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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
Michael LeMayd7200242006-06-22 14:47:17 -070022 - SELinux support
Linus Torvalds1da177e2005-04-16 15:20:36 -070023 - New procfs files
24 - Userspace system call interface
25 - Kernel services
David Howells76d8aea2005-06-23 22:00:49 -070026 - Notes on accessing payload contents
Linus Torvalds1da177e2005-04-16 15:20:36 -070027 - Defining a key type
28 - Request-key callback service
29 - Key access filesystem
30
31
32============
33KEY OVERVIEW
34============
35
36In this context, keys represent units of cryptographic data, authentication
37tokens, keyrings, etc.. These are represented in the kernel by struct key.
38
39Each key has a number of attributes:
40
41 - A serial number.
42 - A type.
43 - A description (for matching a key in a search).
44 - Access control information.
45 - An expiry time.
46 - A payload.
47 - State.
48
49
David Howells76d8aea2005-06-23 22:00:49 -070050 (*) 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.
Linus Torvalds1da177e2005-04-16 15:20:36 -070053
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
David Howells76d8aea2005-06-23 22:00:49 -070058 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.
Linus Torvalds1da177e2005-04-16 15:20:36 -070060
David Howells76d8aea2005-06-23 22:00:49 -070061 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.
Linus Torvalds1da177e2005-04-16 15:20:36 -070063
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
David Howells76d8aea2005-06-23 22:00:49 -070068 type provides an operation to perform a match between the description on a
69 key and a criterion string.
Linus Torvalds1da177e2005-04-16 15:20:36 -070070
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
David Howells76d8aea2005-06-23 22:00:49 -070078 (*) 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.
Linus Torvalds1da177e2005-04-16 15:20:36 -070082
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
David Howells76d8aea2005-06-23 22:00:49 -070096 (*) Uninstantiated. The key exists, but does not have any data attached.
97 Keys being requested from userspace will be in this state.
Linus Torvalds1da177e2005-04-16 15:20:36 -070098
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
117====================
118KEY SERVICE OVERVIEW
119====================
120
121The key service provides a number of features besides keys:
122
123 (*) The key service defines two special key types:
124
125 (+) "keyring"
126
127 Keyrings are special keys that contain a list of other keys. Keyring
128 lists can be modified using various system calls. Keyrings should not
129 be given a payload when created.
130
131 (+) "user"
132
133 A key of this type has a description and a payload that are arbitrary
134 blobs of data. These can be created, updated and read by userspace,
135 and aren't intended for use by kernel services.
136
137 (*) Each process subscribes to three keyrings: a thread-specific keyring, a
138 process-specific keyring, and a session-specific keyring.
139
140 The thread-specific keyring is discarded from the child when any sort of
141 clone, fork, vfork or execve occurs. A new keyring is created only when
142 required.
143
David Howells76d8aea2005-06-23 22:00:49 -0700144 The process-specific keyring is replaced with an empty one in the child on
145 clone, fork, vfork unless CLONE_THREAD is supplied, in which case it is
146 shared. execve also discards the process's process keyring and creates a
147 new one.
Linus Torvalds1da177e2005-04-16 15:20:36 -0700148
149 The session-specific keyring is persistent across clone, fork, vfork and
150 execve, even when the latter executes a set-UID or set-GID binary. A
151 process can, however, replace its current session keyring with a new one
152 by using PR_JOIN_SESSION_KEYRING. It is permitted to request an anonymous
153 new one, or to attempt to create or join one of a specific name.
154
155 The ownership of the thread keyring changes when the real UID and GID of
156 the thread changes.
157
158 (*) Each user ID resident in the system holds two special keyrings: a user
159 specific keyring and a default user session keyring. The default session
160 keyring is initialised with a link to the user-specific keyring.
161
162 When a process changes its real UID, if it used to have no session key, it
163 will be subscribed to the default session key for the new UID.
164
165 If a process attempts to access its session key when it doesn't have one,
166 it will be subscribed to the default for its current UID.
167
168 (*) Each user has two quotas against which the keys they own are tracked. One
169 limits the total number of keys and keyrings, the other limits the total
170 amount of description and payload space that can be consumed.
171
172 The user can view information on this and other statistics through procfs
173 files.
174
175 Process-specific and thread-specific keyrings are not counted towards a
176 user's quota.
177
178 If a system call that modifies a key or keyring in some way would put the
179 user over quota, the operation is refused and error EDQUOT is returned.
180
David Howells76d8aea2005-06-23 22:00:49 -0700181 (*) There's a system call interface by which userspace programs can create and
182 manipulate keys and keyrings.
Linus Torvalds1da177e2005-04-16 15:20:36 -0700183
David Howells76d8aea2005-06-23 22:00:49 -0700184 (*) There's a kernel interface by which services can register types and search
185 for keys.
