linux/Documentation/security/keys.txt
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   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     This file must be enabled at kernel configuration time as it allows anyone
 327     to list the keys database.
 328
 329 (*) /proc/key-users
 330
 331     This file lists the tracking data for each user that has at least one key
 332     on the system.  Such data includes quota information and statistics:
 333
 334        [root@andromeda root]# cat /proc/key-users
 335        0:     46 45/45 1/100 13/10000
 336        29:     2 2/2 2/100 40/10000
 337        32:     2 2/2 2/100 40/10000
 338        38:     2 2/2 2/100 40/10000
 339
 340     The format of each line is
 341        <UID>:                  User ID to which this applies
 342        <usage>                 Structure refcount
 343        <inst>/<keys>           Total number of keys and number instantiated
 344        <keys>/<max>            Key count quota
 345        <bytes>/<max>           Key size quota
 346
 347
 348Four new sysctl files have been added also for the purpose of controlling the
 349quota limits on keys:
 350
 351 (*) /proc/sys/kernel/keys/root_maxkeys
 352     /proc/sys/kernel/keys/root_maxbytes
 353
 354     These files hold the maximum number of keys that root may have and the
 355     maximum total number of bytes of data that root may have stored in those
 356     keys.
 357
 358 (*) /proc/sys/kernel/keys/maxkeys
 359     /proc/sys/kernel/keys/maxbytes
 360
 361     These files hold the maximum number of keys that each non-root user may
 362     have and the maximum total number of bytes of data that each of those
 363     users may have stored in their keys.
 364
 365Root may alter these by writing each new limit as a decimal number string to
 366the appropriate file.
 367
 368
 369===============================
 370USERSPACE SYSTEM CALL INTERFACE
 371===============================
 372
 373Userspace can manipulate keys directly through three new syscalls: add_key,
 374request_key and keyctl. The latter provides a number of functions for
 375manipulating keys.
 376
 377When referring to a key directly, userspace programs should use the key's
 378serial number (a positive 32-bit integer). However, there are some special
 379values available for referring to special keys and keyrings that relate to the
 380process making the call:
 381
 382        CONSTANT                        VALUE   KEY REFERENCED
 383        ==============================  ======  ===========================
 384        KEY_SPEC_THREAD_KEYRING         -1      thread-specific keyring
 385        KEY_SPEC_PROCESS_KEYRING        -2      process-specific keyring
 386        KEY_SPEC_SESSION_KEYRING        -3      session-specific keyring
 387        KEY_SPEC_USER_KEYRING           -4      UID-specific keyring
 388        KEY_SPEC_USER_SESSION_KEYRING   -5      UID-session keyring
 389        KEY_SPEC_GROUP_KEYRING          -6      GID-specific keyring
 390        KEY_SPEC_REQKEY_AUTH_KEY        -7      assumed request_key()
 391                                                  authorisation key
 392
 393
 394The main syscalls are:
 395
 396 (*) Create a new key of given type, description and payload and add it to the
 397     nominated keyring:
 398
 399        key_serial_t add_key(const char *type, const char *desc,
 400                             const void *payload, size_t plen,
 401                             key_serial_t keyring);
 402
 403     If a key of the same type and description as that proposed already exists
 404     in the keyring, this will try to update it with the given payload, or it
 405     will return error EEXIST if that function is not supported by the key
 406     type. The process must also have permission to write to the key to be able
 407     to update it. The new key will have all user permissions granted and no
 408     group or third party permissions.
 409
 410     Otherwise, this will attempt to create a new key of the specified type and
 411     description, and to instantiate it with the supplied payload and attach it
 412     to the keyring. In this case, an error will be generated if the process
 413     does not have permission to write to the keyring.
 414
 415     If the key type supports it, if the description is NULL or an empty
 416     string, the key type will try and generate a description from the content
 417     of the payload.
 418
 419     The payload is optional, and the pointer can be NULL if not required by
 420     the type. The payload is plen in size, and plen can be zero for an empty
 421     payload.
 422
 423     A new keyring can be generated by setting type "keyring", the keyring name
 424     as the description (or NULL) and setting the payload to NULL.
 425
 426     User defined keys can be created by specifying type "user". It is
 427     recommended that a user defined key's description by prefixed with a type
 428     ID and a colon, such as "krb5tgt:" for a Kerberos 5 ticket granting
 429     ticket.
 430
 431     Any other type must have been registered with the kernel in advance by a
 432     kernel service such as a filesystem.
 433
 434     The ID of the new or updated key is returned if successful.
 435
 436
 437 (*) Search the process's keyrings for a key, potentially calling out to
 438     userspace to create it.
 439
 440        key_serial_t request_key(const char *type, const char *description,
 441                                 const char *callout_info,
 442                                 key_serial_t dest_keyring);
 443
 444     This function searches all the process's keyrings in the order thread,
 445     process, session for a matching key. This works very much like
 446     KEYCTL_SEARCH, including the optional attachment of the discovered key to
 447     a keyring.
