1 ============================ 2 KERNEL KEY RETENTION SERVICE 3 ============================ 4 5This service allows cryptographic keys, authentication tokens, cross-domain 6user mappings, and similar to be cached in the kernel for the use of 7filesystems and other kernel services. 8 9Keyrings are permitted; these are a special type of key that can hold links to 10other keys. Processes each have three standard keyring subscriptions that a 11kernel service can search for relevant keys. 12 13The key service can be configured on by enabling: 14 15 "Security options"/"Enable access key retention support" (CONFIG_KEYS) 16 17This document has the following sections: 18 19 - Key overview 20 - Key service overview 21 - Key access permissions 22 - SELinux support 23 - New procfs files 24 - Userspace system call interface 25 - Kernel services 26 - Notes on accessing payload contents 27 - Defining a key type 28 - Request-key callback service 29 - Garbage collection 30 31 32============ 33KEY OVERVIEW 34============ 35 36In this context, keys represent units of cryptographic data, authentication 37tokens, keyrings, etc.. These are represented in the kernel by struct key. 38 39Each key has a number of attributes: 40 41 - A serial number. 42 - A type. 43 - A description (for matching a key in a search). 44 - Access control information. 45 - An expiry time. 46 - A payload. 47 - State. 48 49 50 (*) Each key is issued a serial number of type key_serial_t that is unique for 51 the lifetime of that key. All serial numbers are positive non-zero 32-bit 52 integers. 53 54 Userspace programs can use a key's serial numbers as a way to gain access 55 to it, subject to permission checking. 56 57 (*) Each key is of a defined "type". Types must be registered inside the 58 kernel by a kernel service (such as a filesystem) before keys of that type 59 can be added or used. Userspace programs cannot define new types directly. 60 61 Key types are represented in the kernel by struct key_type. This defines a 62 number of operations that can be performed on a key of that type. 63 64 Should a type be removed from the system, all the keys of that type will 65 be invalidated. 66 67 (*) Each key has a description. This should be a printable string. The key 68 type provides an operation to perform a match between the description on a 69 key and a criterion string. 70 71 (*) Each key has an owner user ID, a group ID and a permissions mask. These 72 are used to control what a process may do to a key from userspace, and 73 whether a kernel service will be able to find the key. 74 75 (*) Each key can be set to expire at a specific time by the key type's 76 instantiation function. Keys can also be immortal. 77 78 (*) Each key can have a payload. This is a quantity of data that represent the 79 actual "key". In the case of a keyring, this is a list of keys to which 80 the keyring links; in the case of a user-defined key, it's an arbitrary 81 blob of data. 82 83 Having a payload is not required; and the payload can, in fact, just be a 84 value stored in the struct key itself. 85 86 When a key is instantiated, the key type's instantiation function is 87 called with a blob of data, and that then creates the key's payload in 88 some way. 89 90 Similarly, when userspace wants to read back the contents of the key, if 91 permitted, another key type operation will be called to convert the key's 92 attached payload back into a blob of data. 93 94 (*) Each key can be in one of a number of basic states: 95 96 (*) Uninstantiated. The key exists, but does not have any data attached. 97 Keys being requested from userspace will be in this state. 98 99 (*) Instantiated. This is the normal state. The key is fully formed, and 100 has data attached. 101 102 (*) Negative. This is a relatively short-lived state. The key acts as a 103 note saying that a previous call out to userspace failed, and acts as 104 a throttle on key lookups. A negative key can be updated to a normal 105 state. 106 107 (*) Expired. Keys can have lifetimes set. If their lifetime is exceeded, 108 they traverse to this state. An expired key can be updated back to a 109 normal state. 110 111 (*) Revoked. A key is put in this state by userspace action. It can't be 112 found or operated upon (apart from by unlinking it). 113 114 (*) Dead. The key's type was unregistered, and so the key is now useless. 115 116Keys in the last three states are subject to garbage collection. See the 117section on "Garbage collection". 118 119 120==================== 121KEY SERVICE OVERVIEW 122==================== 123 124The key service provides a number of features besides keys: 125 126 (*) The key service defines three special key types: 127 128 (+) "keyring" 129 130 Keyrings are special keys that contain a list of other keys. Keyring 131 lists can be modified using various system calls. Keyrings should not 132 be given a payload when created. 133 134 (+) "user" 135 136 A key of this type has a description and a payload that are arbitrary 137 blobs of data. These can be created, updated and read by userspace, 138 and aren't intended for use by kernel services. 139 140 (+) "logon" 141 142 Like a "user" key, a "logon" key has a payload that is an arbitrary 143 blob of data. It is intended as a place to store secrets which are 144 accessible to the kernel but not to userspace programs. 145 146 The description can be arbitrary, but must be prefixed with a non-zero 147 length string that describes the key "subclass". The subclass is 148 separated from the rest of the description by a ':'. "logon" keys can 149 be created and updated from userspace, but the payload is only 150 readable from kernel space. 151 152 (*) Each process subscribes to three keyrings: a thread-specific keyring, a 153 process-specific keyring, and a session-specific keyring. 154 155 The thread-specific keyring is discarded from the child when any sort of 156 clone, fork, vfork or execve occurs. A new keyring is created only when 157 required. 158 159 The process-specific keyring is replaced with an empty one in the child on 160 clone, fork, vfork unless CLONE_THREAD is supplied, in which case it is 161 shared. execve also discards the process's process keyring and creates a 162 new one. 163 164 The session-specific keyring is persistent across clone, fork, vfork and 165 execve, even when the latter executes a set-UID or set-GID binary. A 166 process can, however, replace its current session keyring with a new one 167 by using PR_JOIN_SESSION_KEYRING. It is permitted to request an anonymous 168 new one, or to attempt to create or join one of a specific name. 169 170 The ownership of the thread keyring changes when the real UID and GID of 171 the thread changes. 