1 ============================ 2 KERNEL KEY RETENTION SERVICE 3 ============================ 4 5This service allows cryptographic keys, authentication tokens, cross-domain 6user mappings, and similar to be cached in the kernel for the use of 7filesystems and other kernel services. 8 9Keyrings are permitted; these are a special type of key that can hold links to 10other keys. Processes each have three standard keyring subscriptions that a 11kernel service can search for relevant keys. 12 13The key service can be configured on by enabling: 14 15 "Security options"/"Enable access key retention support" (CONFIG_KEYS) 16 17This 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 - Garbage collection 30 31 32============ 33KEY OVERVIEW 34============ 35 36In this context, keys represent units of cryptographic data, authentication 37tokens, keyrings, etc.. These are represented in the kernel by struct key. 38 39Each 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 116Keys in the last three states are subject to garbage collection. See the 117section on "Garbage collection". 118 119 120==================== 121KEY SERVICE OVERVIEW 122==================== 123 124The key service provides a number of features besides keys: 125 126 (*) The key service defines three special key types: 127 128 (+) "keyring" 129 130 Keyrings are special keys that contain a list of other keys. Keyring 131 lists can be modified using various system calls. Keyrings should not 132 be given a payload when created. 133 134 (+) "user" 135 136 A key of this type has a description and a payload that are arbitrary 137 blobs of data. These can be created, updated and read by userspace, 138 and aren't intended for use by kernel services. 139 140 (+) "logon" 141 142 Like a "user" key, a "logon" key has a payload that is an arbitrary 143 blob of data. It is intended as a place to store secrets which are 144 accessible to the kernel but not to userspace programs. 145 146 The description can be arbitrary, but must be prefixed with a non-zero 147 length string that describes the key "subclass". The subclass is 148 separated from the rest of the description by a ':'. "logon" keys can 149 be created and updated from userspace, but the payload is only 150 readable from kernel space. 151 152 (*) Each process subscribes to three keyrings: a thread-specific keyring, a 153 process-specific keyring, and a session-specific keyring. 154 155 The thread-specific keyring is discarded from the child when any sort of 156 clone, fork, vfork or execve occurs. A new keyring is created only when 157 required. 158 159 The process-specific keyring is replaced with an empty one in the child on 160 clone, fork, vfork unless CLONE_THREAD is supplied, in which case it is 161 shared. execve also discards the process's process keyring and creates a 162 new one. 163 164 The session-specific keyring is persistent across clone, fork, vfork and 165 execve, even when the latter executes a set-UID or set-GID binary. A 166 process can, however, replace its current session keyring with a new one 167 by using PR_JOIN_SESSION_KEYRING. It is permitted to request an anonymous 168 new one, or to attempt to create or join one of a specific name. 169 170 The ownership of the thread keyring changes when the real UID and GID of 171 the thread changes. 172 173 (*) Each user ID resident in the system holds two special keyrings: a user 174 specific keyring and a default user session keyring. The default session 175 keyring is initialised with a link to the user-specific keyring. 176 177 When a process changes its real UID, if it used to have no session key, it 178 will be subscribed to the default session key for the new UID. 179 180 If a process attempts to access its session key when it doesn't have one, 181 it will be subscribed to the default for its current UID. 182 183 (*) Each user has two quotas against which the keys they own are tracked. One 184 limits the total number of keys and keyrings, the other limits the total 185 amount of description and payload space that can be consumed. 186 187 The user can view information on this and other statistics through procfs 188 files. The root user may also alter the quota limits through sysctl files 189 (see the section "New procfs files"). 190 191 Process-specific and thread-specific keyrings are not counted towards a 192 user's quota. 193 194 If a system call that modifies a key or keyring in some way would put the 195 user over quota, the operation is refused and error EDQUOT is returned. 196 197 (*) There's a system call interface by which userspace programs can create and 198 manipulate keys and keyrings. 199 200 (*) There's a kernel interface by which services can register types and search 201 for keys. 202 203 (*) There's a way for the a search done from the kernel to call back to 204 userspace to request a key that can't be found in a process's keyrings. 205 206 (*) An optional filesystem is available through which the key database can be 207 viewed and manipulated. 208 209 210====================== 211KEY ACCESS PERMISSIONS 212====================== 213 214Keys have an owner user ID, a group access ID, and a permissions mask. The mask 215has up to eight bits each for possessor, user, group and other access. Only 216six of each set of eight bits are defined. These permissions granted are: 217 218 (*) View 219 220 This permits a key or keyring's attributes to be viewed - including key 221 type and description. 222 223 (*) Read 224 225 This permits a key's payload to be viewed or a keyring's list of linked 226 keys. 227 228 (*) Write 229 230 This permits a key's payload to be instantiated or updated, or it allows a 231 link to be added to or removed from a keyring. 232 233 (*) Search 234 235 This permits keyrings to be searched and keys to be found. Searches can 236 only recurse into nested keyrings that have search permission set. 237 238 (*) Link 239 240 This permits a key or keyring to be linked to. To create a link from a 241 keyring to a key, a process must have Write permission on the keyring and 242 Link permission on the key. 243 244 (*) Set Attribute 245 246 This permits a key's UID, GID and permissions mask to be changed. 247 248For changing the ownership, group ID or permissions mask, being the owner of 249the key or having the sysadmin capability is sufficient. 250 251 252=============== 253SELINUX SUPPORT 254=============== 255 256The security class "key" has been added to SELinux so that mandatory access 257controls can be applied to keys created within various contexts. This support 258is preliminary, and is likely to change quite significantly in the near future. 259Currently, all of the basic permissions explained above are provided in SELinux 260as well; SELinux is simply invoked after all basic permission checks have been 261performed. 