| 1 | ============================ |
| 2 | KERNEL KEY RETENTION SERVICE |
| 3 | ============================ |
| 4 | |
| 5 | This service allows cryptographic keys, authentication tokens, cross-domain |
| 6 | user mappings, and similar to be cached in the kernel for the use of |
| 7 | filesystems and other kernel services. |
| 8 | |
| 9 | Keyrings are permitted; these are a special type of key that can hold links to |
| 10 | other keys. Processes each have three standard keyring subscriptions that a |
| 11 | kernel service can search for relevant keys. |
| 12 | |
| 13 | The key service can be configured on by enabling: |
| 14 | |
| 15 | "Security options"/"Enable access key retention support" (CONFIG_KEYS) |
| 16 | |
| 17 | This document has the following sections: |
| 18 | |
| 19 | - Key overview |
| 20 | - Key service overview |
| 21 | - Key access permissions |
| 22 | - SELinux support |
| 23 | - New procfs files |
| 24 | - Userspace system call interface |
| 25 | - Kernel services |
| 26 | - Notes on accessing payload contents |
| 27 | - Defining a key type |
| 28 | - Request-key callback service |
| 29 | - Key access filesystem |
| 30 | |
| 31 | |
| 32 | ============ |
| 33 | KEY OVERVIEW |
| 34 | ============ |
| 35 | |
| 36 | In this context, keys represent units of cryptographic data, authentication |
| 37 | tokens, keyrings, etc.. These are represented in the kernel by struct key. |
| 38 | |
| 39 | Each key has a number of attributes: |
| 40 | |
| 41 | - A serial number. |
| 42 | - A type. |
| 43 | - A description (for matching a key in a search). |
| 44 | - Access control information. |
| 45 | - An expiry time. |
| 46 | - A payload. |
| 47 | - State. |
| 48 | |
| 49 | |
| 50 | (*) Each key is issued a serial number of type key_serial_t that is unique for |
| 51 | the lifetime of that key. All serial numbers are positive non-zero 32-bit |
| 52 | integers. |
| 53 | |
| 54 | Userspace programs can use a key's serial numbers as a way to gain access |
| 55 | to it, subject to permission checking. |
| 56 | |
| 57 | (*) Each key is of a defined "type". Types must be registered inside the |
| 58 | kernel by a kernel service (such as a filesystem) before keys of that type |
| 59 | can be added or used. Userspace programs cannot define new types directly. |
| 60 | |
| 61 | Key types are represented in the kernel by struct key_type. This defines a |
| 62 | number of operations that can be performed on a key of that type. |
| 63 | |
| 64 | Should a type be removed from the system, all the keys of that type will |
| 65 | be invalidated. |
| 66 | |
| 67 | (*) Each key has a description. This should be a printable string. The key |
| 68 | type provides an operation to perform a match between the description on a |
| 69 | key and a criterion string. |
| 70 | |
| 71 | (*) Each key has an owner user ID, a group ID and a permissions mask. These |
| 72 | are used to control what a process may do to a key from userspace, and |
| 73 | whether a kernel service will be able to find the key. |
| 74 | |
| 75 | (*) Each key can be set to expire at a specific time by the key type's |
| 76 | instantiation function. Keys can also be immortal. |
| 77 | |
| 78 | (*) Each key can have a payload. This is a quantity of data that represent the |
| 79 | actual "key". In the case of a keyring, this is a list of keys to which |
| 80 | the keyring links; in the case of a user-defined key, it's an arbitrary |
| 81 | blob of data. |
| 82 | |
| 83 | Having a payload is not required; and the payload can, in fact, just be a |
| 84 | value stored in the struct key itself. |
| 85 | |
| 86 | When a key is instantiated, the key type's instantiation function is |
| 87 | called with a blob of data, and that then creates the key's payload in |
| 88 | some way. |
| 89 | |
| 90 | Similarly, when userspace wants to read back the contents of the key, if |
| 91 | permitted, another key type operation will be called to convert the key's |
| 92 | attached payload back into a blob of data. |
| 93 | |
| 94 | (*) Each key can be in one of a number of basic states: |
| 95 | |
| 96 | (*) Uninstantiated. The key exists, but does not have any data attached. |
| 97 | Keys being requested from userspace will be in this state. |
| 98 | |
| 99 | (*) Instantiated. This is the normal state. The key is fully formed, and |
| 100 | has data attached. |
| 101 | |
| 102 | (*) Negative. This is a relatively short-lived state. The key acts as a |
| 103 | note saying that a previous call out to userspace failed, and acts as |
| 104 | a throttle on key lookups. A negative key can be updated to a normal |
| 105 | state. |
| 106 | |
| 107 | (*) Expired. Keys can have lifetimes set. If their lifetime is exceeded, |
| 108 | they traverse to this state. An expired key can be updated back to a |
| 109 | normal state. |
| 110 | |
| 111 | (*) Revoked. A key is put in this state by userspace action. It can't be |
| 112 | found or operated upon (apart from by unlinking it). |
| 113 | |
| 114 | (*) Dead. The key's type was unregistered, and so the key is now useless. |
| 115 | |
| 116 | |
| 117 | ==================== |
| 118 | KEY SERVICE OVERVIEW |
| 119 | ==================== |
| 120 | |
| 121 | The key service provides a number of features besides keys: |
| 122 | |
| 123 | (*) The key service defines two special key types: |
| 124 | |
| 125 | (+) "keyring" |
| 126 | |
| 127 | Keyrings are special keys that contain a list of other keys. Keyring |
| 128 | lists can be modified using various system calls. Keyrings should not |
| 129 | be given a payload when created. |
| 130 | |
| 131 | (+) "user" |
| 132 | |
| 133 | A key of this type has a description and a payload that are arbitrary |
| 134 | blobs of data. These can be created, updated and read by userspace, |
| 135 | and aren't intended for use by kernel services. |
| 136 | |
| 137 | (*) Each process subscribes to three keyrings: a thread-specific keyring, a |
| 138 | process-specific keyring, and a session-specific keyring. |
| 139 | |
| 140 | The thread-specific keyring is discarded from the child when any sort of |
| 141 | clone, fork, vfork or execve occurs. A new keyring is created only when |
| 142 | required. |
| 143 | |
| 144 | The process-specific keyring is replaced with an empty one in the child on |
| 145 | clone, fork, vfork unless CLONE_THREAD is supplied, in which case it is |
| 146 | shared. execve also discards the process's process keyring and creates a |
| 147 | new one. |
| 148 | |
| 149 | The session-specific keyring is persistent across clone, fork, vfork and |
| 150 | execve, even when the latter executes a set-UID or set-GID binary. A |
| 151 | process can, however, replace its current session keyring with a new one |
| 152 | by using PR_JOIN_SESSION_KEYRING. It is permitted to request an anonymous |
| 153 | new one, or to attempt to create or join one of a specific name. |
| 154 | |
| 155 | The ownership of the thread keyring changes when the real UID and GID of |
| 156 | the thread changes. |
| 157 | |
