8. Debugging SELinux | SELinux Cookbook

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SELinux Cookbook

By : Sven Vermeulen

4.3 (7)

SELinux Cookbook

4.3 (7)

By: Sven Vermeulen

Overview of this book

If you are a Linux system administrator or a Linux-based service administrator and want to fine-tune SELinux to implement a supported, mature, and proven access control system, then this book is for you. Basic experience with SELinux enabled distributions is expected.

Preface

Preface

What this book covers

What you need for this book

Who this book is for

Conventions

Reader feedback

Customer support

Free Chapter

1. The SELinux Development Environment

1. The SELinux Development Environment

Introduction

Creating the development environment

Building a simple SELinux module

Calling refpolicy interfaces

Creating our own interface

Using the refpolicy naming convention

Distributing SELinux policy modules

2. Dealing with File Labels

2. Dealing with File Labels

Introduction

Defining file contexts through patterns

Using substitution definitions

Enhancing an SELinux policy with file transitions

Setting resource-sensitivity labels

Configuring sensitivity categories

3. Confining Web Applications

3. Confining Web Applications

Introduction

Listing conditional policy support

Enabling user directory support

Assigning web content types

Using different web server ports

Using custom content types

Creating a custom CGI domain

Setting up mod_selinux

Starting Apache with limited clearance

Mapping HTTP users to contexts

Using source address mapping to decide on contexts

Separating virtual hosts with mod_selinux

4. Creating a Desktop Application Policy

4. Creating a Desktop Application Policy

Introduction

Researching the application's logical design

Creating a skeleton policy

Setting context definitions

Defining application role interfaces

Testing and enhancing the policy

Ignoring permissions we don't need

Creating application resource interfaces

Adding conditional policy rules

Adding build-time policy decisions

5. Creating a Server Policy

5. Creating a Server Policy

Introduction

Understanding the service

Choosing resource types wisely

Differentiating policies based on use cases

Creating resource-access interfaces

Creating exec, run, and transition interfaces

Creating a stream-connect interface

Creating the administrative interface

6. Setting Up Separate Roles

6. Setting Up Separate Roles

Introduction

Managing SELinux users

Mapping Linux users to SELinux users

Running commands in a specified role with sudo

Running commands in a specified role with runcon

Switching roles

Creating a new role

Initial role based on entry

Defining role transitions

Looking into access privileges

7. Choosing the Confinement Level

7. Choosing the Confinement Level

Introduction

Finding common resources

Defining common helper domains

Documenting common privileges

Granting privileges to all clients

Creating a generic application domain

Building application-specific domains using templates

Using fine-grained application domain definitions

8. Debugging SELinux

8. Debugging SELinux

Introduction

Identifying whether SELinux is to blame

Analyzing SELINUX_ERR messages

Logging positive policy decisions

Looking through SELinux constraints

Ensuring an SELinux rule is never allowed

Using strace to clarify permission issues

Using strace against daemons

Auditing system behavior

9. Aligning SELinux with DAC

9. Aligning SELinux with DAC

Introduction

Assigning a different root location to regular services

Using a different root location for SELinux-aware applications

Sharing user content with file ACLs

Enabling polyinstantiated directories

Configuring capabilities instead of setuid binaries

Using group membership for role-based access

Backing up and restoring files

Governing application network access

10. Handling SELinux-aware Applications

10. Handling SELinux-aware Applications

Introduction

Controlling D-Bus message flows

Restricting service ownership

Understanding udev's SELinux integration

Using cron with SELinux

Checking the SELinux state programmatically

Querying SELinux userland configuration in C

Interrogating the SELinux subsystem code-wise

Running new processes in a new context

Reading the context of a resource

Index

Index

Ensuring an SELinux rule is never allowed

It is possible to include statements in the SELinux policy that ensure that a particular access vector cannot be allowed, not even by enhancing the SELinux policy later. This is done with the neverallow statement.

How to do it…

To include the neverallow statements in the policy and enforce them, go through the following steps:

In /etc/selinux/semanage.conf, enable support for the neverallow statements by setting the expand-check variable to 1:
```
expand-check=1
```
Create an SELinux policy in which the access vectors that should be explicitly forbidden are listed. Consider the following instance:
```
neverallow user_t system_mail_t:process transition;
```
Build and load the policy.

Generate another policy that will allow the statement and attempt to load it:

~$ semodule -i mytest.pp
libsepol.check_assertion_helper:  neverallow violated by allow user_t system_mail_t:process { transition };
libsemanage.semanage_expand_sandbox: Expand module failed
semodule: Failed!

How it...

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