1============================ 2Kernel 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.. contents:: :local: 20 21 22Key Overview 23============ 24 25In this context, keys represent units of cryptographic data, authentication 26tokens, keyrings, etc.. These are represented in the kernel by struct key. 27 28Each key has a number of attributes: 29 30 - A serial number. 31 - A type. 32 - A description (for matching a key in a search). 33 - Access control information. 34 - An expiry time. 35 - A payload. 36 - State. 37 38 39 * Each key is issued a serial number of type key_serial_t that is unique for 40 the lifetime of that key. All serial numbers are positive non-zero 32-bit 41 integers. 42 43 Userspace programs can use a key's serial numbers as a way to gain access 44 to it, subject to permission checking. 45 46 * Each key is of a defined "type". Types must be registered inside the 47 kernel by a kernel service (such as a filesystem) before keys of that type 48 can be added or used. Userspace programs cannot define new types directly. 49 50 Key types are represented in the kernel by struct key_type. This defines a 51 number of operations that can be performed on a key of that type. 52 53 Should a type be removed from the system, all the keys of that type will 54 be invalidated. 55 56 * Each key has a description. This should be a printable string. The key 57 type provides an operation to perform a match between the description on a 58 key and a criterion string. 59 60 * Each key has an owner user ID, a group ID and a permissions mask. These 61 are used to control what a process may do to a key from userspace, and 62 whether a kernel service will be able to find the key. 63 64 * Each key can be set to expire at a specific time by the key type's 65 instantiation function. Keys can also be immortal. 66 67 * Each key can have a payload. This is a quantity of data that represent the 68 actual "key". In the case of a keyring, this is a list of keys to which 69 the keyring links; in the case of a user-defined key, it's an arbitrary 70 blob of data. 71 72 Having a payload is not required; and the payload can, in fact, just be a 73 value stored in the struct key itself. 74 75 When a key is instantiated, the key type's instantiation function is 76 called with a blob of data, and that then creates the key's payload in 77 some way. 78 79 Similarly, when userspace wants to read back the contents of the key, if 80 permitted, another key type operation will be called to convert the key's 81 attached payload back into a blob of data. 82 83 * Each key can be in one of a number of basic states: 84 85 * Uninstantiated. The key exists, but does not have any data attached. 86 Keys being requested from userspace will be in this state. 87 88 * Instantiated. This is the normal state. The key is fully formed, and 89 has data attached. 90 91 * Negative. This is a relatively short-lived state. The key acts as a 92 note saying that a previous call out to userspace failed, and acts as 93 a throttle on key lookups. A negative key can be updated to a normal 94 state. 95 96 * Expired. Keys can have lifetimes set. If their lifetime is exceeded, 97 they traverse to this state. An expired key can be updated back to a 98 normal state. 99 100 * Revoked. A key is put in this state by userspace action. It can't be 101 found or operated upon (apart from by unlinking it). 102 103 * Dead. The key's type was unregistered, and so the key is now useless. 104 105Keys in the last three states are subject to garbage collection. See the 106section on "Garbage collection". 107 108 109Key Service Overview 110==================== 111 112The key service provides a number of features besides keys: 113 114 * The key service defines three special key types: 115 116 (+) "keyring" 117 118 Keyrings are special keys that contain a list of other keys. Keyring 119 lists can be modified using various system calls. Keyrings should not 120 be given a payload when created. 121 122 (+) "user" 123 124 A key of this type has a description and a payload that are arbitrary 125 blobs of data. These can be created, updated and read by userspace, 126 and aren't intended for use by kernel services. 127 128 (+) "logon" 129 130 Like a "user" key, a "logon" key has a payload that is an arbitrary 131 blob of data. It is intended as a place to store secrets which are 132 accessible to the kernel but not to userspace programs. 133 134 The description can be arbitrary, but must be prefixed with a non-zero 135 length string that describes the key "subclass". The subclass is 136 separated from the rest of the description by a ':'. "logon" keys can 137 be created and updated from userspace, but the payload is only 138 readable from kernel space. 139 140 * Each process subscribes to three keyrings: a thread-specific keyring, a 141 process-specific keyring, and a session-specific keyring. 142 143 The thread-specific keyring is discarded from the child when any sort of 144 clone, fork, vfork or execve occurs. A new keyring is created only when 145 required. 146 147 The process-specific keyring is replaced with an empty one in the child on 148 clone, fork, vfork unless CLONE_THREAD is supplied, in which case it is 149 shared. execve also discards the process's process keyring and creates a 150 new one. 151 152 The session-specific keyring is persistent across clone, fork, vfork and 153 execve, even when the latter executes a set-UID or set-GID binary. A 154 process can, however, replace its current session keyring with a new one 155 by using PR_JOIN_SESSION_KEYRING. It is permitted to request an anonymous 156 new one, or to attempt to create or join one of a specific name. 157 158 The ownership of the thread keyring changes when the real UID and GID of 159 the thread changes. 160 161 * Each user ID resident in the system holds two special keyrings: a user 162 specific keyring and a default user session keyring. The default session 163 keyring is initialised with a link to the user-specific keyring. 164 165 When a process changes its real UID, if it used to have no session key, it 166 will be subscribed to the default session key for the new UID. 167 168 If a process attempts to access its session key when it doesn't have one, 169 it will be subscribed to the default for its current UID. 170 171 * Each user has two quotas against which the keys they own are tracked. One 172 limits the total number of keys and keyrings, the other limits the total 173 amount of description and payload space that can be consumed. 174 175 The user can view information on this and other statistics through procfs 176 files. The root user may also alter the quota limits through sysctl files 177 (see the section "New procfs files"). 178 179 Process-specific and thread-specific keyrings are not counted towards a 180 user's quota. 181 182 If a system call that modifies a key or keyring in some way would put the 183 user over quota, the operation is refused and error EDQUOT is returned. 184 185 * There's a system call interface by which userspace programs can create and 186 manipulate keys and keyrings. 187 188 * There's a kernel interface by which services can register types and search 189 for keys. 190 191 * There's a way for the a search done from the kernel to call back to 192 userspace to request a key that can't be found in a process's keyrings. 193 194 * An optional filesystem is available through which the key database can be 195 viewed and manipulated. 196 197 198Key Access Permissions 199====================== 200 201Keys have an owner user ID, a group access ID, and a permissions mask. The mask 202has up to eight bits each for possessor, user, group and other access. Only 203six of each set of eight bits are defined. These permissions granted are: 204 205 * View 206 207 This permits a key or keyring's attributes to be viewed - including key 208 type and description. 209 210 * Read 211 212 This permits a key's payload to be viewed or a keyring's list of linked 213 keys. 214 215 * Write 216 217 This permits a key's payload to be instantiated or updated, or it allows a 218 link to be added to or removed from a keyring. 219 220 * Search 221 222 This permits keyrings to be searched and keys to be found. Searches can 223 only recurse into nested keyrings that have search permission set. 224 225 * Link 226 227 This permits a key or keyring to be linked to. To create a link from a 228 keyring to a key, a process must have Write permission on the keyring and 229 Link permission on the key. 230 231 * Set Attribute 232 233 This permits a key's UID, GID and permissions mask to be changed. 234 235For changing the ownership, group ID or permissions mask, being the owner of 236the key or having the sysadmin capability is sufficient. 237 238 239SELinux Support 240=============== 241 242The security class "key" has been added to SELinux so that mandatory access 243controls can be applied to keys created within various contexts. This support 244is preliminary, and is likely to change quite significantly in the near future. 