SELinux: A Key Component in Secure Infrastructures
A presentation at IBM SHARE Orlando in in Orlando, FL, USA by Shawn Wells
Red Hat Deep Dive Sessions SELinux: A Key Component in Secure Infrastructures Shawn D. Wells, RHCE EMail: swells@redhat.com Solutions Architect @ Red Hat 1
Agenda 1) Why do we need SELinux? What are the principal concepts? 2) SELinux Details Type Enforcement What are the available policies? What’s a policy actually made of? How do I {add, change} a policy? What’s the associated overhead? 3) Usage User Perspective Admin Perspective 4) Scenarios Fixing the RHT Corporate VPN “update” 2
Name of Presentation Why do we need Red Hat Presenter SELinux?
Linux Access Control Problems 1) Access is based off users’ access Example: Firefox can read SSH keys # ps -x | grep firefox shawn 21375 1 35 11:38 ? 00:00:01 firefox-bin # ls -l id_rsa -rw———- 1 shawn shawn 1743 2008-08-10 id_rsa Fundamental Problem: Security properties not specific enough. Kernel can’t distinguish applications from users. 4
Linux Access Control Problems 2) Processes can change security properties Example: Mail files are readable only by me….. but Thunderbird could make them world readable Fundamental Problems: Standard access control is discretionary Includes concept of “resource ownership” Processes can escape security policy 5
Linux Access Control Problems 3) Only two privilege levels: User & root Example: Apache gets hacked, allowing remote access to root. Entire system is compromised. Fundamental Problems: Simplistic security policy No way to enforce least-privilege 6
Linux Access Control Introduction Linux access control involves the kernel controling Processes (running programs), which try to access… Resources (files, directories, sockets, etc) For example: Apache (process) can read web files But not the /etc/shadow file (resource) Traditional methods do not clearly separate the privileges of users and applications acting on the users behalf, increasing the damage that can be caused by application exploits. So, how should these decisions be made? 7
Security Architecture Every subject (i.e process) and object (i.e. data files) are assigned collections of security attributes, called a security context 1) Security context of subject & object passed to SELinux 2) Kernel/SELinux check, verify access 2a) Grant access. Record allowance in AVC (Access Vector Cache) 2b) Deny access, log error 8
Security Architecture Or in picture view… 9
Role Based Access Control (RBAC) “root” really isn’t “root” i.e: root_u:WebServerAdmin_r:SysAdmin_t root_u:OracleDBAdmin_r:SysAdmin_t 10
Name of Presentation SELinux Details Red Hat Presenter
Type Enforcement ● ● ● ● ● SELinux implements the MAC model through type enforcement. In RHEL5, SELinux also provides RBAC and Bell-LaPadula (MLS), but it uses type enforcement to implement them. Type Enforcement involves defining a type for every subject, that is, process, and object on the system. Permissions are checked between the source type and the target type for each access. Objects include (but are not limited to): ● Network Sockets ● Shared Memory Segments ● Files ● Processes ● etc. 12
SELinux Contexts root:object r:sysadm home t:s0:c0 ● ● ● ● ● ● The above is an SELinux context user_t role_t file_t Sensitivity category 13
SELinux Contexts 14
DAC vs MAC ● ● Application can change attributes User privileges = Process privileges ● ● Orthogonal to DAC Roles, Contexts, Types 15
How does SELinux Work? /etc/selinux/{targeted,strict}/policy /var/log/messages 16
SELinux Policy ● ● Policies are matrices of statements which tell SELinux if certain actions are allowed based on the context of the objects attempting those actions. There are three SELinux Policy Types 17
