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Linus Torvalds1da177e2005-04-16 15:20:36 -07001 ============================
2 KERNEL KEY RETENTION SERVICE
3 ============================
4
5This service allows cryptographic keys, authentication tokens, cross-domain
6user mappings, and similar to be cached in the kernel for the use of
7filesystems other kernel services.
8
9Keyrings are permitted; these are a special type of key that can hold links to
10other keys. Processes each have three standard keyring subscriptions that a
11kernel service can search for relevant keys.
12
13The key service can be configured on by enabling:
14
15 "Security options"/"Enable access key retention support" (CONFIG_KEYS)
16
17This document has the following sections:
18
19 - Key overview
20 - Key service overview
21 - Key access permissions
22 - New procfs files
23 - Userspace system call interface
24 - Kernel services
David Howells76d8aea2005-06-23 22:00:49 -070025 - Notes on accessing payload contents
Linus Torvalds1da177e2005-04-16 15:20:36 -070026 - Defining a key type
27 - Request-key callback service
28 - Key access filesystem
29
30
31============
32KEY OVERVIEW
33============
34
35In this context, keys represent units of cryptographic data, authentication
36tokens, keyrings, etc.. These are represented in the kernel by struct key.
37
38Each key has a number of attributes:
39
40 - A serial number.
41 - A type.
42 - A description (for matching a key in a search).
43 - Access control information.
44 - An expiry time.
45 - A payload.
46 - State.
47
48
David Howells76d8aea2005-06-23 22:00:49 -070049 (*) Each key is issued a serial number of type key_serial_t that is unique for
50 the lifetime of that key. All serial numbers are positive non-zero 32-bit
51 integers.
Linus Torvalds1da177e2005-04-16 15:20:36 -070052
53 Userspace programs can use a key's serial numbers as a way to gain access
54 to it, subject to permission checking.
55
56 (*) Each key is of a defined "type". Types must be registered inside the
David Howells76d8aea2005-06-23 22:00:49 -070057 kernel by a kernel service (such as a filesystem) before keys of that type
58 can be added or used. Userspace programs cannot define new types directly.
Linus Torvalds1da177e2005-04-16 15:20:36 -070059
David Howells76d8aea2005-06-23 22:00:49 -070060 Key types are represented in the kernel by struct key_type. This defines a
61 number of operations that can be performed on a key of that type.
Linus Torvalds1da177e2005-04-16 15:20:36 -070062
63 Should a type be removed from the system, all the keys of that type will
64 be invalidated.
65
66 (*) Each key has a description. This should be a printable string. The key
David Howells76d8aea2005-06-23 22:00:49 -070067 type provides an operation to perform a match between the description on a
68 key and a criterion string.
Linus Torvalds1da177e2005-04-16 15:20:36 -070069
70 (*) Each key has an owner user ID, a group ID and a permissions mask. These
71 are used to control what a process may do to a key from userspace, and
72 whether a kernel service will be able to find the key.
73
74 (*) Each key can be set to expire at a specific time by the key type's
75 instantiation function. Keys can also be immortal.
76
David Howells76d8aea2005-06-23 22:00:49 -070077 (*) Each key can have a payload. This is a quantity of data that represent the
78 actual "key". In the case of a keyring, this is a list of keys to which
79 the keyring links; in the case of a user-defined key, it's an arbitrary
80 blob of data.
Linus Torvalds1da177e2005-04-16 15:20:36 -070081
82 Having a payload is not required; and the payload can, in fact, just be a
83 value stored in the struct key itself.
84
85 When a key is instantiated, the key type's instantiation function is
86 called with a blob of data, and that then creates the key's payload in
87 some way.
88
89 Similarly, when userspace wants to read back the contents of the key, if
90 permitted, another key type operation will be called to convert the key's
91 attached payload back into a blob of data.
92
93 (*) Each key can be in one of a number of basic states:
94
David Howells76d8aea2005-06-23 22:00:49 -070095 (*) Uninstantiated. The key exists, but does not have any data attached.
96 Keys being requested from userspace will be in this state.
Linus Torvalds1da177e2005-04-16 15:20:36 -070097
98 (*) Instantiated. This is the normal state. The key is fully formed, and
99 has data attached.
100
101 (*) Negative. This is a relatively short-lived state. The key acts as a
102 note saying that a previous call out to userspace failed, and acts as
103 a throttle on key lookups. A negative key can be updated to a normal
104 state.
105
106 (*) Expired. Keys can have lifetimes set. If their lifetime is exceeded,
107 they traverse to this state. An expired key can be updated back to a
108 normal state.
109
110 (*) Revoked. A key is put in this state by userspace action. It can't be
111 found or operated upon (apart from by unlinking it).
112
113 (*) Dead. The key's type was unregistered, and so the key is now useless.