245Currently, all of the basic permissions explained above are provided in SELinux 246as well; SELinux is simply invoked after all basic permission checks have been 247performed. 248 249The value of the file /proc/self/attr/keycreate influences the labeling of 250newly-created keys. If the contents of that file correspond to an SELinux 251security context, then the key will be assigned that context. Otherwise, the 252key will be assigned the current context of the task that invoked the key 253creation request. Tasks must be granted explicit permission to assign a 254particular context to newly-created keys, using the "create" permission in the 255key security class. 256 257The default keyrings associated with users will be labeled with the default 258context of the user if and only if the login programs have been instrumented to 259properly initialize keycreate during the login process. Otherwise, they will 260be labeled with the context of the login program itself. 261 262Note, however, that the default keyrings associated with the root user are 263labeled with the default kernel context, since they are created early in the 264boot process, before root has a chance to log in. 265 266The keyrings associated with new threads are each labeled with the context of 267their associated thread, and both session and process keyrings are handled 268similarly. 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 310 311 312 * /proc/key-users 313 314 This file lists the tracking data for each user that has at least one key 315 on the system. Such data includes quota information and statistics:: 316 317 [root@andromeda root]# cat /proc/key-users 318 0: 46 45/45 1/100 13/10000 319 29: 2 2/2 2/100 40/10000 320 32: 2 2/2 2/100 40/10000 321 38: 2 2/2 2/100 40/10000 322 323 The format of each line is:: 324 325 <UID>: User ID to which this applies 326 <usage> Structure refcount 327 <inst>/<keys> Total number of keys and number instantiated 328 <keys>/<max> Key count quota 329 <bytes>/<max> Key size quota 330 331 332Four new sysctl files have been added also for the purpose of controlling the 333quota limits on keys: 334 335 * /proc/sys/kernel/keys/root_maxkeys 336 /proc/sys/kernel/keys/root_maxbytes 337 338 These files hold the maximum number of keys that root may have and the 339 maximum total number of bytes of data that root may have stored in those 340 keys. 341 342 * /proc/sys/kernel/keys/maxkeys 343 /proc/sys/kernel/keys/maxbytes 344 345 These files hold the maximum number of keys that each non-root user may 346 have and the maximum total number of bytes of data that each of those 347 users may have stored in their keys. 348 349Root may alter these by writing each new limit as a decimal number string to 350the appropriate file. 351 352 353Userspace System Call Interface 354=============================== 355 356Userspace can manipulate keys directly through three new syscalls: add_key, 357request_key and keyctl. The latter provides a number of functions for 358manipulating keys. 359 360When referring to a key directly, userspace programs should use the key's 361serial number (a positive 32-bit integer). However, there are some special 362values available for referring to special keys and keyrings that relate to the 363process making the call:: 364 365 CONSTANT VALUE KEY REFERENCED 366 ============================== ====== =========================== 367 KEY_SPEC_THREAD_KEYRING -1 thread-specific keyring 368 KEY_SPEC_PROCESS_KEYRING -2 process-specific keyring 369 KEY_SPEC_SESSION_KEYRING -3 session-specific keyring 370 KEY_SPEC_USER_KEYRING -4 UID-specific keyring 371 KEY_SPEC_USER_SESSION_KEYRING -5 UID-session keyring 372 KEY_SPEC_GROUP_KEYRING -6 GID-specific keyring 373 KEY_SPEC_REQKEY_AUTH_KEY -7 assumed request_key() 374 authorisation key 375 376 377The main syscalls are: 378 379 * Create a new key of given type, description and payload and add it to the 380 nominated keyring:: 381 382 key_serial_t add_key(const char *type, const char *desc, 383 const void *payload, size_t plen, 384 key_serial_t keyring); 385 386 If a key of the same type and description as that proposed already exists 387 in the keyring, this will try to update it with the given payload, or it 388 will return error EEXIST if that function is not supported by the key 389 type. The process must also have permission to write to the key to be able 390 to update it. The new key will have all user permissions granted and no 391 group or third party permissions. 392 393 Otherwise, this will attempt to create a new key of the specified type and 394 description, and to instantiate it with the supplied payload and attach it 395 to the keyring. In this case, an error will be generated if the process 396 does not have permission to write to the keyring. 397 398 If the key type supports it, if the description is NULL or an empty 399 string, the key type will try and generate a description from the content 400 of the payload. 401 402 The payload is optional, and the pointer can be NULL if not required by 403 the type. The payload is plen in size, and plen can be zero for an empty 404 payload. 405 406 A new keyring can be generated by setting type "keyring", the keyring name 407 as the description (or NULL) and setting the payload to NULL. 408 409 User defined keys can be created by specifying type "user". It is 410 recommended that a user defined key's description by prefixed with a type 411 ID and a colon, such as "krb5tgt:" for a Kerberos 5 ticket granting 412 ticket. 413 414 Any other type must have been registered with the kernel in advance by a 415 kernel service such as a filesystem. 416 417 The ID of the new or updated key is returned if successful. 418 419 420 * Search the process's keyrings for a key, potentially calling out to 421 userspace to create it:: 422 423 key_serial_t request_key(const char *type, const char *description, 424 const char *callout_info, 425 key_serial_t dest_keyring); 426 427 This function searches all the process's keyrings in the order thread, 428 process, session for a matching key. This works very much like 429 KEYCTL_SEARCH, including the optional attachment of the discovered key to 430 a keyring. 431 432 If a key cannot be found, and if callout_info is not NULL, then 433 /sbin/request-key will be invoked in an attempt to obtain a key. The 434 callout_info string will be passed as an argument to the program. 435 436 To link a key into the destination keyring the key must grant link 437 permission on the key to the caller and the keyring must grant write 438 permission. 439 440 See also Documentation/security/keys/request-key.rst. 441 442 443The keyctl syscall functions are: 444 445 * Map a special key ID to a real key ID for this process:: 446 447 key_serial_t keyctl(KEYCTL_GET_KEYRING_ID, key_serial_t id, 448 int create); 449 450 The special key specified by "id" is looked up (with the key being created 451 if necessary) and the ID of the key or keyring thus found is returned if 452 it exists. 453 454 If the key does not yet exist, the key will be created if "create" is 455 non-zero; and the error ENOKEY will be returned if "create" is zero. 456 457 458 * Replace the session keyring this process subscribes to with a new one:: 459 460 key_serial_t keyctl(KEYCTL_JOIN_SESSION_KEYRING, const char *name); 461 462 If name is NULL, an anonymous keyring is created attached to the process 463 as its session keyring, displacing the old session keyring. 464 465 If name is not NULL, if a keyring of that name exists, the process 466 attempts to attach it as the session keyring, returning an error if that 467 is not permitted; otherwise a new keyring of that name is created and 468 attached as the session keyring. 469 470 To attach to a named keyring, the keyring must have search permission for 471 the process's ownership. 472 473 The ID of the new session keyring is returned if successful. 474 475 476 * Update the specified key:: 477 478 long keyctl(KEYCTL_UPDATE, key_serial_t key, const void *payload, 479 size_t plen); 480 481 This will try to update the specified key with the given payload, or it 482 will return error EOPNOTSUPP if that function is not supported by the key 483 type. The process must also have permission to write to the key to be able 484 to update it. 485 486 The payload is of length plen, and may be absent or empty as for 487 add_key(). 488 489 490 * Revoke a key:: 491 492 long keyctl(KEYCTL_REVOKE, key_serial_t key); 493 494 This makes a key unavailable for further operations. Further attempts to 495 use the key will be met with error EKEYREVOKED, and the key will no longer 496 be findable. 497 498 499 * Change the ownership of a key:: 500 501 long keyctl(KEYCTL_CHOWN, key_serial_t key, uid_t uid, gid_t gid); 502 503 This function permits a key's owner and group ID to be changed. Either one 504 of uid or gid can be set to -1 to suppress that change. 505 506 Only the superuser can change a key's owner to something other than the 507 key's current owner. Similarly, only the superuser can change a key's 508 group ID to something other than the calling process's group ID or one of 509 its group list members. 