The Three SELinux Policy Types 1) Targeted Policy Default policy in RHEL5. Supported by HelpDesk. Targets specific applications to lock down. Allows all other applications to run in the unconfined domain (unconfined_t) Applications running in the unconfined domain run as if SELinux were disabled 18
The Three SELinux Policy Types 2) Strict Policy Denies access to everything by default Complete protection for all processes on the system Requires that policies be written for all applications, often requires customization Strict is type enforcement with added types for users (e.g. user_t and user_firefox_t). Not enabled by Red Hat as default 19
The Three SELinux Policy Types 3) Multi-Level Security (MLS) Focuses on confidentiality (i.e. separation of multiple classifications of data) Ability to manage {processes, users} with varying levels of access. (i.e. “the need to know”) Uses category & sensitivity levels 20
The Three SELinux Policy Types 3) Multi-Level Security (MLS) (a) Sensitivity Labels Mostly used by the government – Top Secret, Secret, Unclassified, etc s0 Unclassified s1 Secret s2 Top Secret 21
The Three SELinux Policy Types 3) Multi-Level Security (MLS) (b) Category Labels Separation of data types, compartments, projects, etc s0 s1 s1 Unclassified Secret Top Secret c0 Project A c0 Alpha c1 Project B c1 Bravo c2 Charlie c3 Delta 22
The Three SELinux Policy Types 3) Multi-Level Security (MLS) (b) Polyinstantiation & pam_namespace The pam_namespace PAM module sets up a private namespace for a session with polyinstantiated directories A polyinstantiated directory provides a different instance of itself based on user name, or when using SELinux, user name, security context or both 23
The Three SELinux Policy Types 3) Multi-Level Security (MLS) (b) Polyinstantiation & pam_namespace # id Z staff_u:WebServer_Admin_r:WebServer_Admin_t:s0:c0 # ls l /data secretfile1 secretfile 2 # id Z staff_u:WebServer_Admin_r:WebServer_Admin_t:s1:c0 # ls l /data secretfile1 secretfile 2 topsecretfile1 24
The Three SELinux Policy Types Multi-Level Security (MLS) & Common Criteria The Common Criteria (CC) is an international security standard against which systems are evaluated. Many government customers require CC evaluated systems. Red Hat Enterprise Linux 5 meets EAL4+ with RBAC/LSPP/CAPP endorcements 25
What’s the Performance Overhead? 26
What’s the Performance Overhead? 27
What’s the Performance Overhead? 28
What’s the Performance Overhead? ● Not official statistics ● Laptop = 2GHz, 2x 1GB RAM ● Workstation = 2.13GHz, 4x 1GB RAM ● Apache = Lots of threads ● MySQL = Lots of disk I/O 29
Name of Presentation SELinux Usage Red Hat Presenter (GUI & console)
End-User Perspective ● sealert Notifications 31
End-User Perspective ● sealert Browser 32
System Administrator Perspective ● sealert + EMail Notifications 33
System Administrator Perspective ● system-config-selinux 34
System Administrator Perspective ● sediffx 35
System Administrator Perspective ● apol 36
System Administrator Perspective ● semanage Configure elements of SELinux policy without modification/recompilation of policy sources … . aka on the fly Example: Dynamically Allowing Apache to listen on port 1234 # semanage port a t httpd_port_t p tcp 1234 37
System Administrator Perspective ● semanage (more examples) Example: Allow shawn to join “webadmin_u” group # semanage login a s webadmin_u shawn Example: Relabel files for access by Apache # semanage fcontext a t \ httpd_sys_content_t “/data/webpages(/.*)?” 38
