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.
9 The process starts by either the kernel requesting a service by calling
12 struct key *request_key(const struct key_type *type,
13 const char *description,
14 const char *callout_string);
16 Or by userspace invoking the request_key system call:
18 key_serial_t request_key(const char *type,
19 const char *description,
20 const char *callout_info,
21 key_serial_t dest_keyring);
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.
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
37 A request proceeds in the following manner:
39 (1) Process A calls request_key() [the userspace syscall calls the kernel
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.
47 (3) request_key() sees that A doesn't have the desired key yet, so it creates
50 (a) An uninstantiated key U of requested type and description.
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.
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.
59 (5) /sbin/request-key execs an appropriate program to perform the actual
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.
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.
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.
74 (8) Upon instantiating key U, auth key V is automatically revoked so that it
75 may not be used again.
77 (9) The program then exits 0 and request_key() deletes key V and returns key
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.
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.
90 ======================
91 NEGATIVE INSTANTIATION
92 ======================
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.
99 This is provided to prevent excessive repeated spawning of /sbin/request-key
100 processes for a key that will never be obtainable.
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.
111 A search of any particular keyring proceeds in the following fashion:
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.
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.
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
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
132 When search_process_keyrings() is invoked, it performs the following searches
135 (1) If extant, the process's thread keyring is searched.
137 (2) If extant, the process's process keyring is searched.
139 (3) The process's session keyring is searched.
141 (4) If the process has a request_key() authorisation key in its session
144 (a) If extant, the calling process's thread keyring is searched.
146 (b) If extant, the calling process's process keyring is searched.
148 (c) The calling process's session keyring is searched.
150 The moment one succeeds, all pending errors are discarded and the found key is
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.
156 The error priority is:
158 EKEYREVOKED > EKEYEXPIRED > ENOKEY
160 EACCES/EPERM are only returned on a direct search of a specific keyring where
161 the basal keyring does not grant Search permission.