| 158 | (*) Each user ID resident in the system holds two special keyrings: a user |
| 159 | specific keyring and a default user session keyring. The default session |
| 160 | keyring is initialised with a link to the user-specific keyring. |
| 161 | |
| 162 | When a process changes its real UID, if it used to have no session key, it |
| 163 | will be subscribed to the default session key for the new UID. |
| 164 | |
| 165 | If a process attempts to access its session key when it doesn't have one, |
| 166 | it will be subscribed to the default for its current UID. |
| 167 | |
| 168 | (*) Each user has two quotas against which the keys they own are tracked. One |
| 169 | limits the total number of keys and keyrings, the other limits the total |
| 170 | amount of description and payload space that can be consumed. |
| 171 | |
| 172 | The user can view information on this and other statistics through procfs |
| 173 | files. The root user may also alter the quota limits through sysctl files |
| 174 | (see the section "New procfs files"). |
| 175 | |
| 176 | Process-specific and thread-specific keyrings are not counted towards a |
| 177 | user's quota. |
| 178 | |
| 179 | If a system call that modifies a key or keyring in some way would put the |
| 180 | user over quota, the operation is refused and error EDQUOT is returned. |
| 181 | |
| 182 | (*) There's a system call interface by which userspace programs can create and |
| 183 | manipulate keys and keyrings. |
| 184 | |
| 185 | (*) There's a kernel interface by which services can register types and search |
| 186 | for keys. |
| 187 | |
| 188 | (*) There's a way for the a search done from the kernel to call back to |
| 189 | userspace to request a key that can't be found in a process's keyrings. |
| 190 | |
| 191 | (*) An optional filesystem is available through which the key database can be |
| 192 | viewed and manipulated. |
| 193 | |
| 194 | |
| 195 | ====================== |
| 196 | KEY ACCESS PERMISSIONS |
| 197 | ====================== |
| 198 | |
| 199 | Keys have an owner user ID, a group access ID, and a permissions mask. The mask |
| 200 | has up to eight bits each for possessor, user, group and other access. Only |
| 201 | six of each set of eight bits are defined. These permissions granted are: |
| 202 | |
| 203 | (*) View |
| 204 | |
| 205 | This permits a key or keyring's attributes to be viewed - including key |
| 206 | type and description. |
| 207 | |
| 208 | (*) Read |
| 209 | |
| 210 | This permits a key's payload to be viewed or a keyring's list of linked |
| 211 | keys. |
| 212 | |
| 213 | (*) Write |
| 214 | |
| 215 | This permits a key's payload to be instantiated or updated, or it allows a |
| 216 | link to be added to or removed from a keyring. |
| 217 | |
| 218 | (*) Search |
| 219 | |
| 220 | This permits keyrings to be searched and keys to be found. Searches can |
| 221 | only recurse into nested keyrings that have search permission set. |
| 222 | |
| 223 | (*) Link |
| 224 | |
| 225 | This permits a key or keyring to be linked to. To create a link from a |
| 226 | keyring to a key, a process must have Write permission on the keyring and |
| 227 | Link permission on the key. |
| 228 | |
| 229 | (*) Set Attribute |
| 230 | |
| 231 | This permits a key's UID, GID and permissions mask to be changed. |
| 232 | |
| 233 | For changing the ownership, group ID or permissions mask, being the owner of |
| 234 | the key or having the sysadmin capability is sufficient. |
| 235 | |
| 236 | |
| 237 | =============== |
| 238 | SELINUX SUPPORT |
| 239 | =============== |
| 240 | |
| 241 | The security class "key" has been added to SELinux so that mandatory access |
| 242 | controls can be applied to keys created within various contexts. This support |
| 243 | is preliminary, and is likely to change quite significantly in the near future. |
| 244 | Currently, all of the basic permissions explained above are provided in SELinux |
| 245 | as well; SELinux is simply invoked after all basic permission checks have been |
| 246 | performed. |
| 247 | |
| 248 | The value of the file /proc/self/attr/keycreate influences the labeling of |
| 249 | newly-created keys. If the contents of that file correspond to an SELinux |
| 250 | security context, then the key will be assigned that context. Otherwise, the |
| 251 | key will be assigned the current context of the task that invoked the key |
| 252 | creation request. Tasks must be granted explicit permission to assign a |
| 253 | particular context to newly-created keys, using the "create" permission in the |
| 254 | key security class. |
| 255 | |
| 256 | The default keyrings associated with users will be labeled with the default |
| 257 | context of the user if and only if the login programs have been instrumented to |
| 258 | properly initialize keycreate during the login process. Otherwise, they will |
| 259 | be labeled with the context of the login program itself. |
| 260 | |
| 261 | Note, however, that the default keyrings associated with the root user are |
| 262 | labeled with the default kernel context, since they are created early in the |
| 263 | boot process, before root has a chance to log in. |
| 264 | |
| 265 | The keyrings associated with new threads are each labeled with the context of |
| 266 | their associated thread, and both session and process keyrings are handled |
| 267 | similarly. |
| 268 | |
| 269 | |
| 270 | ================ |
| 271 | NEW PROCFS FILES |
| 272 | ================ |
| 273 | |
| 274 | Two files have been added to procfs by which an administrator can find out |
| 275 | about the status of the key service: |
| 276 | |
| 277 | (*) /proc/keys |
| 278 | |
| 279 | This lists the keys that are currently viewable by the task reading the |
| 280 | file, giving information about their type, description and permissions. |
| 281 | It is not possible to view the payload of the key this way, though some |
| 282 | information about it may be given. |
| 283 | |
| 284 | The only keys included in the list are those that grant View permission to |
| 285 | the reading process whether or not it possesses them. Note that LSM |
| 286 | security checks are still performed, and may further filter out keys that |
| 287 | the current process is not authorised to view. |
| 288 | |
| 289 | The contents of the file look like this: |
| 290 | |
| 291 | SERIAL FLAGS USAGE EXPY PERM UID GID TYPE DESCRIPTION: SUMMARY |
| 292 | 00000001 I----- 39 perm 1f3f0000 0 0 keyring _uid_ses.0: 1/4 |
| 293 | 00000002 I----- 2 perm 1f3f0000 0 0 keyring _uid.0: empty |
| 294 | 00000007 I----- 1 perm 1f3f0000 0 0 keyring _pid.1: empty |
| 295 | 0000018d I----- 1 perm 1f3f0000 0 0 keyring _pid.412: empty |
| 296 | 000004d2 I--Q-- 1 perm 1f3f0000 32 -1 keyring _uid.32: 1/4 |
| 297 | 000004d3 I--Q-- 3 perm 1f3f0000 32 -1 keyring _uid_ses.32: empty |
| 298 | 00000892 I--QU- 1 perm 1f000000 0 0 user metal:copper: 0 |
| 299 | 00000893 I--Q-N 1 35s 1f3f0000 0 0 user metal:silver: 0 |
| 300 | 00000894 I--Q-- 1 10h 003f0000 0 0 user metal:gold: 0 |
| 301 | |
