maandag 12 juli 2010

Targeted configured semi-strict with UBAC for Fedora/Redhat distros.

I am maintaining an SELinux policy that is based off of Fedoras' SELinux policy and it aims to merge changes refpolicy as much as possible. It use to have only rather minor differences compared to refpolicy and Fedora but lately it has been diverging pretty much. The source policy is available in my personal shared Git repository and depending on my time it is updated pretty often. I aim to merge any Fedora and refpolicy updates as much as possible as soon as possible. In theory it is latest Fedora policy with refpolicy changes and my own changes. In practice its Fedora plus/minus some bugs/features that i introduced.

The main property of my policy is that, besides the usual, it aims to confine the user space by default.
As opposed to Fedora, my policy has user based access control enabled. This feature aims to isolate selinux users. In practice it has some rough edges and gothas but it is worth it. So one of the issues is that one has to make sure that the selinux identity field in the security context tuple of user home directories is labeled properly. This is a problem when you add new users. So if a new user is added one should chcon -R -u newuser_u /home/newuser. The second issue is system administration. In my policy, root is mapped to sysadm_u selinux identity, but usually logins as root are not encouraged, and so one logs in as staff_u and then role transitions to either sysadm_r or unconfined_r. This means that you will be root in a sufficient domain but with a ubac constraints. Fortunately unconfined_t has access to the system_r role and can use runcon to run commands using the system_u selinux identity. This is required for many sysadm commands. Example: runcon -u system_u useradd joe.

My policy installs with root mapped to the selinux identity of root but the root selinux identity does not have default contexts for unconfined. Meaning root cannot login as unconfined_t. Root logs in, in the sysadm_t domain. That should be sufficient. On installation the unconfined module is disabled. It can optionally manually be enabled after installation. updating policy will not try to disable it again if you did. The unconfined_u selinux user mapping was removed by default. You can manually add it if required but my policy does not encourage unconfined logins. This is also why unconfined_login boolean is set to false by default.

Users that need to have access to privileged domains and dacs root should be mapped to staff_u. These users should use sudo to change to root and to role transition:

echo "joe ALL=(ALL) TYPE=unconfined_t ROLE=unconfined_r ALL" >> /etc/sudoers

My policy adds some confinement for some user apps. Most notably currently:
firefox, nsplugin, totem, telepathy, mutt, thunderbird, elinks, irssi, metacity, gnome dvb, seahorse-daemon, vino_server and others.

So it is targeted policy that is configured in a semi strict fashion. unconfined logins by default are prohibited but the unconfined role/domain can be used as an replacement for sysadm_r/sysadm_t to use with sudo. Also the policy has selinux user isolation, so the various selinux users cannot interfere with eachother.

To make an rpm of my policy is pretty straight forward on fedora.
You can clone my refpolicy repository:

git clone git://217.19.30.59/refpolicy.git

or update it after you cloned it:

cd refpolicy; git pull

(Mind you that my IP address is dynamic. It does not change often but it can change. Just "/whois" me on irc.freenode.org to get my current IP address and use that.

To determine my main policy version either look at the tags or see "Version ; .." in the spec file in "redhat/selinux-policy.spec"

Replace the version number with the version number in the following commands:

git archive --format=tar --prefix=refpolicy-3.8.6/ refpolicy | gzip > ~/rpmbuild/SOURCES/refpolicy-3.8.6.tar.gz

git diff refpolicy master > ~/rpmbuild/SOURCES/refpolicy-3.8.6.patch

Make sure the path exists (install rpmdevtools) run rpmdev-setuptree.

Next copy the redhat/selinux-policy.spec from the Git repository:

cp redhat/selinux-policy.spec ~/rpmbuild/SPECS/

And build the packages:

cd ~/rpmbuild/SOURCES/; rpmbuild -ba ../SPECS/selinux-policy.spec

If you're installing my policy for the first time. e.g. if your current installed policy is stock redhat/fedora policy, then it is encouraged to start clean.

This means you will lose any modifications that you may have made:

setenforce 0
yum erase selinux-policy selinux-policy-targeted
mv /etc/selinux /etc/selinux.backup
yum install selinux*.rpm (the build rpms)
touch /.autorelabel && reboot

on reboot login as root, you should be in sysadm_t domain and you should be able to add/ fix your login user mapping to staff_u. Once youre login user is mapped to staff_u be sure to fix your login users home and tmp directories context:
semanage login -a -s staff_u -r s0-s0:c0.c1023 joe
chcon -R -u staff_u /home/joe
chcon -R -u staff_u /tmp/joe
chcon -R -u staff_u /var/tmp/joe

That should be sufficient.

If pulseadio does not start, then remove ~/.esd_auth ~/.pulse /tmp/.esd* and relogin or restart pulse.
Also you may when you first log in get AVC denials for stuff like metacity trying to access ~/.config (staff_wm_t -> user_home_t), and maybe others as well, ignore it. This is because that directory and others at that point are not restored yet. restorecond -u runs in a gnome-session and should have restored your home directory contexts in the mean time.

Be prepared to stumble on bugs, issues etcetera. Feel free to contact me if you have questions or comments. Patches welcome.

I am using this policy myself on two desktop systems (fedora13/Gnome) and several headless servers.

Warning: Only consider trying this if you are familiar to, and not intimidated by SELinux.

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