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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
David Howells664cceb2005-09-28 17:03:15 +0100198has up to eight bits each for possessor, user, group and other access. Only
David Howells29db9192005-10-30 15:02:44 -0800199six of each set 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
David Howells29db9192005-10-30 15:02:44 -0800227 (*) Set Attribute
228
229 This permits a key's UID, GID and permissions mask to be changed.
230
Linus Torvalds1da177e2005-04-16 15:20:36 -0700231For changing the ownership, group ID or permissions mask, being the owner of
232the key or having the sysadmin capability is sufficient.
233
234
235================
236NEW PROCFS FILES
237================
238
239Two files have been added to procfs by which an administrator can find out
240about the status of the key service:
241
242 (*) /proc/keys
243
244 This lists all the keys on the system, giving information about their
David Howells76d8aea2005-06-23 22:00:49 -0700245 type, description and permissions. The payload of the key is not available
246 this way:
Linus Torvalds1da177e2005-04-16 15:20:36 -0700247
David Howells664cceb2005-09-28 17:03:15 +0100248 SERIAL FLAGS USAGE EXPY PERM UID GID TYPE DESCRIPTION: SUMMARY
David Howells29db9192005-10-30 15:02:44 -0800249 00000001 I----- 39 perm 1f3f0000 0 0 keyring _uid_ses.0: 1/4
250 00000002 I----- 2 perm 1f3f0000 0 0 keyring _uid.0: empty
251 00000007 I----- 1 perm 1f3f0000 0 0 keyring _pid.1: empty
252 0000018d I----- 1 perm 1f3f0000 0 0 keyring _pid.412: empty
253 000004d2 I--Q-- 1 perm 1f3f0000 32 -1 keyring _uid.32: 1/4
254 000004d3 I--Q-- 3 perm 1f3f0000 32 -1 keyring _uid_ses.32: empty
David Howells664cceb2005-09-28 17:03:15 +0100255 00000892 I--QU- 1 perm 1f000000 0 0 user metal:copper: 0
David Howells29db9192005-10-30 15:02:44 -0800256 00000893 I--Q-N 1 35s 1f3f0000 0 0 user metal:silver: 0
257 00000894 I--Q-- 1 10h 003f0000 0 0 user metal:gold: 0
Linus Torvalds1da177e2005-04-16 15:20:36 -0700258
259 The flags are:
260
261 I Instantiated
262 R Revoked
263 D Dead
264 Q Contributes to user's quota
265 U Under contruction by callback to userspace
266 N Negative key
267
268 This file must be enabled at kernel configuration time as it allows anyone
269 to list the keys database.
270
271 (*) /proc/key-users
272
273 This file lists the tracking data for each user that has at least one key
274 on the system. Such data includes quota information and statistics:
275
276 [root@andromeda root]# cat /proc/key-users
277 0: 46 45/45 1/100 13/10000
278 29: 2 2/2 2/100 40/10000
279 32: 2 2/2 2/100 40/10000
280 38: 2 2/2 2/100 40/10000
281
282 The format of each line is
283 <UID>: User ID to which this applies
284 <usage> Structure refcount
285 <inst>/<keys> Total number of keys and number instantiated
286 <keys>/<max> Key count quota
287 <bytes>/<max> Key size quota
288
289
290===============================
291USERSPACE SYSTEM CALL INTERFACE
292===============================
293
294Userspace can manipulate keys directly through three new syscalls: add_key,
295request_key and keyctl. The latter provides a number of functions for
296manipulating keys.
297
298When referring to a key directly, userspace programs should use the key's
299serial number (a positive 32-bit integer). However, there are some special
300values available for referring to special keys and keyrings that relate to the
301process making the call:
302
303 CONSTANT VALUE KEY REFERENCED
304 ============================== ====== ===========================
305 KEY_SPEC_THREAD_KEYRING -1 thread-specific keyring
306 KEY_SPEC_PROCESS_KEYRING -2 process-specific keyring
307 KEY_SPEC_SESSION_KEYRING -3 session-specific keyring
308 KEY_SPEC_USER_KEYRING -4 UID-specific keyring
309 KEY_SPEC_USER_SESSION_KEYRING -5 UID-session keyring
310 KEY_SPEC_GROUP_KEYRING -6 GID-specific keyring
311
312
313The main syscalls are:
314
315 (*) Create a new key of given type, description and payload and add it to the
316 nominated keyring:
317
318 key_serial_t add_key(const char *type, const char *desc,
319 const void *payload, size_t plen,
320 key_serial_t keyring);
321
322 If a key of the same type and description as that proposed already exists
323 in the keyring, this will try to update it with the given payload, or it
324 will return error EEXIST if that function is not supported by the key
David Howells76d8aea2005-06-23 22:00:49 -0700325 type. The process must also have permission to write to the key to be able
326 to update it. The new key will have all user permissions granted and no
327 group or third party permissions.