| 302 | The flags are: |
| 303 | |
| 304 | I Instantiated |
| 305 | R Revoked |
| 306 | D Dead |
| 307 | Q Contributes to user's quota |
| 308 | U Under construction by callback to userspace |
| 309 | N Negative key |
| 310 | |
| 311 | This file must be enabled at kernel configuration time as it allows anyone |
| 312 | to list the keys database. |
| 313 | |
| 314 | (*) /proc/key-users |
| 315 | |
| 316 | This file lists the tracking data for each user that has at least one key |
| 317 | on the system. Such data includes quota information and statistics: |
| 318 | |
| 319 | [root@andromeda root]# cat /proc/key-users |
| 320 | 0: 46 45/45 1/100 13/10000 |
| 321 | 29: 2 2/2 2/100 40/10000 |
| 322 | 32: 2 2/2 2/100 40/10000 |
| 323 | 38: 2 2/2 2/100 40/10000 |
| 324 | |
| 325 | The format of each line is |
| 326 | <UID>: User ID to which this applies |
| 327 | <usage> Structure refcount |
| 328 | <inst>/<keys> Total number of keys and number instantiated |
| 329 | <keys>/<max> Key count quota |
| 330 | <bytes>/<max> Key size quota |
| 331 | |
| 332 | |
| 333 | Four new sysctl files have been added also for the purpose of controlling the |
| 334 | quota limits on keys: |
| 335 | |
| 336 | (*) /proc/sys/kernel/keys/root_maxkeys |
| 337 | /proc/sys/kernel/keys/root_maxbytes |
| 338 | |
| 339 | These files hold the maximum number of keys that root may have and the |
| 340 | maximum total number of bytes of data that root may have stored in those |
| 341 | keys. |
| 342 | |
| 343 | (*) /proc/sys/kernel/keys/maxkeys |
| 344 | /proc/sys/kernel/keys/maxbytes |
| 345 | |
| 346 | These files hold the maximum number of keys that each non-root user may |
| 347 | have and the maximum total number of bytes of data that each of those |
| 348 | users may have stored in their keys. |
| 349 | |
| 350 | Root may alter these by writing each new limit as a decimal number string to |
| 351 | the appropriate file. |
| 352 | |
| 353 | |
| 354 | =============================== |
| 355 | USERSPACE SYSTEM CALL INTERFACE |
| 356 | =============================== |
| 357 | |
| 358 | Userspace can manipulate keys directly through three new syscalls: add_key, |
| 359 | request_key and keyctl. The latter provides a number of functions for |
| 360 | manipulating keys. |
| 361 | |
| 362 | When referring to a key directly, userspace programs should use the key's |
| 363 | serial number (a positive 32-bit integer). However, there are some special |
| 364 | values available for referring to special keys and keyrings that relate to the |
| 365 | process making the call: |
| 366 | |
| 367 | CONSTANT VALUE KEY REFERENCED |
| 368 | ============================== ====== =========================== |
| 369 | KEY_SPEC_THREAD_KEYRING -1 thread-specific keyring |
| 370 | KEY_SPEC_PROCESS_KEYRING -2 process-specific keyring |
| 371 | KEY_SPEC_SESSION_KEYRING -3 session-specific keyring |
| 372 | KEY_SPEC_USER_KEYRING -4 UID-specific keyring |
| 373 | KEY_SPEC_USER_SESSION_KEYRING -5 UID-session keyring |
| 374 | KEY_SPEC_GROUP_KEYRING -6 GID-specific keyring |
| 375 | KEY_SPEC_REQKEY_AUTH_KEY -7 assumed request_key() |
| 376 | authorisation key |
| 377 | |
| 378 | |
| 379 | The main syscalls are: |
| 380 | |
| 381 | (*) Create a new key of given type, description and payload and add it to the |
| 382 | nominated keyring: |
| 383 | |
| 384 | key_serial_t add_key(const char *type, const char *desc, |
| 385 | const void *payload, size_t plen, |
| 386 | key_serial_t keyring); |
| 387 | |
| 388 | If a key of the same type and description as that proposed already exists |
| 389 | in the keyring, this will try to update it with the given payload, or it |
| 390 | will return error EEXIST if that function is not supported by the key |
| 391 | type. The process must also have permission to write to the key to be able |
| 392 | to update it. The new key will have all user permissions granted and no |
| 393 | group or third party permissions. |
| 394 | |
| 395 | Otherwise, this will attempt to create a new key of the specified type and |
| 396 | description, and to instantiate it with the supplied payload and attach it |
| 397 | to the keyring. In this case, an error will be generated if the process |
| 398 | does not have permission to write to the keyring. |
| 399 | |
| 400 | The payload is optional, and the pointer can be NULL if not required by |
| 401 | the type. The payload is plen in size, and plen can be zero for an empty |
| 402 | payload. |
| 403 | |
| 404 | A new keyring can be generated by setting type "keyring", the keyring name |
| 405 | as the description (or NULL) and setting the payload to NULL. |
| 406 | |
| 407 | User defined keys can be created by specifying type "user". It is |
| 408 | recommended that a user defined key's description by prefixed with a type |
| 409 | ID and a colon, such as "krb5tgt:" for a Kerberos 5 ticket granting |
| 410 | ticket. |
| 411 | |
| 412 | Any other type must have been registered with the kernel in advance by a |
| 413 | kernel service such as a filesystem. |
| 414 | |
| 415 | The ID of the new or updated key is returned if successful. |
| 416 | |
| 417 | |
| 418 | (*) Search the process's keyrings for a key, potentially calling out to |
| 419 | userspace to create it. |
| 420 | |
| 421 | key_serial_t request_key(const char *type, const char *description, |
| 422 | const char *callout_info, |
| 423 | key_serial_t dest_keyring); |
| 424 | |
| 425 | This function searches all the process's keyrings in the order thread, |
| 426 | process, session for a matching key. This works very much like |
| 427 | KEYCTL_SEARCH, including the optional attachment of the discovered key to |
| 428 | a keyring. |
| 429 | |
| 430 | If a key cannot be found, and if callout_info is not NULL, then |
| 431 | /sbin/request-key will be invoked in an attempt to obtain a key. The |
| 432 | callout_info string will be passed as an argument to the program. |
| 433 | |
| 434 | See also Documentation/keys-request-key.txt. |
| 435 | |
| 436 | |
| 437 | The keyctl syscall functions are: |
| 438 | |
| 439 | (*) Map a special key ID to a real key ID for this process: |
| 440 | |
| 441 | key_serial_t keyctl(KEYCTL_GET_KEYRING_ID, key_serial_t id, |
| 442 | int create); |
| 443 | |
| 444 | The special key specified by "id" is looked up (with the key being created |
| 445 | if necessary) and the ID of the key or keyring thus found is returned if |
| 446 | it exists. |
| 447 | |
| 448 | If the key does not yet exist, the key will be created if "create" is |
| 449 | non-zero; and the error ENOKEY will be returned if "create" is zero. |
| 450 | |
| 451 | |
| 452 | (*) Replace the session keyring this process subscribes to with a new one: |
| 453 | |
| 454 | key_serial_t keyctl(KEYCTL_JOIN_SESSION_KEYRING, const char *name); |
| 455 | |
| 456 | If name is NULL, an anonymous keyring is created attached to the process |
| 457 | as its session keyring, displacing the old session keyring. |
| 458 | |
| 459 | If name is not NULL, if a keyring of that name exists, the process |