Linus Torvalds1da177e2005-04-16 15:20:36 -0700186
187 (*) There's a way for the a search done from the kernel to call back to
188 userspace to request a key that can't be found in a process's keyrings.
189
190 (*) An optional filesystem is available through which the key database can be
191 viewed and manipulated.
192
193
194======================
195KEY ACCESS PERMISSIONS
196======================
197
David Howells76d8aea2005-06-23 22:00:49 -0700198Keys have an owner user ID, a group access ID, and a permissions mask. The mask
David Howells664cceb2005-09-28 17:03:15 +0100199has up to eight bits each for possessor, user, group and other access. Only
David Howells29db9192005-10-30 15:02:44 -0800200six of each set of eight bits are defined. These permissions granted are:
Linus Torvalds1da177e2005-04-16 15:20:36 -0700201
202 (*) View
203
204 This permits a key or keyring's attributes to be viewed - including key
205 type and description.
206
207 (*) Read
208
209 This permits a key's payload to be viewed or a keyring's list of linked
210 keys.
211
212 (*) Write
213
David Howells76d8aea2005-06-23 22:00:49 -0700214 This permits a key's payload to be instantiated or updated, or it allows a
215 link to be added to or removed from a keyring.
Linus Torvalds1da177e2005-04-16 15:20:36 -0700216
217 (*) Search
218
219 This permits keyrings to be searched and keys to be found. Searches can
220 only recurse into nested keyrings that have search permission set.
221
222 (*) Link
223
224 This permits a key or keyring to be linked to. To create a link from a
225 keyring to a key, a process must have Write permission on the keyring and
226 Link permission on the key.
227
David Howells29db9192005-10-30 15:02:44 -0800228 (*) Set Attribute
229
230 This permits a key's UID, GID and permissions mask to be changed.
231
Linus Torvalds1da177e2005-04-16 15:20:36 -0700232For changing the ownership, group ID or permissions mask, being the owner of
233the key or having the sysadmin capability is sufficient.
234
235
Michael LeMayd7200242006-06-22 14:47:17 -0700236===============
237SELINUX SUPPORT
238===============
239
240The security class "key" has been added to SELinux so that mandatory access
241controls can be applied to keys created within various contexts. This support
242is preliminary, and is likely to change quite significantly in the near future.
243Currently, all of the basic permissions explained above are provided in SELinux
Michael LeMay4eb582c2006-06-26 00:24:57 -0700244as well; SELinux is simply invoked after all basic permission checks have been
Michael LeMayd7200242006-06-22 14:47:17 -0700245performed.
246
Michael LeMay4eb582c2006-06-26 00:24:57 -0700247The value of the file /proc/self/attr/keycreate influences the labeling of
248newly-created keys. If the contents of that file correspond to an SELinux
249security context, then the key will be assigned that context. Otherwise, the
250key will be assigned the current context of the task that invoked the key
251creation request. Tasks must be granted explicit permission to assign a
252particular context to newly-created keys, using the "create" permission in the
253key security class.
Michael LeMayd7200242006-06-22 14:47:17 -0700254
Michael LeMay4eb582c2006-06-26 00:24:57 -0700255The default keyrings associated with users will be labeled with the default
256context of the user if and only if the login programs have been instrumented to
257properly initialize keycreate during the login process. Otherwise, they will
258be labeled with the context of the login program itself.
Michael LeMayd7200242006-06-22 14:47:17 -0700259
260Note, however, that the default keyrings associated with the root user are
261labeled with the default kernel context, since they are created early in the
262boot process, before root has a chance to log in.
263
Michael LeMay4eb582c2006-06-26 00:24:57 -0700264The keyrings associated with new threads are each labeled with the context of
265their associated thread, and both session and process keyrings are handled
266similarly.
267
Michael LeMayd7200242006-06-22 14:47:17 -0700268
Linus Torvalds1da177e2005-04-16 15:20:36 -0700269================
270NEW PROCFS FILES
271================
272
273Two files have been added to procfs by which an administrator can find out
274about the status of the key service:
275
276 (*) /proc/keys
277
Michael LeMay06ec7be2006-06-26 00:24:56 -0700278 This lists the keys that are currently viewable by the task reading the
279 file, giving information about their type, description and permissions.
280 It is not possible to view the payload of the key this way, though some
281 information about it may be given.
282
283 The only keys included in the list are those that grant View permission to
284 the reading process whether or not it possesses them. Note that LSM
285 security checks are still performed, and may further filter out keys that
286 the current process is not authorised to view.