510 511 512 * Change the permissions mask on a key:: 513 514 long keyctl(KEYCTL_SETPERM, key_serial_t key, key_perm_t perm); 515 516 This function permits the owner of a key or the superuser to change the 517 permissions mask on a key. 518 519 Only bits the available bits are permitted; if any other bits are set, 520 error EINVAL will be returned. 521 522 523 * Describe a key:: 524 525 long keyctl(KEYCTL_DESCRIBE, key_serial_t key, char *buffer, 526 size_t buflen); 527 528 This function returns a summary of the key's attributes (but not its 529 payload data) as a string in the buffer provided. 530 531 Unless there's an error, it always returns the amount of data it could 532 produce, even if that's too big for the buffer, but it won't copy more 533 than requested to userspace. If the buffer pointer is NULL then no copy 534 will take place. 535 536 A process must have view permission on the key for this function to be 537 successful. 538 539 If successful, a string is placed in the buffer in the following format:: 540 541 <type>;<uid>;<gid>;<perm>;<description> 542 543 Where type and description are strings, uid and gid are decimal, and perm 544 is hexadecimal. A NUL character is included at the end of the string if 545 the buffer is sufficiently big. 546 547 This can be parsed with:: 548 549 sscanf(buffer, "%[^;];%d;%d;%o;%s", type, &uid, &gid, &mode, desc); 550 551 552 * Clear out a keyring:: 553 554 long keyctl(KEYCTL_CLEAR, key_serial_t keyring); 555 556 This function clears the list of keys attached to a keyring. The calling 557 process must have write permission on the keyring, and it must be a 558 keyring (or else error ENOTDIR will result). 559 560 This function can also be used to clear special kernel keyrings if they 561 are appropriately marked if the user has CAP_SYS_ADMIN capability. The 562 DNS resolver cache keyring is an example of this. 563 564 565 * Link a key into a keyring:: 566 567 long keyctl(KEYCTL_LINK, key_serial_t keyring, key_serial_t key); 568 569 This function creates a link from the keyring to the key. The process must 570 have write permission on the keyring and must have link permission on the 571 key. 572 573 Should the keyring not be a keyring, error ENOTDIR will result; and if the 574 keyring is full, error ENFILE will result. 575 576 The link procedure checks the nesting of the keyrings, returning ELOOP if 577 it appears too deep or EDEADLK if the link would introduce a cycle. 578 579 Any links within the keyring to keys that match the new key in terms of 580 type and description will be discarded from the keyring as the new one is 581 added. 582 583 584 * Move a key from one keyring to another:: 585 586 long keyctl(KEYCTL_MOVE, 587 key_serial_t id, 588 key_serial_t from_ring_id, 589 key_serial_t to_ring_id, 590 unsigned int flags); 591 592 Move the key specified by "id" from the keyring specified by 593 "from_ring_id" to the keyring specified by "to_ring_id". If the two 594 keyrings are the same, nothing is done. 595 596 "flags" can have KEYCTL_MOVE_EXCL set in it to cause the operation to fail 597 with EEXIST if a matching key exists in the destination keyring, otherwise 598 such a key will be replaced. 599 600 A process must have link permission on the key for this function to be 601 successful and write permission on both keyrings. Any errors that can 602 occur from KEYCTL_LINK also apply on the destination keyring here. 603 604 605 * Unlink a key or keyring from another keyring:: 606 607 long keyctl(KEYCTL_UNLINK, key_serial_t keyring, key_serial_t key); 608 609 This function looks through the keyring for the first link to the 610 specified key, and removes it if found. Subsequent links to that key are 611 ignored. The process must have write permission on the keyring. 612 613 If the keyring is not a keyring, error ENOTDIR will result; and if the key 614 is not present, error ENOENT will be the result. 615 616 617 * Search a keyring tree for a key:: 618 619 key_serial_t keyctl(KEYCTL_SEARCH, key_serial_t keyring, 620 const char *type, const char *description, 621 key_serial_t dest_keyring); 622 623 This searches the keyring tree headed by the specified keyring until a key 624 is found that matches the type and description criteria. Each keyring is 625 checked for keys before recursion into its children occurs. 626 627 The process must have search permission on the top level keyring, or else 628 error EACCES will result. Only keyrings that the process has search 629 permission on will be recursed into, and only keys and keyrings for which 630 a process has search permission can be matched. If the specified keyring 631 is not a keyring, ENOTDIR will result. 632 633 If the search succeeds, the function will attempt to link the found key 634 into the destination keyring if one is supplied (non-zero ID). All the 635 constraints applicable to KEYCTL_LINK apply in this case too. 636 637 Error ENOKEY, EKEYREVOKED or EKEYEXPIRED will be returned if the search 638 fails. On success, the resulting key ID will be returned. 639 640 641 * Read the payload data from a key:: 642 643 long keyctl(KEYCTL_READ, key_serial_t keyring, char *buffer, 644 size_t buflen); 645 646 This function attempts to read the payload data from the specified key 647 into the buffer. The process must have read permission on the key to 648 succeed. 649 650 The returned data will be processed for presentation by the key type. For 651 instance, a keyring will return an array of key_serial_t entries 652 representing the IDs of all the keys to which it is subscribed. The user 653 defined key type will return its data as is. If a key type does not 654 implement this function, error EOPNOTSUPP will result. 655 656 If the specified buffer is too small, then the size of the buffer required 657 will be returned. Note that in this case, the contents of the buffer may 658 have been overwritten in some undefined way. 659 660 Otherwise, on success, the function will return the amount of data copied 661 into the buffer. 662 663 * Instantiate a partially constructed key:: 664 665 long keyctl(KEYCTL_INSTANTIATE, key_serial_t key, 666 const void *payload, size_t plen, 667 key_serial_t keyring); 668 long keyctl(KEYCTL_INSTANTIATE_IOV, key_serial_t key, 669 const struct iovec *payload_iov, unsigned ioc, 670 key_serial_t keyring); 671 672 If the kernel calls back to userspace to complete the instantiation of a 673 key, userspace should use this call to supply data for the key before the 674 invoked process returns, or else the key will be marked negative 675 automatically. 676 677 The process must have write access on the key to be able to instantiate 678 it, and the key must be uninstantiated. 679 680 If a keyring is specified (non-zero), the key will also be linked into 681 that keyring, however all the constraints applying in KEYCTL_LINK apply in 682 this case too. 683 684 The payload and plen arguments describe the payload data as for add_key(). 685 686 The payload_iov and ioc arguments describe the payload data in an iovec 687 array instead of a single buffer. 688 689 690 * Negatively instantiate a partially constructed key:: 691 692 long keyctl(KEYCTL_NEGATE, key_serial_t key, 693 unsigned timeout, key_serial_t keyring); 694 long keyctl(KEYCTL_REJECT, key_serial_t key, 695 unsigned timeout, unsigned error, key_serial_t keyring); 696 697 If the kernel calls back to userspace to complete the instantiation of a 698 key, userspace should use this call mark the key as negative before the 699 invoked process returns if it is unable to fulfill the request. 700 701 The process must have write access on the key to be able to instantiate 702 it, and the key must be uninstantiated. 703 704 If a keyring is specified (non-zero), the key will also be linked into 705 that keyring, however all the constraints applying in KEYCTL_LINK apply in 706 this case too. 707 708 If the key is rejected, future searches for it will return the specified 709 error code until the rejected key expires. Negating the key is the same 710 as rejecting the key with ENOKEY as the error code. 711 712 713 * Set the default request-key destination keyring:: 714 715 long keyctl(KEYCTL_SET_REQKEY_KEYRING, int reqkey_defl); 716 717 This sets the default keyring to which implicitly requested keys will be 718 attached for this thread. reqkey_defl should be one of these constants:: 719 720 CONSTANT VALUE NEW DEFAULT KEYRING 721 ====================================== ====== ======================= 722 KEY_REQKEY_DEFL_NO_CHANGE -1 No change 723 KEY_REQKEY_DEFL_DEFAULT 0 Default[1] 724 KEY_REQKEY_DEFL_THREAD_KEYRING 1 Thread keyring 725 KEY_REQKEY_DEFL_PROCESS_KEYRING 2 Process keyring 726 KEY_REQKEY_DEFL_SESSION_KEYRING 3 Session keyring 727 KEY_REQKEY_DEFL_USER_KEYRING 4 User keyring 728 KEY_REQKEY_DEFL_USER_SESSION_KEYRING 5 User session keyring 729 KEY_REQKEY_DEFL_GROUP_KEYRING 6 Group keyring 730 731 The old default will be returned if successful and error EINVAL will be 732 returned if reqkey_defl is not one of the above values. 