| 460 | attempts to attach it as the session keyring, returning an error if that |
| 461 | is not permitted; otherwise a new keyring of that name is created and |
| 462 | attached as the session keyring. |
| 463 | |
| 464 | To attach to a named keyring, the keyring must have search permission for |
| 465 | the process's ownership. |
| 466 | |
| 467 | The ID of the new session keyring is returned if successful. |
| 468 | |
| 469 | |
| 470 | (*) Update the specified key: |
| 471 | |
| 472 | long keyctl(KEYCTL_UPDATE, key_serial_t key, const void *payload, |
| 473 | size_t plen); |
| 474 | |
| 475 | This will try to update the specified key with the given payload, or it |
| 476 | will return error EOPNOTSUPP if that function is not supported by the key |
| 477 | type. The process must also have permission to write to the key to be able |
| 478 | to update it. |
| 479 | |
| 480 | The payload is of length plen, and may be absent or empty as for |
| 481 | add_key(). |
| 482 | |
| 483 | |
| 484 | (*) Revoke a key: |
| 485 | |
| 486 | long keyctl(KEYCTL_REVOKE, key_serial_t key); |
| 487 | |
| 488 | This makes a key unavailable for further operations. Further attempts to |
| 489 | use the key will be met with error EKEYREVOKED, and the key will no longer |
| 490 | be findable. |
| 491 | |
| 492 | |
| 493 | (*) Change the ownership of a key: |
| 494 | |
| 495 | long keyctl(KEYCTL_CHOWN, key_serial_t key, uid_t uid, gid_t gid); |
| 496 | |
| 497 | This function permits a key's owner and group ID to be changed. Either one |
| 498 | of uid or gid can be set to -1 to suppress that change. |
| 499 | |
| 500 | Only the superuser can change a key's owner to something other than the |
| 501 | key's current owner. Similarly, only the superuser can change a key's |
| 502 | group ID to something other than the calling process's group ID or one of |
| 503 | its group list members. |
| 504 | |
| 505 | |
| 506 | (*) Change the permissions mask on a key: |
| 507 | |
| 508 | long keyctl(KEYCTL_SETPERM, key_serial_t key, key_perm_t perm); |
| 509 | |
| 510 | This function permits the owner of a key or the superuser to change the |
| 511 | permissions mask on a key. |
| 512 | |
| 513 | Only bits the available bits are permitted; if any other bits are set, |
| 514 | error EINVAL will be returned. |
| 515 | |
| 516 | |
| 517 | (*) Describe a key: |
| 518 | |
| 519 | long keyctl(KEYCTL_DESCRIBE, key_serial_t key, char *buffer, |
| 520 | size_t buflen); |
| 521 | |
| 522 | This function returns a summary of the key's attributes (but not its |
| 523 | payload data) as a string in the buffer provided. |
| 524 | |
| 525 | Unless there's an error, it always returns the amount of data it could |
| 526 | produce, even if that's too big for the buffer, but it won't copy more |
| 527 | than requested to userspace. If the buffer pointer is NULL then no copy |
| 528 | will take place. |
| 529 | |
| 530 | A process must have view permission on the key for this function to be |
| 531 | successful. |
| 532 | |
| 533 | If successful, a string is placed in the buffer in the following format: |
| 534 | |
| 535 | <type>;<uid>;<gid>;<perm>;<description> |
| 536 | |
| 537 | Where type and description are strings, uid and gid are decimal, and perm |
| 538 | is hexadecimal. A NUL character is included at the end of the string if |
| 539 | the buffer is sufficiently big. |
| 540 | |
| 541 | This can be parsed with |
| 542 | |
| 543 | sscanf(buffer, "%[^;];%d;%d;%o;%s", type, &uid, &gid, &mode, desc); |
| 544 | |
| 545 | |
| 546 | (*) Clear out a keyring: |
| 547 | |
| 548 | long keyctl(KEYCTL_CLEAR, key_serial_t keyring); |
| 549 | |
| 550 | This function clears the list of keys attached to a keyring. The calling |
| 551 | process must have write permission on the keyring, and it must be a |
| 552 | keyring (or else error ENOTDIR will result). |
| 553 | |
| 554 | |
| 555 | (*) Link a key into a keyring: |
| 556 | |
| 557 | long keyctl(KEYCTL_LINK, key_serial_t keyring, key_serial_t key); |
| 558 | |
| 559 | This function creates a link from the keyring to the key. The process must |
| 560 | have write permission on the keyring and must have link permission on the |
| 561 | key. |
| 562 | |
| 563 | Should the keyring not be a keyring, error ENOTDIR will result; and if the |
| 564 | keyring is full, error ENFILE will result. |
| 565 | |
| 566 | The link procedure checks the nesting of the keyrings, returning ELOOP if |
| 567 | it appears too deep or EDEADLK if the link would introduce a cycle. |
| 568 | |
| 569 | Any links within the keyring to keys that match the new key in terms of |
| 570 | type and description will be discarded from the keyring as the new one is |
| 571 | added. |
| 572 | |
| 573 | |
| 574 | (*) Unlink a key or keyring from another keyring: |
| 575 | |
| 576 | long keyctl(KEYCTL_UNLINK, key_serial_t keyring, key_serial_t key); |
| 577 | |
| 578 | This function looks through the keyring for the first link to the |
| 579 | specified key, and removes it if found. Subsequent links to that key are |
| 580 | ignored. The process must have write permission on the keyring. |
| 581 | |
| 582 | If the keyring is not a keyring, error ENOTDIR will result; and if the key |
| 583 | is not present, error ENOENT will be the result. |
| 584 | |
| 585 | |
| 586 | (*) Search a keyring tree for a key: |
| 587 | |
| 588 | key_serial_t keyctl(KEYCTL_SEARCH, key_serial_t keyring, |
| 589 | const char *type, const char *description, |
| 590 | key_serial_t dest_keyring); |
| 591 | |
| 592 | This searches the keyring tree headed by the specified keyring until a key |
| 593 | is found that matches the type and description criteria. Each keyring is |
| 594 | checked for keys before recursion into its children occurs. |
| 595 | |
| 596 | The process must have search permission on the top level keyring, or else |
| 597 | error EACCES will result. Only keyrings that the process has search |
| 598 | permission on will be recursed into, and only keys and keyrings for which |
| 599 | a process has search permission can be matched. If the specified keyring |
| 600 | is not a keyring, ENOTDIR will result. |
| 601 | |
| 602 | If the search succeeds, the function will attempt to link the found key |
| 603 | into the destination keyring if one is supplied (non-zero ID). All the |
| 604 | constraints applicable to KEYCTL_LINK apply in this case too. |
| 605 | |
| 606 | Error ENOKEY, EKEYREVOKED or EKEYEXPIRED will be returned if the search |
| 607 | fails. On success, the resulting key ID will be returned. |
| 608 | |
| 609 | |
| 610 | (*) Read the payload data from a key: |
| 611 | |
| 612 | long keyctl(KEYCTL_READ, key_serial_t keyring, char *buffer, |
| 613 | size_t buflen); |
| 614 | |
| 615 | This function attempts to read the payload data from the specified key |
| 616 | into the buffer. The process must have read permission on the key to |
| 617 | succeed. |
| 618 | |
| 619 | The returned data will be processed for presentation by the key type. For |
| 620 | instance, a keyring will return an array of key_serial_t entries |
