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SELinux Policy Development: Modifying Policy
By: Pax Dickenson

Once you have your list of all your allow statements, examine them carefully and try to understand what you are allowing before adding them to policy. One weakness of audit2allow is that it is unaware of macros contained in the policy, so grep through your policy sources for allow statements close to the ones you'd like to add and try to find appropriate macros to use instead. If you're planning on doing a lot of policy customization it's a good idea to familiarize yourself with the existing policy sources so you're aware what macros are available.

The $policy/policy/support/obj_perm_sets.spt is one good place to start, it contains macros that expand out to useful permissions groupings. For example, rather than allowing a domain the ioctl, read, getattr, lock, write, and append permissions to a given type, you can simply assign it the rw_file_perms macro instead. This helps keep policy readable later on.

Once you have generated your needed allow statements, add them to the $policy/policy/modules/admin/local.te file and recompile the policy. If your application still won't work in enforcing mode, just repeat the process until you can run it with no SELinux audit errors.

Always keep your policy changes in the: $policy/policy/modules/admin/local.* files. T

hese files are included in the package empty and intended for local policy customization. If you change a file that belongs to a service and contains rules already your changes will be lost when the policy is upgraded, so keep local changes in the local.te and local.fc files where they belong.

If you find a problem in existing policy, add your changes to local.* but provide a patch to the policy maintainers so they can include it in a later build. Most SELinux policies are being constantly developed and revised since the technology is still fairly new, and your upstream maintainers will thank you for your help.

Policy development can be difficult at the beginning, but I think you'll find that as you make progress you'll be learning not only about SELinux but about the details of what your applications are really doing under the hood. You'll not only be making your system more secure, you'll be learning about the low level details of your system and its services. SELinux development has already resulted in upstream patches to many applications that had hidden bugs that were only found because SELinux alerted policy developers to the kernel level actions the applications were attempting.

I hope you enjoyed reading this SELinux series as much as I enjoyed writing it. Until next time, stay secure and keep your policy locked down tight.

Read Entire Aricle:
http://www.linuxsecurity.com/content/view/120837/49/

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