262 263The value of the file /proc/self/attr/keycreate influences the labeling of 264newly-created keys. If the contents of that file correspond to an SELinux 265security context, then the key will be assigned that context. Otherwise, the 266key will be assigned the current context of the task that invoked the key 267creation request. Tasks must be granted explicit permission to assign a 268particular context to newly-created keys, using the "create" permission in the 269key security class. 270 271The default keyrings associated with users will be labeled with the default 272context of the user if and only if the login programs have been instrumented to 273properly initialize keycreate during the login process. Otherwise, they will 274be labeled with the context of the login program itself. 275 276Note, however, that the default keyrings associated with the root user are 277labeled with the default kernel context, since they are created early in the 278boot process, before root has a chance to log in. 279 280The keyrings associated with new threads are each labeled with the context of 281their associated thread, and both session and process keyrings are handled 282similarly. 283 284 285================ 286NEW PROCFS FILES 287================ 288 289Two files have been added to procfs by which an administrator can find out 290about the status of the key service: 291 292 (*) /proc/keys 293 294 This lists the keys that are currently viewable by the task reading the 295 file, giving information about their type, description and permissions. 296 It is not possible to view the payload of the key this way, though some 297 information about it may be given. 298 299 The only keys included in the list are those that grant View permission to 300 the reading process whether or not it possesses them. Note that LSM 301 security checks are still performed, and may further filter out keys that 302 the current process is not authorised to view. 303 304 The contents of the file look like this: 305 306 SERIAL FLAGS USAGE EXPY PERM UID GID TYPE DESCRIPTION: SUMMARY 307 00000001 I----- 39 perm 1f3f0000 0 0 keyring _uid_ses.0: 1/4 308 00000002 I----- 2 perm 1f3f0000 0 0 keyring _uid.0: empty 309 00000007 I----- 1 perm 1f3f0000 0 0 keyring _pid.1: empty 310 0000018d I----- 1 perm 1f3f0000 0 0 keyring _pid.412: empty 311 000004d2 I--Q-- 1 perm 1f3f0000 32 -1 keyring _uid.32: 1/4 312 000004d3 I--Q-- 3 perm 1f3f0000 32 -1 keyring _uid_ses.32: empty 313 00000892 I--QU- 1 perm 1f000000 0 0 user metal:copper: 0 314 00000893 I--Q-N 1 35s 1f3f0000 0 0 user metal:silver: 0 315 00000894 I--Q-- 1 10h 003f0000 0 0 user metal:gold: 0 316 317 The flags are: 318 319 I Instantiated 320 R Revoked 321 D Dead 322 Q Contributes to user's quota 323 U Under construction by callback to userspace 324 N Negative key 325 326 327 (*) /proc/key-users 328 329 This file lists the tracking data for each user that has at least one key 330 on the system. Such data includes quota information and statistics: 331 332 [root@andromeda root]# cat /proc/key-users 333 0: 46 45/45 1/100 13/10000 334 29: 2 2/2 2/100 40/10000 335 32: 2 2/2 2/100 40/10000 336 38: 2 2/2 2/100 40/10000 337 338 The format of each line is 339 <UID>: User ID to which this applies 340 <usage> Structure refcount 341 <inst>/<keys> Total number of keys and number instantiated 342 <keys>/<max> Key count quota 343 <bytes>/<max> Key size quota 344 345 346Four new sysctl files have been added also for the purpose of controlling the 347quota limits on keys: 348 349 (*) /proc/sys/kernel/keys/root_maxkeys 350 /proc/sys/kernel/keys/root_maxbytes 351 352 These files hold the maximum number of keys that root may have and the 353 maximum total number of bytes of data that root may have stored in those 354 keys. 355 356 (*) /proc/sys/kernel/keys/maxkeys 357 /proc/sys/kernel/keys/maxbytes 358 359 These files hold the maximum number of keys that each non-root user may 360 have and the maximum total number of bytes of data that each of those 361 users may have stored in their keys. 362 363Root may alter these by writing each new limit as a decimal number string to 364the appropriate file. 365 366 367=============================== 368USERSPACE SYSTEM CALL INTERFACE 369=============================== 370 371Userspace can manipulate keys directly through three new syscalls: add_key, 372request_key and keyctl. The latter provides a number of functions for 373manipulating keys. 374 375When referring to a key directly, userspace programs should use the key's 376serial number (a positive 32-bit integer). However, there are some special 377values available for referring to special keys and keyrings that relate to the 378process making the call: 379 380 CONSTANT VALUE KEY REFERENCED 381 ============================== ====== =========================== 382 KEY_SPEC_THREAD_KEYRING -1 thread-specific keyring 383 KEY_SPEC_PROCESS_KEYRING -2 process-specific keyring 384 KEY_SPEC_SESSION_KEYRING -3 session-specific keyring 385 KEY_SPEC_USER_KEYRING -4 UID-specific keyring 386 KEY_SPEC_USER_SESSION_KEYRING -5 UID-session keyring 387 KEY_SPEC_GROUP_KEYRING -6 GID-specific keyring 388 KEY_SPEC_REQKEY_AUTH_KEY -7 assumed request_key() 389 authorisation key 390 391 392The main syscalls are: 393 394 (*) Create a new key of given type, description and payload and add it to the 395 nominated keyring: 396 397 key_serial_t add_key(const char *type, const char *desc, 398 const void *payload, size_t plen, 399 key_serial_t keyring); 400 401 If a key of the same type and description as that proposed already exists 402 in the keyring, this will try to update it with the given payload, or it 403 will return error EEXIST if that function is not supported by the key 404 type. The process must also have permission to write to the key to be able 405 to update it. The new key will have all user permissions granted and no 406 group or third party permissions. 407 408 Otherwise, this will attempt to create a new key of the specified type and 409 description, and to instantiate it with the supplied payload and attach it 410 to the keyring. In this case, an error will be generated if the process 411 does not have permission to write to the keyring. 412 413 If the key type supports it, if the description is NULL or an empty 414 string, the key type will try and generate a description from the content 415 of the payload. 416 417 The payload is optional, and the pointer can be NULL if not required by 418 the type. The payload is plen in size, and plen can be zero for an empty 419 payload. 420 421 A new keyring can be generated by setting type "keyring", the keyring name 422 as the description (or NULL) and setting the payload to NULL. 423 424 User defined keys can be created by specifying type "user". It is 425 recommended that a user defined key's description by prefixed with a type 426 ID and a colon, such as "krb5tgt:" for a Kerberos 5 ticket granting 427 ticket. 