Linus Torvalds1da177e2005-04-16 15:20:36 -0700328
David Howells76d8aea2005-06-23 22:00:49 -0700329 Otherwise, this will attempt to create a new key of the specified type and
330 description, and to instantiate it with the supplied payload and attach it
331 to the keyring. In this case, an error will be generated if the process
332 does not have permission to write to the keyring.
Linus Torvalds1da177e2005-04-16 15:20:36 -0700333
334 The payload is optional, and the pointer can be NULL if not required by
335 the type. The payload is plen in size, and plen can be zero for an empty
336 payload.
337
David Howells76d8aea2005-06-23 22:00:49 -0700338 A new keyring can be generated by setting type "keyring", the keyring name
339 as the description (or NULL) and setting the payload to NULL.
Linus Torvalds1da177e2005-04-16 15:20:36 -0700340
341 User defined keys can be created by specifying type "user". It is
342 recommended that a user defined key's description by prefixed with a type
343 ID and a colon, such as "krb5tgt:" for a Kerberos 5 ticket granting
344 ticket.
345
346 Any other type must have been registered with the kernel in advance by a
347 kernel service such as a filesystem.
348
349 The ID of the new or updated key is returned if successful.
350
351
352 (*) Search the process's keyrings for a key, potentially calling out to
353 userspace to create it.
354
355 key_serial_t request_key(const char *type, const char *description,
356 const char *callout_info,
357 key_serial_t dest_keyring);
358
359 This function searches all the process's keyrings in the order thread,
360 process, session for a matching key. This works very much like
361 KEYCTL_SEARCH, including the optional attachment of the discovered key to
362 a keyring.
363
364 If a key cannot be found, and if callout_info is not NULL, then
365 /sbin/request-key will be invoked in an attempt to obtain a key. The
366 callout_info string will be passed as an argument to the program.
367
David Howellsf1a9bad2005-10-07 15:04:52 +0100368 See also Documentation/keys-request-key.txt.
369
Linus Torvalds1da177e2005-04-16 15:20:36 -0700370
371The keyctl syscall functions are:
372
373 (*) Map a special key ID to a real key ID for this process:
374
375 key_serial_t keyctl(KEYCTL_GET_KEYRING_ID, key_serial_t id,
376 int create);
377
David Howells76d8aea2005-06-23 22:00:49 -0700378 The special key specified by "id" is looked up (with the key being created
379 if necessary) and the ID of the key or keyring thus found is returned if
380 it exists.
Linus Torvalds1da177e2005-04-16 15:20:36 -0700381
382 If the key does not yet exist, the key will be created if "create" is
383 non-zero; and the error ENOKEY will be returned if "create" is zero.
384
385
386 (*) Replace the session keyring this process subscribes to with a new one:
387
388 key_serial_t keyctl(KEYCTL_JOIN_SESSION_KEYRING, const char *name);
389
390 If name is NULL, an anonymous keyring is created attached to the process
391 as its session keyring, displacing the old session keyring.
392
393 If name is not NULL, if a keyring of that name exists, the process
394 attempts to attach it as the session keyring, returning an error if that
395 is not permitted; otherwise a new keyring of that name is created and
396 attached as the session keyring.
397
398 To attach to a named keyring, the keyring must have search permission for
399 the process's ownership.
400
401 The ID of the new session keyring is returned if successful.
402
403
404 (*) Update the specified key:
405
406 long keyctl(KEYCTL_UPDATE, key_serial_t key, const void *payload,
407 size_t plen);
408
409 This will try to update the specified key with the given payload, or it
410 will return error EOPNOTSUPP if that function is not supported by the key
David Howells76d8aea2005-06-23 22:00:49 -0700411 type. The process must also have permission to write to the key to be able
412 to update it.
