Security-Enhanced Linux (SELinux) is a Linux kernel security module that limits "the malicious things" that certain processes, including OVS, can do to the system in case they get compromised. In our case SELinux basically serves as the "second line of defense" that limits the things that OVS processes are allowed to do. The "first line of defense" is proper input validation that eliminates code paths that could be used by attacker to do any sort of "escape attacks", such as file name escape, shell escape, command line argument escape, buffer escape. Since developers don't always implement proper input validation, then SELinux Access Control's goal is to confine damage of such attacks, if they turned out to be possible.
Besides Type Enforcement there are other SELinux features, but they are out of scope for this document.
Currently there are two SELinux policies for Open vSwitch:
- the one that ships with your Linux distribution (i.e. selinux-policy-targeted package)
- the one that ships with OVS (i.e. openvswitch-selinux-policy package)
If Open vSwitch is directly started from command line, then it will run under
unconfined_t SELinux domain that basically lets daemon to do whatever it
likes. This is very important for developers to understand, because they might
introduced code in OVS that invokes new system calls that SELinux policy did
not anticipate. This means that their feature may have worked out just fine
for them. However, if someone else would try to run the same code when Open
vSwitch is started through systemctl, then Open vSwitch would get Permission
Denied errors.
Currently the only distributions that enforce SELinux on OVS by default are
RHEL, CentOS and Fedora. While Ubuntu and Debian also have some SELinux
support, they run Open vSwitch under the unrestricted unconfined domain.
Also, it seems that Ubuntu is leaning towards Apparmor that works slightly
differently than SELinux.
SELinux and Open vSwitch are moving targets. What this means is that, if you solely rely on your Linux distribution's SELinux policy, then this policy might not have correctly anticipated that a newer Open vSwitch version needs extra white list rules. However, if you solely rely on SELinux policy that ships with Open vSwitch, then Open vSwitch developers might not have correctly anticipated the feature set that your SELinux implementation supports.
Refer to the Fedora installation guide for instructions on how to build all Open vSwitch rpm packages.
Once the package is built, install it on your Linux distribution:
$ dnf install openvswitch-selinux-policy-2.4.1-1.el7.centos.noarch.rpm
Restart Open vSwitch:
$ systemctl restart openvswitch
When SELinux was implemented some of the standard system utilities acquired
-Z flag (e.g. ps -Z, ls -Z). For example, to find out under which
SELinux security domain process runs, use:
$ ps -AZ | grep ovs-vswitchd system_u:system_r:openvswitch_t:s0 854 ? ovs-vswitchd
To find out the SELinux label of file or directory, use:
$ ls -Z /etc/openvswitch/conf.db system_u:object_r:openvswitch_rw_t:s0 /etc/openvswitch/conf.db
If, for example, SELinux policy for Open vSwitch is too strict, then you might see in Open vSwitch log files "Permission Denied" errors:
$ cat /var/log/openvswitch/ovs-vswitchd.log
vlog|INFO|opened log file /var/log/openvswitch/ovs-vswitchd.log
ovs_numa|INFO|Discovered 2 CPU cores on NUMA node 0
ovs_numa|INFO|Discovered 1 NUMA nodes and 2 CPU cores
reconnect|INFO|unix:/var/run/openvswitch/db.sock: connecting...
reconnect|INFO|unix:/var/run/openvswitch/db.sock: connected
netlink_socket|ERR|fcntl: Permission denied
dpif_netlink|ERR|Generic Netlink family 'ovs_datapath' does not exist.
The Open vSwitch kernel module is probably not loaded.
dpif|WARN|failed to enumerate system datapaths: Permission denied
dpif|WARN|failed to create datapath ovs-system: Permission denied
However, not all "Permission denied" errors are caused by SELinux. So, before blaming too strict SELinux policy, make sure that indeed SELinux was the one that denied OVS access to certain resources, for example, run:
$ grep "openvswitch_t" /var/log/audit/audit.log | tail type=AVC msg=audit(1453235431.640:114671): avc: denied { getopt } for pid=4583 comm="ovs-vswitchd" scontext=system_u:system_r:openvswitch_t:s0 tcontext=system_u:system_r:openvswitch_t:s0 tclass=netlink_generic_socket permissive=0
If SELinux denied OVS access to certain resources, then make sure that you have installed our SELinux policy package that "loosens" up distribution's SELinux policy:
$ rpm -qa | grep openvswitch-selinux openvswitch-selinux-policy-2.4.1-1.el7.centos.noarch
Then verify that this module was indeed loaded:
# semodule -l | grep openvswitch openvswitch-custom 1.0 openvswitch 1.1.1
If you still see Permission denied errors, then take a look into
selinux/openvswitch.te file in the OVS source tree and try to add white
list rules. This is really simple, just run SELinux audit2allow tool:
$ grep "openvswitch_t" /var/log/audit/audit.log | audit2allow -M ovslocal
Here are few things to consider before proposing SELinux policy patches to Open vSwitch developer mailing list:
The SELinux policy that resides in Open vSwitch source tree amends SELinux policy that ships with your distributions.
Implications of this are that it is assumed that the distribution's Open vSwitch SELinux module must be already loaded to satisfy dependencies.
The SELinux policy that resides in Open vSwitch source tree must work on all currently relevant Linux distributions.
Implications of this are that you should use only those SELinux policy features that are supported by the lowest SELinux version out there. Typically this means that you should test your SELinux policy changes on the oldest RHEL or CentOS version that this OVS version supports. Check INSTALL.Fedora.rst file to find out this.
The SELinux policy is enforced only when state transition to
openvswitch_tdomain happens.Implications of this are that perhaps instead of loosening SELinux policy you can do certain things at the time rpm package is installed.
Report problems to [email protected].