172 173 (*) Each user ID resident in the system holds two special keyrings: a user 174 specific keyring and a default user session keyring. The default session 175 keyring is initialised with a link to the user-specific keyring. 176 177 When a process changes its real UID, if it used to have no session key, it 178 will be subscribed to the default session key for the new UID. 179 180 If a process attempts to access its session key when it doesn't have one, 181 it will be subscribed to the default for its current UID. 182 183 (*) Each user has two quotas against which the keys they own are tracked. One 184 limits the total number of keys and keyrings, the other limits the total 185 amount of description and payload space that can be consumed. 186 187 The user can view information on this and other statistics through procfs 188 files. The root user may also alter the quota limits through sysctl files 189 (see the section "New procfs files"). 190 191 Process-specific and thread-specific keyrings are not counted towards a 192 user's quota. 193 194 If a system call that modifies a key or keyring in some way would put the 195 user over quota, the operation is refused and error EDQUOT is returned. 196 197 (*) There's a system call interface by which userspace programs can create and 198 manipulate keys and keyrings. 199 200 (*) There's a kernel interface by which services can register types and search 201 for keys. 202 203 (*) There's a way for the a search done from the kernel to call back to 204 userspace to request a key that can't be found in a process's keyrings. 205 206 (*) An optional filesystem is available through which the key database can be 207 viewed and manipulated. 208 209 210====================== 211KEY ACCESS PERMISSIONS 212====================== 213 214Keys have an owner user ID, a group access ID, and a permissions mask. The mask 215has up to eight bits each for possessor, user, group and other access. Only 216six of each set of eight bits are defined. These permissions granted are: 217 218 (*) View 219 220 This permits a key or keyring's attributes to be viewed - including key 221 type and description. 222 223 (*) Read 224 225 This permits a key's payload to be viewed or a keyring's list of linked 226 keys. 227 228 (*) Write 229 230 This permits a key's payload to be instantiated or updated, or it allows a 231 link to be added to or removed from a keyring. 232 233 (*) Search 234 235 This permits keyrings to be searched and keys to be found. Searches can 236 only recurse into nested keyrings that have search permission set. 237 238 (*) Link 239 240 This permits a key or keyring to be linked to. To create a link from a 241 keyring to a key, a process must have Write permission on the keyring and 242 Link permission on the key. 243 244 (*) Set Attribute 245 246 This permits a key's UID, GID and permissions mask to be changed. 247 248For changing the ownership, group ID or permissions mask, being the owner of 249the key or having the sysadmin capability is sufficient. 250 251 252=============== 253SELINUX SUPPORT 254=============== 255 256The security class "key" has been added to SELinux so that mandatory access 257controls can be applied to keys created within various contexts. This support 258is preliminary, and is likely to change quite significantly in the near future. 259Currently, all of the basic permissions explained above are provided in SELinux 260as well; SELinux is simply invoked after all basic permission checks have been 261performed. 262 263The value of the file /proc/self/attr/keycreate influences the labeling of 264newly-created keys. If the contents of that file correspond to an SELinux 265security context, then the key will be assigned that context. Otherwise, the 266key will be assigned the current context of the task that invoked the key 267creation request. Tasks must be granted explicit permission to assign a 268particular context to newly-created keys, using the "create" permission in the 269key security class. 270 271The default keyrings associated with users will be labeled with the default 272context of the user if and only if the login programs have been instrumented to 273properly initialize keycreate during the login process. Otherwise, they will 274be labeled with the context of the login program itself. 275 276Note, however, that the default keyrings associated with the root user are 277labeled with the default kernel context, since they are created early in the 278boot process, before root has a chance to log in. 279 280The keyrings associated with new threads are each labeled with the context of 281their associated thread, and both session and process keyrings are handled 282similarly. 283 284 285================ 286NEW PROCFS FILES 287================ 288 289Two files have been added to procfs by which an administrator can find out 290about the status of the key service: 291 292 (*) /proc/keys 293 294 This lists the keys that are currently viewable by the task reading the 295 file, giving information about their type, description and permissions. 296 It is not possible to view the payload of the key this way, though some 297 information about it may be given. 298 299 The only keys included in the list are those that grant View permission to 300 the reading process whether or not it possesses them. Note that LSM 301 security checks are still performed, and may further filter out keys that 302 the current process is not authorised to view. 303 304 The contents of the file look like this: 305 306 SERIAL FLAGS USAGE EXPY PERM UID GID TYPE DESCRIPTION: SUMMARY 307 00000001 I----- 39 perm 1f3f0000 0 0 keyring _uid_ses.0: 1/4 308 00000002 I----- 2 perm 1f3f0000 0 0 keyring _uid.0: empty 309 00000007 I----- 1 perm 1f3f0000 0 0 keyring _pid.1: empty 310 0000018d I----- 1 perm 1f3f0000 0 0 keyring _pid.412: empty 311 000004d2 I--Q-- 1 perm 1f3f0000 32 -1 keyring _uid.32: 1/4 312 000004d3 I--Q-- 3 perm 1f3f0000 32 -1 keyring _uid_ses.32: empty 313 00000892 I--QU- 1 perm 1f000000 0 0 user metal:copper: 0 314 00000893 I--Q-N 1 35s 1f3f0000 0 0 user metal:silver: 0 315 00000894 I--Q-- 1 10h 003f0000 0 0 user metal:gold: 0 316 317 The flags are: 318 319 I Instantiated 320 R Revoked 321 D Dead 322 Q Contributes to user's quota 323 U Under construction by callback to userspace 324 N Negative key 325 326 This file must be enabled at kernel configuration time as it allows anyone 327 to list the keys database. 