Podcast
Questions and Answers
What does the possession of values by a user process indicate?
What does the possession of values by a user process indicate?
- The access may not be legitimate. (correct)
- The user has all necessary permissions.
- The access is guaranteed to be secure.
- The access is always legitimate.
The possession of values by a user process guarantees legitimate access.
The possession of values by a user process guarantees legitimate access.
False (B)
What is the implication of a user program executing a statement successfully?
What is the implication of a user program executing a statement successfully?
It indicates that the user program has access but does not confirm the legitimacy of that access.
The user process's possession of access values says nothing about whether that access would actually be __________.
The user process's possession of access values says nothing about whether that access would actually be __________.
Match the following concepts with their descriptions:
Match the following concepts with their descriptions:
What is the primary advantage of using back-pointers for revoking capabilities?
What is the primary advantage of using back-pointers for revoking capabilities?
Reacquisition involves periodically deleting capabilities to reduce the number of active capabilities.
Reacquisition involves periodically deleting capabilities to reduce the number of active capabilities.
What happens to capabilities when a master key is changed?
What happens to capabilities when a master key is changed?
The method of __________ allows capabilities to be revoked by deleting an intermediate table entry.
The method of __________ allows capabilities to be revoked by deleting an intermediate table entry.
Which method involves using unique bits for each capability?
Which method involves using unique bits for each capability?
Match the revocation method with its description.
Match the revocation method with its description.
Distributed tables provide a way to centrally modify access and capability information across multiple locations.
Distributed tables provide a way to centrally modify access and capability information across multiple locations.
What fundamental challenge is associated with the use of back-pointers in capability revocation?
What fundamental challenge is associated with the use of back-pointers in capability revocation?
In the __________ method, all capabilities are invalidated when the master key is altered.
In the __________ method, all capabilities are invalidated when the master key is altered.
What happens if a word's high-order bit is 1?
What happens if a word's high-order bit is 1?
A word with a high-order bit of 0 is treated as an address.
A word with a high-order bit of 0 is treated as an address.
What is the outcome of following pointers when a high-order bit is 1?
What is the outcome of following pointers when a high-order bit is 1?
If the high-order bit is 1, the word is treated as an ______.
If the high-order bit is 1, the word is treated as an ______.
Match the terms with their meanings:
Match the terms with their meanings:
What defines a word's classification as data?
What defines a word's classification as data?
The process stops when a word with a high-order bit of 1 is reached.
The process stops when a word with a high-order bit of 1 is reached.
What does a high-order bit of 1 indicate when processing a word?
What does a high-order bit of 1 indicate when processing a word?
Following pointers stops at a word whose high-order bit is ______.
Following pointers stops at a word whose high-order bit is ______.
What is the significance of the high-order bit in this context?
What is the significance of the high-order bit in this context?
What is the primary purpose of creating an alias in the context described?
What is the primary purpose of creating an alias in the context described?
An alias allows an object to be permanently accessed without further permissions.
An alias allows an object to be permanently accessed without further permissions.
Who creates the alias for the object?
Who creates the alias for the object?
To allow B to access the object, A creates an ______ for the object.
To allow B to access the object, A creates an ______ for the object.
Match the following terms with their descriptions:
Match the following terms with their descriptions:
What action does A take to grant B access?
What action does A take to grant B access?
B can access the object directly without the alias created by A.
B can access the object directly without the alias created by A.
What does A provide to B after creating the alias?
What does A provide to B after creating the alias?
The alias allows B to access the object ______ the rights granted by A.
The alias allows B to access the object ______ the rights granted by A.
Which of the following best describes a capability in this context?
Which of the following best describes a capability in this context?
The process described involves giving away ownership of an object.
The process described involves giving away ownership of an object.
What is the main purpose of using an alias in capability management?
What is the main purpose of using an alias in capability management?
Indirection allows a capability to contain the actual address of an object.
Indirection allows a capability to contain the actual address of an object.
Describe the role of the high-order bit in indirect addressing.
Describe the role of the high-order bit in indirect addressing.
The process of __________ is used to access objects through an indirect reference.
The process of __________ is used to access objects through an indirect reference.
Which of the following best describes a capability?
Which of the following best describes a capability?
Match the following terms with their definitions:
Match the following terms with their definitions:
A capability must always point directly to the memory address of an object.
A capability must always point directly to the memory address of an object.
Why might a system implement indirection for capabilities?
Why might a system implement indirection for capabilities?
In the example of capability revocation, __________ revokes access by invalidating an alias.
In the example of capability revocation, __________ revokes access by invalidating an alias.
What happens when an alias is invalidated?
What happens when an alias is invalidated?
Study Notes
Revocation of Access Rights
-
Access List
- Delete access rights from an access list
- Simple to remove entries, by searching and deleting
- Can be immediate, general or selective, total or partial, permanent or temporary
-
Capability List
- Requires a scheme to locate the capability within the system before it can be revoked
- Reacquisition
- Periodically deletes capabilities
- Requires denial and reacquisition if a capability is revoked
- Back-pointers
- Set of pointers from each object to all capabilities of that object (Multics)
- Maintains a synchronized and updated set of pointers
- Indirection
- Capability points to a global table entry, which points to the object
- Deletes the entry from the global table.
- Not selective (CAL)
- Keys
- Unique bits associated with a capability, generated when the capability is created
- Master key associated with each object, and the key matches the master key for access
- Revocation: Create a new master key
- Requires a policy decision on who can create and modify the keys (object owner or others)
- Distributed Tables
- Centralized tables containing access information and capabilities
- Can be modified centrally for revocation
Example of Capability Revocation (Indirection)
- Indirection - using an indirect reference to access objects.
- A capability does not directly point to the object, but to an intermediate level, such as an address or alias.
- Can be used to revoke access easily by invalidating the alias.
- Case: Object owned by subject A, who wants to give revocable access to subject B.
- Create a new kind of capability called an alias.
- A creates an alias for the object and gives the capability to access through the alias to B.
- A revokes the access by invalidating the alias, so B cannot access the object without going through the alias first.
- Example: In indirect addressing, a register can contain the address of a word. If the high-order (tag) bit is 1, then it is also treated as an address. This pointer search continues until reaching a word with a high-order bit of 0, which is treated as data.
- Benefits: Allows for flexible revocation without modifying every capability.
Examples on Capability List
- String as a Capability: A user process has a string in memory space, identifying a unique object on the system. It does not specify access rights, so it is not a capability.
- Pair of Values: A pair of values identifies an object with a set of access rights. This pair is still not a capability, because the user's possession of the values does not mean that access is legitimate.
- Successful Execution: When a user program successfully executes an access statement, the system verifies the validity of the capability, by checking the level of indirection and revoking the capability if the values are not legitimate or the alias has been revoked.
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Description
This quiz covers the intricacies of revoking access rights in computing systems, focusing on access lists and capability lists. It discusses methods for immediate, general, or selective revocation, including key concepts like reacquisition and back-pointers. Test your understanding of how capabilities are managed and revoked!
