Based on kernel version 188.8.131.52. Page generated on 2011-06-03 13:47 EST.
1 ============================ 2 KERNEL KEY RETENTION SERVICE 3 ============================ 4 5 This service allows cryptographic keys, authentication tokens, cross-domain 6 user mappings, and similar to be cached in the kernel for the use of 7 filesystems and other kernel services. 8 9 Keyrings are permitted; these are a special type of key that can hold links to 10 other keys. Processes each have three standard keyring subscriptions that a 11 kernel service can search for relevant keys. 12 13 The key service can be configured on by enabling: 14 15 "Security options"/"Enable access key retention support" (CONFIG_KEYS) 16 17 This 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 ============ 33 KEY OVERVIEW 34 ============ 35 36 In this context, keys represent units of cryptographic data, authentication 37 tokens, keyrings, etc.. These are represented in the kernel by struct key. 38 39 Each key has a number of attributes: 40 41 - A serial number. 42 - A type. 43 - A description (for matching a key in a search). 44 - Access control information. 45 - An expiry time. 46 - A payload. 47 - State. 48 49 50 (*) Each key is issued a serial number of type key_serial_t that is unique for 51 the lifetime of that key. All serial numbers are positive non-zero 32-bit 52 integers. 53 54 Userspace programs can use a key's serial numbers as a way to gain access 55 to it, subject to permission checking. 56 57 (*) Each key is of a defined "type". Types must be registered inside the 58 kernel by a kernel service (such as a filesystem) before keys of that type 59 can be added or used. Userspace programs cannot define new types directly. 60 61 Key types are represented in the kernel by struct key_type. This defines a 62 number of operations that can be performed on a key of that type. 63 64 Should a type be removed from the system, all the keys of that type will 65 be invalidated. 66 67 (*) Each key has a description. This should be a printable string. The key 68 type provides an operation to perform a match between the description on a 69 key and a criterion string. 70 71 (*) Each key has an owner user ID, a group ID and a permissions mask. These 72 are used to control what a process may do to a key from userspace, and 73 whether a kernel service will be able to find the key. 74 75 (*) Each key can be set to expire at a specific time by the key type's 76 instantiation function. Keys can also be immortal. 77 78 (*) Each key can have a payload. This is a quantity of data that represent the 79 actual "key". In the case of a keyring, this is a list of keys to which 80 the keyring links; in the case of a user-defined key, it's an arbitrary 81 blob of data. 82 83 Having a payload is not required; and the payload can, in fact, just be a 84 value stored in the struct key itself. 85 86 When a key is instantiated, the key type's instantiation function is 87 called with a blob of data, and that then creates the key's payload in 88 some way. 89 90 Similarly, when userspace wants to read back the contents of the key, if 91 permitted, another key type operation will be called to convert the key's 92 attached payload back into a blob of data. 93 94 (*) Each key can be in one of a number of basic states: 95 96 (*) Uninstantiated. The key exists, but does not have any data attached. 97 Keys being requested from userspace will be in this state. 98 99 (*) Instantiated. This is the normal state. The key is fully formed, and 100 has data attached. 101 102 (*) Negative. This is a relatively short-lived state. The key acts as a 103 note saying that a previous call out to userspace failed, and acts as 104 a throttle on key lookups. A negative key can be updated to a normal 105 state. 106 107 (*) Expired. Keys can have lifetimes set. If their lifetime is exceeded, 108 they traverse to this state. An expired key can be updated back to a 109 normal state. 110 111 (*) Revoked. A key is put in this state by userspace action. It can't be 112 found or operated upon (apart from by unlinking it). 113 114 (*) Dead. The key's type was unregistered, and so the key is now useless. 115 116 Keys in the last three states are subject to garbage collection. See the 117 section on "Garbage collection". 118 119 120 ==================== 121 KEY SERVICE OVERVIEW 122 ==================== 123 124 The key service provides a number of features besides keys: 125 126 (*) The key service defines two special key types: 127 128 (+) "keyring" 129 130 Keyrings are special keys that contain a list of other keys. Keyring 131 lists can be modified using various system calls. Keyrings should not 132 be given a payload when created. 133 134 (+) "user" 135 136 A key of this type has a description and a payload that are arbitrary 137 blobs of data. These can be created, updated and read by userspace, 138 and aren't intended for use by kernel services. 139 140 (*) Each process subscribes to three keyrings: a thread-specific keyring, a 141 process-specific keyring, and a session-specific keyring. 142 143 The thread-specific keyring is discarded from the child when any sort of 144 clone, fork, vfork or execve occurs. A new keyring is created only when 145 required. 146 147 The process-specific keyring is replaced with an empty one in the child on 148 clone, fork, vfork unless CLONE_THREAD is supplied, in which case it is 149 shared. execve also discards the process's process keyring and creates a 150 new one. 151 152 The session-specific keyring is persistent across clone, fork, vfork and 153 execve, even when the latter executes a set-UID or set-GID binary. A 154 process can, however, replace its current session keyring with a new one 155 by using PR_JOIN_SESSION_KEYRING. It is permitted to request an anonymous 156 new one, or to attempt to create or join one of a specific name. 157 158 The ownership of the thread keyring changes when the real UID and GID of 159 the thread changes. 160 161 (*) Each user ID resident in the system holds two special keyrings: a user 162 specific keyring and a default user session keyring. The default session 163 keyring is initialised with a link to the user-specific keyring. 