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A Hardware Architecture For Implementing Protection Rings
A Hardware Architecture For Implementing Protection Rings
A Hardware Architecture For Implementing Protection Rings
A Hardware Architecture For Implementing Protection Rings
A Hardware Architecture For Implementing Protection Rings
A Hardware Architecture For Implementing Protection Rings
A Hardware Architecture For Implementing Protection Rings
A Hardware Architecture For Implementing Protection Rings
A Hardware Architecture For Implementing Protection Rings
A Hardware Architecture For Implementing Protection Rings
A Hardware Architecture For Implementing Protection Rings
A Hardware Architecture For Implementing Protection Rings
A Hardware Architecture For Implementing Protection Rings
A Hardware Architecture For Implementing Protection Rings
A Hardware Architecture For Implementing Protection Rings
A Hardware Architecture For Implementing Protection Rings
A Hardware Architecture For Implementing Protection Rings
A Hardware Architecture For Implementing Protection Rings
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A Hardware Architecture For Implementing Protection Rings

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My take on this famous paper on protection rings made for my graduate OS class

My take on this famous paper on protection rings made for my graduate OS class

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  • 1. A Hardware Architecture for Implementing Protection Rings Schroeder & Saltzer: Mighty Institute of Terminology (m.i.t.) Brought to you by: Chris Sosa I <3 Cornell
  • 2. Overview
    • Introduction of Protection Rings
      • Allows multiple domains to be associated with a process and process domain movement during execution
    • Hardware a.k.a. Computer Architecture Implementation of Protection Rings for Multics
  • 3. Motivation
    • Basic access control mechanisms only allow one set of access permissions (a domain) for a user per resource
    • Intrinsic need to change access capabilities of a user as his/her process runs
      • User A may wish to allow user B to access sensitive data but only through a special program
  • 4. Four Sets of Criteria to Judge Protection Mechanisms
    • Functional Capability
    • Economy
    • Simplicity
    • Programming Generality
  • 5. What is a Protection Ring?
    • This does not meet our economy criteria!
    • … let’s try again
  • 6. Protection Rings
    • Each process associated with 0 -> r-1 of domains called rings
      • Concentric = subset of privileges of n for m when m > n
    • Ring 0 =
  • 7. Protection Rings (cont’d)
    • Typical R/W/E privileges divided into brackets
    • R/W brackets must start at ring 0, why?
    • Allows for lowest-numbered ring of execution to be specified (not necessarily 0). Why might this be a good thing?
  • 8. Moving Between Protection Rings
    • Downward movement restricted to “gates”
      • Specific program locations
      • During execution allows a process to enter a lower domain
      • Example of gates?
    • Upward movement is unrestricted (but you must use a special call to do it)
  • 9. Revisiting Gate Extension
    • Bracket that defines from which rings gate movement is allowed
    • Possible use of this in Windows?
  • 10. Call and Return
    • Procedure transfer = subroutine call
    • Easy to validate protection rings
    • If Call goes through gate => validate caller’s ring w.r.t. gate extension
    • Return restores caller’s ring
  • 11. Three Issues with Downward Calls
    • Find new stack area
      • Each process has a stack segment per ring
    • Argument validation
      • Procedure assumes more restricted access capabilities of caller when accessing operand references
    • Knowledge of caller’s ring
      • Processor leaves this in a read-only register
  • 12. Other calls?
    • Dealing with a call and return that doesn’t change rings is trivial … duh!
    • Upward calls are hard
      • Their solution, let’s not support it!
      • What’s wrong with keeping the protection ring of the caller?
        • Internet Explorer bugs anyone?
  • 13. Computer Architecture Support
    • Hot or Not?
  • 14. Quick Review of the Multics before Protection Rings
    • Each user has individual VM
    • A segment is the unit of Access Control
    • Users has R/W/E privileges defined per segment (flags)
    • Flags stored in with segment descriptor in H/W
    • … for more information of hardware: see paper  (for those Clint’s out there)
  • 15. Quick Review of Multics AFTER Protection Rings
    • Eight Rings
    • Modification of SDW to include three 3-bit values for bracket management
      • R0 -> SDW.R1 = Write
      • SDW.R1 -> SDW.R2 = Execute
      • SDW.R2+1 -> SDW.R3 = Gate Extension
      • Read = Execute, loses some flexibility
    • Other modifications that only involve adding a 3-bit ring field denoting required or current ring
  • 16. An Example
    • Retrieval of Next Instruction to be executed
  • 17. Issues
    • Complicated to design user programs with rings
    • Rare to see more than two rings used
      • We have seen some exceptions … see Xen
    • Implementation lacked some features
      • Execution end bracket same as read bracket
      • Didn’t implement upward movement in terms of ring movement
  • 18. Questions?

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