Synnefo » snf-ganeti

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Improvements of Node Security

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

This document describes an enhancement of Ganeti's security by restricting

the distribution of security-sensitive data to the master and master

candidates only.

Note: In this document, we will use the term 'normal node' for a node that

is neither master nor master-candidate.

.. contents:: :depth: 4

Objective

=========

Up till 2.10, Ganeti distributes security-relevant keys to all nodes,

including nodes that are neither master nor master-candidates. Those

keys are the private and public SSH keys for node communication and the

SSL certficate and private key for RPC communication. Objective of this

design is to limit the set of nodes that can establish ssh and RPC

connections to the master and master candidates.

As pointed out in

`issue 377 <https://code.google.com/p/ganeti/issues/detail?id=377>`_, this

is a security risk. Since all nodes have these keys, compromising

any of those nodes would possibly give an attacker access to all other

machines in the cluster. Reducing the set of nodes that are able to

make ssh and RPC connections to the master and master candidates would

significantly reduce the risk simply because fewer machines would be a

valuable target for attackers.

Note: For bigger installations of Ganeti, it is advisable to run master

candidate nodes as non-vm-capable nodes. This would reduce the attack

surface for the hypervisor exploitation.

Detailed design

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

Current state and shortcomings

------------------------------

Currently (as of 2.10), all nodes hold the following information:

- the ssh host keys (public and private)

- the ssh root keys (public and private)

- node daemon certificate (the SSL client certificate and its

  corresponding private key)

Concerning ssh, this setup contains the following security issue. Since

all nodes of a cluster can ssh as root into any other cluster node, one

compromised node can harm all other nodes of a cluster.

Regarding the SSL encryption of the RPC communication with the node

daemon, we currently have the following setup. There is only one

certificate which is used as both, client and server certificate. Besides

the SSL client verification, we check if the used client certificate is

the same as the certificate stored on the server.

This means that any node running a node daemon can also act as an RPC

client and use it to issue RPC calls to other cluster nodes. This in

turn means that any compromised node could be used to make RPC calls to

any node (including itself) to gain full control over VMs. This could

be used by an attacker to for example bring down the VMs or exploit bugs

in the virtualization stacks to gain access to the host machines as well.

Proposal concerning SSH host key distribution

---------------------------------------------

We propose the following design regarding the SSH host key handling. The

root keys are untouched by this design.

Each node gets its own ssh private/public key pair, but only the public

keys of the master candidates get added to the ``authorized_keys`` file

of all nodes. This has the advantages, that:

- Only master candidates can ssh into other nodes, thus compromised

  nodes cannot compromise the cluster further.

- One can remove a compromised master candidate from a cluster

  (including removing it's public key from all nodes' ``authorized_keys``

  file) without having to regenerate and distribute new ssh keys for all

  master candidates. (Even though it is be good practice to do that anyway,

  since the compromising of the other master candidates might have taken

  place already.)

- If a (uncompromised) master candidate is offlined to be sent for

  repair due to a hardware failure before Ganeti can remove any keys

  from it (for example when the network adapter of the machine is broken),

  we don't have to worry about the keys being on a machine that is

  physically accessible.

To ensure security while transferring public key information and

updating the ``authorized_keys``, there are several other changes

necessary:

- Any distribution of keys (in this case only public keys) is done via

  SSH and not via RPC. An attacker who has RPC control should not be

  able to get SSH access where he did not have SSH access before

  already.

- The only RPC calls that are made in this context are from the master

  daemon to the node daemon on its own host and noded ensures as much

  as possible that the change to be made does not harm the cluster's

  security boundary.

- The nodes that are potential master candidates keep a list of public

  keys of potential master candidates of the cluster in a separate

  file called ``ganeti_pub_keys`` to keep track of which keys could

  possibly be added ``authorized_keys`` files of the nodes. We come

  to what "potential" means in this case in the next section. The key

  list is only transferred via SSH or written directly by noded. It

  is not stored in the cluster config, because the config is

  distributed via RPC.

The following sections describe in detail which Ganeti commands are

affected by the proposed changes.