164 165 When a process changes its real UID, if it used to have no session key, it 166 will be subscribed to the default session key for the new UID. 167 168 If a process attempts to access its session key when it doesn't have one, 169 it will be subscribed to the default for its current UID. 170 171 (*) Each user has two quotas against which the keys they own are tracked. One 172 limits the total number of keys and keyrings, the other limits the total 173 amount of description and payload space that can be consumed. 174 175 The user can view information on this and other statistics through procfs 176 files. The root user may also alter the quota limits through sysctl files 177 (see the section "New procfs files"). 178 179 Process-specific and thread-specific keyrings are not counted towards a 180 user's quota. 181 182 If a system call that modifies a key or keyring in some way would put the 183 user over quota, the operation is refused and error EDQUOT is returned. 184 185 (*) There's a system call interface by which userspace programs can create and 186 manipulate keys and keyrings. 187 188 (*) There's a kernel interface by which services can register types and search 189 for keys. 190 191 (*) There's a way for the a search done from the kernel to call back to 192 userspace to request a key that can't be found in a process's keyrings. 193 194 (*) An optional filesystem is available through which the key database can be 195 viewed and manipulated. 196 197 198 ====================== 199 KEY ACCESS PERMISSIONS 200 ====================== 201 202 Keys have an owner user ID, a group access ID, and a permissions mask. The mask 203 has up to eight bits each for possessor, user, group and other access. Only 204 six of each set of eight bits are defined. These permissions granted are: 205 206 (*) View 207 208 This permits a key or keyring's attributes to be viewed - including key 209 type and description. 210 211 (*) Read 212 213 This permits a key's payload to be viewed or a keyring's list of linked 214 keys. 215 216 (*) Write 217 218 This permits a key's payload to be instantiated or updated, or it allows a 219 link to be added to or removed from a keyring. 220 221 (*) Search 222 223 This permits keyrings to be searched and keys to be found. Searches can 224 only recurse into nested keyrings that have search permission set. 225 226 (*) Link 227 228 This permits a key or keyring to be linked to. To create a link from a 229 keyring to a key, a process must have Write permission on the keyring and 230 Link permission on the key. 231 232 (*) Set Attribute 233 234 This permits a key's UID, GID and permissions mask to be changed. 235 236 For changing the ownership, group ID or permissions mask, being the owner of 237 the key or having the sysadmin capability is sufficient. 238 239 240 =============== 241 SELINUX SUPPORT 242 =============== 243 244 The security class "key" has been added to SELinux so that mandatory access 245 controls can be applied to keys created within various contexts. This support 246 is preliminary, and is likely to change quite significantly in the near future. 247 Currently, all of the basic permissions explained above are provided in SELinux 248 as well; SELinux is simply invoked after all basic permission checks have been 249 performed. 250 251 The value of the file /proc/self/attr/keycreate influences the labeling of 252 newly-created keys. If the contents of that file correspond to an SELinux 253 security context, then the key will be assigned that context. Otherwise, the 254 key will be assigned the current context of the task that invoked the key 255 creation request. Tasks must be granted explicit permission to assign a 256 particular context to newly-created keys, using the "create" permission in the 257 key security class. 258 259 The default keyrings associated with users will be labeled with the default 260 context of the user if and only if the login programs have been instrumented to 261 properly initialize keycreate during the login process. Otherwise, they will 262 be labeled with the context of the login program itself. 263 264 Note, however, that the default keyrings associated with the root user are 265 labeled with the default kernel context, since they are created early in the 266 boot process, before root has a chance to log in. 267 268 The keyrings associated with new threads are each labeled with the context of 269 their associated thread, and both session and process keyrings are handled 270 similarly. 271 272 273 ================ 274 NEW PROCFS FILES 275 ================ 276 277 Two files have been added to procfs by which an administrator can find out 278 about the status of the key service: 279 280 (*) /proc/keys 281 282 This lists the keys that are currently viewable by the task reading the 283 file, giving information about their type, description and permissions. 284 It is not possible to view the payload of the key this way, though some 285 information about it may be given. 286 287 The only keys included in the list are those that grant View permission to 288 the reading process whether or not it possesses them. Note that LSM 289 security checks are still performed, and may further filter out keys that 290 the current process is not authorised to view. 291 292 The contents of the file look like this: 293 294 SERIAL FLAGS USAGE EXPY PERM UID GID TYPE DESCRIPTION: SUMMARY 295 00000001 I----- 39 perm 1f3f0000 0 0 keyring _uid_ses.0: 1/4 296 00000002 I----- 2 perm 1f3f0000 0 0 keyring _uid.0: empty 297 00000007 I----- 1 perm 1f3f0000 0 0 keyring _pid.1: empty 298 0000018d I----- 1 perm 1f3f0000 0 0 keyring _pid.412: empty 299 000004d2 I--Q-- 1 perm 1f3f0000 32 -1 keyring _uid.32: 1/4 300 000004d3 I--Q-- 3 perm 1f3f0000 32 -1 keyring _uid_ses.32: empty 301 00000892 I--QU- 1 perm 1f000000 0 0 user metal:copper: 0 302 00000893 I--Q-N 1 35s 1f3f0000 0 0 user metal:silver: 0 303 00000894 I--Q-- 1 10h 003f0000 0 0 user metal:gold: 0 304 305 The flags are: 306 307 I Instantiated 308 R Revoked 309 D Dead 310 Q Contributes to user's quota 311 U Under construction by callback to userspace 312 N Negative key 313 314 This file must be enabled at kernel configuration time as it allows anyone 315 to list the keys database. 