Technowine

Security-enhanced Linux (SELinux) is an implementation of a mandatory access control mechanism. This mechanism is in the Linux kernel, checking for allowed operations after standard Linux discretionary access controls are checked.

To understand the benefit of mandatory access control (MAC) over traditional discretionary access control (DAC), you need to first understand the limitations of DAC.

Under DAC, ownership of a file object provides potentially crippling or risky control over the object. A user can expose a file or directory to a security or confidentiality breach with a misconfigured chmod command and an unexpected propagation of access rights. A process started by that user, such as a CGI script, can do anything it wants to the files owned by the user. A compromised Apache HTTP server can perform any operation on files in the Web group. Malicious or broken software can have root-level access to the entire system, either by running as a root process or using setuid or setgid.

Under DAC, there are really only two major categories of users, administrators and non-administrators. In order for services and programs to run with any level of elevated privilege, the choices are few and course grained, and typically resolve to just giving full administrator access. Solutions such as ACLs (access control lists) can provide some additional security for allowing non-administrators expanded privileges, but for the most part a root account has complete discretion over the file system.

A MAC or non-discretionary access control framework allows you to define permissions for how all processes (called subjects) interact with other parts of the system such as files, devices, sockets, ports, and other processes (called objects in SELinux). This is done through an administratively-defined security policy over all processes and objects. These processes and objects are controlled through the kernel, and security decisions are made on all available information rather than just user identity. With this model, a process can be granted just the permissions it needs to be functional. This follows the principle of least privilege. Under MAC, for example, users who have exposed their data using chmod are protected by the fact that their data is a kind only associated with user home directories, and confined processes cannot touch those files without permission and purpose written into the policy.

SELinux is implemented in the Linux kernel using the LSM (Linux Security Modules) framework.

Type Enforcement involves defining a type for every subject, that is, process, and object on the system. These types are defined by the SELinux policy and are contained in security labels on the files themselves, stored in the extended attributes (xattrs) of the file. When a type is associated with a processes, the type is called a domain, as in, “httpd is in the domain of httpd_t.” This is a terminology difference leftover from other models when domains and types were handled separately.

All interactions between subjects and objects are disallowed by default on an SELinux system. The policy specifically allows certain operations. To know what to allow, TE uses a matrix of domains and object types derived from the policy. The matrix is derived from the policy rules. For example, allow httpd_t net_conf_t:file { read getattr lock ioctl }; gives the domain associated with httpd the permissions to read data out of specific network configuration files such as/etc/resolv.conf. The matrix clearly defines all the interactions of processes and the targets of their operations.

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