Chapter 7. Using ACLs to Protect ZFS Files

Table of Contents

New Solaris ACL Model
Syntax Descriptions for Setting ACLs
ACL Inheritance
ACL Property Modes
Setting ACLs on ZFS Files
Setting and Displaying ACLs on ZFS Files in Verbose Format
Setting ACL Inheritance on ZFS Files in Verbose Format
Setting and Displaying ACLs on ZFS Files in Compact Format

New Solaris ACL Model

Recent previous versions of Solaris supported an ACL implementation that was primarily based on the POSIX-draft ACL specification. The POSIX-draft based ACLs are used to protect UFS files and are translated by versions of NFS prior to NFSv4.

With the introduction of NFSv4, a new ACL model fully supports the interoperability that NFSv4 offers between UNIX and non-UNIX clients. The new ACL implementation, as defined in the NFSv4 specification, provides much richer semantics that are based on NT-style ACLs.

The main differences of the new ACL model are as follows:

  • Based on the NFSv4 specification and similar to NT-style ACLs.

  • Provide much more granular set of access privileges. For more information, see Table 7–2.

  • Set and displayed with the chmod and ls commands rather than the setfacl and getfacl commands.

  • Provide richer inheritance semantics for designating how access privileges are applied from directory to subdirectories, and so on. For more information, see ACL Inheritance.

Both ACL models provide more fine-grained access control than is available with the standard file permissions. Much like POSIX-draft ACLs, the new ACLs are composed of multiple Access Control Entries (ACEs).

POSIX-draft style ACLs use a single entry to define what permissions are allowed and what permissions are denied. The new ACL model has two types of ACEs that affect access checking: ALLOW and DENY. As such, you cannot infer from any single ACE that defines a set of permissions whether or not the permissions that weren't defined in that ACE are allowed or denied.

Translation between NFSv4-style ACLs and POSIX-draft ACLs is as follows:

  • If you use any ACL-aware utility, such as the cp, mv, tar, cpio, or rcp commands, to transfer UFS files with ACLs to a ZFS file system, the POSIX-draft ACLs are translated into the equivalent NFSv4-style ACLs.

  • Some NFSv4-style ACLs are translated to POSIX-draft ACLs. You see a message similar to the following if an NFSv4–style ACL isn't translated to a POSIX-draft ACL:

    # cp -p filea /var/tmp
    cp: failed to set acl entries on /var/tmp/filea
  • If you create a UFS tar or cpio archive with the preserve ACL option (tar p or cpio P) on a system that runs a current Solaris release, you will lose the ACLs when the archive is extracted on a system that runs a previous Solaris release.

    All of the files are extracted with the correct file modes, but the ACL entries are ignored.

  • You can use the ufsrestore command to restore data into a ZFS file system, but the ACLs will be lost.

  • If you attempt to set an NFSv4-style ACL on a UFS file, you see a message similar to the following:

    chmod: ERROR: ACL type's are different
  • If you attempt to set a POSIX-style ACL on a ZFS file, you will see messages similar to the following:

    # getfacl filea
    File system doesn't support aclent_t style ACL's.
    See acl(5) for more information on Solaris ACL support.

For information about other limitations with ACLs and backup products, see Saving ZFS Data With Other Backup Products.

Syntax Descriptions for Setting ACLs

Two basic ACL formats are provided as follows:

Syntax for Setting Trivial ACLs

chmod [options] A[index]{+|=}[email protected], [email protected], [email protected]: access-permissions/...[:inheritance-flags]:deny | allow file

chmod [options] [email protected], [email protected], [email protected]: access-permissions/...[:inheritance-flags]:deny | allow file ...

chmod [options] A[index]- file

Syntax for Setting Explicit ACLs

chmod [options] A[index]{+|=}user|group: access-permissions/...[:inheritance-flags]:deny | allow file

chmod [options] A-user|group: access-permissions/...[:inheritance-flags]:deny | allow file ...

chmod [options] A[index]- file

[email protected], [email protected], [email protected]

Identifies the ACL-entry-type for trivial ACL syntax . For a description of ACL-entry-types, see Table 7–1.

user or group:ACL-entry-ID=username or groupname

Identifies the ACL-entry-type for explicit ACL syntax. The user and group ACL-entry-type must also contain the ACL-entry-ID, username or groupname. For a description of ACL-entry-types, see Table 7–1.

access-permissions/.../

Identifies the access permissions that are granted or denied. For a description of ACL access privileges, see Table 7–2.

inheritance-flags

Identifies an optional list of ACL inheritance flags. For a description of the ACL inheritance flags, see Table 7–3.

deny | allow

Identifies whether the access permissions are granted or denied.

In the following example, the ACL-entry-ID value is not relevant.

[email protected]:write_data/append_data/execute:deny

The following example includes an ACL-entry-ID because a specific user (ACL-entry-type) is included in the ACL.

