Based on kernel version 4.9. Page generated on 2016-12-21 14:37 EST.
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.txt). 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_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 24 or: 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 31 or: 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 39 Or 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 46 The main difference between the access points is that the in-kernel interface 47 does not need to link the key to a keyring to prevent it from being immediately 48 destroyed. The kernel interface returns a pointer directly to the key, and 49 it's up to the caller to destroy the key. 50 51 The request_key*_with_auxdata() calls are like the in-kernel request_key*() 52 calls, except that they permit auxiliary data to be passed to the upcaller (the 53 default is NULL). This is only useful for those key types that define their 54 own upcall mechanism rather than using /sbin/request-key. 55 56 The two async in-kernel calls may return keys that are still in the process of 57 being constructed. The two non-async ones will wait for construction to 58 complete first. 59 60 The userspace interface links the key to a keyring associated with the process 61 to prevent the key from going away, and returns the serial number of the key to 62 the caller. 63 64 65 The following example assumes that the key types involved don't define their 66 own upcall mechanisms. If they do, then those should be substituted for the 67 forking and execution of /sbin/request-key. 68 69 70 =========== 71 THE PROCESS 72 =========== 73 74 A 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. 118 119 This also extends further. If key W (step 7 above) didn't exist, key W would 120 be created uninstantiated, another auth key (X) would be created (as per step 121 3) and another copy of /sbin/request-key spawned (as per step 4); but the 122 context specified by auth key X will still be process A, as it was in auth key 123 V. 124 125 This 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 127 of them, and (b) it requires the same UID/GID/Groups all the way through. 128 129 130 ==================================== 131 NEGATIVE INSTANTIATION AND REJECTION 132 ==================================== 133 134 Rather than instantiating a key, it is possible for the possessor of an 135 authorisation key to negatively instantiate a key that's under construction. 136 This is a short duration placeholder that causes any attempt at re-requesting 137 the key whilst it exists to fail with error ENOKEY if negated or the specified 138 error if rejected. 139 140 This is provided to prevent excessive repeated spawning of /sbin/request-key 141 processes for a key that will never be obtainable. 142 143 Should the /sbin/request-key process exit anything other than 0 or die on a 144 signal, the key under construction will be automatically negatively 145 instantiated for a short amount of time. 146 147 148 ==================== 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_aux) 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 search_process_keyrings() is invoked, it performs the following searches 174 until one succeeds: 175 176 (1) If extant, the process's thread keyring is searched. 177 178 (2) If extant, the process's process keyring is searched. 179 180 (3) The process's session keyring is searched. 181 182 (4) If the process has assumed the authority associated with a request_key() 183 authorisation key then: 184 185 (a) If extant, the calling process's thread keyring is searched. 186 187 (b) If extant, the calling process's process keyring is searched. 188 189 (c) The calling process's session keyring is searched. 190 191 The moment one succeeds, all pending errors are discarded and the found key is 192 returned. 193 194 Only if all these fail does the whole thing fail with the highest priority 195 error. Note that several errors may have come from LSM. 196 197 The error priority is: 198 199 EKEYREVOKED > EKEYEXPIRED > ENOKEY 200 201 EACCES/EPERM are only returned on a direct search of a specific keyring where 202 the basal keyring does not grant Search permission.