System Administrator Perspective ● semanage (most important example) You don’t need to disable SELinux to fix a single error! type=SYSCALL msg=audit(1204719775.306:738): arch=40000003 syscall=54 success=no exit=19 a0=4 a1=8933 a2=bfcec1bc a3=bfcec1bc items=0 ppid=3900 pid=5003 auid=501 uid=0 gid=0 euid=0 suid=0 fsuid=0 egid=0 sgid=0 fsgid=0 tty=(none) comm=”ip” exe=”/sbin/ip” subj=user_u:system_r:ifconfig_t:s0 key=(null) The Fix: # semanage permissive a ifconfig_t 39
System Administrator Perspective ● audit2allow Allows generation of SELinux policy rules from logs of denied operations Example: Fix all the errors on the system (completely not a good idea on a real system)
cat /var/log/audit/audit.log | audit2allow M FixAll Generating type enforcment file: FixAll.te Compiling policy: checkmodule M m o FixAll.mod FixAll.te Building package: semodule_package o FixAll.pp m FixAll.mod
semodule i FixAll.pp
40
Name of Presentation Red Hat Presenter Scenarios
Scenario: Fixing the RHT corporate VPN “update” ● Red Hat has a Corporate Standard Build (CSB) for desktop environments ● Red Hat pushes updates to said CSB ● I “tweak” my configuration files ● When RHT pushed a CSB update, it broke my VPN settings 42
Scenario: Fixing the RHT corporate VPN “update” /var/log/messages: type=SYSCALL msg=audit(1204719775.306:738): arch=40000003 syscall=54 success=no exit=19 a0=4 a1=8933 a2=bfcec1bc a3=bfcec1bc items=0 ppid=3900 pid=5003 auid=501 uid=0 gid=0 euid=0 suid=0 fsuid=0 egid=0 sgid=0 fsgid=0 tty=(none) comm=”ip” exe=”/sbin/ip” subj=user_u:system_r:ifconfig_t:s0 key=(null) Now what? 43
Scenario: Fixing the RHT corporate VPN “update” type=SYSCALL msg=audit(1204719775.306:738): arch=40000003 syscall=54 success=no exit=19 a0=4 a1=8933 a2=bfcec1bc a3=bfcec1bc items=0 ppid=3900 pid=5003 auid=501 uid=0 sgid=0 fsgid=0 tty=(none) comm=”ip” gid=0 euid=0 suid=0 fsuid=0 egid=0 exe=”/sbin/ip” subj=user_u:system_r:ifconfig_t:s0 key=(null) What I Know: 1) 2) 3) AVC Event ID 738 syscall=54 (I’d have to google this) root (or an application on its behalf) was running /sbin/ip 4) context = user_u:system_r:ifconfig_t:s0 44
Scenario: Fixing the RHT corporate VPN “update” type=SYSCALL msg=audit(1204719775.306:738): arch=40000003 syscall=54 success=no exit=19 a0=4 a1=8933 a2=bfcec1bc a3=bfcec1bc items=0 ppid=3900 pid=5003 auid=501 uid=0 sgid=0 fsgid=0 tty=(none) comm=”ip” gid=0 euid=0 suid=0 fsuid=0 egid=0 exe=”/sbin/ip” subj=user_u:system_r:ifconfig_t:s0 key=(null) My Options: 1) Create a SELinux Policy Module # ausearch x “/sbin/ip” | audit2allow M MyVPNFix 45
Scenario: Fixing the RHT corporate VPN “update” type=SYSCALL msg=audit(1204719775.306:738): arch=40000003 syscall=54 success=no exit=19 a0=4 a1=8933 a2=bfcec1bc a3=bfcec1bc items=0 ppid=3900 pid=5003 auid=501 uid=0 sgid=0 fsgid=0 tty=(none) comm=”ip” gid=0 euid=0 suid=0 fsuid=0 egid=0 exe=”/sbin/ip” subj=user_u:system_r:ifconfig_t:s0 key=(null) My Options: 1) Create a SELinux Policy Module # ausearch x “/sbin/ip” | audit2allow M MyVPNFix # semodule i MyVPNFix.pp 46
Scenario: Fixing the RHT corporate VPN “update” type=SYSCALL msg=audit(1204719775.306:738): arch=40000003 syscall=54 success=no exit=19 a0=4 a1=8933 a2=bfcec1bc a3=bfcec1bc items=0 ppid=3900 pid=5003 auid=501 uid=0 sgid=0 fsgid=0 tty=(none) comm=”ip” gid=0 euid=0 suid=0 fsuid=0 egid=0 exe=”/sbin/ip” subj=user_u:system_r:ifconfig_t:s0 key=(null) My Options: 2) Disable enforcement of ifconfig_t (there is no need to turn SELinux completely off!) # semanage permissive a ifconfig_t 47
Name of Presentation Red Hat Presenter Questions
Why do we need SELinux? What are the principal concepts? Session discussed SELinux type enforcement, available policies, and how to write your own SELinux policy. Covered SELinux from the perspective of a system user and system administrator. Performed live demo of creating an SELinux policy based on errors in syslog.