733 734 The default keyring can be overridden by the keyring indicated to the 735 request_key() system call. 736 737 Note that this setting is inherited across fork/exec. 738 739 [1] The default is: the thread keyring if there is one, otherwise 740 the process keyring if there is one, otherwise the session keyring if 741 there is one, otherwise the user default session keyring. 742 743 744 * Set the timeout on a key:: 745 746 long keyctl(KEYCTL_SET_TIMEOUT, key_serial_t key, unsigned timeout); 747 748 This sets or clears the timeout on a key. The timeout can be 0 to clear 749 the timeout or a number of seconds to set the expiry time that far into 750 the future. 751 752 The process must have attribute modification access on a key to set its 753 timeout. Timeouts may not be set with this function on negative, revoked 754 or expired keys. 755 756 757 * Assume the authority granted to instantiate a key:: 758 759 long keyctl(KEYCTL_ASSUME_AUTHORITY, key_serial_t key); 760 761 This assumes or divests the authority required to instantiate the 762 specified key. Authority can only be assumed if the thread has the 763 authorisation key associated with the specified key in its keyrings 764 somewhere. 765 766 Once authority is assumed, searches for keys will also search the 767 requester's keyrings using the requester's security label, UID, GID and 768 groups. 769 770 If the requested authority is unavailable, error EPERM will be returned, 771 likewise if the authority has been revoked because the target key is 772 already instantiated. 773 774 If the specified key is 0, then any assumed authority will be divested. 775 776 The assumed authoritative key is inherited across fork and exec. 777 778 779 * Get the LSM security context attached to a key:: 780 781 long keyctl(KEYCTL_GET_SECURITY, key_serial_t key, char *buffer, 782 size_t buflen) 783 784 This function returns a string that represents the LSM security context 785 attached to a key in the buffer provided. 786 787 Unless there's an error, it always returns the amount of data it could 788 produce, even if that's too big for the buffer, but it won't copy more 789 than requested to userspace. If the buffer pointer is NULL then no copy 790 will take place. 791 792 A NUL character is included at the end of the string if the buffer is 793 sufficiently big. This is included in the returned count. If no LSM is 794 in force then an empty string will be returned. 795 796 A process must have view permission on the key for this function to be 797 successful. 798 799 800 * Install the calling process's session keyring on its parent:: 801 802 long keyctl(KEYCTL_SESSION_TO_PARENT); 803 804 This functions attempts to install the calling process's session keyring 805 on to the calling process's parent, replacing the parent's current session 806 keyring. 807 808 The calling process must have the same ownership as its parent, the 809 keyring must have the same ownership as the calling process, the calling 810 process must have LINK permission on the keyring and the active LSM module 811 mustn't deny permission, otherwise error EPERM will be returned. 812 813 Error ENOMEM will be returned if there was insufficient memory to complete 814 the operation, otherwise 0 will be returned to indicate success. 815 816 The keyring will be replaced next time the parent process leaves the 817 kernel and resumes executing userspace. 818 819 820 * Invalidate a key:: 821 822 long keyctl(KEYCTL_INVALIDATE, key_serial_t key); 823 824 This function marks a key as being invalidated and then wakes up the 825 garbage collector. The garbage collector immediately removes invalidated 826 keys from all keyrings and deletes the key when its reference count 827 reaches zero. 828 829 Keys that are marked invalidated become invisible to normal key operations 830 immediately, though they are still visible in /proc/keys until deleted 831 (they're marked with an 'i' flag). 832 833 A process must have search permission on the key for this function to be 834 successful. 835 836 * Compute a Diffie-Hellman shared secret or public key:: 837 838 long keyctl(KEYCTL_DH_COMPUTE, struct keyctl_dh_params *params, 839 char *buffer, size_t buflen, struct keyctl_kdf_params *kdf); 840 841 The params struct contains serial numbers for three keys:: 842 843 - The prime, p, known to both parties 844 - The local private key 845 - The base integer, which is either a shared generator or the 846 remote public key 847 848 The value computed is:: 849 850 result = base ^ private (mod prime) 851 852 If the base is the shared generator, the result is the local 853 public key. If the base is the remote public key, the result is 854 the shared secret. 855 856 If the parameter kdf is NULL, the following applies: 857 858 - The buffer length must be at least the length of the prime, or zero. 859 860 - If the buffer length is nonzero, the length of the result is 861 returned when it is successfully calculated and copied in to the 862 buffer. When the buffer length is zero, the minimum required 863 buffer length is returned. 864 865 The kdf parameter allows the caller to apply a key derivation function 866 (KDF) on the Diffie-Hellman computation where only the result 867 of the KDF is returned to the caller. The KDF is characterized with 868 struct keyctl_kdf_params as follows: 869 870 - ``char *hashname`` specifies the NUL terminated string identifying 871 the hash used from the kernel crypto API and applied for the KDF 872 operation. The KDF implemenation complies with SP800-56A as well 873 as with SP800-108 (the counter KDF). 874 875 - ``char *otherinfo`` specifies the OtherInfo data as documented in 876 SP800-56A section 5.8.1.2. The length of the buffer is given with 877 otherinfolen. The format of OtherInfo is defined by the caller. 878 The otherinfo pointer may be NULL if no OtherInfo shall be used. 879 880 This function will return error EOPNOTSUPP if the key type is not 881 supported, error ENOKEY if the key could not be found, or error 882 EACCES if the key is not readable by the caller. In addition, the 883 function will return EMSGSIZE when the parameter kdf is non-NULL 884 and either the buffer length or the OtherInfo length exceeds the 885 allowed length. 886 887 888 * Restrict keyring linkage:: 889 890 long keyctl(KEYCTL_RESTRICT_KEYRING, key_serial_t keyring, 891 const char *type, const char *restriction); 892 893 An existing keyring can restrict linkage of additional keys by evaluating 894 the contents of the key according to a restriction scheme. 895 896 "keyring" is the key ID for an existing keyring to apply a restriction 897 to. It may be empty or may already have keys linked. Existing linked keys 898 will remain in the keyring even if the new restriction would reject them. 899 900 "type" is a registered key type. 901 902 "restriction" is a string describing how key linkage is to be restricted. 903 The format varies depending on the key type, and the string is passed to 904 the lookup_restriction() function for the requested type. It may specify 905 a method and relevant data for the restriction such as signature 906 verification or constraints on key payload. If the requested key type is 907 later unregistered, no keys may be added to the keyring after the key type 908 is removed. 909 910 To apply a keyring restriction the process must have Set Attribute 911 permission and the keyring must not be previously restricted. 912 913 One application of restricted keyrings is to verify X.509 certificate 914 chains or individual certificate signatures using the asymmetric key type. 915 See Documentation/crypto/asymmetric-keys.txt for specific restrictions 916 applicable to the asymmetric key type. 917 918 919 * Query an asymmetric key:: 920 921 long keyctl(KEYCTL_PKEY_QUERY, 922 key_serial_t key_id, unsigned long reserved, 923 struct keyctl_pkey_query *info); 924 925 Get information about an asymmetric key. The information is returned in 926 the keyctl_pkey_query struct:: 927 928 __u32 supported_ops; 929 __u32 key_size; 930 __u16 max_data_size; 931 __u16 max_sig_size; 932 __u16 max_enc_size; 933 __u16 max_dec_size; 934 __u32 __spare[10]; 935 936 ``supported_ops`` contains a bit mask of flags indicating which ops are 937 supported. This is constructed from a bitwise-OR of:: 938 939 KEYCTL_SUPPORTS_{ENCRYPT,DECRYPT,SIGN,VERIFY} 940 941 ``key_size`` indicated the size of the key in bits. 942 943 ``max_*_size`` indicate the maximum sizes in bytes of a blob of data to be 944 signed, a signature blob, a blob to be encrypted and a blob to be 945 decrypted. 946 947 ``__spare[]`` must be set to 0. This is intended for future use to hand 948 over one or more passphrases needed unlock a key. 949 950 If successful, 0 is returned. If the key is not an asymmetric key, 951 EOPNOTSUPP is returned. 