<?xml version="1.0" encoding="UTF-8"?> <!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Strict//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-strict.dtd"> <html xmlns="http://www.w3.org/1999/xhtml"> <body> <h1>Polkit access control</h1> <p> Libvirt's client <a href="acl.html">access control framework</a> allows administrators to setup fine grained permission rules across client users, managed objects and API operations. This allows client connections to be locked down to a minimal set of privileges. The polkit driver provides a simple implementation of the access control framework. </p> <ul id="toc"></ul> <h2><a name="intro">Introduction</a></h2> <p> A default install of libvirt will typically use <a href="http://www.freedesktop.org/wiki/Software/polkit/">polkit</a> to authenticate the initial user connection to libvirtd. This is a very coarse grained check though, either allowing full read-write access to all APIs, or just read-only access. The polkit access control driver in libvirt builds on this capability to allow for fine grained control over the operations a user may perform on an object. </p> <h2><a name="perms">Permission names</a></h2> <p> The libvirt <a href="acl.html#perms">object names and permission names</a> are mapped onto polkit action names using the simple pattern: </p> <pre>org.libvirt.api.$object.$permission </pre> <p> The only caveat is that any underscore characters in the object or permission names are converted to hyphens. So, for example, the <code>search_storage_vols</code> permission on the <code>storage_pool</code> object maps to the polkit action: </p> <pre>org.libvirt.api.storage-pool.search-storage-vols </pre> <p> The default policy for any permission which corresponds to a "read only" operation, is to allow access. All other permissions default to deny access. </p> <h2><a name="attrs">Object identity attributes</a></h2> <p> To allow polkit authorization rules to be written to match against individual object instances, libvirt provides a number of authorization detail attributes when performing a permission check. The set of attributes varies according to the type of object being checked </p> <h3><a name="object_connect">virConnectPtr</a></h3> <table class="acl"> <thead> <tr> <th>Attribute</th> <th>Description</th> </tr> </thead> <tbody> <tr> <td>connect_driver</td> <td>Name of the libvirt connection driver</td> </tr> </tbody> </table> <h3><a name="object_domain">virDomainPtr</a></h3> <table class="acl"> <thead> <tr> <th>Attribute</th> <th>Description</th> </tr> </thead> <tbody> <tr> <td>connect_driver</td> <td>Name of the libvirt connection driver</td> </tr> <tr> <td>domain_name</td> <td>Name of the domain, unique to the local host</td> </tr> <tr> <td>domain_uuid</td> <td>UUID of the domain, globally unique</td> </tr> </tbody> </table> <h3><a name="object_interface">virInterfacePtr</a></h3> <table class="acl"> <thead> <tr> <th>Attribute</th> <th>Description</th> </tr> </thead> <tbody> <tr> <td>connect_driver</td> <td>Name of the libvirt connection driver</td> </tr> <tr> <td>interface_name</td> <td>Name of the network interface, unique to the local host</td> </tr> <tr> <td>interface_mac</td> <td>MAC address of the network interface, not unique</td> </tr> </tbody> </table> <h3><a name="object_network">virNetworkPtr</a></h3> <table class="acl"> <thead> <tr> <th>Attribute</th> <th>Description</th> </tr> </thead> <tbody> <tr> <td>connect_driver</td> <td>Name of the libvirt connection driver</td> </tr> <tr> <td>network_name</td> <td>Name of the network, unique to the local host</td> </tr> <tr> <td>network_uuid</td> <td>UUID of the network, globally unique</td> </tr> </tbody> </table> <h3><a name="object_node_device">virNodeDevicePtr</a></h3> <table class="acl"> <thead> <tr> <th>Attribute</th> <th>Description</th> </tr> </thead> <tbody> <tr> <td>connect_driver</td> <td>Name of the libvirt connection driver</td> </tr> <tr> <td>node_device_name</td> <td>Name of the node device, unique to the local host</td> </tr> </tbody> </table> <h3><a name="object_nwfilter">virNWFilterPtr</a></h3> <table class="acl"> <thead> <tr> <th>Attribute</th> <th>Description</th> </tr> </thead> <tbody> <tr> <td>connect_driver</td> <td>Name of the libvirt connection driver</td> </tr> <tr> <td>nwfilter_name</td> <td>Name of the network filter, unique to the local host</td> </tr> <tr> <td>nwfilter_uuid</td> <td>UUID of the network filter, globally unique</td> </tr> </tbody> </table> <h3><a name="object_secret">virSecretPtr</a></h3> <table class="acl"> <thead> <tr> <th>Attribute</th> <th>Description</th> </tr> </thead> <tbody> <tr> <td>connect_driver</td> <td>Name of the libvirt connection driver</td> </tr> <tr> <td>secret_uuid</td> <td>UUID of the secret, globally unique</td> </tr> <tr> <td>secret_usage_volume</td> <td>Name of the associated volume, if any</td> </tr> <tr> <td>secret_usage_ceph</td> <td>Name of the associated Ceph server, if any</td> </tr> <tr> <td>secret_usage_target</td> <td>Name of the associated iSCSI target, if any</td> </tr> </tbody> </table> <h3><a name="object_storage_pool">virStoragePoolPtr</a></h3> <table class="acl"> <thead> <tr> <th>Attribute</th> <th>Description</th> </tr> </thead> <tbody> <tr> <td>connect_driver</td> <td>Name