Secure Files/Directories using ACLs (Access Control Lists) in Linux

TABLE OF CONTENTS

As a System Admin, our first priority will be to protect and secure data from unauthorized access. We all are aware of the permissions that we set using some helpful Linux commands like chmod, chown, chgrp… etc. However, these default permission sets have some limitation and sometimes may not work as per our needs. For example, we cannot set up different permission sets for different users on same directory or file. Thus, Access Control Lists (ACLs) were implemented.

Let’s say, you have three users, ‘tecmint1‘, ‘tecmint2‘ and ‘tecmint3‘. Each having common group say ‘acl’. User ‘tecmint1‘ want that only ‘tecmint2‘ user can read and access files owned by ‘tecmint1‘ and no one else should have any access on that.

ACLs (Access Control Lists) allows us doing the same trick. These ACLs allow us to grant permissions for a user, group and any group of any users which are not in the group list of a user.

Note: As per Redhat Product Documentation, it provides ACL support for ext3 file system and NFS exported file systems.

1. Check Kernel for ACL Support

Run the following command to check ACL Support for file system and POSIX_ACL=Y option (if there is N instead of Y, then it means Kernel doesn’t support ACL and need to be recompiled).

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[root@linux ~]# grep -i acl /boot/config*

CONFIG_EXT4_FS_POSIX_ACL=y
CONFIG_REISERFS_FS_POSIX_ACL=y
CONFIG_JFS_POSIX_ACL=y
CONFIG_XFS_POSIX_ACL=y
CONFIG_BTRFS_FS_POSIX_ACL=y
CONFIG_FS_POSIX_ACL=y
CONFIG_GENERIC_ACL=y
CONFIG_TMPFS_POSIX_ACL=y
CONFIG_NFS_V3_ACL=y
CONFIG_NFSD_V2_ACL=y
CONFIG_NFSD_V3_ACL=y
CONFIG_NFS_ACL_SUPPORT=m
CONFIG_CIFS_ACL=y
CONFIG_9P_FS_POSIX_ACL=y

2. Check Required Packages

Before starting playing with ACLs make sure that you have required packages installed. Below are the required packages that needs to be installed using yum or apt-get.

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[root@linux ~]# yum install nfs4-acl-tools acl libacl        [on RedHat based systems]

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[tecmint@linux ~]$ sudo apt-get install nfs4-acl-tools acl    [on Debian based systems]

3. Check Mounted File System for ACLs Support

Now, check the mounted file system that whether it is mounted with ACL option or not. We can use ‘mount‘ command for checking the the same as shown below.

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[root@linux ~]# mount  | grep -i root

/dev/mapper/fedora-root on / type ext4 (rw,relatime,data=ordered)

But in our case its not showing acl by default. So, next we have option to remount the mounted partition again using acl option. But, before moving ahead, we have another option to make sure that partition is mounted with acl option or not, because for recent system it may be integrated with default mount option

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[root@linux ~]# tune2fs -l /dev/mapper/fedora-root | grep acl

Default mount options:    user_xattr acl

In the above output, you can see that default mount option already have support for acl. Another option is to remount the partition as shown below.

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[root@linux ~]# mount -o remount,acl /

Next, add the below entry to ‘/etc/fstab’ file to make it permanent.

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/dev/mapper/fedora-root /    ext4    defaults,acl 1 1

Again, remount the partition.

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[root@linux ~]# mount -o remount  /

4. For NFS Server

On NFS server, if file system which is exported by NSF server supports ACL and ACLs can be read by NFS Clients, then ACLs are utilized by client System.

For disabling ACLs on NFS share, you have to add option “no_acl” in ‘/etc/exportfs‘ file on NFS Server. To disable it on NSF client side again use “no_acl” option during mount time.

How to Implement ACL Support in Linux Systems

There are two types of ACLs:

1. Access ACLs: Access ACLs are used for granting permissions on any file or directory.

2. Default ACLs: Default ACLs are used for granting/setting access control list on a specific directory only.

Difference between Access ACL and Default ACL:

1. Default ACL can be used on directory level only.

2. Any sub directory or file created within that directory will inherit the ACLs from its parent directory. On the other hand a file inherits the default ACLs as its access ACLs.

3. We make use of “–d” for setting default ACLs and Default ACLs are optionals.

Before Setting Default ACLs

To determine the default ACLs for a specific file or directory, use the ‘getfacl‘ command. In the example below, the getfacl is used to get the default ACLs for a folder ‘Music‘.

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[root@linux ~]# getfacl Music/

# file: Music/
# owner: root
# group: root
user::rwx
group::r-x
other::r-x
default:user::rwx
default:group::r-x
default:other::rw-

After Setting Default ACLs

To set the default ACLs for a specific file or directory, use the ‘setfacl‘ command. In the example below, the setfacl command will set a new ACLs (read and execute) on a folder ‘Music’.

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[root@linux ~]# setfacl -m d:o:rx Music/
[root@linux ~]# getfacl Music/
# file: Music/
# owner: root
# group: root
user::rwx
group::r-x
other::r-x
default:user::rwx
default:group::r-x
default:other::r-x

How to Set New ACLs

Use the ‘setfacl’ command for setting or modifying on any file or directory. For example, to give read and write permissions to user ‘tecmint1‘.

