CS 6238 Midterm Exam Questions
With Verified Answers
Reference Monitor - ANSWER Enforces an access control policy over a subjects' ability
to perform operations on objects on a system
TCB Requirements - ANSWER Tamper-proof: cannot alter
Complete Mediation: cannot be bypassed
Correctness: no vulnerabilities
Reflections on Trusting Trust - ANSWER Cannot trust software system unless the
people behind the system and entire tool chain can be trusted
Ware Report - ANSWER Created to recommend safeguards that would protect
information in multiaccess resource sharing systems
Economy of Mechanism - ANSWER Design a system to use only a small number of
different types of security enforcement mechanisms. More features there are more ways
to attack the system
Least Privilege - ANSWER subjects should be granted the minimum level of access
needed
3 principals of security + 1 - ANSWER Security should be transparent
It should be easy to give access
It should be easy to restrict access
Establish reasonable defaults
Cyber Risk = - ANSWER Attack likelihood* attack impact
, Design Principles - ANSWER Economy of Mechanism
Open Design
Least Privilege
Separation of Privilege
Fail-safe default
User Acceptability
Layered Defense
Hardware provides two mechanisms for isolating TCB and apps - ANSWER 1)
Processor execution modes (privilege levels)
2) Privileged Instructions
A processor does the following: - ANSWER Executes instructions by:
EIP holds pointer to next instruction
Operands point to data which is operated on by an instruction
Executes next instruction
Physical Address = - ANSWER (SGTBR (base reg.) + STE(segment table entryto locate
segment within reg) * STE size) + displacement
First half tells us address of segment table and displacement tells where to read
Descriptor Protection Level (DPL) - ANSWER Contained in each segment, contains
protection level information about the segment
Protection Levels - ANSWER Goes from 0 - 3 where 0 is more privileged. Specified by 2
bits
Current protection level (CPL) - ANSWER DPL of the code segment being executed
Logical Address - ANSWER Address a program actually specifies. It gets converted to
linear address which gets converted to physical address
Requestor Privilege Level (RPL) - ANSWER Stored in the segment selector and used to
stop privilege escalation attacks
Conforming Code Segment - ANSWER If you move to a more privileged code segment,
execution can continue. DPL represents lowest privilege level (RPL is not checked)
Non-conforming code segment - ANSWER Transfer to a different privilege segment
generates a protection fault unless call or task gate is used. CPL and DPL must be
equal
Page Protection Level (PPL) - ANSWER Can be either 0 or 1. 0 is privileged and 1 us
non-privileged. If CPL is 3 we can only access PPL of 1
With Verified Answers
Reference Monitor - ANSWER Enforces an access control policy over a subjects' ability
to perform operations on objects on a system
TCB Requirements - ANSWER Tamper-proof: cannot alter
Complete Mediation: cannot be bypassed
Correctness: no vulnerabilities
Reflections on Trusting Trust - ANSWER Cannot trust software system unless the
people behind the system and entire tool chain can be trusted
Ware Report - ANSWER Created to recommend safeguards that would protect
information in multiaccess resource sharing systems
Economy of Mechanism - ANSWER Design a system to use only a small number of
different types of security enforcement mechanisms. More features there are more ways
to attack the system
Least Privilege - ANSWER subjects should be granted the minimum level of access
needed
3 principals of security + 1 - ANSWER Security should be transparent
It should be easy to give access
It should be easy to restrict access
Establish reasonable defaults
Cyber Risk = - ANSWER Attack likelihood* attack impact
, Design Principles - ANSWER Economy of Mechanism
Open Design
Least Privilege
Separation of Privilege
Fail-safe default
User Acceptability
Layered Defense
Hardware provides two mechanisms for isolating TCB and apps - ANSWER 1)
Processor execution modes (privilege levels)
2) Privileged Instructions
A processor does the following: - ANSWER Executes instructions by:
EIP holds pointer to next instruction
Operands point to data which is operated on by an instruction
Executes next instruction
Physical Address = - ANSWER (SGTBR (base reg.) + STE(segment table entryto locate
segment within reg) * STE size) + displacement
First half tells us address of segment table and displacement tells where to read
Descriptor Protection Level (DPL) - ANSWER Contained in each segment, contains
protection level information about the segment
Protection Levels - ANSWER Goes from 0 - 3 where 0 is more privileged. Specified by 2
bits
Current protection level (CPL) - ANSWER DPL of the code segment being executed
Logical Address - ANSWER Address a program actually specifies. It gets converted to
linear address which gets converted to physical address
Requestor Privilege Level (RPL) - ANSWER Stored in the segment selector and used to
stop privilege escalation attacks
Conforming Code Segment - ANSWER If you move to a more privileged code segment,
execution can continue. DPL represents lowest privilege level (RPL is not checked)
Non-conforming code segment - ANSWER Transfer to a different privilege segment
generates a protection fault unless call or task gate is used. CPL and DPL must be
equal
Page Protection Level (PPL) - ANSWER Can be either 0 or 1. 0 is privileged and 1 us
non-privileged. If CPL is 3 we can only access PPL of 1
