Document revision date: 30 March 2001 | |
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For the most part, the OpenVMS operating system bases its security controls on user identity. Protected objects, such as files and devices, are accessible to individual users or groups of users. If an object's ACL or protection code allows a user the necessary access, then the user can use that object by using any available software. (See Chapter 4 for a description of OpenVMS object protection.)
In a protected subsystem, an application protected by normal access controls serves as a gatekeeper to objects belonging to the subsystem. Users have no access to the subsystem's objects unless they execute the application serving as gatekeeper. Once users run the application, their process rights list acquires identifiers giving them access to objects owned by the subsystem. As soon as they exit from the application, these identifiers and, therefore, the users' access rights to objects are taken away.
This chapter describes protected subsystems and explains how to build
them.
13.1 Advantages of Protected Subsystems
Using protected subsystems offers several advantages:
Protected subsystems have many applications, from databases to common system management situations.
One use for a protected subsystem might be a group membership list that you want to make available to all group members. The list contains the names, addresses, personnel numbers, and interests of group members. When the membership list is set up as a protected subsystem, all members of the group can read selected information and update specific types of information.
A protected subsystem might also solve the problem of confidential
information being sent to printers in public areas. You could write an
application to filter data for sensitive information. Confidential
files would be sent to printers in restricted areas, while public files
would be sent to any available printer. Any user with execute access to
the application could use the restricted printers, but only through the
protected subsystem.
13.3 How Protected Subsystems Work
A protected subsystem is an application that, when run, causes the process running the application to be granted one or more identifiers. For as long as a user runs the subsystem, the user's process rights list carries these additional identifiers. Figure 13-1 shows how a protected subsystem adds a second level of access control to traditional controls.
Figure 13-1 How Protected Subsystems Differ from Normal Access Control
Users with execute access to the application gain access to the subsystem. Once in the subsystem, users can work with the data files and other resources of the subsystem.
A subsystem can have several identifiers because the resources consumed by the subsystem (the files, printers, and so forth) can be protected differently.
Possession of subsystem identifiers is limited to the period users are executing the application. Once the users exit from the application, the identifiers are removed from their process rights lists. Subsystem identifiers are also removed from the rights list whenever users enter a Ctrl/Y sequence or attempt to create a subprocess with the DCL command SPAWN. (In this respect, use of the subsystem identifiers is identical to the operation of images installed with privileges.)
The following identifiers are reserved for use in the security subsystem and should not be granted to any user:
Someone developing an application for a protected subsystem must link the application images without the /DEBUG or /TRACEBACK qualifiers.
Although this kind of subsystem often precludes the need for privilege, applications can be installed with privilege. For example, some applications may need the PRMGBL privilege to create permanent global sections, or they may need the AUDIT privilege to send security audit records to the system security audit log file. Compaq does discourage the installation of a protected subsystem application with privileges in the All category. This category includes such privileges as BYPASS, CMKRNL, and SYSPRV---privileges that allow a user to subvert OpenVMS access controls. See Table 8-2 for a list of OpenVMS privileges and Appendix A for a description of the privileges.
Subsystem designers need to generate a list of identifiers that are
necessary for it to operate as intended. Then the designers approach
you, as the security administrator, to make the preparations described
in Section 13.5.
13.5 System Management Requirements
Although an unprivileged user can build and manage a protected subsystem, you need to be involved at two points in the process: at the beginning to create the necessary identifiers for the subsystem and at the end to mount the volume with the protected subsystem.
You need to perform the following tasks:
The following example shows how you can set up identifiers and the necessary application access so that users can manage a membership list:
$ SET DEFAULT SYS$SYSTEM $ RUN AUTHORIZE UAF> ADD/IDENTIFIER MEMBERS_SUBSYSTEM- (1) _UAF> /ATTRIBUTES=(SUBSYSTEM,RESOURCE) UAF> GRANT/IDENTIFIER MEMBERS_SUBSYSTEM - (2) _UAF> /ATTRIBUTES=(SUBSYSTEM,RESOURCE) LOUIS UAF> GRANT/IDENTIFIER MEMBERS_SUBSYSTEM - _UAF> /ATTRIBUTES=(SUBSYSTEM,RESOURCE) WU $ SET SECURITY/ACL=(IDENTIFIER=MEMBERS_SUBSYSTEM,- (3) _$ ACCESS=CONTROL) MEMBER_LIST.EXE |
Note that you create the subsystem identifier MEMBERS_SUBSYSTEM with
the Resource attribute. This allows disk space to be charged to the
identifier MEMBERS_SUBSYSTEM and not the individuals accessing the
subsystem. (When using the Resource attribute, be careful to set the
appropriate ACLs on directories [see Section 8.8.1.2.3].)
