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David Howellsf1a9bad2005-10-07 15:04:52 +01001 ===================
2 KEY REQUEST SERVICE
3 ===================
4
5The key request service is part of the key retention service (refer to
David Howells4e54f082006-06-29 02:24:28 -07006Documentation/keys.txt). This document explains more fully how the requesting
7algorithm works.
David Howellsf1a9bad2005-10-07 15:04:52 +01008
9The process starts by either the kernel requesting a service by calling
David Howells4e54f082006-06-29 02:24:28 -070010request_key*():
David Howellsf1a9bad2005-10-07 15:04:52 +010011
12 struct key *request_key(const struct key_type *type,
13 const char *description,
14 const char *callout_string);
15
David Howells4e54f082006-06-29 02:24:28 -070016or:
17
18 struct key *request_key_with_auxdata(const struct key_type *type,
19 const char *description,
20 const char *callout_string,
21 void *aux);
22
David Howells76181c12007-10-16 23:29:46 -070023or:
24
25 struct key *request_key_async(const struct key_type *type,
26 const char *description,
27 const char *callout_string);
28
29or:
30
31 struct key *request_key_async_with_auxdata(const struct key_type *type,
32 const char *description,
33 const char *callout_string,
34 void *aux);
35
David Howellsf1a9bad2005-10-07 15:04:52 +010036Or by userspace invoking the request_key system call:
37
38 key_serial_t request_key(const char *type,
39 const char *description,
40 const char *callout_info,
41 key_serial_t dest_keyring);
42
David Howells4e54f082006-06-29 02:24:28 -070043The main difference between the access points is that the in-kernel interface
44does not need to link the key to a keyring to prevent it from being immediately
45destroyed. The kernel interface returns a pointer directly to the key, and
46it's up to the caller to destroy the key.
47
David Howells76181c12007-10-16 23:29:46 -070048The request_key*_with_auxdata() calls are like the in-kernel request_key*()
49calls, except that they permit auxiliary data to be passed to the upcaller (the
50default is NULL). This is only useful for those key types that define their
51own upcall mechanism rather than using /sbin/request-key.
52
53The two async in-kernel calls may return keys that are still in the process of
54being constructed. The two non-async ones will wait for construction to
55complete first.
David Howellsf1a9bad2005-10-07 15:04:52 +010056
57The userspace interface links the key to a keyring associated with the process
58to prevent the key from going away, and returns the serial number of the key to
59the caller.
60
61
David Howells4e54f082006-06-29 02:24:28 -070062The following example assumes that the key types involved don't define their
63own upcall mechanisms. If they do, then those should be substituted for the
64forking and execution of /sbin/request-key.
65
66
David Howellsf1a9bad2005-10-07 15:04:52 +010067===========
68THE PROCESS
69===========
70
71A request proceeds in the following manner:
72
73 (1) Process A calls request_key() [the userspace syscall calls the kernel
74 interface].
75
76 (2) request_key() searches the process's subscribed keyrings to see if there's
David Howells4e54f082006-06-29 02:24:28 -070077 a suitable key there. If there is, it returns the key. If there isn't,
78 and callout_info is not set, an error is returned. Otherwise the process
David Howellsf1a9bad2005-10-07 15:04:52 +010079 proceeds to the next step.
80
81 (3) request_key() sees that A doesn't have the desired key yet, so it creates
82 two things:
83
84 (a) An uninstantiated key U of requested type and description.
85
86 (b) An authorisation key V that refers to key U and notes that process A
87 is the context in which key U should be instantiated and secured, and
88 from which associated key requests may be satisfied.
89
90 (4) request_key() then forks and executes /sbin/request-key with a new session
91 keyring that contains a link to auth key V.
92
David Howellsb5f545c2006-01-08 01:02:47 -080093 (5) /sbin/request-key assumes the authority associated with key U.
94
95 (6) /sbin/request-key execs an appropriate program to perform the actual
David Howellsf1a9bad2005-10-07 15:04:52 +010096 instantiation.
97
David Howellsb5f545c2006-01-08 01:02:47 -080098 (7) The program may want to access another key from A's context (say a
David Howells4e54f082006-06-29 02:24:28 -070099 Kerberos TGT key). It just requests the appropriate key, and the keyring
David Howellsf1a9bad2005-10-07 15:04:52 +0100100 search notes that the session keyring has auth key V in its bottom level.
101
102 This will permit it to then search the keyrings of process A with the
103 UID, GID, groups and security info of process A as if it was process A,
104 and come up with key W.
