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