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 the requesting 7algorithm 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_info); 15 16or: 17 18 struct key *request_key_with_auxdata(const struct key_type *type, 19 const char *description, 20 const char *callout_info, 21 size_t callout_len, 22 void *aux); 23 24or: 25 26 struct key *request_key_async(const struct key_type *type, 27 const char *description, 28 const char *callout_info, 29 size_t callout_len); 30 31or: 32 33 struct key *request_key_async_with_auxdata(const struct key_type *type, 34 const char *description, 35 const char *callout_info, 36 size_t callout_len, 37 void *aux); 38 39Or by userspace invoking the request_key system call: 40 41 key_serial_t request_key(const char *type, 42 const char *description, 43 const char *callout_info, 44 key_serial_t dest_keyring); 45 46The main difference between the access points is that the in-kernel interface 47does not need to link the key to a keyring to prevent it from being immediately 48destroyed. The kernel interface returns a pointer directly to the key, and 49it's up to the caller to destroy the key. 50 51The request_key*_with_auxdata() calls are like the in-kernel request_key*() 52calls, except that they permit auxiliary data to be passed to the upcaller (the 53default is NULL). This is only useful for those key types that define their 54own upcall mechanism rather than using /sbin/request-key. 55 56The two async in-kernel calls may return keys that are still in the process of 57being constructed. The two non-async ones will wait for construction to 58complete first. 59 60The userspace interface links the key to a keyring associated with the process 61to prevent the key from going away, and returns the serial number of the key to 62the caller. 63 64 65The following example assumes that the key types involved don't define their 66own upcall mechanisms. If they do, then those should be substituted for the 67forking and execution of /sbin/request-key. 68 69 70=========== 71THE PROCESS 72=========== 73 74A request proceeds in the following manner: 75 76 (1) Process A calls request_key() [the userspace syscall calls the kernel 77 interface]. 78 79 (2) request_key() searches the process's subscribed keyrings to see if there's 80 a suitable key there. If there is, it returns the key. If there isn't, 81 and callout_info is not set, an error is returned. Otherwise the process 82 proceeds to the next step. 83 84 (3) request_key() sees that A doesn't have the desired key yet, so it creates 85 two things: 86 87 (a) An uninstantiated key U of requested type and description. 88 89 (b) An authorisation key V that refers to key U and notes that process A 90 is the context in which key U should be instantiated and secured, and 91 from which associated key requests may be satisfied. 92 93 (4) request_key() then forks and executes /sbin/request-key with a new session 94 keyring that contains a link to auth key V. 95 96 (5) /sbin/request-key assumes the authority associated with key U. 97 98 (6) /sbin/request-key execs an appropriate program to perform the actual 99 instantiation. 100 101 (7) The program may want to access another key from A's context (say a 102 Kerberos TGT key). It just requests the appropriate key, and the keyring 103 search notes that the session keyring has auth key V in its bottom level. 104 105 This will permit it to then search the keyrings of process A with the 106 UID, GID, groups and security info of process A as if it was process A, 107 and come up with key W. 108 109 (8) The program then does what it must to get the data with which to 110 instantiate key U, using key W as a reference (perhaps it contacts a 111 Kerberos server using the TGT) and then instantiates key U. 112 113 (9) Upon instantiating key U, auth key V is automatically revoked so that it 114 may not be used again. 115 116(10) The program then exits 0 and request_key() deletes key V and returns key 117 U to the caller. 118 119This also extends further. If key W (step 7 above) didn't exist, key W would 120be created uninstantiated, another auth key (X) would be created (as per step 1213) and another copy of /sbin/request-key spawned (as per step 4); but the 122context specified by auth key X will still be process A, as it was in auth key 123V. 124 125This is because process A's keyrings can't simply be attached to 126/sbin/request-key at the appropriate places because (a) execve will discard two 127of them, and (b) it requires the same UID/GID/Groups all the way through. 128 129 130====================== 131NEGATIVE INSTANTIATION 132====================== 133 134Rather than instantiating a key, it is possible for the possessor of an 135authorisation key to negatively instantiate a key that's under construction. 136This is a short duration placeholder that causes any attempt at re-requesting 137the key whilst it exists to fail with error ENOKEY. 138 139This is provided to prevent excessive repeated spawning of /sbin/request-key 140processes for a key that will never be obtainable. 141 142Should the /sbin/request-key process exit anything other than 0 or die on a 143signal, the key under construction will be automatically negatively 144instantiated for a short amount of time. 145 146 147==================== 148THE SEARCH ALGORITHM 149==================== 150 151A search of any particular keyring proceeds in the following fashion: 152 153 (1) When the key management code searches for a key (keyring_search_aux) it 154 firstly calls key_permission(SEARCH) on the keyring it's starting with, 155 if this denies permission, it doesn't search further. 156 157 (2) It considers all the non-keyring keys within that keyring and, if any key 158 matches the criteria specified, calls key_permission(SEARCH) on it to see 159 if the key is allowed to be found. If it is, that key is returned; if 160 not, the search continues, and the error code is retained if of higher 161 priority than the one currently set. 162 163 (3) It then considers all the keyring-type keys in the keyring it's currently 164 searching. It calls key_permission(SEARCH) on each keyring, and if this 165 grants permission, it recurses, executing steps (2) and (3) on that 166 keyring. 167 168The process stops immediately a valid key is found with permission granted to 169use it. Any error from a previous match attempt is discarded and the key is 170returned. 171 172When search_process_keyrings() is invoked, it performs the following searches 173until one succeeds: 174 175 (1) If extant, the process's thread keyring is searched. 176 177 (2) If extant, the process's process keyring is searched. 178 179 (3) The process's session keyring is searched. 180 181 (4) If the process has assumed the authority associated with a request_key() 182 authorisation key then: 183 184 (a) If extant, the calling process's thread keyring is searched. 185 186 (b) If extant, the calling process's process keyring is searched. 187 188 (c) The calling process's session keyring is searched. 189 190The moment one succeeds, all pending errors are discarded and the found key is 191returned. 192 193Only if all these fail does the whole thing fail with the highest priority 194error. Note that several errors may have come from LSM. 195 196The error priority is: 197 198 EKEYREVOKED > EKEYEXPIRED > ENOKEY 199 200EACCES/EPERM are only returned on a direct search of a specific keyring where 201the basal keyring does not grant Search permission. 202