RAPI

~~~~

The design goal to limit SSH powers to master candidates conflicts with

the current powers a user of the RAPI interface would have. The

``master_capable`` flag of nodes can be modified via RAPI.

That means, an attacker that has access to the RAPI interface, can make

all non-master-capable nodes master-capable, and then increase the master

candidate pool size till all machines are master candidates (or at least

a particular machine that he is aming for). This means that with RAPI

access and a compromised normal node, one can make this node a master

candidate and then still have the power to compromise the whole cluster.

To mitigate this issue, we propose the following changes:

- Add a flag to the cluster configuration

  ``master_capability_rapi_modifiable`` which indicates whether or

  not it should be possible to modify the ``master_capable`` flag of

  nodes via RAPI. The flag is set to ``False`` by default and can

  itself only be changed on the commandline. In this design doc, we

  refer to the flag as the "rapi flag" from here on.

- Only if the ``master_capabability_rapi_modifiable`` switch is set to

  ``True``, it is possible to modify the master-capability flag of

  nodes.

With this setup, there are the following definitions of "potential

master candidates" depending on the rapi flag:

- If the rapi flag is set to ``True``, all cluster nodes are potential

  master candidates, because as described above, all of them can

  eventually be made master candidates via RAPI and thus security-wise,

  we haven't won anything above the current SSH handling.

- If the rapi flag is set to ``False``, only the master capable nodes

  are considered potential master candidates, as it is not possible to

  make them master candidates via RAPI at all.

Note that when the rapi flag is changed, the state of the

``ganeti_pub_keys`` file on all nodes  has to be updated accordingly.

This should be done in the client script ``gnt_cluster`` before the

RPC call to update the configuration is made, because this way, if

someone would try to perform that RPC call on master to trick it into

thinking that the flag is enabled, this would not help as the content of

the ``ganeti_pub_keys`` file is a crucial part in the design of the

distribution of the SSH keys.

Note: One could think of always allowing to disable the master-capability

via RAPI and just restrict the enabling of it, thus making it possible

to RAPI-"freeze" the nodes' master-capability state once it disabled.

However, we think these are rather confusing semantics of the involved

flags and thus we go with proposed design.

Note that this change will break RAPI compatibility, at least if the

rapi flag is not explicitely set to ``True``. We made this choice to

have the more secure option as default, because otherwise it is

unlikely to be widely used.

Cluster initialization

~~~~~~~~~~~~~~~~~~~~~~

On cluster initialization, the following steps are taken in

bootstrap.py:

- A public/private key pair is generated (as before), but only used

  by the first (and thus master) node. In particular, the private key

  never leaves the node.

- A mapping of node UUIDs to public SSH keys is created and stored

  as text file in ``/var/lib/ganeti/ganeti_pub_keys`` only accessible

  by root (permissions 0600). The master node's uuid and its public

  key is added as first entry. The format of the file is one

  line per node, each line composed as ``node_uuid ssh_key``.

- The node's public key is added to it's own ``authorized_keys`` file.

(Re-)Adding nodes to a cluster

~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

According to :doc:`design-node-add`, Ganeti transfers the ssh keys to

every node that gets added to the cluster.

Adding a new node will require the following steps.

In gnt_node.py:

- On the new node, a new public/private SSH key pair is generated.

- The public key of the new node is fetched (via SSH) to the master

  node and if it is a potential master candidate (see definition above),

  it is added to the ``ganeti_pub_keys`` list on the master node.

- The public keys of all current master candidates are added to the

  new node's ``authorized_keys`` file (also via SSH).

In LUNodeAdd in cmdlib/node.py:

- The LUNodeAdd determines whether or not the new node is a master

  candidate and in any case updates the cluster's configuration with the

  new nodes information. (This is not changed by the proposed design.)

- If the new node is a master candidate, we make an RPC call to the node

  daemon of the master node to add the new node's public key to all

  nodes' ``authorized_keys`` files. The implementation of this RPC call

  has to be extra careful as described in the next steps, because

  compromised RPC security should not compromise SSH security.

RPC call execution in noded (on master node):

- Check that the public key of the new node is in the

  ``ganeti_pub_keys`` file of the master node to make sure that no keys

  of nodes outside the Ganeti cluster and no keys that are not potential

  master candidates gain SSH access in the cluster.

- Via SSH, transfer the new node's public key to all nodes (including

  the new node) and add it to their ``authorized_keys`` file.

- The ``ganeti_pub_keys`` file is transferred via SSH to all

  potential master candidates nodes except the master node

  (including the new one).

In case of readding a node that used to be in the cluster before,

handling of the SSH keys would basically be the same, in particular also

a new SSH key pair is generated for the node, because we cannot be sure

that the old key pair has not been compromised while the node was

offlined.

Pro- and demoting a node to/from master candidate

~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

If the role of a node is changed from 'normal' to 'master_candidate',

the procedure is the same as for adding nodes from the step "In

LUNodeAdd ..." on.

If a node gets demoted to 'normal', the master daemon makes a similar

RPC call to the master node's node daemon as for adding a node.