Linus Torvalds1da177e2005-04-16 15:20:36 -0700413
414 The payload is of length plen, and may be absent or empty as for
415 add_key().
416
417
418 (*) Revoke a key:
419
420 long keyctl(KEYCTL_REVOKE, key_serial_t key);
421
422 This makes a key unavailable for further operations. Further attempts to
423 use the key will be met with error EKEYREVOKED, and the key will no longer
424 be findable.
425
426
427 (*) Change the ownership of a key:
428
429 long keyctl(KEYCTL_CHOWN, key_serial_t key, uid_t uid, gid_t gid);
430
David Howells76d8aea2005-06-23 22:00:49 -0700431 This function permits a key's owner and group ID to be changed. Either one
432 of uid or gid can be set to -1 to suppress that change.
Linus Torvalds1da177e2005-04-16 15:20:36 -0700433
434 Only the superuser can change a key's owner to something other than the
435 key's current owner. Similarly, only the superuser can change a key's
436 group ID to something other than the calling process's group ID or one of
437 its group list members.
438
439
440 (*) Change the permissions mask on a key:
441
442 long keyctl(KEYCTL_SETPERM, key_serial_t key, key_perm_t perm);
443
444 This function permits the owner of a key or the superuser to change the
445 permissions mask on a key.
446
447 Only bits the available bits are permitted; if any other bits are set,
448 error EINVAL will be returned.
449
450
451 (*) Describe a key:
452
453 long keyctl(KEYCTL_DESCRIBE, key_serial_t key, char *buffer,
454 size_t buflen);
455
456 This function returns a summary of the key's attributes (but not its
457 payload data) as a string in the buffer provided.
458
459 Unless there's an error, it always returns the amount of data it could
460 produce, even if that's too big for the buffer, but it won't copy more
461 than requested to userspace. If the buffer pointer is NULL then no copy
462 will take place.
463
464 A process must have view permission on the key for this function to be
465 successful.
466
467 If successful, a string is placed in the buffer in the following format:
468
469 <type>;<uid>;<gid>;<perm>;<description>
470
471 Where type and description are strings, uid and gid are decimal, and perm
472 is hexadecimal. A NUL character is included at the end of the string if
473 the buffer is sufficiently big.
474
475 This can be parsed with
476
477 sscanf(buffer, "%[^;];%d;%d;%o;%s", type, &uid, &gid, &mode, desc);
478
479
480 (*) Clear out a keyring:
481
482 long keyctl(KEYCTL_CLEAR, key_serial_t keyring);
483
484 This function clears the list of keys attached to a keyring. The calling
485 process must have write permission on the keyring, and it must be a
486 keyring (or else error ENOTDIR will result).
487
488
489 (*) Link a key into a keyring:
490
491 long keyctl(KEYCTL_LINK, key_serial_t keyring, key_serial_t key);
492
David Howells76d8aea2005-06-23 22:00:49 -0700493 This function creates a link from the keyring to the key. The process must
494 have write permission on the keyring and must have link permission on the
495 key.
Linus Torvalds1da177e2005-04-16 15:20:36 -0700496
David Howells76d8aea2005-06-23 22:00:49 -0700497 Should the keyring not be a keyring, error ENOTDIR will result; and if the
498 keyring is full, error ENFILE will result.
Linus Torvalds1da177e2005-04-16 15:20:36 -0700499
500 The link procedure checks the nesting of the keyrings, returning ELOOP if
501 it appears to deep or EDEADLK if the link would introduce a cycle.
502
503
504 (*) Unlink a key or keyring from another keyring:
505
506 long keyctl(KEYCTL_UNLINK, key_serial_t keyring, key_serial_t key);
507
508 This function looks through the keyring for the first link to the
509 specified key, and removes it if found. Subsequent links to that key are
510 ignored. The process must have write permission on the keyring.
511
David Howells76d8aea2005-06-23 22:00:49 -0700512 If the keyring is not a keyring, error ENOTDIR will result; and if the key
513 is not present, error ENOENT will be the result.
Linus Torvalds1da177e2005-04-16 15:20:36 -0700514
515
516 (*) Search a keyring tree for a key:
517
518 key_serial_t keyctl(KEYCTL_SEARCH, key_serial_t keyring,
519 const char *type, const char *description,
520 key_serial_t dest_keyring);
521
David Howells76d8aea2005-06-23 22:00:49 -0700522 This searches the keyring tree headed by the specified keyring until a key
523 is found that matches the type and description criteria. Each keyring is
524 checked for keys before recursion into its children occurs.