328 329 (*) /proc/key-users 330 331 This file lists the tracking data for each user that has at least one key 332 on the system. Such data includes quota information and statistics: 333 334 [root@andromeda root]# cat /proc/key-users 335 0: 46 45/45 1/100 13/10000 336 29: 2 2/2 2/100 40/10000 337 32: 2 2/2 2/100 40/10000 338 38: 2 2/2 2/100 40/10000 339 340 The format of each line is 341 <UID>: User ID to which this applies 342 <usage> Structure refcount 343 <inst>/<keys> Total number of keys and number instantiated 344 <keys>/<max> Key count quota 345 <bytes>/<max> Key size quota 346 347 348Four new sysctl files have been added also for the purpose of controlling the 349quota limits on keys: 350 351 (*) /proc/sys/kernel/keys/root_maxkeys 352 /proc/sys/kernel/keys/root_maxbytes 353 354 These files hold the maximum number of keys that root may have and the 355 maximum total number of bytes of data that root may have stored in those 356 keys. 357 358 (*) /proc/sys/kernel/keys/maxkeys 359 /proc/sys/kernel/keys/maxbytes 360 361 These files hold the maximum number of keys that each non-root user may 362 have and the maximum total number of bytes of data that each of those 363 users may have stored in their keys. 364 365Root may alter these by writing each new limit as a decimal number string to 366the appropriate file. 367 368 369=============================== 370USERSPACE SYSTEM CALL INTERFACE 371=============================== 372 373Userspace can manipulate keys directly through three new syscalls: add_key, 374request_key and keyctl. The latter provides a number of functions for 375manipulating keys. 376 377When referring to a key directly, userspace programs should use the key's 378serial number (a positive 32-bit integer). However, there are some special 379values available for referring to special keys and keyrings that relate to the 380process making the call: 381 382 CONSTANT VALUE KEY REFERENCED 383 ============================== ====== =========================== 384 KEY_SPEC_THREAD_KEYRING -1 thread-specific keyring 385 KEY_SPEC_PROCESS_KEYRING -2 process-specific keyring 386 KEY_SPEC_SESSION_KEYRING -3 session-specific keyring 387 KEY_SPEC_USER_KEYRING -4 UID-specific keyring 388 KEY_SPEC_USER_SESSION_KEYRING -5 UID-session keyring 389 KEY_SPEC_GROUP_KEYRING -6 GID-specific keyring 390 KEY_SPEC_REQKEY_AUTH_KEY -7 assumed request_key() 391 authorisation key 392 393 394The main syscalls are: 395 396 (*) Create a new key of given type, description and payload and add it to the 397 nominated keyring: 398 399 key_serial_t add_key(const char *type, const char *desc, 400 const void *payload, size_t plen, 401 key_serial_t keyring); 402 403 If a key of the same type and description as that proposed already exists 404 in the keyring, this will try to update it with the given payload, or it 405 will return error EEXIST if that function is not supported by the key 406 type. The process must also have permission to write to the key to be able 407 to update it. The new key will have all user permissions granted and no 408 group or third party permissions. 409 410 Otherwise, this will attempt to create a new key of the specified type and 411 description, and to instantiate it with the supplied payload and attach it 412 to the keyring. In this case, an error will be generated if the process 413 does not have permission to write to the keyring. 414 415 The payload is optional, and the pointer can be NULL if not required by 416 the type. The payload is plen in size, and plen can be zero for an empty 417 payload. 418 419 A new keyring can be generated by setting type "keyring", the keyring name 420 as the description (or NULL) and setting the payload to NULL. 421 422 User defined keys can be created by specifying type "user". It is 423 recommended that a user defined key's description by prefixed with a type 424 ID and a colon, such as "krb5tgt:" for a Kerberos 5 ticket granting 425 ticket. 426 427 Any other type must have been registered with the kernel in advance by a 428 kernel service such as a filesystem. 429 430 The ID of the new or updated key is returned if successful. 431 432 433 (*) Search the process's keyrings for a key, potentially calling out to 434 userspace to create it. 435 436 key_serial_t request_key(const char *type, const char *description, 437 const char *callout_info, 438 key_serial_t dest_keyring); 439 440 This function searches all the process's keyrings in the order thread, 441 process, session for a matching key. This works very much like 442 KEYCTL_SEARCH, including the optional attachment of the discovered key to 443 a keyring. 444 445 If a key cannot be found, and if callout_info is not NULL, then 446 /sbin/request-key will be invoked in an attempt to obtain a key. The 447 callout_info string will be passed as an argument to the program. 448 449 See also Documentation/security/keys-request-key.txt. 450 451 452The keyctl syscall functions are: 453 454 (*) Map a special key ID to a real key ID for this process: 455 456 key_serial_t keyctl(KEYCTL_GET_KEYRING_ID, key_serial_t id, 457 int create); 458 459 The special key specified by "id" is looked up (with the key being created 460 if necessary) and the ID of the key or keyring thus found is returned if 461 it exists. 462 463 If the key does not yet exist, the key will be created if "create" is 464 non-zero; and the error ENOKEY will be returned if "create" is zero. 465 466 467 (*) Replace the session keyring this process subscribes to with a new one: 468 469 key_serial_t keyctl(KEYCTL_JOIN_SESSION_KEYRING, const char *name); 470 471 If name is NULL, an anonymous keyring is created attached to the process 472 as its session keyring, displacing the old session keyring. 473 474 If name is not NULL, if a keyring of that name exists, the process 475 attempts to attach it as the session keyring, returning an error if that 476 is not permitted; otherwise a new keyring of that name is created and 477 attached as the session keyring. 478 479 To attach to a named keyring, the keyring must have search permission for 480 the process's ownership. 481 482 The ID of the new session keyring is returned if successful. 483 484 485 (*) Update the specified key: 486 487 long keyctl(KEYCTL_UPDATE, key_serial_t key, const void *payload, 488 size_t plen); 489 490 This will try to update the specified key with the given payload, or it 491 will return error EOPNOTSUPP if that function is not supported by the key 492 type. The process must also have permission to write to the key to be able 493 to update it. 494 495 The payload is of length plen, and may be absent or empty as for 496 add_key(). 