Linux Security Modules

linux

Authors: Will Shand

Disclaimer: I threw these notes together for CU Boulder’s Collegiate Cyber Defense Competition team for spring 2020. There may be some inaccuracies, and parts of the notes are still incomplete.

Introduction

The Linux kernel has a built-in discretionary access control (DAC) system for controlling system resources. DACs are characterized by the fact that the owner of a resource may grant somebody else access to that resource; for instance, in Linux, the owner of a file could run chmod +rwx myfile to give every user on the system read, write, and execute permissions to the file. As a result, a process can privesc if it’s able to run commands as a user with higher permissions.

Linux Security Modules (LSM) is a framework for implementating mandatory access control (MAC) in Linux. Under a MAC policy, users cannot modify the permissions of resources; the permissions are set in stone by the system administrator. This makes privilege escalation significantly more difficult. For instance, say an attacker manages to get a reverse shell into an Apache webserver, and is eventually able to login as root. Even as the root user, the attacker is confined by whatever permissions are granted to the webserver process, which may be extremely restrictive.

There are a few other general characteristics of LSMs that differentiate them from traditional Linux DAC:

  • LSMs focus on assigning permissions to processes, whereas the Linux DAC policy assigns permissions to users and groups.
  • LSMs are generally “deny by default”: if access to a resource isn’t explicitly granted, then it’s denied.
  • LSMs can have much more fine-grained permissions than Linux’s DAC. For instance, SELinux doesn’t just control access to files; you can define permissions over ports, syscalls, regions of memory, and more.

Logging and auditd

Introduction

auditd is an auditing daemon that logs system events and provides various utilities for searching logs. It is extremely useful for SELinux as it can be used to record denial to various resources.

auditd logs generally look something like this:

time->Sun Mar  1 00:11:43 2020
type=PROCTITLE msg=audit(1583046703.755:2547): proctitle=636174002F6574632F736861646F77
type=SYSCALL msg=audit(1583046703.755:2547): arch=c000003e syscall=257 success=yes exit=3 a0=ffffff9c a1=7ffeef4fae7b a2=0 a3=0 items=0 ppid=11335 pid=11336 auid=4294967295 uid=33 gid=33 euid=33 suid=33 fsuid=33 egid=33 sgid=33 fsgid=33 tty=(none) ses=4294967295 comm="cat" exe="/usr/bin/cat" subj=system_u:system_r:httpd_t:s0 key=(null)
type=AVC msg=audit(1583046703.755:2547): avc:  denied  { open } for  pid=11336 comm="cat" path="/etc/shadow" dev="sda1" ino=277524 scontext=system_u:system_r:httpd_t:s0 tcontext=system_u:object_r:shadow_t:s0 tclass=file permissive=1
type=AVC msg=audit(1583046703.755:2547): avc:  denied  { read } for  pid=11336 comm="cat" name="shadow" dev="sda1" ino=277524 scontext=system_u:system_r:httpd_t:s0 tcontext=system_u:object_r:shadow_t:s0 tclass=file permissive=1

These logs are fairly verbose, but there are a few pieces of information that are worth looking at:

  • success (under type=SYSCALL): tells us whether or not a given syscall was successful.
  • comm and exe: the command/executable that activated the rule.
  • scontext: the SELinux context of the resource that triggered the rule. Here, it’s the context in which /usr/bin/cat is run.
  • tcontext: the SELinux context of the resource that the source was trying to access. Here, it’s the context of /etc/shadow.

auditctl: create auditd rules

auditctl -w /etc/shadow -p rwa -k shadow_file

  • Checks for reads, writes, and attribute changes to /etc/shadow
  • The -k flag assigns the key shadow_file to this rule

auditctl -w /usr/local/bin -p x -k cron

  • Checks for execution of binaries in the /usr/local/bin directory

auditctl -a always,exit -F arch=b64 -F "auid>=1000" -S rename -S renameat -k rename

  • Log attempts to rename a file using the rename or renameat syscalls
  • Only logs attempts for users with ID greater than or equal to 1000

Note that rules created with auditctl are lost after rebooting. If you want to make them permanent, you need to save them in /etc/audit/rules.d/audit.rules.