287
288 The contents of the file look like this:
Linus Torvalds1da177e2005-04-16 15:20:36 -0700289
David Howells664cceb2005-09-28 17:03:15 +0100290 SERIAL FLAGS USAGE EXPY PERM UID GID TYPE DESCRIPTION: SUMMARY
David Howells29db9192005-10-30 15:02:44 -0800291 00000001 I----- 39 perm 1f3f0000 0 0 keyring _uid_ses.0: 1/4
292 00000002 I----- 2 perm 1f3f0000 0 0 keyring _uid.0: empty
293 00000007 I----- 1 perm 1f3f0000 0 0 keyring _pid.1: empty
294 0000018d I----- 1 perm 1f3f0000 0 0 keyring _pid.412: empty
295 000004d2 I--Q-- 1 perm 1f3f0000 32 -1 keyring _uid.32: 1/4
296 000004d3 I--Q-- 3 perm 1f3f0000 32 -1 keyring _uid_ses.32: empty
David Howells664cceb2005-09-28 17:03:15 +0100297 00000892 I--QU- 1 perm 1f000000 0 0 user metal:copper: 0
David Howells29db9192005-10-30 15:02:44 -0800298 00000893 I--Q-N 1 35s 1f3f0000 0 0 user metal:silver: 0
299 00000894 I--Q-- 1 10h 003f0000 0 0 user metal:gold: 0
Linus Torvalds1da177e2005-04-16 15:20:36 -0700300
301 The flags are:
302
303 I Instantiated
304 R Revoked
305 D Dead
306 Q Contributes to user's quota
307 U Under contruction by callback to userspace
308 N Negative key
309
310 This file must be enabled at kernel configuration time as it allows anyone
311 to list the keys database.
312
313 (*) /proc/key-users
314
315 This file lists the tracking data for each user that has at least one key
Michael LeMay06ec7be2006-06-26 00:24:56 -0700316 on the system. Such data includes quota information and statistics:
Linus Torvalds1da177e2005-04-16 15:20:36 -0700317
318 [root@andromeda root]# cat /proc/key-users
319 0: 46 45/45 1/100 13/10000
320 29: 2 2/2 2/100 40/10000
321 32: 2 2/2 2/100 40/10000
322 38: 2 2/2 2/100 40/10000
323
324 The format of each line is
325 <UID>: User ID to which this applies
326 <usage> Structure refcount
327 <inst>/<keys> Total number of keys and number instantiated
328 <keys>/<max> Key count quota
329 <bytes>/<max> Key size quota
330
331
332===============================
333USERSPACE SYSTEM CALL INTERFACE
334===============================
335
336Userspace can manipulate keys directly through three new syscalls: add_key,
337request_key and keyctl. The latter provides a number of functions for
338manipulating keys.
339
340When referring to a key directly, userspace programs should use the key's
341serial number (a positive 32-bit integer). However, there are some special
342values available for referring to special keys and keyrings that relate to the
343process making the call:
344
345 CONSTANT VALUE KEY REFERENCED
346 ============================== ====== ===========================
347 KEY_SPEC_THREAD_KEYRING -1 thread-specific keyring
348 KEY_SPEC_PROCESS_KEYRING -2 process-specific keyring
349 KEY_SPEC_SESSION_KEYRING -3 session-specific keyring
350 KEY_SPEC_USER_KEYRING -4 UID-specific keyring
351 KEY_SPEC_USER_SESSION_KEYRING -5 UID-session keyring
352 KEY_SPEC_GROUP_KEYRING -6 GID-specific keyring
David Howellsb5f545c2006-01-08 01:02:47 -0800353 KEY_SPEC_REQKEY_AUTH_KEY -7 assumed request_key()
354 authorisation key
Linus Torvalds1da177e2005-04-16 15:20:36 -0700355
356
357The main syscalls are:
358
359 (*) Create a new key of given type, description and payload and add it to the
360 nominated keyring:
361
362 key_serial_t add_key(const char *type, const char *desc,
363 const void *payload, size_t plen,
364 key_serial_t keyring);
365
366 If a key of the same type and description as that proposed already exists
367 in the keyring, this will try to update it with the given payload, or it
368 will return error EEXIST if that function is not supported by the key
David Howells76d8aea2005-06-23 22:00:49 -0700369 type. The process must also have permission to write to the key to be able
370 to update it. The new key will have all user permissions granted and no
371 group or third party permissions.
Linus Torvalds1da177e2005-04-16 15:20:36 -0700372
David Howells76d8aea2005-06-23 22:00:49 -0700373 Otherwise, this will attempt to create a new key of the specified type and
374 description, and to instantiate it with the supplied payload and attach it
375 to the keyring. In this case, an error will be generated if the process
376 does not have permission to write to the keyring.
Linus Torvalds1da177e2005-04-16 15:20:36 -0700377
378 The payload is optional, and the pointer can be NULL if not required by
379 the type. The payload is plen in size, and plen can be zero for an empty
380 payload.
381
David Howells76d8aea2005-06-23 22:00:49 -0700382 A new keyring can be generated by setting type "keyring", the keyring name
383 as the description (or NULL) and setting the payload to NULL.
Linus Torvalds1da177e2005-04-16 15:20:36 -0700384
385 User defined keys can be created by specifying type "user". It is
386 recommended that a user defined key's description by prefixed with a type
387 ID and a colon, such as "krb5tgt:" for a Kerberos 5 ticket granting
388 ticket.
