Linux Device Drivers, Second Edition

Security Issues

Security is an increasingly important concern in modern times. We will discuss security-related issues as they come up throughout the book. There are a few general concepts, however, that are worth mentioning now.

Security has two faces, which can be called deliberate and incidental. One security problem is the damage a user can cause through the misuse of existing programs, or by incidentally exploiting bugs; a different issue is what kind of (mis)functionality a programmer can deliberately implement. The programmer has, obviously, much more power than a plain user. In other words, it’s as dangerous to run a program you got from somebody else from the root account as it is to give him or her a root shell now and then. Although having access to a compiler is not a security hole per se, the hole can appear when compiled code is actually executed; everyone should be careful with modules, because a kernel module can do anything. A module is just as powerful as a superuser shell.

Any security check in the system is enforced by kernel code. If the kernel has security holes, then the system has holes. In the official kernel distribution, only an authorized user can load modules; the system call create_module checks if the invoking process is authorized to load a module into the kernel. Thus, when running an official kernel, only the superuser,^[2] or an intruder who has succeeded in becoming privileged, can exploit the power of privileged code.

When possible, driver ...