428 429 Any other type must have been registered with the kernel in advance by a 430 kernel service such as a filesystem. 431 432 The ID of the new or updated key is returned if successful. 433 434 435 (*) Search the process's keyrings for a key, potentially calling out to 436 userspace to create it. 437 438 key_serial_t request_key(const char *type, const char *description, 439 const char *callout_info, 440 key_serial_t dest_keyring); 441 442 This function searches all the process's keyrings in the order thread, 443 process, session for a matching key. This works very much like 444 KEYCTL_SEARCH, including the optional attachment of the discovered key to 445 a keyring. 446 447 If a key cannot be found, and if callout_info is not NULL, then 448 /sbin/request-key will be invoked in an attempt to obtain a key. The 449 callout_info string will be passed as an argument to the program. 450 451 See also Documentation/security/keys-request-key.txt. 452 453 454The keyctl syscall functions are: 455 456 (*) Map a special key ID to a real key ID for this process: 457 458 key_serial_t keyctl(KEYCTL_GET_KEYRING_ID, key_serial_t id, 459 int create); 460 461 The special key specified by "id" is looked up (with the key being created 462 if necessary) and the ID of the key or keyring thus found is returned if 463 it exists. 464 465 If the key does not yet exist, the key will be created if "create" is 466 non-zero; and the error ENOKEY will be returned if "create" is zero. 467 468 469 (*) Replace the session keyring this process subscribes to with a new one: 470 471 key_serial_t keyctl(KEYCTL_JOIN_SESSION_KEYRING, const char *name); 472 473 If name is NULL, an anonymous keyring is created attached to the process 474 as its session keyring, displacing the old session keyring. 475 476 If name is not NULL, if a keyring of that name exists, the process 477 attempts to attach it as the session keyring, returning an error if that 478 is not permitted; otherwise a new keyring of that name is created and 479 attached as the session keyring. 480 481 To attach to a named keyring, the keyring must have search permission for 482 the process's ownership. 483 484 The ID of the new session keyring is returned if successful. 485 486 487 (*) Update the specified key: 488 489 long keyctl(KEYCTL_UPDATE, key_serial_t key, const void *payload, 490 size_t plen); 491 492 This will try to update the specified key with the given payload, or it 493 will return error EOPNOTSUPP if that function is not supported by the key 494 type. The process must also have permission to write to the key to be able 495 to update it. 496 497 The payload is of length plen, and may be absent or empty as for 498 add_key(). 499 500 501 (*) Revoke a key: 502 503 long keyctl(KEYCTL_REVOKE, key_serial_t key); 504 505 This makes a key unavailable for further operations. Further attempts to 506 use the key will be met with error EKEYREVOKED, and the key will no longer 507 be findable. 508 509 510 (*) Change the ownership of a key: 511 512 long keyctl(KEYCTL_CHOWN, key_serial_t key, uid_t uid, gid_t gid); 513 514 This function permits a key's owner and group ID to be changed. Either one 515 of uid or gid can be set to -1 to suppress that change. 516 517 Only the superuser can change a key's owner to something other than the 518 key's current owner. Similarly, only the superuser can change a key's 519 group ID to something other than the calling process's group ID or one of 520 its group list members. 521 522 523 (*) Change the permissions mask on a key: 524 525 long keyctl(KEYCTL_SETPERM, key_serial_t key, key_perm_t perm); 526 527 This function permits the owner of a key or the superuser to change the 528 permissions mask on a key. 529 530 Only bits the available bits are permitted; if any other bits are set, 531 error EINVAL will be returned. 532 533 534 (*) Describe a key: 535 536 long keyctl(KEYCTL_DESCRIBE, key_serial_t key, char *buffer, 537 size_t buflen); 538 539 This function returns a summary of the key's attributes (but not its 540 payload data) as a string in the buffer provided. 541 542 Unless there's an error, it always returns the amount of data it could 543 produce, even if that's too big for the buffer, but it won't copy more 544 than requested to userspace. If the buffer pointer is NULL then no copy 545 will take place. 546 547 A process must have view permission on the key for this function to be 548 successful. 549 550 If successful, a string is placed in the buffer in the following format: 551 552 <type>;<uid>;<gid>;<perm>;<description> 553 554 Where type and description are strings, uid and gid are decimal, and perm 555 is hexadecimal. A NUL character is included at the end of the string if 556 the buffer is sufficiently big. 557 558 This can be parsed with 559 560 sscanf(buffer, "%[^;];%d;%d;%o;%s", type, &uid, &gid, &mode, desc); 561 562 563 (*) Clear out a keyring: 564 565 long keyctl(KEYCTL_CLEAR, key_serial_t keyring); 566 567 This function clears the list of keys attached to a keyring. The calling 568 process must have write permission on the keyring, and it must be a 569 keyring (or else error ENOTDIR will result). 570 571 This function can also be used to clear special kernel keyrings if they 572 are appropriately marked if the user has CAP_SYS_ADMIN capability. The 573 DNS resolver cache keyring is an example of this. 574 575 576 (*) Link a key into a keyring: 577 578 long keyctl(KEYCTL_LINK, key_serial_t keyring, key_serial_t key); 579 580 This function creates a link from the keyring to the key. The process must 581 have write permission on the keyring and must have link permission on the 582 key. 583 584 Should the keyring not be a keyring, error ENOTDIR will result; and if the 585 keyring is full, error ENFILE will result. 586 587 The link procedure checks the nesting of the keyrings, returning ELOOP if 588 it appears too deep or EDEADLK if the link would introduce a cycle. 589 590 Any links within the keyring to keys that match the new key in terms of 591 type and description will be discarded from the keyring as the new one is 592 added. 593 594 595 (*) Unlink a key or keyring from another keyring: 596 597 long keyctl(KEYCTL_UNLINK, key_serial_t keyring, key_serial_t key); 598 599 This function looks through the keyring for the first link to the 600 specified key, and removes it if found. Subsequent links to that key are 601 ignored. The process must have write permission on the keyring. 602 603 If the keyring is not a keyring, error ENOTDIR will result; and if the key 604 is not present, error ENOENT will be the result. 