KDE su uses the sytem's su for acquiring priviliges. In this section, I explain the details of how KDE su does this.

Because some su implementations (i.e. the one from Red Hat®) don't want to read the password from stdin, KDE su creates a pty/tty pair and executes su with its standard filedescriptors connected to the tty.

To execute the command the user selected, rather than an interactive shell, KDE su uses the -c argument with su. This argument is understood by every shell that I know of so it should work portably. su passes this -c argument to the target user's shell, and the shell executes the program. Example command: su root -c the_program.

Instead of executing the user command directly with su, KDE su executes a little stub program called kdesu_stub. This stub (running as the target user), requests some information from KDE su over the pty/tty channel (the stub's stdin and stdout) and then executes the user's program. The information passed over is: the X display, an X authentication cookie (if available), the PATH and the command to run. The reason why a stub program is used is that the X cookie is private information and therefore cannot be passed on the command line.

KDE su will check the password you entered and gives an error message if it is not correct. The checking is done by executing a test program: /bin/true. If this succeeds, the password is assumed to be correct.

For your comfort, KDE su implements a “keep password” feature. If you are interested in security, you should read this paragraph.

Allowing KDE su to remember passwords opens up a (small) security hole in your system. Obviously, KDE su does not allow anybody but your user id to use the passwords, but, if done without caution, this would lowers root's security level to that of a normal user (you). A hacker who breaks into your account, would get root access. KDE su tries to prevent this. The security scheme it uses is, in my opinion at least, reasonably safe and is explained here.

KDE su uses a daemon, called kdesud. The daemon listens to a UNIX® socket in /tmp for commands. The mode of the socket is 0600 so that only your user id can connect to it. If password keeping is enabled, KDE su executes commands through this daemon. It writes the command and root's password to the socket and the daemon executes the command using su, as describe before. After this, the command and the password are not thrown away. Instead, they are kept for a specified amount of time. This is the timeout value from in the control module. If another request for the same command is coming within this time period, the client does not have to supply the password. To keep hackers who broke into your account from stealing passwords from the daemon (for example, by attaching a debugger), the daemon is installed set-group-id nogroup. This should prevent all normal users (including you) from getting passwords from the kdesud process. Also, the daemon sets the DISPLAY environment variable to the value it had when it was started. The only thing a hacker can do is execute an application on your display.

One weak spot in this scheme is that the programs you execute are probably not written with security in mind (like setuid root programs). This means that they might have buffer overruns or other problems and a hacker could exploit those.

The use of the password keeping feature is a tradeoff between security and comfort. I encourage you to think it over and decide for yourself if you want to use it or not.