389
390 Any other type must have been registered with the kernel in advance by a
391 kernel service such as a filesystem.
392
393 The ID of the new or updated key is returned if successful.
394
395
396 (*) Search the process's keyrings for a key, potentially calling out to
397 userspace to create it.
398
399 key_serial_t request_key(const char *type, const char *description,
400 const char *callout_info,
401 key_serial_t dest_keyring);
402
403 This function searches all the process's keyrings in the order thread,
404 process, session for a matching key. This works very much like
405 KEYCTL_SEARCH, including the optional attachment of the discovered key to
406 a keyring.
407
408 If a key cannot be found, and if callout_info is not NULL, then
409 /sbin/request-key will be invoked in an attempt to obtain a key. The
410 callout_info string will be passed as an argument to the program.
411
David Howellsf1a9bad2005-10-07 15:04:52 +0100412 See also Documentation/keys-request-key.txt.
413
Linus Torvalds1da177e2005-04-16 15:20:36 -0700414
415The keyctl syscall functions are:
416
417 (*) Map a special key ID to a real key ID for this process:
418
419 key_serial_t keyctl(KEYCTL_GET_KEYRING_ID, key_serial_t id,
420 int create);
421
David Howells76d8aea2005-06-23 22:00:49 -0700422 The special key specified by "id" is looked up (with the key being created
423 if necessary) and the ID of the key or keyring thus found is returned if
424 it exists.
Linus Torvalds1da177e2005-04-16 15:20:36 -0700425
426 If the key does not yet exist, the key will be created if "create" is
427 non-zero; and the error ENOKEY will be returned if "create" is zero.
428
429
430 (*) Replace the session keyring this process subscribes to with a new one:
431
432 key_serial_t keyctl(KEYCTL_JOIN_SESSION_KEYRING, const char *name);
433
434 If name is NULL, an anonymous keyring is created attached to the process
435 as its session keyring, displacing the old session keyring.
436
437 If name is not NULL, if a keyring of that name exists, the process
438 attempts to attach it as the session keyring, returning an error if that
439 is not permitted; otherwise a new keyring of that name is created and
440 attached as the session keyring.
441
442 To attach to a named keyring, the keyring must have search permission for
443 the process's ownership.
444
445 The ID of the new session keyring is returned if successful.
446
447
448 (*) Update the specified key:
449
450 long keyctl(KEYCTL_UPDATE, key_serial_t key, const void *payload,
451 size_t plen);
452
453 This will try to update the specified key with the given payload, or it
454 will return error EOPNOTSUPP if that function is not supported by the key
David Howells76d8aea2005-06-23 22:00:49 -0700455 type. The process must also have permission to write to the key to be able
456 to update it.
Linus Torvalds1da177e2005-04-16 15:20:36 -0700457
458 The payload is of length plen, and may be absent or empty as for
459 add_key().
460
461
462 (*) Revoke a key:
463
464 long keyctl(KEYCTL_REVOKE, key_serial_t key);
465
466 This makes a key unavailable for further operations. Further attempts to
467 use the key will be met with error EKEYREVOKED, and the key will no longer
468 be findable.
469
470
471 (*) Change the ownership of a key:
472
473 long keyctl(KEYCTL_CHOWN, key_serial_t key, uid_t uid, gid_t gid);
474
David Howells76d8aea2005-06-23 22:00:49 -0700475 This function permits a key's owner and group ID to be changed. Either one
476 of uid or gid can be set to -1 to suppress that change.
Linus Torvalds1da177e2005-04-16 15:20:36 -0700477
478 Only the superuser can change a key's owner to something other than the
479 key's current owner. Similarly, only the superuser can change a key's
480 group ID to something other than the calling process's group ID or one of
481 its group list members.
482
483
484 (*) Change the permissions mask on a key:
485
486 long keyctl(KEYCTL_SETPERM, key_serial_t key, key_perm_t perm);
487
488 This function permits the owner of a key or the superuser to change the
489 permissions mask on a key.
490
491 Only bits the available bits are permitted; if any other bits are set,
492 error EINVAL will be returned.
493
494
495 (*) Describe a key:
496
497 long keyctl(KEYCTL_DESCRIBE, key_serial_t key, char *buffer,
498 size_t buflen);
499
500 This function returns a summary of the key's attributes (but not its
501 payload data) as a string in the buffer provided.
502
503 Unless there's an error, it always returns the amount of data it could
504 produce, even if that's too big for the buffer, but it won't copy more
505 than requested to userspace. If the buffer pointer is NULL then no copy
506 will take place.
507
508 A process must have view permission on the key for this function to be
509 successful.
510
511 If successful, a string is placed in the buffer in the following format:
512
513 <type>;<uid>;<gid>;<perm>;<description>
514
515 Where type and description are strings, uid and gid are decimal, and perm
516 is hexadecimal. A NUL character is included at the end of the string if
517 the buffer is sufficiently big.