952 953 954 * Encrypt, decrypt, sign or verify a blob using an asymmetric key:: 955 956 long keyctl(KEYCTL_PKEY_ENCRYPT, 957 const struct keyctl_pkey_params *params, 958 const char *info, 959 const void *in, 960 void *out); 961 962 long keyctl(KEYCTL_PKEY_DECRYPT, 963 const struct keyctl_pkey_params *params, 964 const char *info, 965 const void *in, 966 void *out); 967 968 long keyctl(KEYCTL_PKEY_SIGN, 969 const struct keyctl_pkey_params *params, 970 const char *info, 971 const void *in, 972 void *out); 973 974 long keyctl(KEYCTL_PKEY_VERIFY, 975 const struct keyctl_pkey_params *params, 976 const char *info, 977 const void *in, 978 const void *in2); 979 980 Use an asymmetric key to perform a public-key cryptographic operation a 981 blob of data. For encryption and verification, the asymmetric key may 982 only need the public parts to be available, but for decryption and signing 983 the private parts are required also. 984 985 The parameter block pointed to by params contains a number of integer 986 values:: 987 988 __s32 key_id; 989 __u32 in_len; 990 __u32 out_len; 991 __u32 in2_len; 992 993 ``key_id`` is the ID of the asymmetric key to be used. ``in_len`` and 994 ``in2_len`` indicate the amount of data in the in and in2 buffers and 995 ``out_len`` indicates the size of the out buffer as appropriate for the 996 above operations. 997 998 For a given operation, the in and out buffers are used as follows:: 999 1000 Operation ID in,in_len out,out_len in2,in2_len 1001 ======================= =============== =============== =============== 1002 KEYCTL_PKEY_ENCRYPT Raw data Encrypted data - 1003 KEYCTL_PKEY_DECRYPT Encrypted data Raw data - 1004 KEYCTL_PKEY_SIGN Raw data Signature - 1005 KEYCTL_PKEY_VERIFY Raw data - Signature 1006 1007 ``info`` is a string of key=value pairs that supply supplementary 1008 information. These include: 1009 1010 ``enc=<encoding>`` The encoding of the encrypted/signature blob. This 1011 can be "pkcs1" for RSASSA-PKCS1-v1.5 or 1012 RSAES-PKCS1-v1.5; "pss" for "RSASSA-PSS"; "oaep" for 1013 "RSAES-OAEP". If omitted or is "raw", the raw output 1014 of the encryption function is specified. 1015 1016 ``hash=<algo>`` If the data buffer contains the output of a hash 1017 function and the encoding includes some indication of 1018 which hash function was used, the hash function can be 1019 specified with this, eg. "hash=sha256". 1020 1021 The ``__spare[]`` space in the parameter block must be set to 0. This is 1022 intended, amongst other things, to allow the passing of passphrases 1023 required to unlock a key. 1024 1025 If successful, encrypt, decrypt and sign all return the amount of data 1026 written into the output buffer. Verification returns 0 on success. 1027 1028 1029 * Watch a key or keyring for changes:: 1030 1031 long keyctl(KEYCTL_WATCH_KEY, key_serial_t key, int queue_fd, 1032 const struct watch_notification_filter *filter); 1033 1034 This will set or remove a watch for changes on the specified key or 1035 keyring. 1036 1037 "key" is the ID of the key to be watched. 1038 1039 "queue_fd" is a file descriptor referring to an open "/dev/watch_queue" 1040 which manages the buffer into which notifications will be delivered. 1041 1042 "filter" is either NULL to remove a watch or a filter specification to 1043 indicate what events are required from the key. 1044 1045 See Documentation/watch_queue.rst for more information. 1046 1047 Note that only one watch may be emplaced for any particular { key, 1048 queue_fd } combination. 1049 1050 Notification records look like:: 1051 1052 struct key_notification { 1053 struct watch_notification watch; 1054 __u32 key_id; 1055 __u32 aux; 1056 }; 1057 1058 In this, watch::type will be "WATCH_TYPE_KEY_NOTIFY" and subtype will be 1059 one of:: 1060 1061 NOTIFY_KEY_INSTANTIATED 1062 NOTIFY_KEY_UPDATED 1063 NOTIFY_KEY_LINKED 1064 NOTIFY_KEY_UNLINKED 1065 NOTIFY_KEY_CLEARED 1066 NOTIFY_KEY_REVOKED 1067 NOTIFY_KEY_INVALIDATED 1068 NOTIFY_KEY_SETATTR 1069 1070 Where these indicate a key being instantiated/rejected, updated, a link 1071 being made in a keyring, a link being removed from a keyring, a keyring 1072 being cleared, a key being revoked, a key being invalidated or a key 1073 having one of its attributes changed (user, group, perm, timeout, 1074 restriction). 1075 1076 If a watched key is deleted, a basic watch_notification will be issued 1077 with "type" set to WATCH_TYPE_META and "subtype" set to 1078 watch_meta_removal_notification. The watchpoint ID will be set in the 1079 "info" field. 1080 1081 This needs to be configured by enabling: 1082 1083 "Provide key/keyring change notifications" (KEY_NOTIFICATIONS) 1084 1085 1086Kernel Services 1087=============== 1088 1089The kernel services for key management are fairly simple to deal with. They can 1090be broken down into two areas: keys and key types. 1091 1092Dealing with keys is fairly straightforward. Firstly, the kernel service 1093registers its type, then it searches for a key of that type. It should retain 1094the key as long as it has need of it, and then it should release it. For a 1095filesystem or device file, a search would probably be performed during the open 1096call, and the key released upon close. How to deal with conflicting keys due to 1097two different users opening the same file is left to the filesystem author to 1098solve. 1099 1100To access the key manager, the following header must be #included:: 1101 1102 <linux/key.h> 1103 1104Specific key types should have a header file under include/keys/ that should be 1105used to access that type. For keys of type "user", for example, that would be:: 1106 1107 <keys/user-type.h> 1108 1109Note that there are two different types of pointers to keys that may be 1110encountered: 1111 1112 * struct key * 1113 1114 This simply points to the key structure itself. Key structures will be at 1115 least four-byte aligned. 1116 1117 * key_ref_t 1118 1119 This is equivalent to a ``struct key *``, but the least significant bit is set 1120 if the caller "possesses" the key. By "possession" it is meant that the 1121 calling processes has a searchable link to the key from one of its 1122 keyrings. There are three functions for dealing with these:: 1123 1124 key_ref_t make_key_ref(const struct key *key, bool possession); 1125 1126 struct key *key_ref_to_ptr(const key_ref_t key_ref); 1127 1128 bool is_key_possessed(const key_ref_t key_ref); 1129 1130 The first function constructs a key reference from a key pointer and 1131 possession information (which must be true or false). 1132 1133 The second function retrieves the key pointer from a reference and the 1134 third retrieves the possession flag. 1135 1136When accessing a key's payload contents, certain precautions must be taken to 1137prevent access vs modification races. See the section "Notes on accessing 1138payload contents" for more information. 1139 1140 * To search for a key, call:: 1141 1142 struct key *request_key(const struct key_type *type, 1143 const char *description, 1144 const char *callout_info); 1145 1146 This is used to request a key or keyring with a description that matches 1147 the description specified according to the key type's match_preparse() 1148 method. This permits approximate matching to occur. If callout_string is 1149 not NULL, then /sbin/request-key will be invoked in an attempt to obtain 1150 the key from userspace. In that case, callout_string will be passed as an 1151 argument to the program. 1152 1153 Should the function fail error ENOKEY, EKEYEXPIRED or EKEYREVOKED will be 1154 returned. 1155 1156 If successful, the key will have been attached to the default keyring for 1157 implicitly obtained request-key keys, as set by KEYCTL_SET_REQKEY_KEYRING. 1158 1159 See also Documentation/security/keys/request-key.rst. 1160 1161 1162 * To search for a key in a specific domain, call:: 1163 1164 struct key *request_key_tag(const struct key_type *type, 1165 const char *description, 1166 struct key_tag *domain_tag, 1167 const char *callout_info); 1168 1169 This is identical to request_key(), except that a domain tag may be 1170 specifies that causes search algorithm to only match keys matching that 1171 tag. The domain_tag may be NULL, specifying a global domain that is 1172 separate from any nominated domain. 1173 1174 1175 * To search for a key, passing auxiliary data to the upcaller, call:: 1176 1177 struct key *request_key_with_auxdata(const struct key_type *type, 1178 const char *description, 1179 struct key_tag *domain_tag, 1180 const void *callout_info, 1181 size_t callout_len, 1182 void *aux); 1183 1184 This is identical to request_key_tag(), except that the auxiliary data is 1185 passed to the key_type->request_key() op if it exists, and the 1186 callout_info is a blob of length callout_len, if given (the length may be 1187 0). 1188 1189 1190 * To search for a key under RCU conditions, call:: 1191 1192 struct key *request_key_rcu(const struct key_type *type, 1193 const char *description, 1194 struct key_tag *domain_tag); 1195 1196 which is similar to request_key_tag() except that it does not check for 1197 keys that are under construction and it will not call out to userspace to 1198 construct a key if it can't find a match. 