 448
 449     If a key cannot be found, and if callout_info is not NULL, then
 450     /sbin/request-key will be invoked in an attempt to obtain a key. The
 451     callout_info string will be passed as an argument to the program.
 452
 453     See also Documentation/security/keys-request-key.txt.
 454
 455
 456The keyctl syscall functions are:
 457
 458 (*) Map a special key ID to a real key ID for this process:
 459
 460        key_serial_t keyctl(KEYCTL_GET_KEYRING_ID, key_serial_t id,
 461                            int create);
 462
 463     The special key specified by "id" is looked up (with the key being created
 464     if necessary) and the ID of the key or keyring thus found is returned if
 465     it exists.
 466
 467     If the key does not yet exist, the key will be created if "create" is
 468     non-zero; and the error ENOKEY will be returned if "create" is zero.
 469
 470
 471 (*) Replace the session keyring this process subscribes to with a new one:
 472
 473        key_serial_t keyctl(KEYCTL_JOIN_SESSION_KEYRING, const char *name);
 474
 475     If name is NULL, an anonymous keyring is created attached to the process
 476     as its session keyring, displacing the old session keyring.
 477
 478     If name is not NULL, if a keyring of that name exists, the process
 479     attempts to attach it as the session keyring, returning an error if that
 480     is not permitted; otherwise a new keyring of that name is created and
 481     attached as the session keyring.
 482
 483     To attach to a named keyring, the keyring must have search permission for
 484     the process's ownership.
 485
 486     The ID of the new session keyring is returned if successful.
 487
 488
 489 (*) Update the specified key:
 490
 491        long keyctl(KEYCTL_UPDATE, key_serial_t key, const void *payload,
 492                    size_t plen);
 493
 494     This will try to update the specified key with the given payload, or it
 495     will return error EOPNOTSUPP if that function is not supported by the key
 496     type. The process must also have permission to write to the key to be able
 497     to update it.
 498
 499     The payload is of length plen, and may be absent or empty as for
 500     add_key().
 501
 502
 503 (*) Revoke a key:
 504
 505        long keyctl(KEYCTL_REVOKE, key_serial_t key);
 506
 507     This makes a key unavailable for further operations. Further attempts to
 508     use the key will be met with error EKEYREVOKED, and the key will no longer
 509     be findable.
 510
 511
 512 (*) Change the ownership of a key:
 513
 514        long keyctl(KEYCTL_CHOWN, key_serial_t key, uid_t uid, gid_t gid);
 515
 516     This function permits a key's owner and group ID to be changed. Either one
 517     of uid or gid can be set to -1 to suppress that change.
 518
 519     Only the superuser can change a key's owner to something other than the
 520     key's current owner. Similarly, only the superuser can change a key's
 521     group ID to something other than the calling process's group ID or one of
 522     its group list members.
 523
 524
 525 (*) Change the permissions mask on a key:
 526
 527        long keyctl(KEYCTL_SETPERM, key_serial_t key, key_perm_t perm);
 528
 529     This function permits the owner of a key or the superuser to change the
 530     permissions mask on a key.
 531
 532     Only bits the available bits are permitted; if any other bits are set,
 533     error EINVAL will be returned.
 534
 535
 536 (*) Describe a key:
 537
 538        long keyctl(KEYCTL_DESCRIBE, key_serial_t key, char *buffer,
 539                    size_t buflen);
 540
 541     This function returns a summary of the key's attributes (but not its
 542     payload data) as a string in the buffer provided.
 543
 544     Unless there's an error, it always returns the amount of data it could
 545     produce, even if that's too big for the buffer, but it won't copy more
 546     than requested to userspace. If the buffer pointer is NULL then no copy
 547     will take place.
 548
 549     A process must have view permission on the key for this function to be
 550     successful.
 551
 552     If successful, a string is placed in the buffer in the following format:
 553
 554        <type>;<uid>;<gid>;<perm>;<description>
 555
 556     Where type and description are strings, uid and gid are decimal, and perm
 557     is hexadecimal. A NUL character is included at the end of the string if
 558     the buffer is sufficiently big.
 559
 560     This can be parsed with
 561
 562        sscanf(buffer, "%[^;];%d;%d;%o;%s", type, &uid, &gid, &mode, desc);
 563
 564
 565 (*) Clear out a keyring:
 566
 567        long keyctl(KEYCTL_CLEAR, key_serial_t keyring);
 568
 569     This function clears the list of keys attached to a keyring. The calling
 570     process must have write permission on the keyring, and it must be a
 571     keyring (or else error ENOTDIR will result).
