Implementing and Managing File Access Policy (FAP) in Linux

 

Introduction

Ensuring secure file access in a Linux environment is essential for preventing unauthorized data access and protecting system integrity. This guide will help you understand and implement File Access Policy (FAP) strategies on Linux, from standard permissions to advanced tools like Access Control Lists (ACLs) and SELinux.

Implementing and Managing FAP in Linux

Understanding File Access Policies in Linux

Linux File Access Policies (FAP) define who can access what files or directories, how they can interact with them, and ensure system security by restricting unauthorized access. Linux typically enforces file access policies through basic permissions and extended features such as Access Control Lists (ACLs) and SELinux.

Standard Linux File Permissions

The traditional approach to file access control in Linux involves the permission model that assigns read (r), write (w), and execute (x) permissions for three categories:

Category	Description
Owner	The user who owns the file or directory
Group	The group associated with the file or directory
Others (World)	Any user not part of the owner or group category

File Permissions Explained

Each file or directory has a 10-character string that represents permissions, as seen in ls -l output:

-rwxr-xr--

Character	Meaning
1st (File Type)	- (file), d (directory), l (link)
2-4 (Owner)	Read, write, execute for the owner
5-7 (Group)	Read, write, execute for the group
8-10 (Others)	Read, write, execute for others

Managing File Permissions with CLI Examples

Permissions can be modified using the chmod command.

Assigning Read and Write Permission to Owner

chmod u+rw filename

Removing Execute Permission from Group and Others

chmod go-x filename

Setting Exact Permissions with Octal Notation

chmod 755 filename

Octal Code	Permissions
7	Read, write, and execute (rwx)
5	Read and execute (r-x)
4	Read only (r–)
0	No permissions (—)

Advanced Access Control with ACLs

Access Control Lists (ACLs) provide finer-grained control over file access by allowing specific permissions for individual users or groups beyond the owner, group, and others categories.

Enable ACL Support on Filesystems

Use the following command to check if ACL support is enabled:

mount | grep acl

Setting ACLs for a User

setfacl -m u:username:rw filename

Viewing ACLs for a File

getfacl filename

Command	Description
setfacl -m	Modify ACL entry for user/group
getfacl	View ACL entries for a file or directory
setfacl -b	Remove all ACL entries

Enforcing Policies with SELinux

Security-Enhanced Linux (SELinux) provides a robust security model for enforcing mandatory access control (MAC). SELinux assigns labels to files and defines access based on security contexts rather than traditional ownership.

Check SELinux Status

sestatus

Set File Contexts

sudo semanage fcontext -a -t httpd_sys_content_t "/web(/.*)?"

Applying the New Context

sudo restorecon -R /web

SELinux Command	Description
semanage fcontext	Define file context
restorecon	Apply defined context to files
sestatus	Display SELinux status

Using Audit Logs for FAP Monitoring

Audit Logs in Linux help monitor and verify FAP by logging access events for critical files and directories.

Installing Auditd

sudo dnf install audit

Adding a Watch on Sensitive Files

sudo auditctl -w /etc/passwd -p rwxa -k passwd_changes

Viewing Audit Logs

sudo ausearch -k passwd_changes

Audit Command	Description
auditctl	Set up file watches
ausearch	Search audit logs based on keywords
audispd	Dispatch audit events to specific logs