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--- ops102:permissions [2024/01/25 04:51] – chris
+++ ops102:permissions [2024/05/28 21:27] (current) – external edit 127.0.0.1
@@ Line 6: / Line 6: @@
 There are two broad categories of access controls applied to files and directories:
   *  Discretionary Access Controls (DACs) - these are access controls that can be set to any value at the discretion of the users or administrators of the computer system.
-  *  Mandatory Access Controls (MACs) - these are access controls that are applied across the entire system in a uniform way, and cannot be individually overridden by the users or administrators. An example of a Mandatory Access Control system is SELinux (security-enhanced Linux), a system originally developed by the National Security Agency of the US Federal Government and now part of the Linux Kernel (via KSM - Kernel Security Models). SELinux uses type enforcement and labelling of both resources (files, network connections) and processes to determine whether a specific process should have access to a specific resource, and to deny access when it does not. SELinux is used in several operating systems, including Android, Fedora, CentOS, and Red Hat Enterprise Linux.
+  *  Mandatory Access Controls (MACs) - these are access controls that are applied across the entire system in a uniform way, and cannot be individually overridden by the users or administrators. An example of a Mandatory Access Control system is SELinux (security-enhanced Linux), a system originally developed by the National Security Agency of the US Federal Government and now part of the Linux Kernel (via KSM - Kernel Security Modules). SELinux uses //type enforcement// and //labelling// of both resources (such as files and network connections) and processes (running programs) to determine whether a specific process should have access to a specific resource, and to deny access when it does not. SELinux is used in several operating systems, including Android, Fedora, CentOS, and Red Hat Enterprise Linux.
 In this OPS102 course, we will be looking only at DACs.
@@ Line 39: / Line 39: @@
   *  **Read** -- the ability to //see// the names of the files and subdirectories within the directory. This is also called "search" permission.
-  *  **Write** -- the ability to create/delete files and subdirectories within the directory.
+  *  **Write** -- the ability to create/delete/rename files and subdirectories within the directory.
-  *  **eXecute** -- the ability to access files with the directory. If turned off, the files cannot be accessed, and metadata about each file (such as the owner, group owner, file length, permissions, and timestamps) cannot be accessed either. This is sometimes called //passthrough// permission.
+  *  **eXecute** (**access**) -- the ability to access files with the directory. If turned off, the files cannot be accessed, and metadata about each file (such as the owner, group owner, file length, permissions, and timestamps) cannot be accessed either. You can think of this as **access** permission when applied to directories. This is sometimes called //passthrough// permission.
 If execute permission is enabled for a directory but read permission has not been enabled, the affected community cannot view a directory listing to determine filenames, but if they know the name of a file within that directory, they may still access it.
+If read permission is enabled for a directory but execute permission has not been enabled, the affected community can view the names of files in a directory (but only the names, not permissions, file size, ownership, or any other information), but they will have no access to use the files in any way.
+However, read and execute permission are almost always assigned to a directory together.
+In order to access a file, a user must have execute permission on __all__ of the directories from the root directory to the directory containing the file. For example, on the file ''/home/jdoe/ops102/practice/info.txt'', a user must have execute permission on all four directories (home, jdoe, ops102, practice) to access the file.
-In order to access a file, a user must have execute permission on __all__ of the directories from the root directory to the directory containing the file. For example, on the file ''/home/jdoe/ops102/practice/info.txt'', the user must have execute permission on all four directories (home, jdoe, ops102, practice) to access the file.
 ====  Viewing Permissions  ====
@@ Line 253: / Line 258: @@
  $ ls -l example001
  -rw-r-----. 1 chris chris 0 Sep 26 11:45 example001</code>
+=== Recursively Setting Permissions ===
+It is sometimes useful to use the chmod ''-R'' option to recursively set all the permissions on a directory and all of its contents. However, it is quite common to need to set execute permissions on directories but not on files. You can indicate this to ''chmod'' using a capital X for the execute permission.
