Announcements - Operating Systems - Notes | CMSC 412, Study notes of Operating Systems

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Announcements

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Reading Chapter 19

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MT#2 re-grade requests due by end of class

Computer Threat Model

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must consider acceptable risks– value of item to be protected– $2,000 of computer time to steal 50 cents of data

• this is a sufficient deter someone•^ but

computers keep getting faster

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Basic Ideas:– confine access to only the highest level needed

• run programs as root only if needed• don’t give system access to all users

Authentication (cont.)

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How does a user know what computer they are using?

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Need to have

mutual authentication

• computer presents some information that only it could contain– example: Windows -- to login
  • user software can’t trap that information• assumes that the kernel itself is secure

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telephone example:– never give banking/credit card info over the phone unless you

placed the phone call• i.e. you use the telco namespace for authentication

Example (UNIX passwords)

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use a function that is hard to invert–

“easy” to compute f(x) given x– hard to compute x given f(x)– the function used is a variation on the DES algorithm

-^ changes selected items in the transformation matrix to preventhardware attacks -^

store only f(x) in the filesystem

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to login:–

user supplies a password x’– compute f(x’) and compare to f(x) z^

salt–

add an extra two characters to x so that the same x will producedifferent values on different machines z^

dictionary attach–

if its to easy to compute f(x)– can “guess” many passwords and try them out

Viruses

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Most common on systems with little security– easy to write to boot blocks, system software– never run untrusted software with special privileges– Don’t perform daily operations with root/system privileges

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Possible to write system independent viruses– MS Word virus

• uses macros to call into the OS

Access Matrix

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Abstraction of protection for objects in a system.– Rows are domains (users or groups of users)– Columns are objects (files, printers, etc.)– Items are methods permitted by a domain on an objects

• read, write, execute, print, delete, …

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Representing the Table– simple representation (dense matrix) is large– sparse representation possible: each non-zero in the matrix– observation: same column used frequently

• represent groups of users with a name and just store that
  • create a default policy for some objects without a value z^

Revocation of access– when are access rights checked?– selective revocation vs. global

Capabilities

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Un-forgeable Key to access something

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Implementation: a string– I.e. a long numeric sequence for a copier)

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Implementation: A protected memory region

• tag memory (or procedures) with access rights
  • example - x86 call gate abstraction
    • permit rights amplification