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Chapter: Security in Computing : Protection in General-Purpose Operating Systems

File Protection Mechanisms

Basic Forms of Protection : We noted earlier that all multiuser operating systems must provide some minimal protection to keep one user from maliciously or inadvertently accessing or modifying the files of another. As the number of users has grown, so also has the complexity of these protection schemes.

File Protection Mechanisms


Until now, we have examined approaches to protecting a general object, no matter the object's nature or type. But some protection schemes are particular to the type. To see how they work, we focus in this section on file protection. The examples we present are only representative; they do not cover all possible means of file protection on the market.


Basic Forms of Protection


We noted earlier that all multiuser operating systems must provide some minimal protection to keep one user from maliciously or inadvertently accessing or modifying the files of another. As the number of users has grown, so also has the complexity of these protection schemes.


AllNone Protection


In the original IBM OS operating systems, files were by default public. Any user could read, modify, or delete a file belonging to any other user. Instead of software- or hardware-based protection, the principal protection involved trust combined with ignorance. System designers supposed that users could be trusted not to read or modify others' files because the users would expect the same respect from others. Ignorance helped this situation, because a user could access a file only by name; presumably users knew the names only of those files to which they had legitimate access.


However, it was acknowledged that certain system files were sensitive and that the system administrator could protect them with a password. A normal user could exercise this feature, but passwords were viewed as most valuable for protecting operating system files. Two philosophies guided password use. Sometimes, passwords controlled all accesses (read, write, or delete), giving the system administrator complete control over all files. But at other times passwords controled only write and delete accesses because only these two actions affected other users. In either case, the password mechanism required a system operator's intervention each time access to the file began.


However, this all-or-none protection is unacceptable for several reasons.


  Lack of trust. The assumption of trustworthy users is not necessarily justified. For systems with few users who all know each other, mutual respect might suffice; but in large systems where not every user knows every other user, there is no basis for trust.


  Too coarse. Even if a user identifies a set of trustworthy users, there is no convenient way to allow access only to them.


  Rise of sharing. This protection scheme is more appropriate for a batch environment, in which users have little chance to interact with other users and in which users do their thinking and exploring when not interacting with the system. However, on shared-use systems, users interact with other users and programs representing other classes of users.


  Complexity. Because (human) operator intervention is required for this file protection, operating system performance is degraded. For this reason, this type of file protection is discouraged by computing centers for all but the most sensitive data sets.


  File listings. For accounting purposes and to help users remember for what files they are responsible, various system utilities can produce a list of all files. Thus, users are not necessarily ignorant of what files reside on the system. Interactive users may try to browse through any unprotected files.


Group Protection


Because the all-or-nothing approach has so many drawbacks, researchers sought an improved way to protect files. They focused on identifying groups of users who had some common relationship. In a typical Unix+ implementation, the world is divided into three classes: the user, a trusted working group associated with the user, and the rest of the users. For simplicity we can call these classes user, group, and world. Windows NT+ uses groups such as Administrators, Power Users, Users, and Guests. (NT+ administrators can also create other groups.)


All authorized users are separated into groups. A group may consist of several members working on a common project, a department, a class, or a single user. The basis for group membership is need to share. The group members have some common interest and therefore are assumed to have files to share with the other group members. In this approach, no user belongs to more than one group. (Otherwise, a member belonging to groups A and B could pass along an A file to another B group member.)


When creating a file, a user defines access rights to the file for the user, for other members of the same group, and for all other users in general. Typically, the choices for access rights are a limited set, such as {update, readexecute, read, writecreatedelete}. For a particular file, a user might declare read-only access to the general world, read and update access to the group, and all rights to the user. This approach would be suitable for a paper being developed by a group, whereby the different members of the group might modify sections being written within the group. The paper itself should be available for people outside the group to review but not change.


A key advantage of the group protection approach is its ease of implementation. A user is recognized by two identifiers (usually numbers): a user ID and a group ID. These identifiers are stored in the file directory entry for each file and are obtained by the operating system when a user logs in. Therefore, the operating system can easily check whether a proposed access to a file is requested from someone whose group ID matches the group ID for the file to be accessed.


Although this protection scheme overcomes some of the shortcomings of the all-or-nothing scheme, it introduces some new difficulties of its own.


Group affiliation. A single user cannot belong to two groups. Suppose Tom belongs to one group with Ann and to a second group with Bill. If Tom indicates that a file is to be readable by the group, to which group(s) does this permission refer? Suppose a file of Ann's is readable by the group; does Bill have access to it? These ambiguities are most simply resolved by declaring that every user belongs to exactly one group. (This restriction does not mean that all users belong to the same group.)



