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1===================2Key Request Service3===================4 5The key request service is part of the key retention service (refer to6Documentation/security/keys/core.rst). This document explains more fully how7the requesting algorithm works.8 9The process starts by either the kernel requesting a service by calling10``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_tag(const struct key_type *type,19 const char *description,20 const struct key_tag *domain_tag,21 const char *callout_info);22 23or::24 25 struct key *request_key_with_auxdata(const struct key_type *type,26 const char *description,27 const struct key_tag *domain_tag,28 const char *callout_info,29 size_t callout_len,30 void *aux);31 32or::33 34 struct key *request_key_rcu(const struct key_type *type,35 const char *description,36 const struct key_tag *domain_tag);37 38Or by userspace invoking the request_key system call::39 40 key_serial_t request_key(const char *type,41 const char *description,42 const char *callout_info,43 key_serial_t dest_keyring);44 45The main difference between the access points is that the in-kernel interface46does not need to link the key to a keyring to prevent it from being immediately47destroyed. The kernel interface returns a pointer directly to the key, and48it's up to the caller to destroy the key.49 50The request_key_tag() call is like the in-kernel request_key(), except that it51also takes a domain tag that allows keys to be separated by namespace and52killed off as a group.53 54The request_key_with_auxdata() calls is like the request_key_tag() call, except55that they permit auxiliary data to be passed to the upcaller (the default is56NULL). This is only useful for those key types that define their own upcall57mechanism rather than using /sbin/request-key.58 59The request_key_rcu() call is like the request_key_tag() call, except that it60doesn't check for keys that are under construction and doesn't attempt to61construct missing keys.62 63The userspace interface links the key to a keyring associated with the process64to prevent the key from going away, and returns the serial number of the key to65the caller.66 67 68The following example assumes that the key types involved don't define their69own upcall mechanisms. If they do, then those should be substituted for the70forking and execution of /sbin/request-key.71 72 73The Process74===========75 76A request proceeds in the following manner:77 78 1) Process A calls request_key() [the userspace syscall calls the kernel79 interface].80 81 2) request_key() searches the process's subscribed keyrings to see if there's82 a suitable key there. If there is, it returns the key. If there isn't,83 and callout_info is not set, an error is returned. Otherwise the process84 proceeds to the next step.85 86 3) request_key() sees that A doesn't have the desired key yet, so it creates87 two things:88 89 a) An uninstantiated key U of requested type and description.90 91 b) An authorisation key V that refers to key U and notes that process A92 is the context in which key U should be instantiated and secured, and93 from which associated key requests may be satisfied.94 95 4) request_key() then forks and executes /sbin/request-key with a new session96 keyring that contains a link to auth key V.97 98 5) /sbin/request-key assumes the authority associated with key U.99 100 6) /sbin/request-key execs an appropriate program to perform the actual101 instantiation.102 103 7) The program may want to access another key from A's context (say a104 Kerberos TGT key). It just requests the appropriate key, and the keyring105 search notes that the session keyring has auth key V in its bottom level.106 107 This will permit it to then search the keyrings of process A with the108 UID, GID, groups and security info of process A as if it was process A,109 and come up with key W.110 111 8) The program then does what it must to get the data with which to112 instantiate key U, using key W as a reference (perhaps it contacts a113 Kerberos server using the TGT) and then instantiates key U.114 115 9) Upon instantiating key U, auth key V is automatically revoked so that it116 may not be used again.117 118 10) The program then exits 0 and request_key() deletes key V and returns key119 U to the caller.120 121This also extends further. If key W (step 7 above) didn't exist, key W would122be created uninstantiated, another auth key (X) would be created (as per step1233) and another copy of /sbin/request-key spawned (as per step 4); but the124context specified by auth key X will still be process A, as it was in auth key125V.126 127This is because process A's keyrings can't simply be attached to128/sbin/request-key at the appropriate places because (a) execve will discard two129of them, and (b) it requires the same UID/GID/Groups all the way through.130 131 132Negative Instantiation And Rejection133====================================134 135Rather than instantiating a key, it is possible for the possessor of an136authorisation key to negatively instantiate a key that's under construction.137This is a short duration placeholder that causes any attempt at re-requesting138the key while it exists to fail with error ENOKEY if negated or the specified139error if rejected.140 141This is provided to prevent excessive repeated spawning of /sbin/request-key142processes for a key that will never be obtainable.143 144Should the /sbin/request-key process exit anything other than 0 or die on a145signal, the key under construction will be automatically negatively146instantiated for a short amount of time.147 148 149The Search Algorithm150====================151 152A search of any particular keyring proceeds in the following fashion:153 154 1) When the key management code searches for a key (keyring_search_rcu) it155 firstly calls key_permission(SEARCH) on the keyring it's starting with,156 if this denies permission, it doesn't search further.157 158 2) It considers all the non-keyring keys within that keyring and, if any key159 matches the criteria specified, calls key_permission(SEARCH) on it to see160 if the key is allowed to be found. If it is, that key is returned; if161 not, the search continues, and the error code is retained if of higher162 priority than the one currently set.163 164 3) It then considers all the keyring-type keys in the keyring it's currently165 searching. It calls key_permission(SEARCH) on each keyring, and if this166 grants permission, it recurses, executing steps (2) and (3) on that167 keyring.168 169The process stops immediately a valid key is found with permission granted to170use it. Any error from a previous match attempt is discarded and the key is171returned.172 173When request_key() is invoked, if CONFIG_KEYS_REQUEST_CACHE=y, a per-task174one-key cache is first checked for a match.175 176When search_process_keyrings() is invoked, it performs the following searches177until one succeeds:178 179 1) If extant, the process's thread keyring is searched.180 181 2) If extant, the process's process keyring is searched.182 183 3) The process's session keyring is searched.184 185 4) If the process has assumed the authority associated with a request_key()186 authorisation key then:187 188 a) If extant, the calling process's thread keyring is searched.189 190 b) If extant, the calling process's process keyring is searched.191 192 c) The calling process's session keyring is searched.193 194The moment one succeeds, all pending errors are discarded and the found key is195returned. If CONFIG_KEYS_REQUEST_CACHE=y, then that key is placed in the196per-task cache, displacing the previous key. The cache is cleared on exit or197just prior to resumption of userspace.198 199Only if all these fail does the whole thing fail with the highest priority200error. Note that several errors may have come from LSM.201 202The error priority is::203 204 EKEYREVOKED > EKEYEXPIRED > ENOKEY205 206EACCES/EPERM are only returned on a direct search of a specific keyring where207the basal keyring does not grant Search permission.208