605 606 607 (*) Search a keyring tree for a key: 608 609 key_serial_t keyctl(KEYCTL_SEARCH, key_serial_t keyring, 610 const char *type, const char *description, 611 key_serial_t dest_keyring); 612 613 This searches the keyring tree headed by the specified keyring until a key 614 is found that matches the type and description criteria. Each keyring is 615 checked for keys before recursion into its children occurs. 616 617 The process must have search permission on the top level keyring, or else 618 error EACCES will result. Only keyrings that the process has search 619 permission on will be recursed into, and only keys and keyrings for which 620 a process has search permission can be matched. If the specified keyring 621 is not a keyring, ENOTDIR will result. 622 623 If the search succeeds, the function will attempt to link the found key 624 into the destination keyring if one is supplied (non-zero ID). All the 625 constraints applicable to KEYCTL_LINK apply in this case too. 626 627 Error ENOKEY, EKEYREVOKED or EKEYEXPIRED will be returned if the search 628 fails. On success, the resulting key ID will be returned. 629 630 631 (*) Read the payload data from a key: 632 633 long keyctl(KEYCTL_READ, key_serial_t keyring, char *buffer, 634 size_t buflen); 635 636 This function attempts to read the payload data from the specified key 637 into the buffer. The process must have read permission on the key to 638 succeed. 639 640 The returned data will be processed for presentation by the key type. For 641 instance, a keyring will return an array of key_serial_t entries 642 representing the IDs of all the keys to which it is subscribed. The user 643 defined key type will return its data as is. If a key type does not 644 implement this function, error EOPNOTSUPP will result. 645 646 As much of the data as can be fitted into the buffer will be copied to 647 userspace if the buffer pointer is not NULL. 648 649 On a successful return, the function will always return the amount of data 650 available rather than the amount copied. 651 652 653 (*) Instantiate a partially constructed key. 654 655 long keyctl(KEYCTL_INSTANTIATE, key_serial_t key, 656 const void *payload, size_t plen, 657 key_serial_t keyring); 658 long keyctl(KEYCTL_INSTANTIATE_IOV, key_serial_t key, 659 const struct iovec *payload_iov, unsigned ioc, 660 key_serial_t keyring); 661 662 If the kernel calls back to userspace to complete the instantiation of a 663 key, userspace should use this call to supply data for the key before the 664 invoked process returns, or else the key will be marked negative 665 automatically. 666 667 The process must have write access on the key to be able to instantiate 668 it, and the key must be uninstantiated. 669 670 If a keyring is specified (non-zero), the key will also be linked into 671 that keyring, however all the constraints applying in KEYCTL_LINK apply in 672 this case too. 673 674 The payload and plen arguments describe the payload data as for add_key(). 675 676 The payload_iov and ioc arguments describe the payload data in an iovec 677 array instead of a single buffer. 678 679 680 (*) Negatively instantiate a partially constructed key. 681 682 long keyctl(KEYCTL_NEGATE, key_serial_t key, 683 unsigned timeout, key_serial_t keyring); 684 long keyctl(KEYCTL_REJECT, key_serial_t key, 685 unsigned timeout, unsigned error, key_serial_t keyring); 686 687 If the kernel calls back to userspace to complete the instantiation of a 688 key, userspace should use this call mark the key as negative before the 689 invoked process returns if it is unable to fulfill the request. 690 691 The process must have write access on the key to be able to instantiate 692 it, and the key must be uninstantiated. 693 694 If a keyring is specified (non-zero), the key will also be linked into 695 that keyring, however all the constraints applying in KEYCTL_LINK apply in 696 this case too. 697 698 If the key is rejected, future searches for it will return the specified 699 error code until the rejected key expires. Negating the key is the same 700 as rejecting the key with ENOKEY as the error code. 701 702 703 (*) Set the default request-key destination keyring. 704 705 long keyctl(KEYCTL_SET_REQKEY_KEYRING, int reqkey_defl); 706 707 This sets the default keyring to which implicitly requested keys will be 708 attached for this thread. reqkey_defl should be one of these constants: 709 710 CONSTANT VALUE NEW DEFAULT KEYRING 711 ====================================== ====== ======================= 712 KEY_REQKEY_DEFL_NO_CHANGE -1 No change 713 KEY_REQKEY_DEFL_DEFAULT 0 Default[1] 714 KEY_REQKEY_DEFL_THREAD_KEYRING 1 Thread keyring 715 KEY_REQKEY_DEFL_PROCESS_KEYRING 2 Process keyring 716 KEY_REQKEY_DEFL_SESSION_KEYRING 3 Session keyring 717 KEY_REQKEY_DEFL_USER_KEYRING 4 User keyring 718 KEY_REQKEY_DEFL_USER_SESSION_KEYRING 5 User session keyring 719 KEY_REQKEY_DEFL_GROUP_KEYRING 6 Group keyring 720 721 The old default will be returned if successful and error EINVAL will be 722 returned if reqkey_defl is not one of the above values. 723 724 The default keyring can be overridden by the keyring indicated to the 725 request_key() system call. 726 727 Note that this setting is inherited across fork/exec. 728 729 [1] The default is: the thread keyring if there is one, otherwise 730 the process keyring if there is one, otherwise the session keyring if 731 there is one, otherwise the user default session keyring. 732 733 734 (*) Set the timeout on a key. 735 736 long keyctl(KEYCTL_SET_TIMEOUT, key_serial_t key, unsigned timeout); 737 738 This sets or clears the timeout on a key. The timeout can be 0 to clear 739 the timeout or a number of seconds to set the expiry time that far into 740 the future. 741 742 The process must have attribute modification access on a key to set its 743 timeout. Timeouts may not be set with this function on negative, revoked 744 or expired keys. 745 746 747 (*) Assume the authority granted to instantiate a key 748 749 long keyctl(KEYCTL_ASSUME_AUTHORITY, key_serial_t key); 750 751 This assumes or divests the authority required to instantiate the 752 specified key. Authority can only be assumed if the thread has the 753 authorisation key associated with the specified key in its keyrings 754 somewhere. 755 756 Once authority is assumed, searches for keys will also search the 757 requester's keyrings using the requester's security label, UID, GID and 758 groups. 759 760 If the requested authority is unavailable, error EPERM will be returned, 761 likewise if the authority has been revoked because the target key is 762 already instantiated. 763 764 If the specified key is 0, then any assumed authority will be divested. 