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1 | =================== |
2 | KEY REQUEST SERVICE | |
3 | =================== | |
4 | ||
5 | The key request service is part of the key retention service (refer to | |
6 | Documentation/keys.txt). This document explains more fully how that the | |
7 | requesting algorithm works. | |
8 | ||
9 | The 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_string); | |
15 | ||
16 | Or 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 | ||
23 | The main difference between the two access points is that the in-kernel | |
24 | interface does not need to link the key to a keyring to prevent it from being | |
25 | immediately destroyed. The kernel interface returns a pointer directly to the | |
26 | key, and it's up to the caller to destroy the key. | |
27 | ||
28 | The userspace interface links the key to a keyring associated with the process | |
29 | to prevent the key from going away, and returns the serial number of the key to | |
30 | the caller. | |
31 | ||
32 | ||
33 | =========== | |
34 | THE PROCESS | |
35 | =========== | |
36 | ||
37 | A 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 | ||
80 | This also extends further. If key W (step 5 above) didn't exist, key W would be | |
81 | created uninstantiated, another auth key (X) would be created [as per step 3] | |
82 | and another copy of /sbin/request-key spawned [as per step 4]; but the context | |
83 | specified by auth key X will still be process A, as it was in auth key V. | |
84 | ||
85 | This 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 | |
87 | of them, and (b) it requires the same UID/GID/Groups all the way through. | |
88 | ||
89 | ||
90 | ====================== | |
91 | NEGATIVE INSTANTIATION | |
92 | ====================== | |
93 | ||
94 | Rather than instantiating a key, it is possible for the possessor of an | |
95 | authorisation key to negatively instantiate a key that's under construction. | |
96 | This is a short duration placeholder that causes any attempt at re-requesting | |
97 | the key whilst it exists to fail with error ENOKEY. | |
98 | ||
99 | This is provided to prevent excessive repeated spawning of /sbin/request-key | |
100 | processes for a key that will never be obtainable. | |
101 | ||
102 | Should the /sbin/request-key process exit anything other than 0 or die on a | |
103 | signal, the key under construction will be automatically negatively | |
104 | instantiated for a short amount of time. | |
105 | ||
106 | ||
107 | ==================== | |
108 | THE SEARCH ALGORITHM | |
109 | ==================== | |
110 | ||
111 | A 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 | ||
128 | The process stops immediately a valid key is found with permission granted to | |
129 | use it. Any error from a previous match attempt is discarded and the key is | |
130 | returned. | |
131 | ||
132 | When search_process_keyrings() is invoked, it performs the following searches | |
133 | until 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 | ||
150 | The moment one succeeds, all pending errors are discarded and the found key is | |
151 | returned. | |
152 | ||
153 | Only if all these fail does the whole thing fail with the highest priority | |
154 | error. Note that several errors may have come from LSM. | |
155 | ||
156 | The error priority is: | |
157 | ||
158 | EKEYREVOKED > EKEYEXPIRED > ENOKEY | |
159 | ||
160 | EACCES/EPERM are only returned on a direct search of a specific keyring where | |
161 | the basal keyring does not grant Search permission. |