518
519 This can be parsed with
520
521 sscanf(buffer, "%[^;];%d;%d;%o;%s", type, &uid, &gid, &mode, desc);
522
523
524 (*) Clear out a keyring:
525
526 long keyctl(KEYCTL_CLEAR, key_serial_t keyring);
527
528 This function clears the list of keys attached to a keyring. The calling
529 process must have write permission on the keyring, and it must be a
530 keyring (or else error ENOTDIR will result).
531
532
533 (*) Link a key into a keyring:
534
535 long keyctl(KEYCTL_LINK, key_serial_t keyring, key_serial_t key);
536
David Howells76d8aea2005-06-23 22:00:49 -0700537 This function creates a link from the keyring to the key. The process must
538 have write permission on the keyring and must have link permission on the
539 key.
Linus Torvalds1da177e2005-04-16 15:20:36 -0700540
David Howells76d8aea2005-06-23 22:00:49 -0700541 Should the keyring not be a keyring, error ENOTDIR will result; and if the
542 keyring is full, error ENFILE will result.
Linus Torvalds1da177e2005-04-16 15:20:36 -0700543
544 The link procedure checks the nesting of the keyrings, returning ELOOP if
David Howells017679c2006-01-08 01:02:43 -0800545 it appears too deep or EDEADLK if the link would introduce a cycle.
Linus Torvalds1da177e2005-04-16 15:20:36 -0700546
David Howellscab8eb52006-01-08 01:02:45 -0800547 Any links within the keyring to keys that match the new key in terms of
548 type and description will be discarded from the keyring as the new one is
549 added.
550
Linus Torvalds1da177e2005-04-16 15:20:36 -0700551
552 (*) Unlink a key or keyring from another keyring:
553
554 long keyctl(KEYCTL_UNLINK, key_serial_t keyring, key_serial_t key);
555
556 This function looks through the keyring for the first link to the
557 specified key, and removes it if found. Subsequent links to that key are
558 ignored. The process must have write permission on the keyring.
559
David Howells76d8aea2005-06-23 22:00:49 -0700560 If the keyring is not a keyring, error ENOTDIR will result; and if the key
561 is not present, error ENOENT will be the result.
Linus Torvalds1da177e2005-04-16 15:20:36 -0700562
563
564 (*) Search a keyring tree for a key:
565
566 key_serial_t keyctl(KEYCTL_SEARCH, key_serial_t keyring,
567 const char *type, const char *description,
568 key_serial_t dest_keyring);
569
David Howells76d8aea2005-06-23 22:00:49 -0700570 This searches the keyring tree headed by the specified keyring until a key
571 is found that matches the type and description criteria. Each keyring is
572 checked for keys before recursion into its children occurs.
Linus Torvalds1da177e2005-04-16 15:20:36 -0700573
574 The process must have search permission on the top level keyring, or else
575 error EACCES will result. Only keyrings that the process has search
576 permission on will be recursed into, and only keys and keyrings for which
577 a process has search permission can be matched. If the specified keyring
578 is not a keyring, ENOTDIR will result.
579
580 If the search succeeds, the function will attempt to link the found key
581 into the destination keyring if one is supplied (non-zero ID). All the
582 constraints applicable to KEYCTL_LINK apply in this case too.
583
584 Error ENOKEY, EKEYREVOKED or EKEYEXPIRED will be returned if the search
585 fails. On success, the resulting key ID will be returned.
586
587
588 (*) Read the payload data from a key:
589
David Howellsf1a9bad2005-10-07 15:04:52 +0100590 long keyctl(KEYCTL_READ, key_serial_t keyring, char *buffer,
591 size_t buflen);
Linus Torvalds1da177e2005-04-16 15:20:36 -0700592
593 This function attempts to read the payload data from the specified key
594 into the buffer. The process must have read permission on the key to
595 succeed.
596
597 The returned data will be processed for presentation by the key type. For
598 instance, a keyring will return an array of key_serial_t entries
599 representing the IDs of all the keys to which it is subscribed. The user
600 defined key type will return its data as is. If a key type does not
601 implement this function, error EOPNOTSUPP will result.
602
603 As much of the data as can be fitted into the buffer will be copied to
604 userspace if the buffer pointer is not NULL.
605
David Howells76d8aea2005-06-23 22:00:49 -0700606 On a successful return, the function will always return the amount of data
607 available rather than the amount copied.
Linus Torvalds1da177e2005-04-16 15:20:36 -0700608
609
610 (*) Instantiate a partially constructed key.
611
David Howellsf1a9bad2005-10-07 15:04:52 +0100612 long keyctl(KEYCTL_INSTANTIATE, key_serial_t key,
613 const void *payload, size_t plen,
614 key_serial_t keyring);
Linus Torvalds1da177e2005-04-16 15:20:36 -0700615
616 If the kernel calls back to userspace to complete the instantiation of a
617 key, userspace should use this call to supply data for the key before the
618 invoked process returns, or else the key will be marked negative
619 automatically.
