Today's Topics

• Preventing Unauthorized Access
• ACM Storage
• Access Control Lists
• Access Control in UNIX

Preventing Unauthorized Access:

• protect the integrity of the ACM
• authenticate requests

ACM Storage:

The ACM can be stored as a set of rows where each row, called a capability list (C-list), is maintained by the relevant subject. Alternatively, the ACM can be stored as a set of columns where each column, called an access control list (ACL), is maintained by the relevant object.

Access Control Lists:

The time to add, remove, or verify rights is linear with respect to the length of the list.
This time can be reduced by combining subjects with the same rights into a single group.
Example:
ACL = ((i, r_ijk), (j, r_ijk), (k, r_ijk), (m, r_mn), (n, r_mn)) becomes
ACL = ((group_ijk, r_ijk), (group_mn, r_mn)), group_ijk = (i, j, k), group_mn = (m, n)
The time could also be reduced by caching portions of the list. This approach, however, would have to insure cache consistency.

Access Control in UNIX:

• The system maintains 2 files, /etc/password and /etc/groups.
  • /etc/password maintains an entry for each user. A sample entry with the required fields is given below. uid and gid refer to the user and group ids, respectively. Any salt used to encrypt the password would also be kept here.
    
    
    

    /etc/groups maintains an entry for each group. A sample entry with the required fields is given below. If the encrypted password field contains *, then no password is required for login.
    

    
    
• Each user stores his/her username, uid, and a list of groups to which he/she belongs. (Originally, a user could only belong to 1 group, but this has changed.)
• Each process stores the username, uid, and gid of the user that started it. Processes also maintain an effective uid. Child processes inherit these values.
• All requests are made by processes.
• Most objects are treated as files. Each file maintains an ACL in the form of 12 protection bits which can be modified by the owner. The first 9 bits determine the read (R), write (W), and execute (X) rights for the user/owner (u), owner's group (g), and other users (o). The last 3 bits support domain changes by allowing the uid and gid to be changed from the caller and caller's group to the file owner and file owner's group. (1 of the last 3 bits was not discussed in lecture.)

  A subject's access to a file may be restricted by the directories that contain the file.
  Example 1:
  

  Subject has R, W, and X rights to the file /usr/file.txt
  
  Subject also has X (but not R or W) right to the directory /usr
  
  Subject can only access /usr/file.txt, if the subject knows the file's name.
  Example 2:
  
  Subject has R, W, and X rights to the file /usr/file.txt
  
  Subject also has R (but not W or X) right to the directory /usr
  
  Subject can learn the name of usr/file.txt, but cannot access the file.
  

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