765 766 The assumed authoritative key is inherited across fork and exec. 767 768 769 (*) Get the LSM security context attached to a key. 770 771 long keyctl(KEYCTL_GET_SECURITY, key_serial_t key, char *buffer, 772 size_t buflen) 773 774 This function returns a string that represents the LSM security context 775 attached to a key in the buffer provided. 776 777 Unless there's an error, it always returns the amount of data it could 778 produce, even if that's too big for the buffer, but it won't copy more 779 than requested to userspace. If the buffer pointer is NULL then no copy 780 will take place. 781 782 A NUL character is included at the end of the string if the buffer is 783 sufficiently big. This is included in the returned count. If no LSM is 784 in force then an empty string will be returned. 785 786 A process must have view permission on the key for this function to be 787 successful. 788 789 790 (*) Install the calling process's session keyring on its parent. 791 792 long keyctl(KEYCTL_SESSION_TO_PARENT); 793 794 This functions attempts to install the calling process's session keyring 795 on to the calling process's parent, replacing the parent's current session 796 keyring. 797 798 The calling process must have the same ownership as its parent, the 799 keyring must have the same ownership as the calling process, the calling 800 process must have LINK permission on the keyring and the active LSM module 801 mustn't deny permission, otherwise error EPERM will be returned. 802 803 Error ENOMEM will be returned if there was insufficient memory to complete 804 the operation, otherwise 0 will be returned to indicate success. 805 806 The keyring will be replaced next time the parent process leaves the 807 kernel and resumes executing userspace. 808 809 810 (*) Invalidate a key. 811 812 long keyctl(KEYCTL_INVALIDATE, key_serial_t key); 813 814 This function marks a key as being invalidated and then wakes up the 815 garbage collector. The garbage collector immediately removes invalidated 816 keys from all keyrings and deletes the key when its reference count 817 reaches zero. 818 819 Keys that are marked invalidated become invisible to normal key operations 820 immediately, though they are still visible in /proc/keys until deleted 821 (they're marked with an 'i' flag). 822 823 A process must have search permission on the key for this function to be 824 successful. 825 826 827=============== 828KERNEL SERVICES 829=============== 830 831The kernel services for key management are fairly simple to deal with. They can 832be broken down into two areas: keys and key types. 833 834Dealing with keys is fairly straightforward. Firstly, the kernel service 835registers its type, then it searches for a key of that type. It should retain 836the key as long as it has need of it, and then it should release it. For a 837filesystem or device file, a search would probably be performed during the open 838call, and the key released upon close. How to deal with conflicting keys due to 839two different users opening the same file is left to the filesystem author to 840solve. 841 842To access the key manager, the following header must be #included: 843 844 <linux/key.h> 845 846Specific key types should have a header file under include/keys/ that should be 847used to access that type. For keys of type "user", for example, that would be: 848 849 <keys/user-type.h> 850 851Note that there are two different types of pointers to keys that may be 852encountered: 853 854 (*) struct key * 855 856 This simply points to the key structure itself. Key structures will be at 857 least four-byte aligned. 858 859 (*) key_ref_t 860 861 This is equivalent to a struct key *, but the least significant bit is set 862 if the caller "possesses" the key. By "possession" it is meant that the 863 calling processes has a searchable link to the key from one of its 864 keyrings. There are three functions for dealing with these: 865 866 key_ref_t make_key_ref(const struct key *key, bool possession); 867 868 struct key *key_ref_to_ptr(const key_ref_t key_ref); 869 870 bool is_key_possessed(const key_ref_t key_ref); 871 872 The first function constructs a key reference from a key pointer and 873 possession information (which must be true or false). 874 875 The second function retrieves the key pointer from a reference and the 876 third retrieves the possession flag. 877 878When accessing a key's payload contents, certain precautions must be taken to 879prevent access vs modification races. See the section "Notes on accessing 880payload contents" for more information. 881 882(*) To search for a key, call: 883 884 struct key *request_key(const struct key_type *type, 885 const char *description, 886 const char *callout_info); 887 888 This is used to request a key or keyring with a description that matches 889 the description specified according to the key type's match_preparse() 890 method. This permits approximate matching to occur. If callout_string is 891 not NULL, then /sbin/request-key will be invoked in an attempt to obtain 892 the key from userspace. In that case, callout_string will be passed as an 893 argument to the program. 894 895 Should the function fail error ENOKEY, EKEYEXPIRED or EKEYREVOKED will be 896 returned. 897 898 If successful, the key will have been attached to the default keyring for 899 implicitly obtained request-key keys, as set by KEYCTL_SET_REQKEY_KEYRING. 900 901 See also Documentation/security/keys-request-key.txt. 902 903 904(*) To search for a key, passing auxiliary data to the upcaller, call: 905 906 struct key *request_key_with_auxdata(const struct key_type *type, 907 const char *description, 908 const void *callout_info, 909 size_t callout_len, 910 void *aux); 911 912 This is identical to request_key(), except that the auxiliary data is 913 passed to the key_type->request_key() op if it exists, and the callout_info 914 is a blob of length callout_len, if given (the length may be 0). 915 916 917(*) A key can be requested asynchronously by calling one of: 918 919 struct key *request_key_async(const struct key_type *type, 920 const char *description, 921 const void *callout_info, 922 size_t callout_len); 923 924 or: 925 926 struct key *request_key_async_with_auxdata(const struct key_type *type, 927 const char *description, 928 const char *callout_info, 929 size_t callout_len, 930 void *aux); 931 932 which are asynchronous equivalents of request_key() and 933 request_key_with_auxdata() respectively. 934 935 These two functions return with the key potentially still under 936 construction. To wait for construction completion, the following should be 937 called: 938 939 int wait_for_key_construction(struct key *key, bool intr); 940 941 The function will wait for the key to finish being constructed and then 942 invokes key_validate() to return an appropriate value to indicate the state 943 of the key (0 indicates the key is usable). 