Memory protection

Memory protection is a way to control memory access rights on a computer, and is a part of most modern operating systems. The main purpose of memory protection is to prevent a process from accessing memory that has not been allocated to it. This prevents a bug within a process from affecting other processes, or the operating system itself. Memory protection is a behavior that is distinct from ASLR and the NX bit.

Methods

Segmentation

Segmentation refers to dividing a computer's memory into segments.

The x86 architecture has multiple segmentation features, which are helpful for using protected memory on this architecture.^[1] On the x86 processor architecture, the Global Descriptor Table and Local Descriptor Tables can be used to reference segments in the computer's memory. Pointers to memory segments on x86 processors can also be stored in the processor's segment registers. Initially x86 processors had 4 segment registers, CS (code segment), SS (stack segment), DS (data segment) and ES (extra segment); later another two segment registers were added – FS and GS.^[1]

Paged virtual memory

In paging, the memory address space is divided into equal, small pieces, called pages. Using a virtual memory mechanism, each page can be made to reside in any location of the physical memory, or be flagged as being protected. Virtual memory makes it possible to have a linear virtual memory address space and to use it to access blocks fragmented over physical memory address space.

Most computer architectures based on pages, most notably x86 architecture, also use pages for memory protection.

A page table is used for mapping virtual memory to physical memory. The page table is usually invisible to the process. Page tables make it easier to allocate new memory, as each new page can be allocated from anywhere in physical memory.

By such design, it is impossible for an application to access a page that has not been explicitly allocated to it, simply because any memory address, even a completely random one, that application may decide to use, either points to an allocated page, or generates a page fault (PF). Unallocated pages simply do not have any addresses from the application point of view.

As a side note, a PF may not be a fatal occurrence. Page faults are used not only for memory protection, but also in another interesting way: the OS may intercept the PF, and may load a page that has been previously swapped out to disk, and resume execution of the application which had caused the page fault. This way, the application receives the memory page as needed. This scheme, known as swapped virtual memory, allows in-memory data not currently in use to be moved to disk storage and back in a way which is transparent to applications, to increase overall memory capacity.

Protection keys

A protection key mechanism divides physical memory up into blocks of a particular size (e.g., 2 kiB), each of which has an associated numerical value called a protection key. Each process also has a protection key value associated with it. On a memory access the hardware checks that the current process's protection key matches the value associated with the memory block being accessed; if not, an exception occurs. This mechanism was used in the System/360 architecture.

The System/360 protection keys described above are associated with physical addresses. This is different from the protection key mechanism used by processors such as the Intel Itanium and the Hewlett-Packard Precision Architecture (HP/PA, also known as PA-RISC), which are associated with virtual addresses, and which allow multiple keys per process.

In the Itanium and PA processor architectures, translations (TLB entries) have keys (Itanium) or access ids (PA) associated with them. A running process has several protection key registers (16 for Itanium,^[2] 4 for HP PA^[3]). A translation selected by the virtual address has its key compared to each of the protection key registers. If any of them match (plus other possible checks), the access is permitted. If none match, a fault or exception is generated. The software fault handler can, if desired, check the missing key against a larger list of keys maintained by software; thus, the protection key registers inside the processor may be treated as a software managed cache of a larger list of keys associated with a process.

PA has 15–18 bits of key; Itanium mandates at least 18. Keys are usually associated with protection domains, such as libraries, modules, etc.

Simulated segmentation

Simulation is use of a monitoring program to interpret the machine code instructions of some computer. Such an Instruction Set Simulator can provide memory protection by using a segmentation-like scheme and validating the target address and length of each instruction in real time before actually executing them. The simulator must calculate the target address and length and compare this against a list of valid address ranges that it holds concerning the thread's environment, such as any dynamic memory blocks acquired since the thread's inception plus any valid shared static memory slots. The meaning of "valid" may change throughout the thread's life depending upon context: it may sometimes be allowed to alter a static block of storage, and sometimes not, depending upon the current mode of execution which may or may not depend on a storage key or supervisor state. ^{[citation needed]}

It is generally not advisable to use this method of memory protection where adequate facilities exist on a CPU, as this takes valuable processing power from the computer. However it is generally used for debugging and testing purposes to provide an extra fine level of granularity to otherwise generic storage violations and can indicate precisely which instruction is attempting to overwrite the particular section of storage which may have the same storage key as unprotected storage. Early IBM teleprocessing systems, such as CICS, multi-threaded commercial transactions in shared and unprotected storage for around 20 years.

Capability-based addressing

Capability-based addressing is a method of memory protection, that is unused in modern commercial computers. In this, pointers are replaced by protected objects (called capabilities) that can only be created via using privileged instructions which may only be executed by the kernel, or some other process authorized to do so. This effectively lets the kernel control which processes may access which objects in memory, with no need to use separate address spaces or context switches. Capabilities have never gained mainstream adoption in commercial hardware, but they are widely used in research systems such as KeyKOS and its successors, and are used conceptually as the basis for some virtual machines, most notably Smalltalk and Java.

Measures

A useful estimation of the protection level of a particular implementation, is to measure how closely it adheres to the principle of minimum privilege.^[4]