SunSHIELD Basic Security Module Guide

Chapter 4 Device Allocation

The Trusted Computer System Evaluation Criteria's (TCSEC) object-reuse requirement for computing systems at C2 level and above is fulfilled by the device-allocation mechanism. This chapter describes what you need to know about managing devices.

You must decide whether any devices should be allocatable, and if so, which ones, if the defaults are not appropriate for your site's security policy.

Risks Associated With Device Use

For one example of the security risks associated with the use of various I/O devices, consider how cartridge devices are typically used. Often several users share a single tape drive, which can be located in an office or lab away from where an individual user's own machine is located. This means that, after the user loads a tape into the tape drive, some length of time can elapse before the user can return to the machine to invoke the command that reads or writes data to or from the tape. Then another time lapse occurs before the user is able to take the tape out of the drive. Because tape devices are typically accessible to all users, during the time when the tape is unattended, an unauthorized user can access or overwrite data on the tape. The device-allocation mechanism makes it possible to assign certain devices to one user at a time, so that the device can be accessed only by that user while it is assigned to that user's name.

The device-allocation mechanism ensures the following for tape devices and provides related security services for other allocatable devices:

Prevents simultaneous access to a device

Prevents a user from reading a tape just written to by another user, before the first user has removed the tape from the tape drive

Prevents a user from gleaning any information from the device's or the driver's internal storage after another user is finished with the device

Components of the Device-Allocation Mechanism

The components of the allocation mechanism that you must understand in order to manage device allocation are:

The allocate, deallocate, dminfo, and list_devices commands
The /etc/security/device_allocate file (see the device_allocate(4) man page)
The /etc/security/device_maps file (see the device_maps(4) man page)
The lock files that must exist for each allocatable device in /etc/security/dev
The changed attributes of the device-special files that are associated with each allocatable device
Device-clean scripts for each allocatable device

How any user invokes the allocate, deallocate, dminfo, and list_devices commands is described in "Using the Device-Allocation Utilities". All of the options and other descriptions are defined in the man pages.

The device_allocate file, the device_map file, and the lock files are specific to each machine. The configuration files are not administered as NIS databases because tape drives, diskette drives, and the printers are all connected to specific machines.

Using the Device-Allocation Utilities

This section describes what the administrator can do with the options to allocate, deallocate, and list_devices that are usable only by root. The commands are detailed on their respective man pages.

allocate -F device_special_filename: Reallocates the specified device. This option is often used with the -U option to reallocate the specified device to the specified user. Without the -U option, the device is allocated to root.
allocate -U username: Causes the device to be allocated to the user specified rather than to the current user. This option allows you to allocate a device for another user while you are root, without having to assume that user's identity.
deallocate -F device_special_filename: Devices that a user has allocated are not automatically deallocated when the process terminates or when the user logs out. When a user forgets to deallocate a tape drive, you can force deallocation using the -F option while you are root.
deallocate -I: Forces deallocation of all allocatable devices. This option should be used only at system initialization.
list_devices: Run list_devices to get a listing of all the device-special files that are associated with any device listed in the device_maps file.
list_devices -U username: List the devices that are allocatable or allocated to the user ID associated with the specified user name. This allows you to check which devices are allocatable or allocated to another user while you are root.

The Allocate Error State

The allocate error state is mentioned in the man pages for the allocate components. An allocatable device is in the allocate error state if it is owned by user bin and group bin with a device-special file mode of 0100. If a user wants to allocate a device that is in the allocate error state, you should try to force the deallocation of the device, using the deallocate command with the -F option, or use allocate -U to assign it to the user, then investigate any error messages that appear. When the problems with the device are corrected, you must rerun the deallocate -F or allocate -F commands to clear the allocate error state from the device.

The `device_maps` File

You can look at the /etc/security/device_maps file to determine the device names, device types, and device-special files that are associated with each allocatable device. See the device_maps(4) man page. Device maps are created by the system administrator when setting up device allocation. A rudimentary file is created by bsmconv when the BSM is enabled. This initial map file should be used only as a starting point. The system administrator is expected to augment and customize device_maps for the individual site.

This file defines the device-special file mappings for each device, which in many cases is not intuitive. This file allows various programs to discover which device-special files map to which devices. You can use the dminfo command, for example, to get the device name, the device type, and the device-special files to specify when setting up an allocatable device; dminfo uses the device_maps file.