In the RPC call, noded will perform the following steps:

- Check that the public key of the node to be demoted is indeed in the

  ``ganeti_pub_keys`` file to avoid deleting ssh keys of machines that

  don't belong to the cluster (and thus potentially lock out the

  administrator).

- Via SSH, remove the key from all node's ``authorized_keys`` files.

This affected the behavior of the following commands:

::

  gnt-node modify --master-candidate=yes

  gnt-node modify --master-candidate=no [--auto-promote]

If the node has been master candidate already before the command to promote

it was issued, Ganeti does not do anything.

Note that when you demote a node from master candidate to normal node, another

master-capable and normal node will be promoted to master candidate. For this

newly promoted node, the same changes apply as if it was explicitely promoted.

The same behavior should be ensured for the corresponding rapi command.

Offlining and onlining a node

~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

When offlining a node, it immediately loses its role as master or master

candidate as well. When it is onlined again, it will become master

candidate again if it was so before. The handling of the keys should be done

in the same way as when the node is explicitely promoted or demoted to or from

master candidate. See the previous section for details.

This affects the command:

::

  gnt-node modify --offline=yes

  gnt-node modify --offline=no [--auto-promote]

For offlining, the removal of the keys is particularly important, as the

detection of a compromised node might be the very reason for the offlining.

Of course we cannot guarantee that removal of the key is always successful,

because the node might not be reachable anymore. Even though it is a

best-effort operation, it is still an improvement over the status quo,

because currently Ganeti does not even try to remove any keys.

The same behavior should be ensured for the corresponding rapi command.

Cluster verify

~~~~~~~~~~~~~~

So far, 'gnt-cluster verify' checks the SSH connectivity of all nodes to

all other nodes. We propose to replace this by the following checks:

- For all master candidates, we check if they can connect any other node

  in the cluster (other master candidates and normal nodes).

- We check if the ``ganeti_pub_keys`` file contains keys of nodes that

  are no longer in the cluster or that are not potential master

  candidates.

- For all normal nodes, we check if their key does not appear in other

  node's ``authorized_keys``. For now, we will only emit a warning

  rather than an error if this check fails, because Ganeti might be

  run in a setup where Ganeti is not the only system manipulating the

  SSH keys.

Upgrades

~~~~~~~~

When upgrading from a version that has the previous SSH setup to the one

proposed in this design, the upgrade procedure has to involve the

following steps in the post-upgrade hook:

- For all nodes, new SSH key pairs are generated.

- All nodes and their public keys are added to the ``ganeti_pub_keys``

  file and the file is copied to all nodes.

- All keys of master candidate nodes are added to the

  ``authorized_keys`` files of all other nodes.

Since this upgrade significantly changes the configuration of the

clusters' nodes, we will add a note to the UPGRADE notes to make the

administrator aware of this fact (in case he intends to enable access

from normal nodes to master candidates for other reasons than Ganeti

uses the machines).

Also, in any operation where Ganeti creates new SSH keys, the old keys

will be backed up and not simply overridden.

Downgrades

~~~~~~~~~~

These downgrading steps will be implemtented from 2.12 to 2.11:

- The master node's private/public key pair will be distributed to all

  nodes (via SSH) and the individual SSH keys will be backed up.

- The obsolete individual ssh keys will be removed from all nodes'

  ``authorized_keys`` file.

Renew-Crypto

~~~~~~~~~~~~

The ``gnt-cluster renew-crypto`` command is not affected by the proposed

changes related to SSH.

Proposal regarding node daemon certificates

-------------------------------------------

Regarding the node daemon certificates, we propose the following changes

in the design.

- Instead of using the same certificate for all nodes as both, server

  and client certificate, we generate a common server certificate (and

  the corresponding private key) for all nodes and a different client

  certificate (and the corresponding private key) for each node. All

  those certificates will be self-signed for now. The client

  certificates will use the node UUID as serial number to ensure

  uniqueness within the cluster.

- In addition, we store a mapping of

  (node UUID, client certificate digest) in the cluster's configuration

  and ssconf for hosts that are master or master candidate.

  The client certificate digest is a hash of the client certificate.

  We suggest a 'sha1' hash here. We will call this mapping 'candidate map'

  from here on.

- The node daemon will be modified in a way that on an incoming RPC

  request, it first performs a client verification (same as before) to

  ensure that the requesting host is indeed the holder of the

  corresponding private key. Additionally, it compares the digest of

  the certificate of the incoming request to the respective entry of

  the candidate map. If the digest does not match the entry of the host

  in the mapping or is not included in the mapping at all, the SSL

  connection is refused.

This design has the following advantages:

- A compromised normal node cannot issue RPC calls, because it will

  not be in the candidate map. (See the ``Drawbacks`` section regarding

  an indirect way of achieving this though.)