13.6 Building the Subsystem
Once managers of the subsystem have the appropriate identifiers and access rights as described in Section 13.5, they can add the necessary ACEs to a subsystem image. Two kinds of ACEs are necessary to construct a subsystem: the application image receives a Subsystem ACE, and the objects managed by the subsystem receive Identifier ACEs. Therefore, building a subsystem requires the following steps:
(SUBSYSTEM,{IDENTIFIER=identifier[,ATTRIBUTES=attributes]}) |
(IDENTIFIER=identifier, ACCESS=access-type[+...]) |
In the following example, the subsystem manager uses the DCL command SET SECURITY to associate the subsystem identifier with the images that make up the subsystem. First, the subsystem manager adds a Subsystem ACE with the identifier MEMBERS_SUBSYSTEM to the ACL of the application image MEMBER_LIST.EXE:
$ SET SECURITY/ACL=(SUBSYSTEM,IDENTIFIER=MEMBERS_SUBSYSTEM,- _$ ATTRIBUTES=RESOURCE) MEMBER_LIST.EXE |
Then the subsystem manager adds an Identifier ACE with the subsystem identifier MEMBERS_SUBSYSTEM to the data files managed by the subsystem:
$ SET SECURITY/ACL=(IDENTIFIER=MEMBERS_SUBSYSTEM,- _$ ACCESS=READ+WRITE) MEMBER_DATA*.DAT |
The DCL command SHOW SECURITY displays the security attributes of the files. For example:
$ SHOW SECURITY MEMBER_LIST.EXE MEMBER_LIST.EXE object of class FILE Owner: [STAFF] Protection: (System: RWED, Owner: RWED, Group, World: RE) Access Control List: (SUBSYSTEM,IDENTIFIER=MEMBERS_SUBSYSTEM,ATTRIBUTES=RESOURCE) |
$ SHOW SECURITY MEMBER_DATA*.DAT MEMBER_DATA_1.DAT object of class FILE Owner: MEMBERS_SUBSYSTEM Protection: (System: RWED, Owner: RWED, Group, World) Access Control List: (IDENTIFIER=MEMBERS_SUBSYSTEM,ACCESS=READ+WRITE) MEMBER_DATA_2.DAT object of class FILE Owner: MEMBERS_SUBSYSTEM Protection: (System: RWED, Owner: RWED, Group, World) Access Control List: (IDENTIFIER=MEMBERS_SUBSYSTEM, ACCESS=READ+WRITE) |
A person with the SECURITY privilege can enable subsystems on a volume by using the /SUBSYSTEM qualifier on the MOUNT command. By default, subsystems are enabled only on the system disk. For other disks, you need to enable subsystems every time a volume is mounted.
In the following example, a security administrator uses the MOUNT command with the /SUBSYSTEM qualifier to enable the processing of Subsystem ACEs on device DUA0. Assume that this disk contains the subsystem with the identifier MEMBERS_SUBSYSTEM.
$ MOUNT /SUBSYSTEM /SYSTEM DUA0: DOC WORK8 |
You can turn the processing of Subsystem ACEs on and off dynamically with the DCL command SET VOLUME /SUBSYSTEM. This command is especially useful for the system disk, which is not mounted using the MOUNT command.
Any person mounting a subsystem is responsible for knowing what is on
the volume being mounted. Without this knowledge, an operator or system
manager can inadvertently subvert system security. For example, it is
easy for a user with privileges on one cluster to put an application
holding a subsystem identifier on a volume and then take the volume to
a naive operator on another cluster and request that it be mounted.
Because the application holds an appropriate subsystem identifier, it
feigns membership in a subsystem for which it is unauthorized.
