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_string); 15 16or: 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 23Or by userspace invoking the request_key system call: 24 25 key_serial_t request_key(const char *type, 26 const char *description, 27 const char *callout_info, 28 key_serial_t dest_keyring); 29 30The main difference between the access points is that the in-kernel interface 31does not need to link the key to a keyring to prevent it from being immediately 32destroyed. The kernel interface returns a pointer directly to the key, and 33it's up to the caller to destroy the key. 34 35The request_key_with_auxdata() call is like the in-kernel request_key() call, 36except that it permits auxiliary data to be passed to the upcaller (the default 37is NULL). This is only useful for those key types that define their own upcall 38mechanism rather than using /sbin/request-key. 39 40The userspace interface links the key to a keyring associated with the process 41to prevent the key from going away, and returns the serial number of the key to 42the caller. 43 44 45The following example assumes that the key types involved don't define their 46own upcall mechanisms. If they do, then those should be substituted for the 47forking and execution of /sbin/request-key. 48 49 50=========== 51THE PROCESS 52=========== 53 54A request proceeds in the following manner: 55 56 (1) Process A calls request_key() [the userspace syscall calls the kernel 57 interface]. 58 59 (2) request_key() searches the process's subscribed keyrings to see if there's 60 a suitable key there. If there is, it returns the key. If there isn't, 61 and callout_info is not set, an error is returned. Otherwise the process 62 proceeds to the next step. 63 64 (3) request_key() sees that A doesn't have the desired key yet, so it creates 65 two things: 66 67 (a) An uninstantiated key U of requested type and description. 68 69 (b) An authorisation key V that refers to key U and notes that process A 70 is the context in which key U should be instantiated and secured, and 71 from which associated key requests may be satisfied. 72 73 (4) request_key() then forks and executes /sbin/request-key with a new session 74 keyring that contains a link to auth key V. 75 76 (5) /sbin/request-key assumes the authority associated with key U. 77 78 (6) /sbin/request-key execs an appropriate program to perform the actual 79 instantiation. 80 81 (7) The program may want to access another key from A's context (say a 82 Kerberos TGT key). It just requests the appropriate key, and the keyring 83 search notes that the session keyring has auth key V in its bottom level. 84 85 This will permit it to then search the keyrings of process A with the 86 UID, GID, groups and security info of process A as if it was process A, 87 and come up with key W. 88 89 (8) The program then does what it must to get the data with which to 90 instantiate key U, using key W as a reference (perhaps it contacts a 91 Kerberos server using the TGT) and then instantiates key U. 92 93 (9) Upon instantiating key U, auth key V is automatically revoked so that it 94 may not be used again. 95 96(10) The program then exits 0 and request_key() deletes key V and returns key 97 U to the caller. 98 99This also extends further. If key W (step 7 above) didn't exist, key W would 100be created uninstantiated, another auth key (X) would be created (as per step 1013) and another copy of /sbin/request-key spawned (as per step 4); but the 102context specified by auth key X will still be process A, as it was in auth key 103V. 104 105This is because process A's keyrings can't simply be attached to 106/sbin/request-key at the appropriate places because (a) execve will discard two 107of them, and (b) it requires the same UID/GID/Groups all the way through. 108 109 110====================== 111NEGATIVE INSTANTIATION 112====================== 113 114Rather than instantiating a key, it is possible for the possessor of an 115authorisation key to negatively instantiate a key that's under construction. 116This is a short duration placeholder that causes any attempt at re-requesting 117the key whilst it exists to fail with error ENOKEY. 118 119This is provided to prevent excessive repeated spawning of /sbin/request-key 120processes for a key that will never be obtainable. 121 122Should the /sbin/request-key process exit anything other than 0 or die on a 123signal, the key under construction will be automatically negatively 124instantiated for a short amount of time. 125 126 127==================== 128THE SEARCH ALGORITHM 129==================== 130 131A search of any particular keyring proceeds in the following fashion: 132 133 (1) When the key management code searches for a key (keyring_search_aux) it 134 firstly calls key_permission(SEARCH) on the keyring it's starting with, 135 if this denies permission, it doesn't search further. 136 137 (2) It considers all the non-keyring keys within that keyring and, if any key 138 matches the criteria specified, calls key_permission(SEARCH) on it to see 139 if the key is allowed to be found. If it is, that key is returned; if 140 not, the search continues, and the error code is retained if of higher 141 priority than the one currently set. 142 143 (3) It then considers all the keyring-type keys in the keyring it's currently 144 searching. It calls key_permission(SEARCH) on each keyring, and if this 145 grants permission, it recurses, executing steps (2) and (3) on that 146 keyring. 147 148The process stops immediately a valid key is found with permission granted to 149use it. Any error from a previous match attempt is discarded and the key is 150returned. 151 152When search_process_keyrings() is invoked, it performs the following searches 153until one succeeds: 154 155 (1) If extant, the process's thread keyring is searched. 156 157 (2) If extant, the process's process keyring is searched. 158 159 (3) The process's session keyring is searched. 160 161 (4) If the process has assumed the authority associated with a request_key() 162 authorisation key then: 163 164 (a) If extant, the calling process's thread keyring is searched. 165 166 (b) If extant, the calling process's process keyring is searched. 167 168 (c) The calling process's session keyring is searched. 169 170The moment one succeeds, all pending errors are discarded and the found key is 171returned. 172 173Only if all these fail does the whole thing fail with the highest priority 174error. Note that several errors may have come from LSM. 175 176The error priority is: 177 178 EKEYREVOKED > EKEYEXPIRED > ENOKEY 179 180EACCES/EPERM are only returned on a direct search of a specific keyring where 181the basal keyring does not grant Search permission. 182