497 498 499 (*) Revoke a key: 500 501 long keyctl(KEYCTL_REVOKE, key_serial_t key); 502 503 This makes a key unavailable for further operations. Further attempts to 504 use the key will be met with error EKEYREVOKED, and the key will no longer 505 be findable. 506 507 508 (*) Change the ownership of a key: 509 510 long keyctl(KEYCTL_CHOWN, key_serial_t key, uid_t uid, gid_t gid); 511 512 This function permits a key's owner and group ID to be changed. Either one 513 of uid or gid can be set to -1 to suppress that change. 514 515 Only the superuser can change a key's owner to something other than the 516 key's current owner. Similarly, only the superuser can change a key's 517 group ID to something other than the calling process's group ID or one of 518 its group list members. 519 520 521 (*) Change the permissions mask on a key: 522 523 long keyctl(KEYCTL_SETPERM, key_serial_t key, key_perm_t perm); 524 525 This function permits the owner of a key or the superuser to change the 526 permissions mask on a key. 527 528 Only bits the available bits are permitted; if any other bits are set, 529 error EINVAL will be returned. 530 531 532 (*) Describe a key: 533 534 long keyctl(KEYCTL_DESCRIBE, key_serial_t key, char *buffer, 535 size_t buflen); 536 537 This function returns a summary of the key's attributes (but not its 538 payload data) as a string in the buffer provided. 539 540 Unless there's an error, it always returns the amount of data it could 541 produce, even if that's too big for the buffer, but it won't copy more 542 than requested to userspace. If the buffer pointer is NULL then no copy 543 will take place. 544 545 A process must have view permission on the key for this function to be 546 successful. 547 548 If successful, a string is placed in the buffer in the following format: 549 550 <type>;<uid>;<gid>;<perm>;<description> 551 552 Where type and description are strings, uid and gid are decimal, and perm 553 is hexadecimal. A NUL character is included at the end of the string if 554 the buffer is sufficiently big. 555 556 This can be parsed with 557 558 sscanf(buffer, "%[^;];%d;%d;%o;%s", type, &uid, &gid, &mode, desc); 559 560 561 (*) Clear out a keyring: 562 563 long keyctl(KEYCTL_CLEAR, key_serial_t keyring); 564 565 This function clears the list of keys attached to a keyring. The calling 566 process must have write permission on the keyring, and it must be a 567 keyring (or else error ENOTDIR will result). 568 569 This function can also be used to clear special kernel keyrings if they 570 are appropriately marked if the user has CAP_SYS_ADMIN capability. The 571 DNS resolver cache keyring is an example of this. 572 573 574 (*) Link a key into a keyring: 575 576 long keyctl(KEYCTL_LINK, key_serial_t keyring, key_serial_t key); 577 578 This function creates a link from the keyring to the key. The process must 579 have write permission on the keyring and must have link permission on the 580 key. 581 582 Should the keyring not be a keyring, error ENOTDIR will result; and if the 583 keyring is full, error ENFILE will result. 584 585 The link procedure checks the nesting of the keyrings, returning ELOOP if 586 it appears too deep or EDEADLK if the link would introduce a cycle. 587 588 Any links within the keyring to keys that match the new key in terms of 589 type and description will be discarded from the keyring as the new one is 590 added. 591 592 593 (*) Unlink a key or keyring from another keyring: 594 595 long keyctl(KEYCTL_UNLINK, key_serial_t keyring, key_serial_t key); 596 597 This function looks through the keyring for the first link to the 598 specified key, and removes it if found. Subsequent links to that key are 599 ignored. The process must have write permission on the keyring. 600 601 If the keyring is not a keyring, error ENOTDIR will result; and if the key 602 is not present, error ENOENT will be the result. 603 604 605 (*) Search a keyring tree for a key: 606 607 key_serial_t keyctl(KEYCTL_SEARCH, key_serial_t keyring, 608 const char *type, const char *description, 609 key_serial_t dest_keyring); 610 611 This searches the keyring tree headed by the specified keyring until a key 612 is found that matches the type and description criteria. Each keyring is 613 checked for keys before recursion into its children occurs. 614 615 The process must have search permission on the top level keyring, or else 616 error EACCES will result. Only keyrings that the process has search 617 permission on will be recursed into, and only keys and keyrings for which 618 a process has search permission can be matched. If the specified keyring 619 is not a keyring, ENOTDIR will result. 620 621 If the search succeeds, the function will attempt to link the found key 622 into the destination keyring if one is supplied (non-zero ID). All the 623 constraints applicable to KEYCTL_LINK apply in this case too. 624 625 Error ENOKEY, EKEYREVOKED or EKEYEXPIRED will be returned if the search 626 fails. On success, the resulting key ID will be returned. 627 628 629 (*) Read the payload data from a key: 630 631 long keyctl(KEYCTL_READ, key_serial_t keyring, char *buffer, 632 size_t buflen); 633 634 This function attempts to read the payload data from the specified key 635 into the buffer. The process must have read permission on the key to 636 succeed. 637 638 The returned data will be processed for presentation by the key type. For 639 instance, a keyring will return an array of key_serial_t entries 640 representing the IDs of all the keys to which it is subscribed. The user 641 defined key type will return its data as is. If a key type does not 642 implement this function, error EOPNOTSUPP will result. 643 644 As much of the data as can be fitted into the buffer will be copied to 645 userspace if the buffer pointer is not NULL. 646 647 On a successful return, the function will always return the amount of data 648 available rather than the amount copied. 649 650 651 (*) Instantiate a partially constructed key. 652 653 long keyctl(KEYCTL_INSTANTIATE, key_serial_t key, 654 const void *payload, size_t plen, 655 key_serial_t keyring); 656 long keyctl(KEYCTL_INSTANTIATE_IOV, key_serial_t key, 657 const struct iovec *payload_iov, unsigned ioc, 658 key_serial_t keyring); 659 660 If the kernel calls back to userspace to complete the instantiation of a 661 key, userspace should use this call to supply data for the key before the 662 invoked process returns, or else the key will be marked negative 663 automatically. 