Linus Torvalds1da177e2005-04-16 15:20:36 -0700525
526 The process must have search permission on the top level keyring, or else
527 error EACCES will result. Only keyrings that the process has search
528 permission on will be recursed into, and only keys and keyrings for which
529 a process has search permission can be matched. If the specified keyring
530 is not a keyring, ENOTDIR will result.
531
532 If the search succeeds, the function will attempt to link the found key
533 into the destination keyring if one is supplied (non-zero ID). All the
534 constraints applicable to KEYCTL_LINK apply in this case too.
535
536 Error ENOKEY, EKEYREVOKED or EKEYEXPIRED will be returned if the search
537 fails. On success, the resulting key ID will be returned.
538
539
540 (*) Read the payload data from a key:
541
David Howellsf1a9bad2005-10-07 15:04:52 +0100542 long keyctl(KEYCTL_READ, key_serial_t keyring, char *buffer,
543 size_t buflen);
Linus Torvalds1da177e2005-04-16 15:20:36 -0700544
545 This function attempts to read the payload data from the specified key
546 into the buffer. The process must have read permission on the key to
547 succeed.
548
549 The returned data will be processed for presentation by the key type. For
550 instance, a keyring will return an array of key_serial_t entries
551 representing the IDs of all the keys to which it is subscribed. The user
552 defined key type will return its data as is. If a key type does not
553 implement this function, error EOPNOTSUPP will result.
554
555 As much of the data as can be fitted into the buffer will be copied to
556 userspace if the buffer pointer is not NULL.
557
David Howells76d8aea2005-06-23 22:00:49 -0700558 On a successful return, the function will always return the amount of data
559 available rather than the amount copied.
Linus Torvalds1da177e2005-04-16 15:20:36 -0700560
561
562 (*) Instantiate a partially constructed key.
563
David Howellsf1a9bad2005-10-07 15:04:52 +0100564 long keyctl(KEYCTL_INSTANTIATE, key_serial_t key,
565 const void *payload, size_t plen,
566 key_serial_t keyring);
Linus Torvalds1da177e2005-04-16 15:20:36 -0700567
568 If the kernel calls back to userspace to complete the instantiation of a
569 key, userspace should use this call to supply data for the key before the
570 invoked process returns, or else the key will be marked negative
571 automatically.
572
573 The process must have write access on the key to be able to instantiate
574 it, and the key must be uninstantiated.
575
576 If a keyring is specified (non-zero), the key will also be linked into
David Howells76d8aea2005-06-23 22:00:49 -0700577 that keyring, however all the constraints applying in KEYCTL_LINK apply in
578 this case too.
Linus Torvalds1da177e2005-04-16 15:20:36 -0700579
580 The payload and plen arguments describe the payload data as for add_key().
581
582
583 (*) Negatively instantiate a partially constructed key.
584
David Howellsf1a9bad2005-10-07 15:04:52 +0100585 long keyctl(KEYCTL_NEGATE, key_serial_t key,
586 unsigned timeout, key_serial_t keyring);
Linus Torvalds1da177e2005-04-16 15:20:36 -0700587
588 If the kernel calls back to userspace to complete the instantiation of a
589 key, userspace should use this call mark the key as negative before the
590 invoked process returns if it is unable to fulfil the request.
591
592 The process must have write access on the key to be able to instantiate
593 it, and the key must be uninstantiated.
594
595 If a keyring is specified (non-zero), the key will also be linked into
David Howells76d8aea2005-06-23 22:00:49 -0700596 that keyring, however all the constraints applying in KEYCTL_LINK apply in
597 this case too.
Linus Torvalds1da177e2005-04-16 15:20:36 -0700598
599
David Howells3e301482005-06-23 22:00:56 -0700600 (*) Set the default request-key destination keyring.
