xref: /qemu/docs/system/authz.rst (revision b2a3cbb80c21f38f0cc626ddd642240d7194c8df)
1 .. _client authorization:
2 
3 Client authorization
4 --------------------
5 
6 When configuring a QEMU network backend with either TLS certificates or SASL
7 authentication, access will be granted if the client successfully proves
8 their identity. If the authorization identity database is scoped to the QEMU
9 client this may be sufficient. It is common, however, for the identity database
10 to be much broader and thus authentication alone does not enable sufficient
11 access control. In this case QEMU provides a flexible system for enforcing
12 finer grained authorization on clients post-authentication.
13 
14 Identity providers
15 ~~~~~~~~~~~~~~~~~~
16 
17 At the time of writing there are two authentication frameworks used by QEMU
18 that emit an identity upon completion.
19 
20  * TLS x509 certificate distinguished name.
21 
22    When configuring the QEMU backend as a network server with TLS, there
23    are a choice of credentials to use. The most common scenario is to utilize
24    x509 certificates. The simplest configuration only involves issuing
25    certificates to the servers, allowing the client to avoid a MITM attack
26    against their intended server.
27 
28    It is possible, however, to enable mutual verification by requiring that
29    the client provide a certificate to the server to prove its own identity.
30    This is done by setting the property ``verify-peer=yes`` on the
31    ``tls-creds-x509`` object, which is in fact the default.
32 
33    When peer verification is enabled, client will need to be issued with a
34    certificate by the same certificate authority as the server. If this is
35    still not sufficiently strong access control the Distinguished Name of
36    the certificate can be used as an identity in the QEMU authorization
37    framework.
38 
39  * SASL username.
40 
41    When configuring the QEMU backend as a network server with SASL, upon
42    completion of the SASL authentication mechanism, a username will be
43    provided. The format of this username will vary depending on the choice
44    of mechanism configured for SASL. It might be a simple UNIX style user
45    ``joebloggs``, while if using Kerberos/GSSAPI it can have a realm
46    attached ``joebloggs@QEMU.ORG``.  Whatever format the username is presented
47    in, it can be used with the QEMU authorization framework.
48 
49 Authorization drivers
50 ~~~~~~~~~~~~~~~~~~~~~
51 
52 The QEMU authorization framework is a general purpose design with choice of
53 user customizable drivers. These are provided as objects that can be
54 created at startup using the ``-object`` argument, or at runtime using the
55 ``object_add`` monitor command.
56 
57 Simple
58 ^^^^^^
59 
60 This authorization driver provides a simple mechanism for granting access
61 based on an exact match against a single identity. This is useful when it is
62 known that only a single client is to be allowed access.
63 
64 A possible use case would be when configuring QEMU for an incoming live
65 migration. It is known exactly which source QEMU the migration is expected
66 to arrive from. The x509 certificate associated with this source QEMU would
67 thus be used as the identity to match against. Alternatively if the virtual
68 machine is dedicated to a specific tenant, then the VNC server would be
69 configured with SASL and the username of only that tenant listed.
70 
71 To create an instance of this driver via QMP:
72 
73 ::
74 
75    {
76      "execute": "object-add",
77      "arguments": {
78        "qom-type": "authz-simple",
79        "id": "authz0",
80        "identity": "fred"
81      }
82    }
83 
84 
85 Or via the command line
86 
87 ::
88 
89    -object authz-simple,id=authz0,identity=fred
90 
91 
92 List
93 ^^^^
94 
95 In some network backends it will be desirable to grant access to a range of
96 clients. This authorization driver provides a list mechanism for granting
97 access by matching identities against a list of permitted one. Each match
98 rule has an associated policy and a catch all policy applies if no rule
99 matches. The match can either be done as an exact string comparison, or can
100 use the shell-like glob syntax, which allows for use of wildcards.
101 
102 To create an instance of this class via QMP:
103 
104 ::
105 
106    {
107      "execute": "object-add",
108      "arguments": {
109        "qom-type": "authz-list",
110        "id": "authz0",
111        "rules": [
112           { "match": "fred", "policy": "allow", "format": "exact" },
113           { "match": "bob", "policy": "allow", "format": "exact" },
114           { "match": "danb", "policy": "deny", "format": "exact" },
115           { "match": "dan*", "policy": "allow", "format": "glob" }
116        ],
117        "policy": "deny"
118      }
119    }
120 
121 
122 Due to the way this driver requires setting nested properties, creating
123 it on the command line will require use of the JSON syntax for ``-object``.
