1. CS-3013 Operating Systems Hugh C. Lauer
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CS-3013 Operating Systems Hugh C. Lauer
(Slides include materials from Slides include materials from Modern Operating Systems, 3rd ed., by Andrew Tanenbaum and from Operating System Concepts, 7th ed., by Silbershatz, Galvin, & Gagne)
CS-3013, C-Term 2012
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2. Potpourri Tidbits about protection and security The first virus
Android is based on Linux, but Android is not Linux
CS-3013, C-Term 2012
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3. Protection
Mechanisms built into operating systems, networks, and other environments to prevent erroneous or malicious programs from
doing stuff that they shouldn’t do
accessing information they shouldn’t access
causing harm, inconvenience, etc.
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4. Security
Policies, guidelines, rules about who can access what information or take what actions
E.g., who is allowed to see/know your
grades
health records
salary
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5. Linux File Protection Bits
We are already familiar with rwx bits of Linux/Unix file protection
User (i.e., owner), group, other
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6. Windows File Protection
A bit more complex
Different users and groups can be assigned different privileges.
Need a unifying model
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7. File Protections
File systems are designed so that processes cannot bypass the protect mechanisms
Lots of testing and verification to establish this
Years (and years) of experience
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8. Discussion
What other things does an operating system protect against? How?
Isolation of processes and VMs
System calls
Devices …
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9. Protection and Security
Chapter 9 in Tanenbaum
CS-4401, Software Security Engineering
CS-44xx, Network Security
Proposed for next year
M.Sc. Specialization in Security
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10. Questions? Comments?
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11. The First Computer Virus
Reading assignment:–
Ken Thompson, “Reflections on Trusting Trust,” Communications of ACM, vol.27, #8, August 1984, pp (pdf)
Three steps
Program that prints a copy of itself
Training a compiler to understand a constant
Embedding a Trojan Horse without a trace
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12. Step 1 – Program to print copy of itself
How do we do this?
First, store character array representing text of program
Body of program
Print declaration of character array
Loop through array, printing each character
Print entry array as a string
Result: general method for program to reproduce itself to any destination!
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13. Step 2 – Teaching constant values to compiler…
/* reading string constants */
if (s[i++] == '\\')
if (s[i] == 'n') insert ('\n');
elseif (s[i] == 'v') insert ('\v');
elseif …
Question: How does compiler know what integer values to insert for '\n', '\v', etc.?
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14. Step 2 (continued)
Answer: In the first compiler ever written, insert the actual character code
i.e., 11 (decimal) for ‘\v’, etc.
/* reading string constants */
if (s[i++] == '\\')
if (s[i] == 'n') insert ('\n');
elseif (s[i] == 'v') insert (11);
elseif …
Next: Use the first compiler to compile itself!
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15. Step 2 (continued)
Result: a new compiler that “knows” how to interpret the sequence “\v”, “\n”, etc,
And all compilers derived from this one, forever after!
Finally: replace the value “11” in the source code of the compiler with ‘\v’ and compile itself again
Note: no trace of values of special characters in …
The C Programming Language book
source code of C compiler
I.e., special character values are self-reproducing
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16. Step 3 – Inserting a Trojan Horse
In compiler source, add the text
if (match(sourceString, pattern) insert the Trojan Horse code
where “pattern” is the login code (for example)
In compiler source, add additional text
if (match(sourceString2, pattern2) insert the self-reproducing code
where “pattern2” is a part of the compiler itself
Use this compiler to recompile itself, then remove source
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17. Step 3 – Concluded Result: an infected compiler that will
Insert a Trojan Horse in the login code of any Unix system
Propagate itself to all future compilers
Leave no trace of Trojan Horse in its source code
Like a biological virus:
A small bundle of code that uses the compiler’s own reproductive mechanism to propagate itself
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18. Questions?
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19. Android & Linux Android is based on Linux, but Android is not Linux
Cannot run a Linux application on Android
Cannot run an Android app on Linux
Kernels have a lot of code in common
Libraries have almost nothing in common
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20. Differences Fundamental operation Hardware platform
Mobile and connected vs. stationary
Hardware platform
Devices and interfaces
Security requirements
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21. Fundamentally Different Security Requirements
Linux:–
Protect the system from errant processes
Keep separate processes out of each others’ way
Android:–
Protect user’s information from rogue apps
Manage access to/from network
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22. Android Every app is a separate “user” to kernel
App’s files readable/writeable only by the app-user
Communication among apps via controlled channels
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23. Android (continued) Every app runs in own virtual machine
Dalvik VM:– programmable only in Java
Permissions to access other resources from (human) user
At installation time
System can stop an app at any time to recover memory
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24. Summary Less flexibility in development environment on iOS/Android
iOS and Android require tighter memory management/more controlled access to devices
Mobile app development targeted for a more specific and known set of devices
Application distribution has tighter regulation on iOS and Android
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25. Discussion
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