  Multiple personalities. To overcome the one-person one-group restriction, certain people might obtain multiple accounts, permitting them, in effect, to be multiple users. This hole in the protection approach leads to new problems because a single person can be only one user at a time. To see how problems arise, suppose Tom obtains two accounts, thereby becoming Tom1 in a group with Ann and Tom2 in a group with Bill. Tom1 is not in the same group as Tom2, so any files, programs, or aids developed under the Tom1 account can be available to Tom2 only if they are available to the entire world. Multiple personalities lead to a proliferation of accounts, redundant files, limited protection for files of general interest, and inconvenience to users.


  All groups. To avoid multiple personalities, the system administrator may decide that Tom should have access to all his files any time he is active. This solution puts the responsibility on Tom to control with whom he shares what things. For example, he may be in Group1 with Ann and Group2 with Bill. He creates a Group1 file to share with Ann. But if he is active in Group2 the next time he is logged in, he still sees the Group1 file and may not realize that it is not accessible to Bill, too.


  Limited sharing. Files can be shared only within groups or with the world. Users want to be able to identify sharing partners for a file on a per-file basis; for example, sharing one file with ten people and another file with twenty others.


Individual Permissions


In spite of their drawbacks, the file protection schemes we have described are relatively simple and straightforward. The simplicity of implementing them suggests other easy-to-manage methods that provide finer degrees of security while associating permission with a single file.


Persistent Permission


From other contexts you are familiar with persistent permissions . The usual implementation of such a scheme uses a name (you claim a dinner reservation under the name of Sanders), a token (you show your driver's license or library card), or a secret (you say a secret word or give the club handshake). Similarly, in computing you are allowed access by being on the access list, presenting a token or ticket, or giving a password. User access permissions can be required for any access or only for modifications (write access).


All these approaches present obvious difficulties in revocation: Taking someone off one list is easy, but it is more complicated to find all lists authorizing someone and remove him or her. Reclaiming a token or password is even more challenging.


Temporary Acquired Permission


Unix+ operating systems provide an interesting permission scheme based on a three-level usergroupworld hierarchy. The Unix designers added a permission called set userid (suid). If this protection is set for a file to be executed, the protection level is that of the file's owner, not the executor. To see how it works, suppose Tom owns a file and allows Ann to execute it with suid. When Ann executes the file, she has the protection rights of Tom, not of herself.


This peculiar-sounding permission has a useful application. It permits a user to establish data files to which access is allowed only through specified procedures.


For example, suppose you want to establish a computerized dating service that manipulates a database of people available on particular nights. Sue might be interested in a date for Saturday, but she might have already refused a request from Jeff, saying she had other plans. Sue instructs the service not to reveal to Jeff that she is available. To use the service, Sue, Jeff, and others must be able to read the file and write to it (at least indirectly) to determine who is available or to post their availability. But if Jeff can read the file directly, he would find that Sue has lied. Therefore, your dating service must force Sue and Jeff (and all others) to access this file only through an access program that would screen the data Jeff obtains. But if the file access is limited to read and write by you as its owner, Sue and Jeff will never be able to enter data into it.


The solution is the Unix SUID protection. You create the database file, giving only you access permission. You also write the program that is to access the database, and save it with the SUID protection. Then, when Jeff executes your program, he temporarily acquires your access permission, but only during execution of the program. Jeff never has direct access to the file because your program will do the actual file access. When Jeff exits from your program, he regains his own access rights and loses yours. Thus, your program can access the file, but the program must display to Jeff only the data Jeff is allowed to see.


This mechanism is convenient for system functions that general users should be able to perform only in a prescribed way. For example, only the system should be able to modify the file of users' passwords, but individual users should be able to change their own passwords any time they wish. With the SUID feature, a password change program can be owned by the system, which will therefore have full access to the system password table. The program to change passwords also has SUID protection so that when a normal user executes it, the program can modify the password file in a carefully constrained way on behalf of the user.


Per-Object and Per-User Protection


The primary limitation of these protection schemes is the ability to create meaningful groups of related users who should have similar access to related objects. The access control lists or access control matrices described earlier provide very flexible protection. Their disadvantage is for the user who wants to allow access to many users and to many different data sets; such a user must still specify each data set to be accessed by each user. As a new user is added, that user's special access rights must be specified by all appropriate users.

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