114
115
116====================
117KEY SERVICE OVERVIEW
118====================
119
120The key service provides a number of features besides keys:
121
122 (*) The key service defines two special key types:
123
124 (+) "keyring"
125
126 Keyrings are special keys that contain a list of other keys. Keyring
127 lists can be modified using various system calls. Keyrings should not
128 be given a payload when created.
129
130 (+) "user"
131
132 A key of this type has a description and a payload that are arbitrary
133 blobs of data. These can be created, updated and read by userspace,
134 and aren't intended for use by kernel services.
135
136 (*) Each process subscribes to three keyrings: a thread-specific keyring, a
137 process-specific keyring, and a session-specific keyring.
138
139 The thread-specific keyring is discarded from the child when any sort of
140 clone, fork, vfork or execve occurs. A new keyring is created only when
141 required.
142
David Howells76d8aea2005-06-23 22:00:49 -0700143 The process-specific keyring is replaced with an empty one in the child on
144 clone, fork, vfork unless CLONE_THREAD is supplied, in which case it is
145 shared. execve also discards the process's process keyring and creates a
146 new one.
Linus Torvalds1da177e2005-04-16 15:20:36 -0700147
148 The session-specific keyring is persistent across clone, fork, vfork and
149 execve, even when the latter executes a set-UID or set-GID binary. A
150 process can, however, replace its current session keyring with a new one
151 by using PR_JOIN_SESSION_KEYRING. It is permitted to request an anonymous
152 new one, or to attempt to create or join one of a specific name.
153
154 The ownership of the thread keyring changes when the real UID and GID of
155 the thread changes.
156
157 (*) Each user ID resident in the system holds two special keyrings: a user
158 specific keyring and a default user session keyring. The default session
159 keyring is initialised with a link to the user-specific keyring.
160
161 When a process changes its real UID, if it used to have no session key, it
162 will be subscribed to the default session key for the new UID.
163
164 If a process attempts to access its session key when it doesn't have one,
165 it will be subscribed to the default for its current UID.
166
167 (*) Each user has two quotas against which the keys they own are tracked. One
168 limits the total number of keys and keyrings, the other limits the total
169 amount of description and payload space that can be consumed.
170
171 The user can view information on this and other statistics through procfs
172 files.
173
174 Process-specific and thread-specific keyrings are not counted towards a
175 user's quota.
176
177 If a system call that modifies a key or keyring in some way would put the
178 user over quota, the operation is refused and error EDQUOT is returned.
179
David Howells76d8aea2005-06-23 22:00:49 -0700180 (*) There's a system call interface by which userspace programs can create and
181 manipulate keys and keyrings.
Linus Torvalds1da177e2005-04-16 15:20:36 -0700182
David Howells76d8aea2005-06-23 22:00:49 -0700183 (*) There's a kernel interface by which services can register types and search
184 for keys.
Linus Torvalds1da177e2005-04-16 15:20:36 -0700185
186 (*) There's a way for the a search done from the kernel to call back to
187 userspace to request a key that can't be found in a process's keyrings.
188
189 (*) An optional filesystem is available through which the key database can be
190 viewed and manipulated.
191
192
193======================
194KEY ACCESS PERMISSIONS
195======================
196
David Howells76d8aea2005-06-23 22:00:49 -0700197Keys have an owner user ID, a group access ID, and a permissions mask. The mask
198has up to eight bits each for user, group and other access. Only five of each
199set of eight bits are defined. These permissions granted are:
Linus Torvalds1da177e2005-04-16 15:20:36 -0700200
201 (*) View
202
203 This permits a key or keyring's attributes to be viewed - including key
204 type and description.
205
206 (*) Read
207
208 This permits a key's payload to be viewed or a keyring's list of linked
209 keys.
210
211 (*) Write
212
David Howells76d8aea2005-06-23 22:00:49 -0700213 This permits a key's payload to be instantiated or updated, or it allows a
214 link to be added to or removed from a keyring.
Linus Torvalds1da177e2005-04-16 15:20:36 -0700215
216 (*) Search
217
218 This permits keyrings to be searched and keys to be found. Searches can
219 only recurse into nested keyrings that have search permission set.
220
221 (*) Link
222
223 This permits a key or keyring to be linked to. To create a link from a
224 keyring to a key, a process must have Write permission on the keyring and
225 Link permission on the key.
226
227For changing the ownership, group ID or permissions mask, being the owner of
228the key or having the sysadmin capability is sufficient.