To delete a rule, use -W instead of -w.

ausearch: search audit logs

ausearch -k shadow_file

  • Search for logs with the shadow_file key

ausearch --start recent -k cron

  • Search for logs in the last ten minutes that are tagged with cron
  • You can also choose today, yesterday, etc. instead of recent.
  • You can specify a specific time or date too, e.g. --start 03/01/2020 or --start 03/01/2020 16:00:00
  • There is an --end flag that, in conjunction with --start, allows you to specify the end of a time range.

ausearch -se system_u:system_r:httpd_t:s0

  • Search for all logs where either the source SELinux context or target SELinux context is system_u:system_r:httpd_t:s0

ausearch -c apache2 -f /etc/shadow

  • Search for all logs triggered by the apache2 command related to the /etc/shadow file.
  • You can use -x instead of -x to specify the path to the executable instead of its name.

ausyscall: view available syscalls

ausyscall --dump

  • List all linux syscalls and their numeric codes

ausyscall 4

  • Find the name of the syscall corresponding to the ID 4
  • Useful when reviewing auditd logs

SELinux

Introduction

SELinux is the default LSM enhancement for RHEL, Fedora, and CentOS. It is also installed in Debian, although as of Debian 10 (Buster) it is deactivated by default, with AppArmor enabled in its place. SELinux for Ubuntu has been largely unmaintained since Ubuntu 9.10 and broken since Ubuntu 12.04. Although it is still possible to install and use SELinux via the Debian repositories (where SELinux is still maintained), it is generally recommended that you use AppArmor for Ubuntu instead.

Installation

To get SELinux up and running, you probably want to install the following packages: selinux-basics selinux-policy-default auditd. In addition, depending on your system you may need to explicitly enable SELinux. You can check whether or not SELinux has been enabled by running getenforce; if you get the response Disabled, then you will need to explicitly activate it. Generally, this can be done by running selinux-activate and rebooting.

SELinux utilities

getenforce, sestatus, and seinfo: get information about SELinux

These three utilities each provide information about the current status and configuration of SELinux on your system:

  • getenforce: gets the current SELinux state, which can be “Disabled”, “Permissive”, or “Enforcing”.
  • sestatus: shows the current SELinux state like getenforce, but also provides a little more information (e.g. the name of the loaded policy).
  • seinfo: get information about the current SELinux policy. Here are some example queries you can run with seinfo:
    • seinfo -b: show all of the available SELinux booleans, and whether they are on or off.
    • seinfo -x -u staff_u: show the full definition of the staff_u SELinux user. This will show you what roles are assigned to the staff_u user.
    • seinfo -x -r user_r: show the full definition of the user_r SELinux role. This will show you what types are assigned to the user_r user.

selinux-activate: enable SELinux

Sets SELinux to activate after you reboot. Your machine will reboot into whatever mode has been assigned to the SELINUX variable in /etc/selinux/config.

setenforce: set SELinux state

When SELinux is activated, you can use setenforce to change the current SELinux mode. setenforce 0 will put SELinux in permissive mode, and setenforce 1 will put it in enforcing mode.

sesearch: search SELinux policies

sesearch allows you to search for specific rules in your SELinux policy. Here are some examples of how to use it:

  • sesearch --allow -s auditd_t -t auditd_log_t: search for all allow rules with auditd_t as their source type and auditd_log_t as their target type.
  • sesearch --allow -s httpd_t -p read,write: find all rules with source type httpd_t that grant the read and write permissions.
  • sesearch --allow -s httpd_t -t auditd_log_t -ds -dt: find all rules that exactly have httpd_t as their source type, and auditd_log_t as their destination type. Without the -ds and -dt flags, sesearch typically just matches by attribute contents.

chcon: change context of a file

Suppose that we want to change the SELinux context of /var/www/html/index.html to match that of the directory /var/www/html/. Running ls -dZ /var/www/html, we see that this directory has context system_u:object_r:httpd_sys_content_t. To change the context of /var/www/html/index.html, we use chcon as follows:

