1 =================== 2 Key Request Service 3 =================== 4 5 The key request service is part of the key retention service (refer to 6 Documentation/security/keys/core.rst). This document explains more fully how 7 the requesting 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_info); 15 16 or:: 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 23 or:: 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 32 or:: 33 34 struct key *request_key_rcu(const struct key_type *type, 35 const char *description, 36 const struct key_tag *domain_tag); 37 38 Or 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 45 The main difference between the access points is that the in-kernel interface 46 does not need to link the key to a keyring to prevent it from being immediately 47 destroyed. The kernel interface returns a pointer directly to the key, and 48 it's up to the caller to destroy the key. 49 50 The request_key_tag() call is like the in-kernel request_key(), except that it 51 also takes a domain tag that allows keys to be separated by namespace and 52 killed off as a group. 53 54 The request_key_with_auxdata() calls is like the request_key_tag() call, except 55 that they permit auxiliary data to be passed to the upcaller (the default is 56 NULL). This is only useful for those key types that define their own upcall 57 mechanism rather than using /sbin/request-key. 58 59 The request_key_rcu() call is like the request_key_tag() call, except that it 60 doesn't check for keys that are under construction and doesn't attempt to 61 construct missing keys. 62 63 The userspace interface links the key to a keyring associated with the process 64 to prevent the key from going away, and returns the serial number of the key to 65 the caller. 66 67 68 The following example assumes that the key types involved don't define their 69 own upcall mechanisms. If they do, then those should be substituted for the 70 forking and execution of /sbin/request-key. 71 72 73 The Process 74 =========== 75 76 A request proceeds in the following manner: 77 78 1) Process A calls request_key() [the userspace syscall calls the kernel 79 interface]. 80 81 2) request_key() searches the process's subscribed keyrings to see if there's 82 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 process 84 proceeds to the next step. 85 86 3) request_key() sees that A doesn't have the desired key yet, so it creates 87 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 A 92 is the context in which key U should be instantiated and secured, and 93 from which associated key requests may be satisfied. 94 95 4) request_key() then forks and executes /sbin/request-key with a new session 96 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 actual 101 instantiation. 102 103 7) The program may want to access another key from A's context (say a 104 Kerberos TGT key). It just requests the appropriate key, and the keyring 105 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 the 108 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 to 112 instantiate key U, using key W as a reference (perhaps it contacts a 113 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 it 116 may not be used again. 117 118 10) The program then exits 0 and request_key() deletes key V and returns key 119 U to the caller. 120 121 This also extends further. If key W (step 7 above) didn't exist, key W would 122 be created uninstantiated, another auth key (X) would be created (as per step 123 3) and another copy of /sbin/request-key spawned (as per step 4); but the 124 context specified by auth key X will still be process A, as it was in auth key 125 V. 126 127 This is because process A's keyrings can't simply be attached to 128 /sbin/request-key at the appropriate places because (a) execve will discard two 129 of them, and (b) it requires the same UID/GID/Groups all the way through. 130 131 132 Negative Instantiation And Rejection 133 ==================================== 134 135 Rather than instantiating a key, it is possible for the possessor of an 136 authorisation key to negatively instantiate a key that's under construction. 137 This is a short duration placeholder that causes any attempt at re-requesting 138 the key while it exists to fail with error ENOKEY if negated or the specified 139 error if rejected. 140 141 This is provided to prevent excessive repeated spawning of /sbin/request-key 142 processes for a key that will never be obtainable. 143 144 Should the /sbin/request-key process exit anything other than 0 or die on a 145 signal, the key under construction will be automatically negatively 146 instantiated for a short amount of time. 147 148 149 The Search Algorithm 150 ==================== 151 152 A 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) it 155 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 key 159 matches the criteria specified, calls key_permission(SEARCH) on it to see 160 if the key is allowed to be found. If it is, that key is returned; if 161 not, the search continues, and the error code is retained if of higher 162 priority than the one currently set. 163 164 3) It then considers all the keyring-type keys in the keyring it's currently 165 searching. It calls key_permission(SEARCH) on each keyring, and if this 166 grants permission, it recurses, executing steps (2) and (3) on that 167 keyring. 168 169 The process stops immediately a valid key is found with permission granted to 170 use it. Any error from a previous match attempt is discarded and the key is 171 returned. 172 173 When request_key() is invoked, if CONFIG_KEYS_REQUEST_CACHE=y, a per-task 174 one-key cache is first checked for a match. 175 176 When search_process_keyrings() is invoked, it performs the following searches 177 until 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. 193 194 The moment one succeeds, all pending errors are discarded and the found key is 195 returned. If CONFIG_KEYS_REQUEST_CACHE=y, then that key is placed in the 196 per-task cache, displacing the previous key. The cache is cleared on exit or 197 just prior to resumption of userspace. 198 199 Only if all these fail does the whole thing fail with the highest priority 200 error. Note that several errors may have come from LSM. 201 202 The error priority is:: 203 204 EKEYREVOKED > EKEYEXPIRED > ENOKEY 205 206 EACCES/EPERM are only returned on a direct search of a specific keyring where 207 the basal keyring does not grant Search permission.
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