229
230
231================
232NEW PROCFS FILES
233================
234
235Two files have been added to procfs by which an administrator can find out
236about the status of the key service:
237
238 (*) /proc/keys
239
240 This lists all the keys on the system, giving information about their
David Howells76d8aea2005-06-23 22:00:49 -0700241 type, description and permissions. The payload of the key is not available
242 this way:
Linus Torvalds1da177e2005-04-16 15:20:36 -0700243
244 SERIAL FLAGS USAGE EXPY PERM UID GID TYPE DESCRIPTION: SUMMARY
245 00000001 I----- 39 perm 1f0000 0 0 keyring _uid_ses.0: 1/4
246 00000002 I----- 2 perm 1f0000 0 0 keyring _uid.0: empty
247 00000007 I----- 1 perm 1f0000 0 0 keyring _pid.1: empty
248 0000018d I----- 1 perm 1f0000 0 0 keyring _pid.412: empty
249 000004d2 I--Q-- 1 perm 1f0000 32 -1 keyring _uid.32: 1/4
250 000004d3 I--Q-- 3 perm 1f0000 32 -1 keyring _uid_ses.32: empty
251 00000892 I--QU- 1 perm 1f0000 0 0 user metal:copper: 0
252 00000893 I--Q-N 1 35s 1f0000 0 0 user metal:silver: 0
253 00000894 I--Q-- 1 10h 1f0000 0 0 user metal:gold: 0
254
255 The flags are:
256
257 I Instantiated
258 R Revoked
259 D Dead
260 Q Contributes to user's quota
261 U Under contruction by callback to userspace
262 N Negative key
263
264 This file must be enabled at kernel configuration time as it allows anyone
265 to list the keys database.
266
267 (*) /proc/key-users
268
269 This file lists the tracking data for each user that has at least one key
270 on the system. Such data includes quota information and statistics:
271
272 [root@andromeda root]# cat /proc/key-users
273 0: 46 45/45 1/100 13/10000
274 29: 2 2/2 2/100 40/10000
275 32: 2 2/2 2/100 40/10000
276 38: 2 2/2 2/100 40/10000
277
278 The format of each line is
279 <UID>: User ID to which this applies
280 <usage> Structure refcount
281 <inst>/<keys> Total number of keys and number instantiated
282 <keys>/<max> Key count quota
283 <bytes>/<max> Key size quota
284
285
286===============================
287USERSPACE SYSTEM CALL INTERFACE
288===============================
289
290Userspace can manipulate keys directly through three new syscalls: add_key,
291request_key and keyctl. The latter provides a number of functions for
292manipulating keys.
293
294When referring to a key directly, userspace programs should use the key's
295serial number (a positive 32-bit integer). However, there are some special
296values available for referring to special keys and keyrings that relate to the
297process making the call:
298
299 CONSTANT VALUE KEY REFERENCED
300 ============================== ====== ===========================
301 KEY_SPEC_THREAD_KEYRING -1 thread-specific keyring
302 KEY_SPEC_PROCESS_KEYRING -2 process-specific keyring
303 KEY_SPEC_SESSION_KEYRING -3 session-specific keyring
304 KEY_SPEC_USER_KEYRING -4 UID-specific keyring
305 KEY_SPEC_USER_SESSION_KEYRING -5 UID-session keyring
306 KEY_SPEC_GROUP_KEYRING -6 GID-specific keyring
307
308
309The main syscalls are:
310
311 (*) Create a new key of given type, description and payload and add it to the
312 nominated keyring:
313
314 key_serial_t add_key(const char *type, const char *desc,
315 const void *payload, size_t plen,
316 key_serial_t keyring);
317
318 If a key of the same type and description as that proposed already exists
319 in the keyring, this will try to update it with the given payload, or it
320 will return error EEXIST if that function is not supported by the key
David Howells76d8aea2005-06-23 22:00:49 -0700321 type. The process must also have permission to write to the key to be able
322 to update it. The new key will have all user permissions granted and no
323 group or third party permissions.
Linus Torvalds1da177e2005-04-16 15:20:36 -0700324
David Howells76d8aea2005-06-23 22:00:49 -0700325 Otherwise, this will attempt to create a new key of the specified type and
326 description, and to instantiate it with the supplied payload and attach it
327 to the keyring. In this case, an error will be generated if the process
328 does not have permission to write to the keyring.
Linus Torvalds1da177e2005-04-16 15:20:36 -0700329
330 The payload is optional, and the pointer can be NULL if not required by
331 the type. The payload is plen in size, and plen can be zero for an empty
332 payload.
333
David Howells76d8aea2005-06-23 22:00:49 -0700334 A new keyring can be generated by setting type "keyring", the keyring name
335 as the description (or NULL) and setting the payload to NULL.
Linus Torvalds1da177e2005-04-16 15:20:36 -0700336
337 User defined keys can be created by specifying type "user". It is
338 recommended that a user defined key's description by prefixed with a type
339 ID and a colon, such as "krb5tgt:" for a Kerberos 5 ticket granting
340 ticket.
341
342 Any other type must have been registered with the kernel in advance by a
343 kernel service such as a filesystem.
