother to do that are debuggers and tracers. Before the advent of MMUs and virtual memory everybody shared the same address space and everybody could write everywhere. Fun times. You still have MMU-less system nowadays but it's generally limited to really low power (or really old) microcontrollers.