601
602 long keyctl(KEYCTL_SET_REQKEY_KEYRING, int reqkey_defl);
603
604 This sets the default keyring to which implicitly requested keys will be
605 attached for this thread. reqkey_defl should be one of these constants:
606
607 CONSTANT VALUE NEW DEFAULT KEYRING
608 ====================================== ====== =======================
609 KEY_REQKEY_DEFL_NO_CHANGE -1 No change
610 KEY_REQKEY_DEFL_DEFAULT 0 Default[1]
611 KEY_REQKEY_DEFL_THREAD_KEYRING 1 Thread keyring
612 KEY_REQKEY_DEFL_PROCESS_KEYRING 2 Process keyring
613 KEY_REQKEY_DEFL_SESSION_KEYRING 3 Session keyring
614 KEY_REQKEY_DEFL_USER_KEYRING 4 User keyring
615 KEY_REQKEY_DEFL_USER_SESSION_KEYRING 5 User session keyring
616 KEY_REQKEY_DEFL_GROUP_KEYRING 6 Group keyring
617
618 The old default will be returned if successful and error EINVAL will be
619 returned if reqkey_defl is not one of the above values.
620
621 The default keyring can be overridden by the keyring indicated to the
622 request_key() system call.
623
624 Note that this setting is inherited across fork/exec.
625
626 [1] The default default is: the thread keyring if there is one, otherwise
627 the process keyring if there is one, otherwise the session keyring if
628 there is one, otherwise the user default session keyring.
629
630
Linus Torvalds1da177e2005-04-16 15:20:36 -0700631===============
632KERNEL SERVICES
633===============
634
635The kernel services for key managment are fairly simple to deal with. They can
636be broken down into two areas: keys and key types.
637
638Dealing with keys is fairly straightforward. Firstly, the kernel service
639registers its type, then it searches for a key of that type. It should retain
640the key as long as it has need of it, and then it should release it. For a
David Howells76d8aea2005-06-23 22:00:49 -0700641filesystem or device file, a search would probably be performed during the open
642call, and the key released upon close. How to deal with conflicting keys due to
643two different users opening the same file is left to the filesystem author to
644solve.
Linus Torvalds1da177e2005-04-16 15:20:36 -0700645
David Howells664cceb2005-09-28 17:03:15 +0100646Note that there are two different types of pointers to keys that may be
647encountered:
648
649 (*) struct key *
650
651 This simply points to the key structure itself. Key structures will be at
652 least four-byte aligned.
653
654 (*) key_ref_t
655
656 This is equivalent to a struct key *, but the least significant bit is set
657 if the caller "possesses" the key. By "possession" it is meant that the
658 calling processes has a searchable link to the key from one of its
659 keyrings. There are three functions for dealing with these:
660
661 key_ref_t make_key_ref(const struct key *key,
662 unsigned long possession);
663
664 struct key *key_ref_to_ptr(const key_ref_t key_ref);
665
666 unsigned long is_key_possessed(const key_ref_t key_ref);
667
668 The first function constructs a key reference from a key pointer and
669 possession information (which must be 0 or 1 and not any other value).
670
671 The second function retrieves the key pointer from a reference and the
672 third retrieves the possession flag.
673
David Howells76d8aea2005-06-23 22:00:49 -0700674When accessing a key's payload contents, certain precautions must be taken to
675prevent access vs modification races. See the section "Notes on accessing
676payload contents" for more information.
Linus Torvalds1da177e2005-04-16 15:20:36 -0700677
678(*) To search for a key, call:
679
680 struct key *request_key(const struct key_type *type,
681 const char *description,
682 const char *callout_string);
683
684 This is used to request a key or keyring with a description that matches
685 the description specified according to the key type's match function. This
686 permits approximate matching to occur. If callout_string is not NULL, then
687 /sbin/request-key will be invoked in an attempt to obtain the key from
688 userspace. In that case, callout_string will be passed as an argument to
689 the program.
690
691 Should the function fail error ENOKEY, EKEYEXPIRED or EKEYREVOKED will be
692 returned.
693
David Howells3e301482005-06-23 22:00:56 -0700694 If successful, the key will have been attached to the default keyring for
695 implicitly obtained request-key keys, as set by KEYCTL_SET_REQKEY_KEYRING.
696
David Howellsf1a9bad2005-10-07 15:04:52 +0100697 See also Documentation/keys-request-key.txt.
698
Linus Torvalds1da177e2005-04-16 15:20:36 -0700699
700(*) When it is no longer required, the key should be released using:
701
702 void key_put(struct key *key);
703
David Howells664cceb2005-09-28 17:03:15 +0100704 Or:
705
706 void key_ref_put(key_ref_t key_ref);
707
708 These can be called from interrupt context. If CONFIG_KEYS is not set then
Linus Torvalds1da177e2005-04-16 15:20:36 -0700709 the argument will not be parsed.
