1===================
2Key Request Service
3===================
4
5The key request service is part of the key retention service (refer to
6Documentation/security/keys.txt).  This document explains more fully how
7the requesting algorithm works.
8
9The 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
16or::
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
24or::
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
31or::
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
39Or 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
46The main difference between the access points is that the in-kernel interface
47does not need to link the key to a keyring to prevent it from being immediately
48destroyed.  The kernel interface returns a pointer directly to the key, and
49it's up to the caller to destroy the key.
50
51The request_key*_with_auxdata() calls are like the in-kernel request_key*()
52calls, except that they permit auxiliary data to be passed to the upcaller (the
53default is NULL).  This is only useful for those key types that define their
54own upcall mechanism rather than using /sbin/request-key.
55
56The two async in-kernel calls may return keys that are still in the process of
57being constructed.  The two non-async ones will wait for construction to
58complete first.
59
60The userspace interface links the key to a keyring associated with the process
61to prevent the key from going away, and returns the serial number of the key to
62the caller.
63
64
65The following example assumes that the key types involved don't define their
66own upcall mechanisms.  If they do, then those should be substituted for the
67forking and execution of /sbin/request-key.
68
69
70The Process
71===========
72
73A request proceeds in the following manner:
74
75  1) Process A calls request_key() [the userspace syscall calls the kernel
76     interface].
77
78  2) request_key() searches the process's subscribed keyrings to see if there's
79     a suitable key there.  If there is, it returns the key.  If there isn't,
80     and callout_info is not set, an error is returned.  Otherwise the process
81     proceeds to the next step.
82
83  3) request_key() sees that A doesn't have the desired key yet, so it creates
84     two things:
85
86      a) An uninstantiated key U of requested type and description.
87
88      b) An authorisation key V that refers to key U and notes that process A
89     	 is the context in which key U should be instantiated and secured, and
90     	 from which associated key requests may be satisfied.
91
92  4) request_key() then forks and executes /sbin/request-key with a new session
93     keyring that contains a link to auth key V.
94
95  5) /sbin/request-key assumes the authority associated with key U.
96
97  6) /sbin/request-key execs an appropriate program to perform the actual
98     instantiation.
99
100  7) The program may want to access another key from A's context (say a
101     Kerberos TGT key).  It just requests the appropriate key, and the keyring
102     search notes that the session keyring has auth key V in its bottom level.
103
104     This will permit it to then search the keyrings of process A with the
105     UID, GID, groups and security info of process A as if it was process A,
106     and come up with key W.
107
108  8) The program then does what it must to get the data with which to
109     instantiate key U, using key W as a reference (perhaps it contacts a
110     Kerberos server using the TGT) and then instantiates key U.
111
112  9) Upon instantiating key U, auth key V is automatically revoked so that it
113     may not be used again.
114
115  10) The program then exits 0 and request_key() deletes key V and returns key
116      U to the caller.
117
118This also extends further.  If key W (step 7 above) didn't exist, key W would
119be created uninstantiated, another auth key (X) would be created (as per step
1203) and another copy of /sbin/request-key spawned (as per step 4); but the
121context specified by auth key X will still be process A, as it was in auth key
122V.
123
124This is because process A's keyrings can't simply be attached to
125/sbin/request-key at the appropriate places because (a) execve will discard two
126of them, and (b) it requires the same UID/GID/Groups all the way through.
127
128
129Negative Instantiation And Rejection
130====================================
131
132Rather than instantiating a key, it is possible for the possessor of an
133authorisation key to negatively instantiate a key that's under construction.
134This is a short duration placeholder that causes any attempt at re-requesting
135the key whilst it exists to fail with error ENOKEY if negated or the specified
136error if rejected.
137
138This is provided to prevent excessive repeated spawning of /sbin/request-key
139processes for a key that will never be obtainable.
140
141Should the /sbin/request-key process exit anything other than 0 or die on a
142signal, the key under construction will be automatically negatively
143instantiated for a short amount of time.
144
145
146The Search Algorithm
147====================
148
149A search of any particular keyring proceeds in the following fashion:
150
151  1) When the key management code searches for a key (keyring_search_aux) it
152     firstly calls key_permission(SEARCH) on the keyring it's starting with,
153     if this denies permission, it doesn't search further.
154
155  2) It considers all the non-keyring keys within that keyring and, if any key
156     matches the criteria specified, calls key_permission(SEARCH) on it to see
157     if the key is allowed to be found.  If it is, that key is returned; if
158     not, the search continues, and the error code is retained if of higher
159     priority than the one currently set.
160
161  3) It then considers all the keyring-type keys in the keyring it's currently
162     searching.  It calls key_permission(SEARCH) on each keyring, and if this
163     grants permission, it recurses, executing steps (2) and (3) on that
164     keyring.
165
166The process stops immediately a valid key is found with permission granted to
167use it.  Any error from a previous match attempt is discarded and the key is
168returned.
169
170When search_process_keyrings() is invoked, it performs the following searches
171until one succeeds:
172
173  1) If extant, the process's thread keyring is searched.
174
175  2) If extant, the process's process keyring is searched.
176
177  3) The process's session keyring is searched.
178
179  4) If the process has assumed the authority associated with a request_key()
180     authorisation key then:
181
182      a) If extant, the calling process's thread keyring is searched.
183
184      b) If extant, the calling process's process keyring is searched.
185
186      c) The calling process's session keyring is searched.
187
188The moment one succeeds, all pending errors are discarded and the found key is
189returned.
190
191Only if all these fail does the whole thing fail with the highest priority
192error.  Note that several errors may have come from LSM.
193
194The error priority is::
195
196	EKEYREVOKED > EKEYEXPIRED > ENOKEY
197
198EACCES/EPERM are only returned on a direct search of a specific keyring where
199the basal keyring does not grant Search permission.
200