Synnefo » snf-ganeti

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Ganeti OS installation redesign

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.. contents:: :depth: 3

This is a design document detailing a new OS installation procedure, which is

more secure, able to provide more features and easier to use for many common

tasks w.r.t. the current one.

Current state and shortcomings

==============================

As of Ganeti 2.10, each instance is associated with an OS definition. An OS

definition is a set of scripts (``create``, ``export``, ``import``, ``rename``)

that are executed with root privileges on the primary host of the instance to

perform all the OS-related functionality (setting up an operating system inside

the disks of the instance being created, exporting/importing the instance,

renaming it).

These scripts receive through environment variables a fixed set of parameters

related to the instance (such as the hypervisor, the name of the instance, the

number of disks, and their location) and a set of user defined parameters.

These parameters are also written in the configuration file of Ganeti, to allow

future reinstalls of the instance, and in various log files, namely:

* node daemon log file: contains DEBUG strings of the ``/os_validate``,

  ``/instance_os_add`` and ``/instance_start`` RPC calls.

* master daemon log file: DEBUG strings related to the same RPC calls are stored

  here as well.

* commands log: the CLI commands that create a new instance, including their

  parameters, are logged here.

* RAPI log: the RAPI commands that create a new instance, including their

  parameters, are logged here.

* job logs: the job files stored in the job queue, or in its archive, contain

  the parameters.

The current situation presents a number of shortcomings:

* Having the installation scripts run as root on the nodes doesn't allow

  user-defined OS scripts, as they would pose a huge security issue.

  Furthermore, even a script without malicious intentions might end up

  distrupting a node because of a bug in it.

* Ganeti cannot be used to create instances starting from user provided disk

  images: even in the (hypothetical) case where the scripts are completely

  secure and run not by root but by an unprivileged user with only the power to

  mount arbitrary files as disk images, this is a security issue. It has been

  proven that a carefully crafted file system might exploit kernel

  vulnerabilities to gain control of the system. Therefore, directly mounting

  images on the Ganeti nodes is not an option.

* There is no way to inject files into an existing disk image. A common use case

  is for the system administrator to provide a standard image of the system, to

  be later personalized with the network configuration, private keys identifying

  the machine, ssh keys of the users and so on. A possible workaround would be

  for the scripts to mount the image (only if this is trusted!) and to receive

  the configurations and ssh keys as user defined OS parameters. Unfortunately,

  this is also not an option for security sensitive material (such as the ssh

  keys) because the OS parameters are stored in many places on the system, as

  already described above.

* Most other virtualization software simply work with instance images, not with

  installation scripts. This difference makes the interaction of Ganeti with

  other software difficult.

Proposed changes

================

In order to fix the shortcomings of the current state, we plan to introduce the

following changes.

OS parameters categories

++++++++++++++++++++++++

Change the OS parameters to have three categories:

* ``public``: the current behavior. The parameter is logged and stored freely.

* ``private``: the parameter is saved inside the Ganeti configuration (to allow

  for instance reinstall) but it is not shown in logs, job logs, or passed back

  via RAPI.

* ``secret``: the parameter is not saved inside the Ganeti configuration.

  Reinstalls are impossible unless the data is passed again. The parameter will

  not appear in any log file. When a functionality is performed jointly by

  multiple daemons (such as MasterD and LuxiD), currently Ganeti sometimes

  serializes jobs on disk and later reloads them. Secret parameters will not be

  serialized to disk. They will be passed around as part of the LUXI calls

  exchanged by the daemons, and only kept in memory, in order to reduce their

  accessibility as much as possible. In case of failure of the master node,

  these parameters will be lost and cannot be recovered because they are not

  serialized. As a result, the job cannot be taken over by the new master.  This

  is an expected and accepted side effect of jobs with secret parameters: if

  they fail, they'll have to be restarted manually.

Metadata

++++++++

In order to allow metadata to be sent inside the instance, a communication

mechanism between the instance and the host will be created.  This mechanism

will be bidirectional (e.g.: to allow the setup process going on inside the

instance to communicate its progress to the host). Each instance will have

access exclusively to its own metadata, and it will be only able to communicate

with its host over this channel.  This is the approach followed the

``cloud-init`` tool and more details will be provided in the `Communication

mechanism and metadata service`_ section.

