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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 assumes the authority associated with key U.
60
61 (6) /sbin/request-key execs an appropriate program to perform the actual
62 instantiation.
63
64 (7) The program may want to access another key from A's context (say a
65 Kerberos TGT key). It just requests the appropriate key, and the keyring
66 search notes that the session keyring has auth key V in its bottom level.
67
68 This will permit it to then search the keyrings of process A with the
69 UID, GID, groups and security info of process A as if it was process A,
70 and come up with key W.
71
72 (8) The program then does what it must to get the data with which to
73 instantiate key U, using key W as a reference (perhaps it contacts a
74 Kerberos server using the TGT) and then instantiates key U.
75
76 (9) Upon instantiating key U, auth key V is automatically revoked so that it
77 may not be used again.
78
79(10) The program then exits 0 and request_key() deletes key V and returns key
80 U to the caller.
81
82This also extends further. If key W (step 7 above) didn't exist, key W would be
83created uninstantiated, another auth key (X) would be created (as per step 3)
84and another copy of /sbin/request-key spawned (as per step 4); but the context
85specified by auth key X will still be process A, as it was in auth key V.
86
87This is because process A's keyrings can't simply be attached to
88/sbin/request-key at the appropriate places because (a) execve will discard two
89of them, and (b) it requires the same UID/GID/Groups all the way through.
90
91
92======================
93NEGATIVE INSTANTIATION
94======================
95
96Rather than instantiating a key, it is possible for the possessor of an
97authorisation key to negatively instantiate a key that's under construction.
98This is a short duration placeholder that causes any attempt at re-requesting
99the key whilst it exists to fail with error ENOKEY.
100
101This is provided to prevent excessive repeated spawning of /sbin/request-key
102processes for a key that will never be obtainable.
103
104Should the /sbin/request-key process exit anything other than 0 or die on a
105signal, the key under construction will be automatically negatively
106instantiated for a short amount of time.
107
108
109====================
110THE SEARCH ALGORITHM
111====================
112
113A search of any particular keyring proceeds in the following fashion:
114
115 (1) When the key management code searches for a key (keyring_search_aux) it
116 firstly calls key_permission(SEARCH) on the keyring it's starting with,
117 if this denies permission, it doesn't search further.
118
119 (2) It considers all the non-keyring keys within that keyring and, if any key
120 matches the criteria specified, calls key_permission(SEARCH) on it to see
121 if the key is allowed to be found. If it is, that key is returned; if
122 not, the search continues, and the error code is retained if of higher
123 priority than the one currently set.
124
125 (3) It then considers all the keyring-type keys in the keyring it's currently
126 searching. It calls key_permission(SEARCH) on each keyring, and if this
127 grants permission, it recurses, executing steps (2) and (3) on that
128 keyring.
129
130The process stops immediately a valid key is found with permission granted to
131use it. Any error from a previous match attempt is discarded and the key is
132returned.
133
134When search_process_keyrings() is invoked, it performs the following searches
135until one succeeds:
136
137 (1) If extant, the process's thread keyring is searched.
138
139 (2) If extant, the process's process keyring is searched.
140
141 (3) The process's session keyring is searched.
142
143 (4) If the process has assumed the authority associated with a request_key()
144 authorisation key then:
145
146 (a) If extant, the calling process's thread keyring is searched.
147
148 (b) If extant, the calling process's process keyring is searched.
149
150 (c) The calling process's session keyring is searched.
151
152The moment one succeeds, all pending errors are discarded and the found key is
153returned.
154
155Only if all these fail does the whole thing fail with the highest priority
156error. Note that several errors may have come from LSM.
157
158The error priority is:
159
160 EKEYREVOKED > EKEYEXPIRED > ENOKEY
161
162EACCES/EPERM are only returned on a direct search of a specific keyring where
163the basal keyring does not grant Search permission.