| 621 | representing the IDs of all the keys to which it is subscribed. The user |
| 622 | defined key type will return its data as is. If a key type does not |
| 623 | implement this function, error EOPNOTSUPP will result. |
| 624 | |
| 625 | As much of the data as can be fitted into the buffer will be copied to |
| 626 | userspace if the buffer pointer is not NULL. |
| 627 | |
| 628 | On a successful return, the function will always return the amount of data |
| 629 | available rather than the amount copied. |
| 630 | |
| 631 | |
| 632 | (*) Instantiate a partially constructed key. |
| 633 | |
| 634 | long keyctl(KEYCTL_INSTANTIATE, key_serial_t key, |
| 635 | const void *payload, size_t plen, |
| 636 | key_serial_t keyring); |
| 637 | |
| 638 | If the kernel calls back to userspace to complete the instantiation of a |
| 639 | key, userspace should use this call to supply data for the key before the |
| 640 | invoked process returns, or else the key will be marked negative |
| 641 | automatically. |
| 642 | |
| 643 | The process must have write access on the key to be able to instantiate |
| 644 | it, and the key must be uninstantiated. |
| 645 | |
| 646 | If a keyring is specified (non-zero), the key will also be linked into |
| 647 | that keyring, however all the constraints applying in KEYCTL_LINK apply in |
| 648 | this case too. |
| 649 | |
| 650 | The payload and plen arguments describe the payload data as for add_key(). |
| 651 | |
| 652 | |
| 653 | (*) Negatively instantiate a partially constructed key. |
| 654 | |
| 655 | long keyctl(KEYCTL_NEGATE, key_serial_t key, |
| 656 | unsigned timeout, key_serial_t keyring); |
| 657 | |
| 658 | If the kernel calls back to userspace to complete the instantiation of a |
| 659 | key, userspace should use this call mark the key as negative before the |
| 660 | invoked process returns if it is unable to fulfil the request. |
| 661 | |
| 662 | The process must have write access on the key to be able to instantiate |
| 663 | it, and the key must be uninstantiated. |
| 664 | |
| 665 | If a keyring is specified (non-zero), the key will also be linked into |
| 666 | that keyring, however all the constraints applying in KEYCTL_LINK apply in |
| 667 | this case too. |
| 668 | |
| 669 | |
| 670 | (*) Set the default request-key destination keyring. |
| 671 | |
| 672 | long keyctl(KEYCTL_SET_REQKEY_KEYRING, int reqkey_defl); |
| 673 | |
| 674 | This sets the default keyring to which implicitly requested keys will be |
| 675 | attached for this thread. reqkey_defl should be one of these constants: |
| 676 | |
| 677 | CONSTANT VALUE NEW DEFAULT KEYRING |
| 678 | ====================================== ====== ======================= |
| 679 | KEY_REQKEY_DEFL_NO_CHANGE -1 No change |
| 680 | KEY_REQKEY_DEFL_DEFAULT 0 Default[1] |
| 681 | KEY_REQKEY_DEFL_THREAD_KEYRING 1 Thread keyring |
| 682 | KEY_REQKEY_DEFL_PROCESS_KEYRING 2 Process keyring |
| 683 | KEY_REQKEY_DEFL_SESSION_KEYRING 3 Session keyring |
| 684 | KEY_REQKEY_DEFL_USER_KEYRING 4 User keyring |
| 685 | KEY_REQKEY_DEFL_USER_SESSION_KEYRING 5 User session keyring |
| 686 | KEY_REQKEY_DEFL_GROUP_KEYRING 6 Group keyring |
| 687 | |
| 688 | The old default will be returned if successful and error EINVAL will be |
| 689 | returned if reqkey_defl is not one of the above values. |
| 690 | |
| 691 | The default keyring can be overridden by the keyring indicated to the |
| 692 | request_key() system call. |
| 693 | |
| 694 | Note that this setting is inherited across fork/exec. |
| 695 | |
| 696 | [1] The default is: the thread keyring if there is one, otherwise |
| 697 | the process keyring if there is one, otherwise the session keyring if |
| 698 | there is one, otherwise the user default session keyring. |
| 699 | |
| 700 | |
| 701 | (*) Set the timeout on a key. |
| 702 | |
| 703 | long keyctl(KEYCTL_SET_TIMEOUT, key_serial_t key, unsigned timeout); |
| 704 | |
| 705 | This sets or clears the timeout on a key. The timeout can be 0 to clear |
| 706 | the timeout or a number of seconds to set the expiry time that far into |
| 707 | the future. |
| 708 | |
| 709 | The process must have attribute modification access on a key to set its |
| 710 | timeout. Timeouts may not be set with this function on negative, revoked |
| 711 | or expired keys. |
| 712 | |
| 713 | |
| 714 | (*) Assume the authority granted to instantiate a key |
| 715 | |
| 716 | long keyctl(KEYCTL_ASSUME_AUTHORITY, key_serial_t key); |
| 717 | |
| 718 | This assumes or divests the authority required to instantiate the |
| 719 | specified key. Authority can only be assumed if the thread has the |
| 720 | authorisation key associated with the specified key in its keyrings |
| 721 | somewhere. |
| 722 | |
| 723 | Once authority is assumed, searches for keys will also search the |
| 724 | requester's keyrings using the requester's security label, UID, GID and |
| 725 | groups. |
| 726 | |
| 727 | If the requested authority is unavailable, error EPERM will be returned, |
| 728 | likewise if the authority has been revoked because the target key is |
| 729 | already instantiated. |
| 730 | |
| 731 | If the specified key is 0, then any assumed authority will be divested. |
| 732 | |
| 733 | The assumed authoritative key is inherited across fork and exec. |
| 734 | |
| 735 | |
| 736 | (*) Get the LSM security context attached to a key. |
| 737 | |
| 738 | long keyctl(KEYCTL_GET_SECURITY, key_serial_t key, char *buffer, |
| 739 | size_t buflen) |
| 740 | |
| 741 | This function returns a string that represents the LSM security context |
| 742 | attached to a key in the buffer provided. |
| 743 | |
| 744 | Unless there's an error, it always returns the amount of data it could |
| 745 | produce, even if that's too big for the buffer, but it won't copy more |
| 746 | than requested to userspace. If the buffer pointer is NULL then no copy |
| 747 | will take place. |
| 748 | |
| 749 | A NUL character is included at the end of the string if the buffer is |
| 750 | sufficiently big. This is included in the returned count. If no LSM is |
| 751 | in force then an empty string will be returned. |
| 752 | |
| 753 | A process must have view permission on the key for this function to be |
| 754 | successful. |
| 755 | |
| 756 | |
| 757 | =============== |
| 758 | KERNEL SERVICES |
| 759 | =============== |
| 760 | |
| 761 | The kernel services for key management are fairly simple to deal with. They can |
| 762 | be broken down into two areas: keys and key types. |
| 763 | |
| 764 | Dealing with keys is fairly straightforward. Firstly, the kernel service |
| 765 | registers its type, then it searches for a key of that type. It should retain |
| 766 | the key as long as it has need of it, and then it should release it. For a |
| 767 | filesystem or device file, a search would probably be performed during the open |
| 768 | call, and the key released upon close. How to deal with conflicting keys due to |
| 769 | two different users opening the same file is left to the filesystem author to |
