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(a)Explain what is meant
by the terms ‘operating system kernel ’and ‘security
kernel ’.
[2 marks ]
The
kernel of an operating system performs the lowest-level functions,
such as interprocess communication and interrupt handling (1 mark).
A security kernel is responsible for implementing the security mechanisms
for the entire operating system; it is typically a part of the operating
system kernel (1 mark). [2 marks]
(b)Describe three features
that distinguish a security kernel from a distributed collection of
security mechanisms,and explain the benefits that derive from these
features.
[6 marks ]
Features
of a security kernel:
- isolates
security mechanisms from user space (makes protection from penetration
easier);
- groups security
mechanisms together (makes changes easier to perform and to test);
- contains
only security mechanisms (makes it compact, and so easier to verify);
- single point
of entry (simplifies guaranteeing total coverage).
(1
mark for a description, 1 mark for its benefit, for up to three features.)
[6
marks]
(c)Describe two possible
disadvantages of security kernels.[2 marks ]
An
overhead on every operation may degrade system performance (1 mark),
lack of modularity in a heterogeneous distributed system, as the security
features are spread across several systems (1 mark). [2
marks]
(d)Give five reasons
why simply deleting a file on a PC does not prevent that file from
being recovered.[5 marks ]
Deleting
a file on disk usually consists simply of marking its location as
free for subsequent use; that location must be overwritten with new
data (1 mark). Backup copies may have been kept, for example
on magnetic tape or removable hard disk; these too must be destroyed
(1 mark). Temporary copies may remain in computer memory or
disk caches, which should also be wiped (1 mark). Even printer
ribbons or laser-printer drums may reveal what has been printed (1
mark). And of course, any paper copies must be carefully destroyed.
(1 mark). [5 marks]
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