344
345 The ID of the new or updated key is returned if successful.
346
347
348 (*) Search the process's keyrings for a key, potentially calling out to
349 userspace to create it.
350
351 key_serial_t request_key(const char *type, const char *description,
352 const char *callout_info,
353 key_serial_t dest_keyring);
354
355 This function searches all the process's keyrings in the order thread,
356 process, session for a matching key. This works very much like
357 KEYCTL_SEARCH, including the optional attachment of the discovered key to
358 a keyring.
359
360 If a key cannot be found, and if callout_info is not NULL, then
361 /sbin/request-key will be invoked in an attempt to obtain a key. The
362 callout_info string will be passed as an argument to the program.
363
364
365The keyctl syscall functions are:
366
367 (*) Map a special key ID to a real key ID for this process:
368
369 key_serial_t keyctl(KEYCTL_GET_KEYRING_ID, key_serial_t id,
370 int create);
371
David Howells76d8aea2005-06-23 22:00:49 -0700372 The special key specified by "id" is looked up (with the key being created
373 if necessary) and the ID of the key or keyring thus found is returned if
374 it exists.
Linus Torvalds1da177e2005-04-16 15:20:36 -0700375
376 If the key does not yet exist, the key will be created if "create" is
377 non-zero; and the error ENOKEY will be returned if "create" is zero.
378
379
380 (*) Replace the session keyring this process subscribes to with a new one:
381
382 key_serial_t keyctl(KEYCTL_JOIN_SESSION_KEYRING, const char *name);
383
384 If name is NULL, an anonymous keyring is created attached to the process
385 as its session keyring, displacing the old session keyring.
386
387 If name is not NULL, if a keyring of that name exists, the process
388 attempts to attach it as the session keyring, returning an error if that
389 is not permitted; otherwise a new keyring of that name is created and
390 attached as the session keyring.
391
392 To attach to a named keyring, the keyring must have search permission for
393 the process's ownership.
394
395 The ID of the new session keyring is returned if successful.
396
397
398 (*) Update the specified key:
399
400 long keyctl(KEYCTL_UPDATE, key_serial_t key, const void *payload,
401 size_t plen);
402
403 This will try to update the specified key with the given payload, or it
404 will return error EOPNOTSUPP if that function is not supported by the key
David Howells76d8aea2005-06-23 22:00:49 -0700405 type. The process must also have permission to write to the key to be able
406 to update it.
Linus Torvalds1da177e2005-04-16 15:20:36 -0700407
408 The payload is of length plen, and may be absent or empty as for
409 add_key().
410
411
412 (*) Revoke a key:
413
414 long keyctl(KEYCTL_REVOKE, key_serial_t key);
415
416 This makes a key unavailable for further operations. Further attempts to
417 use the key will be met with error EKEYREVOKED, and the key will no longer
418 be findable.
419
420
421 (*) Change the ownership of a key:
422
423 long keyctl(KEYCTL_CHOWN, key_serial_t key, uid_t uid, gid_t gid);
424
David Howells76d8aea2005-06-23 22:00:49 -0700425 This function permits a key's owner and group ID to be changed. Either one
426 of uid or gid can be set to -1 to suppress that change.
Linus Torvalds1da177e2005-04-16 15:20:36 -0700427
428 Only the superuser can change a key's owner to something other than the
429 key's current owner. Similarly, only the superuser can change a key's
430 group ID to something other than the calling process's group ID or one of
431 its group list members.
432
433
434 (*) Change the permissions mask on a key:
435
436 long keyctl(KEYCTL_SETPERM, key_serial_t key, key_perm_t perm);
437
438 This function permits the owner of a key or the superuser to change the
439 permissions mask on a key.
440
441 Only bits the available bits are permitted; if any other bits are set,
442 error EINVAL will be returned.
443
444
445 (*) Describe a key:
446
447 long keyctl(KEYCTL_DESCRIBE, key_serial_t key, char *buffer,
448 size_t buflen);
449
450 This function returns a summary of the key's attributes (but not its
451 payload data) as a string in the buffer provided.
452
453 Unless there's an error, it always returns the amount of data it could
454 produce, even if that's too big for the buffer, but it won't copy more
455 than requested to userspace. If the buffer pointer is NULL then no copy
456 will take place.
457
458 A process must have view permission on the key for this function to be
459 successful.
460
461 If successful, a string is placed in the buffer in the following format:
462
463 <type>;<uid>;<gid>;<perm>;<description>
464
465 Where type and description are strings, uid and gid are decimal, and perm
466 is hexadecimal. A NUL character is included at the end of the string if
467 the buffer is sufficiently big.