316 317 (*) /proc/key-users 318 319 This file lists the tracking data for each user that has at least one key 320 on the system. Such data includes quota information and statistics: 321 322 [root@andromeda root]# cat /proc/key-users 323 0: 46 45/45 1/100 13/10000 324 29: 2 2/2 2/100 40/10000 325 32: 2 2/2 2/100 40/10000 326 38: 2 2/2 2/100 40/10000 327 328 The format of each line is 329 <UID>: User ID to which this applies 330 <usage> Structure refcount 331 <inst>/<keys> Total number of keys and number instantiated 332 <keys>/<max> Key count quota 333 <bytes>/<max> Key size quota 334 335 336 Four new sysctl files have been added also for the purpose of controlling the 337 quota limits on keys: 338 339 (*) /proc/sys/kernel/keys/root_maxkeys 340 /proc/sys/kernel/keys/root_maxbytes 341 342 These files hold the maximum number of keys that root may have and the 343 maximum total number of bytes of data that root may have stored in those 344 keys. 345 346 (*) /proc/sys/kernel/keys/maxkeys 347 /proc/sys/kernel/keys/maxbytes 348 349 These files hold the maximum number of keys that each non-root user may 350 have and the maximum total number of bytes of data that each of those 351 users may have stored in their keys. 352 353 Root may alter these by writing each new limit as a decimal number string to 354 the appropriate file. 355 356 357 =============================== 358 USERSPACE SYSTEM CALL INTERFACE 359 =============================== 360 361 Userspace can manipulate keys directly through three new syscalls: add_key, 362 request_key and keyctl. The latter provides a number of functions for 363 manipulating keys. 364 365 When referring to a key directly, userspace programs should use the key's 366 serial number (a positive 32-bit integer). However, there are some special 367 values available for referring to special keys and keyrings that relate to the 368 process making the call: 369 370 CONSTANT VALUE KEY REFERENCED 371 ============================== ====== =========================== 372 KEY_SPEC_THREAD_KEYRING -1 thread-specific keyring 373 KEY_SPEC_PROCESS_KEYRING -2 process-specific keyring 374 KEY_SPEC_SESSION_KEYRING -3 session-specific keyring 375 KEY_SPEC_USER_KEYRING -4 UID-specific keyring 376 KEY_SPEC_USER_SESSION_KEYRING -5 UID-session keyring 377 KEY_SPEC_GROUP_KEYRING -6 GID-specific keyring 378 KEY_SPEC_REQKEY_AUTH_KEY -7 assumed request_key() 379 authorisation key 380 381 382 The main syscalls are: 383 384 (*) Create a new key of given type, description and payload and add it to the 385 nominated keyring: 386 387 key_serial_t add_key(const char *type, const char *desc, 388 const void *payload, size_t plen, 389 key_serial_t keyring); 390 391 If a key of the same type and description as that proposed already exists 392 in the keyring, this will try to update it with the given payload, or it 393 will return error EEXIST if that function is not supported by the key 394 type. The process must also have permission to write to the key to be able 395 to update it. The new key will have all user permissions granted and no 396 group or third party permissions. 397 398 Otherwise, this will attempt to create a new key of the specified type and 399 description, and to instantiate it with the supplied payload and attach it 400 to the keyring. In this case, an error will be generated if the process 401 does not have permission to write to the keyring. 402 403 The payload is optional, and the pointer can be NULL if not required by 404 the type. The payload is plen in size, and plen can be zero for an empty 405 payload. 406 407 A new keyring can be generated by setting type "keyring", the keyring name 408 as the description (or NULL) and setting the payload to NULL. 409 410 User defined keys can be created by specifying type "user". It is 411 recommended that a user defined key's description by prefixed with a type 412 ID and a colon, such as "krb5tgt:" for a Kerberos 5 ticket granting 413 ticket. 414 415 Any other type must have been registered with the kernel in advance by a 416 kernel service such as a filesystem. 417 418 The ID of the new or updated key is returned if successful. 419 420 421 (*) Search the process's keyrings for a key, potentially calling out to 422 userspace to create it. 423 424 key_serial_t request_key(const char *type, const char *description, 425 const char *callout_info, 426 key_serial_t dest_keyring); 427 428 This function searches all the process's keyrings in the order thread, 429 process, session for a matching key. This works very much like 430 KEYCTL_SEARCH, including the optional attachment of the discovered key to 431 a keyring. 432 433 If a key cannot be found, and if callout_info is not NULL, then 434 /sbin/request-key will be invoked in an attempt to obtain a key. The 435 callout_info string will be passed as an argument to the program. 436 437 See also Documentation/keys-request-key.txt. 438 439 440 The keyctl syscall functions are: 441 442 (*) Map a special key ID to a real key ID for this process: 443 444 key_serial_t keyctl(KEYCTL_GET_KEYRING_ID, key_serial_t id, 445 int create); 446 447 The special key specified by "id" is looked up (with the key being created 448 if necessary) and the ID of the key or keyring thus found is returned if 449 it exists. 450 451 If the key does not yet exist, the key will be created if "create" is 452 non-zero; and the error ENOKEY will be returned if "create" is zero. 453 454 455 (*) Replace the session keyring this process subscribes to with a new one: 456 457 key_serial_t keyctl(KEYCTL_JOIN_SESSION_KEYRING, const char *name); 458 459 If name is NULL, an anonymous keyring is created attached to the process 460 as its session keyring, displacing the old session keyring. 461 462 If name is not NULL, if a keyring of that name exists, the process 463 attempts to attach it as the session keyring, returning an error if that 464 is not permitted; otherwise a new keyring of that name is created and 465 attached as the session keyring. 466 467 To attach to a named keyring, the keyring must have search permission for 468 the process's ownership. 469 470 The ID of the new session keyring is returned if successful. 471 472 473 (*) Update the specified key: 474 475 long keyctl(KEYCTL_UPDATE, key_serial_t key, const void *payload, 476 size_t plen); 477 478 This will try to update the specified key with the given payload, or it 479 will return error EOPNOTSUPP if that function is not supported by the key 480 type. The process must also have permission to write to the key to be able 481 to update it. 482 483 The payload is of length plen, and may be absent or empty as for 484 add_key(). 