Computer and Network Security Lecture 4 Richard Newman

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Computer and Network Security Lecture 4
Richard Newman

• Covert channels
• Broad Definition
• Mechanism by which a process may transmit data
  to another process in unconventional and (
  pretty much ) non-obvious ways.
• Narrow Definition
• In multi level secure system, a non-conventional
  (not usually monitored/mediated by the reference
  monitor) way for a high-level process to
  communicate to a low-level process.
• Classification
• Timing channels (modulation of resource
  utilization)
• Storage channels (alteration state of system
  variables)

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Computer and Network Security Lecture 4
Richard Newman

• Example of a Timing channel
• Two processes share a resource ( here CPU ) to
  communicate
• Long interval - 1 Short interval - 0
• CPU usage Hi uses full time quantum if 1 or
  voluntarily releases CPU if 0

Normal CPU schedule
Hi
Lo
Hi
Hi
Lo
Lo
Hi
Hi
Lo
Hi communicating to Lo by releasing its time
quantum earlier for a 0 and using its full time
quantum for 1
Hi
Hi
Hi
Lo
Lo
Lo
Lo
Lo
Hi
Hi
1
1
0
0
Start t1
End t2
t3
t4
t2- t1 gt q1 gt 1
t4 - t3 lt q0 gt 0
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Computer and Network Security Lecture 4
Richard Newman

• Example of a Storage channel
• Fill table FULL
• Hi fills FT -----------gt FT 1
• Hi opens a space in FT -------gt FT 0
• Lo tried to create a new file
• if FAIL -------gt 1
• if SUCCED ---gt 0 and delete file
• Semaphores, locks can also be used

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Computer and Network Security Lecture 4
Richard Newman

• Controls
• Find covert channel ( using shared resource
  matrix , examining using compiler ), estimate
  capacity, handle them
• Fixed allocation for low/medium/high - partition
  resources
• Reallocate the quantum for low
• Purposely introduce noise
• Change the allocation
• Mode based security
• Do not allow Hi to change modes in any fashion.
  Allow Hi to change mode only step by step.
• If there are P partition values, then there
  can be log 2 P bits/symbol
• If only F mode changes /second is allowed then
  only F(log 2 P) bits/sec can be conveyed.

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Computer and Network Security Lecture 4
Richard Newman

• Identifying Potential covert channels using
  Shared Resource Matrix
• Construct a Shared Resource Matrix for the
  concerned process

Service Process Spys Process Data file A
R,M R,M Confidential data
R

• Check for pattern like the one shown
  below.
• If such a pattern exists then the service
  process can pass information to the spys
  process.
• In the above e.g. the service process can
  read confidential data and write that in file A
  which can be
• read by the spys process.

M
R
R
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Computer and Network Security Lecture 4
Richard Newman

• Controls against program threats
• Programming controls
• Description of programming task
• Peer reviews
• Modularity,encapsulation and information hiding
• Independent testing
• Configuration management
• Proofs of program correctness
• Process improvement

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Computer and Network Security Lecture 4
Richard Newman

• Controls against program threats continuation...
  
• Operating system controls
• Trusted software
• Mutual suspicion
• Confinement
• Access log
• Administrative controls
• Enforcement of standards
• Separation of duty
• Physical and timing separation
• Other controls
• Fire walls
• Cryptography