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The pane ACL Permissions

Introduction

Every file and every folder accessed by your computer is associated with a specific set of rights that define which users are allowed to perform what operations with these objects, e.g. reading the contents of a file, or removing a file from a folder. This set of rights associated with a file system object is called permissions. Mac OS X uses the classic permissions found on every UNIX system, the so-called POSIX Permissions, an extended set of permission-like markers, called Special Permissions, and an advanced set of right definitions used by Microsoft® Windows, most modern UNIX systems, and many other operating systems, the Access Control Lists, abbreviated ACLs. ACLs are also called POSIX.1e permissions, because they behave very similar to a draft document called POSIX.1e which was planned to become an industry-wide standard for permissions one day. However, the 1e documents have been officially withdrawn for various reasons, so actually no standard exists by that name. The 1e draft contained very good ideas, however, so permissions very similar to the intentions of 1e exist in most operating systems today. But you should keep in mind that exact semantics of ACL permissions may differ slightly between different OS vendors.

POSIX Permissions

The minimum set of permission definitions used in all UNIX systems and many other operating systems which are compliant to the POSIX standard (IEEE 1003) is based on three predefined “parties” for which rights can be granted:

• the owner of the object: By default, the user who created the object automatically becomes its owner.
• the group owner of the object: a named group of users who are also considered to be special owners of the object. In a UNIX system, each user must be member of at least one user group. Although a user can be member of many different groups, she or he always has one preferred group, which is called primary group. By default, the primary group of the user who created the object automatically becomes its group owner.
• all other users: this access party is defined by the “rest”, namely all remaining users who are neither owner, nor members of the group owner group, respectively. All unidentified users, e.g. users from other computers on the Internet, who have not been identified by their names and passwords yet (or cannot be identified at all), are automatically considered to be users of a special user account called unknown, who is also member of the primary group with the same name unknown. This means any other users, no matter if the operating system could identify them or not, will be included in the category other. This access party indeed refers to “the rest of the world”.

Apple identifies the third category by the term everyone. Unfortunately, this term is incorrect, because this category does explicitly not include the owner or any member of the primary group. If you grant or deny a right for “everyone” via the Finder, those users won't be included, which is not really what the word everyone suggests. For this reason, TinkerTool System only uses the correct term other.

For each of the three categories, the following permissions can be granted:

• read: the permission to open the object and to read its contents.
• write: the permission to write to this object which includes creating it, changing its contents, appending data, etc.
• execute: the permission to execute this object. For programs, this means that the respective party can actually launch and run the program, for folders, it means that the affected users are permitted to “pass” through that folder. Note that this right has also the characteristics of a marker which allows to differentiate between executable and non-executable files, i.e. programs and other data files.

If one of these rights is not explicitly granted for a user, this will mean that the user doesn't have permission to access. The right is denied, although there is no possibility to explicitly define denials in this model.

By default, most applications create files with the following permission settings:

• the current user is made owner and has read and write permissions,
• the current primary user group is made group owner and has read permission,
• others have read permission.
• If the object is a program or folder, the execute rights for user, group, and others will be granted additionally.

Applications can grant less rights for specific files if they have been programmed to do so. For example, an e-mail application is designed to “know” that a new mail folder should be kept confidential, so it won't grant any group and other permissions when creating it. Only the owner should have read/write permission in this case.

Additional Permission Markers

Mac OS X supports some other special permission settings. They can be found on most other UNIX systems as well.

• the SUID setting: SUID is the abbreviation for “set user identification”. Under normal circumstances, every program which is started by a certain user will have the rights of that user. (Actually, starting and running programs is what a user does when working with a computer, so, as a matter of fact, the sentence “user A has permission to do B” really means “all applications started by user A have permission to do B”.) The SUID setting allows that certain marked programs break that rule. If a SUID marker is set for a program, this will mean “when running, the program should have the permissions of the file owner, not the permissions of the user who started the program”. Such an exception rule is needed for very special cases where small, restricted programs need access to system resources which are normally protected. For example, when a user likes to change her own password, the program performing this operation must have temporary permission to modify the system file containing all encrypted passwords, although — in all other cases — no user ever has permission to read or even write the password file via “normal” programs. The use of the SUID marker should only be restricted to very special cases. Very serious security problems will arise if the SUID marker is being misused.
• the SGID setting: SGID is the abbreviation for “set group identification”. This is basically the same as the SUID marker, but does not apply to user and file owner, but to user group and the program's group owner.
• the sticky setting: This flag was originally used to mark resident programs, i.e. programs that should always “stick in RAM” and must not be removed from memory even when the program quits. For programs used very often, this could result in a speed gain, because on later starts, the program could just run from memory and did not need to be loaded from disk again. In today's computers, such mechanisms are usually counter-productive. For this reason it doesn't make sense to use this marker for program files any longer. However, the sticky bit has a different meaning when being applied to folders, and this aspect is in active use by Mac OS X: A folder whose sticky marker is set becomes an “append-only” folder, or, more accurately, a folder in which the deletion of files is restricted. A file in a sticky folder may only be removed or renamed by a user if the user has write permission for the folder and the user is either owner of the file, or owner of the folder. The sticky setting is typically used for “public” system folders where everyone should have write permission, but users should not have permission to delete each others files.