485 486 487 (*) Revoke a key: 488 489 long keyctl(KEYCTL_REVOKE, key_serial_t key); 490 491 This makes a key unavailable for further operations. Further attempts to 492 use the key will be met with error EKEYREVOKED, and the key will no longer 493 be findable. 494 495 496 (*) Change the ownership of a key: 497 498 long keyctl(KEYCTL_CHOWN, key_serial_t key, uid_t uid, gid_t gid); 499 500 This function permits a key's owner and group ID to be changed. Either one 501 of uid or gid can be set to -1 to suppress that change. 502 503 Only the superuser can change a key's owner to something other than the 504 key's current owner. Similarly, only the superuser can change a key's 505 group ID to something other than the calling process's group ID or one of 506 its group list members. 507 508 509 (*) Change the permissions mask on a key: 510 511 long keyctl(KEYCTL_SETPERM, key_serial_t key, key_perm_t perm); 512 513 This function permits the owner of a key or the superuser to change the 514 permissions mask on a key. 515 516 Only bits the available bits are permitted; if any other bits are set, 517 error EINVAL will be returned. 518 519 520 (*) Describe a key: 521 522 long keyctl(KEYCTL_DESCRIBE, key_serial_t key, char *buffer, 523 size_t buflen); 524 525 This function returns a summary of the key's attributes (but not its 526 payload data) as a string in the buffer provided. 527 528 Unless there's an error, it always returns the amount of data it could 529 produce, even if that's too big for the buffer, but it won't copy more 530 than requested to userspace. If the buffer pointer is NULL then no copy 531 will take place. 532 533 A process must have view permission on the key for this function to be 534 successful. 535 536 If successful, a string is placed in the buffer in the following format: 537 538 <type>;<uid>;<gid>;<perm>;<description> 539 540 Where type and description are strings, uid and gid are decimal, and perm 541 is hexadecimal. A NUL character is included at the end of the string if 542 the buffer is sufficiently big. 543 544 This can be parsed with 545 546 sscanf(buffer, "%[^;];%d;%d;%o;%s", type, &uid, &gid, &mode, desc); 547 548 549 (*) Clear out a keyring: 550 551 long keyctl(KEYCTL_CLEAR, key_serial_t keyring); 552 553 This function clears the list of keys attached to a keyring. The calling 554 process must have write permission on the keyring, and it must be a 555 keyring (or else error ENOTDIR will result). 556 557 558 (*) Link a key into a keyring: 559 560 long keyctl(KEYCTL_LINK, key_serial_t keyring, key_serial_t key); 561 562 This function creates a link from the keyring to the key. The process must 563 have write permission on the keyring and must have link permission on the 564 key. 565 566 Should the keyring not be a keyring, error ENOTDIR will result; and if the 567 keyring is full, error ENFILE will result. 568 569 The link procedure checks the nesting of the keyrings, returning ELOOP if 570 it appears too deep or EDEADLK if the link would introduce a cycle. 571 572 Any links within the keyring to keys that match the new key in terms of 573 type and description will be discarded from the keyring as the new one is 574 added. 575 576 577 (*) Unlink a key or keyring from another keyring: 578 579 long keyctl(KEYCTL_UNLINK, key_serial_t keyring, key_serial_t key); 580 581 This function looks through the keyring for the first link to the 582 specified key, and removes it if found. Subsequent links to that key are 583 ignored. The process must have write permission on the keyring. 584 585 If the keyring is not a keyring, error ENOTDIR will result; and if the key 586 is not present, error ENOENT will be the result. 587 588 589 (*) Search a keyring tree for a key: 590 591 key_serial_t keyctl(KEYCTL_SEARCH, key_serial_t keyring, 592 const char *type, const char *description, 593 key_serial_t dest_keyring); 594 595 This searches the keyring tree headed by the specified keyring until a key 596 is found that matches the type and description criteria. Each keyring is 597 checked for keys before recursion into its children occurs. 598 599 The process must have search permission on the top level keyring, or else 600 error EACCES will result. Only keyrings that the process has search 601 permission on will be recursed into, and only keys and keyrings for which 602 a process has search permission can be matched. If the specified keyring 603 is not a keyring, ENOTDIR will result. 604 605 If the search succeeds, the function will attempt to link the found key 606 into the destination keyring if one is supplied (non-zero ID). All the 607 constraints applicable to KEYCTL_LINK apply in this case too. 608 609 Error ENOKEY, EKEYREVOKED or EKEYEXPIRED will be returned if the search 610 fails. On success, the resulting key ID will be returned. 611 612 613 (*) Read the payload data from a key: 614 615 long keyctl(KEYCTL_READ, key_serial_t keyring, char *buffer, 616 size_t buflen); 617 618 This function attempts to read the payload data from the specified key 619 into the buffer. The process must have read permission on the key to 620 succeed. 621 622 The returned data will be processed for presentation by the key type. For 623 instance, a keyring will return an array of key_serial_t entries 624 representing the IDs of all the keys to which it is subscribed. The user 625 defined key type will return its data as is. If a key type does not 626 implement this function, error EOPNOTSUPP will result. 627 628 As much of the data as can be fitted into the buffer will be copied to 629 userspace if the buffer pointer is not NULL. 630 631 On a successful return, the function will always return the amount of data 632 available rather than the amount copied. 633 634 635 (*) Instantiate a partially constructed key. 636 637 long keyctl(KEYCTL_INSTANTIATE, key_serial_t key, 638 const void *payload, size_t plen, 639 key_serial_t keyring); 640 long keyctl(KEYCTL_INSTANTIATE_IOV, key_serial_t key, 641 const struct iovec *payload_iov, unsigned ioc, 642 key_serial_t keyring); 643 644 If the kernel calls back to userspace to complete the instantiation of a 645 key, userspace should use this call to supply data for the key before the 646 invoked process returns, or else the key will be marked negative 647 automatically. 648 649 The process must have write access on the key to be able to instantiate 650 it, and the key must be uninstantiated. 