468
469 This can be parsed with
470
471 sscanf(buffer, "%[^;];%d;%d;%o;%s", type, &uid, &gid, &mode, desc);
472
473
474 (*) Clear out a keyring:
475
476 long keyctl(KEYCTL_CLEAR, key_serial_t keyring);
477
478 This function clears the list of keys attached to a keyring. The calling
479 process must have write permission on the keyring, and it must be a
480 keyring (or else error ENOTDIR will result).
481
482
483 (*) Link a key into a keyring:
484
485 long keyctl(KEYCTL_LINK, key_serial_t keyring, key_serial_t key);
486
David Howells76d8aea2005-06-23 22:00:49 -0700487 This function creates a link from the keyring to the key. The process must
488 have write permission on the keyring and must have link permission on the
489 key.
Linus Torvalds1da177e2005-04-16 15:20:36 -0700490
David Howells76d8aea2005-06-23 22:00:49 -0700491 Should the keyring not be a keyring, error ENOTDIR will result; and if the
492 keyring is full, error ENFILE will result.
Linus Torvalds1da177e2005-04-16 15:20:36 -0700493
494 The link procedure checks the nesting of the keyrings, returning ELOOP if
495 it appears to deep or EDEADLK if the link would introduce a cycle.
496
497
498 (*) Unlink a key or keyring from another keyring:
499
500 long keyctl(KEYCTL_UNLINK, key_serial_t keyring, key_serial_t key);
501
502 This function looks through the keyring for the first link to the
503 specified key, and removes it if found. Subsequent links to that key are
504 ignored. The process must have write permission on the keyring.
505
David Howells76d8aea2005-06-23 22:00:49 -0700506 If the keyring is not a keyring, error ENOTDIR will result; and if the key
507 is not present, error ENOENT will be the result.
Linus Torvalds1da177e2005-04-16 15:20:36 -0700508
509
510 (*) Search a keyring tree for a key:
511
512 key_serial_t keyctl(KEYCTL_SEARCH, key_serial_t keyring,
513 const char *type, const char *description,
514 key_serial_t dest_keyring);
515
David Howells76d8aea2005-06-23 22:00:49 -0700516 This searches the keyring tree headed by the specified keyring until a key
517 is found that matches the type and description criteria. Each keyring is
518 checked for keys before recursion into its children occurs.
Linus Torvalds1da177e2005-04-16 15:20:36 -0700519
520 The process must have search permission on the top level keyring, or else
521 error EACCES will result. Only keyrings that the process has search
522 permission on will be recursed into, and only keys and keyrings for which
523 a process has search permission can be matched. If the specified keyring
524 is not a keyring, ENOTDIR will result.
525
526 If the search succeeds, the function will attempt to link the found key
527 into the destination keyring if one is supplied (non-zero ID). All the
528 constraints applicable to KEYCTL_LINK apply in this case too.
529
530 Error ENOKEY, EKEYREVOKED or EKEYEXPIRED will be returned if the search
531 fails. On success, the resulting key ID will be returned.
532
533
534 (*) Read the payload data from a key:
535
536 key_serial_t keyctl(KEYCTL_READ, key_serial_t keyring, char *buffer,
537 size_t buflen);
538
539 This function attempts to read the payload data from the specified key
540 into the buffer. The process must have read permission on the key to
541 succeed.
542
543 The returned data will be processed for presentation by the key type. For
544 instance, a keyring will return an array of key_serial_t entries
545 representing the IDs of all the keys to which it is subscribed. The user
546 defined key type will return its data as is. If a key type does not
547 implement this function, error EOPNOTSUPP will result.
548
549 As much of the data as can be fitted into the buffer will be copied to
550 userspace if the buffer pointer is not NULL.
551
David Howells76d8aea2005-06-23 22:00:49 -0700552 On a successful return, the function will always return the amount of data
553 available rather than the amount copied.
Linus Torvalds1da177e2005-04-16 15:20:36 -0700554
555
556 (*) Instantiate a partially constructed key.
557
558 key_serial_t keyctl(KEYCTL_INSTANTIATE, key_serial_t key,
559 const void *payload, size_t plen,
560 key_serial_t keyring);
561
562 If the kernel calls back to userspace to complete the instantiation of a
563 key, userspace should use this call to supply data for the key before the
564 invoked process returns, or else the key will be marked negative
565 automatically.
566
567 The process must have write access on the key to be able to instantiate
568 it, and the key must be uninstantiated.
569
570 If a keyring is specified (non-zero), the key will also be linked into
David Howells76d8aea2005-06-23 22:00:49 -0700571 that keyring, however all the constraints applying in KEYCTL_LINK apply in
572 this case too.
Linus Torvalds1da177e2005-04-16 15:20:36 -0700573
574 The payload and plen arguments describe the payload data as for add_key().
575
576
577 (*) Negatively instantiate a partially constructed key.
