SLIDE 1
1 Operating System Security Why secure the OS?
Works directly on the hardware but can
be adapted during runtime
Data and process are directly visible Application security can be
circumvented from lower layers = > good scope because:
Hardware is too narrow and inflexible Application and Network is too broad
The basis of protection: Seperation
Physical separation
Processes use different physically devices
Temporal separation
Process (with different security demands) execute
at different time
Logical separation
Processes and objects are logical separated so
that they are not aware of each other
Cryptographic separation
Data and computation is concealed by
cryptography to the outside
Privilege levels in 80X86 processors
Procedures can
access objects only in their own ring or in outer rings
Procedures can
invoke subroutines
- nly in their own
ring
OS function I/O . . .
Privilege levels - A problem
Outer ring procedures have a need to
execute procedures on a lower ring
e.g. An application (level 4) want to save a file
(level 3)
Solution: A gate (well defined access way)
allow the call (execute only) of lower ring procedures
To prevent abuse (outer ring asks lower ring to
transfer an object to the outer ring) both the current privilege level and the calling privilege level need to be verified
Reference Monitor
Reference Monitors control access to
- bjects
Physical - Memory Management Logical - File Management