Polkit access control
Libvirt's client access control framework 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.
- Introduction
- Permission names
- Object identity attributes
- Hypervisor Driver connect_driver
- User identity attributes
- Writing access control policies
Introduction ¶
A default install of libvirt will typically use polkit 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.
Permission names ¶
The libvirt object names and permission names are mapped onto polkit action names using the simple pattern:
org.libvirt.api.$object.$permission
The only caveat is that any underscore characters in the
object or permission names are converted to hyphens. So,
for example, the search_storage_vols permission
on the storage_pool object maps to the polkit
action:
org.libvirt.api.storage-pool.search-storage-vols
The default policy for any permission which corresponds to a "read only" operation, is to allow access. All other permissions default to deny access.
Object identity attributes ¶
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
virConnectPtr ¶
| Attribute | Description |
|---|---|
| connect_driver | Name of the libvirt connection driver |
virDomainPtr ¶
| Attribute | Description |
|---|---|
| connect_driver | Name of the libvirt connection driver |
| domain_name | Name of the domain, unique to the local host |
| domain_uuid | UUID of the domain, globally unique |
virInterfacePtr ¶
| Attribute | Description |
|---|---|
| connect_driver | Name of the libvirt connection driver |
| interface_name | Name of the network interface, unique to the local host |
| interface_macaddr | MAC address of the network interface, not unique |
virNetworkPtr ¶
| Attribute | Description |
|---|---|
| connect_driver | Name of the libvirt connection driver |
| network_name | Name of the network, unique to the local host |
| network_uuid | UUID of the network, globally unique |
virNodeDevicePtr ¶
| Attribute | Description |
|---|---|
| connect_driver | Name of the libvirt connection driver |
| node_device_name | Name of the node device, unique to the local host |
virNWFilterPtr ¶
| Attribute | Description |
|---|---|
| connect_driver | Name of the libvirt connection driver |
| nwfilter_name | Name of the network filter, unique to the local host |
| nwfilter_uuid | UUID of the network filter, globally unique |
virSecretPtr ¶
| Attribute | Description |
|---|---|
| connect_driver | Name of the libvirt connection driver |
| secret_uuid | UUID of the secret, globally unique |
| secret_usage_volume | Name of the associated volume, if any |
| secret_usage_ceph | Name of the associated Ceph server, if any |
| secret_usage_target | Name of the associated iSCSI target, if any |
| secret_usage_name | Name of the associated TLS secret, if any |
virStoragePoolPtr ¶
| Attribute | Description |
|---|---|
| connect_driver | Name of the libvirt connection driver |
| pool_name | Name of the storage pool, unique to the local host |
| pool_uuid | UUID of the storage pool, globally unique |
virStorageVolPtr ¶
| Attribute | Description |
|---|---|
| connect_driver | Name of the libvirt connection driver |
| pool_name | Name of the storage pool, unique to the local host |
| pool_uuid | UUID of the storage pool, globally unique |
| vol_name | Name of the storage volume, unique to the pool |
| vol_key | Key of the storage volume, globally unique |
Hypervisor Driver connect_driver ¶
The connect_driver parameter describes the
client's remote Connection Driver
name based on the URI used for the
connection.
Since 4.1.0, when calling an API
outside the scope of the primary connection driver, the
primary driver will attempt to open a secondary connection
to the specific API driver in order to process the API. For
example, when hypervisor domain processing needs to make an
API call within the storage driver or the network filter driver
an attempt to open a connection to the "storage" or "nwfilter"
driver will be made. Similarly, a "storage" primary connection
may need to create a connection to the "secret" driver in order
to process secrets for the API. If successful, then calls to
those API's will occur in the connect_driver context
of the secondary connection driver rather than in the context of
the primary driver. This affects the connect_driver
returned from rule generation from the action.loookup
function. The following table provides a list of the various
connection drivers and the connect_driver name
used by each regardless of primary or secondary connection.
The access denied error message from libvirt will list the
connection driver by name that denied the access.
Connection Driver Name ¶
| Connection Driver | connect_driver name |
|---|---|
| bhyve | bhyve |
| esx | ESX |
| hyperv | Hyper-V |
| interface | interface |
| libxl | xenlight |
| lxc | LXC |
| network | network |
| nodedev | nodedev |
| nwfilter | NWFilter |
| openvz | OPENVZ |
| phyp | PHYP |
| qemu | QEMU |
| secret | secret |
| storage | storage |
| vbox | VBOX |
| vmware | VMWARE |
| vz | vz |
| xenapi | XenAPI |
User identity attributes ¶
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.
Writing access control policies ¶
If using versions of polkit prior to 0.106 then it is only
possible to validate (user, permission) pairs via the .pkla
files. Fully validation of the (user, permission, object) triple
requires the new JavaScript .rules support that
was introduced in version 0.106. The latter is what will be
described here.
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
/etc/polkit-1/rules.d directory with a name
chosen by the administrator (100-libvirt-acl.rules
would be a reasonable choice). See the polkit(8)
manual page for a description of how to write these files
in general. The key idea is to create a file containing
something like
polkit.addRule(function(action, subject) {
....logic to check 'action' and 'subject'...
});
In this code snippet above, the action object
instance will represent the libvirt permission being checked
along with identifying attributes for the object it is being
applied to. The subject 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 action object, the permission name is
accessible via the id attribute, while the
object identifying attributes are exposed via the
lookup method.
See source code for a more complex example.
Example: restricting ability to connect to drivers ¶
Consider a local user berrange
who has been granted permission to connect to libvirt in
full read-write mode. The goal is to only allow them to
use the QEMU 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 connect_driver attribute
is QEMU, and match on an action
name of org.libvirt.api.connect.getattr. Using
the javascript rules format, this ends up written as
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;
}
}
});
Example: restricting access to a single domain ¶
Consider a local user berrange
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 demo on the LXC driver.
To achieve this we need to write a rule which checks
whether the connect_driver attribute
is LXC and the domain_name
attribute is demo, and match on an action
name of org.libvirt.api.domain.getattr. Using
the javascript rules format, this ends up written as
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;
}
}
});