Installation procedure

++++++++++++++++++++++

A new installation procedure will be introduced, with which it will be possible

to use an installation medium and run the OS scripts in an optional virtualized

environment and with an optional personalization package.  There will be two

sets of parameters, namely, installation parameters, which are used mainly for

installs and reinstalls, and execution parameters, which are used in all the

other runs that are not part of an installation procedure.

This set of installation parameters will allow, e.g., to attach an installation

floppy/cdrom/network, change the boot device order, or specify a disk image to

be used.  Through this set of parameters, the administrator will have to provide

the hypervisor a location for an installation medium for the instance (e.g., a

boot disk, a network image, etc).  This medium will carry out the installation

of the instance onto the instance's disks and will then be responsible for

getting the parameters for configuring the instance, such as, network

interfaces, IP address, and hostname.  These parameters are taken from the

metadata.  The installation parameters will be stored in the configuration of

Ganeti and used in future reinstalls, but not during normal execution.

The instance is reinstalled using the same installation parameters from the

first installation.  However, it will be the administrator's responsibility to

ensure that the any installation media is still available at the proper location

when a reinstall occurs.

The parameter ``--os-parameters`` can still be used to specify the OS

parameters.  However, without OS scripts, Ganeti cannot do more than a syntactic

check to validate the supplied OS parameters string.  As a result, this string

will be directly passed to the instance as part of the metadata.  If the

installation procedure is running inside a virtualized environment, then Ganeti

will take these parameters from the metadata and pass them to the OS scripts as

environment variables.

* Use a disk image:

  Currently, it is already possible to specify an installation medium, such as,

  a cdrom, but not a disk image.  Therefore, a new parameter ``--os-image`` will

  be used to specify the location of a disk image which will be dumped to the

  instance's first disk before the instance is started.  The location of the

  image can be a URL and, if this is the case, Ganeti will download this image.

* Run OS scripts:

  The parameter ``--os-type`` (short version: ``-o``), is currently used to

  specify the OS scripts.  This parameter will still be used to specify the OS

  scripts with the difference that these OS scripts may optionally run inside a

  virtualized environment for safety reasons, depending on whether they are

  trusted or not.  For more details on trusted and untrusted OS scripts, refer

  to the `Installation process in a virtualized environment`_ section.

* Personalization package

  As part of the instance creation command, it will be possible to indicate a

  URL for a "personalization package", which is an archive containing a set of

  files meant to be overlayed on top of the OS file system at the end of the

  setup process and before the VM is started for the first time in normal mode.

  Ganeti will provide a mechanism for receiving and unpacking this archive

  whether the installation is being performed inside the virtualized environment

  or not.

  The archive will be in TAR-GZIP format (with extension ``.tar.gz`` or

  ``.tgz``) and contain the files according to the directory structure that will

  be recreated on the installation disk.  Files contained in this archive will

  overwrite files with the same path created during the installation procedure

  (if any).  The URL of the "personalization package" will have to specify an

  extension to identify the file format (in order to allow for more formats to

  be supported in the future).  The URL will be stored as part of the

  configuration of the instance (therefore, the URL should not contain

  confidential information, but the files there available can).

  It is up to the system administrator to ensure that a package is actually

  available at that URL at install and reinstall time.  The contents of the

  package are allowed to change.  E.g.: a system administrator might create a

  package containing the private keys of the instance being created.  When the

  instance is reinstalled, a new package with new keys can be made available

  there, thus allowing instance reinstall without the need to store keys.  A

  username and a password can be specified together with the URL.  If the URL is

  a HTTP(S) URL, they will be used as basic access authentication credentials to

  access that URL.  The username and password will not be saved in the config,

  and will have to be provided again in case a reinstall is requested.

  The downloaded personalization package will not be stored locally on the node

  for longer than it is needed while unpacking it and adding its files to the

  instance being created.  The personalization package will be overlayed on top

  of the instance filesystem after the scripts that created it have been

  executed.  In order for the files in the package to be automatically overlayed

  on top of the instance filesystem, it is required that the appliance is

  actually able to mount the instance's disks.  As a result, this will not work

  for every filesystem.

