Add the document describing a new design for the OS installation process for new instances.
Signed-off-by: Michele Tartara <[email protected]> --- doc/design-draft.rst | 1 + doc/design-os.rst | 399 +++++++++++++++++++++++++++++++++++++++++++++++++++ 2 files changed, 400 insertions(+) create mode 100644 doc/design-os.rst diff --git a/doc/design-draft.rst b/doc/design-draft.rst index c821292..3ed3852 100644 --- a/doc/design-draft.rst +++ b/doc/design-draft.rst @@ -20,6 +20,7 @@ Design document drafts design-daemons.rst design-hsqueeze.rst design-ssh-ports.rst + design-os.rst .. vim: set textwidth=72 : .. Local Variables: diff --git a/doc/design-os.rst b/doc/design-os.rst new file mode 100644 index 0000000..a26801a --- /dev/null +++ b/doc/design-os.rst @@ -0,0 +1,399 @@ +=============================== +Ganeti OS installation redesign +=============================== + +.. contents:: :depth: 3 + +This is a design document detailing a new OS installation procedure, 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, as environment variables, a fixed set of parameters +describing 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. Each +of these parameters is also written into the configuration file of Ganeti, to +allow for 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 instances, 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 with root power 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: + +* 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. + Reinstall 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 on 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 a failure of the master node, + these parameters will be lost and cannot be recovered because they are not + serialized on file, therefore the job cannot 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. + +* A new OS installation procedure, based on a safe virtualized environment. + This virtualized environment will run with the same hardware parameter as the + actual instance being installed, as much as possible. This will also allow to + reduce the memory usage in the host (specifically, in Dom0 for Xen + installations). Each instance will have these possible execution modes: + + * ``default``: the default mode, used when the machine is running normally and + the OS installation procedure is run before starting the instance for the + first time. + + * ``self_install``: the first run of the instance will be with a different set + of parameters w.r.t. all the successive runs. This set of "install + parameters" will allow, e.g., to attach an installation + floppy/cdrom/network, change the boot device order, or specify an OS image + to be used. Through this set of parameters, the administrator will have to + provide the hypervisor a way to find an installation medium for the instance + (e.g., a boot disk, a network image, etc). This medium will then install the + instance itself on the disks and will then be responsible to get the + parameters for configuring it (its network interfaces, IP address, hostname, + etc.) from a set of metadata provided by Ganeti (e.g.: using an approach + comparable to the one of the ``cloud-init`` tool). When this installation + mode is used, no OS installation script is required. In order for + installation of an OS from an image to be possible, the ``--os-type`` + parameter will be extended to support a new additional format: ``--os-type + image:<URL>`` will instruct ganeti to take an image from the specified + position. For the initial implementation, URL can be either a filename or a + publically accessible http or ftp resource. Once the instance image is + received, it will be dd-ed on the first disk of the instance. + When an image is specified, ``--os-parameters`` can still be used, + and its content will be passed to the instance as part of the metadata. Nota + that as part of the OS scripts there is a file specifying what parameters + are expected. With OS images, though, none of the traditional structure of + OS scripts is in place, so there will be no check regarding what parameters + can be specified: they will all be passed, as long as the + ``--os-parameters`` string is syntactically valid. + The set of ``self_install`` parameters will be stored as part of the + instance configuration, so that they can be used to reinstall the instance. + It will be the user's responsibility to ensure that the OS image or any + installation media is still available in the proper position when a + reinstall happens. After the first run, the instance will revert to + ``default`` mode. + + * ``install``: Ganeti will start the instance using a virtual appliance + specifically made for installing Ganeti instances. Scripts analogous to the + current ones will run inside this instance. The disks of the instance being + installed will be connected to this virtual appliance, so that the scripts + can mount them and modify them as needed, as currently happens, but with the + additional protection given by this happening in a VM. The disk of the + virtual appliance will be read only, so that a pristine copy of the + appliance can be started every time a new instance needs to be created, 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. Metadata will be provided also to this virtual applicance, that + will take care of converting them to environment variables for the + installation scripts. After the first run, the instance will revert to + ``default`` mode. + +* In order to allow for the 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. More details will be provided in + the `Communication mechanism and metadata service`_ section. + +* As part of the instance creation command it will be possible to indicate a URL + for a "personalization package", that is an archive containing a set of files + meant to be overlayed on top of the operating system file system at the end of + the setup process, before the VM is started for the first time in ``run`` + mode. Ganeti will provide a mechanism for receiving and unpacking this + archive as part of the ``install`` execution mode, whereas in ``self_install`` + mode it will only be provided as a metadata for the instance to use. The + archive will be in TAR-GZIP format (with extension ``.tar.gz`` or ``.tgz``) + and will 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 install procedure (if + any). The URL of the "personalization package" will have to specify an + extesion 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 file 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 content of the package is 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, therefore allowing instance + reinstall without the need to store keys. Together with the URL, a username + and a password can be specified to. 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 disks, therefore this will not work for every filesystem. + +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) +#. ``self_install`` mode +#. ``install`` mode (inside a virtualization 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. On the other hand, the communication mechanism +will need to be more generic because it can be used for other reasons 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 when the instance is +in ``self_install`` or ``install`` mode, but for backwards compatibility it will +be disabled when the instance is in ``run`` mode unless it is explicitly +requested. Specifically, a new parameter ``--communication`` (short version: +``-C``), with possible values ``true`` or ``false`` will be added to +``gnt-instance add`` and ``gnt-instance modify``. It will determine whether the +instance will have a communication channel set up to interact with the host and +to 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 extra network interface will be the last one +of the instance, after all the user defined ones. On the host side, this +interface will be only accessible to the host itself, and not be routed outside +the machine. +On this network interface, the instance will connect using the IP: +169.254.169.1 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 interface 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 to ensure that 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 request 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 while the +instance is run in ``install`` mode will be available at:: + + http://169.254.169.254/<version>/ganeti/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 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 environment, 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. + +.. vim: set textwidth=72 : +.. Local Variables: +.. mode: rst +.. fill-column: 72 +.. End: -- 1.8.5.1