710
711
712(*) Extra references can be made to a key by calling the following function:
713
714 struct key *key_get(struct key *key);
715
716 These need to be disposed of by calling key_put() when they've been
717 finished with. The key pointer passed in will be returned. If the pointer
718 is NULL or CONFIG_KEYS is not set then the key will not be dereferenced and
719 no increment will take place.
720
721
722(*) A key's serial number can be obtained by calling:
723
724 key_serial_t key_serial(struct key *key);
725
726 If key is NULL or if CONFIG_KEYS is not set then 0 will be returned (in the
727 latter case without parsing the argument).
728
729
730(*) If a keyring was found in the search, this can be further searched by:
731
David Howells664cceb2005-09-28 17:03:15 +0100732 key_ref_t keyring_search(key_ref_t keyring_ref,
733 const struct key_type *type,
734 const char *description)
Linus Torvalds1da177e2005-04-16 15:20:36 -0700735
736 This searches the keyring tree specified for a matching key. Error ENOKEY
David Howells664cceb2005-09-28 17:03:15 +0100737 is returned upon failure (use IS_ERR/PTR_ERR to determine). If successful,
738 the returned key will need to be released.
739
740 The possession attribute from the keyring reference is used to control
741 access through the permissions mask and is propagated to the returned key
742 reference pointer if successful.
Linus Torvalds1da177e2005-04-16 15:20:36 -0700743
744
745(*) To check the validity of a key, this function can be called:
746
747 int validate_key(struct key *key);
748
749 This checks that the key in question hasn't expired or and hasn't been
750 revoked. Should the key be invalid, error EKEYEXPIRED or EKEYREVOKED will
751 be returned. If the key is NULL or if CONFIG_KEYS is not set then 0 will be
752 returned (in the latter case without parsing the argument).
753
754
755(*) To register a key type, the following function should be called:
756
757 int register_key_type(struct key_type *type);
758
759 This will return error EEXIST if a type of the same name is already
760 present.
761
762
763(*) To unregister a key type, call:
764
765 void unregister_key_type(struct key_type *type);
766
767
David Howells76d8aea2005-06-23 22:00:49 -0700768===================================
769NOTES ON ACCESSING PAYLOAD CONTENTS
770===================================
771
772The simplest payload is just a number in key->payload.value. In this case,
773there's no need to indulge in RCU or locking when accessing the payload.
774
775More complex payload contents must be allocated and a pointer to them set in
776key->payload.data. One of the following ways must be selected to access the
777data:
778
David Howells664cceb2005-09-28 17:03:15 +0100779 (1) Unmodifiable key type.
David Howells76d8aea2005-06-23 22:00:49 -0700780
781 If the key type does not have a modify method, then the key's payload can
782 be accessed without any form of locking, provided that it's known to be
783 instantiated (uninstantiated keys cannot be "found").
784
785 (2) The key's semaphore.
786
787 The semaphore could be used to govern access to the payload and to control
788 the payload pointer. It must be write-locked for modifications and would
789 have to be read-locked for general access. The disadvantage of doing this
790 is that the accessor may be required to sleep.
791
792 (3) RCU.
793
794 RCU must be used when the semaphore isn't already held; if the semaphore
795 is held then the contents can't change under you unexpectedly as the
796 semaphore must still be used to serialise modifications to the key. The
797 key management code takes care of this for the key type.
798
799 However, this means using:
800
801 rcu_read_lock() ... rcu_dereference() ... rcu_read_unlock()
802
803 to read the pointer, and:
804
805 rcu_dereference() ... rcu_assign_pointer() ... call_rcu()
806
807 to set the pointer and dispose of the old contents after a grace period.
808 Note that only the key type should ever modify a key's payload.
809
810 Furthermore, an RCU controlled payload must hold a struct rcu_head for the
811 use of call_rcu() and, if the payload is of variable size, the length of
812 the payload. key->datalen cannot be relied upon to be consistent with the
813 payload just dereferenced if the key's semaphore is not held.
