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Understanding File System Permissions

The technologies collectively known as “file system permissions” are used to control file and folder authorization for Mac OS X. File system permissions work alongside the user account technologies, which control user identification and authentication, to provide the Mac’s secure multiuser environment. File system permissions—again just like user accounts—permeate every level of the operating system, so a thorough investigation of this system is required to fully understand Mac OS X.

In short, every single item on the system volume has permissions rules applied to it by the operating system. Only users and processes with root account access can ignore file system permissions rules. Thus, these rules are used to define file and folder access for every normal, administrative, guest, and sharing user. Any user can easily identify the permissions of a file or folder with the Finder’s Get Info window.

Viewing File System Permissions

To identify file system permissions from the Finder:

  1. In the Finder, select the file or folder for which you wish to identify the permissions. You can select multiple items to open multiple Get Info windows.
  2. Open the Get Info window.

    There are several methods for doing this. You can choose File > Get Info from the menu bar; use the Command-I keyboard combination; choose Get Info from the Action pop-up menu in a Finder window toolbar; or in the Finder, right-click or Control-click on an item and choose Get Info.

  3. Once you have opened a Get Info window, click the Sharing & Permissions disclosure triangle to reveal the item’s permissions.

    Note that the permissions list is broken into two columns. To the left is a list of users or groups with access to this item, and to the right is the associated privilege assigned per user or group. Modifying these settings is covered in the “Managing Permissions” sections later in this chapter.

Ownership for Permissions

Every file and folder belongs to at least one owner and one group, and also has an ownership setting for everyone else. This three-tiered ownership structure provides the basis for file system permissions:

  • Owner—By default, the owner of an item is the user who created or copied the item to the Mac. For example, the user owns most of the items in his home folder. The system or root user almost always owns system software items, including system resources and applications. Traditionally, only the owner can change the item’s ownership or permissions. Despite this, Mac OS X makes management easier by giving every administrative user the ability to change ownership and permissions regardless of who the item’s owner is.

  • Group—By default, the group of an item is inherited from the folder it was created in. Thus, most items belong to the staff, wheel, or admin groups. Group ownership is designated to allow users other than the owner to have access to an item. For instance, even though root owns the /Applications folder, the group is set to admin so administrative users can make changes to the contents of this folder.

  • Everyone—The Everyone setting is used to define access for anyone who isn’t the owner and isn’t part of the item’s group. In other words, this means everyone else. This includes local, sharing, and guest users.

The simple three-tiered ownership structure presented here has been part of traditional UNIX operating systems for decades. However, with only three levels of permissions to choose from, it is quite difficult to define appropriate access settings for a computer with many user accounts and shared files, as is the case with many servers. Fortunately, as you’ll see later, access control lists (ACLs) were developed to allow for nearly limitless ownership and permissions configurations.

Traditional UNIX Permissions

Mac OS X’s basic file system permissions structure is based on decades-old UNIX-style permissions. This system also sometimes goes by POSIX-style permissions. The system may be old, but for most Mac users it is quite adequate because you can define privilege rules separately at each ownership tier. In other words, the owner, the group, and everyone else has individually specified access to each file or folder. Further, because of the inherent hierarchy built into the file system, where folders can reside inside of other folders, you easily create a complex file structure that allows for varying levels of sharing and security.

There is a variety of UNIX privilege combinations available from the command line, as discussed in the “Managing Permissions via Command Line” section later in this chapter. However, Apple has streamlined the Finder to allow only the most common permissions options.

Permissions that you can assign to a file using the Finder are:

  • Read and Write—The user or group members can open the file and save changes.

  • Read Only—The user or group members can open the file but cannot save any changes.

  • No Access—The user or group members have no access to the file at all.

Permissions that you can assign to a folder using the Finder are:

  • Read and Write—The user or group members can browse and make changes to the contents of the folder.

  • Read Only—The user or group members can browse the contents of the folder but cannot make changes to the contents of the folder.

  • Write Only (Drop Box)—The user or group members cannot browse the folder but can copy or move items into it.

  • No Access—The user or group members have no access to the contents of the folder.

Access Control Lists (ACLs)

Access control lists (ACLs) were developed to expand the traditional UNIX-style permissions architecture to allow more control of file and folder access. Though there is no common standard for ACLs, Mac OS X has adopted a style of ACL similar to that available on Windows-based NTFS file systems and UNIX systems that support NFSv4. This ACL implementation is extremely flexible but increases complexity by adding more than a dozen unique privilege and inheritance attribute types. Further, this implementation supports an unlimited number of ACL attributes for any user or group. Finally, it’s important to note that if an ACL rule applies to a user or group, this rule will trump traditional UNIX permissions. However, any users or groups that don’t apply to a specific ACL will still be bound by the UNIX permissions currently in place.

Apple does not expect average users to navigate through all the options available using ACLs, so once again the Finder has been streamlined to allow only the most common ACL configurations. In fact, the Finder only allows you to assign ACL attributes that match the most common UNIX permissions configurations that were previously listed in this chapter. The only feature of ACLs that the Finder actually implements is the ability to have an unlimited number of user or group privilege rules. In other words, the Finder uses the ACL architecture to let you configure unique privileges for an unlimited number of users or groups. Prior to Mac OS X v10.5, the Finder would only allow you to assign permissions using the standard three-tiered ownership style, with one owner, one group, and one setting for everyone else.