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 The payload and plen arguments describe the payload data as for add_key(). 673 674 The payload_iov and ioc arguments describe the payload data in an iovec 675 array instead of a single buffer. 676 677 678 (*) Negatively instantiate a partially constructed key. 679 680 long keyctl(KEYCTL_NEGATE, key_serial_t key, 681 unsigned timeout, key_serial_t keyring); 682 long keyctl(KEYCTL_REJECT, key_serial_t key, 683 unsigned timeout, unsigned error, key_serial_t keyring); 684 685 If the kernel calls back to userspace to complete the instantiation of a 686 key, userspace should use this call mark the key as negative before the 687 invoked process returns if it is unable to fulfill the request. 688 689 The process must have write access on the key to be able to instantiate 690 it, and the key must be uninstantiated. 691 692 If a keyring is specified (non-zero), the key will also be linked into 693 that keyring, however all the constraints applying in KEYCTL_LINK apply in 694 this case too. 695 696 If the key is rejected, future searches for it will return the specified 697 error code until the rejected key expires. Negating the key is the same 698 as rejecting the key with ENOKEY as the error code. 699 700 701 (*) Set the default request-key destination keyring. 702 703 long keyctl(KEYCTL_SET_REQKEY_KEYRING, int reqkey_defl); 704 705 This sets the default keyring to which implicitly requested keys will be 706 attached for this thread. reqkey_defl should be one of these constants: 707 708 CONSTANT VALUE NEW DEFAULT KEYRING 709 ====================================== ====== ======================= 710 KEY_REQKEY_DEFL_NO_CHANGE -1 No change 711 KEY_REQKEY_DEFL_DEFAULT 0 Default 712 KEY_REQKEY_DEFL_THREAD_KEYRING 1 Thread keyring 713 KEY_REQKEY_DEFL_PROCESS_KEYRING 2 Process keyring 714 KEY_REQKEY_DEFL_SESSION_KEYRING 3 Session keyring 715 KEY_REQKEY_DEFL_USER_KEYRING 4 User keyring 716 KEY_REQKEY_DEFL_USER_SESSION_KEYRING 5 User session keyring 717 KEY_REQKEY_DEFL_GROUP_KEYRING 6 Group keyring 718 719 The old default will be returned if successful and error EINVAL will be 720 returned if reqkey_defl is not one of the above values. 721 722 The default keyring can be overridden by the keyring indicated to the 723 request_key() system call. 724 725 Note that this setting is inherited across fork/exec. 726 727  The default is: the thread keyring if there is one, otherwise 728 the process keyring if there is one, otherwise the session keyring if 729 there is one, otherwise the user default session keyring. 730 731 732 (*) Set the timeout on a key. 733 734 long keyctl(KEYCTL_SET_TIMEOUT, key_serial_t key, unsigned timeout); 735 736 This sets or clears the timeout on a key. The timeout can be 0 to clear 737 the timeout or a number of seconds to set the expiry time that far into 738 the future. 739 740 The process must have attribute modification access on a key to set its 741 timeout. Timeouts may not be set with this function on negative, revoked 742 or expired keys. 743 744 745 (*) Assume the authority granted to instantiate a key 746 747 long keyctl(KEYCTL_ASSUME_AUTHORITY, key_serial_t key); 748 749 This assumes or divests the authority required to instantiate the 750 specified key. Authority can only be assumed if the thread has the 751 authorisation key associated with the specified key in its keyrings 752 somewhere. 753 754 Once authority is assumed, searches for keys will also search the 755 requester's keyrings using the requester's security label, UID, GID and 756 groups. 757 758 If the requested authority is unavailable, error EPERM will be returned, 759 likewise if the authority has been revoked because the target key is 760 already instantiated. 761 762 If the specified key is 0, then any assumed authority will be divested. 763 764 The assumed authoritative key is inherited across fork and exec. 765 766 767 (*) Get the LSM security context attached to a key. 768 769 long keyctl(KEYCTL_GET_SECURITY, key_serial_t key, char *buffer, 770 size_t buflen) 771 772 This function returns a string that represents the LSM security context 773 attached to a key in the buffer provided. 774 775 Unless there's an error, it always returns the amount of data it could 776 produce, even if that's too big for the buffer, but it won't copy more 777 than requested to userspace. If the buffer pointer is NULL then no copy 778 will take place. 779 780 A NUL character is included at the end of the string if the buffer is 781 sufficiently big. This is included in the returned count. If no LSM is 782 in force then an empty string will be returned. 783 784 A process must have view permission on the key for this function to be 785 successful. 786 787 788 (*) Install the calling process's session keyring on its parent. 789 790 long keyctl(KEYCTL_SESSION_TO_PARENT); 791 792 This functions attempts to install the calling process's session keyring 793 on to the calling process's parent, replacing the parent's current session 794 keyring. 795 796 The calling process must have the same ownership as its parent, the 797 keyring must have the same ownership as the calling process, the calling 798 process must have LINK permission on the keyring and the active LSM module 799 mustn't deny permission, otherwise error EPERM will be returned. 800 801 Error ENOMEM will be returned if there was insufficient memory to complete 802 the operation, otherwise 0 will be returned to indicate success. 803 804 The keyring will be replaced next time the parent process leaves the 805 kernel and resumes executing userspace. 806 807 808 (*) Invalidate a key. 809 810 long keyctl(KEYCTL_INVALIDATE, key_serial_t key); 811 812 This function marks a key as being invalidated and then wakes up the 813 garbage collector. The garbage collector immediately removes invalidated 814 keys from all keyrings and deletes the key when its reference count 815 reaches zero. 816 817 Keys that are marked invalidated become invisible to normal key operations 818 immediately, though they are still visible in /proc/keys until deleted 819 (they're marked with an 'i' flag). 820 821 A process must have search permission on the key for this function to be 822 successful. 823 824 825=============== 826KERNEL SERVICES 827=============== 828 829The kernel services for key management are fairly simple to deal with. They can 830be broken down into two areas: keys and key types. 