- A compromised master candidate would be able to issue RPC requests,

  but on detection of its compromised state, it can be removed from the

  cluster (and thus from the candidate map) without the need for

  redistribution of any certificates, because the other master candidates

  can continue using their own certificates. However, it is best

  practice to issue a complete key renewal even in this case, unless one

  can ensure no actions compromising other nodes have not already been

  carried out.

- A compromised node would not be able to use the other (possibly master

  candidate) nodes' information from the candidate map to issue RPCs,

  because the config just stores the digests and not the certificate

  itself.

- A compromised node would be able to obtain another node's certificate

  by waiting for incoming RPCs from this other node. However, the node

  cannot use the certificate to issue RPC calls, because the SSL client

  verification would require the node to hold the corresponding private

  key as well.

Drawbacks of this design:

- Complexity of node and certificate management will be increased (see

  following sections for details).

- If the candidate map is not distributed fast enough to all nodes after

  an update of the configuration, it might be possible to issue RPC calls

  from a compromised master candidate node that has been removed

  from the Ganeti cluster already. However, this is still a better

  situation than before and an inherent problem when one wants to

  distinguish between master candidates and normal nodes.

- A compromised master candidate would still be able to issue RPC calls,

  if it uses ssh to retrieve another master candidate's client

  certificate and the corresponding private SSL key. This is an issue

  even with the first part of the improved handling of ssh keys in this

  design (limiting ssh keys to master candidates), but it will be

  eliminated with the second part of the design (separate ssh keys for

  each master candidate).

- Even though this proposal is an improvement towards the previous

  situation in Ganeti, it still does not use the full power of SSL. For

  further improvements, see Section "Related and future work".

Alternative proposals:

- Instead of generating a client certificate per node, one could think

  of just generating two different client certificates, one for normal

  nodes and one for master candidates. Noded could then just check if

  the requesting node has the master candidate certificate. Drawback of

  this proposal is that once one master candidate gets compromised, all

  master candidates would need to get a new certificate even if the

  compromised master candidate had not yet fetched the certificates

  from the other master candidates via ssh.

- In addition to our main proposal, one could think of including a

  piece of data (for example the node's host name or UUID) in the RPC

  call which is encrypted with the requesting node's private key. The

  node daemon could check if the datum can be decrypted using the node's

  certificate. However, this would ensure similar functionality as

  SSL's built-in client verification and add significant complexity

  to Ganeti's RPC protocol.

In the following sections, we describe how our design affects various

Ganeti operations.

Cluster initialization

~~~~~~~~~~~~~~~~~~~~~~

On cluster initialization, so far only the node daemon certificate was

created. With our design, two certificates (and corresponding keys)

need to be created, a server certificate to be distributed to all nodes

and a client certificate only to be used by this particular node. In the

following, we use the term node daemon certificate for the server

certficate only.

In the cluster configuration, the candidate map is created. It is

populated with the respective entry for the master node. It is also

written to ssconf.

(Re-)Adding nodes

~~~~~~~~~~~~~~~~~

When a node is added, the server certificate is copied to the node (as

before). Additionally, a new client certificate (and the corresponding

private key) is created on the new node to be used only by the new node

as client certifcate.

If the new node is a master candidate, the candidate map is extended by

the new node's data. As before, the updated configuration is distributed

to all nodes (as complete configuration on the master candidates and

ssconf on all nodes). Note that distribution of the configuration after

adding a node is already implemented, since all nodes hold the list of

nodes in the cluster in ssconf anyway.

If the configuration for whatever reason already holds an entry for this

node, it will be overriden.

When readding a node, the procedure is the same as for adding a node.

Promotion and demotion of master candidates

~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

When a normal node gets promoted to be master candidate, an entry to the

candidate map has to be added and the updated configuration has to be

distributed to all nodes. If there was already an entry for the node,

we override it.

On demotion of a master candidate, the node's entry in the candidate map

gets removed and the updated configuration gets redistibuted.

The same procedure applied to onlining and offlining master candidates.

Cluster verify

~~~~~~~~~~~~~~

Cluster verify will be extended by the following checks:

- Whether each entry in the candidate map indeed corresponds to a master

  candidate.

- Whether the master candidate's certificate digest match their entry

  in the candidate map.

- Whether no node tries to use the certificate of another node. In

  particular, it is important to check that no normal node tries to

  use the certificate of a master candidate.

Crypto renewal

~~~~~~~~~~~~~~

Currently, when the cluster's cryptographic tokens are renewed using the

``gnt-cluster renew-crypto`` command the node daemon certificate is

renewed (among others). Option ``--new-cluster-certificate`` renews the

node daemon certificate only.