578
579 key_serial_t keyctl(KEYCTL_NEGATE, key_serial_t key,
580 unsigned timeout, key_serial_t keyring);
581
582 If the kernel calls back to userspace to complete the instantiation of a
583 key, userspace should use this call mark the key as negative before the
584 invoked process returns if it is unable to fulfil the request.
585
586 The process must have write access on the key to be able to instantiate
587 it, and the key must be uninstantiated.
588
589 If a keyring is specified (non-zero), the key will also be linked into
David Howells76d8aea2005-06-23 22:00:49 -0700590 that keyring, however all the constraints applying in KEYCTL_LINK apply in
591 this case too.
Linus Torvalds1da177e2005-04-16 15:20:36 -0700592
593
594===============
595KERNEL SERVICES
596===============
597
598The kernel services for key managment are fairly simple to deal with. They can
599be broken down into two areas: keys and key types.
600
601Dealing with keys is fairly straightforward. Firstly, the kernel service
602registers its type, then it searches for a key of that type. It should retain
603the key as long as it has need of it, and then it should release it. For a
David Howells76d8aea2005-06-23 22:00:49 -0700604filesystem or device file, a search would probably be performed during the open
605call, and the key released upon close. How to deal with conflicting keys due to
606two different users opening the same file is left to the filesystem author to
607solve.
Linus Torvalds1da177e2005-04-16 15:20:36 -0700608
David Howells76d8aea2005-06-23 22:00:49 -0700609When accessing a key's payload contents, certain precautions must be taken to
610prevent access vs modification races. See the section "Notes on accessing
611payload contents" for more information.
Linus Torvalds1da177e2005-04-16 15:20:36 -0700612
613(*) To search for a key, call:
614
615 struct key *request_key(const struct key_type *type,
616 const char *description,
617 const char *callout_string);
618
619 This is used to request a key or keyring with a description that matches
620 the description specified according to the key type's match function. This
621 permits approximate matching to occur. If callout_string is not NULL, then
622 /sbin/request-key will be invoked in an attempt to obtain the key from
623 userspace. In that case, callout_string will be passed as an argument to
624 the program.
625
626 Should the function fail error ENOKEY, EKEYEXPIRED or EKEYREVOKED will be
627 returned.
628
629
630(*) When it is no longer required, the key should be released using:
631
632 void key_put(struct key *key);
633
634 This can be called from interrupt context. If CONFIG_KEYS is not set then
635 the argument will not be parsed.
636
637
638(*) Extra references can be made to a key by calling the following function:
639
640 struct key *key_get(struct key *key);
641
642 These need to be disposed of by calling key_put() when they've been
643 finished with. The key pointer passed in will be returned. If the pointer
644 is NULL or CONFIG_KEYS is not set then the key will not be dereferenced and
645 no increment will take place.
646
647
648(*) A key's serial number can be obtained by calling:
649
650 key_serial_t key_serial(struct key *key);
651
652 If key is NULL or if CONFIG_KEYS is not set then 0 will be returned (in the
653 latter case without parsing the argument).
654
655
656(*) If a keyring was found in the search, this can be further searched by:
657
658 struct key *keyring_search(struct key *keyring,
659 const struct key_type *type,
660 const char *description)
661
662 This searches the keyring tree specified for a matching key. Error ENOKEY
663 is returned upon failure. If successful, the returned key will need to be
664 released.
665
666
667(*) To check the validity of a key, this function can be called:
668
669 int validate_key(struct key *key);
670
671 This checks that the key in question hasn't expired or and hasn't been
672 revoked. Should the key be invalid, error EKEYEXPIRED or EKEYREVOKED will
673 be returned. If the key is NULL or if CONFIG_KEYS is not set then 0 will be
674 returned (in the latter case without parsing the argument).
675
676
677(*) To register a key type, the following function should be called:
678
679 int register_key_type(struct key_type *type);
680
681 This will return error EEXIST if a type of the same name is already
682 present.
683
684
685(*) To unregister a key type, call:
686
687 void unregister_key_type(struct key_type *type);
688
689
David Howells76d8aea2005-06-23 22:00:49 -0700690===================================
691NOTES ON ACCESSING PAYLOAD CONTENTS
692===================================
693
694The simplest payload is just a number in key->payload.value. In this case,
695there's no need to indulge in RCU or locking when accessing the payload.
696
697More complex payload contents must be allocated and a pointer to them set in
698key->payload.data. One of the following ways must be selected to access the
699data:
700
701 (1) Unmodifyable key type.
702
703 If the key type does not have a modify method, then the key's payload can
704 be accessed without any form of locking, provided that it's known to be
705 instantiated (uninstantiated keys cannot be "found").
706
707 (2) The key's semaphore.
708
709 The semaphore could be used to govern access to the payload and to control
710 the payload pointer. It must be write-locked for modifications and would
711 have to be read-locked for general access. The disadvantage of doing this
712 is that the accessor may be required to sleep.
713
714 (3) RCU.