 572
 573     This function can also be used to clear special kernel keyrings if they
 574     are appropriately marked if the user has CAP_SYS_ADMIN capability.  The
 575     DNS resolver cache keyring is an example of this.
 576
 577
 578 (*) Link a key into a keyring:
 579
 580        long keyctl(KEYCTL_LINK, key_serial_t keyring, key_serial_t key);
 581
 582     This function creates a link from the keyring to the key. The process must
 583     have write permission on the keyring and must have link permission on the
 584     key.
 585
 586     Should the keyring not be a keyring, error ENOTDIR will result; and if the
 587     keyring is full, error ENFILE will result.
 588
 589     The link procedure checks the nesting of the keyrings, returning ELOOP if
 590     it appears too deep or EDEADLK if the link would introduce a cycle.
 591
 592     Any links within the keyring to keys that match the new key in terms of
 593     type and description will be discarded from the keyring as the new one is
 594     added.
 595
 596
 597 (*) Unlink a key or keyring from another keyring:
 598
 599        long keyctl(KEYCTL_UNLINK, key_serial_t keyring, key_serial_t key);
 600
 601     This function looks through the keyring for the first link to the
 602     specified key, and removes it if found. Subsequent links to that key are
 603     ignored. The process must have write permission on the keyring.
 604
 605     If the keyring is not a keyring, error ENOTDIR will result; and if the key
 606     is not present, error ENOENT will be the result.
 607
 608
 609 (*) Search a keyring tree for a key:
 610
 611        key_serial_t keyctl(KEYCTL_SEARCH, key_serial_t keyring,
 612                            const char *type, const char *description,
 613                            key_serial_t dest_keyring);
 614
 615     This searches the keyring tree headed by the specified keyring until a key
 616     is found that matches the type and description criteria. Each keyring is
 617     checked for keys before recursion into its children occurs.
 618
 619     The process must have search permission on the top level keyring, or else
 620     error EACCES will result. Only keyrings that the process has search
 621     permission on will be recursed into, and only keys and keyrings for which
 622     a process has search permission can be matched. If the specified keyring
 623     is not a keyring, ENOTDIR will result.
 624
 625     If the search succeeds, the function will attempt to link the found key
 626     into the destination keyring if one is supplied (non-zero ID). All the
 627     constraints applicable to KEYCTL_LINK apply in this case too.
 628
 629     Error ENOKEY, EKEYREVOKED or EKEYEXPIRED will be returned if the search
 630     fails. On success, the resulting key ID will be returned.
 631
 632
 633 (*) Read the payload data from a key:
 634
 635        long keyctl(KEYCTL_READ, key_serial_t keyring, char *buffer,
 636                    size_t buflen);
 637
 638     This function attempts to read the payload data from the specified key
 639     into the buffer. The process must have read permission on the key to
 640     succeed.
 641
 642     The returned data will be processed for presentation by the key type. For
 643     instance, a keyring will return an array of key_serial_t entries
 644     representing the IDs of all the keys to which it is subscribed. The user
 645     defined key type will return its data as is. If a key type does not
 646     implement this function, error EOPNOTSUPP will result.
 647
 648     As much of the data as can be fitted into the buffer will be copied to
 649     userspace if the buffer pointer is not NULL.
 650
 651     On a successful return, the function will always return the amount of data
 652     available rather than the amount copied.
 653
 654
 655 (*) Instantiate a partially constructed key.
 656
 657        long keyctl(KEYCTL_INSTANTIATE, key_serial_t key,
 658                    const void *payload, size_t plen,
 659                    key_serial_t keyring);
 660        long keyctl(KEYCTL_INSTANTIATE_IOV, key_serial_t key,
 661                    const struct iovec *payload_iov, unsigned ioc,
 662                    key_serial_t keyring);
 663
 664     If the kernel calls back to userspace to complete the instantiation of a
 665     key, userspace should use this call to supply data for the key before the
 666     invoked process returns, or else the key will be marked negative
 667     automatically.
 668
 669     The process must have write access on the key to be able to instantiate
 670     it, and the key must be uninstantiated.
 671
 672     If a keyring is specified (non-zero), the key will also be linked into
 673     that keyring, however all the constraints applying in KEYCTL_LINK apply in
 674     this case too.
 675
 676     The payload and plen arguments describe the payload data as for add_key().
 677
 678     The payload_iov and ioc arguments describe the payload data in an iovec
 679     array instead of a single buffer.
 680
 681
 682 (*) Negatively instantiate a partially constructed key.
 683
 684        long keyctl(KEYCTL_NEGATE, key_serial_t key,
 685                    unsigned timeout, key_serial_t keyring);
 686        long keyctl(KEYCTL_REJECT, key_serial_t key,
 687                    unsigned timeout, unsigned error, key_serial_t keyring);
 688
 689     If the kernel calls back to userspace to complete the instantiation of a
 690     key, userspace should use this call mark the key as negative before the
 691     invoked process returns if it is unable to fulfill the request.
 692
 693     The process must have write access on the key to be able to instantiate
 694     it, and the key must be uninstantiated.