944 945 If intr is true, then the wait can be interrupted by a signal, in which 946 case error ERESTARTSYS will be returned. 947 948 949(*) When it is no longer required, the key should be released using: 950 951 void key_put(struct key *key); 952 953 Or: 954 955 void key_ref_put(key_ref_t key_ref); 956 957 These can be called from interrupt context. If CONFIG_KEYS is not set then 958 the argument will not be parsed. 959 960 961(*) Extra references can be made to a key by calling one of the following 962 functions: 963 964 struct key *__key_get(struct key *key); 965 struct key *key_get(struct key *key); 966 967 Keys so references will need to be disposed of by calling key_put() when 968 they've been finished with. The key pointer passed in will be returned. 969 970 In the case of key_get(), if the pointer is NULL or CONFIG_KEYS is not set 971 then the key will not be dereferenced and no increment will take place. 972 973 974(*) A key's serial number can be obtained by calling: 975 976 key_serial_t key_serial(struct key *key); 977 978 If key is NULL or if CONFIG_KEYS is not set then 0 will be returned (in the 979 latter case without parsing the argument). 980 981 982(*) If a keyring was found in the search, this can be further searched by: 983 984 key_ref_t keyring_search(key_ref_t keyring_ref, 985 const struct key_type *type, 986 const char *description) 987 988 This searches the keyring tree specified for a matching key. Error ENOKEY 989 is returned upon failure (use IS_ERR/PTR_ERR to determine). If successful, 990 the returned key will need to be released. 991 992 The possession attribute from the keyring reference is used to control 993 access through the permissions mask and is propagated to the returned key 994 reference pointer if successful. 995 996 997(*) A keyring can be created by: 998 999 struct key *keyring_alloc(const char *description, uid_t uid, gid_t gid, 1000 const struct cred *cred, 1001 key_perm_t perm, 1002 unsigned long flags, 1003 struct key *dest); 1004 1005 This creates a keyring with the given attributes and returns it. If dest 1006 is not NULL, the new keyring will be linked into the keyring to which it 1007 points. No permission checks are made upon the destination keyring. 1008 1009 Error EDQUOT can be returned if the keyring would overload the quota (pass 1010 KEY_ALLOC_NOT_IN_QUOTA in flags if the keyring shouldn't be accounted 1011 towards the user's quota). Error ENOMEM can also be returned. 1012 1013 1014(*) To check the validity of a key, this function can be called: 1015 1016 int validate_key(struct key *key); 1017 1018 This checks that the key in question hasn't expired or and hasn't been 1019 revoked. Should the key be invalid, error EKEYEXPIRED or EKEYREVOKED will 1020 be returned. If the key is NULL or if CONFIG_KEYS is not set then 0 will be 1021 returned (in the latter case without parsing the argument). 1022 1023 1024(*) To register a key type, the following function should be called: 1025 1026 int register_key_type(struct key_type *type); 1027 1028 This will return error EEXIST if a type of the same name is already 1029 present. 1030 1031 1032(*) To unregister a key type, call: 1033 1034 void unregister_key_type(struct key_type *type); 1035 1036 1037Under some circumstances, it may be desirable to deal with a bundle of keys. 1038The facility provides access to the keyring type for managing such a bundle: 1039 1040 struct key_type key_type_keyring; 1041 1042This can be used with a function such as request_key() to find a specific 1043keyring in a process's keyrings. A keyring thus found can then be searched 1044with keyring_search(). Note that it is not possible to use request_key() to 1045search a specific keyring, so using keyrings in this way is of limited utility. 1046 1047 1048=================================== 1049NOTES ON ACCESSING PAYLOAD CONTENTS 1050=================================== 1051 1052The simplest payload is just data stored in key->payload directly. In this 1053case, there's no need to indulge in RCU or locking when accessing the payload. 1054 1055More complex payload contents must be allocated and pointers to them set in the 1056key->payload.data[] array. One of the following ways must be selected to 1057access the data: 1058 1059 (1) Unmodifiable key type. 1060 1061 If the key type does not have a modify method, then the key's payload can 1062 be accessed without any form of locking, provided that it's known to be 1063 instantiated (uninstantiated keys cannot be "found"). 1064 1065 (2) The key's semaphore. 1066 1067 The semaphore could be used to govern access to the payload and to control 1068 the payload pointer. It must be write-locked for modifications and would 1069 have to be read-locked for general access. The disadvantage of doing this 1070 is that the accessor may be required to sleep. 1071 1072 (3) RCU. 1073 1074 RCU must be used when the semaphore isn't already held; if the semaphore 1075 is held then the contents can't change under you unexpectedly as the 1076 semaphore must still be used to serialise modifications to the key. The 1077 key management code takes care of this for the key type. 1078 1079 However, this means using: 1080 1081 rcu_read_lock() ... rcu_dereference() ... rcu_read_unlock() 1082 1083 to read the pointer, and: 1084 1085 rcu_dereference() ... rcu_assign_pointer() ... call_rcu() 1086 1087 to set the pointer and dispose of the old contents after a grace period. 1088 Note that only the key type should ever modify a key's payload. 1089 1090 Furthermore, an RCU controlled payload must hold a struct rcu_head for the 1091 use of call_rcu() and, if the payload is of variable size, the length of 1092 the payload. key->datalen cannot be relied upon to be consistent with the 1093 payload just dereferenced if the key's semaphore is not held. 1094 1095 Note that key->payload.data[0] has a shadow that is marked for __rcu 1096 usage. This is called key->payload.rcu_data0. The following accessors 1097 wrap the RCU calls to this element: 1098 1099 rcu_assign_keypointer(struct key *key, void *data); 1100 void *rcu_dereference_key(struct key *key); 1101 1102 1103=================== 1104DEFINING A KEY TYPE 1105=================== 1106 1107A kernel service may want to define its own key type. For instance, an AFS 1108filesystem might want to define a Kerberos 5 ticket key type. To do this, it 1109author fills in a key_type struct and registers it with the system. 1110 1111Source files that implement key types should include the following header file: 1112 1113 <linux/key-type.h> 1114 1115The structure has a number of fields, some of which are mandatory: 1116 1117 (*) const char *name 1118 1119 The name of the key type. This is used to translate a key type name 1120 supplied by userspace into a pointer to the structure. 