Permissions in a Hierarchical Context

It is important to remember that permissions do not exist in isolation; rather, permissions are applied in the context of folder hierarchy. In other words, your access to an item is based on an item’s permissions in combination with the permissions of the folder in which it resides. If you’re still confused, it’s easiest to think of permissions as defining access to an item’s content, not the item itself. Remember the word “content” as you consider the following three simplified examples.

Example 1: Your permissions to Example Folder 1 are read and write. It’s obvious that you should have full access to Example File 1.1, as your permissions here are read and write as well. You can also view and copy Example File 1.2, but you can’t make changes to the file’s content because your permissions are read only. Yet you can still move, delete, or rename File 1.2 because you have read and write access to the folder’s contents. Thus, File 1.2 isn’t secure in this example because you can make a copy of the original file, change the copied file’s content, delete the original file, and finally replace it with the modified copy. In fact, this is how most graphical applications save document changes; thus the file can indeed be edited.

Example 2: You have read-only permission to Example Folder 2. You can edit the content of Example File 2.1 because you have read and write access to it, but you can’t move, delete, or rename it because you have read-only access to the folder’s contents. On the other hand, you can effectively delete the file by erasing its contents. Example File 2.2 is the only truly secure file, as you’re only allowed to view or copy the file. Granted, you can make changes to the contents of a copied file, but you still can’t replace the original.

Example 3: Your permissions are identical to the first example, with one significant change. The owner of Example File 3.1 has enabled the locked attribute. Even though you have read and write access to Example Folder 3 and File 3.1, the locked attribute prevents all users who aren’t the file’s owner from modifying, moving, deleting, or renaming the file. From most applications, only the owner is allowed to change the file’s content or delete it, but the owner can also disable the locked attribute to return the file to normal. You can still make a copy of the locked file, but the copy will be locked as well. However, you will own the copy, so you can disable the locked attribute on the copy, but you still can’t delete the original locked file unless you’re the owner.

Permissions for Sharing

Once you have an understanding of the permissions options available to you in Mac OS X, you should explore how the local file system is set up by default to provide a secure environment that still allows for users to share files.

If you don’t have fast user switching enabled as outlined in Chapter 2, “User Accounts,” you should enable it now to make it easy to test file system permissions as different users. Further, to aid in your exploration of the file system you should use the Finder’s Inspector window. This single floating window, which automatically refreshes as you select different items in the Finder, allows you to quickly explore the default permissions settings without having to open multiple Finder Get Info windows. Open the Inspector from the Finder by using the Option-Command-I keyboard combination, and then click the disclosure triangle to reveal the Sharing & Permissions section.

Home Folder Sharing

Mac OS X protects the user’s files by default and allows them to be shared easily when needed. This starts with the user’s home folder. You’ll notice that users are allowed read and write access to their home folder, while the staff group and everyone is allowed only read access.

This means that every local user or guest can view the first level of every other user’s home folder. (As a reminder, guests are allowed access to your computer without a password. This is why you can disable guest access in the Accounts preferences.) The default home folder permissions may seem insecure until you look at the permissions in context. Most user data is actually stored inside a subfolder in the user’s home folder, and if you inspect those subfolders you’ll notice that other users are not allowed to access most of them.

There are a few subfolders in a user’s home folder, however, that are specifically designed for sharing. The Public and Sites folders remain readable to everyone. A user can easily share files without having to mess with permissions by simply moving the files into those two folders. Others will be able to read those files, but they still cannot make any changes to them.

Looking deeper, you’ll notice a subfolder of the Public folder is the Drop Box. This folder’s permissions allow all other users to copy files into the folder even though they cannot actually see other files in the Drop Box folder. This allows other users to discreetly transfer files without others knowing.

The Shared Folder

An additional folder set aside for sharing is the /User/Shared folder. You’ll notice that this is a general sharing location that allows all users to read and write items to the folder. Normally this permissions setting would also allow any user to delete another user’s item in this folder. Yet the Finder’s Inspector window is not showing you the full permissions picture here. There is a unique permissions setting on the Shared folder that prevents other users from being able to delete items that they don’t own. This permission setting, known as the “sticky bit,” can only be set using a command-line tool. Inspecting and changing permissions from the command line is covered in the “Managing Permissions via Command Line” section later in this chapter, and the sticky bit is specifically covered in the “Using the Sticky Bit” section later in this chapter.

Securing New Items

Once you understand how Mac OS X’s file system security architecture works with the folder hierarchy, it’s time to consider how this technology is used to secure new items. You’ve learned previously in this chapter that Mac OS X is already preconfigured for secure file and folder sharing, but you will find that new items are created with unrestricted read access.

For example, when a user creates a new file or folder at the root of her home folder, by default all other users, including guest users, are allowed to view this item’s contents. The same is true for new items created by administrators in local areas such as the root of the system volume and the local Library and Applications folders.

New items are created this way to facilitate sharing, so you do not have to change any permissions to share a new item. All that is required of you is to place the new item in a folder that other users can access; like the pre-defined sharing folders covered in the previous section. It’s assumed that if you want to secure a new item, you will place it inside a folder that no one else has access to, like your home Desktop or Documents folders.

On the other hand, this default behavior is inconvenient if you want to safely store your items in an otherwise public area, like the root of the system volume. To store items in a public area so they are only accessible to the owner requires you to change the item’s permissions using either the Finder or the command line, as outlined later in this chapter.

Specifically, from the Finder’s Sharing & Permissions section of the Get Info window, you must remove all other users and all group accounts from the permissions list. You cannot remove the Everyone permission, so you will have to set it to No Access. Once you have made these permissions changes, only the owner will have access to the item.

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