 695
 696     If a keyring is specified (non-zero), the key will also be linked into
 697     that keyring, however all the constraints applying in KEYCTL_LINK apply in
 698     this case too.
 699
 700     If the key is rejected, future searches for it will return the specified
 701     error code until the rejected key expires.  Negating the key is the same
 702     as rejecting the key with ENOKEY as the error code.
 703
 704
 705 (*) Set the default request-key destination keyring.
 706
 707        long keyctl(KEYCTL_SET_REQKEY_KEYRING, int reqkey_defl);
 708
 709     This sets the default keyring to which implicitly requested keys will be
 710     attached for this thread. reqkey_defl should be one of these constants:
 711
 712        CONSTANT                                VALUE   NEW DEFAULT KEYRING
 713        ======================================  ======  =======================
 714        KEY_REQKEY_DEFL_NO_CHANGE               -1      No change
 715        KEY_REQKEY_DEFL_DEFAULT                 0       Default[1]
 716        KEY_REQKEY_DEFL_THREAD_KEYRING          1       Thread keyring
 717        KEY_REQKEY_DEFL_PROCESS_KEYRING         2       Process keyring
 718        KEY_REQKEY_DEFL_SESSION_KEYRING         3       Session keyring
 719        KEY_REQKEY_DEFL_USER_KEYRING            4       User keyring
 720        KEY_REQKEY_DEFL_USER_SESSION_KEYRING    5       User session keyring
 721        KEY_REQKEY_DEFL_GROUP_KEYRING           6       Group keyring
 722
 723     The old default will be returned if successful and error EINVAL will be
 724     returned if reqkey_defl is not one of the above values.
 725
 726     The default keyring can be overridden by the keyring indicated to the
 727     request_key() system call.
 728
 729     Note that this setting is inherited across fork/exec.
 730
 731     [1] The default is: the thread keyring if there is one, otherwise
 732     the process keyring if there is one, otherwise the session keyring if
 733     there is one, otherwise the user default session keyring.
 734
 735
 736 (*) Set the timeout on a key.
 737
 738        long keyctl(KEYCTL_SET_TIMEOUT, key_serial_t key, unsigned timeout);
 739
 740     This sets or clears the timeout on a key. The timeout can be 0 to clear
 741     the timeout or a number of seconds to set the expiry time that far into
 742     the future.
 743
 744     The process must have attribute modification access on a key to set its
 745     timeout. Timeouts may not be set with this function on negative, revoked
 746     or expired keys.
 747
 748
 749 (*) Assume the authority granted to instantiate a key
 750
 751        long keyctl(KEYCTL_ASSUME_AUTHORITY, key_serial_t key);
 752
 753     This assumes or divests the authority required to instantiate the
 754     specified key. Authority can only be assumed if the thread has the
 755     authorisation key associated with the specified key in its keyrings
 756     somewhere.
 757
 758     Once authority is assumed, searches for keys will also search the
 759     requester's keyrings using the requester's security label, UID, GID and
 760     groups.
 761
 762     If the requested authority is unavailable, error EPERM will be returned,
 763     likewise if the authority has been revoked because the target key is
 764     already instantiated.
 765
 766     If the specified key is 0, then any assumed authority will be divested.
 767
 768     The assumed authoritative key is inherited across fork and exec.
 769
 770
 771 (*) Get the LSM security context attached to a key.
 772
 773        long keyctl(KEYCTL_GET_SECURITY, key_serial_t key, char *buffer,
 774                    size_t buflen)
 775
 776     This function returns a string that represents the LSM security context
 777     attached to a key in the buffer provided.
 778
 779     Unless there's an error, it always returns the amount of data it could
 780     produce, even if that's too big for the buffer, but it won't copy more
 781     than requested to userspace. If the buffer pointer is NULL then no copy
 782     will take place.
 783
 784     A NUL character is included at the end of the string if the buffer is
 785     sufficiently big.  This is included in the returned count.  If no LSM is
 786     in force then an empty string will be returned.
 787
 788     A process must have view permission on the key for this function to be
 789     successful.
 790
 791
 792 (*) Install the calling process's session keyring on its parent.
 793
 794        long keyctl(KEYCTL_SESSION_TO_PARENT);
 795
 796     This functions attempts to install the calling process's session keyring
 797     on to the calling process's parent, replacing the parent's current session
 798     keyring.
 799
 800     The calling process must have the same ownership as its parent, the
 801     keyring must have the same ownership as the calling process, the calling
 802     process must have LINK permission on the keyring and the active LSM module
 803     mustn't deny permission, otherwise error EPERM will be returned.
 804
 805     Error ENOMEM will be returned if there was insufficient memory to complete
 806     the operation, otherwise 0 will be returned to indicate success.
 807
 808     The keyring will be replaced next time the parent process leaves the
 809     kernel and resumes executing userspace.