814
815
Linus Torvalds1da177e2005-04-16 15:20:36 -0700816===================
817DEFINING A KEY TYPE
818===================
819
820A kernel service may want to define its own key type. For instance, an AFS
821filesystem might want to define a Kerberos 5 ticket key type. To do this, it
822author fills in a struct key_type and registers it with the system.
823
824The structure has a number of fields, some of which are mandatory:
825
826 (*) const char *name
827
828 The name of the key type. This is used to translate a key type name
829 supplied by userspace into a pointer to the structure.
830
831
832 (*) size_t def_datalen
833
834 This is optional - it supplies the default payload data length as
835 contributed to the quota. If the key type's payload is always or almost
836 always the same size, then this is a more efficient way to do things.
837
838 The data length (and quota) on a particular key can always be changed
839 during instantiation or update by calling:
840
841 int key_payload_reserve(struct key *key, size_t datalen);
842
David Howells76d8aea2005-06-23 22:00:49 -0700843 With the revised data length. Error EDQUOT will be returned if this is not
844 viable.
Linus Torvalds1da177e2005-04-16 15:20:36 -0700845
846
847 (*) int (*instantiate)(struct key *key, const void *data, size_t datalen);
848
849 This method is called to attach a payload to a key during construction.
David Howells76d8aea2005-06-23 22:00:49 -0700850 The payload attached need not bear any relation to the data passed to this
851 function.
Linus Torvalds1da177e2005-04-16 15:20:36 -0700852
853 If the amount of data attached to the key differs from the size in
854 keytype->def_datalen, then key_payload_reserve() should be called.
855
856 This method does not have to lock the key in order to attach a payload.
857 The fact that KEY_FLAG_INSTANTIATED is not set in key->flags prevents
858 anything else from gaining access to the key.
859
David Howells76d8aea2005-06-23 22:00:49 -0700860 It is safe to sleep in this method.
Linus Torvalds1da177e2005-04-16 15:20:36 -0700861
862
863 (*) int (*duplicate)(struct key *key, const struct key *source);
864
865 If this type of key can be duplicated, then this method should be
David Howells76d8aea2005-06-23 22:00:49 -0700866 provided. It is called to copy the payload attached to the source into the
867 new key. The data length on the new key will have been updated and the
868 quota adjusted already.
Linus Torvalds1da177e2005-04-16 15:20:36 -0700869
870 This method will be called with the source key's semaphore read-locked to
David Howells76d8aea2005-06-23 22:00:49 -0700871 prevent its payload from being changed, thus RCU constraints need not be
872 applied to the source key.
873
874 This method does not have to lock the destination key in order to attach a
875 payload. The fact that KEY_FLAG_INSTANTIATED is not set in key->flags
876 prevents anything else from gaining access to the key.
877
878 It is safe to sleep in this method.
Linus Torvalds1da177e2005-04-16 15:20:36 -0700879
880
881 (*) int (*update)(struct key *key, const void *data, size_t datalen);
882
David Howells76d8aea2005-06-23 22:00:49 -0700883 If this type of key can be updated, then this method should be provided.
884 It is called to update a key's payload from the blob of data provided.
Linus Torvalds1da177e2005-04-16 15:20:36 -0700885
886 key_payload_reserve() should be called if the data length might change
David Howells76d8aea2005-06-23 22:00:49 -0700887 before any changes are actually made. Note that if this succeeds, the type
888 is committed to changing the key because it's already been altered, so all
889 memory allocation must be done first.
Linus Torvalds1da177e2005-04-16 15:20:36 -0700890
David Howells76d8aea2005-06-23 22:00:49 -0700891 The key will have its semaphore write-locked before this method is called,
892 but this only deters other writers; any changes to the key's payload must
893 be made under RCU conditions, and call_rcu() must be used to dispose of
894 the old payload.
Linus Torvalds1da177e2005-04-16 15:20:36 -0700895
David Howells76d8aea2005-06-23 22:00:49 -0700896 key_payload_reserve() should be called before the changes are made, but
897 after all allocations and other potentially failing function calls are
898 made.
899
900 It is safe to sleep in this method.
Linus Torvalds1da177e2005-04-16 15:20:36 -0700901
902
903 (*) int (*match)(const struct key *key, const void *desc);
904
905 This method is called to match a key against a description. It should
906 return non-zero if the two match, zero if they don't.
907
908 This method should not need to lock the key in any way. The type and
909 description can be considered invariant, and the payload should not be
910 accessed (the key may not yet be instantiated).
