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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 Howellsf1a9bad2005-10-07 15:04:52 +010023Or by userspace invoking the request_key system call:
24
25 key_serial_t request_key(const char *type,
26 const char *description,
27 const char *callout_info,
28 key_serial_t dest_keyring);
29
David Howells4e54f082006-06-29 02:24:28 -070030The main difference between the access points is that the in-kernel interface
31does not need to link the key to a keyring to prevent it from being immediately
32destroyed. The kernel interface returns a pointer directly to the key, and
33it's up to the caller to destroy the key.
34
35The request_key_with_auxdata() call is like the in-kernel request_key() call,
36except that it permits auxiliary data to be passed to the upcaller (the default
37is NULL). This is only useful for those key types that define their own upcall
38mechanism rather than using /sbin/request-key.
David Howellsf1a9bad2005-10-07 15:04:52 +010039
40The userspace interface links the key to a keyring associated with the process
41to prevent the key from going away, and returns the serial number of the key to
42the caller.
43
44
David Howells4e54f082006-06-29 02:24:28 -070045The following example assumes that the key types involved don't define their
46own upcall mechanisms. If they do, then those should be substituted for the
47forking and execution of /sbin/request-key.
48
49
David Howellsf1a9bad2005-10-07 15:04:52 +010050===========
51THE PROCESS
52===========
53
54A request proceeds in the following manner:
55
56 (1) Process A calls request_key() [the userspace syscall calls the kernel
57 interface].
58
59 (2) request_key() searches the process's subscribed keyrings to see if there's
David Howells4e54f082006-06-29 02:24:28 -070060 a suitable key there. If there is, it returns the key. If there isn't,
61 and callout_info is not set, an error is returned. Otherwise the process
David Howellsf1a9bad2005-10-07 15:04:52 +010062 proceeds to the next step.
63
64 (3) request_key() sees that A doesn't have the desired key yet, so it creates
65 two things:
66
67 (a) An uninstantiated key U of requested type and description.
68
69 (b) An authorisation key V that refers to key U and notes that process A
70 is the context in which key U should be instantiated and secured, and
71 from which associated key requests may be satisfied.
72
73 (4) request_key() then forks and executes /sbin/request-key with a new session
74 keyring that contains a link to auth key V.
75
David Howellsb5f545c2006-01-08 01:02:47 -080076 (5) /sbin/request-key assumes the authority associated with key U.
77
78 (6) /sbin/request-key execs an appropriate program to perform the actual
David Howellsf1a9bad2005-10-07 15:04:52 +010079 instantiation.
80
David Howellsb5f545c2006-01-08 01:02:47 -080081 (7) The program may want to access another key from A's context (say a
David Howells4e54f082006-06-29 02:24:28 -070082 Kerberos TGT key). It just requests the appropriate key, and the keyring
David Howellsf1a9bad2005-10-07 15:04:52 +010083 search notes that the session keyring has auth key V in its bottom level.
84
85 This will permit it to then search the keyrings of process A with the
86 UID, GID, groups and security info of process A as if it was process A,
87 and come up with key W.
88
David Howellsb5f545c2006-01-08 01:02:47 -080089 (8) The program then does what it must to get the data with which to
David Howellsf1a9bad2005-10-07 15:04:52 +010090 instantiate key U, using key W as a reference (perhaps it contacts a
91 Kerberos server using the TGT) and then instantiates key U.
92
David Howellsb5f545c2006-01-08 01:02:47 -080093 (9) Upon instantiating key U, auth key V is automatically revoked so that it
David Howellsf1a9bad2005-10-07 15:04:52 +010094 may not be used again.
95
David Howellsb5f545c2006-01-08 01:02:47 -080096(10) The program then exits 0 and request_key() deletes key V and returns key
David Howellsf1a9bad2005-10-07 15:04:52 +010097 U to the caller.
98
David Howells4e54f082006-06-29 02:24:28 -070099This also extends further. If key W (step 7 above) didn't exist, key W would
100be created uninstantiated, another auth key (X) would be created (as per step
1013) and another copy of /sbin/request-key spawned (as per step 4); but the
102context specified by auth key X will still be process A, as it was in auth key
103V.
David Howellsf1a9bad2005-10-07 15:04:52 +0100104
105This is because process A's keyrings can't simply be attached to
106/sbin/request-key at the appropriate places because (a) execve will discard two
107of them, and (b) it requires the same UID/GID/Groups all the way through.
108
109
110======================
111NEGATIVE INSTANTIATION
112======================
113
114Rather than instantiating a key, it is possible for the possessor of an
115authorisation key to negatively instantiate a key that's under construction.
116This is a short duration placeholder that causes any attempt at re-requesting
117the key whilst it exists to fail with error ENOKEY.
118
119This is provided to prevent excessive repeated spawning of /sbin/request-key
120processes for a key that will never be obtainable.
121
122Should the /sbin/request-key process exit anything other than 0 or die on a
123signal, the key under construction will be automatically negatively
124instantiated for a short amount of time.
125
126
127====================
128THE SEARCH ALGORITHM
129====================
130
131A search of any particular keyring proceeds in the following fashion:
132
133 (1) When the key management code searches for a key (keyring_search_aux) it
134 firstly calls key_permission(SEARCH) on the keyring it's starting with,
135 if this denies permission, it doesn't search further.
136
137 (2) It considers all the non-keyring keys within that keyring and, if any key
138 matches the criteria specified, calls key_permission(SEARCH) on it to see
David Howells4e54f082006-06-29 02:24:28 -0700139 if the key is allowed to be found. If it is, that key is returned; if
David Howellsf1a9bad2005-10-07 15:04:52 +0100140 not, the search continues, and the error code is retained if of higher
141 priority than the one currently set.
142
143 (3) It then considers all the keyring-type keys in the keyring it's currently
David Howells4e54f082006-06-29 02:24:28 -0700144 searching. It calls key_permission(SEARCH) on each keyring, and if this
David Howellsf1a9bad2005-10-07 15:04:52 +0100145 grants permission, it recurses, executing steps (2) and (3) on that
146 keyring.
147
148The process stops immediately a valid key is found with permission granted to
David Howells4e54f082006-06-29 02:24:28 -0700149use it. Any error from a previous match attempt is discarded and the key is
David Howellsf1a9bad2005-10-07 15:04:52 +0100150returned.
151
152When search_process_keyrings() is invoked, it performs the following searches
153until one succeeds:
154
155 (1) If extant, the process's thread keyring is searched.
156
157 (2) If extant, the process's process keyring is searched.
158
159 (3) The process's session keyring is searched.
160
David Howellsb5f545c2006-01-08 01:02:47 -0800161 (4) If the process has assumed the authority associated with a request_key()
162 authorisation key then:
David Howellsf1a9bad2005-10-07 15:04:52 +0100163
164 (a) If extant, the calling process's thread keyring is searched.
165
166 (b) If extant, the calling process's process keyring is searched.
167
168 (c) The calling process's session keyring is searched.
169
170The moment one succeeds, all pending errors are discarded and the found key is
171returned.
172
173Only if all these fail does the whole thing fail with the highest priority
David Howells4e54f082006-06-29 02:24:28 -0700174error. Note that several errors may have come from LSM.
David Howellsf1a9bad2005-10-07 15:04:52 +0100175
176The error priority is:
177
178 EKEYREVOKED > EKEYEXPIRED > ENOKEY
179
180EACCES/EPERM are only returned on a direct search of a specific keyring where
181the basal keyring does not grant Search permission.