 810
 811
 812 (*) Invalidate a key.
 813
 814        long keyctl(KEYCTL_INVALIDATE, key_serial_t key);
 815
 816     This function marks a key as being invalidated and then wakes up the
 817     garbage collector.  The garbage collector immediately removes invalidated
 818     keys from all keyrings and deletes the key when its reference count
 819     reaches zero.
 820
 821     Keys that are marked invalidated become invisible to normal key operations
 822     immediately, though they are still visible in /proc/keys until deleted
 823     (they're marked with an 'i' flag).
 824
 825     A process must have search permission on the key for this function to be
 826     successful.
 827
 828
 829===============
 830KERNEL SERVICES
 831===============
 832
 833The kernel services for key management are fairly simple to deal with. They can
 834be broken down into two areas: keys and key types.
 835
 836Dealing with keys is fairly straightforward. Firstly, the kernel service
 837registers its type, then it searches for a key of that type. It should retain
 838the key as long as it has need of it, and then it should release it. For a
 839filesystem or device file, a search would probably be performed during the open
 840call, and the key released upon close. How to deal with conflicting keys due to
 841two different users opening the same file is left to the filesystem author to
 842solve.
 843
 844To access the key manager, the following header must be #included:
 845
 846        <linux/key.h>
 847
 848Specific key types should have a header file under include/keys/ that should be
 849used to access that type.  For keys of type "user", for example, that would be:
 850
 851        <keys/user-type.h>
 852
 853Note that there are two different types of pointers to keys that may be
 854encountered:
 855
 856 (*) struct key *
 857
 858     This simply points to the key structure itself. Key structures will be at
 859     least four-byte aligned.
 860
 861 (*) key_ref_t
 862
 863     This is equivalent to a struct key *, but the least significant bit is set
 864     if the caller "possesses" the key. By "possession" it is meant that the
 865     calling processes has a searchable link to the key from one of its
 866     keyrings. There are three functions for dealing with these:
 867
 868        key_ref_t make_key_ref(const struct key *key,
 869                               unsigned long possession);
 870
 871        struct key *key_ref_to_ptr(const key_ref_t key_ref);
 872
 873        unsigned long is_key_possessed(const key_ref_t key_ref);
 874
 875     The first function constructs a key reference from a key pointer and
 876     possession information (which must be 0 or 1 and not any other value).
 877
 878     The second function retrieves the key pointer from a reference and the
 879     third retrieves the possession flag.
 880
 881When accessing a key's payload contents, certain precautions must be taken to
 882prevent access vs modification races. See the section "Notes on accessing
 883payload contents" for more information.
 884
 885(*) To search for a key, call:
 886
 887        struct key *request_key(const struct key_type *type,
 888                                const char *description,
 889                                const char *callout_info);
 890
 891    This is used to request a key or keyring with a description that matches
 892    the description specified according to the key type's match function. This
 893    permits approximate matching to occur. If callout_string is not NULL, then
 894    /sbin/request-key will be invoked in an attempt to obtain the key from
 895    userspace. In that case, callout_string will be passed as an argument to
 896    the program.
 897
 898    Should the function fail error ENOKEY, EKEYEXPIRED or EKEYREVOKED will be
 899    returned.
 900
 901    If successful, the key will have been attached to the default keyring for
 902    implicitly obtained request-key keys, as set by KEYCTL_SET_REQKEY_KEYRING.
 903
 904    See also Documentation/security/keys-request-key.txt.
 905
 906
 907(*) To search for a key, passing auxiliary data to the upcaller, call:
 908
 909        struct key *request_key_with_auxdata(const struct key_type *type,
 910                                             const char *description,
 911                                             const void *callout_info,
 912                                             size_t callout_len,
 913                                             void *aux);
 914
 915    This is identical to request_key(), except that the auxiliary data is
 916    passed to the key_type->request_key() op if it exists, and the callout_info
 917    is a blob of length callout_len, if given (the length may be 0).
 918
 919
 920(*) A key can be requested asynchronously by calling one of:
 921
 922        struct key *request_key_async(const struct key_type *type,
 923                                      const char *description,
 924                                      const void *callout_info,
 925                                      size_t callout_len);
 926
 927    or:
 928
 929        struct key *request_key_async_with_auxdata(const struct key_type *type,
 930                                                   const char *description,
 931                                                   const char *callout_info,
 932                                                   size_t callout_len,
 933                                                   void *aux);
 934
 935    which are asynchronous equivalents of request_key() and
 936    request_key_with_auxdata() respectively.
 937
 938    These two functions return with the key potentially still under
 939    construction.  To wait for construction completion, the following should be
 940    called:
 941
 942        int wait_for_key_construction(struct key *key, bool intr);
 943
 944    The function will wait for the key to finish being constructed and then
 945    invokes key_validate() to return an appropriate value to indicate the state
 946    of the key (0 indicates the key is usable).