1199 1200 1201 * When it is no longer required, the key should be released using:: 1202 1203 void key_put(struct key *key); 1204 1205 Or:: 1206 1207 void key_ref_put(key_ref_t key_ref); 1208 1209 These can be called from interrupt context. If CONFIG_KEYS is not set then 1210 the argument will not be parsed. 1211 1212 1213 * Extra references can be made to a key by calling one of the following 1214 functions:: 1215 1216 struct key *__key_get(struct key *key); 1217 struct key *key_get(struct key *key); 1218 1219 Keys so references will need to be disposed of by calling key_put() when 1220 they've been finished with. The key pointer passed in will be returned. 1221 1222 In the case of key_get(), if the pointer is NULL or CONFIG_KEYS is not set 1223 then the key will not be dereferenced and no increment will take place. 1224 1225 1226 * A key's serial number can be obtained by calling:: 1227 1228 key_serial_t key_serial(struct key *key); 1229 1230 If key is NULL or if CONFIG_KEYS is not set then 0 will be returned (in the 1231 latter case without parsing the argument). 1232 1233 1234 * If a keyring was found in the search, this can be further searched by:: 1235 1236 key_ref_t keyring_search(key_ref_t keyring_ref, 1237 const struct key_type *type, 1238 const char *description, 1239 bool recurse) 1240 1241 This searches the specified keyring only (recurse == false) or keyring tree 1242 (recurse == true) specified for a matching key. Error ENOKEY is returned 1243 upon failure (use IS_ERR/PTR_ERR to determine). If successful, the returned 1244 key will need to be released. 1245 1246 The possession attribute from the keyring reference is used to control 1247 access through the permissions mask and is propagated to the returned key 1248 reference pointer if successful. 1249 1250 1251 * A keyring can be created by:: 1252 1253 struct key *keyring_alloc(const char *description, uid_t uid, gid_t gid, 1254 const struct cred *cred, 1255 key_perm_t perm, 1256 struct key_restriction *restrict_link, 1257 unsigned long flags, 1258 struct key *dest); 1259 1260 This creates a keyring with the given attributes and returns it. If dest 1261 is not NULL, the new keyring will be linked into the keyring to which it 1262 points. No permission checks are made upon the destination keyring. 1263 1264 Error EDQUOT can be returned if the keyring would overload the quota (pass 1265 KEY_ALLOC_NOT_IN_QUOTA in flags if the keyring shouldn't be accounted 1266 towards the user's quota). Error ENOMEM can also be returned. 1267 1268 If restrict_link is not NULL, it should point to a structure that contains 1269 the function that will be called each time an attempt is made to link a 1270 key into the new keyring. The structure may also contain a key pointer 1271 and an associated key type. The function is called to check whether a key 1272 may be added into the keyring or not. The key type is used by the garbage 1273 collector to clean up function or data pointers in this structure if the 1274 given key type is unregistered. Callers of key_create_or_update() within 1275 the kernel can pass KEY_ALLOC_BYPASS_RESTRICTION to suppress the check. 1276 An example of using this is to manage rings of cryptographic keys that are 1277 set up when the kernel boots where userspace is also permitted to add keys 1278 - provided they can be verified by a key the kernel already has. 1279 1280 When called, the restriction function will be passed the keyring being 1281 added to, the key type, the payload of the key being added, and data to be 1282 used in the restriction check. Note that when a new key is being created, 1283 this is called between payload preparsing and actual key creation. The 1284 function should return 0 to allow the link or an error to reject it. 1285 1286 A convenience function, restrict_link_reject, exists to always return 1287 -EPERM to in this case. 1288 1289 1290 * To check the validity of a key, this function can be called:: 1291 1292 int validate_key(struct key *key); 1293 1294 This checks that the key in question hasn't expired or and hasn't been 1295 revoked. Should the key be invalid, error EKEYEXPIRED or EKEYREVOKED will 1296 be returned. If the key is NULL or if CONFIG_KEYS is not set then 0 will be 1297 returned (in the latter case without parsing the argument). 1298 1299 1300 * To register a key type, the following function should be called:: 1301 1302 int register_key_type(struct key_type *type); 1303 1304 This will return error EEXIST if a type of the same name is already 1305 present. 1306 1307 1308 * To unregister a key type, call:: 1309 1310 void unregister_key_type(struct key_type *type); 1311 1312 1313Under some circumstances, it may be desirable to deal with a bundle of keys. 1314The facility provides access to the keyring type for managing such a bundle:: 1315 1316 struct key_type key_type_keyring; 1317 1318This can be used with a function such as request_key() to find a specific 1319keyring in a process's keyrings. A keyring thus found can then be searched 1320with keyring_search(). Note that it is not possible to use request_key() to 1321search a specific keyring, so using keyrings in this way is of limited utility. 1322 1323 1324Notes On Accessing Payload Contents 1325=================================== 1326 1327The simplest payload is just data stored in key->payload directly. In this 1328case, there's no need to indulge in RCU or locking when accessing the payload. 1329 1330More complex payload contents must be allocated and pointers to them set in the 1331key->payload.data[] array. One of the following ways must be selected to 1332access the data: 1333 1334 1) Unmodifiable key type. 1335 1336 If the key type does not have a modify method, then the key's payload can 1337 be accessed without any form of locking, provided that it's known to be 1338 instantiated (uninstantiated keys cannot be "found"). 1339 1340 2) The key's semaphore. 1341 1342 The semaphore could be used to govern access to the payload and to control 1343 the payload pointer. It must be write-locked for modifications and would 1344 have to be read-locked for general access. The disadvantage of doing this 1345 is that the accessor may be required to sleep. 1346 1347 3) RCU. 1348 1349 RCU must be used when the semaphore isn't already held; if the semaphore 1350 is held then the contents can't change under you unexpectedly as the 1351 semaphore must still be used to serialise modifications to the key. The 1352 key management code takes care of this for the key type. 1353 1354 However, this means using:: 1355 1356 rcu_read_lock() ... rcu_dereference() ... rcu_read_unlock() 1357 1358 to read the pointer, and:: 1359 1360 rcu_dereference() ... rcu_assign_pointer() ... call_rcu() 1361 1362 to set the pointer and dispose of the old contents after a grace period. 1363 Note that only the key type should ever modify a key's payload. 1364 1365 Furthermore, an RCU controlled payload must hold a struct rcu_head for the 1366 use of call_rcu() and, if the payload is of variable size, the length of 1367 the payload. key->datalen cannot be relied upon to be consistent with the 1368 payload just dereferenced if the key's semaphore is not held. 1369 1370 Note that key->payload.data[0] has a shadow that is marked for __rcu 1371 usage. This is called key->payload.rcu_data0. The following accessors 1372 wrap the RCU calls to this element: 1373 1374 a) Set or change the first payload pointer:: 1375 1376 rcu_assign_keypointer(struct key *key, void *data); 1377 1378 b) Read the first payload pointer with the key semaphore held:: 1379 1380 [const] void *dereference_key_locked([const] struct key *key); 1381 1382 Note that the return value will inherit its constness from the key 1383 parameter. Static analysis will give an error if it things the lock 1384 isn't held. 1385 1386 c) Read the first payload pointer with the RCU read lock held:: 1387 1388 const void *dereference_key_rcu(const struct key *key); 1389 1390 1391Defining a Key Type 1392=================== 1393 1394A kernel service may want to define its own key type. For instance, an AFS 1395filesystem might want to define a Kerberos 5 ticket key type. To do this, it 1396author fills in a key_type struct and registers it with the system. 1397 1398Source files that implement key types should include the following header file:: 1399 1400 <linux/key-type.h> 1401 1402The structure has a number of fields, some of which are mandatory: 1403 1404 * ``const char *name`` 1405 1406 The name of the key type. This is used to translate a key type name 1407 supplied by userspace into a pointer to the structure. 1408 1409 1410 * ``size_t def_datalen`` 1411 1412 This is optional - it supplies the default payload data length as 1413 contributed to the quota. If the key type's payload is always or almost 1414 always the same size, then this is a more efficient way to do things. 1415 1416 The data length (and quota) on a particular key can always be changed 1417 during instantiation or update by calling:: 1418 1419 int key_payload_reserve(struct key *key, size_t datalen); 1420 1421 With the revised data length. Error EDQUOT will be returned if this is not 1422 viable. 