See this page as a slide show

Access Control and Root

Original slides from Dr. James Walden at Northern Kentucky University. Access Control

Access Control

• Access control refers to exerting control over who can interact with a resource. Often but not always, this involves an authority, who does the controlling
• Access control is, in reality, an everyday phenomenon. A lock on a car door is a form of access control. A PIN on an ATM system at a bank is another means of access control.
• Access control is of prime importance when persons seek to secure important, confidential, or sensitive information and equipment.

https://en.wikipedia.org/wiki/Access_control

Control Mechanisms

• System Access is implemented through password protection. The resources associated with a user are accessible only after logging in with the correct password.
• Filesystem Access is implemented by every file having an owner and a group. The owner can have a different set of privileges from group members and other users.

Control Mechanisms

• Process Ownership also depends on ownership. Only the owner of a process can send it signals or change its scheduling priority.
• Root Privileges gives broad privileges to certain classes of users so that administrative functions such as shutdown and reboot are restricted from ordinary users.

System Access

• The most basic form of system access is the management of users accounts by administrative users. Only users with valid usernames and passwords can login to a system.
• Linux implements system access through the /etc/passwd file that stores passwords and maps usernames to user identification numbers (UIDs).
• Linux allows shared access to certain resources through the /etc/group file that maps group names to group identification numbers (GIDs).
• Users can belong to an arbitrary number of groups, but root privileges are required to add or remove users from groups. Similarly, only root can change passwords for other users.

Access Commands

• useradd: add new user, associate with group, create home directory, set default shell, set initial password
• userdel: remove existing user, delete home directory and files, edit associated groups, assuming no processes!
• usermod: modify existing users, including initial group, home directory, user identification number
• groupadd/groupdel/groupmod: functions for groups instead of users, assigns group identification numbers
• passwd: modify or delete a password, users can modify their own password, root can modify any password
• login: authenticate a username and password before allowing user access

Filesystem Protection

• Every file has an owner and group. File access varies depending on whether you are the owner, belong to the group, or are neither.
• Read, write, and execute privileges for each file are distinct for owner, group, and world. The latter defines privileges for users that are not owners and are not in the group.
• A listing command shows the each of these protections, which can only be modified by the owner to the root user, as shown on the next page.
• Also Linux stores a sticky bit for each directory, that tells who can rename or delete files. In addition there are SUID and SGID, which will be explained later.

Protection Commands

• ls -l: listing directory in long format show protections

    drwxr-----  5 applin fac 4096 Aug 22 10:35 Desktop
    drwxr-xr-x 13 applin fac 4096 Aug 21 16:57 Documents
    drwx------  3 applin fac 4096  Sep 2  2013 Cheaters

• chown: change file owner

    $ chown applin Desktop

• chgrp: change file group

    $ chgrp fac Desktop

• chmod: change file privileges

    $ chmod 755 foo
    $ chmod ug+rw bar

Protection Commands

• umask: set up default privileges, e.g. umask 664
• pwconv, punconv, grpconv, grpunconv: more commands having to do with shadow passwords and groups.

More on Permissions

• Must have execute to use a directory
• Permission bits stored in the parent’s directory
  • Delete and rename controlled by the parent’s permissions
• setuid and setgid
  • Bits with octal value 4000 and 2000
    • Only losers memorize octal values
• sticky bit
  • If set, cannot delete or rename unless you are directory owner
    • Historical for regular files – ignored these days
  • Bit with octal 1000

ACLS

Features of an access control list (ACL)

• Defines a list of permissions per object
• Permission specifications for multiple users or groups
• More complex systems have inheritance
• More complex to administer and develop for
• Can also apply to network file access

Linux ACL support

Linux can support ACL mode

• Sits on top of the 9-bit (rwxrwxrwx) model
• Not required for many administrative situations
• ACL is disabled in most Linux systes by default
• Turn on using mount –o acl option
• Use the setfacl command to define permissions

Process Ownership

• Linux assigns user identifiers (UIDs) and group identifiers (GIDs) to processes. When a child process is created, by default it inherits the identifiers from the parent process.
• The login process launches the initial shell process with the UID and GID of the user that logged on, so commands launched by that user will have the same identifiers.
• An exception is made if the setuid and setgid flags are set on the process. In this case ownership of the process follows the ownership of the executable instead of the user.

    $ ls –l /bin/passwd
    -rwsr—xr—x 1 root root 25000 Feb 8 2011 passwd

Root Privileges

A special root account exists that represents the omnipotent administrative user, often called the superuser account, that can perform tasks that are restricted to other users:

• Shutting down or rebooting the system (shutdown, reboot)
• Setting the system or domain name
• Changing the system date and time
• Creating or deleting device files
• Configuring network interfaces
• Raising resource usage limits
• Raising process priorities

Root Privileges

Several ways exist in which root privileges can be accessed, and a number of concerns should be taken into account when deciding which method to use:

• Logging in to the root account
• Substituting user identity (su command)
• Execute command as root (sudo command)

Root

• Operating from the root account gives unfettered access, but leaves no record of which operations are performed. Also can be extremely dangerous to always be root!
• Substituting identify can apply to root or other users, for example makes it possible to change passwords.
• Executing single commands as root with sudo maintains a log of all operations performed with root privileges, thus making it easy to monitor system administration activities.

Access Control Problems

• Root account represents a single point of failure. If compromised, the integrity of the whole system is violated, and there is not limit to the damage that can be inflicted.
• There is no way to subdivide the special privileges of the root account, i.e. allow one administrator to manage accounts and another to mount devices.
• Access control rules must be embedded in the code of individual commands and daemons, so modifying access behavior requires significant source code modification.
• The security model is not strong enough for use on a network, since user and group identifiers can be hacked on systems to which an unprivileged user has access.

Common Extensions

• Role Based Access Control (RBAC)
  • Users are associated with roles, which in turn define access
• Security-Enhanced Linux (SELinux)
  • National Security Agency project
• POSIX Capabilities
  • Subdivides privileges of the root account
• Access Control Lists (ACLs)
  • A generalization of the user/group/other model
• Pluggable Authentication Modules (PAM)
  • Authentication framework
• Kerberos: Cryptographic Authentication
  • Authentication method