Access Control Lists

Introduction

Access Control Lists, or, in short, ACLs, are a supplement to the existing POSIX permissions, so you don't necessarily need to use ACLs. The conventional rules for access rights outlined above still apply, but some optional new rules can be added.

Technically seen, an ACL is a list of individual rights which can be attached to a file system object. The ACL can either be empty — in this case, the conventional POSIX permissions apply only —, or it can contain one or more objects called Access Control Entries (ACEs). An Access Control Entry includes the following information:

• to which users does this entry apply (this can be an individual user or a user group)?
• does this entry allow or deny access?
• which right in particular is allowed or denied, respectively?
• how should this entry be inherited from a folder to the contents of this folder?

ACL Rights

ACLs allow the definition of 13 different rights to access a file-system object:

• read data / list folder contents: the right to read data from a file, or to list the contents of a folder.
• execute file / traverse folder: the right to execute a file as a program, or —if the object is a folder— the right to traverse this folder to open an enclosed folder.
• read attributes: the right to read the attributes of a file or folder, e.g. its creation date.
• read extended attributes: the right to read extended attributes of a file or folder. Extended attributes are for example Spotlight comments or the quarantine info of a file.
• read permissions: the right to read the permission settings of a file or folder.
• write data / create files: the right to write data into a file, or —if the object is a folder— the right to create a new file in the folder.
• append data / create folders: the right to append additional data to a file, or —if the object is a folder— the right to create a new folder in this folder.
• write attributes: the right to write attributes of a file or folder, e.g. its creation date.
• write extended attributes: the right to write extended attributes of a file or folder. Extended attributes are for example Spotlight comments or the quarantine info of a file.
• delete: the right to delete this file or folder.
• delete subfolders and files: if this is a folder, the right to delete enclosed objects.
• change permissions: the right to change permission settings for this file or folder.
• change owner: the right to change the owner of this file or folder.

These rights can be joined in any possible combination.

ACL Inheritance Settings

Each Access Control Entry is allowed to contain additional information that specifies how this entry is inherited to objects located at deeper levels in the file system hierarchy, for example, a file in a folder which is enclosed in another folder. The top folder may have an ACL which is automatically inherited to objects inside this folder.

Inheritance operations take only place when new objects are being created. For example, when a file B is created in a folder A, the file B will inherit ACEs from A only at that moment. When somebody changes the permissions of B at a later time, the system will not automatically reinforce a new inheritance operation from A to B. However, when somebody changes a setting of folder A, and this setting is marked for inheritance, the changes will also apply to the objects that initially inherited settings from A.

There are four different settings which control how ACE permissions should be inherited from a certain folder onto the objects that will later be created in that folder. The settings basically control how “deep” the inheritance should take effect.

• apply to this folder: the ACL permission settings should take effect on the folder itself.
• apply to subfolders: The ACL permission settings should be inherited to folders inside the current folder.
• apply to enclosed files: The ACL permission settings should be inherited to files in the current folder.
• apply to all subfolder levels: The inheritance of ACL permission settings should not stop at the level of the current folder, it should also take effect on all deeper levels of nested folders.

There are 16 possible combinations of these four settings, but only 12 of them really make sense in practice.

Inherited and Explicit Entries

Because ACE settings can be inherited from folders to the objects they contain, the system has to keep track which ACEs in an ACL are inherited and which are not. Only ACEs which are not inherited can be changed. Non-inherited entries are called explicit. To change an inherited entry, it is either necessary to change the entry at the parent level (where this inherited entry came from), or to delete the ACL for this object (hereby breaking the inheritance), replacing the inherited entries by explicit entries.