651 652 If a keyring is specified (non-zero), the key will also be linked into 653 that keyring, however all the constraints applying in KEYCTL_LINK apply in 654 this case too. 655 656 The payload and plen arguments describe the payload data as for add_key(). 657 658 The payload_iov and ioc arguments describe the payload data in an iovec 659 array instead of a single buffer. 660 661 662 (*) Negatively instantiate a partially constructed key. 663 664 long keyctl(KEYCTL_NEGATE, key_serial_t key, 665 unsigned timeout, key_serial_t keyring); 666 long keyctl(KEYCTL_REJECT, key_serial_t key, 667 unsigned timeout, unsigned error, key_serial_t keyring); 668 669 If the kernel calls back to userspace to complete the instantiation of a 670 key, userspace should use this call mark the key as negative before the 671 invoked process returns if it is unable to fulfil the request. 672 673 The process must have write access on the key to be able to instantiate 674 it, and the key must be uninstantiated. 675 676 If a keyring is specified (non-zero), the key will also be linked into 677 that keyring, however all the constraints applying in KEYCTL_LINK apply in 678 this case too. 679 680 If the key is rejected, future searches for it will return the specified 681 error code until the rejected key expires. Negating the key is the same 682 as rejecting the key with ENOKEY as the error code. 683 684 685 (*) Set the default request-key destination keyring. 686 687 long keyctl(KEYCTL_SET_REQKEY_KEYRING, int reqkey_defl); 688 689 This sets the default keyring to which implicitly requested keys will be 690 attached for this thread. reqkey_defl should be one of these constants: 691 692 CONSTANT VALUE NEW DEFAULT KEYRING 693 ====================================== ====== ======================= 694 KEY_REQKEY_DEFL_NO_CHANGE -1 No change 695 KEY_REQKEY_DEFL_DEFAULT 0 Default 696 KEY_REQKEY_DEFL_THREAD_KEYRING 1 Thread keyring 697 KEY_REQKEY_DEFL_PROCESS_KEYRING 2 Process keyring 698 KEY_REQKEY_DEFL_SESSION_KEYRING 3 Session keyring 699 KEY_REQKEY_DEFL_USER_KEYRING 4 User keyring 700 KEY_REQKEY_DEFL_USER_SESSION_KEYRING 5 User session keyring 701 KEY_REQKEY_DEFL_GROUP_KEYRING 6 Group keyring 702 703 The old default will be returned if successful and error EINVAL will be 704 returned if reqkey_defl is not one of the above values. 705 706 The default keyring can be overridden by the keyring indicated to the 707 request_key() system call. 708 709 Note that this setting is inherited across fork/exec. 710 711  The default is: the thread keyring if there is one, otherwise 712 the process keyring if there is one, otherwise the session keyring if 713 there is one, otherwise the user default session keyring. 714 715 716 (*) Set the timeout on a key. 717 718 long keyctl(KEYCTL_SET_TIMEOUT, key_serial_t key, unsigned timeout); 719 720 This sets or clears the timeout on a key. The timeout can be 0 to clear 721 the timeout or a number of seconds to set the expiry time that far into 722 the future. 723 724 The process must have attribute modification access on a key to set its 725 timeout. Timeouts may not be set with this function on negative, revoked 726 or expired keys. 727 728 729 (*) Assume the authority granted to instantiate a key 730 731 long keyctl(KEYCTL_ASSUME_AUTHORITY, key_serial_t key); 732 733 This assumes or divests the authority required to instantiate the 734 specified key. Authority can only be assumed if the thread has the 735 authorisation key associated with the specified key in its keyrings 736 somewhere. 737 738 Once authority is assumed, searches for keys will also search the 739 requester's keyrings using the requester's security label, UID, GID and 740 groups. 741 742 If the requested authority is unavailable, error EPERM will be returned, 743 likewise if the authority has been revoked because the target key is 744 already instantiated. 745 746 If the specified key is 0, then any assumed authority will be divested. 747 748 The assumed authoritative key is inherited across fork and exec. 749 750 751 (*) Get the LSM security context attached to a key. 752 753 long keyctl(KEYCTL_GET_SECURITY, key_serial_t key, char *buffer, 754 size_t buflen) 755 756 This function returns a string that represents the LSM security context 757 attached to a key in the buffer provided. 758 759 Unless there's an error, it always returns the amount of data it could 760 produce, even if that's too big for the buffer, but it won't copy more 761 than requested to userspace. If the buffer pointer is NULL then no copy 762 will take place. 763 764 A NUL character is included at the end of the string if the buffer is 765 sufficiently big. This is included in the returned count. If no LSM is 766 in force then an empty string will be returned. 767 768 A process must have view permission on the key for this function to be 769 successful. 770 771 772 (*) Install the calling process's session keyring on its parent. 773 774 long keyctl(KEYCTL_SESSION_TO_PARENT); 775 776 This functions attempts to install the calling process's session keyring 777 on to the calling process's parent, replacing the parent's current session 778 keyring. 779 780 The calling process must have the same ownership as its parent, the 781 keyring must have the same ownership as the calling process, the calling 782 process must have LINK permission on the keyring and the active LSM module 783 mustn't deny permission, otherwise error EPERM will be returned. 784 785 Error ENOMEM will be returned if there was insufficient memory to complete 786 the operation, otherwise 0 will be returned to indicate success. 787 788 The keyring will be replaced next time the parent process leaves the 789 kernel and resumes executing userspace. 790 791 792 =============== 793 KERNEL SERVICES 794 =============== 795 796 The kernel services for key management are fairly simple to deal with. They can 797 be broken down into two areas: keys and key types. 798 799 Dealing with keys is fairly straightforward. Firstly, the kernel service 800 registers its type, then it searches for a key of that type. It should retain 801 the key as long as it has need of it, and then it should release it. For a 802 filesystem or device file, a search would probably be performed during the open 803 call, and the key released upon close. How to deal with conflicting keys due to 804 two different users opening the same file is left to the filesystem author to 805 solve. 