+  # Don't do this! It will set Execute permission on files and directories.
+  chmod -R go+rx publicdir
+  # Instead, do this: it will set Execute permission on directories only, and not on files
+  # (Notice the capital X in the symbolic permissions)
+  chmod -R go+rX publicdir
+=== Other useful chmod Options ===
+<code>
+  -v    Verbose: show information about each file processed (whether changed or not)
+  -c    Changes: show information about each change made (no output for unchanged files)</code>
 ====  Controlling Permissions on New Files and Directories  ====
@@ Line 262: / Line 284: @@
   -  The modes prohibited by the umask value.
-For example, a umask value of 0022 represents the permissions ''----w--w-'' (write permission for group and other). Therefore, any new files or directories will be created //without// these permissions.
+For example, a umask value of 0022 represents the permissions ''----w--w-'' (write permission for group and other). Therefore, any new files or directories will be created //without// these permissions. This is the default permission on Matrix.
 The umask value can be viewed or set with the ''umask'' command.
@@ Line 283: / Line 305: @@
  -rw-r-----. 1 chris chris 0 Sep 26 11:55 testfile0027</code>
-Note that the umask value is specific to the process that is currently running, and it inherited by child processes. That means that if you're using multiple shells (perhaps in multiple windows), each shell's umask value is specific to that shell, and changing it will not affect the other active shells. However, any command or program that you run from that shell will inherit the umask value. In a future topic in this course, we'll look at how to set up a umask value so that it is applied to every new shell.
+To ensure that no one else has __any__ access to new files and directories that you create on Matrix (unless you change the permission mode of the file after it is created), set your umask to 0077: ''umask 0077''
+Note that the umask value is specific to the process that is currently running, and it inherited by child processes. That means that if you're using multiple shells (perhaps in multiple windows), each shell's umask value is specific to that shell, and changing it will not affect the other active shells. However, any command or program that you run from that shell will inherit the umask value. You can set the default umask value for all future bash sessions by placing a umask command into your ''~/.bashrc'' file.
+**Warning:** be __extremely__ careful editing your ''~/.bashrc'' file, since an error may prevent you from logging in to your Matrix account. Always stay logged in to Matrix on one terminal while using a second terminal to confirm that you are able to successfully log in to the system. If you are not able to login, fix the problem using the first terminal and then re-test.
 ====  Special Permissions  ====
@@ Line 327: / Line 353: @@
  drwxrwxrwt. 26 root root   600 Sep 26 12:21 /tmp
  -rwsr-xr-x.  1 root root 32760 Jan 18  2023 /usr/bin/passwd</code>
+==== Securing Your Account ====
+There are two ways to secure your account on Matrix:
+. If you do not want to share //any// of your files with other users, you can disable access to your home directory by turning off all permissions for the group and other communities. The command ''chmod go=  ~'' will set this. Since this turns off access permission to that directory, other users will not be able to access any of the files within your home directory, regardless of the permission on the individual file.
+. If you want to share access to //some// of your files with other users, turn on the appropriate permissions for group and/or other users, and use the ''umask'' to limit the permissions on new files and directories.
+For example, you might:
+  * Start by turning off all permissions on the files and directories that are currently in your home directory:
+  chmod -r go= ~/*
+  * Create a directory of files you wish to share, called ''~/public''. Set the permission on this directory so that group and others can access it:
+  mkdir ~/public
+  chmod go=rx ~/public
+  * Place any files that you want to share in the ''~/public'' directory and set appropriate permissions:
+  cp anyFilesYouWantToShare ~/public
+  chmod go=r ~/public
+  * Set up you umask so that, by default, other users have no access to any new files you create (place this in your ''~/.bashrc'' file to ensure that it is applied to all new bash shells that you start in the future):
+  umask 0077
+  * Ensure that users can access your ''~/public'' directory through your home directory:
+  chmod go=rx ~
 =====  Access Control Lists  =====