By adding an option ``--new-node-certificates`` we offer to renew the

client certificate. Whenever the client certificates are renewed, the

candidate map has to be updated and redistributed.

If for whatever reason, the candidate map becomes inconsistent, for example

due inconsistent updating after a demotion or offlining), the user can use

this option to renew the client certificates and update the candidate

certificate map.

Further considerations

----------------------

Watcher

~~~~~~~

The watcher is a script that is run on all nodes in regular intervals. The

changes proposed in this design will not affect the watcher's implementation,

because it behaves differently on the master than on non-master nodes.

Only on the master, it issues query calls which would require a client

certificate of a node in the candidate mapping. This is the case for the

master node. On non-master node, it's only external communication is done via

the ConfD protocol, which uses the hmac key, which is present on all nodes.

Besides that, the watcher does not make any ssh connections, and thus is

not affected by the changes in ssh key handling either.

Other Keys and Daemons

~~~~~~~~~~~~~~~~~~~~~~

Ganeti handles a couple of other keys/certificates that have not been mentioned

in this design so far. Also, other daemons than the ones mentioned so far

perform intra-cluster communication. Neither the keys nor the daemons will

be affected by this design for several reasons:

- The hmac key used by ConfD (see :doc:`design-2.1`): the hmac key is still

  distributed to all nodes, because it was designed to be used for

  communicating with ConfD, which should be possible from all nodes.

  For example, the monitoring daemon which runs on all nodes uses it to

  retrieve information from ConfD. However, since communication with ConfD

  is read-only, a compromised node holding the hmac key does not enable an

  attacker to change the cluster's state.

- The WConfD daemon writes the configuration to all master candidates

  via RPC. Since it only runs on the master node, it's ability to run

  RPC requests is maintained with this design.

- The rapi SSL key certificate and rapi user/password file 'rapi_users' is

  already only copied to the master candidates (see :doc:`design-2.1`,

  Section ``Redistribute Config``).

- The spice certificates are still distributed to all nodes, since it should

  be possible to use spice to access VMs on any cluster node.

- The cluster domain secret is used for inter-cluster instance moves.

  Since instances can be moved from any normal node of the source cluster to

  any normal node of the destination cluster, the presence of this

  secret on all nodes is necessary.

Related and Future Work

~~~~~~~~~~~~~~~~~~~~~~~

There a couple of suggestions on how to improve the SSL setup even more.

As a trade-off wrt to complexity and implementation effort, we did not

implement them yet (as of version 2.11) but describe them here for

future reference.

- All SSL certificates that Ganeti uses so far are self-signed. It would

  increase the security if they were signed by a common CA. There is

  already a design doc for a Ganeti CA which was suggested in a

  different context (related to import/export). This would also be a

  benefit for the RPC calls. See design doc :doc:`design-impexp2` for

  more information. Implementing a CA is rather complex, because it

  would mean also to support renewing the CA certificate and providing

  and supporting infrastructure to revoke compromised certificates.

- An extension of the previous suggestion would be to even enable the

  system administrator to use an external CA. Especially in bigger

  setups, where already an SSL infrastructure exists, it would be useful

  if Ganeti can simply be integrated with it, rather than forcing the

  user to use the Ganeti CA.

- A lighter version of using a CA would be to use the server certificate

  to sign the client certificate instead of using self-signed

  certificates for both. The probleme here is that this would make

  renewing the server certificate rather complicated, because all client

  certificates would need to be resigned and redistributed as well,

  which leads to interesting chicken-and-egg problems when this is done

  via RPC calls.

- Ganeti RPC calls are currently done without checking if the hostname

  of the node complies with the common name of the certificate. This

  might be a desirable feature, but would increase the effort when a

  node is renamed.

- The typical use case for SSL is to have one certificate per node

  rather than one shared certificate (Ganeti's noded server certificate)

  and a client certificate. One could change the design in a way that

  only one certificate per node is used, but this would require a common

  CA so that the validity of the certificate can be established by every

  node in the cluster.

- With the proposed design, the serial numbers of the client

  certificates are set to the node UUIDs. This is technically also not

  complying to how SSL is supposed to be used, as the serial numbers

  should reflect the enumeration of certificates created by the CA. Once

  a CA is implemented, it might be reasonable to change this

  accordingly. The implementation of the proposed design also has the

  drawback of the serial number not changing even if the certificate is

  replaced by a new one (for example when calling ``gnt-cluster renew-

  crypt``), which also does not comply to way SSL was designed to be

  used.

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