 947
 948    If intr is true, then the wait can be interrupted by a signal, in which
 949    case error ERESTARTSYS will be returned.
 950
 951
 952(*) When it is no longer required, the key should be released using:
 953
 954        void key_put(struct key *key);
 955
 956    Or:
 957
 958        void key_ref_put(key_ref_t key_ref);
 959
 960    These can be called from interrupt context. If CONFIG_KEYS is not set then
 961    the argument will not be parsed.
 962
 963
 964(*) Extra references can be made to a key by calling the following function:
 965
 966        struct key *key_get(struct key *key);
 967
 968    These need to be disposed of by calling key_put() when they've been
 969    finished with. The key pointer passed in will be returned. If the pointer
 970    is NULL or CONFIG_KEYS is not set then the key will not be dereferenced and
 971    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(*) To check the validity of a key, this function can be called:
 998
 999        int validate_key(struct key *key);
1000
1001    This checks that the key in question hasn't expired or and hasn't been
1002    revoked. Should the key be invalid, error EKEYEXPIRED or EKEYREVOKED will
1003    be returned. If the key is NULL or if CONFIG_KEYS is not set then 0 will be
1004    returned (in the latter case without parsing the argument).
1005
1006
1007(*) To register a key type, the following function should be called:
1008
1009        int register_key_type(struct key_type *type);
1010
1011    This will return error EEXIST if a type of the same name is already
1012    present.
1013
1014
1015(*) To unregister a key type, call:
1016
1017        void unregister_key_type(struct key_type *type);
1018
1019
1020Under some circumstances, it may be desirable to deal with a bundle of keys.
1021The facility provides access to the keyring type for managing such a bundle:
1022
1023        struct key_type key_type_keyring;
1024
1025This can be used with a function such as request_key() to find a specific
1026keyring in a process's keyrings.  A keyring thus found can then be searched
1027with keyring_search().  Note that it is not possible to use request_key() to
1028search a specific keyring, so using keyrings in this way is of limited utility.
1029
1030
1031===================================
1032NOTES ON ACCESSING PAYLOAD CONTENTS
1033===================================
1034
1035The simplest payload is just a number in key->payload.value. In this case,
1036there's no need to indulge in RCU or locking when accessing the payload.
1037
1038More complex payload contents must be allocated and a pointer to them set in
1039key->payload.data. One of the following ways must be selected to access the
1040data:
1041
1042 (1) Unmodifiable key type.
1043
1044     If the key type does not have a modify method, then the key's payload can
1045     be accessed without any form of locking, provided that it's known to be
1046     instantiated (uninstantiated keys cannot be "found").
1047
1048 (2) The key's semaphore.
1049
1050     The semaphore could be used to govern access to the payload and to control
1051     the payload pointer. It must be write-locked for modifications and would
1052     have to be read-locked for general access. The disadvantage of doing this
1053     is that the accessor may be required to sleep.
1054
1055 (3) RCU.
1056
1057     RCU must be used when the semaphore isn't already held; if the semaphore
1058     is held then the contents can't change under you unexpectedly as the
1059     semaphore must still be used to serialise modifications to the key. The
1060     key management code takes care of this for the key type.
1061
1062     However, this means using:
1063
1064        rcu_read_lock() ... rcu_dereference() ... rcu_read_unlock()
1065
1066     to read the pointer, and:
1067
1068        rcu_dereference() ... rcu_assign_pointer() ... call_rcu()
1069
1070     to set the pointer and dispose of the old contents after a grace period.
1071     Note that only the key type should ever modify a key's payload.
1072
1073     Furthermore, an RCU controlled payload must hold a struct rcu_head for the
1074     use of call_rcu() and, if the payload is of variable size, the length of
1075     the payload. key->datalen cannot be relied upon to be consistent with the
1076     payload just dereferenced if the key's semaphore is not held.
1077
1078
1079===================
1080DEFINING A KEY TYPE
1081===================
1082
1083A kernel service may want to define its own key type. For instance, an AFS
1084filesystem might want to define a Kerberos 5 ticket key type. To do this, it
1085author fills in a key_type struct and registers it with the system.
1086
1087Source files that implement key types should include the following header file:
1088
1089        <linux/key-type.h>
1090
1091The structure has a number of fields, some of which are mandatory:
1092
1093 (*) const char *name
1094
1095     The name of the key type. This is used to translate a key type name
1096     supplied by userspace into a pointer to the structure.
1097
1098
1099 (*) size_t def_datalen
1100
1101     This is optional - it supplies the default payload data length as
1102     contributed to the quota. If the key type's payload is always or almost
1103     always the same size, then this is a more efficient way to do things.
1104
1105     The data length (and quota) on a particular key can always be changed
1106     during instantiation or update by calling:
1107
1108        int key_payload_reserve(struct key *key, size_t datalen);
1109
1110     With the revised data length. Error EDQUOT will be returned if this is not
1111     viable.
