Linux Permissions

Basic

Permissions are a mechanism to restrict the access to resources.

Each file has specific permissions, owner, and owner group.

Each process is executed as a user. The process has the same privileges as the user does.

Root user has no permission restrictions.

Categories

There are 3 types of permissions:

  • Read (r)
    • read the content of a file
    • list the contents of a directory
    • 2^2 = 4
  • Write (w)
    • change the content of a file
    • create or delete files in a directory
    • 2^1 = 2
  • Execute (x)
    • run a file as command
    • access the content in a directory
      • Directories without x cannot be ‘opened’ (check the content inside a directory)
    • 2^0 = 1

Usually permissions are encoded using octal numbers. For a number ‘rwx’, r is the 3rd bit, w is the 2nd bit, x is the 1st bit.

Therefore ‘rw-‘ is ‘110’ and thus 6. Same thing for other encodings.

UGO

There are 3 parts of permissions for the 9 permission bits:

User Group Other

  • Owner of the file: User, the first 3 bits
  • Group of the file: Group, the middle 3 bits
  • Other users: Other, the last 3 bits

There are also 3 special bits for a file.

Inspect Permissions

$ ls -l
drwxr-xr-- 2 some-user some-group 208 Oct 1 13:50 some-directory
# UGO | number of links | owner | group | size | time of last modification | name
drwxr-xr--

# d:    is directory
# rwx : owner permission
# r-x : group permission
# r-- : other permission

Modify Permissions

chown

chown some-user some-file
chown -R some-user some-directory

Change the owner

-R: recursively change all subdirectories and files

chgrp

Change the owner group

Same usage as chown

chmod

chmod <new_permission> <some_file>

chmod u+rw ./a.out
chmod g-x ./a.out
chmod go+r ./a.out
chmod a-x ./a.out
chmod u+x ./a.sh

chmod 660 ./a.out # rw-rw----
chmod 775 ./a.out # rwxrwxr-x

Change the permissions

Default Permissions

There is something controlling the default permission for a new file created.

umask

Subtraction

  • Permission for a newly created file: 666 - umask
  • Permission for a newly created directory: 777 - umask

Each user has a umask property

There are 4 bits for umask. The first 3 bits are UGO and the last bit is special permission.

umask value by default:

  • normal user
    • 002
    • 666 - 002 = 664
    • rw-rw-r–
  • root
    • 022
    • 666 - 022 = 644
    • rw-r–r–

It’s actually not subtraction but a bitwise XOR (?)

umask                   # inspect
umask <new_umask_value> # set

Special Permissions

What about the ‘extra’ bit, or the 4th bit, of umask?

The real permission in binary is 12bits. UGO uses 9 bits. The last 3 bits, namely the leading bit of umask, is the special permission bit.

suid

Run the command with the access permissions of the owner, not the user executing this command.

$ ls -l /usr/bin/passwd
-rwsr-xr-x 1 root root 53K May 16  2017 /usr/bin/passwd*

The ‘x’ bit of U becomes ‘s’.

The user executing this command will potentially gain extra access.

Usually, the file name will be colored and highlighted on a terminal with color support.

chmod u+s  xxx

sgid

Same as ‘suid’ but uses the permission of the group owner.

The ‘x’ bit of G becomes ‘s’.

Usually set for directories. It is commonly used to inherit the permissions of parent directory.

chmod g+s xxx

Sticky Bit

Users with write access to a directory can only delete the files owned by this user. They can not delete other files in this directory which are owned by other users.

Used to protect a directory shared by multiple people. Usually within same group.

Usually the directory name will be highlighted using blue color.

chmod o+t xxx

Octal Numbers for Special Permissions

suid = 4 sgid = 2 sticky = 1

chmod 4755 xxx