* Combine a disk image, OS scripts, and a personalization package

  It will possible to combine a disk image, OS scripts, and a personalization

  package, both with or without a virtualized environment.  There is one

  exception which is if there are untrusted OS scripts.  At least, an

  installation medium or OS scripts should be specified.

  The disk image of the actual virtual appliance, which bootstraps the virtual

  environment used in the installation procedure, will be read only, so that a

  pristine copy of the appliance can be started every time a new instance needs

  to be created and to further increase security.  The data the instance needs

  to write at runtime will only be stored in RAM and disappear as soon as the

  instance is stopped.

  The parameter ``--enable-safe-install=yes|no`` will be used to give the

  administrator control over whether to use a virtualized environment for the

  installation procedure.  By default, a virtualized environment will be used.

  Note that some feature combinations, such as, using untrusted scripts, will

  require the virtualized environment.  In this case, Ganeti will not allow

  disabling the virtualized environment.

Implementation

==============

The implementation of this design will happen as an ordered sequence of steps,

of increasing impact on the system and, in some cases, dependent on each other:

#. Private and secret instance parameters

#. Communication mechanism between host and instance

#. Metadata service

#. Personalization package (inside a virtualization environment)

#. Instance creation via a disk image

#. Instance creation inside a virtualized environment

Some of these steps need to be more deeply specified w.r.t. what is already

written in the `Proposed changes`_ Section. Extra details will be provided in

the following subsections.

Communication mechanism and metadata service

++++++++++++++++++++++++++++++++++++++++++++

The communication mechanism and the metadata service are described together

because they are deeply tied. The communication mechanism will be made more

generic because it can be used for other purposes in the future (like allowing

instances to explicitly send commands to Ganeti, or to let Ganeti control a

helper instance, like the one hereby introduced for performing OS installs

inside a safe environment).

The communication mechanism will be enabled automatically during an installation

procedure that requires a virtualized environment, but for backwards

compatibility it will be disabled when the instance is running normally, unless

it is explicitly requested. Specifically, a new parameter

``--communication=yes|no`` (short version: ``-C``) will be added to

``gnt-instance add`` and ``gnt-instance modify``. It will determine whether the

instance has a communication channel set to interact with the host and receive

metadata. The value of this parameter will be saved as part of the configuration

of the instance.

When the communication mechanism is enabled, Ganeti will create a new network

interface inside the instance. This additional network interface will be the

last one in the instance, after all the user defined ones. On the host side,

this interface will only be accessible to the host itself, and not routed

outside the machine.

On this network interface, the instance will connect using the IP:

169.254.169.253 and netmask 255.255.255.0.

The host will be on the same network, with the IP address: 169.254.169.254.

The way to create this interface depends on the specific hypervisor being used.

In KVM, it is possible to create a network interface inside the instance without

having a corresponding interface created on the host. Using a command like::

  kvm -net nic -net \

    user,restrict=on,net=169.254.169.0/24,host=169.254.169.253,

    guestfwd=tcp:169.254.169.254:80-tcp:127.0.0.1:8080

a network interface will be created inside the VM, part of the 169.254.169.0/24

network, where the VM will have IP address .253 and the host port 8080 will be

reachable on port 80.

In Xen, unfortunately, such a capability is not present, and an actual network

interface has to be created on the host (using the ``vif`` parameter of the Xen

configuration file). Each instance will have its corresponding ``vif`` network

interface on the host. These interfaces will not be connected to each other in

any way, and Ganeti will not configure them to allow traffic to be forwarded

beyond the host machine. The ``vif-route`` script of Xen might be helpful in

implementing this.

It will be the system administrator's responsibility to ensure that the extra

firewalling and routing rules specified on the host don't allow this

accidentally.

The instance will be able to connect to 169.254.169.254:80, and issue GET

requests to an HTTP server that will provide the instance metadata.

The choice of this IP address and port for accessing the metadata is done for

compatibility reasons with OpenStack's and Amazon EC2's ways of providing

metadata to the instance. The metadata will be provided by a single daemon,

which will determine what instance the request comes from and reply with the

metadata specific for that instance.

Where possible, the metadata will be provided in a way compatible with Amazon

EC2, at::

  http://169.254.169.254/<version>/meta-data/*

If some metadata are Ganeti-specific and don't fit this structure, they will be

provided at::

  http://169.254.169.254/ganeti/<version>/meta_data.json

``<version>`` is either a date in YYYY-MM-DD format, or ``latest`` to indicate

the most recent available protocol version.