620
621 The process must have write access on the key to be able to instantiate
622 it, and the key must be uninstantiated.
623
624 If a keyring is specified (non-zero), the key will also be linked into
David Howells76d8aea2005-06-23 22:00:49 -0700625 that keyring, however all the constraints applying in KEYCTL_LINK apply in
626 this case too.
Linus Torvalds1da177e2005-04-16 15:20:36 -0700627
628 The payload and plen arguments describe the payload data as for add_key().
629
630
631 (*) Negatively instantiate a partially constructed key.
632
David Howellsf1a9bad2005-10-07 15:04:52 +0100633 long keyctl(KEYCTL_NEGATE, key_serial_t key,
634 unsigned timeout, key_serial_t keyring);
Linus Torvalds1da177e2005-04-16 15:20:36 -0700635
636 If the kernel calls back to userspace to complete the instantiation of a
637 key, userspace should use this call mark the key as negative before the
638 invoked process returns if it is unable to fulfil the request.
639
640 The process must have write access on the key to be able to instantiate
641 it, and the key must be uninstantiated.
642
643 If a keyring is specified (non-zero), the key will also be linked into
David Howells76d8aea2005-06-23 22:00:49 -0700644 that keyring, however all the constraints applying in KEYCTL_LINK apply in
645 this case too.
Linus Torvalds1da177e2005-04-16 15:20:36 -0700646
647
David Howells3e301482005-06-23 22:00:56 -0700648 (*) Set the default request-key destination keyring.
649
650 long keyctl(KEYCTL_SET_REQKEY_KEYRING, int reqkey_defl);
651
652 This sets the default keyring to which implicitly requested keys will be
653 attached for this thread. reqkey_defl should be one of these constants:
654
655 CONSTANT VALUE NEW DEFAULT KEYRING
656 ====================================== ====== =======================
657 KEY_REQKEY_DEFL_NO_CHANGE -1 No change
658 KEY_REQKEY_DEFL_DEFAULT 0 Default[1]
659 KEY_REQKEY_DEFL_THREAD_KEYRING 1 Thread keyring
660 KEY_REQKEY_DEFL_PROCESS_KEYRING 2 Process keyring
661 KEY_REQKEY_DEFL_SESSION_KEYRING 3 Session keyring
662 KEY_REQKEY_DEFL_USER_KEYRING 4 User keyring
663 KEY_REQKEY_DEFL_USER_SESSION_KEYRING 5 User session keyring
664 KEY_REQKEY_DEFL_GROUP_KEYRING 6 Group keyring
665
666 The old default will be returned if successful and error EINVAL will be
667 returned if reqkey_defl is not one of the above values.
668
669 The default keyring can be overridden by the keyring indicated to the
670 request_key() system call.
671
672 Note that this setting is inherited across fork/exec.
673
674 [1] The default default is: the thread keyring if there is one, otherwise
675 the process keyring if there is one, otherwise the session keyring if
676 there is one, otherwise the user default session keyring.
677
678
David Howells017679c2006-01-08 01:02:43 -0800679 (*) Set the timeout on a key.
680
681 long keyctl(KEYCTL_SET_TIMEOUT, key_serial_t key, unsigned timeout);
682
683 This sets or clears the timeout on a key. The timeout can be 0 to clear
684 the timeout or a number of seconds to set the expiry time that far into
685 the future.
686
687 The process must have attribute modification access on a key to set its
688 timeout. Timeouts may not be set with this function on negative, revoked
689 or expired keys.
690
691
David Howellsb5f545c2006-01-08 01:02:47 -0800692 (*) Assume the authority granted to instantiate a key
693
694 long keyctl(KEYCTL_ASSUME_AUTHORITY, key_serial_t key);
695
696 This assumes or divests the authority required to instantiate the
697 specified key. Authority can only be assumed if the thread has the
698 authorisation key associated with the specified key in its keyrings
699 somewhere.
700
701 Once authority is assumed, searches for keys will also search the
702 requester's keyrings using the requester's security label, UID, GID and
703 groups.
704
705 If the requested authority is unavailable, error EPERM will be returned,
706 likewise if the authority has been revoked because the target key is
707 already instantiated.
708
709 If the specified key is 0, then any assumed authority will be divested.
710
Matt LaPlante3f6dee92006-10-03 22:45:33 +0200711 The assumed authoritative key is inherited across fork and exec.
David Howellsb5f545c2006-01-08 01:02:47 -0800712
713
Linus Torvalds1da177e2005-04-16 15:20:36 -0700714===============
715KERNEL SERVICES
716===============
717
Matt LaPlante2fe0ae72006-10-03 22:50:39 +0200718The kernel services for key management are fairly simple to deal with. They can
Linus Torvalds1da177e2005-04-16 15:20:36 -0700719be broken down into two areas: keys and key types.