Each device is represented by a one-line entry of the form:

device-name:device-type:device-list

Lines in the file can end with a \ to continue an entry on the next line. Comments can also be included. A # makes a comment of all further text until the next newline not immediately preceded by a \. Leading and trailing blanks are allowed in any of the fields.

device-name: The name of the device, for example st0, fd0, or audio. The device name specified here must correspond to the name of the lockfile used in the /etc/security/dev directory.
device-type: The generic device type (the name for the class of devices, such as st, fd, audio). The device-type logically groups related devices.
device-list: A list of the device-special files associated with the physical device. The device-list must contain all of the special files that allow access to a particular device. If the list is incomplete, a malevolent user can still obtain or modify private information. Also, as in the example below, either the real device files located under /devices or the symbolic links in /dev, provided for binary compatibility, are valid entries for the device-list field.

For an example of entries for SCSI tape st0 and diskette fd0 in a device_maps file, see the following screen.

fd0:\
	fd:\
	/dev/fd0 /dev/fd0a /dev/fd0b /dev/rfd0 /dev/rfd0a /dev/rfd0b:\
					.
					.
					.
st0:\
	st:\
	/dev/rst0 /dev/rst8 /dev/rst16 /dev/nrst0 /dev/nrst8 /dev/nrst16:\

The `device_allocate` File

Modify the device_allocate file to change devices from allocatable to non-allocatable or to add new devices. Table 4-1 shows a sample device_allocate file.

Table 4-1 Sample device_allocate File


st0;st;;;;/etc/security/lib/st_clean fd0;fd;;;;/etc/security/lib/fd_clean sr0;sr;;;;/etc/security/lib/sr_clean audio;audio;;;*;/etc/security/lib/audio_clean

The administrator defines which devices should be allocatable during initial configuration of the Basic Security Module. You can decide to accept the default devices and their defined characteristics, as shown in Table 4-1. Whenever you add a device to any machine after the system is up and running, you must decide whether to make the new device allocatable.

The entries for devices in the device_allocate file can be modified by the administrator after installation. Any device that needs to be allocated before use must be defined in the device_allocate file on each machine. Currently, cartridge tape drives, diskette drives, CD-ROM devices, and audio chips are considered allocatable and have device-clean scripts.

Note -

If you add a Xylogics^TM tape drive or an Archive tape drive, they can also use the st_clean script supplied for SCSI devices. Other devices that you can make allocatable are modems, terminals, graphics tablets, and the like, but you need to create your own device-clean scripts for such devices, and the script must fulfill object-reuse requirements for that type of device.

An entry in the device_allocate file does not mean the device is allocatable, unless the entry specifically states the device is allocatable. Notice in Table 4-1 an asterisk (*) in the fifth field of the audio device entry. An asterisk in the fifth field indicates to the system that the device is not allocatable; that is, the system administrator does not require a user to allocate the device before it is used nor to deallocate it afterward. Any other string placed in this field indicates that the device is allocatable.

In the device_allocate file, represent each device by a one-line entry of the form:

device-name;device-type;reserved;reserved;alloc;device-clean

For example, the following line shows the entry for device name st0:

st0;st;;;;;/etc/security/lib/st_clean

Lines in device_allocate can end with a \ to continue an entry on the next line. Comments can also be included. A # makes a comment of all further text until the next newline not immediately preceded by a \. Leading and trailing blanks are allowed in any of the fields.

The following paragraphs describe each field in the device_allocate file in detail.

device-name: Specifies the name of the device; for example, st0, fd0, or sr0. When making a new allocatable device, look up the device-name from the device-name field in the device_maps file or use the dminfo command. (The name is also the DAC file name for the device.)
device-type: Specifies the generic device type (the name for the class of devices, such as st, fd, and sr). This field groups related devices. When making a new allocatable device, look up the device-type from the device-type field in the device_maps file or use the dminfo command.
reserved: These fields are reserved for future use.
alloc: Specifies whether or not the device is allocatable. An asterisk (*) in this field indicates that the device is not allocatable. Any other string, or an empty field, indicates that the device is allocatable.
device-clean: Supplies the path name of a program to be invoked for special handling, such as cleanup and object-reuse protection during the allocation process. The device-clean program is run any time the device is acted on by deallocate, such as when a device is forcibly deallocated with deallocate -F.

Device-Clean Scripts

The device-clean scripts address the security requirement that all usable data is purged from a physical device before reuse. By default, cartridge tape drives, diskette drives, CD-ROM devices, and audio devices require device-clean scripts, which are provided. This section describes what the device-clean scripts do.

Object Reuse

Device allocation satisfies part of the object-reuse requirement. The device-clean scripts make sure that data left on a device by one user is cleared before the device is allocatable by another user.

Device-Clean Script for Tapes

The three supported tape devices and the device-clean script for each are shown in Table 4-2.