1121 1122 1123 (*) size_t def_datalen 1124 1125 This is optional - it supplies the default payload data length as 1126 contributed to the quota. If the key type's payload is always or almost 1127 always the same size, then this is a more efficient way to do things. 1128 1129 The data length (and quota) on a particular key can always be changed 1130 during instantiation or update by calling: 1131 1132 int key_payload_reserve(struct key *key, size_t datalen); 1133 1134 With the revised data length. Error EDQUOT will be returned if this is not 1135 viable. 1136 1137 1138 (*) int (*vet_description)(const char *description); 1139 1140 This optional method is called to vet a key description. If the key type 1141 doesn't approve of the key description, it may return an error, otherwise 1142 it should return 0. 1143 1144 1145 (*) int (*preparse)(struct key_preparsed_payload *prep); 1146 1147 This optional method permits the key type to attempt to parse payload 1148 before a key is created (add key) or the key semaphore is taken (update or 1149 instantiate key). The structure pointed to by prep looks like: 1150 1151 struct key_preparsed_payload { 1152 char *description; 1153 union key_payload payload; 1154 const void *data; 1155 size_t datalen; 1156 size_t quotalen; 1157 time_t expiry; 1158 }; 1159 1160 Before calling the method, the caller will fill in data and datalen with 1161 the payload blob parameters; quotalen will be filled in with the default 1162 quota size from the key type; expiry will be set to TIME_T_MAX and the 1163 rest will be cleared. 1164 1165 If a description can be proposed from the payload contents, that should be 1166 attached as a string to the description field. This will be used for the 1167 key description if the caller of add_key() passes NULL or "". 1168 1169 The method can attach anything it likes to payload. This is merely passed 1170 along to the instantiate() or update() operations. If set, the expiry 1171 time will be applied to the key if it is instantiated from this data. 1172 1173 The method should return 0 if successful or a negative error code 1174 otherwise. 1175 1176 1177 (*) void (*free_preparse)(struct key_preparsed_payload *prep); 1178 1179 This method is only required if the preparse() method is provided, 1180 otherwise it is unused. It cleans up anything attached to the description 1181 and payload fields of the key_preparsed_payload struct as filled in by the 1182 preparse() method. It will always be called after preparse() returns 1183 successfully, even if instantiate() or update() succeed. 1184 1185 1186 (*) int (*instantiate)(struct key *key, struct key_preparsed_payload *prep); 1187 1188 This method is called to attach a payload to a key during construction. 1189 The payload attached need not bear any relation to the data passed to this 1190 function. 1191 1192 The prep->data and prep->datalen fields will define the original payload 1193 blob. If preparse() was supplied then other fields may be filled in also. 1194 1195 If the amount of data attached to the key differs from the size in 1196 keytype->def_datalen, then key_payload_reserve() should be called. 1197 1198 This method does not have to lock the key in order to attach a payload. 1199 The fact that KEY_FLAG_INSTANTIATED is not set in key->flags prevents 1200 anything else from gaining access to the key. 1201 1202 It is safe to sleep in this method. 1203 1204 generic_key_instantiate() is provided to simply copy the data from 1205 prep->payload.data[] to key->payload.data[], with RCU-safe assignment on 1206 the first element. It will then clear prep->payload.data[] so that the 1207 free_preparse method doesn't release the data. 1208 1209 1210 (*) int (*update)(struct key *key, const void *data, size_t datalen); 1211 1212 If this type of key can be updated, then this method should be provided. 1213 It is called to update a key's payload from the blob of data provided. 1214 1215 The prep->data and prep->datalen fields will define the original payload 1216 blob. If preparse() was supplied then other fields may be filled in also. 1217 1218 key_payload_reserve() should be called if the data length might change 1219 before any changes are actually made. Note that if this succeeds, the type 1220 is committed to changing the key because it's already been altered, so all 1221 memory allocation must be done first. 1222 1223 The key will have its semaphore write-locked before this method is called, 1224 but this only deters other writers; any changes to the key's payload must 1225 be made under RCU conditions, and call_rcu() must be used to dispose of 1226 the old payload. 1227 1228 key_payload_reserve() should be called before the changes are made, but 1229 after all allocations and other potentially failing function calls are 1230 made. 1231 1232 It is safe to sleep in this method. 1233 1234 1235 (*) int (*match_preparse)(struct key_match_data *match_data); 1236 1237 This method is optional. It is called when a key search is about to be 1238 performed. It is given the following structure: 1239 1240 struct key_match_data { 1241 bool (*cmp)(const struct key *key, 1242 const struct key_match_data *match_data); 1243 const void *raw_data; 1244 void *preparsed; 1245 unsigned lookup_type; 1246 }; 1247 1248 On entry, raw_data will be pointing to the criteria to be used in matching 1249 a key by the caller and should not be modified. (*cmp)() will be pointing 1250 to the default matcher function (which does an exact description match 1251 against raw_data) and lookup_type will be set to indicate a direct lookup. 1252 1253 The following lookup_type values are available: 1254 1255 [*] KEYRING_SEARCH_LOOKUP_DIRECT - A direct lookup hashes the type and 1256 description to narrow down the search to a small number of keys. 1257 1258 [*] KEYRING_SEARCH_LOOKUP_ITERATE - An iterative lookup walks all the 1259 keys in the keyring until one is matched. This must be used for any 1260 search that's not doing a simple direct match on the key description. 1261 1262 The method may set cmp to point to a function of its choice that does some 1263 other form of match, may set lookup_type to KEYRING_SEARCH_LOOKUP_ITERATE 1264 and may attach something to the preparsed pointer for use by (*cmp)(). 1265 (*cmp)() should return true if a key matches and false otherwise. 1266 1267 If preparsed is set, it may be necessary to use the match_free() method to 1268 clean it up. 1269 1270 The method should return 0 if successful or a negative error code 1271 otherwise. 1272 1273 It is permitted to sleep in this method, but (*cmp)() may not sleep as 1274 locks will be held over it. 1275 1276 If match_preparse() is not provided, keys of this type will be matched 1277 exactly by their description. 