Computer Security: Principles and Practice, 1/e

Directories are structured in a hierarchical tree

UNIX File Access Control

• Unique user identification number (user ID)
• Member of a primary group identified by a group ID
• Belongs to a specific group
• 12 protection bits
• 9 of the protection bits specify read, write, and execute permission for the owner of the file, members of the group and all other users
• The owner ID, group ID, and protection bits are part of the file’s inode

Traditional UNIX File Access Control

• “Set user ID”(SetUID)
• Any program that is owned by, and SetUID to, the “superuser” potentially granted unrestricted access to the system to any user executing that program
• “Set group ID”(SetGID)
• System temporarily uses rights of the file owner/group in addition to the real user’s rights when making access control decisions
• Enables privileged programs to access files/resources not generally accessible
• Sticky bit
• When applied to a directory it specifies that only the owner of the file in the directory can rename, move, or delete that file
• Superuser (root)
• Is exempt from usual access control restrictions
• Has system-wide access

When a process requests access to a file:

• Step 1 selects the ACL entry that most closely matches the requesting process. ACL entries are looked at the order: owner, named users, groups, & others. Only a single entry determines access.
• Step 2 checks if the matching entry contains sufficient permissions. A process can be a member of more than one group; so more than one group entry can match. If any of these matching group entries contain the requested permissions, one that contains the requested permissions is picked (the result is the same no matter which entry is picked). If none of the matching group entries contains the requested permissions, access will be denied no matter which entry is picked.