SELinux policy developers already have a number of file formats to work with. Currently, policy code is written in a set of three files:

  • The .te file contains the SELinux policy code (type enforcement rules)
  • The .if file contains functions which turn a set of arguments into blocks of SELinux policy code (interfaces). These functions are called by other interface files or type enforcement files
  • The .fc file contains mappings of file path expressions towards labels (file contexts)

These files are compiled into loadable modules (or a base module) which are then transformed to an active policy. But this is not a single-step approach.

Transforming policy code into policy file

For the Linux kernel SELinux subsystem, only a single file matters - the policy.## file (for instance policy.29). The suffix denotes the binary format used as higher numbers mean that additional SELinux features are supported which require different binary formats for the SELinux code in the Linux kernel.

With the 2.4 userspace, the transformation of the initial files as mentioned above towards a policy file is done as follows:

SELinux transformation diagram

When a developer builds a policy module, first checkmodule is used to build a .mod intermediate file. This file contains the type enforcement rules with the expanded rules of the various interface files. Next, semodule_package is called which transforms this intermediate file, together with the file context file, into a .pp file.

This .pp file is, in the 2.4 userspace, called a "high level language" file. There is little high-level about it, but the idea is that such high-level language files are then transformed into .cil files (CIL stands for Common Intermediate Language). If at any moment other frameworks come around, they could create high-level languages themselves and provide a transformation engine to convert these HLL files into CIL files.

For the current .pp files, this transformation is supported through the /usr/libexec/selinux/hll/pp binary which, given a .pp file, outputs CIL code.

Finally, all CIL files (together) are compiled into a binary policy.29 file. All the steps coming from a .pp file towards the final binary file are handled by the semodule command. For instance, if an administrator loads an additional .pp file, its (generated) CIL code is added to the other active CIL code and together, a new policy binary file is created.

Adding some CIL code

The SELinux userspace development repository contains a secilc command which can compile CIL code into a binary policy file. As such, it can perform the (very) last step of the file conversions above. However, it is not integrated in the sense that, if additional code is added, the administrator can "play" with it as he would with SELinux policy modules.

Still, that shouldn't prohibit us from playing around with it to experiment with the CIL language construct. Consider the following CIL SELinux policy code:

; Declare a test_port_t type
(type test_port_t)
; Assign the type to the object_r role
(roletype object_r test_port_t)

; Assign the right set of attributes to the port
(typeattributeset defined_port_type test_port_t)
(typeattributeset port_type test_port_t)

; Declare tcp:1440 as test_port_t
(portcon tcp 1440 (system_u object_r test_port_t ((s0) (s0))))

The code declares a port type (test_port_t) and uses it for the TCP port 1440.

In order to use this code, we have to build a policy file which includes all currently active CIL code, together with the test code:

~$ secilc -c 29 /var/lib/selinux/mcs/active/modules/400/*/cil testport.cil

The result is a policy.29 (the command forces version 29 as the current Linux kernel used on this system does not support version 30) file, which can now be copied to /etc/selinux/mcs/policy. Then, after having copied the file, load the new policy file using load_policy.

And lo and behold, the port type is now available:

~# semanage port -l | grep 1440
test_port_t           tcp      1440

To verify that it really is available and not just parsed by the userspace, let's connect to it and hope for a nice denial message:

~$ ssh -p 1440 localhost
ssh: connect to host localhost port 1440: Permission denied

~$ sudo ausearch -ts recent
time->Thu Jun 11 19:35:45 2015
type=PROCTITLE msg=audit(1434044145.829:296): proctitle=737368002D700031343430006C6F63616C686F7374
type=SOCKADDR msg=audit(1434044145.829:296): saddr=0A0005A0000000000000000000000000000000000000000100000000
type=SYSCALL msg=audit(1434044145.829:296): arch=c000003e syscall=42 success=no exit=-13 a0=3 a1=6d4d1ce050 a2=1c a3=0 items=0 ppid=2005 pid=18045 auid=1001 uid=1001 gid=1001 euid=1001 suid=1001 fsuid=1001 egid=1001 sgid=1001 fsgid=1001 tty=pts0 ses=1 comm="ssh" exe="/usr/bin/ssh" subj=staff_u:staff_r:ssh_t:s0 key=(null)
type=AVC msg=audit(1434044145.829:296): avc:  denied  { name_connect } for  pid=18045 comm="ssh" dest=1440 scontext=staff_u:staff_r:ssh_t:s0 tcontext=system_u:object_r:test_port_t:s0 tclass=tcp_socket permissive=0

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