# chcon -u system_u -r object_r -t httpd_sys_content_t /var/www/html/index.html

But if you just want to change the type, you only have to use the -t flag, e.g. chcon -t $selinux_type $target

You can make things even easier by using the --reference flag:

# chcon --reference /var/www/html/ /var/www/html/index.html

This gives /var/www/html/index.html the same SELinux context as the /var/www/html/ directory.

restorecon: restore context of a file

Recursively restore context of all files in /var/www/html to have the same context as the /var/www/html/ directory:

# restorecon -R /var/www/html

You can use the -v flag for verbose output.

semanage: main SELinux configuration tool

Can be used to manage SELinux settings for

  • node
  • file context
  • booleans
  • permissive state
  • dontaudit

Unlike chcon or restorecon, rules set via semanage are saved so that they become reapplied after reboot.

Examples:

  • semanage fcontext -l: List all of the SELinux policy rules around file contexts.
  • semanage fcontext -a -t httpd_sys_content_t "/foo(/.*)?": add the type httpd_sys_content_t to the /foo directory and all of its contents.
  • semanage fcontext -a -e /var/www/html /foo: add a rule to make the context of /foo match that of /var/www/html. After this, you can run restorecon -vR /foo to apply the new label to /foo.

audit2why

audit2allow: generate SELinux policies from logs of denied operations

audit2allow uses log files (/var/log/audit/audit.log if auditd is installed, and /var/log/messages otherwise) to generate SELinux rules. It’s an extremely useful tool for fixing broken policies.

For example, suppose you run ip addr, and it appears to be denied by SELinux (a situation I encountered on Debian 10). To figure out where the issue is, let’s start by reviewing the audit logs:

# ausearch --start recent
..
time->Wed Mar  4 21:20:21 2020
type=AVC msg=audit(1583360421.558:1732): avc:  denied  { signull } for  pid=305 comm="systemd-journal" scontext=system_u:system_r:syslogd_t:s0 tcontext=system_u:system_r:NetworkManager_t:s0 tclass=process permissive=0

We can immediately make a few notes about the log:

  • The source context is system_u:system_r:syslogd_t:s0
  • The target context is system_u:system_r:NetworkManager_t:s0
  • The command that triggered the denial was systemd-journal

Overall, it looks like the command was denied because systemd-journal sent SIGNULL to a process with a different context, and SELinux blocked it. We can use sesearch to try and see what rules are defined for syslogd_t and NetworkManager_t:

# sesearch -s syslogd_t -t NetworkManager_t -A
allow syslogd_t domain:dir { getattr ioctl lock open read search };
allow syslogd_t domain:file { getattr ioctl lock open read };
allow syslogd_t domain:lnk_file { getattr read };
allow syslogd_t domain:process getattr;

It looks like there aren’t any rules directly related to both syslogd_t and NetworkManager_t. We’re going to need to add a new rule that allows processes of type syslogd_t to send SIGNULL to processes of type NetworkManager_t.

To do this, we use the audit2allow command. First, we use audit2allow to give us more information about how to fix this issue:

# sudo ausearch -se system_u:system_r:NetworkManager_t | tail -n 1 | audit2allow -w
type=AVC msg=audit(1583407237.188:26): avc:  denied  { search } for  pid=478 comm="NetworkManager" name=".cache" dev="vda1" ino=410827 scontext=system_u:system_r:NetworkManager_t:s0 tcontext=system_u:object_r:xdg_cache_t:s0 tclass=dir permissive=1

	Was caused by:
		Missing type enforcement (TE) allow rule.

		You can use audit2allow to generate a loadable module to allow this access.

With the -a flag, audit2allow will tell us what rule needs to be added in order to fix the error:

# sudo ausearch -se system_u:system_r:NetworkManager_t | tail -n 1 | audit2allow -a


#============= NetworkManager_t ==============
allow NetworkManager_t xdg_cache_t:dir search;

(Note: you can pipe in multiple logs at once in order to generate multiple SELinux rules.)

Finally, we create a new policy module with the rule:

After running semodule -i syslogd.pp and rebooting, a new SELinux rule will be added to your policy that permits the previously forbidden action.