715
716 RCU must be used when the semaphore isn't already held; if the semaphore
717 is held then the contents can't change under you unexpectedly as the
718 semaphore must still be used to serialise modifications to the key. The
719 key management code takes care of this for the key type.
720
721 However, this means using:
722
723 rcu_read_lock() ... rcu_dereference() ... rcu_read_unlock()
724
725 to read the pointer, and:
726
727 rcu_dereference() ... rcu_assign_pointer() ... call_rcu()
728
729 to set the pointer and dispose of the old contents after a grace period.
730 Note that only the key type should ever modify a key's payload.
731
732 Furthermore, an RCU controlled payload must hold a struct rcu_head for the
733 use of call_rcu() and, if the payload is of variable size, the length of
734 the payload. key->datalen cannot be relied upon to be consistent with the
735 payload just dereferenced if the key's semaphore is not held.
736
737
Linus Torvalds1da177e2005-04-16 15:20:36 -0700738===================
739DEFINING A KEY TYPE
740===================
741
742A kernel service may want to define its own key type. For instance, an AFS
743filesystem might want to define a Kerberos 5 ticket key type. To do this, it
744author fills in a struct key_type and registers it with the system.
745
746The structure has a number of fields, some of which are mandatory:
747
748 (*) const char *name
749
750 The name of the key type. This is used to translate a key type name
751 supplied by userspace into a pointer to the structure.
752
753
754 (*) size_t def_datalen
755
756 This is optional - it supplies the default payload data length as
757 contributed to the quota. If the key type's payload is always or almost
758 always the same size, then this is a more efficient way to do things.
759
760 The data length (and quota) on a particular key can always be changed
761 during instantiation or update by calling:
762
763 int key_payload_reserve(struct key *key, size_t datalen);
764
David Howells76d8aea2005-06-23 22:00:49 -0700765 With the revised data length. Error EDQUOT will be returned if this is not
766 viable.
Linus Torvalds1da177e2005-04-16 15:20:36 -0700767
768
769 (*) int (*instantiate)(struct key *key, const void *data, size_t datalen);
770
771 This method is called to attach a payload to a key during construction.
David Howells76d8aea2005-06-23 22:00:49 -0700772 The payload attached need not bear any relation to the data passed to this
773 function.
Linus Torvalds1da177e2005-04-16 15:20:36 -0700774
775 If the amount of data attached to the key differs from the size in
776 keytype->def_datalen, then key_payload_reserve() should be called.
777
778 This method does not have to lock the key in order to attach a payload.
779 The fact that KEY_FLAG_INSTANTIATED is not set in key->flags prevents
780 anything else from gaining access to the key.
781
David Howells76d8aea2005-06-23 22:00:49 -0700782 It is safe to sleep in this method.
Linus Torvalds1da177e2005-04-16 15:20:36 -0700783
784
785 (*) int (*duplicate)(struct key *key, const struct key *source);
786
787 If this type of key can be duplicated, then this method should be
David Howells76d8aea2005-06-23 22:00:49 -0700788 provided. It is called to copy the payload attached to the source into the
789 new key. The data length on the new key will have been updated and the
790 quota adjusted already.
Linus Torvalds1da177e2005-04-16 15:20:36 -0700791
792 This method will be called with the source key's semaphore read-locked to
David Howells76d8aea2005-06-23 22:00:49 -0700793 prevent its payload from being changed, thus RCU constraints need not be
794 applied to the source key.
795
796 This method does not have to lock the destination key in order to attach a
797 payload. The fact that KEY_FLAG_INSTANTIATED is not set in key->flags
798 prevents anything else from gaining access to the key.
799
800 It is safe to sleep in this method.
Linus Torvalds1da177e2005-04-16 15:20:36 -0700801
802
803 (*) int (*update)(struct key *key, const void *data, size_t datalen);
804
David Howells76d8aea2005-06-23 22:00:49 -0700805 If this type of key can be updated, then this method should be provided.
806 It is called to update a key's payload from the blob of data provided.
Linus Torvalds1da177e2005-04-16 15:20:36 -0700807
808 key_payload_reserve() should be called if the data length might change
David Howells76d8aea2005-06-23 22:00:49 -0700809 before any changes are actually made. Note that if this succeeds, the type
810 is committed to changing the key because it's already been altered, so all
811 memory allocation must be done first.
Linus Torvalds1da177e2005-04-16 15:20:36 -0700812
David Howells76d8aea2005-06-23 22:00:49 -0700813 The key will have its semaphore write-locked before this method is called,
814 but this only deters other writers; any changes to the key's payload must
815 be made under RCU conditions, and call_rcu() must be used to dispose of
816 the old payload.
Linus Torvalds1da177e2005-04-16 15:20:36 -0700817
David Howells76d8aea2005-06-23 22:00:49 -0700818 key_payload_reserve() should be called before the changes are made, but
819 after all allocations and other potentially failing function calls are
820 made.