911
912 It is not safe to sleep in this method; the caller may hold spinlocks.
913
914
915 (*) void (*destroy)(struct key *key);
916
David Howells76d8aea2005-06-23 22:00:49 -0700917 This method is optional. It is called to discard the payload data on a key
918 when it is being destroyed.
Linus Torvalds1da177e2005-04-16 15:20:36 -0700919
David Howells76d8aea2005-06-23 22:00:49 -0700920 This method does not need to lock the key to access the payload; it can
921 consider the key as being inaccessible at this time. Note that the key's
922 type may have been changed before this function is called.
Linus Torvalds1da177e2005-04-16 15:20:36 -0700923
924 It is not safe to sleep in this method; the caller may hold spinlocks.
925
926
927 (*) void (*describe)(const struct key *key, struct seq_file *p);
928
929 This method is optional. It is called during /proc/keys reading to
930 summarise a key's description and payload in text form.
931
David Howells76d8aea2005-06-23 22:00:49 -0700932 This method will be called with the RCU read lock held. rcu_dereference()
933 should be used to read the payload pointer if the payload is to be
934 accessed. key->datalen cannot be trusted to stay consistent with the
935 contents of the payload.
936
937 The description will not change, though the key's state may.
938
939 It is not safe to sleep in this method; the RCU read lock is held by the
940 caller.
Linus Torvalds1da177e2005-04-16 15:20:36 -0700941
942
943 (*) long (*read)(const struct key *key, char __user *buffer, size_t buflen);
944
945 This method is optional. It is called by KEYCTL_READ to translate the
David Howells76d8aea2005-06-23 22:00:49 -0700946 key's payload into something a blob of data for userspace to deal with.
947 Ideally, the blob should be in the same format as that passed in to the
948 instantiate and update methods.
Linus Torvalds1da177e2005-04-16 15:20:36 -0700949
950 If successful, the blob size that could be produced should be returned
951 rather than the size copied.
952
David Howells76d8aea2005-06-23 22:00:49 -0700953 This method will be called with the key's semaphore read-locked. This will
954 prevent the key's payload changing. It is not necessary to use RCU locking
955 when accessing the key's payload. It is safe to sleep in this method, such
956 as might happen when the userspace buffer is accessed.
Linus Torvalds1da177e2005-04-16 15:20:36 -0700957
958
959============================
960REQUEST-KEY CALLBACK SERVICE
961============================
962
963To create a new key, the kernel will attempt to execute the following command
964line:
965
966 /sbin/request-key create <key> <uid> <gid> \
967 <threadring> <processring> <sessionring> <callout_info>
968
969<key> is the key being constructed, and the three keyrings are the process
970keyrings from the process that caused the search to be issued. These are
971included for two reasons:
972
973 (1) There may be an authentication token in one of the keyrings that is
974 required to obtain the key, eg: a Kerberos Ticket-Granting Ticket.
975
976 (2) The new key should probably be cached in one of these rings.
977
978This program should set it UID and GID to those specified before attempting to
979access any more keys. It may then look around for a user specific process to
980hand the request off to (perhaps a path held in placed in another key by, for
981example, the KDE desktop manager).
982
983The program (or whatever it calls) should finish construction of the key by
984calling KEYCTL_INSTANTIATE, which also permits it to cache the key in one of
985the keyrings (probably the session ring) before returning. Alternatively, the
986key can be marked as negative with KEYCTL_NEGATE; this also permits the key to
987be cached in one of the keyrings.
988
989If it returns with the key remaining in the unconstructed state, the key will
990be marked as being negative, it will be added to the session keyring, and an
991error will be returned to the key requestor.
992
David Howells76d8aea2005-06-23 22:00:49 -0700993Supplementary information may be provided from whoever or whatever invoked this
994service. This will be passed as the <callout_info> parameter. If no such
Linus Torvalds1da177e2005-04-16 15:20:36 -0700995information was made available, then "-" will be passed as this parameter
996instead.
997
998
999Similarly, the kernel may attempt to update an expired or a soon to expire key
1000by executing:
1001
1002 /sbin/request-key update <key> <uid> <gid> \
1003 <threadring> <processring> <sessionring>
1004
1005In this case, the program isn't required to actually attach the key to a ring;
1006the rings are provided for reference.