1=================== 2Key Request Service 3=================== 4 5The key request service is part of the key retention service (refer to 6Documentation/security/keys/core.rst). This document explains more fully how 7the requesting algorithm works. 8 9The 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_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 70The Process 71=========== 72 73A request proceeds in the following manner: 74 75 1) Process A calls request_key() [the userspace syscall calls the kernel 76 interface]. 77 78 2) request_key() searches the process's subscribed keyrings to see if there's 79 a suitable key there. If there is, it returns the key. If there isn't, 80 and callout_info is not set, an error is returned. Otherwise the process 81 proceeds to the next step. 82 83 3) request_key() sees that A doesn't have the desired key yet, so it creates 84 two things: 85 86 a) An uninstantiated key U of requested type and description. 87 88 b) An authorisation key V that refers to key U and notes that process A 89 is the context in which key U should be instantiated and secured, and 90 from which associated key requests may be satisfied. 91 92 4) request_key() then forks and executes /sbin/request-key with a new session 93 keyring that contains a link to auth key V. 94 95 5) /sbin/request-key assumes the authority associated with key U. 96 97 6) /sbin/request-key execs an appropriate program to perform the actual 98 instantiation. 99 100 7) The program may want to access another key from A's context (say a 101 Kerberos TGT key). It just requests the appropriate key, and the keyring 102 search notes that the session keyring has auth key V in its bottom level. 103 104 This will permit it to then search the keyrings of process A with the 105 UID, GID, groups and security info of process A as if it was process A, 106 and come up with key W. 107 108 8) The program then does what it must to get the data with which to 109 instantiate key U, using key W as a reference (perhaps it contacts a 110 Kerberos server using the TGT) and then instantiates key U. 111 112 9) Upon instantiating key U, auth key V is automatically revoked so that it 113 may not be used again. 114 115 10) The program then exits 0 and request_key() deletes key V and returns key 116 U to the caller. 117 118This also extends further. If key W (step 7 above) didn't exist, key W would 119be created uninstantiated, another auth key (X) would be created (as per step 1203) and another copy of /sbin/request-key spawned (as per step 4); but the 121context specified by auth key X will still be process A, as it was in auth key 122V. 123 124This is because process A's keyrings can't simply be attached to 125/sbin/request-key at the appropriate places because (a) execve will discard two 126of them, and (b) it requires the same UID/GID/Groups all the way through. 127 128 129Negative Instantiation And Rejection 130==================================== 131 132Rather than instantiating a key, it is possible for the possessor of an 133authorisation key to negatively instantiate a key that's under construction. 134This is a short duration placeholder that causes any attempt at re-requesting 135the key while it exists to fail with error ENOKEY if negated or the specified 136error if rejected. 137 138This is provided to prevent excessive repeated spawning of /sbin/request-key 139processes for a key that will never be obtainable. 140 141Should the /sbin/request-key process exit anything other than 0 or die on a 142signal, the key under construction will be automatically negatively 143instantiated for a short amount of time. 144 145 146The Search Algorithm 147==================== 148 149A search of any particular keyring proceeds in the following fashion: 150 151 1) When the key management code searches for a key (keyring_search_aux) it 152 firstly calls key_permission(SEARCH) on the keyring it's starting with, 153 if this denies permission, it doesn't search further. 154 155 2) It considers all the non-keyring keys within that keyring and, if any key 156 matches the criteria specified, calls key_permission(SEARCH) on it to see 157 if the key is allowed to be found. If it is, that key is returned; if 158 not, the search continues, and the error code is retained if of higher 159 priority than the one currently set. 160 161 3) It then considers all the keyring-type keys in the keyring it's currently 162 searching. It calls key_permission(SEARCH) on each keyring, and if this 163 grants permission, it recurses, executing steps (2) and (3) on that 164 keyring. 165 166The process stops immediately a valid key is found with permission granted to 167use it. Any error from a previous match attempt is discarded and the key is 168returned. 169 170When search_process_keyrings() is invoked, it performs the following searches 171until one succeeds: 172 173 1) If extant, the process's thread keyring is searched. 174 175 2) If extant, the process's process keyring is searched. 176 177 3) The process's session keyring is searched. 178 179 4) If the process has assumed the authority associated with a request_key() 180 authorisation key then: 181 182 a) If extant, the calling process's thread keyring is searched. 183 184 b) If extant, the calling process's process keyring is searched. 185 186 c) The calling process's session keyring is searched. 187 188The moment one succeeds, all pending errors are discarded and the found key is 189returned. 190 191Only if all these fail does the whole thing fail with the highest priority 192error. Note that several errors may have come from LSM. 193 194The error priority is:: 195 196 EKEYREVOKED > EKEYEXPIRED > ENOKEY 197 198EACCES/EPERM are only returned on a direct search of a specific keyring where 199the basal keyring does not grant Search permission. 200