831 832Dealing with keys is fairly straightforward. Firstly, the kernel service 833registers its type, then it searches for a key of that type. It should retain 834the key as long as it has need of it, and then it should release it. For a 835filesystem or device file, a search would probably be performed during the open 836call, and the key released upon close. How to deal with conflicting keys due to 837two different users opening the same file is left to the filesystem author to 838solve. 839 840To access the key manager, the following header must be #included: 841 842 <linux/key.h> 843 844Specific key types should have a header file under include/keys/ that should be 845used to access that type. For keys of type "user", for example, that would be: 846 847 <keys/user-type.h> 848 849Note that there are two different types of pointers to keys that may be 850encountered: 851 852 (*) struct key * 853 854 This simply points to the key structure itself. Key structures will be at 855 least four-byte aligned. 856 857 (*) key_ref_t 858 859 This is equivalent to a struct key *, but the least significant bit is set 860 if the caller "possesses" the key. By "possession" it is meant that the 861 calling processes has a searchable link to the key from one of its 862 keyrings. There are three functions for dealing with these: 863 864 key_ref_t make_key_ref(const struct key *key, 865 unsigned long possession); 866 867 struct key *key_ref_to_ptr(const key_ref_t key_ref); 868 869 unsigned long is_key_possessed(const key_ref_t key_ref); 870 871 The first function constructs a key reference from a key pointer and 872 possession information (which must be 0 or 1 and not any other value). 873 874 The second function retrieves the key pointer from a reference and the 875 third retrieves the possession flag. 876 877When accessing a key's payload contents, certain precautions must be taken to 878prevent access vs modification races. See the section "Notes on accessing 879payload contents" for more information. 880 881(*) To search for a key, call: 882 883 struct key *request_key(const struct key_type *type, 884 const char *description, 885 const char *callout_info); 886 887 This is used to request a key or keyring with a description that matches 888 the description specified according to the key type's match function. This 889 permits approximate matching to occur. If callout_string is not NULL, then 890 /sbin/request-key will be invoked in an attempt to obtain the key from 891 userspace. In that case, callout_string will be passed as an argument to 892 the program. 893 894 Should the function fail error ENOKEY, EKEYEXPIRED or EKEYREVOKED will be 895 returned. 896 897 If successful, the key will have been attached to the default keyring for 898 implicitly obtained request-key keys, as set by KEYCTL_SET_REQKEY_KEYRING. 899 900 See also Documentation/security/keys-request-key.txt. 901 902 903(*) To search for a key, passing auxiliary data to the upcaller, call: 904 905 struct key *request_key_with_auxdata(const struct key_type *type, 906 const char *description, 907 const void *callout_info, 908 size_t callout_len, 909 void *aux); 910 911 This is identical to request_key(), except that the auxiliary data is 912 passed to the key_type->request_key() op if it exists, and the callout_info 913 is a blob of length callout_len, if given (the length may be 0). 914 915 916(*) A key can be requested asynchronously by calling one of: 917 918 struct key *request_key_async(const struct key_type *type, 919 const char *description, 920 const void *callout_info, 921 size_t callout_len); 922 923 or: 924 925 struct key *request_key_async_with_auxdata(const struct key_type *type, 926 const char *description, 927 const char *callout_info, 928 size_t callout_len, 929 void *aux); 930 931 which are asynchronous equivalents of request_key() and 932 request_key_with_auxdata() respectively. 933 934 These two functions return with the key potentially still under 935 construction. To wait for construction completion, the following should be 936 called: 937 938 int wait_for_key_construction(struct key *key, bool intr); 939 940 The function will wait for the key to finish being constructed and then 941 invokes key_validate() to return an appropriate value to indicate the state 942 of the key (0 indicates the key is usable). 943 944 If intr is true, then the wait can be interrupted by a signal, in which 945 case error ERESTARTSYS will be returned. 946 947 948(*) When it is no longer required, the key should be released using: 949 950 void key_put(struct key *key); 951 952 Or: 953 954 void key_ref_put(key_ref_t key_ref); 955 956 These can be called from interrupt context. If CONFIG_KEYS is not set then 957 the argument will not be parsed. 958 959 960(*) Extra references can be made to a key by calling the following function: 961 962 struct key *key_get(struct key *key); 963 964 These need to be disposed of by calling key_put() when they've been 965 finished with. The key pointer passed in will be returned. If the pointer 966 is NULL or CONFIG_KEYS is not set then the key will not be dereferenced and 967 no increment will take place. 968 969 970(*) A key's serial number can be obtained by calling: 971 972 key_serial_t key_serial(struct key *key); 973 974 If key is NULL or if CONFIG_KEYS is not set then 0 will be returned (in the 975 latter case without parsing the argument). 976 977 978(*) If a keyring was found in the search, this can be further searched by: 979 980 key_ref_t keyring_search(key_ref_t keyring_ref, 981 const struct key_type *type, 982 const char *description) 983 984 This searches the keyring tree specified for a matching key. Error ENOKEY 985 is returned upon failure (use IS_ERR/PTR_ERR to determine). If successful, 986 the returned key will need to be released. 987 988 The possession attribute from the keyring reference is used to control 989 access through the permissions mask and is propagated to the returned key 990 reference pointer if successful. 991 992 993(*) To check the validity of a key, this function can be called: 994 995 int validate_key(struct key *key); 996 997 This checks that the key in question hasn't expired or and hasn't been 998 revoked. Should the key be invalid, error EKEYEXPIRED or EKEYREVOKED will 999 be returned. If the key is NULL or if CONFIG_KEYS is not set then 0 will be 1000 returned (in the latter case without parsing the argument).