Table 4-2 Device-Clean Script for the Three Supported Tape Devices


Tape Device Type	Device-Clean Script
SCSI 1/4-inch tape	`st_clean`
Archive 1/4-inch tape	`st_clean`
Open-reel 1/2-inch tape	`st_clean`

The script uses the rewoffl option to mt to affect the device cleanup. See the mt(1) man page. If the script runs during system boot, it queries the device to see if the device is online and has media in it. The 1/4-inch tape devices that have media remaining, are placed in the allocate error state to force the administrator to clean up the device manually.

During normal system operation, when allocate or deallocate is executed in the interactive mode, the user is prompted to remove the media from the device being deallocated. The script pauses until the media is removed from the device.

Device-Clean Scripts for Diskettes and CD-ROM Devices

The device-clean scripts for the diskettes and CD-ROM devices are shown in Table 4-3.

Table 4-3 Device-Clean Scripts for the Diskette and CD-ROM Device


Disk Device Type	Device-Clean Script
diskette	`fd_clean`
CD-ROM	`sr_clean`

The scripts use the eject command to remove the media from the drive. See the eject(1) man page. If eject fails, the device is placed in the allocate error state.

Device-Clean Script for Audio

The audio device is cleaned up with an audio-clean script. The script performs an AUDIO_DRAIN ioctl system call to flush the device, then an AUDIO_SETINFO ioctl system call to reset the device configuration to default. In addition, the script retrieves the audio chip registers using the AUDIOGETREG ioctl system call. Any registers deviating from default are reset using the AUDIOSETREG ioctl system call.

Writing New Device-Clean Scripts

If you add more allocatable devices to the system, you might need to create your own device-clean scripts. The deallocate command passes a parameter to the device-clean scripts. The parameter, shown here, is a string that contains the device name (see the device_allocate(4) man page):

st_clean -[I|F|S] device-name

Device-clean scripts must return 0 for success and greater than 0 for failure. The options -I, -F, and -S help the script determine its running mode.

-I is needed during system boot only. All output must go to the system console. Failure or inability to forcibly eject the media must put the device in the allocate error state.

-F is for forced cleanup. This option is interactive and assumes that the user is available to respond to prompts. A script with this option must attempt to complete the cleanup if one part of the cleanup fails.

-S is for standard cleanup. This option is interactive and assumes that the user is available to respond to prompts.

Setting Up Lock Files

The lock files are zero-length files created in /etc/security/dev -- one for each allocatable device. If no lock file exists for an allocatable device, the device cannot be allocated, and no one can access the device.

How to Set Up Lock Files for a Device to Be Made Allocatable

Use the dminfo command to get the device name for the device from its entry in the device_maps file.

See "The device_maps File" and the dminfo(1M) and device_maps(4) man pages. For example, the device name for device type st is st0. Use the device name as the name of the lock file.

Use the touch command to create an empty lock file for the device, using the device name.

untouchable# cd /etc/security/dev
untouchable# touch device-name
untouchable# chmod 600 device-name
untouchable# chown bin device-name
untouchable# chgrp bin device-name

How the Allocate Mechanism Works

This section gives an example of how the allocate mechanism works.

The allocate command first checks for the presence of a lock file under the device name for the specified device in the /etc/security/dev directory. If the file is owned by allocate, then the ownership of the lock file is changed to the name of the user entering the allocate command.

The allocate command then checks for an entry for the device in the device_allocate file, and checks whether the entry shows the device as allocatable.

The first listing in the screen example below shows that a lock file exists with owner bin, group bin, and mode 600 for the st0 device in /etc/security/dev. The second listing shows that the associated device-special files are set up properly, with owner bin, group bin, and mode 000:

untouchable% ls -lg /etc/security/dev/st0
-rw------- 1 bin bin      		      0 Dec 6 15:21 /etc/security/dev/st0
untouchable% ls -lg /devices/sbus@1,f8000000/esp@0,800000
c--------- 1 bin bin		       18,  4 May 12 13:11 st@4,0:
c--------- 1 bin bin	       18, 20 May 12 13:11 st@4,0:b
c--------- 1 bin bin	       18, 28 May 12 13:11 st@4,0:bn
c--------- 1 bin bin	       18, 12 May 12 13:11 st@4,0:c
					 .
					 .
					 .
c--------- 1 bin bin	       18,  0 May 12 13:11 st@4,0:u
c--------- 1 bin bin	       18, 16 May 12 13:11 st@4,0:ub
c--------- 1 bin bin	       18, 24 May 12 13:11 st@4,0:ubn
c--------- 1 bin bin	       18,  8 May 12 13:11 st@4,0:un

In this screen, user vanessa allocates device st0.

untouchable% whoami
vanessa
untouchable% allocate st0

When the user vanessa enters the allocate command to allocate the tape st0, allocate first checks for the existence of an /etc/security/dev/st0 file. If no lock file exists or if the lock file is owned by a user other than allocate, then vanessa could not allocate the device.

