quartz/content/notes/24-protection-of-file-systems.md

title	aliases	tags
24-protection-of-file-systems		cosc204 lectures

protection model

file system shared by many users

• users should be able to keep them private

need to define

• mechanisms
• policies

types of file access users can have

• read
  • includes list
• write
  • includes delete
• execute
  • if a directory is not executable you cannot get into it (run it)

Access control bits

three classes for each file

• owner: user who created the file
• group : set of users who are sharing the file: a group is defined by the systems
• others: all other in the system

when a file is create it has an owner id and a group id to identify the users of the file

3 permission bits for each class

• r: readable
• w: writeable
• x: executble

advanced file attributes

• setuid bit: if set for executable file, regardless of who runs the program, it should be run with the priviledge of the owner of the file
• setgid bit: same as setuid but run with priviledge of group
• sticky bit: a directory with this bit set restricts the deletionof files within it

soft vs hard link

access control models

Discretionary access control (DAC)

• a means of restricting access to objects based on the identity of subjects and/or groups to which they belong. The controls are discretionary in the sense that a subject with a certain access permission is capable of passing that permission (perhaps indirectly) on to any other subject (unless restrained by mandatory access control).
• subject centered

Mandatory access control

• a means of restricting access to objects based on the sensitivity (as represented by a label) of the information contained in the objects and the formal authorization (i.e., clearance) of subjects to access information of such sensitivity
• object centred
• more overhead

Access control list

• list of permission attahed to an object (file)
• speifies who or what is allowed to access the objdct and what operation a re allowed to be performed on the object
• consists of entries like [user, operations] where the operation can be r, w, x, d etc
• more secure and convenient that discretionary access control
• can implement DAC and MAC

Role based

• similar to ACL except RBAC aggregates a grou of users with the same priviledges as roles
• can implement DAC and MAC
• policy neutral and defined around roles and priviledges