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# setfacl -m u:tecmint1:rw /tecmint1/example

How to View ACLs

Use the ‘getfacl‘ command for viewing ACL on any file or directory. For example, to view ACL on ‘/tecmint1/example‘ use below command.

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# getfacl /tecmint1/example

# file: tecmint1/example/
# owner: tecmint1
# group: tecmint1
user::rwx
user:tecmint1:rwx
user:tecmint2:r--
group::rwx
mask::rwx
other::---

How to Remove ACLs

For removing ACL from any file/directory, we use x and b options as shown below.

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# setfacl -x ACL file/directory      # remove only specified ACL from file/directory.

# setfacl -b  file/directory           #removing all ACL from file/direcoty

Let’s implement ACL’s on following scenario’s.

Two Users (tecmint1 and tecmint2), both having common secondary group named ‘acl‘. We will create one directory owned by ‘tecmint1‘ and will provide the read and execute permission on that directory to user ‘tecmint2‘.

Step 1: Create two users and remove password from both

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[root@linux ~]# for user in tecmint1 tecmint2

> do
> useradd $user
> passwd -d $user
> done
Removing password for user tecmint1.
passwd: Success
Removing password for user tecmint2.
passwd: Success

Step 2: Create a Group and Users to Secondary Group.

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[root@linux ~]# groupadd acl
[root@linux ~]# usermod -G acl tecmint1
[root@linux ~]# usermod -G acl tecmint2

Step 3: Create a Directory /tecmint and change ownership to tecmint1.

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[root@linux ~]# mkdir /tecmint1
[root@linux ~]# chown tecmint1 /tecmint1/

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[root@linux ~]# ls -ld /tecmint1/

drwxr-xr-x 2 tecmint1 root 4096 Apr 17 14:46 /tecmint1/

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[root@linux ~]# getfacl /tecmint1

getfacl: Removing leading '/' from absolute path names
# file: tecmint1
# owner: tecmint1
# group: root
user::rwx
group::r-x
other::r-x

Step 4: Login with tecmint1 and create a Directory in /tecmint folder.

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[tecmint@linux ~]$ su - tecmint1

Last login: Thu Apr 17 14:49:16 IST 2014 on pts/4

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[tecmint1@linux ~]$ cd /tecmint1/
[tecmint1@linux tecmint1]$ mkdir example

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[tecmint1@linux tecmint1]$ ll

total 4
drwxrwxr-x 2 tecmint1 tecmint1 4096 Apr 17 14:50 example

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[tecmint1@linux tecmint1]$ whoami 
tecmint1

Step 5: Now set ACL using ‘setfacl‘, so that ‘tecmint1‘ will have all rwx permissions, ‘tecmint2‘ will have only read permission on ‘example‘ folder and other will have no permissions.

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$ setfacl -m u:tecmint1:rwx example/
$ setfacl -m u:tecmint2:r-- example/
$ setfacl -m  other:--- example/
$ getfacl example/

# file: example
# owner: tecmint1
# group: tecmint1
user::rwx
user:tecmint1:rwx
user:tecmint2:r--
group::r-x
mask::rwx
other::---

Step 6: Now login with other user i.e. ‘tecmint2‘ on another terminal and change directory to ‘/tecmint1‘. Now try to view the contents using ‘ls‘ command and then try to change directory and see the difference as below.

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[tecmint@linux ~]$ su - tecmint2

Last login: Thu Apr 17 15:03:31 IST 2014 on pts/5

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[tecmint2@linux ~]$ cd /tecmint1/
[tecmint2@linux tecmint1]$ ls -lR example/
example/:
total 0

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[tecmint2@linux tecmint1]$ cd example/

-bash: cd: example/: Permission denied

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[tecmint2@linux tecmint1]$ getfacl example/

# file: example
# owner: tecmint1
# group: tecmint1
user::rwx
user:tecmint1:rwx
user:tecmint2:r--
group::rwx
mask::rwx
other::---

Step 7: Now give ‘execute‘ permission to ‘tecmint2‘ on ‘example‘ folder and then use ‘cd‘ command to see the effect. Now ‘tecmint2‘ have the permissions to view and change directory, but don’t have permissions for writing anything.

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[tecmint1@linux tecmint1]$ setfacl -m u:tecmint2:r-x example/
[tecmint1@linux tecmint1]$ getfacl example/

# file: example
# owner: tecmint1
# group: tecmint1
user::rwx
user:tecmint1:rwx
user:tecmint2:r-x
group::rwx
mask::rwx
other::---

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[tecmint@linux ~]$ su - tecmint2

Last login: Thu Apr 17 15:09:49 IST 2014 on pts/5

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[tecmint2@linux ~]$ cd /tecmint1/
[tecmint2@linux tecmint1]$ cd example/
[tecmint2@linux example]$ getfacl .

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[tecmint2@linux example]$ mkdir test

mkdir: cannot create directory ‘test’: Permission denied

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[tecmint2@linux example]$ touch test

touch: cannot touch ‘test’: Permission denied

Note: After implementing ACL, you will see a extra ‘+‘ sign for ‘ls –l’ output as below.

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[root@linux tecmint1]# ll

total 4
drwxrwx---+ 2 tecmint1 tecmint1 4096 Apr 17 17:01 example