Therefore, mount volumes of only those users whom you trust, or
thoroughly search a volume for Subsystem ACEs before you mount it with
subsystems enabled.
13.8 Giving Users Access
All users with execute access to the main application image of the subsystem can use the data files and other objects under control of the subsystem if the subsystem allows the access. However, managers of the subsystem can restrict access to objects of the subsystem in the following ways:
(ID=MEMBERS_SUBSYSTEM+MEMBERS_ADMIN, ACCESS=READ+WRITE) |
Remember that as long as users are executing the application image for
the subsystem, their process rights list contains the subsystem
identifier as well as their normal identifiers. However, as soon as
users interrupt or exit from the application, their process rights list
loses the subsystem identifier, and they lose access rights to the
objects in the subsystem. Subsystem identifiers are not propagated by
default when subprocesses are spawned.
13.9 Example of a Protected Subsystem
R. D. Taylor Inc., a company specializing in building supplies, decides to set up a protected subsystem for its purchasing and accounts payable departments. Although the departments are in different parts of the company, they share a common database for recording purchases from suppliers.
When the company's inventory drops below the desired level, the purchasing department is directed to order required supplies. Purchasing personnel find suppliers (if necessary), assign purchase order numbers, and issue a purchase orders.
When the goods arrive, the receiving and quality control departments check the contents against what was ordered, ensure the goods meet quality standards, and put the goods into inventory. Once the shipment is processed, the information goes to the accounts payable department, which settles the invoices.
Administrators in the accounts payable department check the invoices against purchase orders and run a payments program to calculate the monies due to suppliers each week. Payments are recorded in a database, and checks are printed on a printer loaded with company checks.
Using the subsystem lets the company meet two objectives:
The company appoints one employee, McGrey, to design and manage the subsystem. Figure 13-2 illustrates the directory structure of the Taylor subsystem, and Example 13-1 shows the command procedure McGrey wrote to implement it.
Figure 13-2 Directory Structure of the Taylor Company's Subsystem
McGrey implements a directory structure in which users can gain access to the subsystem only by holding an appropriate identifier: purchasing personnel hold the identifier SUPPLIERS_ORDERS, and the accounts payable administrators hold the identifier ACCOUNTS_PAYABLE. As subsystem manager, McGrey holds the identifier SUPPLIERS_SUBSYSTEM.
The top-level directory SUPPLIERS_SUBSYSTEM.DIR has the following protection:
$ DIRECTORY/SECURITY SYS$SYSDEVICE:[000000]SUPPLIERS_SUBSYSTEM.DIR |
Directory SYS$SYSDEVICE:[000000] SUPPLIERS_SUBSYSTEM.DIR;1 SUPPLIERS_SUBSYSTEM (RWE,RWE,,) (1) (CREATOR,ACCESS=NONE) (2) (DEFAULT_PROTECTION,SYSTEM:RWED,OWNER:RWED,GROUP:,WORLD:) (3) (IDENTIFIER=SUPPLIERS_SUBSYSTEM,ACCESS=READ+WRITE+CONTROL) (4) (IDENTIFIER=SUPPLIERS_ORDERS,ACCESS=EXECUTE) (5) (IDENTIFIER=ACCOUNTS_PAYABLE,ACCESS=EXECUTE) (6) (IDENTIFIER=*,ACCESS=NONE) (7) (IDENTIFIER=SUPPLIERS_SUBSYSTEM,OPTIONS=DEFAULT,ACCESS=READ+WRITE+CONTROL) (8) (IDENTIFIER=SUPPLIERS_ORDERS,OPTIONS=DEFAULT,ACCESS=EXECUTE) (IDENTIFIER=ACCOUNTS_PAYABLE,OPTIONS=DEFAULT,ACCESS=EXECUTE) (IDENTIFIER=*,OPTIONS=DEFAULT,ACCESS=NONE) Total of 1 file. |
The directory EXE.DIR has the same protection as the top-level directory because subsystem users need to access the subsystem images: ORDERS.EXE and PAYMENTS.EXE. The other directory, LIB.DIR, is more restricted because only the subsystem images and McGrey need access.