of the libvirt connection driver</td> </tr> <tr> <td>pool_name</td> <td>Name of the storage pool, unique to the local host</td> </tr> <tr> <td>pool_uuid</td> <td>UUID of the storage pool, globally unique</td> </tr> </tbody> </table> <h3><a name="object_storage_vol">virStorageVolPtr</a></h3> <table class="acl"> <thead> <tr> <th>Attribute</th> <th>Description</th> </tr> </thead> <tbody> <tr> <td>connect_driver</td> <td>Name of the libvirt connection driver</td> </tr> <tr> <td>pool_name</td> <td>Name of the storage pool, unique to the local host</td> </tr> <tr> <td>pool_uuid</td> <td>UUID of the storage pool, globally unique</td> </tr> <tr> <td>vol_name</td> <td>Name of the storage volume, unique to the pool</td> </tr> <tr> <td>vol_key</td> <td>Key of the storage volume, globally unique</td> </tr> </tbody> </table> <h2><a name="user">User identity attributes</a></h2> <p> At this point in time, the only attribute provided by libvirt to identify the user invoking the operation is the PID of the client program. This means that the polkit access control driver is only useful if connections to libvirt are restricted to its UNIX domain socket. If connections are being made to a TCP socket, no identifying information is available and access will be denied. Also note that if the client is connecting via an SSH tunnel, it is the local SSH user that will be identified. In future versions, it is expected that more information about the client user will be provided, including the SASL / Kerberos username and/or x509 distinguished name obtained from the authentication provider in use. </p> <h2><a name="checks">Writing access control policies</a></h2> <p> If using versions of polkit prior to 0.106 then it is only possible to validate (user, permission) pairs via the <code>.pkla</code> files. Fully validation of the (user, permission, object) triple requires the new JavaScript <code>.rules</code> support that was introduced in version 0.106. The latter is what will be described here. </p> <p> Libvirt does not ship any rules files by default. It merely provides a definition of the default behaviour for each action (permission). As noted earlier, permissions which correspond to read-only operations in libvirt will be allowed to all users by default; everything else is denied by default. Defining custom rules requires creation of a file in the <code>/etc/polkit-1/rules.d</code> directory with a name chosen by the administrator (<code>100-libvirt-acl.rules</code> would be a reasonable choice). See the <code>polkit(8)</code> manual page for a description of how to write these files in general. The key idea is to create a file containing something like </p> <pre> polkit.addRule(function(action, subject) { ....logic to check 'action' and 'subject'... }); </pre> <p> In this code snippet above, the <code>action</code> object instance will represent the libvirt permission being checked along with identifying attributes for the object it is being applied to. The <code>subject</code> meanwhile will identify the libvirt client app (with the caveat above about it only dealing with local clients connected via the UNIX socket). On the <code>action</code> object, the permission name is accessible via the <code>id</code> attribute, while the object identifying attributes are exposed via the <code>lookup</code> method. </p> <h3><a name="exconnect">Example: restricting ability to connect to drivers</a></h3> <p> Consider a local user <code>berrange</code> who has been granted permission to connect to libvirt in full read-write mode. The goal is to only allow them to use the <code>QEMU</code> driver and not the Xen or LXC drivers which are also available in libvirtd. To achieve this we need to write a rule which checks whether the <code>connect_driver</code> attribute is <code>QEMU</code>, and match on an action name of <code>org.libvirt.api.connect.getattr</code>. Using the javascript rules format, this ends up written as </p> <pre> polkit.addRule(function(action, subject) { if (action.id == "org.libvirt.api.connect.getattr" && subject.user == "berrange") { if (action.lookup("connect_driver") == 'QEMU') { return polkit.Result.YES; } else { return polkit.Result.NO; } } }); </pre> <h3><a name="exdomain">Example: restricting access to a single domain</a></h3> <p> Consider a local user <code>berrange</code> who has been granted permission to connect to libvirt in full read-write mode. The goal is to only allow them to see the domain called <code>demo</code> on the LXC driver. To achieve this we need to write a rule which checks whether the <code>connect_driver</code> attribute is <code>LXC</code> and the <code>domain_name</code> attribute is <code>demo</code>, and match on a action name of <code>org.libvirt.api.domain.getattr</code>. Using the javascript rules format, this ends up written as </p> <pre> polkit.addRule(function(action, subject) { if (action.id == "org.libvirt.api.domain.getattr" && subject.user == "berrange") { if (action.lookup("connect_driver") == 'LXC' && action.lookup("domain_name") == 'demo') { return polkit.Result.YES; } else { return polkit.Result.NO; } } }); </pre> </body> </html>