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