The Evaluation Rules for Access Control Entries

As mentioned before, an Access Control List consists of several Access Control Entries. Certain rules define how Mac OS X evaluates the entries when a specific user wants to access an object in the file system. Note that ACEs could contradict each other. For example, if user A is allowed to access the file B, but user A is also member of a user group which is denied access to file B, we have a contradiction which must be resolved. The following rules apply:

• The ACEs in the ACL are processed in top-down fashion. The first ACE rule that matches the particular user in question will “win” and take effect, either granting or denying access.
• The conventional POSIX permissions will be checked after the ACL has been processed. If a file system object has no ACL, the POSIX permissions will take effect only.

Important Recommendations

Access Control Lists are a powerful tool to define specific rights at a low level of granularity. However, you should keep in mind that ACLs are also very complex.

There are 13 different permissions which can be granted or denied, and 12 possible ways to define inheritance. This results in a total of 2^13 * 12 = 98,304 different concepts of access rights you can define.

Each of these nearly 100,000 different access rights can be applied to a user or a user group to form an ACE, and a nearly unlimited number of ACEs can be combined into an ACL. Each file or folder in your system can be attached to a different ACL, so maintaining all these entries can easily become a nightmare. For this reason you should define ACL permissions with greatest care only.

• Use ACL permissions only when it is necessary, which means only when you have a permission problem which cannot be solved by using conventional POSIX settings.
• Use as few user groups as possible. Don't over-organize your users.
• Avoid to define Access Control Entries for users. Apply ACEs to user groups instead, whenever possible.
• If you want to protect certain files, use POSIX permissions to define very limited access rights, then use as few ACEs as possible to grant permissions to the user groups which should have access.
• Use inheritance whenever possible. If you inherit permissions, you only need to maintain ACLs for a small list of top folders.
• Avoid Access Control Entries of the deny type. Denials can easily create unexpected side effects. You might inadvertently lose the right to access some objects yourself, or worse, also lose the right to release this restriction.
• Never apply ACLs to parts of Mac OS X, and never try to redefine the access permissions on system files. The computer might become unusable.

File Systems Supporting ACLs

Access Control Lists can only be used on file systems which are capable of storing them. Mac OS X allows using ACLs when working with the following types of file systems, under the prerequisite that the computers hosting these file systems are using an operating system version generally capable of handling ACLs:

• disk volumes formatted with the Mac OS Extended file system (HFS+),
• network volumes accessed via the Apple Filing Protocol (AFP, AppleShare)
• network volumes accessed via the SMB/CIFS Protocol (Microsoft® Windows)
• network volumes accessed via the NFS version 4 protocol (UNIX systems, available in Lion only).

Other file systems, e.g. disk volumes formatted using UFS, FAT, VFAT, FAT32, NTFS, or ZFS, and network volumes accessed via NFSv2, NFSv3, FTP, or WebDAV cannot support Access Control Lists. Not supporting ACLs over a file server connection means that the client computer cannot “see” or modify ACLs stored on the server. However, if the file server is capable of using ACLs, it will still respect them, no matter if the accessing computer may notice this or not.

Show or Set Permissions

Displaying Permissions

TinkerTool System can display the full set of POSIX and ACL permissions which are currently set for a specific file or folder. The settings are displayed in a clear table, sorted by the same order in which evaluation of effective rights takes place. The table can also be used to change permission settings.

The Finder of Mac OS X is not capable of displaying the “true” permission settings of a file system object. Due to several design flaws, the section Sharing & Permissions in the Get Info panel of the Finder may show a very simplified or even wrong summary of the permission settings. TinkerTool System, however, will display the true settings, as they are defined and stored by the core operating system. For this reason, some permission details shown can differ between the two applications. In such a case you should not trust the display of the Finder.

To display or change the current permission settings of a file system object, perform the following steps:

1. Open the tab item Show or Set Permissions in the pane ACL Permissions.
2. Drag the file or folder from the Finder into the field File or folder. You can also click the button […] to navigate to the object, or click on the white area to enter the UNIX path of the object.
3. The current settings will be shown in the table.

Header lines in the table show which rights are ACEs of an ACL, and which are based on the conventional POSIX settings. The columns specify the following information:

• the user or group for which an entry takes effect,
• the type of entry, namely to allow or deny permission,
• the permission setting, in simple terms,
• a marker if the entry has been inherited or is explicit,
• the inheritance settings.

If a permission is being displayed as Custom, it will indicate that the rights cannot be described by simple terms, like read only. Remember that there are 98,304 different concepts of permissions which can be defined by combining ACL rights. To see the 13 detail rights and 4 inheritance settings (for folders) exactly, double-click a line of the table. Alternatively, you can click on the button with the pencil icon directly below the table.

Changing permissions

After you have chosen an item and TinkerTool System is displaying its permission settings in the table, every aspect of the settings can be changed. After you have made all desired changes, you can press the button Apply in the lower right corner to save the current settings. The button Revert will discard all changes you have made and TinkerTool System will go back to the original settings currently stored for the object in question.