806 807 To access the key manager, the following header must be #included: 808 809 <linux/key.h> 810 811 Specific key types should have a header file under include/keys/ that should be 812 used to access that type. For keys of type "user", for example, that would be: 813 814 <keys/user-type.h> 815 816 Note that there are two different types of pointers to keys that may be 817 encountered: 818 819 (*) struct key * 820 821 This simply points to the key structure itself. Key structures will be at 822 least four-byte aligned. 823 824 (*) key_ref_t 825 826 This is equivalent to a struct key *, but the least significant bit is set 827 if the caller "possesses" the key. By "possession" it is meant that the 828 calling processes has a searchable link to the key from one of its 829 keyrings. There are three functions for dealing with these: 830 831 key_ref_t make_key_ref(const struct key *key, 832 unsigned long possession); 833 834 struct key *key_ref_to_ptr(const key_ref_t key_ref); 835 836 unsigned long is_key_possessed(const key_ref_t key_ref); 837 838 The first function constructs a key reference from a key pointer and 839 possession information (which must be 0 or 1 and not any other value). 840 841 The second function retrieves the key pointer from a reference and the 842 third retrieves the possession flag. 843 844 When accessing a key's payload contents, certain precautions must be taken to 845 prevent access vs modification races. See the section "Notes on accessing 846 payload contents" for more information. 847 848 (*) To search for a key, call: 849 850 struct key *request_key(const struct key_type *type, 851 const char *description, 852 const char *callout_info); 853 854 This is used to request a key or keyring with a description that matches 855 the description specified according to the key type's match function. This 856 permits approximate matching to occur. If callout_string is not NULL, then 857 /sbin/request-key will be invoked in an attempt to obtain the key from 858 userspace. In that case, callout_string will be passed as an argument to 859 the program. 860 861 Should the function fail error ENOKEY, EKEYEXPIRED or EKEYREVOKED will be 862 returned. 863 864 If successful, the key will have been attached to the default keyring for 865 implicitly obtained request-key keys, as set by KEYCTL_SET_REQKEY_KEYRING. 866 867 See also Documentation/keys-request-key.txt. 868 869 870 (*) To search for a key, passing auxiliary data to the upcaller, call: 871 872 struct key *request_key_with_auxdata(const struct key_type *type, 873 const char *description, 874 const void *callout_info, 875 size_t callout_len, 876 void *aux); 877 878 This is identical to request_key(), except that the auxiliary data is 879 passed to the key_type->request_key() op if it exists, and the callout_info 880 is a blob of length callout_len, if given (the length may be 0). 881 882 883 (*) A key can be requested asynchronously by calling one of: 884 885 struct key *request_key_async(const struct key_type *type, 886 const char *description, 887 const void *callout_info, 888 size_t callout_len); 889 890 or: 891 892 struct key *request_key_async_with_auxdata(const struct key_type *type, 893 const char *description, 894 const char *callout_info, 895 size_t callout_len, 896 void *aux); 897 898 which are asynchronous equivalents of request_key() and 899 request_key_with_auxdata() respectively. 900 901 These two functions return with the key potentially still under 902 construction. To wait for construction completion, the following should be 903 called: 904 905 int wait_for_key_construction(struct key *key, bool intr); 906 907 The function will wait for the key to finish being constructed and then 908 invokes key_validate() to return an appropriate value to indicate the state 909 of the key (0 indicates the key is usable). 910 911 If intr is true, then the wait can be interrupted by a signal, in which 912 case error ERESTARTSYS will be returned. 913 914 915 (*) When it is no longer required, the key should be released using: 916 917 void key_put(struct key *key); 918 919 Or: 920 921 void key_ref_put(key_ref_t key_ref); 922 923 These can be called from interrupt context. If CONFIG_KEYS is not set then 924 the argument will not be parsed. 925 926 927 (*) Extra references can be made to a key by calling the following function: 928 929 struct key *key_get(struct key *key); 930 931 These need to be disposed of by calling key_put() when they've been 932 finished with. The key pointer passed in will be returned. If the pointer 933 is NULL or CONFIG_KEYS is not set then the key will not be dereferenced and 934 no increment will take place. 935 936 937 (*) A key's serial number can be obtained by calling: 938 939 key_serial_t key_serial(struct key *key); 940 941 If key is NULL or if CONFIG_KEYS is not set then 0 will be returned (in the 942 latter case without parsing the argument). 943 944 945 (*) If a keyring was found in the search, this can be further searched by: 946 947 key_ref_t keyring_search(key_ref_t keyring_ref, 948 const struct key_type *type, 949 const char *description) 950 951 This searches the keyring tree specified for a matching key. Error ENOKEY 952 is returned upon failure (use IS_ERR/PTR_ERR to determine). If successful, 953 the returned key will need to be released. 954 955 The possession attribute from the keyring reference is used to control 956 access through the permissions mask and is propagated to the returned key 957 reference pointer if successful. 958 959 960 (*) To check the validity of a key, this function can be called: 961 962 int validate_key(struct key *key); 963 964 This checks that the key in question hasn't expired or and hasn't been 965 revoked. Should the key be invalid, error EKEYEXPIRED or EKEYREVOKED will 966 be returned. If the key is NULL or if CONFIG_KEYS is not set then 0 will be 967 returned (in the latter case without parsing the argument). 968 969 970 (*) To register a key type, the following function should be called: 971 972 int register_key_type(struct key_type *type); 973 974 This will return error EEXIST if a type of the same name is already 975 present. 