SELinux Johannesburg Linux User Group (JoziJUg)

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SELinux presentation given at the Jozi Lug in March. If you are in Johannesburg, South Africa and want to join us see our page on meetup.com. Search for JLug.
http://www.meetup.com/Jozi-Linux-User-Group-JLUG/

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SELinux Johannesburg Linux User Group (JoziJUg)

  1. 1. Jozi LUG - SELinuxIntroduction to Security Enhanced Linux 26th March 2012 Sponsored by LPI South Africa
  2. 2. Topics● What is SELinux?● Computer Security Models● Mandatory Access Control & Discretionary Access Control● SELinux Policy● Object Classes and actions/permissions
  3. 3. Topics● Security Context● File Security Context● Troubleshooting & Tools● SELinux Booleans● SELinux Managing Ports● SELinux Writing Policy
  4. 4. What is SELinux● A mechanism for supporting mandatory access control (mac),role based access control (rbac) & multi-level security (msl/mcs)● Implemented as a Linux Security Module(LSM)● LSM allows kernel to support different security models used by: ● AppArmor,Smack,SELinux
  5. 5. Computer Security Models● Three security models possible with SELinux ● MLS/MCS – multilevel security, multi category security. Mainly about file access. Every subject must have clearance level and also every file (not covered) Top Secret, Secret, Confidential and Unclassified ● RBAC – role base access control, how users transition between roles and domains to which roles have rights, roles aggregate permissions
  6. 6. Computer Security Models ● Mandatory Access Control via Type Enforcement – First step before MLS/MCS. Good for daemons, services● This presentations focuses on MAC via TE in SELinux. Although other security models can be used they are too restrictive for most situation there limited TE used. MAC mainly useful for daemons and processes not users
  7. 7. Mandatory Access Control Definition● Mandatory Access Control (MAC) – security policy sets access controls and cannot be changed by system users or processes,● Discretionary Access Control (DAC) – underlying unix permissions can be changed at the discretion of the file owner
  8. 8. Mandatory/Discretionary Access Control● DAC makes system vulnerable, users can change permissions and no protection from broken software, i.e. process has complete control over all resources owned by user,● MAC - provides control over interactions of software by defined policies and does not allow users to do anything that breaks these policies. Prevents compromised processes from affecting other processes and files
  9. 9. Mandatory Access Control● Subject performs actions on an object● Subject always a process● Object can be file, device,users, processes,sockets,x_cursor..● Action is a system function call, i.e permissions
  10. 10. How is MAC Implemented?● How is MAC implemented? ● Security context given to objects and processes aka labeling for file system ● A Security context just free format strings “label” ● By policy file which contain rules about what domains/type enforcements subject and object must have to allow requested action. I.e provides meaning to security context strings. Policies limit what a daemon can access and how
  11. 11. SELinux Policy● Rules for how source context of subject evaluated against target security context of object● By default if not defined, then deny action. Difficult for general purpose computing. To improve use less restrictive policy provided,
  12. 12. SELinux Policy● Two policies packages – ● Targeted – doesnt use users & roles, only restricts certain services, uses type enforcement only. Unaffected subjects and objects run in unconfined_t domain ● Strict – deny all by default lots of tweaking● We will look at a policy file later
  13. 13. Objects Classes● Object classes (categories) – more then 70@● Object classes have set of permissions (actions) – dir, – socket – tcp_socket – filesystem – node – x_cursor
  14. 14. Object Class Permissions (Actions)● Each object class has its list of permissions or actions e.g. dir: (see slide on seinfo later) ● getattr/setattr, ● unlink ● execute ● read ● search ● rmdir
  15. 15. Security Context● Security Context or labels set of security attributes associated with a subject or an object● <user>:<role>:<type>● e.g system_u:object_r:httpd_exec_t ● system_u – standard for system daemon ● object_r standard for system objects such as devices and files ● Targets policy – unrestricted_u, unrestricted_r
  16. 16. Security Context● User – individual or process, SELinux maintains own list of users. For subjects the user is the user the process is run as, for objects its the owner of the object,● Role – similar to group, but user can only have 1 role at a time, can switch roles if authorised to do so● Type/Domain -Type used for files, domain used for processes. Manages access control
  17. 17. Security Context● Standard command come with add -Z option to see security context ● ls -Z ● ps -Z ● netstat -Z
  18. 18. File Security Context● Most common SELinux problem – file labels ● restorecon – restores defined context for a file ● chcon -t $tye ${file|dir} name – temporary ● semanage fcontext -a -t $type ${file|dir} name● /etc/selinux/targeted/contexts/files/files_contexts
  19. 19. Troubleshooting & Tools● /var/log/audit/audit.log● Create policy files from audit2allow● avc = access vector cache
  20. 20. SELinux Tools● setroubleshooter – can help with friendlier error messages and suggestions of how to fix the problem● “cat /var/log/audit/audit.log | sedispatch” → will send the error messages to setroubleshooter for lookup & formatting
  21. 21. SELinux Tools● Seinfo ● List all classes “seinfo -c” ● List all permissions for a class “seinfo -cdir -x” for dir premissions/actions ● List all types with permissions “seinfo -txx -x” ● List all users/roles with permissions “seinfo -{u| r}xx -x” ● List all port context “seinfo --portcon”
  22. 22. SELinux - Booleans● Booleans ● getsebool -a ● semanage boolean -l ● setsebool xxx on| off ● setsebool -P xxx on|off
  23. 23. Manage Ports● semanage port -l● Add a port ● semanage port -at [-p proto] port |port-range● Delete a port ● semanage port -dt [-p proto] port|port-range
  24. 24. Writing SELinux Policy● The policy is compiled in user space● The m4 macro preprocessor is used prior to compilation (optional)● The initial policy binary is loaded by init at boot● Policy modules (binaries) can be loaded and unloaded at any time
  25. 25. Writing SELinux Policy● “cat /var/log/audit/audit.log | audit2allow -m mymod > mymod.te● checkmodule -M -m -o mymod.mod mymod.te● semodule package -o mymod.pp -m mymod.mod● semodule -i mymod.p
  26. 26. Questions?● Visit us at – www.JumpingBean.co.za – www.LinuxCertification.co.za

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