| 770 | solve. |
| 771 | |
| 772 | To access the key manager, the following header must be #included: |
| 773 | |
| 774 | <linux/key.h> |
| 775 | |
| 776 | Specific key types should have a header file under include/keys/ that should be |
| 777 | used to access that type. For keys of type "user", for example, that would be: |
| 778 | |
| 779 | <keys/user-type.h> |
| 780 | |
| 781 | Note that there are two different types of pointers to keys that may be |
| 782 | encountered: |
| 783 | |
| 784 | (*) struct key * |
| 785 | |
| 786 | This simply points to the key structure itself. Key structures will be at |
| 787 | least four-byte aligned. |
| 788 | |
| 789 | (*) key_ref_t |
| 790 | |
| 791 | This is equivalent to a struct key *, but the least significant bit is set |
| 792 | if the caller "possesses" the key. By "possession" it is meant that the |
| 793 | calling processes has a searchable link to the key from one of its |
| 794 | keyrings. There are three functions for dealing with these: |
| 795 | |
| 796 | key_ref_t make_key_ref(const struct key *key, |
| 797 | unsigned long possession); |
| 798 | |
| 799 | struct key *key_ref_to_ptr(const key_ref_t key_ref); |
| 800 | |
| 801 | unsigned long is_key_possessed(const key_ref_t key_ref); |
| 802 | |
| 803 | The first function constructs a key reference from a key pointer and |
| 804 | possession information (which must be 0 or 1 and not any other value). |
| 805 | |
| 806 | The second function retrieves the key pointer from a reference and the |
| 807 | third retrieves the possession flag. |
| 808 | |
| 809 | When accessing a key's payload contents, certain precautions must be taken to |
| 810 | prevent access vs modification races. See the section "Notes on accessing |
| 811 | payload contents" for more information. |
| 812 | |
| 813 | (*) To search for a key, call: |
| 814 | |
| 815 | struct key *request_key(const struct key_type *type, |
| 816 | const char *description, |
| 817 | const char *callout_info); |
| 818 | |
| 819 | This is used to request a key or keyring with a description that matches |
| 820 | the description specified according to the key type's match function. This |
| 821 | permits approximate matching to occur. If callout_string is not NULL, then |
| 822 | /sbin/request-key will be invoked in an attempt to obtain the key from |
| 823 | userspace. In that case, callout_string will be passed as an argument to |
| 824 | the program. |
| 825 | |
| 826 | Should the function fail error ENOKEY, EKEYEXPIRED or EKEYREVOKED will be |
| 827 | returned. |
| 828 | |
| 829 | If successful, the key will have been attached to the default keyring for |
| 830 | implicitly obtained request-key keys, as set by KEYCTL_SET_REQKEY_KEYRING. |
| 831 | |
| 832 | See also Documentation/keys-request-key.txt. |
| 833 | |
| 834 | |
| 835 | (*) To search for a key, passing auxiliary data to the upcaller, call: |
| 836 | |
| 837 | struct key *request_key_with_auxdata(const struct key_type *type, |
| 838 | const char *description, |
| 839 | const void *callout_info, |
| 840 | size_t callout_len, |
| 841 | void *aux); |
| 842 | |
| 843 | This is identical to request_key(), except that the auxiliary data is |
| 844 | passed to the key_type->request_key() op if it exists, and the callout_info |
| 845 | is a blob of length callout_len, if given (the length may be 0). |
| 846 | |
| 847 | |
| 848 | (*) A key can be requested asynchronously by calling one of: |
| 849 | |
| 850 | struct key *request_key_async(const struct key_type *type, |
| 851 | const char *description, |
| 852 | const void *callout_info, |
| 853 | size_t callout_len); |
| 854 | |
| 855 | or: |
| 856 | |
| 857 | struct key *request_key_async_with_auxdata(const struct key_type *type, |
| 858 | const char *description, |
| 859 | const char *callout_info, |
| 860 | size_t callout_len, |
| 861 | void *aux); |
| 862 | |
| 863 | which are asynchronous equivalents of request_key() and |
| 864 | request_key_with_auxdata() respectively. |
| 865 | |
| 866 | These two functions return with the key potentially still under |
| 867 | construction. To wait for contruction completion, the following should be |
| 868 | called: |
| 869 | |
| 870 | int wait_for_key_construction(struct key *key, bool intr); |
| 871 | |
| 872 | The function will wait for the key to finish being constructed and then |
| 873 | invokes key_validate() to return an appropriate value to indicate the state |
| 874 | of the key (0 indicates the key is usable). |
| 875 | |
| 876 | If intr is true, then the wait can be interrupted by a signal, in which |
| 877 | case error ERESTARTSYS will be returned. |
| 878 | |
| 879 | |
| 880 | (*) When it is no longer required, the key should be released using: |
| 881 | |
| 882 | void key_put(struct key *key); |
| 883 | |
| 884 | Or: |
| 885 | |
| 886 | void key_ref_put(key_ref_t key_ref); |
| 887 | |
| 888 | These can be called from interrupt context. If CONFIG_KEYS is not set then |
| 889 | the argument will not be parsed. |
| 890 | |
| 891 | |
| 892 | (*) Extra references can be made to a key by calling the following function: |
| 893 | |
| 894 | struct key *key_get(struct key *key); |
| 895 | |
| 896 | These need to be disposed of by calling key_put() when they've been |
| 897 | finished with. The key pointer passed in will be returned. If the pointer |
| 898 | is NULL or CONFIG_KEYS is not set then the key will not be dereferenced and |
| 899 | no increment will take place. |
| 900 | |
| 901 | |
| 902 | (*) A key's serial number can be obtained by calling: |
| 903 | |
| 904 | key_serial_t key_serial(struct key *key); |
| 905 | |
| 906 | If key is NULL or if CONFIG_KEYS is not set then 0 will be returned (in the |
| 907 | latter case without parsing the argument). |
| 908 | |
| 909 | |
| 910 | (*) If a keyring was found in the search, this can be further searched by: |
| 911 | |
| 912 | key_ref_t keyring_search(key_ref_t keyring_ref, |
| 913 | const struct key_type *type, |
| 914 | const char *description) |
| 915 | |
| 916 | This searches the keyring tree specified for a matching key. Error ENOKEY |
| 917 | is returned upon failure (use IS_ERR/PTR_ERR to determine). If successful, |
| 918 | the returned key will need to be released. |
| 919 | |
| 920 | The possession attribute from the keyring reference is used to control |
| 921 | access through the permissions mask and is propagated to the returned key |
| 922 | reference pointer if successful. |
| 923 | |
| 924 | |
| 925 | (*) To check the validity of a key, this function can be called: |
| 926 | |
| 927 | int validate_key(struct key *key); |
| 928 | |
| 929 | This checks that the key in question hasn't expired or and hasn't been |
| 930 | revoked. Should the key be invalid, error EKEYEXPIRED or EKEYREVOKED will |
| 931 | be returned. If the key is NULL or if CONFIG_KEYS is not set then 0 will be |
| 932 | returned (in the latter case without parsing the argument). |
| 933 | |
| 934 | |
| 935 | (*) To register a key type, the following function should be called: |