1001 1002 1003(*) To register a key type, the following function should be called: 1004 1005 int register_key_type(struct key_type *type); 1006 1007 This will return error EEXIST if a type of the same name is already 1008 present. 1009 1010 1011(*) To unregister a key type, call: 1012 1013 void unregister_key_type(struct key_type *type); 1014 1015 1016Under some circumstances, it may be desirable to deal with a bundle of keys. 1017The facility provides access to the keyring type for managing such a bundle: 1018 1019 struct key_type key_type_keyring; 1020 1021This can be used with a function such as request_key() to find a specific 1022keyring in a process's keyrings. A keyring thus found can then be searched 1023with keyring_search(). Note that it is not possible to use request_key() to 1024search a specific keyring, so using keyrings in this way is of limited utility. 1025 1026 1027=================================== 1028NOTES ON ACCESSING PAYLOAD CONTENTS 1029=================================== 1030 1031The simplest payload is just a number in key->payload.value. In this case, 1032there's no need to indulge in RCU or locking when accessing the payload. 1033 1034More complex payload contents must be allocated and a pointer to them set in 1035key->payload.data. One of the following ways must be selected to access the 1036data: 1037 1038 (1) Unmodifiable key type. 1039 1040 If the key type does not have a modify method, then the key's payload can 1041 be accessed without any form of locking, provided that it's known to be 1042 instantiated (uninstantiated keys cannot be "found"). 1043 1044 (2) The key's semaphore. 1045 1046 The semaphore could be used to govern access to the payload and to control 1047 the payload pointer. It must be write-locked for modifications and would 1048 have to be read-locked for general access. The disadvantage of doing this 1049 is that the accessor may be required to sleep. 1050 1051 (3) RCU. 1052 1053 RCU must be used when the semaphore isn't already held; if the semaphore 1054 is held then the contents can't change under you unexpectedly as the 1055 semaphore must still be used to serialise modifications to the key. The 1056 key management code takes care of this for the key type. 1057 1058 However, this means using: 1059 1060 rcu_read_lock() ... rcu_dereference() ... rcu_read_unlock() 1061 1062 to read the pointer, and: 1063 1064 rcu_dereference() ... rcu_assign_pointer() ... call_rcu() 1065 1066 to set the pointer and dispose of the old contents after a grace period. 1067 Note that only the key type should ever modify a key's payload. 1068 1069 Furthermore, an RCU controlled payload must hold a struct rcu_head for the 1070 use of call_rcu() and, if the payload is of variable size, the length of 1071 the payload. key->datalen cannot be relied upon to be consistent with the 1072 payload just dereferenced if the key's semaphore is not held. 1073 1074 1075=================== 1076DEFINING A KEY TYPE 1077=================== 1078 1079A kernel service may want to define its own key type. For instance, an AFS 1080filesystem might want to define a Kerberos 5 ticket key type. To do this, it 1081author fills in a key_type struct and registers it with the system. 1082 1083Source files that implement key types should include the following header file: 1084 1085 <linux/key-type.h> 1086 1087The structure has a number of fields, some of which are mandatory: 1088 1089 (*) const char *name 1090 1091 The name of the key type. This is used to translate a key type name 1092 supplied by userspace into a pointer to the structure. 1093 1094 1095 (*) size_t def_datalen 1096 1097 This is optional - it supplies the default payload data length as 1098 contributed to the quota. If the key type's payload is always or almost 1099 always the same size, then this is a more efficient way to do things. 1100 1101 The data length (and quota) on a particular key can always be changed 1102 during instantiation or update by calling: 1103 1104 int key_payload_reserve(struct key *key, size_t datalen); 1105 1106 With the revised data length. Error EDQUOT will be returned if this is not 1107 viable. 1108 1109 1110 (*) int (*vet_description)(const char *description); 1111 1112 This optional method is called to vet a key description. If the key type 1113 doesn't approve of the key description, it may return an error, otherwise 1114 it should return 0. 1115 1116 1117 (*) int (*instantiate)(struct key *key, const void *data, size_t datalen); 1118 1119 This method is called to attach a payload to a key during construction. 1120 The payload attached need not bear any relation to the data passed to this 1121 function. 1122 1123 If the amount of data attached to the key differs from the size in 1124 keytype->def_datalen, then key_payload_reserve() should be called. 1125 1126 This method does not have to lock the key in order to attach a payload. 1127 The fact that KEY_FLAG_INSTANTIATED is not set in key->flags prevents 1128 anything else from gaining access to the key. 1129 1130 It is safe to sleep in this method. 1131 1132 1133 (*) int (*update)(struct key *key, const void *data, size_t datalen); 1134 1135 If this type of key can be updated, then this method should be provided. 1136 It is called to update a key's payload from the blob of data provided. 1137 1138 key_payload_reserve() should be called if the data length might change 1139 before any changes are actually made. Note that if this succeeds, the type 1140 is committed to changing the key because it's already been altered, so all 1141 memory allocation must be done first. 1142 1143 The key will have its semaphore write-locked before this method is called, 1144 but this only deters other writers; any changes to the key's payload must 1145 be made under RCU conditions, and call_rcu() must be used to dispose of 1146 the old payload. 1147 1148 key_payload_reserve() should be called before the changes are made, but 1149 after all allocations and other potentially failing function calls are 1150 made. 1151 1152 It is safe to sleep in this method. 1153 1154 1155 (*) int (*match)(const struct key *key, const void *desc); 1156 1157 This method is called to match a key against a description. It should 1158 return non-zero if the two match, zero if they don't. 1159 1160 This method should not need to lock the key in any way. The type and 1161 description can be considered invariant, and the payload should not be 1162 accessed (the key may not yet be instantiated). 1163 1164 It is not safe to sleep in this method; the caller may hold spinlocks. 