1112
1113
1114 (*) int (*vet_description)(const char *description);
1115
1116     This optional method is called to vet a key description.  If the key type
1117     doesn't approve of the key description, it may return an error, otherwise
1118     it should return 0.
1119
1120
1121 (*) int (*preparse)(struct key_preparsed_payload *prep);
1122
1123     This optional method permits the key type to attempt to parse payload
1124     before a key is created (add key) or the key semaphore is taken (update or
1125     instantiate key).  The structure pointed to by prep looks like:
1126
1127        struct key_preparsed_payload {
1128                char            *description;
1129                void            *type_data[2];
1130                void            *payload;
1131                const void      *data;
1132                size_t          datalen;
1133                size_t          quotalen;
1134        };
1135
1136     Before calling the method, the caller will fill in data and datalen with
1137     the payload blob parameters; quotalen will be filled in with the default
1138     quota size from the key type and the rest will be cleared.
1139
1140     If a description can be proposed from the payload contents, that should be
1141     attached as a string to the description field.  This will be used for the
1142     key description if the caller of add_key() passes NULL or "".
1143
1144     The method can attach anything it likes to type_data[] and payload.  These
1145     are merely passed along to the instantiate() or update() operations.
1146
1147     The method should return 0 if success ful or a negative error code
1148     otherwise.
1149
1150     
1151 (*) void (*free_preparse)(struct key_preparsed_payload *prep);
1152
1153     This method is only required if the preparse() method is provided,
1154     otherwise it is unused.  It cleans up anything attached to the
1155     description, type_data and payload fields of the key_preparsed_payload
1156     struct as filled in by the preparse() method.
1157
1158
1159 (*) int (*instantiate)(struct key *key, struct key_preparsed_payload *prep);
1160
1161     This method is called to attach a payload to a key during construction.
1162     The payload attached need not bear any relation to the data passed to this
1163     function.
1164
1165     The prep->data and prep->datalen fields will define the original payload
1166     blob.  If preparse() was supplied then other fields may be filled in also.
1167
1168     If the amount of data attached to the key differs from the size in
1169     keytype->def_datalen, then key_payload_reserve() should be called.
1170
1171     This method does not have to lock the key in order to attach a payload.
1172     The fact that KEY_FLAG_INSTANTIATED is not set in key->flags prevents
1173     anything else from gaining access to the key.
1174
1175     It is safe to sleep in this method.
1176
1177
1178 (*) int (*update)(struct key *key, const void *data, size_t datalen);
1179
1180     If this type of key can be updated, then this method should be provided.
1181     It is called to update a key's payload from the blob of data provided.
1182
1183     The prep->data and prep->datalen fields will define the original payload
1184     blob.  If preparse() was supplied then other fields may be filled in also.
1185
1186     key_payload_reserve() should be called if the data length might change
1187     before any changes are actually made. Note that if this succeeds, the type
1188     is committed to changing the key because it's already been altered, so all
1189     memory allocation must be done first.
1190
1191     The key will have its semaphore write-locked before this method is called,
1192     but this only deters other writers; any changes to the key's payload must
1193     be made under RCU conditions, and call_rcu() must be used to dispose of
1194     the old payload.
1195
1196     key_payload_reserve() should be called before the changes are made, but
1197     after all allocations and other potentially failing function calls are
1198     made.
1199
1200     It is safe to sleep in this method.
1201
1202
1203 (*) int (*match)(const struct key *key, const void *desc);
1204
1205     This method is called to match a key against a description. It should
1206     return non-zero if the two match, zero if they don't.
1207
1208     This method should not need to lock the key in any way. The type and
1209     description can be considered invariant, and the payload should not be
1210     accessed (the key may not yet be instantiated).
1211
1212     It is not safe to sleep in this method; the caller may hold spinlocks.
1213
1214
1215 (*) void (*revoke)(struct key *key);
1216
1217     This method is optional.  It is called to discard part of the payload
1218     data upon a key being revoked.  The caller will have the key semaphore
1219     write-locked.
1220
1221     It is safe to sleep in this method, though care should be taken to avoid
1222     a deadlock against the key semaphore.
1223
1224
1225 (*) void (*destroy)(struct key *key);
1226
1227     This method is optional. It is called to discard the payload data on a key
1228     when it is being destroyed.
1229
1230     This method does not need to lock the key to access the payload; it can
1231     consider the key as being inaccessible at this time. Note that the key's
1232     type may have been changed before this function is called.
1233
1234     It is not safe to sleep in this method; the caller may hold spinlocks.
1235
1236
1237 (*) void (*describe)(const struct key *key, struct seq_file *p);
1238
1239     This method is optional. It is called during /proc/keys reading to
1240     summarise a key's description and payload in text form.