720
721Dealing with keys is fairly straightforward. Firstly, the kernel service
722registers its type, then it searches for a key of that type. It should retain
723the key as long as it has need of it, and then it should release it. For a
David Howells76d8aea2005-06-23 22:00:49 -0700724filesystem or device file, a search would probably be performed during the open
725call, and the key released upon close. How to deal with conflicting keys due to
726two different users opening the same file is left to the filesystem author to
727solve.
Linus Torvalds1da177e2005-04-16 15:20:36 -0700728
David Howells664cceb2005-09-28 17:03:15 +0100729Note that there are two different types of pointers to keys that may be
730encountered:
731
732 (*) struct key *
733
734 This simply points to the key structure itself. Key structures will be at
735 least four-byte aligned.
736
737 (*) key_ref_t
738
739 This is equivalent to a struct key *, but the least significant bit is set
740 if the caller "possesses" the key. By "possession" it is meant that the
741 calling processes has a searchable link to the key from one of its
742 keyrings. There are three functions for dealing with these:
743
744 key_ref_t make_key_ref(const struct key *key,
745 unsigned long possession);
746
747 struct key *key_ref_to_ptr(const key_ref_t key_ref);
748
749 unsigned long is_key_possessed(const key_ref_t key_ref);
750
751 The first function constructs a key reference from a key pointer and
752 possession information (which must be 0 or 1 and not any other value).
753
754 The second function retrieves the key pointer from a reference and the
755 third retrieves the possession flag.
756
David Howells76d8aea2005-06-23 22:00:49 -0700757When accessing a key's payload contents, certain precautions must be taken to
758prevent access vs modification races. See the section "Notes on accessing
759payload contents" for more information.
Linus Torvalds1da177e2005-04-16 15:20:36 -0700760
761(*) To search for a key, call:
762
763 struct key *request_key(const struct key_type *type,
764 const char *description,
765 const char *callout_string);
766
767 This is used to request a key or keyring with a description that matches
768 the description specified according to the key type's match function. This
769 permits approximate matching to occur. If callout_string is not NULL, then
770 /sbin/request-key will be invoked in an attempt to obtain the key from
771 userspace. In that case, callout_string will be passed as an argument to
772 the program.
773
774 Should the function fail error ENOKEY, EKEYEXPIRED or EKEYREVOKED will be
775 returned.
776
David Howells3e301482005-06-23 22:00:56 -0700777 If successful, the key will have been attached to the default keyring for
778 implicitly obtained request-key keys, as set by KEYCTL_SET_REQKEY_KEYRING.
779
David Howellsf1a9bad2005-10-07 15:04:52 +0100780 See also Documentation/keys-request-key.txt.
781
Linus Torvalds1da177e2005-04-16 15:20:36 -0700782
David Howells4e54f082006-06-29 02:24:28 -0700783(*) To search for a key, passing auxiliary data to the upcaller, call:
784
785 struct key *request_key_with_auxdata(const struct key_type *type,
786 const char *description,
787 const char *callout_string,
788 void *aux);
789
790 This is identical to request_key(), except that the auxiliary data is
791 passed to the key_type->request_key() op if it exists.
792
793
Linus Torvalds1da177e2005-04-16 15:20:36 -0700794(*) When it is no longer required, the key should be released using:
795
796 void key_put(struct key *key);
797
David Howells664cceb2005-09-28 17:03:15 +0100798 Or:
799
800 void key_ref_put(key_ref_t key_ref);
801
802 These can be called from interrupt context. If CONFIG_KEYS is not set then
Linus Torvalds1da177e2005-04-16 15:20:36 -0700803 the argument will not be parsed.
804
805
806(*) Extra references can be made to a key by calling the following function:
807
808 struct key *key_get(struct key *key);
809
810 These need to be disposed of by calling key_put() when they've been
811 finished with. The key pointer passed in will be returned. If the pointer
812 is NULL or CONFIG_KEYS is not set then the key will not be dereferenced and
813 no increment will take place.
814
815
816(*) A key's serial number can be obtained by calling:
817
818 key_serial_t key_serial(struct key *key);
819
820 If key is NULL or if CONFIG_KEYS is not set then 0 will be returned (in the
821 latter case without parsing the argument).
822
823
824(*) If a keyring was found in the search, this can be further searched by:
825
David Howells664cceb2005-09-28 17:03:15 +0100826 key_ref_t keyring_search(key_ref_t keyring_ref,
827 const struct key_type *type,
828 const char *description)
Linus Torvalds1da177e2005-04-16 15:20:36 -0700829
830 This searches the keyring tree specified for a matching key. Error ENOKEY
David Howells664cceb2005-09-28 17:03:15 +0100831 is returned upon failure (use IS_ERR/PTR_ERR to determine). If successful,
832 the returned key will need to be released.