1423 1424 1425 * ``int (*vet_description)(const char *description);`` 1426 1427 This optional method is called to vet a key description. If the key type 1428 doesn't approve of the key description, it may return an error, otherwise 1429 it should return 0. 1430 1431 1432 * ``int (*preparse)(struct key_preparsed_payload *prep);`` 1433 1434 This optional method permits the key type to attempt to parse payload 1435 before a key is created (add key) or the key semaphore is taken (update or 1436 instantiate key). The structure pointed to by prep looks like:: 1437 1438 struct key_preparsed_payload { 1439 char *description; 1440 union key_payload payload; 1441 const void *data; 1442 size_t datalen; 1443 size_t quotalen; 1444 time_t expiry; 1445 }; 1446 1447 Before calling the method, the caller will fill in data and datalen with 1448 the payload blob parameters; quotalen will be filled in with the default 1449 quota size from the key type; expiry will be set to TIME_T_MAX and the 1450 rest will be cleared. 1451 1452 If a description can be proposed from the payload contents, that should be 1453 attached as a string to the description field. This will be used for the 1454 key description if the caller of add_key() passes NULL or "". 1455 1456 The method can attach anything it likes to payload. This is merely passed 1457 along to the instantiate() or update() operations. If set, the expiry 1458 time will be applied to the key if it is instantiated from this data. 1459 1460 The method should return 0 if successful or a negative error code 1461 otherwise. 1462 1463 1464 * ``void (*free_preparse)(struct key_preparsed_payload *prep);`` 1465 1466 This method is only required if the preparse() method is provided, 1467 otherwise it is unused. It cleans up anything attached to the description 1468 and payload fields of the key_preparsed_payload struct as filled in by the 1469 preparse() method. It will always be called after preparse() returns 1470 successfully, even if instantiate() or update() succeed. 1471 1472 1473 * ``int (*instantiate)(struct key *key, struct key_preparsed_payload *prep);`` 1474 1475 This method is called to attach a payload to a key during construction. 1476 The payload attached need not bear any relation to the data passed to this 1477 function. 1478 1479 The prep->data and prep->datalen fields will define the original payload 1480 blob. If preparse() was supplied then other fields may be filled in also. 1481 1482 If the amount of data attached to the key differs from the size in 1483 keytype->def_datalen, then key_payload_reserve() should be called. 1484 1485 This method does not have to lock the key in order to attach a payload. 1486 The fact that KEY_FLAG_INSTANTIATED is not set in key->flags prevents 1487 anything else from gaining access to the key. 1488 1489 It is safe to sleep in this method. 1490 1491 generic_key_instantiate() is provided to simply copy the data from 1492 prep->payload.data[] to key->payload.data[], with RCU-safe assignment on 1493 the first element. It will then clear prep->payload.data[] so that the 1494 free_preparse method doesn't release the data. 1495 1496 1497 * ``int (*update)(struct key *key, const void *data, size_t datalen);`` 1498 1499 If this type of key can be updated, then this method should be provided. 1500 It is called to update a key's payload from the blob of data provided. 1501 1502 The prep->data and prep->datalen fields will define the original payload 1503 blob. If preparse() was supplied then other fields may be filled in also. 1504 1505 key_payload_reserve() should be called if the data length might change 1506 before any changes are actually made. Note that if this succeeds, the type 1507 is committed to changing the key because it's already been altered, so all 1508 memory allocation must be done first. 1509 1510 The key will have its semaphore write-locked before this method is called, 1511 but this only deters other writers; any changes to the key's payload must 1512 be made under RCU conditions, and call_rcu() must be used to dispose of 1513 the old payload. 1514 1515 key_payload_reserve() should be called before the changes are made, but 1516 after all allocations and other potentially failing function calls are 1517 made. 1518 1519 It is safe to sleep in this method. 1520 1521 1522 * ``int (*match_preparse)(struct key_match_data *match_data);`` 1523 1524 This method is optional. It is called when a key search is about to be 1525 performed. It is given the following structure:: 1526 1527 struct key_match_data { 1528 bool (*cmp)(const struct key *key, 1529 const struct key_match_data *match_data); 1530 const void *raw_data; 1531 void *preparsed; 1532 unsigned lookup_type; 1533 }; 1534 1535 On entry, raw_data will be pointing to the criteria to be used in matching 1536 a key by the caller and should not be modified. ``(*cmp)()`` will be pointing 1537 to the default matcher function (which does an exact description match 1538 against raw_data) and lookup_type will be set to indicate a direct lookup. 1539 1540 The following lookup_type values are available: 1541 1542 * KEYRING_SEARCH_LOOKUP_DIRECT - A direct lookup hashes the type and 1543 description to narrow down the search to a small number of keys. 1544 1545 * KEYRING_SEARCH_LOOKUP_ITERATE - An iterative lookup walks all the 1546 keys in the keyring until one is matched. This must be used for any 1547 search that's not doing a simple direct match on the key description. 1548 1549 The method may set cmp to point to a function of its choice that does some 1550 other form of match, may set lookup_type to KEYRING_SEARCH_LOOKUP_ITERATE 1551 and may attach something to the preparsed pointer for use by ``(*cmp)()``. 1552 ``(*cmp)()`` should return true if a key matches and false otherwise. 1553 1554 If preparsed is set, it may be necessary to use the match_free() method to 1555 clean it up. 1556 1557 The method should return 0 if successful or a negative error code 1558 otherwise. 1559 1560 It is permitted to sleep in this method, but ``(*cmp)()`` may not sleep as 1561 locks will be held over it. 1562 1563 If match_preparse() is not provided, keys of this type will be matched 1564 exactly by their description. 1565 1566 1567 * ``void (*match_free)(struct key_match_data *match_data);`` 1568 1569 This method is optional. If given, it called to clean up 1570 match_data->preparsed after a successful call to match_preparse(). 1571 1572 1573 * ``void (*revoke)(struct key *key);`` 1574 1575 This method is optional. It is called to discard part of the payload 1576 data upon a key being revoked. The caller will have the key semaphore 1577 write-locked. 1578 1579 It is safe to sleep in this method, though care should be taken to avoid 1580 a deadlock against the key semaphore. 1581 1582 1583 * ``void (*destroy)(struct key *key);`` 1584 1585 This method is optional. It is called to discard the payload data on a key 1586 when it is being destroyed. 1587 1588 This method does not need to lock the key to access the payload; it can 1589 consider the key as being inaccessible at this time. Note that the key's 1590 type may have been changed before this function is called. 1591 1592 It is not safe to sleep in this method; the caller may hold spinlocks. 1593 1594 1595 * ``void (*describe)(const struct key *key, struct seq_file *p);`` 1596 1597 This method is optional. It is called during /proc/keys reading to 1598 summarise a key's description and payload in text form. 1599 1600 This method will be called with the RCU read lock held. rcu_dereference() 1601 should be used to read the payload pointer if the payload is to be 1602 accessed. key->datalen cannot be trusted to stay consistent with the 1603 contents of the payload. 1604 1605 The description will not change, though the key's state may. 1606 1607 It is not safe to sleep in this method; the RCU read lock is held by the 1608 caller. 1609 1610 1611 * ``long (*read)(const struct key *key, char __user *buffer, size_t buflen);`` 1612 1613 This method is optional. It is called by KEYCTL_READ to translate the 1614 key's payload into something a blob of data for userspace to deal with. 1615 Ideally, the blob should be in the same format as that passed in to the 1616 instantiate and update methods. 1617 1618 If successful, the blob size that could be produced should be returned 1619 rather than the size copied. 1620 1621 This method will be called with the key's semaphore read-locked. This will 1622 prevent the key's payload changing. It is not necessary to use RCU locking 1623 when accessing the key's payload. It is safe to sleep in this method, such 1624 as might happen when the userspace buffer is accessed. 1625 1626 1627 * ``int (*request_key)(struct key_construction *cons, const char *op, void *aux);`` 1628 1629 This method is optional. If provided, request_key() and friends will 1630 invoke this function rather than upcalling to /sbin/request-key to operate 1631 upon a key of this type. 1632 1633 The aux parameter is as passed to request_key_async_with_auxdata() and 1634 similar or is NULL otherwise. Also passed are the construction record for 1635 the key to be operated upon and the operation type (currently only 1636 "create"). 