$ DIRECTORY/SECURITY SYS$SYSDEVICE:[SUPPLIERS_SUBSYSTEM...] |
Directory SYS$SYSDEVICE:[SUPPLIERS_SUBSYSTEM] EXE.DIR;1 SUPPLIERS_SUBSYSTEM (RWE,RWE,,) (1) (CREATOR,ACCESS=NONE) (DEFAULT_PROTECTION,SYSTEM:RWED,OWNER:RWED,GROUP:,WORLD:) (IDENTIFIER=SUPPLIERS_SUBSYSTEM,ACCESS=READ+WRITE+CONTROL) (IDENTIFIER=SUPPLIERS_ORDERS,ACCESS=EXECUTE) (IDENTIFIER=ACCOUNTS_PAYABLE,ACCESS=EXECUTE) (IDENTIFIER=*,ACCESS=NONE) (IDENTIFIER=SUPPLIERS_SUBSYSTEM,OPTIONS=DEFAULT,ACCESS=READ+WRITE+CONTROL) (IDENTIFIER=SUPPLIERS_ORDERS,OPTIONS=DEFAULT,ACCESS=EXECUTE) (IDENTIFIER=ACCOUNTS_PAYABLE,OPTIONS=DEFAULT,ACCESS=EXECUTE) (IDENTIFIER=*,OPTIONS=DEFAULT,ACCESS=NONE) LIB.DIR;1 SUPPLIERS_SUBSYSTEM (RWE,RWE,,) (2) (CREATOR,ACCESS=NONE) (DEFAULT_PROTECTION,SYSTEM:RWED,OWNER:RWED,GROUP:,WORLD:) (IDENTIFIER=SUPPLIERS_SUBSYSTEM,ACCESS=READ+WRITE+CONTROL) (IDENTIFIER=*,ACCESS=NONE) (IDENTIFIER=SUPPLIERS_SUBSYSTEM,OPTIONS=DEFAULT,ACCESS=READ+WRITE+CONTROL) (IDENTIFIER=*,OPTIONS=DEFAULT,ACCESS=NONE) Total of 2 files. . . . |
As the following listing shows, the necessary company personnel can access the subsystem's images, ORDERS.EXE and PAYMENTS.EXE, but only the images can update the data files:
Directory SYS$SYSDEVICE:[SUPPLIERS_SUBSYSTEM.EXE] ORDERS.EXE;1 SUPPLIERS_SUBSYSTEM (RWED,RWED,,) (1) (SUBSYSTEM,IDENTIFIER=SUPPLIERS_SUBSYSTEM,ATTRIBUTES=RESOURCE) (IDENTIFIER=SUPPLIERS_SUBSYSTEM,ACCESS=READ+WRITE+CONTROL) (IDENTIFIER=SUPPLIERS_ORDERS,ACCESS=EXECUTE) (IDENTIFIER=ACCOUNTS_PAYABLE,ACCESS=EXECUTE) (IDENTIFIER=*,ACCESS=NONE) PAYMENTS.EXE;1 SUPPLIERS_SUBSYSTEM (RWED,RWED,,) (2) (SUBSYSTEM,IDENTIFIER=SUPPLIERS_SUBSYSTEM,ATTRIBUTES=RESOURCE) (IDENTIFIER=SUPPLIERS_SUBSYSTEM,ACCESS=READ+WRITE+CONTROL) (IDENTIFIER=ACCOUNTS_PAYABLE,ACCESS=EXECUTE) (IDENTIFIER=*,ACCESS=NONE) Total of 2 files. Directory SYS$SYSDEVICE:[SUPPLIERS_SUBSYSTEM.LIB] (3) ORDERS.DAT;1 SUPPLIERS_SUBSYSTEM (RWED,RWED,,) (IDENTIFIER=SUPPLIERS_SUBSYSTEM,ACCESS=READ+WRITE) (IDENTIFIER=*,ACCESS=NONE) PAYMENTS.DAT;1 SUPPLIERS_SUBSYSTEM (RWED,RWED,,) (IDENTIFIER=SUPPLIERS_SUBSYSTEM,ACCESS=READ+WRITE) (IDENTIFIER=*,ACCESS=NONE) Total of 2 files. Grand total of 3 directories, 6 files. |
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