1278 1279 1280 (*) void (*match_free)(struct key_match_data *match_data); 1281 1282 This method is optional. If given, it called to clean up 1283 match_data->preparsed after a successful call to match_preparse(). 1284 1285 1286 (*) void (*revoke)(struct key *key); 1287 1288 This method is optional. It is called to discard part of the payload 1289 data upon a key being revoked. The caller will have the key semaphore 1290 write-locked. 1291 1292 It is safe to sleep in this method, though care should be taken to avoid 1293 a deadlock against the key semaphore. 1294 1295 1296 (*) void (*destroy)(struct key *key); 1297 1298 This method is optional. It is called to discard the payload data on a key 1299 when it is being destroyed. 1300 1301 This method does not need to lock the key to access the payload; it can 1302 consider the key as being inaccessible at this time. Note that the key's 1303 type may have been changed before this function is called. 1304 1305 It is not safe to sleep in this method; the caller may hold spinlocks. 1306 1307 1308 (*) void (*describe)(const struct key *key, struct seq_file *p); 1309 1310 This method is optional. It is called during /proc/keys reading to 1311 summarise a key's description and payload in text form. 1312 1313 This method will be called with the RCU read lock held. rcu_dereference() 1314 should be used to read the payload pointer if the payload is to be 1315 accessed. key->datalen cannot be trusted to stay consistent with the 1316 contents of the payload. 1317 1318 The description will not change, though the key's state may. 1319 1320 It is not safe to sleep in this method; the RCU read lock is held by the 1321 caller. 1322 1323 1324 (*) long (*read)(const struct key *key, char __user *buffer, size_t buflen); 1325 1326 This method is optional. It is called by KEYCTL_READ to translate the 1327 key's payload into something a blob of data for userspace to deal with. 1328 Ideally, the blob should be in the same format as that passed in to the 1329 instantiate and update methods. 1330 1331 If successful, the blob size that could be produced should be returned 1332 rather than the size copied. 1333 1334 This method will be called with the key's semaphore read-locked. This will 1335 prevent the key's payload changing. It is not necessary to use RCU locking 1336 when accessing the key's payload. It is safe to sleep in this method, such 1337 as might happen when the userspace buffer is accessed. 1338 1339 1340 (*) int (*request_key)(struct key_construction *cons, const char *op, 1341 void *aux); 1342 1343 This method is optional. If provided, request_key() and friends will 1344 invoke this function rather than upcalling to /sbin/request-key to operate 1345 upon a key of this type. 1346 1347 The aux parameter is as passed to request_key_async_with_auxdata() and 1348 similar or is NULL otherwise. Also passed are the construction record for 1349 the key to be operated upon and the operation type (currently only 1350 "create"). 1351 1352 This method is permitted to return before the upcall is complete, but the 1353 following function must be called under all circumstances to complete the 1354 instantiation process, whether or not it succeeds, whether or not there's 1355 an error: 1356 1357 void complete_request_key(struct key_construction *cons, int error); 1358 1359 The error parameter should be 0 on success, -ve on error. The 1360 construction record is destroyed by this action and the authorisation key 1361 will be revoked. If an error is indicated, the key under construction 1362 will be negatively instantiated if it wasn't already instantiated. 1363 1364 If this method returns an error, that error will be returned to the 1365 caller of request_key*(). complete_request_key() must be called prior to 1366 returning. 1367 1368 The key under construction and the authorisation key can be found in the 1369 key_construction struct pointed to by cons: 1370 1371 (*) struct key *key; 1372 1373 The key under construction. 1374 1375 (*) struct key *authkey; 1376 1377 The authorisation key. 1378 1379 1380============================ 1381REQUEST-KEY CALLBACK SERVICE 1382============================ 1383 1384To create a new key, the kernel will attempt to execute the following command 1385line: 1386 1387 /sbin/request-key create <key> <uid> <gid> \ 1388 <threadring> <processring> <sessionring> <callout_info> 1389 1390<key> is the key being constructed, and the three keyrings are the process 1391keyrings from the process that caused the search to be issued. These are 1392included for two reasons: 1393 1394 (1) There may be an authentication token in one of the keyrings that is 1395 required to obtain the key, eg: a Kerberos Ticket-Granting Ticket. 1396 1397 (2) The new key should probably be cached in one of these rings. 1398 1399This program should set it UID and GID to those specified before attempting to 1400access any more keys. It may then look around for a user specific process to 1401hand the request off to (perhaps a path held in placed in another key by, for 1402example, the KDE desktop manager). 1403 1404The program (or whatever it calls) should finish construction of the key by 1405calling KEYCTL_INSTANTIATE or KEYCTL_INSTANTIATE_IOV, which also permits it to 1406cache the key in one of the keyrings (probably the session ring) before 1407returning. Alternatively, the key can be marked as negative with KEYCTL_NEGATE 1408or KEYCTL_REJECT; this also permits the key to be cached in one of the 1409keyrings. 1410 1411If it returns with the key remaining in the unconstructed state, the key will 1412be marked as being negative, it will be added to the session keyring, and an 1413error will be returned to the key requestor. 1414 1415Supplementary information may be provided from whoever or whatever invoked this 1416service. This will be passed as the <callout_info> parameter. If no such 1417information was made available, then "-" will be passed as this parameter 1418instead. 1419 1420 1421Similarly, the kernel may attempt to update an expired or a soon to expire key 1422by executing: 1423 1424 /sbin/request-key update <key> <uid> <gid> \ 1425 <threadring> <processring> <sessionring> 1426 1427In this case, the program isn't required to actually attach the key to a ring; 1428the rings are provided for reference. 1429 1430 1431================== 1432GARBAGE COLLECTION 1433================== 1434 1435Dead keys (for which the type has been removed) will be automatically unlinked 1436from those keyrings that point to them and deleted as soon as possible by a 1437background garbage collector. 1438 1439Similarly, revoked and expired keys will be garbage collected, but only after a 1440certain amount of time has passed. This time is set as a number of seconds in: 1441 1442 /proc/sys/kernel/keys/gc_delay 1443