# sudo ausearch -se system_u:system_r:NetworkManager_t | tail -n 1 | audit2allow -a -M syslogd
 ************************** IMPORTANT ******************************
To make this policy package active, execute:

semodule -i syslogd.pp

Additional references

AppArmor

Introduction

AppArmor is another Linux Security Module for implementing MAC that is enabled by default on Ubuntu and recent Debian distributions. Its chief goal is to be more user-friendly than SELinux.

Whereas SELinux is label-based, AppArmor is path-based: access to files and executables is based on their path in the filesystem. In addition, AppArmor rules are defined in profiles (e.g. /etc/apparmor.d/usr.sbin.apache2), in contrast to SELinux’s concept of a system-wide policy. In order to restrict a binary, we must define a new profile for it, and then put that profile in enforcing mode.

Installation

AppArmor is enabled by default on Ubuntu since version 7.10, and on Debian since Debian 10 (Buster). However, like SELinux, AppArmor has a few packages with convenient files and utilities that you should install if you want to use it:

  • apt install apparmor-profiles apparmor-easyprof apparmor-utils

Using AppArmor

In this section, we’ll create an AppArmor profile for the apache2 binary using the following steps:

  1. Create a template profile for the binary using aa-easyprof;
  2. Load that profile using apparmor_parser;
  3. Put the profile in complain mode using aa-complain;
  4. Add new AppArmor rules using aa-logprof; and
  5. Enforce the profile with aa-enforce.

Note that you may not necessarily need to go through all of these steps, e.g. if you already have a default profile for the binary. There are also some tools that wrap a few of these steps together, such as aa-genprof.

1. Create a template apache2 profile

Call aa-easyprof /usr/sbin/apache2 > /etc/apparmor.d/usr.sbin.apache2. The newly created profile will be mostly empty; it may look something like this:

/etc/apparmor.d/usr.sbin.apache2:

#include <tunables/global>

# No template variables specified

"/usr/sbin/apache2" {
  #include <abstractions/base>
  # No abstractions specified
  # No policy groups specified
  # No read paths specified
  # No write paths specified
}

2. Load the profile

Now call apparmor_parser /etc/apparmor.d/usr.sbin.apache2 to read the new profile. Note that if you’ve previously loaded in /etc/apparmor.d/usr.sbin.apache, you’ll need to reload it by adding the -r flag.

3. Put the profile in complain mode

Put the binary in complain mode by calling aa-complain apache2. In complain mode, AppArmor will scan log files to try to identify what capabilities the apache2 binary needs to have.

Visit your Apache webserver, and start trying to simulate how it would be used in real life. As you test the webserver more and more, AppArmor will get an increasingly good idea of what capabilities the server is using.

4. Add new AppArmor rules using aa-logprof

After you’ve tested your webserver sufficiently, run aa-logprof. This will read various logfiles in /var/log to see what files the Apache webserver accessed, and what capabilities it used. aa-logprof will then present you with a list of capabilities, which you can choose to accept, deny, ignore, and so on. Choose whichever capabilities your webserver needs; any capabilities that are not explicitly granted to the webserver will be denied.

5. Enforce the profile

Call aa-enforce apache2 to put the profile in enforcing mode. AppArmor is now running, and will deny capabilities not granted in /etc/apparmor.d/usr.sbin.apache2.

Customizing profiles

It is often desirable to hand-edit your AppArmor profiles to make them a little more specific. Here are some general syntax rules for AppArmor profiles:

Deny rules: To write a deny rule, you just write deny /path/to/file perms.

  • deny /etc/shadow rw: deny read and write access to /etc/shadow.
  • deny /etc/sudo x: deny execute permission to /etc/sudo.

Note that AppArmor is deny-by-default, so you don’t generally need to explicitly state every resource that you want to deny access to.

Accept rules:

Tunables: where possible, you should avoid using explicit paths like /home/, and instead use tunables, which are configurable variables defined in /etc/apparmor.d/tunables. For instance, instead of writing a rule like /home/*/ r, you should use @{HOME}/ r.

Debugging AppArmor

Additional references

Miscellaneous references