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Introduction to Access Control in Operating Systems

News Recommended Links Rainbow Books Unix Access Control Lists Unix permissions model The umask PAM Primary Group User private groups
Group administration The su Command Sudo Root Account Root Security System Accounts Accounts security /etc/passwd File Shadow Passwords

Introduction

The access control is just another name for compartmentalization of resources.

It is useful to group general problems involved in making certain that files are not read or modified by unauthorized personnel under common umbrella -- access control. There are two aspect of access control:

  1. Access control policies. Access control policy defined "whose data is to be protected from whom"
  2. Access control mechanisms. the manner by which the operating system enforces the access control policy. Among them the following are the most important

Classic Unix systems have at least one user with right to access (privilege) any file of the system -- root user. A protection domain is defined by its UID and GID. Provided with any (UID, GID) combination it is possible to build a complete list of all objects that can be accessed and each objects rights. Two processes with the same (uid, gid) combination, have access to exactly the same set of objects. Processes with different (uid, gid) combinations, have access to a different set of files.

Additionally, in UNIX each process has two halves: the user part and the kernel part. When a process does a system call it switches from the user role to the kernel role. The kernel role has access rights to a different set of objects from the user part. A system call therefore invokes a domain switch, from user to kernel.

At every instant in time each process runs in a protection domain. Therefore, there is some collection of objects that the process can access. In addition, each object has a set of access rights. Processes can switch from domain to domain during program execution.

The UNIX process division into user and kernel parts is a legacy of a more powerful domain switching mechanism that was used in MULTICS. In the MULTICS system the hardware supported up to 64 domains for each process, not the two (kernel and user) like in UNIX or MVS. A MULTICS process could be considered as a collection of procedures, each running in a domain, called a ring. The innermost ring was the most powerful, the operating system kernel. Radically, moving outwards from the operating system kernel the rings became successively less powerful. When a procedure in one ring called a procedure in a different ring a trap occurred. Once a trap occurred the system had the option to change the protection domain for that process.

Unix permissions model

Unix introduced a simple model of file permissions in the 70's which has proven to be quite effective and easy to understand. In this model each file has three attributes for each of three access categories (owner, group and world):

There is also one implicit attribute: the ability to delete file/directory. It is property not a file/directory in question, but the directory in which it contained. It the user has write access to the directory he/she can delete any file in it.

Moreover the execution bit on the directory can work as partial access blocker to files in this directory. It in is not set there is no way to obtain a listing of the directory althouth if you know file name you can access file that is contains in the directory.

Due to popularity of Samba, recently there was a half-baked attempt to extend Unix "xwr" model using ACLs, that are used in Windows.

The result is a mess and very few organization have adopted this approach on Unix servers due to the complexity and bad integration with classic Unix model. Not only existence of the second, by-and-large, indeoendent model  confuse administrators, it actually represents a serious security risk.

In the original Unix model, each file has three "rwx" access categories: User (u), Group (g) and Other (o). Group is essentially a role and primary group is a primary role. Any user can be a member of any number of groups, but in Unix groups are atomic -- they cannot contain other groups. This is a serious shortcoming of the classical Unix model.

System groups and pseudo-groups

There is also auxiliary concept of system groups which is similar to the concept of privileged ports. For example, all groups with GID below 100 are usually considered to be system groups. Some system groups are designed mainly for not to providing access to files for a group of users, but for partitioning of permission space. Among them are adm, sys,  daemon, lp, mail, uucp, games, ftp. nobody, etc. Typically users which have such "pseudo-group" as primary group have no legitimate shell. Instead /bin/noshell  or /bin/false  is used; the former logs the access attempts), so nobody can login as such a user. They can be called pseudo-users.

Another severe limitation of this model is that a file can only be a member of one group. That can be partially rectified by usage of "metagroups" -- groups that are just  aggregations of existing groups, but that solution requires additional efforts and discipline (in this case /etc/group needs to be automatically generated from some template using macroprocessor). If we assume that the number of group allowed is large (approximately the same as the number of files/directories) metagroup approach is as powerful as ACL model and is much simpler. It requires relatively simple modification of the /etc/group file.

User Private Groups (UPG)

One interesting step toward this model is the concept of User Private Groups (UPG) introduced in Red Hat. UPG scheme makes UNIX groups easier to use. It does not add or change anything in the standard UNIX way of handling groups. This is simply a new convention for handling classic Unix groups groups: whenever you create a new user, by default, he or she has a unique group with GID identical to UID.

One of limitations of the Unix model is that ordinary users do not have the right to make their own groups. In this way, the model shoots itself in the foot. There are ways around this, but Unix has not introduced a standard solution to the problem. We can think of the Unix model as being a coarse approximation to the model of ACLs.

For more information see User Private Groups

Inheritance of permissions

One of the most interesting aspects of file security of a particular OS is the mechanism by which security attributes are inherited by new files. Newly created files have to start out with some kind of permissions and that this "default set" of permissions is a question that is answered differently by different flavors of Unix. One advance implemented by BSD is  usage of setgid bit in directories  to inherit permissions set for the directory for all new files created in it.

Full inheritance of permissions is possible only under root account. For user account inheritance is restricted. Permissions granted are further restricted by the current value of umask variable 

Most systems are hybrid and  support BSD behavior via setgid bit in directories.

Process permissions

Access rights apply not only to secondary storage but to any resource. In particular, the right to communicate with, control or request services from a process. The same basic ideas apply, but access controls usually have different attributes. Process permissions are usually set by access control lists, or on the basis of understood protocols, such as passwords, keys, or cookies (the message in a fortune cookie).

Unix processes have an owner and a group membership. These are normally inherited from the the user who starts the process and from the group attribute of the program file respectively (though see below about setuid/setgid programs). Each process has the privileges afforded to it by these labels.

Kernel two modes operation

The basic security of multiuser operating systems lies in the ability to restrict privilege. One of our design criteria was to try to prevent malicious users from circumventing security mechanisms. In some operating systems this has been possible by accidental or deliberate memory corruption. Since software is often buggy, we should not rely on software to behave properly. Stronger measures are needed to protect the system. This was the idea behind two-mode operation (see the operating system course notes). By having system protection hard-coded in hardware, the system is more secure. The Multics operating system generalized this idea to more than two-modes. A series of protection levels or rings was used, each of which encapsulated the inner rings. A typical use of protection rings would be the following:

In Unix's two-mode operation, only rings 0 (kernel) and 3 (user processes) are used. This has made it easy to port Unix to a variety of architectures which support 2-mode operation (e.g PC's including and later than i386).

Role based permissions

In a dynamical, interactive situation we could generalize the notion of access to allow users to use different permissions depending on what they are doing. This increase complexity of the model and can be done in several different ways: 

If you think about it global static permission used in Unix are anachronism. There should be a mechanism that prevent process from accessing files in, say, /etc directory even if part of the process is running under the root privileges. This can be done in several different ways:

See also



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