Difference between revisions of "Access Control Lists"

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(Set ACL: Added --test as possible recommendation)
(Granting execution permissions for private files to a web server: you don't want *all* files in your home directory to be accessible by http, so -d is questionable (especially without explanation))
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The first step is granting execution permission to {{ic|http}} so it can access {{ic|geoffrey}}'s home:  
 
The first step is granting execution permission to {{ic|http}} so it can access {{ic|geoffrey}}'s home:  
  # setfacl -dm "u:http:--x" /home/geoffrey
+
  # setfacl -m "u:http:--x" /home/geoffrey
 
''Remember'': Execution permissions to a directory are necessary for a process to list the directory's content.
 
''Remember'': Execution permissions to a directory are necessary for a process to list the directory's content.
  
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}}
 
}}
  
As the above output shows, {{ic|other}}'s no longer have any permissions, but the user {{ic|http}} is still able to execute the files, thus security might be considered increased.
+
As the above output shows, {{ic|other}}'s no longer have any permissions, but the user {{ic|http}} is still able to access the files, thus security might be considered increased.
  
 
== See also ==
 
== See also ==

Revision as of 08:51, 6 December 2018

Access control list (ACL) provides an additional, more flexible permission mechanism for file systems. It is designed to assist with UNIX file permissions. ACL allows you to give permissions for any user or group to any disk resource.

Installation

The acl package is a dependency of systemd, it should already be installed.

Configuration

Enabling ACL

To enable ACL, the filesystem must be mounted with the acl option. You can use fstab to make it permanent on your system.

There is a possibility that the acl option is already active as default mount option on the filesystem. Btrfs does and Ext2/3/4 filesystems do too. Use the following command to check ext* formatted partitions for the option:

# tune2fs -l /dev/sdXY | grep "Default mount options:"
Default mount options:    user_xattr acl

Also check that the default mount option is not overridden, in such case you will see noacl in /proc/mounts in the relevant line.

You can set the default mount options of a filesystem using the tune2fs -o option partition command, for example:

# tune2fs -o acl /dev/sdXY

Using the default mount options instead of an entry in /etc/fstab is very useful for external drives, such partition will be mounted with acl option also on other Linux machines. There is no need to edit /etc/fstab on every machine.

Note:
  • acl is specified as default mount option when creating an ext2/3/4 filesystem. This is configured in /etc/mke2fs.conf.
  • The default mount options are not listed in /proc/mounts.

Set ACL

The ACL can be modified using the setfacl command.

Note: It is recommended to list files/directory changes first (i.e. dry-run) by appending the --test flag.

To add permissions for a user (user is either the user name or ID):

# setfacl -m "u:user:permissions" <file/dir>

To add permissions for a group (group is either the group name or ID):

# setfacl -m "g:group:permissions" <file/dir>

To allow all files or directories to inherit ACL entries from the directory it is within:

# setfacl -dm "entry" <dir>

To remove a specific entry:

# setfacl -x "entry" <file/dir>

To remove all extended ACL entries (the base ACL entries of the owner, group and others are retained):

# setfacl -b <file/dir>

To remove the default ACL entries:

# setfacl -k <file/dir>
Tip: To apply operations to all files and directories recursively, append the -R argument.

Show ACL

To show permissions, use:

# getfacl <file/dir>

Examples

Set all permissions for user johny to file named "abc":

# setfacl -m "u:johny:rwx" abc

Check permissions

# getfacl abc
# file: abc
# owner: someone
# group: someone
user::rw-
user:johny:rwx
group::r--
mask::rwx
other::r--

Change permissions for user johny:

# setfacl -m "u:johny:r-x" abc

Check permissions

# getfacl abc
# file: abc
# owner: someone
# group: someone
user::rw-
user:johny:r-x
group::r--
mask::r-x
other::r--

Remove all extended ACL entries:

# setfacl -b abc

Check permissions

# getfacl abc
# file: abc
# owner: someone
# group: someone
user::rw-
group::r--
other::r--

Output of ls command

You will notice that there is an ACL for a given file because it will exhibit a + (plus sign) after its Unix permissions in the output of ls -l.

$ ls -l /dev/audio
crw-rw----+ 1 root audio 14, 4 nov.   9 12:49 /dev/audio
$ getfacl /dev/audio
getfacl: Removing leading '/' from absolute path names
# file: dev/audio
# owner: root
# group: audio
user::rw-
user:solstice:rw-
group::rw-
mask::rw-
other::---

Granting execution permissions for private files to a web server

The following technique describes how a process like a web server can be granted access to files that reside in a user's home directory, without compromising security by giving the whole world access.

In the following we assume that the web server runs as the user http and grant it access to geoffrey's home directory /home/geoffrey.

The first step is granting execution permission to http so it can access geoffrey's home:

# setfacl -m "u:http:--x" /home/geoffrey

Remember: Execution permissions to a directory are necessary for a process to list the directory's content.

Since http is now able to access files in /home/geoffrey, other no longer needs access, so it can be safely removed:

# chmod o-rx /home/geoffrey

Use getfacl to verify the changes:

$ getfacl /home/geoffrey
getfacl: Removing leading '/' from absolute path names
# file: home/geoffrey
# owner: geoffrey
# group: geoffrey
user::rwx
user:http:--x
group::r-x
mask::r-x
other::---

As the above output shows, other's no longer have any permissions, but the user http is still able to access the files, thus security might be considered increased.

See also