1241
1242     This method will be called with the RCU read lock held. rcu_dereference()
1243     should be used to read the payload pointer if the payload is to be
1244     accessed. key->datalen cannot be trusted to stay consistent with the
1245     contents of the payload.
1246
1247     The description will not change, though the key's state may.
1248
1249     It is not safe to sleep in this method; the RCU read lock is held by the
1250     caller.
1251
1252
1253 (*) long (*read)(const struct key *key, char __user *buffer, size_t buflen);
1254
1255     This method is optional. It is called by KEYCTL_READ to translate the
1256     key's payload into something a blob of data for userspace to deal with.
1257     Ideally, the blob should be in the same format as that passed in to the
1258     instantiate and update methods.
1259
1260     If successful, the blob size that could be produced should be returned
1261     rather than the size copied.
1262
1263     This method will be called with the key's semaphore read-locked. This will
1264     prevent the key's payload changing. It is not necessary to use RCU locking
1265     when accessing the key's payload. It is safe to sleep in this method, such
1266     as might happen when the userspace buffer is accessed.
1267
1268
1269 (*) int (*request_key)(struct key_construction *cons, const char *op,
1270                        void *aux);
1271
1272     This method is optional.  If provided, request_key() and friends will
1273     invoke this function rather than upcalling to /sbin/request-key to operate
1274     upon a key of this type.
1275
1276     The aux parameter is as passed to request_key_async_with_auxdata() and
1277     similar or is NULL otherwise.  Also passed are the construction record for
1278     the key to be operated upon and the operation type (currently only
1279     "create").
1280
1281     This method is permitted to return before the upcall is complete, but the
1282     following function must be called under all circumstances to complete the
1283     instantiation process, whether or not it succeeds, whether or not there's
1284     an error:
1285
1286        void complete_request_key(struct key_construction *cons, int error);
1287
1288     The error parameter should be 0 on success, -ve on error.  The
1289     construction record is destroyed by this action and the authorisation key
1290     will be revoked.  If an error is indicated, the key under construction
1291     will be negatively instantiated if it wasn't already instantiated.
1292
1293     If this method returns an error, that error will be returned to the
1294     caller of request_key*().  complete_request_key() must be called prior to
1295     returning.
1296
1297     The key under construction and the authorisation key can be found in the
1298     key_construction struct pointed to by cons:
1299
1300     (*) struct key *key;
1301
1302         The key under construction.
1303
1304     (*) struct key *authkey;
1305
1306         The authorisation key.
1307
1308
1309============================
1310REQUEST-KEY CALLBACK SERVICE
1311============================
1312
1313To create a new key, the kernel will attempt to execute the following command
1314line:
1315
1316        /sbin/request-key create <key> <uid> <gid> \
1317                <threadring> <processring> <sessionring> <callout_info>
1318
1319<key> is the key being constructed, and the three keyrings are the process
1320keyrings from the process that caused the search to be issued. These are
1321included for two reasons:
1322
1323  (1) There may be an authentication token in one of the keyrings that is
1324      required to obtain the key, eg: a Kerberos Ticket-Granting Ticket.
1325
1326  (2) The new key should probably be cached in one of these rings.
1327
1328This program should set it UID and GID to those specified before attempting to
1329access any more keys. It may then look around for a user specific process to
1330hand the request off to (perhaps a path held in placed in another key by, for
1331example, the KDE desktop manager).
1332
1333The program (or whatever it calls) should finish construction of the key by
1334calling KEYCTL_INSTANTIATE or KEYCTL_INSTANTIATE_IOV, which also permits it to
1335cache the key in one of the keyrings (probably the session ring) before
1336returning.  Alternatively, the key can be marked as negative with KEYCTL_NEGATE
1337or KEYCTL_REJECT; this also permits the key to be cached in one of the
1338keyrings.
1339
1340If it returns with the key remaining in the unconstructed state, the key will
1341be marked as being negative, it will be added to the session keyring, and an
1342error will be returned to the key requestor.
1343
1344Supplementary information may be provided from whoever or whatever invoked this
1345service. This will be passed as the <callout_info> parameter. If no such
1346information was made available, then "-" will be passed as this parameter
1347instead.
1348
1349
1350Similarly, the kernel may attempt to update an expired or a soon to expire key
1351by executing:
1352
1353        /sbin/request-key update <key> <uid> <gid> \
1354                <threadring> <processring> <sessionring>
1355
1356In this case, the program isn't required to actually attach the key to a ring;
1357the rings are provided for reference.
1358
1359
1360==================
1361GARBAGE COLLECTION
1362==================
1363
1364Dead keys (for which the type has been removed) will be automatically unlinked
1365from those keyrings that point to them and deleted as soon as possible by a
1366background garbage collector.
1367
1368Similarly, revoked and expired keys will be garbage collected, but only after a
1369certain amount of time has passed.  This time is set as a number of seconds in:
1370
1371        /proc/sys/kernel/keys/gc_delay
1372
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