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