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Documentation / keys-request-key.txt




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Based on kernel version 2.6.39.1. Page generated on 2011-06-03 13:47 EST.

1				      ===================
2				      KEY REQUEST SERVICE
3				      ===================
4	
5	The key request service is part of the key retention service (refer to
6	Documentation/keys.txt).  This document explains more fully how the requesting
7	algorithm works.
8	
9	The process starts by either the kernel requesting a service by calling
10	request_key*():
11	
12		struct key *request_key(const struct key_type *type,
13					const char *description,
14					const char *callout_info);
15	
16	or:
17	
18		struct key *request_key_with_auxdata(const struct key_type *type,
19						     const char *description,
20						     const char *callout_info,
21						     size_t callout_len,
22						     void *aux);
23	
24	or:
25	
26		struct key *request_key_async(const struct key_type *type,
27					      const char *description,
28					      const char *callout_info,
29					      size_t callout_len);
30	
31	or:
32	
33		struct key *request_key_async_with_auxdata(const struct key_type *type,
34							   const char *description,
35							   const char *callout_info,
36						     	   size_t callout_len,
37							   void *aux);
38	
39	Or by userspace invoking the request_key system call:
40	
41		key_serial_t request_key(const char *type,
42					 const char *description,
43					 const char *callout_info,
44					 key_serial_t dest_keyring);
45	
46	The main difference between the access points is that the in-kernel interface
47	does not need to link the key to a keyring to prevent it from being immediately
48	destroyed.  The kernel interface returns a pointer directly to the key, and
49	it's up to the caller to destroy the key.
50	
51	The request_key*_with_auxdata() calls are like the in-kernel request_key*()
52	calls, except that they permit auxiliary data to be passed to the upcaller (the
53	default is NULL).  This is only useful for those key types that define their
54	own upcall mechanism rather than using /sbin/request-key.
55	
56	The two async in-kernel calls may return keys that are still in the process of
57	being constructed.  The two non-async ones will wait for construction to
58	complete first.
59	
60	The userspace interface links the key to a keyring associated with the process
61	to prevent the key from going away, and returns the serial number of the key to
62	the caller.
63	
64	
65	The following example assumes that the key types involved don't define their
66	own upcall mechanisms.  If they do, then those should be substituted for the
67	forking and execution of /sbin/request-key.
68	
69	
70	===========
71	THE PROCESS
72	===========
73	
74	A request proceeds in the following manner:
75	
76	 (1) Process A calls request_key() [the userspace syscall calls the kernel
77	     interface].
78	
79	 (2) request_key() searches the process's subscribed keyrings to see if there's
80	     a suitable key there.  If there is, it returns the key.  If there isn't,
81	     and callout_info is not set, an error is returned.  Otherwise the process
82	     proceeds to the next step.
83	
84	 (3) request_key() sees that A doesn't have the desired key yet, so it creates
85	     two things:
86	
87	     (a) An uninstantiated key U of requested type and description.
88	
89	     (b) An authorisation key V that refers to key U and notes that process A
90	     	 is the context in which key U should be instantiated and secured, and
91	     	 from which associated key requests may be satisfied.
92	
93	 (4) request_key() then forks and executes /sbin/request-key with a new session
94	     keyring that contains a link to auth key V.
95	
96	 (5) /sbin/request-key assumes the authority associated with key U.
97	
98	 (6) /sbin/request-key execs an appropriate program to perform the actual
99	     instantiation.
100	
101	 (7) The program may want to access another key from A's context (say a
102	     Kerberos TGT key).  It just requests the appropriate key, and the keyring
103	     search notes that the session keyring has auth key V in its bottom level.
104	
105	     This will permit it to then search the keyrings of process A with the
106	     UID, GID, groups and security info of process A as if it was process A,
107	     and come up with key W.
108	
109	 (8) The program then does what it must to get the data with which to
110	     instantiate key U, using key W as a reference (perhaps it contacts a
111	     Kerberos server using the TGT) and then instantiates key U.
112	
113	 (9) Upon instantiating key U, auth key V is automatically revoked so that it
114	     may not be used again.
115	
116	(10) The program then exits 0 and request_key() deletes key V and returns key
117	     U to the caller.
118	
119	This also extends further.  If key W (step 7 above) didn't exist, key W would
120	be created uninstantiated, another auth key (X) would be created (as per step
121	3) and another copy of /sbin/request-key spawned (as per step 4); but the
122	context specified by auth key X will still be process A, as it was in auth key
123	V.
124	
125	This is because process A's keyrings can't simply be attached to
126	/sbin/request-key at the appropriate places because (a) execve will discard two
127	of them, and (b) it requires the same UID/GID/Groups all the way through.
128	
129	
130	====================================
131	NEGATIVE INSTANTIATION AND REJECTION
132	====================================
133	
134	Rather than instantiating a key, it is possible for the possessor of an
135	authorisation key to negatively instantiate a key that's under construction.
136	This is a short duration placeholder that causes any attempt at re-requesting
137	the key whilst it exists to fail with error ENOKEY if negated or the specified
138	error if rejected.
139	
140	This is provided to prevent excessive repeated spawning of /sbin/request-key
141	processes for a key that will never be obtainable.
142	
143	Should the /sbin/request-key process exit anything other than 0 or die on a
144	signal, the key under construction will be automatically negatively
145	instantiated for a short amount of time.
146	
147	
148	====================
149	THE SEARCH ALGORITHM
150	====================
151	
152	A search of any particular keyring proceeds in the following fashion:
153	
154	 (1) When the key management code searches for a key (keyring_search_aux) it
155	     firstly calls key_permission(SEARCH) on the keyring it's starting with,
156	     if this denies permission, it doesn't search further.
157	
158	 (2) It considers all the non-keyring keys within that keyring and, if any key
159	     matches the criteria specified, calls key_permission(SEARCH) on it to see
160	     if the key is allowed to be found.  If it is, that key is returned; if
161	     not, the search continues, and the error code is retained if of higher
162	     priority than the one currently set.
163	
164	 (3) It then considers all the keyring-type keys in the keyring it's currently
165	     searching.  It calls key_permission(SEARCH) on each keyring, and if this
166	     grants permission, it recurses, executing steps (2) and (3) on that
167	     keyring.
168	
169	The process stops immediately a valid key is found with permission granted to
170	use it.  Any error from a previous match attempt is discarded and the key is
171	returned.
172	
173	When search_process_keyrings() is invoked, it performs the following searches
174	until one succeeds:
175	
176	 (1) If extant, the process's thread keyring is searched.
177	
178	 (2) If extant, the process's process keyring is searched.
179	
180	 (3) The process's session keyring is searched.
181	
182	 (4) If the process has assumed the authority associated with a request_key()
183	     authorisation key then:
184	
185	     (a) If extant, the calling process's thread keyring is searched.
186	
187	     (b) If extant, the calling process's process keyring is searched.
188	
189	     (c) The calling process's session keyring is searched.
190	
191	The moment one succeeds, all pending errors are discarded and the found key is
192	returned.
193	
194	Only if all these fail does the whole thing fail with the highest priority
195	error.  Note that several errors may have come from LSM.
196	
197	The error priority is:
198	
199		EKEYREVOKED > EKEYEXPIRED > ENOKEY
200	
201	EACCES/EPERM are only returned on a direct search of a specific keyring where
202	the basal keyring does not grant Search permission.
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