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Contents Introduction 1 Fundamental Concepts 2 Security System Calls in Linux 3 Implementation of Security in Linux 4

Introduction Linux, as a clone of MINIX and UNIX, has been a multiuser system almost from beginning. Security and control of information was built in very early on.

Fundamental Concepts A UID is an integer between 0 and 65,535. Files, processes and other resources are marked with UID of their owner. By default, the owner of a file is the person who created the file. Users can be organized into groups, which are also numbered with 16-bit integers called GIDs (Groups IDs). A user could be in one or more groups at the same time.

Fundamental Concepts Potential accesses are read, write, and execute, designated by the letters r, w, and x, respectively. There are three categories of users (owner, group and others). 3 bits per category. 9 bits are sufficient to represent the access rights.

Fundamental Concepts Figure Some example file protection modes.

Fundamental Concepts

The user with UID 0 is special and is called the superuser (or root). Root can read and write all files in the system.

Power of root Create a file. Set permission ( ). It means nobody has any access (Figure 10-37)

Power of root Result:

Power of root Use root user to access:

Set permission to 777 to check again. Power of root When use command :wq!, the file has been updated successfully.

Conclusion The superuser (root) has the power to read and write all files in system, no matter who owns them and no matter how they are protected.

Fundamental Concepts (cont) Special files corresponding to the I/O devices have the same protection bits as regular files. Ex: the printer special file, /dev/lp, could be owned by the root or by a special user, daemon, and have mode rw to keep everyone else from directly accessing the printer. Problem: Sometimes users do have a legitimate need to print something. Solution: use SETUID bit.

Fundamental Concepts When a program with the SETUID bit on is executed, the effective UID for that process becomes the UID of the executable file’s owner instead of the UID of the user who invoked it. By making the program that accesses the printer be owned by daemon but with the SETUID bit on, any user could execute it, and have power of daemon (e.g., access to /dev/lp ) but only run that program (which might queue print jobs for printing in an orderly fashion).

Fundamental Concepts In addition to the SETUID bit there is also a SETGID bit that works analogously. This bit is rarely used.

Fundamental Concepts

How do you set the sticky bit? –In a terminal window: Put the number ‘1′ in front of a normal chmod command. Eg – to make the test file or directory world readable, writeable, executable, and sticky use chmod 1777 test. –In your desktop environment: Right-click the test file or directory and go to properties. Look for a permissions tab or listing and check the sticky bit checkbox. How can you tell if the sticky bit is set on a file or directory? There will be a letter ‘t’ in the last position of the file permissions as in: rwxrwxrw-t

Security System Calls in Linux There are only a small number of system calls relating to security. For example: –Sets newgame to rwxr-xr-x so that everyone can run it Only the owner of a file and the superuser can change its protection bits.

Security System Calls in Linux

Implementation of Security in Linux Login (which is SETUID root) asks for a login name and a password. It hashes the password and then looks in the password file ( /etc/passwd ) to see if the hash matches the one there. If the password is correct, the login program looks in /etc/passwd to see the name of the user’s preferred shell. ( bash, csh, ksh ) The login program then uses setuid and setgid to give itself the user’s UID and GID.

Implementation of Security in Linux Then it opens: –The keyboard for standard input (file descriptor 0) –The screen for standard output (file descriptor 1) –The screen for standard error (file descriptor 2) Finally, it executes the preferred shell. Username:Password:UID:GID:Description:Home directory:Shell

Implementation of Security in Linux The preferred shell is running with the correct UID and GID. All processes that it forks off automatically inherit the shell’s UID and GID -> they also will have the correct owner and group. All files they create also get these values.

Implementation of Security in Linux When any process attempts to open a file, the system first checks the protection bits to see if the access is permitted. –If so, the file is opened and a file descriptor returned. –If not, the file is not opened and -1 returned. No checks are made on subsequent read or write calls. The Linux security model and its implementation are essentially the same as in most other traditional UNIX systems.

References References: –Modern Operating Systems, 3rd Edition – Andrew S. Tanenbaum (Chapter 10.7 – Page 799) –Hoang Duc Quang’s Slides –Internet Examples was made by using Fedora 11

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