Medusa DS9 1.0.0

Medusa DS9 is an application used to increase Linuxs security. It consists of two major parts, Linux kernel changes and the user-space daemon.

Briefly, it supports, at the kernel level, a user-space authorization server (and is thus fully transparent to any user space applications). Before the execution of certain operations, the kernel asks the authorization server for confirmation. The authorization server then permits or rejects the operation.


The authorization server can also affect the way an operation is executed in some cases, which are described later. This method allows the use of almost any security architecture. When the authorization server is properly configured, it can determine access rights within the system to a very fine level and do very good auditing.

Currently, Medusa consists of two basic parts: a small patch to the Linux kernel and a user space security daemon called "Constable". Constable is the current implementation of an authorization server. User space implementation allows kernel changes to be simpler and smaller and thus easier to port to new versions of the Linux kernel and to be more flexible, so improvements to the authorization server should not require changes in the kernel.

Communication between Constable and kernel goes through the special device "/dev/medusa" (char major 111 minor 0), because it should be both fast and flexible. When the kernel needs confirmation, it writes data to this device, makes the current process sleep and wakes up Constable. Constable reads the data from /dev/medusa, chooses a response (depending on his configuration, which is discussed in doc/Constable), sends it back to the kernel and sleeps.

The kernel gets the data, wakes up the process and determine the result of the operation. Constable can also send certain commands to the kernel (even if the kernel doesnt require them), which are then executed by the kernel. The security daemon has to use a specific communication protocol defined in kernel, so it is possible to implement a full-featured authorization server by only knowing this protocol and knowing that the kernel supports it, without worrying whats really happening in the kernel.

Constable is only one example of such an authorization server. The protocol allows communication in the form of packets which carry all necessary data.