| 936 | |
| 937 | int register_key_type(struct key_type *type); |
| 938 | |
| 939 | This will return error EEXIST if a type of the same name is already |
| 940 | present. |
| 941 | |
| 942 | |
| 943 | (*) To unregister a key type, call: |
| 944 | |
| 945 | void unregister_key_type(struct key_type *type); |
| 946 | |
| 947 | |
| 948 | Under some circumstances, it may be desirable to deal with a bundle of keys. |
| 949 | The facility provides access to the keyring type for managing such a bundle: |
| 950 | |
| 951 | struct key_type key_type_keyring; |
| 952 | |
| 953 | This can be used with a function such as request_key() to find a specific |
| 954 | keyring in a process's keyrings. A keyring thus found can then be searched |
| 955 | with keyring_search(). Note that it is not possible to use request_key() to |
| 956 | search a specific keyring, so using keyrings in this way is of limited utility. |
| 957 | |
| 958 | |
| 959 | =================================== |
| 960 | NOTES ON ACCESSING PAYLOAD CONTENTS |
| 961 | =================================== |
| 962 | |
| 963 | The simplest payload is just a number in key->payload.value. In this case, |
| 964 | there's no need to indulge in RCU or locking when accessing the payload. |
| 965 | |
| 966 | More complex payload contents must be allocated and a pointer to them set in |
| 967 | key->payload.data. One of the following ways must be selected to access the |
| 968 | data: |
| 969 | |
| 970 | (1) Unmodifiable key type. |
| 971 | |
| 972 | If the key type does not have a modify method, then the key's payload can |
| 973 | be accessed without any form of locking, provided that it's known to be |
| 974 | instantiated (uninstantiated keys cannot be "found"). |
| 975 | |
| 976 | (2) The key's semaphore. |
| 977 | |
| 978 | The semaphore could be used to govern access to the payload and to control |
| 979 | the payload pointer. It must be write-locked for modifications and would |
| 980 | have to be read-locked for general access. The disadvantage of doing this |
| 981 | is that the accessor may be required to sleep. |
| 982 | |
| 983 | (3) RCU. |
| 984 | |
| 985 | RCU must be used when the semaphore isn't already held; if the semaphore |
| 986 | is held then the contents can't change under you unexpectedly as the |
| 987 | semaphore must still be used to serialise modifications to the key. The |
| 988 | key management code takes care of this for the key type. |
| 989 | |
| 990 | However, this means using: |
| 991 | |
| 992 | rcu_read_lock() ... rcu_dereference() ... rcu_read_unlock() |
| 993 | |
| 994 | to read the pointer, and: |
| 995 | |
| 996 | rcu_dereference() ... rcu_assign_pointer() ... call_rcu() |
| 997 | |
| 998 | to set the pointer and dispose of the old contents after a grace period. |
| 999 | Note that only the key type should ever modify a key's payload. |
| 1000 | |
| 1001 | Furthermore, an RCU controlled payload must hold a struct rcu_head for the |
| 1002 | use of call_rcu() and, if the payload is of variable size, the length of |
| 1003 | the payload. key->datalen cannot be relied upon to be consistent with the |
| 1004 | payload just dereferenced if the key's semaphore is not held. |
| 1005 | |
| 1006 | |
| 1007 | =================== |
| 1008 | DEFINING A KEY TYPE |
| 1009 | =================== |
| 1010 | |
| 1011 | A kernel service may want to define its own key type. For instance, an AFS |
| 1012 | filesystem might want to define a Kerberos 5 ticket key type. To do this, it |
| 1013 | author fills in a key_type struct and registers it with the system. |
| 1014 | |
| 1015 | Source files that implement key types should include the following header file: |
| 1016 | |
| 1017 | <linux/key-type.h> |
| 1018 | |
| 1019 | The structure has a number of fields, some of which are mandatory: |
| 1020 | |
| 1021 | (*) const char *name |
| 1022 | |
| 1023 | The name of the key type. This is used to translate a key type name |
| 1024 | supplied by userspace into a pointer to the structure. |
| 1025 | |
| 1026 | |
| 1027 | (*) size_t def_datalen |
| 1028 | |
| 1029 | This is optional - it supplies the default payload data length as |
| 1030 | contributed to the quota. If the key type's payload is always or almost |
| 1031 | always the same size, then this is a more efficient way to do things. |
| 1032 | |
| 1033 | The data length (and quota) on a particular key can always be changed |
| 1034 | during instantiation or update by calling: |
| 1035 | |
| 1036 | int key_payload_reserve(struct key *key, size_t datalen); |
| 1037 | |
| 1038 | With the revised data length. Error EDQUOT will be returned if this is not |
| 1039 | viable. |
| 1040 | |
| 1041 | |
| 1042 | (*) int (*instantiate)(struct key *key, const void *data, size_t datalen); |
| 1043 | |
| 1044 | This method is called to attach a payload to a key during construction. |
| 1045 | The payload attached need not bear any relation to the data passed to this |
| 1046 | function. |
| 1047 | |
| 1048 | If the amount of data attached to the key differs from the size in |
| 1049 | keytype->def_datalen, then key_payload_reserve() should be called. |
| 1050 | |
| 1051 | This method does not have to lock the key in order to attach a payload. |
| 1052 | The fact that KEY_FLAG_INSTANTIATED is not set in key->flags prevents |
| 1053 | anything else from gaining access to the key. |
| 1054 | |
| 1055 | It is safe to sleep in this method. |
| 1056 | |
| 1057 | |
| 1058 | (*) int (*update)(struct key *key, const void *data, size_t datalen); |
| 1059 | |
| 1060 | If this type of key can be updated, then this method should be provided. |
| 1061 | It is called to update a key's payload from the blob of data provided. |
| 1062 | |
| 1063 | key_payload_reserve() should be called if the data length might change |
| 1064 | before any changes are actually made. Note that if this succeeds, the type |
| 1065 | is committed to changing the key because it's already been altered, so all |
| 1066 | memory allocation must be done first. |
| 1067 | |
| 1068 | The key will have its semaphore write-locked before this method is called, |
| 1069 | but this only deters other writers; any changes to the key's payload must |
| 1070 | be made under RCU conditions, and call_rcu() must be used to dispose of |
| 1071 | the old payload. |
| 1072 | |
| 1073 | key_payload_reserve() should be called before the changes are made, but |
| 1074 | after all allocations and other potentially failing function calls are |
| 1075 | made. |
| 1076 | |
| 1077 | It is safe to sleep in this method. |
| 1078 | |
| 1079 | |
| 1080 | (*) int (*match)(const struct key *key, const void *desc); |
| 1081 | |
| 1082 | This method is called to match a key against a description. It should |
| 1083 | return non-zero if the two match, zero if they don't. |
| 1084 | |
| 1085 | This method should not need to lock the key in any way. The type and |
| 1086 | description can be considered invariant, and the payload should not be |
| 1087 | accessed (the key may not yet be instantiated). |
| 1088 | |