0:user:gozer:list_directory/read_data/execute:allow

When an ACL entry is displayed, it looks similar to the following:

2:[email protected]:write_data/append_data/execute:deny

The 2 or the index-ID designation in this example identifies the ACL entry in the larger ACL, which might have multiple entries for owner, specific UIDs, group, and everyone. You can specify the index-ID with the chmod command to identify which part of the ACL you want to modify. For example, you can identify index ID 3 as A3 to the chmod command, similar to the following:

chmod A3=user:venkman:read_acl:allow filename

ACL entry types, which are the ACL representations of owner, group, and other, are described in the following table.

Table 7.1. ACL Entry Types

ACL Entry Type

Description

[email protected]

Specifies the access granted to the owner of the object.

[email protected]

Specifies the access granted to the owning group of the object.

[email protected]

Specifies the access granted to any user or group that does not match any other ACL entry.

user

With a user name, specifies the access granted to an additional user of the object. Must include the ACL-entry-ID, which contains a username or userID. If the value is not a valid numeric UID or username, the ACL entry type is invalid.

group

With a group name, specifies the access granted to an additional group of the object. Must include the ACL-entry-ID, which contains a groupname or groupID. If the value is not a valid numeric GID or groupname, the ACL entry type is invalid.

ACL access privileges are described in the following table.

Table 7.2. ACL Access Privileges

Access Privilege

Compact Access Privilege

Description

add_file

w

Permission to add a new file to a directory.

add_subdirectory

p

Permission to create a subdirectory in a directory.

append_data

p

On a file, permission to modify the contents of a file.

delete

d

Permission to delete a file.

delete_child

D

Permission to delete a file or directory within a directory.

execute

x

Permission to execute a file or search the contents of a directory.

list_directory

r

Permission to list the contents of a directory.

read_acl

c

Permission to read the ACL (ls).

read_attributes

a

Permission to read basic attributes (non-ACLs) of a file. Think of basic attributes as the stat level attributes. Allowing this access mask bit means the entity can execute ls(1) and stat(2).

read_data

r

Permission to read the contents of the file.

read_xattr

R

Permission to read the extended attributes of a file or perform a lookup in the file's extended attributes directory.

synchronize

s

Placeholder, not used at this time.

write_xattr

A

Permission to create extended attributes or write to the extended attributes directory.

Granting this permission to a user means that the user can create an extended attribute directory for a file. The attribute file's permissions control the user's access to the attribute.

write_data

w

Permission to modify or replace the contents of a file.

write_attributes

W

Permission to change the times associated with a file or directory to an arbitrary value.

write_acl

C

Permission to write the ACL or the ability to modify the ACL by using the chmod command.

write_owner

o

Permission to change the file's owner or group. Or, the ability to execute the chown or chgrp commands on the file.

Permission to take ownership of a file or permission to change the group ownership of the file to a group of which the user is a member. If you want to change the file or group ownership to an arbitrary user or group, then the PRIV_FILE_CHOWN privilege is required.

ACL Inheritance

The purpose of using ACL inheritance is so that a newly created file or directory can inherit the ACLs they are intended to inherit, but without disregarding the existing permission bits on the parent directory.

By default, ACLs are not propagated. If you set an explicit ACL on a directory, it is not inherited to any subsequent directory. You must specify the inheritance of an ACL on a file or directory.

The optional inheritance flags are described in the following table.

Table 7.3. ACL Inheritance Flags

Inheritance Flag

Compact Inheritance Flag

Description

file_inherit

f

Only inherit the ACL from the parent directory to the directory's files.

dir_inherit

d

Only inherit the ACL from the parent directory to the directory's subdirectories.

inherit_only

i

Inherit the ACL from the parent directory but applies only to newly created files or subdirectories and not the directory itself. This flag requires the file_inherit flag, the dir_inherit flag, or both, to indicate what to inherit.

no_propagate

n

Only inherit the ACL from the parent directory to the first-level contents of the directory, not the second-level or subsequent contents. This flag requires the file_inherit flag, the dir_inherit flag, or both, to indicate what to inherit.

In addition, you can set a default ACL inheritance policy on the file system that is more strict or less strict by using the aclinherit file system property. For more information, see the next section.

ACL Property Modes

The ZFS file system includes two property modes related to ACLs:

  • aclinherit – This property determines the behavior of ACL inheritance. Values include the following:

    • discard – For new objects, no ACL entries are inherited when a file or directory is created. The ACL on the file or directory is equal to the permission mode of the file or directory.

    • noallow – For new objects, only inheritable ACL entries that have an access type of deny are inherited.

    • secure – For new objects, the write_owner and write_acl permissions are removed when an ACL entry is inherited.

    • passthrough – For new objects, the inheritable ACL entries are inherited with no changes made to the them. This mode, in effect, disables secure mode.

    The default mode for the aclinherit is secure.

  • aclmode – This property modifies ACL behavior whenever a file or directory's mode is modified by the chmod command or when a file is initially created. Values include the following:

    • discard – All ACL entries are removed except for the entries needed to define the mode of the file or directory.

    • groupmask – User or group ACL permissions are reduced so that they are no greater than the group permission bits, unless it is a user entry that has the same UID as the owner of the file or directory. Then, the ACL permissions are reduced so that they are no greater than owner permission bits.

    • passthrough – For new objects, the inheritable ACL entries are inherited with no changes made to the them.

    The default mode for the aclmode property is groupmask.