Kernel Programming Guide: Security Considerations

Kernel-level security can mean many things, depending on what kind of kernel code you are writing. This chapter points out some common security issues at the kernel or near-kernel level and where applicable, describes ways to avoid them. These issues are covered in the following sections:

Many of these issues are also relevant for application programming, but are crucial for programmers working in the kernel. Others are special considerations that application programers might not expect or anticipate.

Note: The terms cleartext and plaintext both refer to unencrypted text. These terms can generally be used interchangeably, although in some circles, the term cleartext is restricted to unencrypted transmission across a network. However, in other circles, the term plaintext (or sometimes plain text) refers to plain ASCII text (as opposed to HTML or rich text. To avoid any potential confusion, this chapter will use the term cleartext to refer to unencrypted text.

In order to understand security in Mac OS X, it is important to understand that there are two security models at work. One of these is the kernel security model, which is based on users, groups, and very basic per-user and per-group rights, which are, in turn, coupled with access control lists for increased flexibility. The other is a user-level security model, which is based on keys, keychains, groups, users, password-based authentication, and a host of other details that are beyond the scope of this document.

The user level of security contains two basic features that you should be aware of as a kernel programmer: Security Server and Keychain Manager.

The Security Server consists of a daemon and various access libraries for caching permission to do certain tasks, based upon various means of authentication, including passwords and group membership. When a program requests permission to do something, the Security Server basically says “yes” or “no,” and caches that decision so that further requests from that user (for similar actions within a single context) do not require reauthentication for a period of time.

The Keychain Manager is a daemon that provides services related to the keychain, a central repository for a user’s encryption/authentication keys. For more high level information on keys, see “Key-based Authentication and Encryption”.

The details of the user-level security model use are far beyond the scope of this document. However, if you are writing an application that requires services of this nature, you should consider taking advantage of the Security Server and Keychain Manager from the user-space portion of your application, rather than attempting equivalent services in the kernel. More information about these services can be found in Apple’s Developer Documentation website at http://developer.apple.com/documentation.




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