If it finds the lock file for the device with the correct ownership and permissions, the allocate command then checks to make sure the device has an entry in the device_allocate file and that the entry specifies that the device is allocatable.

In this example, the default device_allocate entry for the st0 device specifies that the device is allocatable. Because the allocate command finds that all the above conditions are met, the device is allocated to vanessa.

The allocate command changes the ownership and permissions of the device-special files associated with the device in the /dev directory. To allocate the st0 device to vanessa, the mode on its associated device-special files is changed to 600 and the owner is changed to vanessa.

The allocate command also changes the ownership of the lock file associated with the device in the /etc/security/dev directory. To allocate the st0 device to vanessa, the owner of /etc/security/dev/st0 is changed to vanessa.

After the user vanessa executes the allocate command using the device name st0, the following screen example shows that the owner of /etc/security/dev is changed to vanessa and that the owner of the associated device-special files is now vanessa as well, and that vanessa now has permission to read and write the files.

untouchable% whoami
vanessa
untouchable% allocate st0
untouchable% ls -lg /etc/security/dev/st0
-rw------- 1 vanessa staff 		      0 Dec 6 15:21 /etc/security/dev/st0
untouchable% ls -la /devices/sbus@1,f8000000/esp@0,800000
.
.
.
crw------- 1 vanessa 18,  4 May 12 13:11 st@4,0:
crw------- 1 vanessa 18, 12 May 12 13:11 st@4,0:b
crw------- 1 vanessa 18, 12 May 12 13:11 st@4,0:bn
crw------- 1 vanessa 18, 12 May 12 13:11 st@4,0:c
.
.
.
crw------- 1 vanessa 18,  4 May 12 13:11 st@4,0:u
crw------- 1 vanessa 18, 12 May 12 13:11 st@4,0:ub
crw------- 1 vanessa 18, 12 May 12 13:11 st@4,0:ubn
crw------- 1 vanessa 18, 12 May 12 13:11 st@4,0:un

Managing and Adding Devices

The procedures in this section show how to manage devices and how to add devices.

How to Manage Devices

Determine which devices are listed in the device_allocate file and which devices can be made allocatable.

Define which devices, if any, should be made allocatable.

Decide which normal users, if any, should be allowed to allocate devices.

Edit the device_allocate file and add the new device.

How to Add a New Allocatable Device

Create an entry for any new allocatable device on the machine in the device_allocate file.

This procedure is described in "The device_allocate File".

Create an empty lock file for each allocatable device in the /etc/security/dev directory.

This procedure is described in "Setting Up Lock Files".

Create a device-clean script, if needed, for each new device.

If you add a Xylogics or an Archive tape drive, you can use the st_clean script; otherwise, create your own. How to create a device-handling script is described in "Device-Clean Scripts".

Make all device-special files for the device to be owned by user bin, group bin, and mode 000.

You can run the dminfo command to get a listing from the device_maps file of all the device-special files that are associated with the device you are making allocatable.

Using Device Allocations

The procedures and commands in this section show how to manage devices and how to add devices. The device-allocation and device-deallocation commands are entered from the command line in a Command Tool or Shell Tool window:

allocate assigns a device to a user.

You can specify the device in either of the two ways shown in Table 4-4.

Table 4-4 Device-Specification Options for allocate


Option	Action
`device-name`	Allocate the device that matches the device name
`-g` `device-type`	Allocate the device that matches the device group type

deallocate releases a previously allocated device.

list_devices enables you to see a list of all allocatable devices, devices currently allocated, and allocatable devices not currently allocated.

The list_devices command requires one of the three options shown in Table 4-5.

Table 4-5 Options for the list_devices Command


Option	Action
`-l`	List all allocatable devices or information about the device.
`-n`	List devices not currently allocated or information about the device.
`-u`	List devices currently allocated or information about the device.

How to Allocate a Device

Use the allocate command with a device specified by name, as in the example, or by type, with -g switch.

sar1% allocate st0

If the command cannot allocate the device, an error message displays in the console window. A list of all error messages appears in the allocate(1M) man page.

How to Deallocate a Device

Deallocate a tape drive by using the deallocate command followed by the device file name.

sar1% deallocate st0

Deallocation allows other users to allocate the device when you are finished.