If needed in the future, this structure also allows us to support OpenStack's

metadata at::

  http://169.254.169.254/openstack/<version>/meta_data.json

A bi-directional, pipe-like communication channel will be provided. The instance

will be able to receive data from the host by a GET request at::

  http://169.254.169.254/ganeti/<version>/read

and to send data to the host by a POST request at::

  http://169.254.169.254/ganeti/<version>/write

As in a pipe, once the data are read, they will not be in the buffer anymore, so

subsequent GET requests to ``read`` will not return the same data twice.

Unlike a pipe, though, it will not be possible to perform blocking I/O

operations.

The OS parameters will be accessible through a GET

request at::

  http://169.254.169.254/ganeti/<version>/os/parameters.json

as a JSON serialized dictionary having the parameter name as the key, and the

pair ``(<value>, <visibility>)`` as the value, where ``<value>`` is the

user-provided value of the parameter, and ``<visibility>`` is either ``public``,

``private`` or ``secret``.

The installation scripts to be run inside the virtualized environment will be

available at::

  http://169.254.169.254/ganeti/<version>/os/scripts/<script_name>

where ``<script_name>`` is the name of the script.

Rationale

---------

The choice of using a network interface for instance-host communication, as

opposed to VirtIO, XenBus or other methods, is due to the will of having a

generic, hypervisor-independent way of creating a communication channel, that

doesn't require unusual (para)virtualization drivers.

At the same time, a network interface was preferred over solutions involving

virtual floppy or USB devices because the latter tend to be detected and

configured by the guest operating systems, sometimes even in prominent positions

in the user interface, whereas it is fairly common to have an unconfigured

network interface in a system, usually without any negative side effects.

Installation process in a virtualized environment

+++++++++++++++++++++++++++++++++++++++++++++++++

In the new OS installation scenario, we distinguish between trusted and

untrusted code.

The trusted installation code maintains the behavior of the current one and

requires no modifications, with the scripts running on the node the instance is

being created on. The untrusted code is stored in a subdirectory of the OS

definition called ``untrusted``.  This directory contains scripts that are

equivalent to the already existing ones (``create``, ``export``, ``import``,

``rename``) but that will be run inside an virtualized environment, to protect

the host from malicious tampering.

The ``untrusted`` code is meant to either be untrusted itself, or to be trusted

code running operations that might be dangerous (such as mounting a

user-provided image).

By default, all new OS definitions will have to be explicitly marked as trusted

by the cluster administrator (with a new ``gnt-os modify`` command) before they

can run code on the host. Otherwise, only the untrusted part of the code will be

allowed to run, inside the virtual appliance. For backwards compatibility

reasons, when upgrading an existing cluster, all the installed OSes will be

marked as trusted, so that they can keep running with no changes.

In order to allow for the highest flexibility, if both a trusted and an

untrusted script are provided for the same operation (i.e. ``create``), both of

them will be executed at the same time, one on the host, and one inside the

installation appliance. They will be allowed to communicate with each other

through the already described communication mechanism, in order to orchestrate

their execution (e.g.: the untrusted code might execute the installation, while

the trusted one receives status updates from it and delivers them to a user

interface).

The cluster administrator will have an option to completely disable scripts

running on the host, leaving only the ones running in the VM.

Ganeti will provide a script to be run at install time that can be used to

create the virtualized environment that will perform the OS installation of new

instances.

This script will build a debootstrapped basic debian system including a software

that will read the metadata, setup the environment variables and launch the

installation scripts inside the virtualized environment. The script will also

provide hooks for personalization.

It will also be possible to use other self-made virtualized environments, as

long as they connect to Ganeti over the described communication mechanism and

they know how to read and use the provided metadata to create a new instance.

While performing an installation in the virtualized environment, a

personalizable timeout will be used to detect possible problems with the

installation process, and to kill the virtualized environment. The timeout will

be optional and set on a cluster basis by the administrator. If set, it will be

the total time allowed to setup an instance inside the appliance. It is mainly

meant as a safety measure to prevent an instance taken over by malicious scripts

to be available for a long time.

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