1637 1638 This method is permitted to return before the upcall is complete, but the 1639 following function must be called under all circumstances to complete the 1640 instantiation process, whether or not it succeeds, whether or not there's 1641 an error:: 1642 1643 void complete_request_key(struct key_construction *cons, int error); 1644 1645 The error parameter should be 0 on success, -ve on error. The 1646 construction record is destroyed by this action and the authorisation key 1647 will be revoked. If an error is indicated, the key under construction 1648 will be negatively instantiated if it wasn't already instantiated. 1649 1650 If this method returns an error, that error will be returned to the 1651 caller of request_key*(). complete_request_key() must be called prior to 1652 returning. 1653 1654 The key under construction and the authorisation key can be found in the 1655 key_construction struct pointed to by cons: 1656 1657 * ``struct key *key;`` 1658 1659 The key under construction. 1660 1661 * ``struct key *authkey;`` 1662 1663 The authorisation key. 1664 1665 1666 * ``struct key_restriction *(*lookup_restriction)(const char *params);`` 1667 1668 This optional method is used to enable userspace configuration of keyring 1669 restrictions. The restriction parameter string (not including the key type 1670 name) is passed in, and this method returns a pointer to a key_restriction 1671 structure containing the relevant functions and data to evaluate each 1672 attempted key link operation. If there is no match, -EINVAL is returned. 1673 1674 1675 * ``asym_eds_op`` and ``asym_verify_signature``:: 1676 1677 int (*asym_eds_op)(struct kernel_pkey_params *params, 1678 const void *in, void *out); 1679 int (*asym_verify_signature)(struct kernel_pkey_params *params, 1680 const void *in, const void *in2); 1681 1682 These methods are optional. If provided the first allows a key to be 1683 used to encrypt, decrypt or sign a blob of data, and the second allows a 1684 key to verify a signature. 1685 1686 In all cases, the following information is provided in the params block:: 1687 1688 struct kernel_pkey_params { 1689 struct key *key; 1690 const char *encoding; 1691 const char *hash_algo; 1692 char *info; 1693 __u32 in_len; 1694 union { 1695 __u32 out_len; 1696 __u32 in2_len; 1697 }; 1698 enum kernel_pkey_operation op : 8; 1699 }; 1700 1701 This includes the key to be used; a string indicating the encoding to use 1702 (for instance, "pkcs1" may be used with an RSA key to indicate 1703 RSASSA-PKCS1-v1.5 or RSAES-PKCS1-v1.5 encoding or "raw" if no encoding); 1704 the name of the hash algorithm used to generate the data for a signature 1705 (if appropriate); the sizes of the input and output (or second input) 1706 buffers; and the ID of the operation to be performed. 1707 1708 For a given operation ID, the input and output buffers are used as 1709 follows:: 1710 1711 Operation ID in,in_len out,out_len in2,in2_len 1712 ======================= =============== =============== =============== 1713 kernel_pkey_encrypt Raw data Encrypted data - 1714 kernel_pkey_decrypt Encrypted data Raw data - 1715 kernel_pkey_sign Raw data Signature - 1716 kernel_pkey_verify Raw data - Signature 1717 1718 asym_eds_op() deals with encryption, decryption and signature creation as 1719 specified by params->op. Note that params->op is also set for 1720 asym_verify_signature(). 1721 1722 Encrypting and signature creation both take raw data in the input buffer 1723 and return the encrypted result in the output buffer. Padding may have 1724 been added if an encoding was set. In the case of signature creation, 1725 depending on the encoding, the padding created may need to indicate the 1726 digest algorithm - the name of which should be supplied in hash_algo. 1727 1728 Decryption takes encrypted data in the input buffer and returns the raw 1729 data in the output buffer. Padding will get checked and stripped off if 1730 an encoding was set. 1731 1732 Verification takes raw data in the input buffer and the signature in the 1733 second input buffer and checks that the one matches the other. Padding 1734 will be validated. Depending on the encoding, the digest algorithm used 1735 to generate the raw data may need to be indicated in hash_algo. 1736 1737 If successful, asym_eds_op() should return the number of bytes written 1738 into the output buffer. asym_verify_signature() should return 0. 1739 1740 A variety of errors may be returned, including EOPNOTSUPP if the operation 1741 is not supported; EKEYREJECTED if verification fails; ENOPKG if the 1742 required crypto isn't available. 1743 1744 1745 * ``asym_query``:: 1746 1747 int (*asym_query)(const struct kernel_pkey_params *params, 1748 struct kernel_pkey_query *info); 1749 1750 This method is optional. If provided it allows information about the 1751 public or asymmetric key held in the key to be determined. 1752 1753 The parameter block is as for asym_eds_op() and co. but in_len and out_len 1754 are unused. The encoding and hash_algo fields should be used to reduce 1755 the returned buffer/data sizes as appropriate. 1756 1757 If successful, the following information is filled in:: 1758 1759 struct kernel_pkey_query { 1760 __u32 supported_ops; 1761 __u32 key_size; 1762 __u16 max_data_size; 1763 __u16 max_sig_size; 1764 __u16 max_enc_size; 1765 __u16 max_dec_size; 1766 }; 1767 1768 The supported_ops field will contain a bitmask indicating what operations 1769 are supported by the key, including encryption of a blob, decryption of a 1770 blob, signing a blob and verifying the signature on a blob. The following 1771 constants are defined for this:: 1772 1773 KEYCTL_SUPPORTS_{ENCRYPT,DECRYPT,SIGN,VERIFY} 1774 1775 The key_size field is the size of the key in bits. max_data_size and 1776 max_sig_size are the maximum raw data and signature sizes for creation and 1777 verification of a signature; max_enc_size and max_dec_size are the maximum 1778 raw data and signature sizes for encryption and decryption. The 1779 max_*_size fields are measured in bytes. 1780 1781 If successful, 0 will be returned. If the key doesn't support this, 1782 EOPNOTSUPP will be returned. 1783 1784 1785Request-Key Callback Service 1786============================ 1787 1788To create a new key, the kernel will attempt to execute the following command 1789line:: 1790 1791 /sbin/request-key create <key> <uid> <gid> \ 1792 <threadring> <processring> <sessionring> <callout_info> 1793 1794<key> is the key being constructed, and the three keyrings are the process 1795keyrings from the process that caused the search to be issued. These are 1796included for two reasons: 1797 1798 1 There may be an authentication token in one of the keyrings that is 1799 required to obtain the key, eg: a Kerberos Ticket-Granting Ticket. 1800 1801 2 The new key should probably be cached in one of these rings. 1802 1803This program should set it UID and GID to those specified before attempting to 1804access any more keys. It may then look around for a user specific process to 1805hand the request off to (perhaps a path held in placed in another key by, for 1806example, the KDE desktop manager). 1807 1808The program (or whatever it calls) should finish construction of the key by 1809calling KEYCTL_INSTANTIATE or KEYCTL_INSTANTIATE_IOV, which also permits it to 1810cache the key in one of the keyrings (probably the session ring) before 1811returning. Alternatively, the key can be marked as negative with KEYCTL_NEGATE 1812or KEYCTL_REJECT; this also permits the key to be cached in one of the 1813keyrings. 1814 1815If it returns with the key remaining in the unconstructed state, the key will 1816be marked as being negative, it will be added to the session keyring, and an 1817error will be returned to the key requestor. 1818 1819Supplementary information may be provided from whoever or whatever invoked this 1820service. This will be passed as the <callout_info> parameter. If no such 1821information was made available, then "-" will be passed as this parameter 1822instead. 1823 1824 1825Similarly, the kernel may attempt to update an expired or a soon to expire key 1826by executing:: 1827 1828 /sbin/request-key update <key> <uid> <gid> \ 1829 <threadring> <processring> <sessionring> 1830 1831In this case, the program isn't required to actually attach the key to a ring; 1832the rings are provided for reference. 1833 1834 1835Garbage Collection 1836================== 1837 1838Dead keys (for which the type has been removed) will be automatically unlinked 1839from those keyrings that point to them and deleted as soon as possible by a 1840background garbage collector. 1841 1842Similarly, revoked and expired keys will be garbage collected, but only after a 1843certain amount of time has passed. This time is set as a number of seconds in:: 1844 1845 /proc/sys/kernel/keys/gc_delay 1846