1165 1166 1167 (*) void (*revoke)(struct key *key); 1168 1169 This method is optional. It is called to discard part of the payload 1170 data upon a key being revoked. The caller will have the key semaphore 1171 write-locked. 1172 1173 It is safe to sleep in this method, though care should be taken to avoid 1174 a deadlock against the key semaphore. 1175 1176 1177 (*) void (*destroy)(struct key *key); 1178 1179 This method is optional. It is called to discard the payload data on a key 1180 when it is being destroyed. 1181 1182 This method does not need to lock the key to access the payload; it can 1183 consider the key as being inaccessible at this time. Note that the key's 1184 type may have been changed before this function is called. 1185 1186 It is not safe to sleep in this method; the caller may hold spinlocks. 1187 1188 1189 (*) void (*describe)(const struct key *key, struct seq_file *p); 1190 1191 This method is optional. It is called during /proc/keys reading to 1192 summarise a key's description and payload in text form. 1193 1194 This method will be called with the RCU read lock held. rcu_dereference() 1195 should be used to read the payload pointer if the payload is to be 1196 accessed. key->datalen cannot be trusted to stay consistent with the 1197 contents of the payload. 1198 1199 The description will not change, though the key's state may. 1200 1201 It is not safe to sleep in this method; the RCU read lock is held by the 1202 caller. 1203 1204 1205 (*) long (*read)(const struct key *key, char __user *buffer, size_t buflen); 1206 1207 This method is optional. It is called by KEYCTL_READ to translate the 1208 key's payload into something a blob of data for userspace to deal with. 1209 Ideally, the blob should be in the same format as that passed in to the 1210 instantiate and update methods. 1211 1212 If successful, the blob size that could be produced should be returned 1213 rather than the size copied. 1214 1215 This method will be called with the key's semaphore read-locked. This will 1216 prevent the key's payload changing. It is not necessary to use RCU locking 1217 when accessing the key's payload. It is safe to sleep in this method, such 1218 as might happen when the userspace buffer is accessed. 1219 1220 1221 (*) int (*request_key)(struct key_construction *cons, const char *op, 1222 void *aux); 1223 1224 This method is optional. If provided, request_key() and friends will 1225 invoke this function rather than upcalling to /sbin/request-key to operate 1226 upon a key of this type. 1227 1228 The aux parameter is as passed to request_key_async_with_auxdata() and 1229 similar or is NULL otherwise. Also passed are the construction record for 1230 the key to be operated upon and the operation type (currently only 1231 "create"). 1232 1233 This method is permitted to return before the upcall is complete, but the 1234 following function must be called under all circumstances to complete the 1235 instantiation process, whether or not it succeeds, whether or not there's 1236 an error: 1237 1238 void complete_request_key(struct key_construction *cons, int error); 1239 1240 The error parameter should be 0 on success, -ve on error. The 1241 construction record is destroyed by this action and the authorisation key 1242 will be revoked. If an error is indicated, the key under construction 1243 will be negatively instantiated if it wasn't already instantiated. 1244 1245 If this method returns an error, that error will be returned to the 1246 caller of request_key*(). complete_request_key() must be called prior to 1247 returning. 1248 1249 The key under construction and the authorisation key can be found in the 1250 key_construction struct pointed to by cons: 1251 1252 (*) struct key *key; 1253 1254 The key under construction. 1255 1256 (*) struct key *authkey; 1257 1258 The authorisation key. 1259 1260 1261============================ 1262REQUEST-KEY CALLBACK SERVICE 1263============================ 1264 1265To create a new key, the kernel will attempt to execute the following command 1266line: 1267 1268 /sbin/request-key create <key> <uid> <gid> \ 1269 <threadring> <processring> <sessionring> <callout_info> 1270 1271<key> is the key being constructed, and the three keyrings are the process 1272keyrings from the process that caused the search to be issued. These are 1273included for two reasons: 1274 1275 (1) There may be an authentication token in one of the keyrings that is 1276 required to obtain the key, eg: a Kerberos Ticket-Granting Ticket. 1277 1278 (2) The new key should probably be cached in one of these rings. 1279 1280This program should set it UID and GID to those specified before attempting to 1281access any more keys. It may then look around for a user specific process to 1282hand the request off to (perhaps a path held in placed in another key by, for 1283example, the KDE desktop manager). 1284 1285The program (or whatever it calls) should finish construction of the key by 1286calling KEYCTL_INSTANTIATE or KEYCTL_INSTANTIATE_IOV, which also permits it to 1287cache the key in one of the keyrings (probably the session ring) before 1288returning. Alternatively, the key can be marked as negative with KEYCTL_NEGATE 1289or KEYCTL_REJECT; this also permits the key to be cached in one of the 1290keyrings. 1291 1292If it returns with the key remaining in the unconstructed state, the key will 1293be marked as being negative, it will be added to the session keyring, and an 1294error will be returned to the key requestor. 1295 1296Supplementary information may be provided from whoever or whatever invoked this 1297service. This will be passed as the <callout_info> parameter. If no such 1298information was made available, then "-" will be passed as this parameter 1299instead. 1300 1301 1302Similarly, the kernel may attempt to update an expired or a soon to expire key 1303by executing: 1304 1305 /sbin/request-key update <key> <uid> <gid> \ 1306 <threadring> <processring> <sessionring> 1307 1308In this case, the program isn't required to actually attach the key to a ring; 1309the rings are provided for reference. 1310 1311 1312================== 1313GARBAGE COLLECTION 1314================== 1315 1316Dead keys (for which the type has been removed) will be automatically unlinked 1317from those keyrings that point to them and deleted as soon as possible by a 1318background garbage collector. 1319 1320Similarly, revoked and expired keys will be garbage collected, but only after a 1321certain amount of time has passed. This time is set as a number of seconds in: 1322 1323 /proc/sys/kernel/keys/gc_delay 1324