Medusa DS9 Security System Medusa is a package that improves the overall security of the Linux OS by extending the standard Linux (Unix) security architecture while preserving backward compatibility. 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 forbids 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. ARCHITECTURE 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 doesn't 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 what's 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. IMPLEMENTATION The current implementation of Medusa in the kernel, in cooperation with the security daemon in user space, has these features: o full access control to any file in the system (via VFS) o ability to redirect access to the selected file or to another one o complete control of signal sending/receiving o direct control of important process actions o control of execution of any syscalls for specified processes o every process or file is a member of a specified virtual subsystems and every process has assigned access rights to VS. (It is possible to completely hide processes or files from other processes this way.) o every process has a login uid (luid), which is set only on the first call of set{re,s}uid o ability to force execution of specified code for any process (similar to signal handling, see doc/Code_forcing for details) o low level control of any system call Use of these features depends on the implementation of the security daemon and details of its configuration. At least Constable can use all of them. These features are described in detail in doc/Features. To learn more about the implementation of Medusa in the Linux kernel see doc/Kernel or the patched kernel sources. Constable is currently implemented as an interpreter of its own configuration language (very similar to C), so it is possible to implement theoretically any system security model. With Medusa (and Constable), you can almost completely override the current Linux security model, and you can create a small subset of extensions to it, as well. Currently, Medusa works only on the i386 architecture, but we plan to support other architectures that are supported by Linux, as soon as these CPUs are made available to us. LICENSE The entire Medusa package is distributed under under GNU GPL v2 license. See doc/COPYING for details. AUTHORS Programming and everything else: Marek Zelem Milan Pikula Martin Ockajak Helpful testing, bugreports and comments: Jan Varga Robert Varga Robert Dobozy FOOTNOTES ** ** You are strongly encouraged to join Medusa mailing list by ** sending an e-mail with "subscribe medusa" in the body of message ** to listserv@medusa.fornax.sk. ** Homepage of Medusa is at http://medusa.fornax.sk. Medusa was not thoroughly tested on multiprocessor systems, but it seems to work well. Feel free to post any questions, comments, ideas, critics or very nice Constable configuration files to the Medusa mailing list at medusa@medusa.fornax.sk.