124 In most cases, however, the next driver will be more suitable.
125 
126 List file
127 ^^^^^^^^^
128 
129 This is a variant on the previous driver that allows for a more dynamic
130 access control policy by storing the match rules in a standalone file
131 that can be reloaded automatically upon change.
132 
133 To create an instance of this class via QMP:
134 
135 ::
136 
137    {
138      "execute": "object-add",
139      "arguments": {
140        "qom-type": "authz-list-file",
141        "id": "authz0",
142        "filename": "/etc/qemu/myvm-vnc.acl",
143        "refresh": true
144      }
145    }
146 
147 
148 If ``refresh`` is ``yes``, inotify is used to monitor for changes
149 to the file and auto-reload the rules.
150 
151 The ``myvm-vnc.acl`` file should contain the match rules in a format that
152 closely matches the previous driver:
153 
154 ::
155 
156    {
157      "rules": [
158        { "match": "fred", "policy": "allow", "format": "exact" },
159        { "match": "bob", "policy": "allow", "format": "exact" },
160        { "match": "danb", "policy": "deny", "format": "exact" },
161        { "match": "dan*", "policy": "allow", "format": "glob" }
162      ],
163      "policy": "deny"
164    }
165 
166 
167 The object can be created on the command line using
168 
169 ::
170 
171    -object authz-list-file,id=authz0,\
172            filename=/etc/qemu/myvm-vnc.acl,refresh=on
173 
174 
175 PAM
176 ^^^
177 
178 In some scenarios it might be desirable to integrate with authorization
179 mechanisms that are implemented outside of QEMU. In order to allow maximum
180 flexibility, QEMU provides a driver that uses the ``PAM`` framework.
181 
182 To create an instance of this class via QMP:
183 
184 ::
185 
186    {
187      "execute": "object-add",
188      "arguments": {
189        "qom-type": "authz-pam",
190        "id": "authz0",
191        "parameters": {
192          "service": "qemu-vnc-tls"
193        }
194      }
195    }
196 
197 
198 The driver only uses the PAM "account" verification
199 subsystem. The above config would require a config
200 file /etc/pam.d/qemu-vnc-tls. For a simple file
201 lookup it would contain
202 
203 ::
204 
205    account requisite  pam_listfile.so item=user sense=allow \
206            file=/etc/qemu/vnc.allow
207 
208 
209 The external file would then contain a list of usernames.
210 If x509 cert was being used as the username, a suitable
211 entry would match the distinguished name:
212 
213 ::
214 
215    CN=laptop.berrange.com,O=Berrange Home,L=London,ST=London,C=GB
216 
217 
218 On the command line it can be created using
219 
220 ::
221 
222    -object authz-pam,id=authz0,service=qemu-vnc-tls
223 
224 
225 There are a variety of PAM plugins that can be used which are not illustrated
226 here, and it is possible to implement brand new plugins using the PAM API.
227 
228 
229 Connecting backends
230 ~~~~~~~~~~~~~~~~~~~
231 
232 The authorization driver is created using the ``-object`` argument and then
233 needs to be associated with a network service. The authorization driver object
234 will be given a unique ID that needs to be referenced.
235 
236 The property to set in the network service will vary depending on the type of
237 identity to verify. By convention, any network server backend that uses TLS
238 will provide ``tls-authz`` property, while any server using SASL will provide
239 a ``sasl-authz`` property.
240 
241 Thus an example using SASL and authorization for the VNC server would look
242 like:
243 
244 ::
245 
246    $QEMU --object authz-simple,id=authz0,identity=fred \
247          --vnc 0.0.0.0:1,sasl,sasl-authz=authz0
248 
249 While to validate both the x509 certificate and SASL username:
250 
251 ::
252 
253    echo "CN=laptop.qemu.org,O=QEMU Project,L=London,ST=London,C=GB" >> tls.acl
254    $QEMU --object authz-simple,id=authz0,identity=fred \
255          --object authz-list-file,id=authz1,filename=tls.acl \
256 	 --object tls-creds-x509,id=tls0,dir=/etc/qemu/tls,verify-peer=yes \
257          --vnc 0.0.0.0:1,sasl,sasl-authz=auth0,tls-creds=tls0,tls-authz=authz1
258