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Modern Operating Systems by Herbert Bos and Andrew S. Tanenb...
Modern_Operating_Systems_by_Herbert_Bos_and_Andrew_S._Tanenbaum_4th_Ed.pdf
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Modern Operating Systems by Herbert Bos and Andrew...
Modern_Operating_Systems_by_Herbert_Bos_and_Andrew_S._Tanenbaum_4th_Ed.pdf-M ODERN O PERATING S YSTEMS
Modern Operating Systems by Herbert...
Modern_Operating_Systems_by_Herbert_Bos_and_Andrew_S._Tanenbaum_4th_Ed.pdf-M ODERN O PERATING S YSTEMS
Page 703
672
SECURITY
CHAP. 9
Source Code Viruses
Parasitic and boot sector viruses are highly platform specific; document viruses
are somewhat less so (
Word
runs on Windows and Macs, but not on UNIX).
The
most portable viruses of all are
source code viruses
.
Imagine the virus of
Fig. 9-28, but with the modification that instead of looking for binary executable
files, it looks for C programs, a change of only 1 line (the call to
access
). The
infect
procedure should be changed to insert the line
#include <virus.h>
at the top of each C source program. One other insertion is needed, the line
run
virus( );
to activate the virus. Deciding where to put this line requires some ability to parse
C code, since it must be at a place that syntactically allows procedure calls and also
not at a place where the code would be dead (e.g., following a
return
statement).
Putting it in the middle of a comment does not work either, and putting it inside a
loop might be too much of a good thing. Assuming the call can be placed properly
(e.g., just before the end of
main
or before the
return
statement if there is one),
when the program is compiled, it now contains the virus, taken from
virus.h
(al-
though
proj.h
might attract less attention should somebody see it).
When the program runs, the virus will be called. The virus can do anything it
wants to, for example, look for other C programs to infect.
If it finds one, it can in-
clude just the two lines given above, but this will work only on the local machine,
where
virus.h
is assumed to be installed already.
To have this work on a remote
machine, the full source code of the virus must be included. This can be done by
including the source code of the virus as an initialized character string, preferably
as a list of 32-bit hexadecimal integers to prevent anyone from figuring out what it
does. This string will probably be fairly long, but with today’s multimegaline code,
it might easily slip by.
To the uninitiated reader, all of these ways may look fairly complicated. One
can legitimately wonder if they could be made to work in practice.
They can be.
Believe us. Virgil is an excellent programmer and has a lot of free time on his
hands. Check your local newspaper for proof.
How Viruses Spread
There are several scenarios for distribution. Let us start with the classical one.
Virgil writes his virus, inserts it into some program he has written (or stolen), and
starts distributing the program, for example, by putting it on a shareware Website.
Eventually, somebody downloads the program and runs it.
At this point there are
several options.
To start with, the virus probably infects more files on the disk, just
in case the victim decides to share some of these with a friend later.
It can also try
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