If you like to modify the Type of an entry, or want to change one of the Permission concepts to one of the simple standard terms, you can do so by using the pop-up buttons in the table.

To change user or group of an entry, perform the following steps:

1. Double click the respective line of the table, or select the line and press the pencil button.
2. In the detail sheet, press the button Set… at the top of the panel.
3. In the new sheet, select either Users or Groups (if applicable).
4. Select a user or group in the table and press the OK button.
5. In the detail sheet, press the Close button.

The entry type and the detail rights can be changed in the same fashion. Note that the detail sheet is grouping the rights and inheritance settings into four categories. You can enable or disable all rights in a category by setting or removing the check mark in the respective group header. Enabling all rights of an ACE is also possible by selecting the item Full Control in the Permission pop-up. The inheritance settings will be set to appropriate defaults in this case.

To add an ACE, press the button [+] below the table. To remove one or more ACEs, use the [-] button. To reorder an ACL, drag a line in the ACE section of the table and drop it at its intended new position. Note that objects always have well-defined POSIX permissions and that POSIX permissions are always evaluated in the predefined user-group-others order, so it won't be possible to remove or reorder one of the lines below the POSIX headline.

Additional Operations

Additional operations can be performed by selecting one of the items in the pull-down menu Operations at the bottom of the window. The operations vary depending on whether you have selected a file or a folder.

If you have selected a folder, you can:

• Sort Access Control List Canonically: This means that the ACL will be brought into a recommended order which is considered to be “normal”. The canonical sort order is: explicit deny entries, explicit allow entries, inherited deny entries, inherited allow entries.
• Remove Inherited Entries: ACEs inherited from objects at higher levels in the folder hierarchy will be removed.
• Make Inherited Entries Explicit: all inherited ACEs will be replaced by explicit entries of the same contents.
• Propagate Permissions: This feature can be used to transfer the permission settings of the current folder to all objects at deeper levels in the folder hierarchy. TinkerTool System will ask what categories of permissions you want to propagate in detail. You can propagate any combination of owner entry, group owner entry, owner permissions, group permissions, permissions of others, and Access Control List. This will completely reset all selected permission settings of all objects enclosed in the chosen folder. For security reasons, objects with special permissions settings (SUID/GUID) will be excluded from the operation automatically.

When propagating permissions in folders containing symbolic links, the program will operate on the links themselves. The objects referred by the links will remain unchanged. Folders referred by a link won't be traversed.

If you have selected a file, you can:

• Sort Access Control List Canonically: see above.
• Remove Access Control List: this will remove the entire ACL.
• Get Inherited Access Control List: TinkerTool System will load a new ACL based on the Access Control List Mac OS X is creating for new files in that folder, based on the current inheritance settings effective in that folder.

With exception of the propagation feature, the operations will modify the permissions table first, not the actual settings on disk. The changes will take effect after pressing the Apply button.

Effective Permissions

The combination of several Access Control Entries and the POSIX permissions can make it difficult to estimate how the final rights for a certain user will be. TinkerTool System can compute and display the effective permissions of a user. This feature is helpful if you don't have much experience with permission settings yet. To display effective permissions, perform the following steps:

1. Open the tab item Effective Permissions on the pane ACL Permissions.
2. Drag a file or folder from the Finder into the field File or folder. You can also click the button […] to navigate to the object, or click on the white area to enter the UNIX path of the object.
3. Press the button Select… to choose one of the known user accounts of the current computer.
4. TinkerTool System will display the results in the table at the bottom. Rights currently granted to this user will be displayed by a green marker, rights currently denied by a red marker.

Special Permissions

The set of POSIX permissions contains three special settings, named SUID, GUID, and sticky. For their individual meanings, please see the introductory sections earlier in this chapter. TinkerTool System can display and change any of the three settings. Perform the following steps:

1. Open the tab item Special Permissions on the pane ACL Permissions.
2. Drag a file or folder from the Finder into the field File or folder. You can also click the button […] to navigate to the object, or click on the white area to enter the UNIX path of the object.
3. The current settings will be displayed. You can modify the fields Owner, Group owner, SUID, GUID, and Sticky as desired.
4. Press the button Apply to save the new settings.

Warning: As mentioned in the introduction, setting the SUID or GUID markers may cause very serious security problems affecting the whole operating system. It should never be necessary to set the SUID/GUID markers for programs when their installers have not set the flags already. Removing flags can cause the affected programs to malfunction. You should not use this feature if you don't know exactly what you are doing.