976 977 978 (*) To unregister a key type, call: 979 980 void unregister_key_type(struct key_type *type); 981 982 983 Under some circumstances, it may be desirable to deal with a bundle of keys. 984 The facility provides access to the keyring type for managing such a bundle: 985 986 struct key_type key_type_keyring; 987 988 This can be used with a function such as request_key() to find a specific 989 keyring in a process's keyrings. A keyring thus found can then be searched 990 with keyring_search(). Note that it is not possible to use request_key() to 991 search a specific keyring, so using keyrings in this way is of limited utility. 992 993 994 =================================== 995 NOTES ON ACCESSING PAYLOAD CONTENTS 996 =================================== 997 998 The simplest payload is just a number in key->payload.value. In this case, 999 there's no need to indulge in RCU or locking when accessing the payload. 1000 1001 More complex payload contents must be allocated and a pointer to them set in 1002 key->payload.data. One of the following ways must be selected to access the 1003 data: 1004 1005 (1) Unmodifiable key type. 1006 1007 If the key type does not have a modify method, then the key's payload can 1008 be accessed without any form of locking, provided that it's known to be 1009 instantiated (uninstantiated keys cannot be "found"). 1010 1011 (2) The key's semaphore. 1012 1013 The semaphore could be used to govern access to the payload and to control 1014 the payload pointer. It must be write-locked for modifications and would 1015 have to be read-locked for general access. The disadvantage of doing this 1016 is that the accessor may be required to sleep. 1017 1018 (3) RCU. 1019 1020 RCU must be used when the semaphore isn't already held; if the semaphore 1021 is held then the contents can't change under you unexpectedly as the 1022 semaphore must still be used to serialise modifications to the key. The 1023 key management code takes care of this for the key type. 1024 1025 However, this means using: 1026 1027 rcu_read_lock() ... rcu_dereference() ... rcu_read_unlock() 1028 1029 to read the pointer, and: 1030 1031 rcu_dereference() ... rcu_assign_pointer() ... call_rcu() 1032 1033 to set the pointer and dispose of the old contents after a grace period. 1034 Note that only the key type should ever modify a key's payload. 1035 1036 Furthermore, an RCU controlled payload must hold a struct rcu_head for the 1037 use of call_rcu() and, if the payload is of variable size, the length of 1038 the payload. key->datalen cannot be relied upon to be consistent with the 1039 payload just dereferenced if the key's semaphore is not held. 1040 1041 1042 =================== 1043 DEFINING A KEY TYPE 1044 =================== 1045 1046 A kernel service may want to define its own key type. For instance, an AFS 1047 filesystem might want to define a Kerberos 5 ticket key type. To do this, it 1048 author fills in a key_type struct and registers it with the system. 1049 1050 Source files that implement key types should include the following header file: 1051 1052 <linux/key-type.h> 1053 1054 The structure has a number of fields, some of which are mandatory: 1055 1056 (*) const char *name 1057 1058 The name of the key type. This is used to translate a key type name 1059 supplied by userspace into a pointer to the structure. 1060 1061 1062 (*) size_t def_datalen 1063 1064 This is optional - it supplies the default payload data length as 1065 contributed to the quota. If the key type's payload is always or almost 1066 always the same size, then this is a more efficient way to do things. 1067 1068 The data length (and quota) on a particular key can always be changed 1069 during instantiation or update by calling: 1070 1071 int key_payload_reserve(struct key *key, size_t datalen); 1072 1073 With the revised data length. Error EDQUOT will be returned if this is not 1074 viable. 1075 1076 1077 (*) int (*vet_description)(const char *description); 1078 1079 This optional method is called to vet a key description. If the key type 1080 doesn't approve of the key description, it may return an error, otherwise 1081 it should return 0. 1082 1083 1084 (*) int (*instantiate)(struct key *key, const void *data, size_t datalen); 1085 1086 This method is called to attach a payload to a key during construction. 1087 The payload attached need not bear any relation to the data passed to this 1088 function. 1089 1090 If the amount of data attached to the key differs from the size in 1091 keytype->def_datalen, then key_payload_reserve() should be called. 1092 1093 This method does not have to lock the key in order to attach a payload. 1094 The fact that KEY_FLAG_INSTANTIATED is not set in key->flags prevents 1095 anything else from gaining access to the key. 1096 1097 It is safe to sleep in this method. 1098 1099 1100 (*) int (*update)(struct key *key, const void *data, size_t datalen); 1101 1102 If this type of key can be updated, then this method should be provided. 1103 It is called to update a key's payload from the blob of data provided. 1104 1105 key_payload_reserve() should be called if the data length might change 1106 before any changes are actually made. Note that if this succeeds, the type 1107 is committed to changing the key because it's already been altered, so all 1108 memory allocation must be done first. 1109 1110 The key will have its semaphore write-locked before this method is called, 1111 but this only deters other writers; any changes to the key's payload must 1112 be made under RCU conditions, and call_rcu() must be used to dispose of 1113 the old payload. 1114 1115 key_payload_reserve() should be called before the changes are made, but 1116 after all allocations and other potentially failing function calls are 1117 made. 1118 1119 It is safe to sleep in this method. 1120 1121 1122 (*) int (*match)(const struct key *key, const void *desc); 1123 1124 This method is called to match a key against a description. It should 1125 return non-zero if the two match, zero if they don't. 1126 1127 This method should not need to lock the key in any way. The type and 1128 description can be considered invariant, and the payload should not be 1129 accessed (the key may not yet be instantiated). 1130 1131 It is not safe to sleep in this method; the caller may hold spinlocks. 1132 1133 1134 (*) void (*revoke)(struct key *key); 1135 1136 This method is optional. It is called to discard part of the payload 1137 data upon a key being revoked. The caller will have the key semaphore 1138 write-locked. 