105
David Howellsb5f545c2006-01-08 01:02:47 -0800106 (8) The program then does what it must to get the data with which to
David Howellsf1a9bad2005-10-07 15:04:52 +0100107 instantiate key U, using key W as a reference (perhaps it contacts a
108 Kerberos server using the TGT) and then instantiates key U.
109
David Howellsb5f545c2006-01-08 01:02:47 -0800110 (9) Upon instantiating key U, auth key V is automatically revoked so that it
David Howellsf1a9bad2005-10-07 15:04:52 +0100111 may not be used again.
112
David Howellsb5f545c2006-01-08 01:02:47 -0800113(10) The program then exits 0 and request_key() deletes key V and returns key
David Howellsf1a9bad2005-10-07 15:04:52 +0100114 U to the caller.
115
David Howells4e54f082006-06-29 02:24:28 -0700116This also extends further. If key W (step 7 above) didn't exist, key W would
117be created uninstantiated, another auth key (X) would be created (as per step
1183) and another copy of /sbin/request-key spawned (as per step 4); but the
119context specified by auth key X will still be process A, as it was in auth key
120V.
David Howellsf1a9bad2005-10-07 15:04:52 +0100121
122This is because process A's keyrings can't simply be attached to
123/sbin/request-key at the appropriate places because (a) execve will discard two
124of them, and (b) it requires the same UID/GID/Groups all the way through.
125
126
127======================
128NEGATIVE INSTANTIATION
129======================
130
131Rather than instantiating a key, it is possible for the possessor of an
132authorisation key to negatively instantiate a key that's under construction.
133This is a short duration placeholder that causes any attempt at re-requesting
134the key whilst it exists to fail with error ENOKEY.
135
136This is provided to prevent excessive repeated spawning of /sbin/request-key
137processes for a key that will never be obtainable.
138
139Should the /sbin/request-key process exit anything other than 0 or die on a
140signal, the key under construction will be automatically negatively
141instantiated for a short amount of time.
142
143
144====================
145THE SEARCH ALGORITHM
146====================
147
148A search of any particular keyring proceeds in the following fashion:
149
150 (1) When the key management code searches for a key (keyring_search_aux) it
151 firstly calls key_permission(SEARCH) on the keyring it's starting with,
152 if this denies permission, it doesn't search further.
153
154 (2) It considers all the non-keyring keys within that keyring and, if any key
155 matches the criteria specified, calls key_permission(SEARCH) on it to see
David Howells4e54f082006-06-29 02:24:28 -0700156 if the key is allowed to be found. If it is, that key is returned; if
David Howellsf1a9bad2005-10-07 15:04:52 +0100157 not, the search continues, and the error code is retained if of higher
158 priority than the one currently set.
159
160 (3) It then considers all the keyring-type keys in the keyring it's currently
David Howells4e54f082006-06-29 02:24:28 -0700161 searching. It calls key_permission(SEARCH) on each keyring, and if this
David Howellsf1a9bad2005-10-07 15:04:52 +0100162 grants permission, it recurses, executing steps (2) and (3) on that
163 keyring.
164
165The process stops immediately a valid key is found with permission granted to
David Howells4e54f082006-06-29 02:24:28 -0700166use it. Any error from a previous match attempt is discarded and the key is
David Howellsf1a9bad2005-10-07 15:04:52 +0100167returned.
168
169When search_process_keyrings() is invoked, it performs the following searches
170until one succeeds:
171
172 (1) If extant, the process's thread keyring is searched.
173
174 (2) If extant, the process's process keyring is searched.
175
176 (3) The process's session keyring is searched.
177
David Howellsb5f545c2006-01-08 01:02:47 -0800178 (4) If the process has assumed the authority associated with a request_key()
179 authorisation key then:
David Howellsf1a9bad2005-10-07 15:04:52 +0100180
181 (a) If extant, the calling process's thread keyring is searched.
182
183 (b) If extant, the calling process's process keyring is searched.
184
185 (c) The calling process's session keyring is searched.
186
187The moment one succeeds, all pending errors are discarded and the found key is
188returned.
189
190Only if all these fail does the whole thing fail with the highest priority
David Howells4e54f082006-06-29 02:24:28 -0700191error. Note that several errors may have come from LSM.
David Howellsf1a9bad2005-10-07 15:04:52 +0100192
193The error priority is:
194
195 EKEYREVOKED > EKEYEXPIRED > ENOKEY
196
197EACCES/EPERM are only returned on a direct search of a specific keyring where
198the basal keyring does not grant Search permission.