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lecture_3_part_2

lecture_3_part_2 - Capabilities Each subject keeps a set of...

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Lecture 3 Page 1 CS 236 Online Capabilities Each subject keeps a set of data items that specify his allowable accesses Essentially, a set of tickets Possession of the capability for an object implies that access is allowed
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Lecture 3 Page 2 CS 236 Online Properties of Capabilities Must be unforgeable In single machine, keep under control of OS What about in a networked system? In most systems, some capabilities allow creation of other capabilities Process can pass restricted set of capabilities to a subprocess
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Lecture 3 Page 3 CS 236 Online Capabilities and Domains The set of objects a subject can access at a given moment is its domain The subject has a capability for each object in its domain Domains can be expanded by obtaining new capabilities New domains can be created for subprocesses Where do we keep capabilities?
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Lecture 3 Page 4 CS 236 Online Capabilities Protecting a File Read X Subject B Subject C Capabilities for C Capabilities for A File X Read, Write Capabilities for B File X Read File X Subject A Capability Checking File X Read, Write read File X Read, Write Check validity of capability OK!
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Lecture 3 Page 5 CS 236 Online write denied Capabilities Denying Access write User B User C Capabilities for C Capabilities for A File X Read, Write Capabilities for B File X Read File X User A Capability Checking Check validity of capability No Capability Provided!
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Lecture 3 Page 6 CS 236 Online How Will This Work in a Network? Subject B Subject C Capabilities for C Capabilities for B File X Read Capabilities for A File X Read, Write Subject A Capability Checking File X File X Subject A Subject B File X Read Subject C File X Read, Write How can we tell if it’s a good capability? File X Read, Write
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Lecture 3 Page 7 CS 236 Online Revoking Capabilities Fred Nancy Accounts receivable How do we take away Fred’s capability? Without taking away Nancy’s?
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Lecture 3 Page 8 CS 236 Online Revocation By Destroying the Capability Fred Nancy Accounts receivable How can you be sure you’ve destroyed all copies everywhere?
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Lecture 3 Page 9 CS 236 Online Revocation By Invalidation on Use Fred Nancy Accounts receivable Fred Read Accounts Receivable Accounts receivable capability revocation list Ted Anne Costs time to check revocation list Especially if list gets long
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Lecture 3 Page 10 CS 236 Online Revocation By Generation Numbers Fred Nancy Accounts receivable 3 3 3 If generation numbers match, the capability is still valid To invalidate capability, increase generation number 4 Requires some control of capabilities Selective revocation is hard Can replace generation number with some other software token
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Lecture 3 Page 11 CS 236 Online Pros and Cons of Capabilities + Easy to determine what a subject can access +
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