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

• Improvements of Node Security
  • Objective
  • Detailed design
    • Current state and shortcomings
    • Proposal concerning SSH key distribution
      • (Re-)Adding nodes to a cluster
      • Pro- and demoting a node to/from master candidate
      • Offlining and onlining a node
      • Cluster verify
    • Proposal regarding node daemon certificates
      • Cluster initialization
      • (Re-)Adding nodes
      • Promotion and demotion of master candidates
      • Cluster verify
      • Crypto renewal
    • Further considerations
      • Watcher
      • Other Keys and Daemons
      • Related and Future Work

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, 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 key distribution

We propose two improvements regarding the ssh keys:

1. Limit the distribution of the private ssh key to the master candidates.
2. Use different ssh key pairs for each master candidate.

We propose to limit the set of nodes holding the private root user SSH key to the master and the master candidates. This way, the security risk would be limited to a rather small set of nodes even though the cluster could consists of a lot more nodes. The set of master candidates could be protected better than the normal nodes (for example residing in a DMZ) to enhance security even more if the administrator wishes so. The following sections describe in detail which Ganeti commands are affected by this change and in what way.

Security will be even more increased if each master candidate gets its own ssh private/public key pair. This way, 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.) However, this improvement was not part of the original feature request and increases the complexity of node management even more. We therefore consider it as second step in this design and will address this after the other parts of this design are implemented.

The following sections describe in detail which Ganeti commands are affected by the first part of ssh-related improvements, limiting the key distribution to master candidates only.

(Re-)Adding nodes to a cluster

According to Design for adding a node to a cluster, Ganeti transfers the ssh keys to every node that gets added to the cluster.

We propose to change this procedure to treat master candidates and normal nodes differently. For master candidates, the procedure would stay as is. For normal nodes, Ganeti would transfer the public and private ssh host keys (as before) and only the public root key.

A normal node would not be able to connect via ssh to other nodes, but the master (and potentially master candidates) can connect to this node.

In case of readding a node that used to be in the cluster before, handling of the ssh keys would basically be the same with the following additional modifications: if the node used to be a master or master-candidate node, but will be a normal node after readding, Ganeti should make sure that the private root key is deleted if it is still present on the node.

Pro- and demoting a node to/from master candidate

If the role of a node is changed from ‘normal’ to ‘master_candidate’, the master node should at that point copy the private root ssh key. When demoting a node from master candidate to a normal node, the key that have been copied there on promotion or addition should be removed again.

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

To make sure the private root ssh key was not distributed to a normal node, ‘gnt-cluster verify’ will be extended by a check for the key on normal nodes. Additionally, it will check if the private key is indeed present on master candidates.

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 Ganeti 2.1 design): 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 Ganeti 2.1 design, 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 Design for import/export version 2 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.