SELinux workshop

1. 2023-05-27, NUREMBERG SELinux introduction Johannes Segitz <jsegitz@suse.com>

5. Expectation management As always it’s not possible to cover everything in one talk, so: – Mix between – higher level concepts (∼15 minutes) – practice (∼45 minutes) – You will not become a SELinux expert in an hour – We will go as far as possible with the examples. I can stick around afterwards, but we’ll stop officially after the slot ends Copyright © SUSE 2023 5

7. Mandatory access control Discretionary access control (DAC) Usual form of access control in Linux – Typical example: root@workstation ~/ $ ls -l /etc/shadow -rw-r-----. 1 root shadow 1421 /etc/shadow – Discretionary: The owner of an object can control the access of the objects he owns Copyright © SUSE 2023 7

8. Mandatory access control Discretionary access control (DAC) Drawbacks: – Coarse: Basically 3 x rwx – Prone to (user) error johannes@workstation ~/ $ ls -lah ~/.ssh/id_rsa -rw-rw-rw-. 1 jsegitz users 1.7K ~/.ssh/id_rsa – Hard to analyze – root == God (- capabilities) But it’s familiar, easy to use and to understand Copyright © SUSE 2023 8

9. Mandatory access control Mandatory access control (MAC) Mandatory (in this context): – Access control decisions are not made by the owner – Access control rules are managed centrally Advantages: – Access control in the hand of people who know what they’re doing – Centralized control and review is easy – Often very fine grained → compartmentalization Drawbacks: – Harder to understand – Complex to administrate – Missing experience Copyright © SUSE 2023 9

10. SELinux History Security Enhanced Linux – Linux security module (LSM), developed by the National Security Agency (NSA) Don’t panic, it’s open source and reviewed thoroughly – First release 2000, since then integrated in the Linux kernel Didn’t play a big role at SUSE up to this point Will be the MAC system for ALP (and already is for SLE Micro) So very likely it’s also the future MAC system for openSUSE Copyright © SUSE 2023 10

11. SELinux Basic idea – Type Enforcement (TE). Every object has a – user: unconfined_u – role: unconfined_r – type: unconfined_t – sensitivity: s0-s0 – category: c0.c1023 – These form the Security Context (SC) unconfined_u:unconfined_r:unconfined_t:s0-s0:c0.c1023 – In practice the type unconfined_t is all you need Copyright © SUSE 2023 11

13. SELinux Basic idea (Almost) everything has a SC. Sockets, packets, ... 134 security classes – Files root@workstation ~/ $ ls -lZ /etc/shadow ----------. root root system_u:object_r:shadow_t:s0 /etc/shadow – Processes root@workstation ~/ $ ps axZ | grep 'postfix/master' system_u:system_r:postfix_master_t:s0 1250 ? Ss 0:00 /usr/lib/ Copyright © SUSE 2023 13

15. SELinux SELinux log messages Found in the audit.log type=AVC msg=audit(1416499522.810:77): avc: denied { transition } for pid=1282 comm="sshd" path="/usr/bin/zsh" dev="vda2" ino=40462 scontext=system_u:system_r:kernel_t:s0 tcontext=unconfined_u:unconfined_r:unconfined_t:s0 tclass=process Copyright © SUSE 2023 15

17. Practice Initial setup Install packages: $ zypper in selinux-policy-targeted restorecond selinux-policy-devel policycoreutils setools-console policycoreutils-devel selinux-autorelabel podman Set SELinux to enforcing: $ sed -i -e 's/SELINUX=.*/SELINUX=enforcing/' /etc/selinux/config Main config file: /etc/selinux/config # This file controls the state of SELinux on the system. # SELINUX= can take one of these three values: # enforcing - SELinux security policy is enforced. # permissive - SELinux prints warnings instead of enforcing. # disabled - No SELinux policy is loaded. SELINUX=enforcing # SELINUXTYPE= can take one of these two values: # targeted - Targeted processes are protected, # mls - Multi Level Security protection. # minimum - Modification of targeted policy. Only selected processes are protected. SELINUXTYPE=targeted Copyright © SUSE 2023 17

18. Practice Initial setup Set the necessary boot parameter: $ if ! egrep 'GRUB_CMDLINE_LINUX_DEFAULT.*security=selinux selinux=1' /etc/default/grub >/dev/null; then sed -i -E 's/(GRUB_CMDLINE_LINUX_DEFAULT=.*)"/1 security=selinux selinux=1"/' /etc/default/grub fi $ update-bootloader --refresh Remove audit log, reboot: $ rm /var/log/audit/audit.log $ reboot Copyright © SUSE 2023 18

19. Practice Look around Current SELinux status $ sestatus Have a look at the processes $ ps auxZ Check out the filesystem labels $ ls -laZ / $ ls -laZ /var Check for mislabeled files $ restorecon -Rvn /var $ restorecon -Rv /var Copyright © SUSE 2023 19

20. Practice Look around Check your identity $ id -Z Check denials: $ grep -i avc /var/log/audit/audit.log $ tail -f /var/log/audit/audit.log | grep -i avc Proper way to do this: $ ausearch -m avc,user_avc,selinux_err -ts boot -i Copyright © SUSE 2023 20

23. Practice audit2allow audit2allow: – Analyzes SELinux denial messages – Generates rules to allow necessary access – Is aware of interfaces – Suggests booleans that would allow the access But don’t use it with every denial! Copyright © SUSE 2023 23

25. Practice avcs.rb Small wrapper around auserch. Makes reading AVCs easier $ podman run --privileged -v /var/log/audit:/var/log/audit registry.opensuse.org/home/jsegitz/containers/containers/avcs:latest Copyright © SUSE 2023 25

26. Practice Lets cause problems Mislabel some files: $ ls -laZ /usr/sbin/postfix $ chcon -t postfix_map_exec_t /usr/sbin/postfix Check it: $ restorecon -Rvn /usr/sbin/ Restart postfix: $ systemctl restart postfix Copyright © SUSE 2023 26

27. Practice Lets cause problems Check the status: $ systemctl status postfix Why does it fail? You’ll see something like postfix[3427]: fatal: chdir(/var/spool/postfix): Permission denied but no AVCs. How do we approach this? Copyright © SUSE 2023 27

28. Practice How to debug SELinux problems Does it happen in permissive mode? $ setenforce 0 Check for denials. If you don’t see any disable dontaudit rules with $ semodule -DB Enable dontaudit again with $ semodule -B Now give audit2allow a try with this ... Check for mislabled files – either because of unaware scripts/programs – policy paths don’t match Copyright © SUSE 2023 28

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