Update security document for version 2.6

Quite some things were out of date. Some formatting was also updated.

Signed-off-by: Michael Hanselmann <hansmi@google.com>
Reviewed-by: Iustin Pop <iustin@google.com>

doc/security.rst

 Security in Ganeti
 ==================
 
+Documents Ganeti version 2.6
+
 Ganeti was developed to run on internal, trusted systems. As such, the
 security model is all-or-nothing.
 
-All the Ganeti code runs as root, because all the operations that Ganeti
-is doing require privileges: creating logical volumes, drbd devices,
-starting instances, etc. Running as root does not mean setuid, but that
-you need to be root to run the cluster commands.
+Up to version 2.3 all Ganeti code ran as root. Since version 2.4 it is
+possible to run all daemons except the node daemon as non-root users by
+specifying user names and groups at build time. The node daemon
+continues to require root privileges to create logical volumes, DRBD
+devices, start instances, etc. Cluster commands can be run as root or by
+users in a group specified at build time.
 
 Host issues
 -----------
@@ -21,7 +25,7 @@ changes:
 - The host will have its SSH host key replaced with the one of the
   cluster (which is the one the initial node had at the cluster
   creation)
-- A new public key will be added to root's authorized_keys file,
+- A new public key will be added to root's ``authorized_keys`` file,
   granting root access to all nodes of the cluster. The private part of
   the key is also distributed to all nodes. Old files are renamed.
 - Communication between nodes is encrypted using SSL/TLS. A common key
@@ -31,7 +35,7 @@ changes:
   the cluster with the correct certificate, and the operations it will
   do as a result of these requests are:
 
-  - running commands under the /etc/ganeti/hooks directory
+  - running commands under the ``/etc/ganeti/hooks`` directory
   - creating DRBD disks between it and the IP it has been told
   - overwrite a defined list of files on the host
 
@@ -39,23 +43,23 @@ As you can see, as soon as a node is joined, it becomes equal to all
 other nodes in the cluster, and the security of the cluster is
 determined by the weakest node.
 
-Note that only the SSH key will allow other machines to run random
-commands on this node; the RPC method will run only:
+Note that only the SSH key will allow other machines to run any command
+on this node; the RPC method will run only:
 
 - well defined commands to create, remove, activate logical volumes,
   drbd devices, start/stop instances, etc;
-- run SSH commands on other nodes in the cluster, again well-defined
-- scripts under the /etc/ganeti/hooks directory
+- run well-defined SSH commands on other nodes in the cluster
+- scripts under the ``/etc/ganeti/hooks`` directory
 
 It is therefore important to make sure that the contents of the
-/etc/ganeti/hooks directory is supervised and only trusted sources can
-populate it.
+``/etc/ganeti/hooks`` directory is supervised and only trusted sources
+can populate it.
 
 Cluster issues
 --------------
 
-As told above, there are multiple ways of communication between cluster
-nodes:
+As mentioned above, there are multiple ways of communication between
+cluster nodes:
 
 - SSH-based, for high-volume traffic like image dumps or for low-level
   command, e.g. restarting the Ganeti node daemon
@@ -74,17 +78,18 @@ simplify the key handling.
 The DRBD traffic is not protected by encryption, as DRBD does not
 support this. It's therefore recommended to implement host-level
 firewalling or to use a separate range of IP addresses for the DRBD
-traffic (this is supported in Ganeti) which is not routed outside the
-cluster. DRBD connections are protected from connecting due to bugs to
-other machines, and from accepting connections from other machines, by
-using a shared secret, exchanged via RPC requests from the master to the
-nodes when configuring the device.
+traffic (this is supported in Ganeti through the use of a secondary
+interface) which is not routed outside the cluster. DRBD connections are
+protected from erroneous connections to other machines (as may happen
+due to software issues), and from accepting connections from other
+machines, by using a shared secret, exchanged via RPC requests from the
+master to the nodes when configuring the device.
 
 Master daemon
 -------------
 
-The command-line tools to master daemon communication is done via an
-UNIX socket, whose permissions are reset to ``0600`` after listening but
+The command-line tools to master daemon communication is done via a
+UNIX socket, whose permissions are reset to ``0660`` after listening but
 before serving requests. This permission-based protection is documented
 and works on Linux, but is not-portable; however, Ganeti doesn't work on
 non-Linux system at the moment.
@@ -113,15 +118,15 @@ KVM Security
 ------------
 
 When running KVM instances under Ganeti three security models ara
-available: 'none', 'user' and 'pool'.
+available: "none", "user" and "pool".
 
-Under security model 'none' instances run by default as root. This means
+Under security model "none" instances run by default as root. This means
 that, if an instance gets jail broken, it will be able to own the host
 node, and thus the ganeti cluster. This is the default model, and the
 only one available before Ganeti 2.1.2.
 
-Under security model 'user' an instance is run as the user specified by
-the hypervisor parameter 'security_domain'. This makes it easy to run
+Under security model "user" an instance is run as the user specified by
+the hypervisor parameter "security_domain". This makes it easy to run
 all instances as non privileged users, and allows one to manually
 allocate specific users to specific instances or sets of instances. If
 the specified user doesn't have permissions a jail broken instance will
@@ -129,7 +134,7 @@ need some local privilege escalation before being able to take over the
 node and the cluster. It's possible though for a jail broken instance to
 affect other ones running under the same user.
 
-Under security model 'pool' a global cluster-level uid pool is used to
+Under security model "pool" a global cluster-level uid pool is used to
 start each instance on the same node under a different user. The uids in
 the cluster pool can be set with ``gnt-cluster init`` and ``gnt-cluster
 modify``, and must correspond to existing users on all nodes. Ganeti
@@ -137,7 +142,7 @@ will then allocate one to each instance, as needed. This way a jail
 broken instance won't be able to affect any other. Since the users are
 handed out by ganeti in a per-node randomized way, in this mode there is
 no way to make sure a particular instance is always run as a certain
-user. Use mode 'user' for that.
+user. Use mode "user" for that.
 
 In addition to these precautions, if you want to avoid instances sending
 traffic on your node network, you can use an iptables rule such as::