1139 1140 It is safe to sleep in this method, though care should be taken to avoid 1141 a deadlock against the key semaphore. 1142 1143 1144 (*) void (*destroy)(struct key *key); 1145 1146 This method is optional. It is called to discard the payload data on a key 1147 when it is being destroyed. 1148 1149 This method does not need to lock the key to access the payload; it can 1150 consider the key as being inaccessible at this time. Note that the key's 1151 type may have been changed before this function is called. 1152 1153 It is not safe to sleep in this method; the caller may hold spinlocks. 1154 1155 1156 (*) void (*describe)(const struct key *key, struct seq_file *p); 1157 1158 This method is optional. It is called during /proc/keys reading to 1159 summarise a key's description and payload in text form. 1160 1161 This method will be called with the RCU read lock held. rcu_dereference() 1162 should be used to read the payload pointer if the payload is to be 1163 accessed. key->datalen cannot be trusted to stay consistent with the 1164 contents of the payload. 1165 1166 The description will not change, though the key's state may. 1167 1168 It is not safe to sleep in this method; the RCU read lock is held by the 1169 caller. 1170 1171 1172 (*) long (*read)(const struct key *key, char __user *buffer, size_t buflen); 1173 1174 This method is optional. It is called by KEYCTL_READ to translate the 1175 key's payload into something a blob of data for userspace to deal with. 1176 Ideally, the blob should be in the same format as that passed in to the 1177 instantiate and update methods. 1178 1179 If successful, the blob size that could be produced should be returned 1180 rather than the size copied. 1181 1182 This method will be called with the key's semaphore read-locked. This will 1183 prevent the key's payload changing. It is not necessary to use RCU locking 1184 when accessing the key's payload. It is safe to sleep in this method, such 1185 as might happen when the userspace buffer is accessed. 1186 1187 1188 (*) int (*request_key)(struct key_construction *cons, const char *op, 1189 void *aux); 1190 1191 This method is optional. If provided, request_key() and friends will 1192 invoke this function rather than upcalling to /sbin/request-key to operate 1193 upon a key of this type. 1194 1195 The aux parameter is as passed to request_key_async_with_auxdata() and 1196 similar or is NULL otherwise. Also passed are the construction record for 1197 the key to be operated upon and the operation type (currently only 1198 "create"). 1199 1200 This method is permitted to return before the upcall is complete, but the 1201 following function must be called under all circumstances to complete the 1202 instantiation process, whether or not it succeeds, whether or not there's 1203 an error: 1204 1205 void complete_request_key(struct key_construction *cons, int error); 1206 1207 The error parameter should be 0 on success, -ve on error. The 1208 construction record is destroyed by this action and the authorisation key 1209 will be revoked. If an error is indicated, the key under construction 1210 will be negatively instantiated if it wasn't already instantiated. 1211 1212 If this method returns an error, that error will be returned to the 1213 caller of request_key*(). complete_request_key() must be called prior to 1214 returning. 1215 1216 The key under construction and the authorisation key can be found in the 1217 key_construction struct pointed to by cons: 1218 1219 (*) struct key *key; 1220 1221 The key under construction. 1222 1223 (*) struct key *authkey; 1224 1225 The authorisation key. 1226 1227 1228 ============================ 1229 REQUEST-KEY CALLBACK SERVICE 1230 ============================ 1231 1232 To create a new key, the kernel will attempt to execute the following command 1233 line: 1234 1235 /sbin/request-key create <key> <uid> <gid> \ 1236 <threadring> <processring> <sessionring> <callout_info> 1237 1238 <key> is the key being constructed, and the three keyrings are the process 1239 keyrings from the process that caused the search to be issued. These are 1240 included for two reasons: 1241 1242 (1) There may be an authentication token in one of the keyrings that is 1243 required to obtain the key, eg: a Kerberos Ticket-Granting Ticket. 1244 1245 (2) The new key should probably be cached in one of these rings. 1246 1247 This program should set it UID and GID to those specified before attempting to 1248 access any more keys. It may then look around for a user specific process to 1249 hand the request off to (perhaps a path held in placed in another key by, for 1250 example, the KDE desktop manager). 1251 1252 The program (or whatever it calls) should finish construction of the key by 1253 calling KEYCTL_INSTANTIATE or KEYCTL_INSTANTIATE_IOV, which also permits it to 1254 cache the key in one of the keyrings (probably the session ring) before 1255 returning. Alternatively, the key can be marked as negative with KEYCTL_NEGATE 1256 or KEYCTL_REJECT; this also permits the key to be cached in one of the 1257 keyrings. 1258 1259 If it returns with the key remaining in the unconstructed state, the key will 1260 be marked as being negative, it will be added to the session keyring, and an 1261 error will be returned to the key requestor. 1262 1263 Supplementary information may be provided from whoever or whatever invoked this 1264 service. This will be passed as the <callout_info> parameter. If no such 1265 information was made available, then "-" will be passed as this parameter 1266 instead. 1267 1268 1269 Similarly, the kernel may attempt to update an expired or a soon to expire key 1270 by executing: 1271 1272 /sbin/request-key update <key> <uid> <gid> \ 1273 <threadring> <processring> <sessionring> 1274 1275 In this case, the program isn't required to actually attach the key to a ring; 1276 the rings are provided for reference. 1277 1278 1279 ================== 1280 GARBAGE COLLECTION 1281 ================== 1282 1283 Dead keys (for which the type has been removed) will be automatically unlinked 1284 from those keyrings that point to them and deleted as soon as possible by a 1285 background garbage collector. 1286 1287 Similarly, revoked and expired keys will be garbage collected, but only after a 1288 certain amount of time has passed. This time is set as a number of seconds in: 1289 1290 /proc/sys/kernel/keys/gc_delay