Client authorization

When configuring a QEMU network backend with either TLS certificates or SASL authentication, access will be granted if the client successfully proves their identity. If the authorization identity database is scoped to the QEMU client this may be sufficient. It is common, however, for the identity database to be much broader and thus authentication alone does not enable sufficient access control. In this case QEMU provides a flexible system for enforcing finer grained authorization on clients post-authentication.

Identity providers

At the time of writing there are two authentication frameworks used by QEMU that emit an identity upon completion.

TLS x509 certificate distinguished name.

When configuring the QEMU backend as a network server with TLS, there are a choice of credentials to use. The most common scenario is to utilize x509 certificates. The simplest configuration only involves issuing certificates to the servers, allowing the client to avoid a MITM attack against their intended server.

It is possible, however, to enable mutual verification by requiring that the client provide a certificate to the server to prove its own identity. This is done by setting the property verify-peer=yes on the tls-creds-x509 object, which is in fact the default.

When peer verification is enabled, client will need to be issued with a certificate by the same certificate authority as the server. If this is still not sufficiently strong access control the Distinguished Name of the certificate can be used as an identity in the QEMU authorization framework.

SASL username.

When configuring the QEMU backend as a network server with SASL, upon completion of the SASL authentication mechanism, a username will be provided. The format of this username will vary depending on the choice of mechanism configured for SASL. It might be a simple UNIX style user joebloggs, while if using Kerberos/GSSAPI it can have a realm attached joebloggs@QEMU.ORG. Whatever format the username is presented in, it can be used with the QEMU authorization framework.

Authorization drivers

The QEMU authorization framework is a general purpose design with choice of user customizable drivers. These are provided as objects that can be created at startup using the -object argument, or at runtime using the object_add monitor command.

Simple

This authorization driver provides a simple mechanism for granting access based on an exact match against a single identity. This is useful when it is known that only a single client is to be allowed access.

A possible use case would be when configuring QEMU for an incoming live migration. It is known exactly which source QEMU the migration is expected to arrive from. The x509 certificate associated with this source QEMU would thus be used as the identity to match against. Alternatively if the virtual machine is dedicated to a specific tenant, then the VNC server would be configured with SASL and the username of only that tenant listed.

To create an instance of this driver via QMP:

{
  "execute": "object-add",
  "arguments": {
    "qom-type": "authz-simple",
    "id": "authz0",
    "identity": "fred"
  }
}

Or via the command line

-object authz-simple,id=authz0,identity=fred

List

In some network backends it will be desirable to grant access to a range of clients. This authorization driver provides a list mechanism for granting access by matching identities against a list of permitted one. Each match rule has an associated policy and a catch all policy applies if no rule matches. The match can either be done as an exact string comparison, or can use the shell-like glob syntax, which allows for use of wildcards.

To create an instance of this class via QMP:

{
  "execute": "object-add",
  "arguments": {
    "qom-type": "authz-list",
    "id": "authz0",
    "rules": [
       { "match": "fred", "policy": "allow", "format": "exact" },
       { "match": "bob", "policy": "allow", "format": "exact" },
       { "match": "danb", "policy": "deny", "format": "exact" },
       { "match": "dan*", "policy": "allow", "format": "glob" }
    ],
    "policy": "deny"
  }
}

Due to the way this driver requires setting nested properties, creating it on the command line will require use of the JSON syntax for -object. In most cases, however, the next driver will be more suitable.

List file

This is a variant on the previous driver that allows for a more dynamic access control policy by storing the match rules in a standalone file that can be reloaded automatically upon change.

To create an instance of this class via QMP:

{
  "execute": "object-add",
  "arguments": {
    "qom-type": "authz-list-file",
    "id": "authz0",
    "filename": "/etc/qemu/myvm-vnc.acl",
    "refresh": true
  }
}

If refresh is yes, inotify is used to monitor for changes to the file and auto-reload the rules.

The myvm-vnc.acl file should contain the match rules in a format that closely matches the previous driver:

{
  "rules": [
    { "match": "fred", "policy": "allow", "format": "exact" },
    { "match": "bob", "policy": "allow", "format": "exact" },
    { "match": "danb", "policy": "deny", "format": "exact" },
    { "match": "dan*", "policy": "allow", "format": "glob" }
  ],
  "policy": "deny"
}

The object can be created on the command line using

-object authz-list-file,id=authz0,\
        filename=/etc/qemu/myvm-vnc.acl,refresh=on

PAM

In some scenarios it might be desirable to integrate with authorization mechanisms that are implemented outside of QEMU. In order to allow maximum flexibility, QEMU provides a driver that uses the PAM framework.

To create an instance of this class via QMP:

{
  "execute": "object-add",
  "arguments": {
    "qom-type": "authz-pam",
    "id": "authz0",
    "parameters": {
      "service": "qemu-vnc-tls"
    }
  }
}

The driver only uses the PAM “account” verification subsystem. The above config would require a config file /etc/pam.d/qemu-vnc-tls. For a simple file lookup it would contain

account requisite  pam_listfile.so item=user sense=allow \
        file=/etc/qemu/vnc.allow

The external file would then contain a list of usernames. If x509 cert was being used as the username, a suitable entry would match the distinguished name:

CN=laptop.berrange.com,O=Berrange Home,L=London,ST=London,C=GB

On the command line it can be created using

-object authz-pam,id=authz0,service=qemu-vnc-tls

There are a variety of PAM plugins that can be used which are not illustrated here, and it is possible to implement brand new plugins using the PAM API.

Connecting backends

The authorization driver is created using the -object argument and then needs to be associated with a network service. The authorization driver object will be given a unique ID that needs to be referenced.

The property to set in the network service will vary depending on the type of identity to verify. By convention, any network server backend that uses TLS will provide tls-authz property, while any server using SASL will provide a sasl-authz property.

Thus an example using SASL and authorization for the VNC server would look like:

$QEMU --object authz-simple,id=authz0,identity=fred \
      --vnc 0.0.0.0:1,sasl,sasl-authz=authz0

While to validate both the x509 certificate and SASL username:

echo "CN=laptop.qemu.org,O=QEMU Project,L=London,ST=London,C=GB" >> tls.acl
$QEMU --object authz-simple,id=authz0,identity=fred \
      --object authz-list-file,id=authz1,filename=tls.acl \
      --object tls-creds-x509,id=tls0,dir=/etc/qemu/tls,verify-peer=yes \
      --vnc 0.0.0.0:1,sasl,sasl-authz=auth0,tls-creds=tls0,tls-authz=authz1