Based on kernel version 2.6.25. Page generated on 2008-04-18 21:22 EST.
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 the requesting 7 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: 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 23 or: 24 25 struct key *request_key_async(const struct key_type *type, 26 const char *description, 27 const char *callout_string); 28 29 or: 30 31 struct key *request_key_async_with_auxdata(const struct key_type *type, 32 const char *description, 33 const char *callout_string, 34 void *aux); 35 36 Or by userspace invoking the request_key system call: 37 38 key_serial_t request_key(const char *type, 39 const char *description, 40 const char *callout_info, 41 key_serial_t dest_keyring); 42 43 The main difference between the access points is that the in-kernel interface 44 does not need to link the key to a keyring to prevent it from being immediately 45 destroyed. The kernel interface returns a pointer directly to the key, and 46 it's up to the caller to destroy the key. 47 48 The request_key*_with_auxdata() calls are like the in-kernel request_key*() 49 calls, except that they permit auxiliary data to be passed to the upcaller (the 50 default is NULL). This is only useful for those key types that define their 51 own upcall mechanism rather than using /sbin/request-key. 52 53 The two async in-kernel calls may return keys that are still in the process of 54 being constructed. The two non-async ones will wait for construction to 55 complete first. 56 57 The userspace interface links the key to a keyring associated with the process 58 to prevent the key from going away, and returns the serial number of the key to 59 the caller. 60 61 62 The following example assumes that the key types involved don't define their 63 own upcall mechanisms. If they do, then those should be substituted for the 64 forking and execution of /sbin/request-key. 65 66 67 =========== 68 THE PROCESS 69 =========== 70 71 A request proceeds in the following manner: 72 73 (1) Process A calls request_key() [the userspace syscall calls the kernel 74 interface]. 75 76 (2) request_key() searches the process's subscribed keyrings to see if there's 77 a suitable key there. If there is, it returns the key. If there isn't, 78 and callout_info is not set, an error is returned. Otherwise the process 79 proceeds to the next step. 80 81 (3) request_key() sees that A doesn't have the desired key yet, so it creates 82 two things: 83 84 (a) An uninstantiated key U of requested type and description. 85 86 (b) An authorisation key V that refers to key U and notes that process A 87 is the context in which key U should be instantiated and secured, and 88 from which associated key requests may be satisfied. 89 90 (4) request_key() then forks and executes /sbin/request-key with a new session 91 keyring that contains a link to auth key V. 92 93 (5) /sbin/request-key assumes the authority associated with key U. 94 95 (6) /sbin/request-key execs an appropriate program to perform the actual 96 instantiation. 97 98 (7) The program may want to access another key from A's context (say a 99 Kerberos TGT key). It just requests the appropriate key, and the keyring 100 search notes that the session keyring has auth key V in its bottom level. 101 102 This will permit it to then search the keyrings of process A with the 103 UID, GID, groups and security info of process A as if it was process A, 104 and come up with key W. 105 106 (8) The program then does what it must to get the data with which to 107 instantiate key U, using key W as a reference (perhaps it contacts a 108 Kerberos server using the TGT) and then instantiates key U. 109 110 (9) Upon instantiating key U, auth key V is automatically revoked so that it 111 may not be used again. 112 113 (10) The program then exits 0 and request_key() deletes key V and returns key 114 U to the caller. 115 116 This also extends further. If key W (step 7 above) didn't exist, key W would 117 be created uninstantiated, another auth key (X) would be created (as per step 118 3) and another copy of /sbin/request-key spawned (as per step 4); but the 119 context specified by auth key X will still be process A, as it was in auth key 120 V. 121 122 This is because process A's keyrings can't simply be attached to 123 /sbin/request-key at the appropriate places because (a) execve will discard two 124 of them, and (b) it requires the same UID/GID/Groups all the way through. 125 126 127 ====================== 128 NEGATIVE INSTANTIATION 129 ====================== 130 131 Rather than instantiating a key, it is possible for the possessor of an 132 authorisation key to negatively instantiate a key that's under construction. 133 This is a short duration placeholder that causes any attempt at re-requesting 134 the key whilst it exists to fail with error ENOKEY. 135 136 This is provided to prevent excessive repeated spawning of /sbin/request-key 137 processes for a key that will never be obtainable. 138 139 Should the /sbin/request-key process exit anything other than 0 or die on a 140 signal, the key under construction will be automatically negatively 141 instantiated for a short amount of time. 142 143 144 ==================== 145 THE SEARCH ALGORITHM 146 ==================== 147 148 A search of any particular keyring proceeds in the following fashion: 149 150 (1) When the key management code searches for a key (keyring_search_aux) it 151 firstly calls key_permission(SEARCH) on the keyring it's starting with, 152 if this denies permission, it doesn't search further. 153 154 (2) It considers all the non-keyring keys within that keyring and, if any key 155 matches the criteria specified, calls key_permission(SEARCH) on it to see 156 if the key is allowed to be found. If it is, that key is returned; if 157 not, the search continues, and the error code is retained if of higher 158 priority than the one currently set. 159 160 (3) It then considers all the keyring-type keys in the keyring it's currently 161 searching. It calls key_permission(SEARCH) on each keyring, and if this 162 grants permission, it recurses, executing steps (2) and (3) on that 163 keyring. 164 165 The process stops immediately a valid key is found with permission granted to 166 use it. Any error from a previous match attempt is discarded and the key is 167 returned. 168 169 When search_process_keyrings() is invoked, it performs the following searches 170 until one succeeds: 171 172 (1) If extant, the process's thread keyring is searched. 173 174 (2) If extant, the process's process keyring is searched. 175 176 (3) The process's session keyring is searched. 177 178 (4) If the process has assumed the authority associated with a request_key() 179 authorisation key then: 180 181 (a) If extant, the calling process's thread keyring is searched. 182 183 (b) If extant, the calling process's process keyring is searched. 184 185 (c) The calling process's session keyring is searched. 186 187 The moment one succeeds, all pending errors are discarded and the found key is 188 returned. 189 190 Only if all these fail does the whole thing fail with the highest priority 191 error. Note that several errors may have come from LSM. 192 193 The error priority is: 194 195 EKEYREVOKED > EKEYEXPIRED > ENOKEY 196 197 EACCES/EPERM are only returned on a direct search of a specific keyring where 198 the basal keyring does not grant Search permission.