| 1089 | It is not safe to sleep in this method; the caller may hold spinlocks. |
| 1090 | |
| 1091 | |
| 1092 | (*) void (*revoke)(struct key *key); |
| 1093 | |
| 1094 | This method is optional. It is called to discard part of the payload |
| 1095 | data upon a key being revoked. The caller will have the key semaphore |
| 1096 | write-locked. |
| 1097 | |
| 1098 | It is safe to sleep in this method, though care should be taken to avoid |
| 1099 | a deadlock against the key semaphore. |
| 1100 | |
| 1101 | |
| 1102 | (*) void (*destroy)(struct key *key); |
| 1103 | |
| 1104 | This method is optional. It is called to discard the payload data on a key |
| 1105 | when it is being destroyed. |
| 1106 | |
| 1107 | This method does not need to lock the key to access the payload; it can |
| 1108 | consider the key as being inaccessible at this time. Note that the key's |
| 1109 | type may have been changed before this function is called. |
| 1110 | |
| 1111 | It is not safe to sleep in this method; the caller may hold spinlocks. |
| 1112 | |
| 1113 | |
| 1114 | (*) void (*describe)(const struct key *key, struct seq_file *p); |
| 1115 | |
| 1116 | This method is optional. It is called during /proc/keys reading to |
| 1117 | summarise a key's description and payload in text form. |
| 1118 | |
| 1119 | This method will be called with the RCU read lock held. rcu_dereference() |
| 1120 | should be used to read the payload pointer if the payload is to be |
| 1121 | accessed. key->datalen cannot be trusted to stay consistent with the |
| 1122 | contents of the payload. |
| 1123 | |
| 1124 | The description will not change, though the key's state may. |
| 1125 | |
| 1126 | It is not safe to sleep in this method; the RCU read lock is held by the |
| 1127 | caller. |
| 1128 | |
| 1129 | |
| 1130 | (*) long (*read)(const struct key *key, char __user *buffer, size_t buflen); |
| 1131 | |
| 1132 | This method is optional. It is called by KEYCTL_READ to translate the |
| 1133 | key's payload into something a blob of data for userspace to deal with. |
| 1134 | Ideally, the blob should be in the same format as that passed in to the |
| 1135 | instantiate and update methods. |
| 1136 | |
| 1137 | If successful, the blob size that could be produced should be returned |
| 1138 | rather than the size copied. |
| 1139 | |
| 1140 | This method will be called with the key's semaphore read-locked. This will |
| 1141 | prevent the key's payload changing. It is not necessary to use RCU locking |
| 1142 | when accessing the key's payload. It is safe to sleep in this method, such |
| 1143 | as might happen when the userspace buffer is accessed. |
| 1144 | |
| 1145 | |
| 1146 | (*) int (*request_key)(struct key_construction *cons, const char *op, |
| 1147 | void *aux); |
| 1148 | |
| 1149 | This method is optional. If provided, request_key() and friends will |
| 1150 | invoke this function rather than upcalling to /sbin/request-key to operate |
| 1151 | upon a key of this type. |
| 1152 | |
| 1153 | The aux parameter is as passed to request_key_async_with_auxdata() and |
| 1154 | similar or is NULL otherwise. Also passed are the construction record for |
| 1155 | the key to be operated upon and the operation type (currently only |
| 1156 | "create"). |
| 1157 | |
| 1158 | This method is permitted to return before the upcall is complete, but the |
| 1159 | following function must be called under all circumstances to complete the |
| 1160 | instantiation process, whether or not it succeeds, whether or not there's |
| 1161 | an error: |
| 1162 | |
| 1163 | void complete_request_key(struct key_construction *cons, int error); |
| 1164 | |
| 1165 | The error parameter should be 0 on success, -ve on error. The |
| 1166 | construction record is destroyed by this action and the authorisation key |
| 1167 | will be revoked. If an error is indicated, the key under construction |
| 1168 | will be negatively instantiated if it wasn't already instantiated. |
| 1169 | |
| 1170 | If this method returns an error, that error will be returned to the |
| 1171 | caller of request_key*(). complete_request_key() must be called prior to |
| 1172 | returning. |
| 1173 | |
| 1174 | The key under construction and the authorisation key can be found in the |
| 1175 | key_construction struct pointed to by cons: |
| 1176 | |
| 1177 | (*) struct key *key; |
| 1178 | |
| 1179 | The key under construction. |
| 1180 | |
| 1181 | (*) struct key *authkey; |
| 1182 | |
| 1183 | The authorisation key. |
| 1184 | |
| 1185 | |
| 1186 | ============================ |
| 1187 | REQUEST-KEY CALLBACK SERVICE |
| 1188 | ============================ |
| 1189 | |
| 1190 | To create a new key, the kernel will attempt to execute the following command |
| 1191 | line: |
| 1192 | |
| 1193 | /sbin/request-key create <key> <uid> <gid> \ |
| 1194 | <threadring> <processring> <sessionring> <callout_info> |
| 1195 | |
| 1196 | <key> is the key being constructed, and the three keyrings are the process |
| 1197 | keyrings from the process that caused the search to be issued. These are |
| 1198 | included for two reasons: |
| 1199 | |
| 1200 | (1) There may be an authentication token in one of the keyrings that is |
| 1201 | required to obtain the key, eg: a Kerberos Ticket-Granting Ticket. |
| 1202 | |
| 1203 | (2) The new key should probably be cached in one of these rings. |
| 1204 | |
| 1205 | This program should set it UID and GID to those specified before attempting to |
| 1206 | access any more keys. It may then look around for a user specific process to |
| 1207 | hand the request off to (perhaps a path held in placed in another key by, for |
| 1208 | example, the KDE desktop manager). |
| 1209 | |
| 1210 | The program (or whatever it calls) should finish construction of the key by |
| 1211 | calling KEYCTL_INSTANTIATE, which also permits it to cache the key in one of |
| 1212 | the keyrings (probably the session ring) before returning. Alternatively, the |
| 1213 | key can be marked as negative with KEYCTL_NEGATE; this also permits the key to |
| 1214 | be cached in one of the keyrings. |
| 1215 | |
| 1216 | If it returns with the key remaining in the unconstructed state, the key will |
| 1217 | be marked as being negative, it will be added to the session keyring, and an |
| 1218 | error will be returned to the key requestor. |
| 1219 | |
| 1220 | Supplementary information may be provided from whoever or whatever invoked this |
| 1221 | service. This will be passed as the <callout_info> parameter. If no such |
| 1222 | information was made available, then "-" will be passed as this parameter |
| 1223 | instead. |
| 1224 | |
| 1225 | |
| 1226 | Similarly, the kernel may attempt to update an expired or a soon to expire key |
| 1227 | by executing: |
| 1228 | |
| 1229 | /sbin/request-key update <key> <uid> <gid> \ |
| 1230 | <threadring> <processring> <sessionring> |
| 1231 | |
| 1232 | In this case, the program isn't required to actually attach the key to a ring; |
| 1233 | the rings are provided for reference. |