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 may 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 may 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 only be accessed 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