After a post about D-Bus comes the inevitable related post about SELinux with D-Bus.

Some users might not know that D-Bus is an SELinux-aware application. That means it has SELinux-specific code in it, which has the D-Bus behavior based on the SELinux policy (and might not necessarily honor the "permissive" flag). This code is used as an additional authentication control within D-Bus.

Inside the SELinux policy, a dbus permission class is supported, even though the Linux kernel doesn't do anything with this class. The class is purely for D-Bus, and it is D-Bus that checks the permission (although work is being made to implement D-Bus in kernel (kdbus)). The class supports two permission checks:

  • acquire_svc which tells the domain(s) allowed to "own" a service (which might, thanks to the SELinux support, be different from the domain itself)
  • send_msg which tells which domain(s) can send messages to a service domain

Inside the D-Bus security configuration (the busconfig XML file, remember) a service configuration might tell D-Bus that the service itself is labeled differently from the process that owned the service. The default is that the service inherits the label from the domain, so when dnsmasq_t registers a service on the system bus, then this service also inherits the dnsmasq_t label.

The necessary permission checks for the sysadm_t user domain to send messages to the dnsmasq service, and the dnsmasq service itself to register it as a service:

allow dnsmasq_t self:dbus { acquire_svc send_msg };
allow sysadm_t dnsmasq_t:dbus send_msg;
allow dnsmasq_t sysadm_t:dbus send_msg;

For the sysadm_t domain, the two rules are needed as we usually not only want to send a message to a D-Bus service, but also receive a reply (which is also handled through a send_msg permission but in the inverse direction).

However, with the following XML snippet inside its service configuration file, owning a certain resource is checked against a different label:

<busconfig>
  <selinux>
    <associate
      own="uk.org.thekelleys.dnsmasq"
      context="system_u:object_r:dnsmasq_dbus_t:s0" />
  </selinux>
</busconfig>

With this, the rules would become as follows:

allow dnsmasq_t dnsmasq_dbus_t:dbus acquire_svc;
allow dnsmasq_t self:dbus send_msg;
allow sysadm_t dnsmasq_t:dbus send_msg;
allow dnsmasq_t sysadm_t:dbus send_msg;

Note that only the access for acquiring a service based on a name (i.e. owning a service) is checked based on the different label. Sending and receiving messages is still handled by the domains of the processes (actually the labels of the connections, but these are always the process domains).

I am not aware of any policy implementation that uses a different label for owning services, and the implementation is more suited to "force" D-Bus to only allow services with a correct label. This ensures that other domains that might have enough privileges to interact with D-Bus and own a service cannot own these particular services. After all, other services don't usually have the privileges (policy-wise) to acquire_svc a service with a different label than their own label.


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