821
822 It is safe to sleep in this method.
Linus Torvalds1da177e2005-04-16 15:20:36 -0700823
824
825 (*) int (*match)(const struct key *key, const void *desc);
826
827 This method is called to match a key against a description. It should
828 return non-zero if the two match, zero if they don't.
829
830 This method should not need to lock the key in any way. The type and
831 description can be considered invariant, and the payload should not be
832 accessed (the key may not yet be instantiated).
833
834 It is not safe to sleep in this method; the caller may hold spinlocks.
835
836
837 (*) void (*destroy)(struct key *key);
838
David Howells76d8aea2005-06-23 22:00:49 -0700839 This method is optional. It is called to discard the payload data on a key
840 when it is being destroyed.
Linus Torvalds1da177e2005-04-16 15:20:36 -0700841
David Howells76d8aea2005-06-23 22:00:49 -0700842 This method does not need to lock the key to access the payload; it can
843 consider the key as being inaccessible at this time. Note that the key's
844 type may have been changed before this function is called.
Linus Torvalds1da177e2005-04-16 15:20:36 -0700845
846 It is not safe to sleep in this method; the caller may hold spinlocks.
847
848
849 (*) void (*describe)(const struct key *key, struct seq_file *p);
850
851 This method is optional. It is called during /proc/keys reading to
852 summarise a key's description and payload in text form.
853
David Howells76d8aea2005-06-23 22:00:49 -0700854 This method will be called with the RCU read lock held. rcu_dereference()
855 should be used to read the payload pointer if the payload is to be
856 accessed. key->datalen cannot be trusted to stay consistent with the
857 contents of the payload.
858
859 The description will not change, though the key's state may.
860
861 It is not safe to sleep in this method; the RCU read lock is held by the
862 caller.
Linus Torvalds1da177e2005-04-16 15:20:36 -0700863
864
865 (*) long (*read)(const struct key *key, char __user *buffer, size_t buflen);
866
867 This method is optional. It is called by KEYCTL_READ to translate the
David Howells76d8aea2005-06-23 22:00:49 -0700868 key's payload into something a blob of data for userspace to deal with.
869 Ideally, the blob should be in the same format as that passed in to the
870 instantiate and update methods.
Linus Torvalds1da177e2005-04-16 15:20:36 -0700871
872 If successful, the blob size that could be produced should be returned
873 rather than the size copied.
874
David Howells76d8aea2005-06-23 22:00:49 -0700875 This method will be called with the key's semaphore read-locked. This will
876 prevent the key's payload changing. It is not necessary to use RCU locking
877 when accessing the key's payload. It is safe to sleep in this method, such
878 as might happen when the userspace buffer is accessed.
Linus Torvalds1da177e2005-04-16 15:20:36 -0700879
880
881============================
882REQUEST-KEY CALLBACK SERVICE
883============================
884
885To create a new key, the kernel will attempt to execute the following command
886line:
887
888 /sbin/request-key create <key> <uid> <gid> \
889 <threadring> <processring> <sessionring> <callout_info>
890
891<key> is the key being constructed, and the three keyrings are the process
892keyrings from the process that caused the search to be issued. These are
893included for two reasons:
894
895 (1) There may be an authentication token in one of the keyrings that is
896 required to obtain the key, eg: a Kerberos Ticket-Granting Ticket.
897
898 (2) The new key should probably be cached in one of these rings.
899
900This program should set it UID and GID to those specified before attempting to
901access any more keys. It may then look around for a user specific process to
902hand the request off to (perhaps a path held in placed in another key by, for
903example, the KDE desktop manager).
904
905The program (or whatever it calls) should finish construction of the key by
906calling KEYCTL_INSTANTIATE, which also permits it to cache the key in one of
907the keyrings (probably the session ring) before returning. Alternatively, the
908key can be marked as negative with KEYCTL_NEGATE; this also permits the key to
909be cached in one of the keyrings.
910
911If it returns with the key remaining in the unconstructed state, the key will
912be marked as being negative, it will be added to the session keyring, and an
913error will be returned to the key requestor.
914
David Howells76d8aea2005-06-23 22:00:49 -0700915Supplementary information may be provided from whoever or whatever invoked this
916service. This will be passed as the <callout_info> parameter. If no such
Linus Torvalds1da177e2005-04-16 15:20:36 -0700917information was made available, then "-" will be passed as this parameter
918instead.
919
920
921Similarly, the kernel may attempt to update an expired or a soon to expire key
922by executing:
923
924 /sbin/request-key update <key> <uid> <gid> \
925 <threadring> <processring> <sessionring>
926
927In this case, the program isn't required to actually attach the key to a ring;
928the rings are provided for reference.