File-based Access Control

Key Concepts

File-based permissions

• Multics and UNIX — treat every object as a logical file — e.g. printing to a printer is printing to a file
• metadata stores permissions
• access to every object is thus “file permissions”
• ACLs have disadvantages
  • long (consumes memory, time)
  • frequent updates needed
  • tedious
  • can use these in Linux!

ugo

• ugo — user, group, other
• userID
• groupID
• see /etc/passwd, /etc/shadow, /etc/group
• superuser: userID = 0, root
• fixed length!
• less expressive than ACLs
• see ls -al
• see Table 5.1, page 136
• initial value for file permissions set by umask — play with this to see how it works
• change permissions using chown, chgrp

setuid bit

• see Figure 5.5, page 135
• can be set by owner
• means the file, if run, executes with the permissons of the owner, rather than the user running the program
• allows the program to do things the owner can do
• attackers look for programs owned by root with setuid bit set
• if you can find a vulnerability in a root-owned program with setuid bit set, you can do anything on the system
• real