833
834 The possession attribute from the keyring reference is used to control
835 access through the permissions mask and is propagated to the returned key
836 reference pointer if successful.
Linus Torvalds1da177e2005-04-16 15:20:36 -0700837
838
839(*) To check the validity of a key, this function can be called:
840
841 int validate_key(struct key *key);
842
843 This checks that the key in question hasn't expired or and hasn't been
844 revoked. Should the key be invalid, error EKEYEXPIRED or EKEYREVOKED will
845 be returned. If the key is NULL or if CONFIG_KEYS is not set then 0 will be
846 returned (in the latter case without parsing the argument).
847
848
849(*) To register a key type, the following function should be called:
850
851 int register_key_type(struct key_type *type);
852
853 This will return error EEXIST if a type of the same name is already
854 present.
855
856
857(*) To unregister a key type, call:
858
859 void unregister_key_type(struct key_type *type);
860
861
David Howells76d8aea2005-06-23 22:00:49 -0700862===================================
863NOTES ON ACCESSING PAYLOAD CONTENTS
864===================================
865
866The simplest payload is just a number in key->payload.value. In this case,
867there's no need to indulge in RCU or locking when accessing the payload.
868
869More complex payload contents must be allocated and a pointer to them set in
870key->payload.data. One of the following ways must be selected to access the
871data:
872
David Howells664cceb2005-09-28 17:03:15 +0100873 (1) Unmodifiable key type.
David Howells76d8aea2005-06-23 22:00:49 -0700874
875 If the key type does not have a modify method, then the key's payload can
876 be accessed without any form of locking, provided that it's known to be
877 instantiated (uninstantiated keys cannot be "found").
878
879 (2) The key's semaphore.
880
881 The semaphore could be used to govern access to the payload and to control
882 the payload pointer. It must be write-locked for modifications and would
883 have to be read-locked for general access. The disadvantage of doing this
884 is that the accessor may be required to sleep.
885
886 (3) RCU.
887
888 RCU must be used when the semaphore isn't already held; if the semaphore
889 is held then the contents can't change under you unexpectedly as the
890 semaphore must still be used to serialise modifications to the key. The
891 key management code takes care of this for the key type.
892
893 However, this means using:
894
895 rcu_read_lock() ... rcu_dereference() ... rcu_read_unlock()
896
897 to read the pointer, and:
898
899 rcu_dereference() ... rcu_assign_pointer() ... call_rcu()
900
901 to set the pointer and dispose of the old contents after a grace period.
902 Note that only the key type should ever modify a key's payload.
903
904 Furthermore, an RCU controlled payload must hold a struct rcu_head for the
905 use of call_rcu() and, if the payload is of variable size, the length of
906 the payload. key->datalen cannot be relied upon to be consistent with the
907 payload just dereferenced if the key's semaphore is not held.
908
909
Linus Torvalds1da177e2005-04-16 15:20:36 -0700910===================
911DEFINING A KEY TYPE
912===================
913
914A kernel service may want to define its own key type. For instance, an AFS
915filesystem might want to define a Kerberos 5 ticket key type. To do this, it
916author fills in a struct key_type and registers it with the system.
917
918The structure has a number of fields, some of which are mandatory:
919
920 (*) const char *name
921
922 The name of the key type. This is used to translate a key type name
923 supplied by userspace into a pointer to the structure.
924
925
926 (*) size_t def_datalen
927
928 This is optional - it supplies the default payload data length as
929 contributed to the quota. If the key type's payload is always or almost
930 always the same size, then this is a more efficient way to do things.
931
932 The data length (and quota) on a particular key can always be changed
933 during instantiation or update by calling:
934
935 int key_payload_reserve(struct key *key, size_t datalen);
936
David Howells76d8aea2005-06-23 22:00:49 -0700937 With the revised data length. Error EDQUOT will be returned if this is not
938 viable.
Linus Torvalds1da177e2005-04-16 15:20:36 -0700939
940
941 (*) int (*instantiate)(struct key *key, const void *data, size_t datalen);
942
943 This method is called to attach a payload to a key during construction.
David Howells76d8aea2005-06-23 22:00:49 -0700944 The payload attached need not bear any relation to the data passed to this
945 function.
Linus Torvalds1da177e2005-04-16 15:20:36 -0700946
947 If the amount of data attached to the key differs from the size in
948 keytype->def_datalen, then key_payload_reserve() should be called.
949
950 This method does not have to lock the key in order to attach a payload.
951 The fact that KEY_FLAG_INSTANTIATED is not set in key->flags prevents
952 anything else from gaining access to the key.
953
David Howells76d8aea2005-06-23 22:00:49 -0700954 It is safe to sleep in this method.
Linus Torvalds1da177e2005-04-16 15:20:36 -0700955
956
Linus Torvalds1da177e2005-04-16 15:20:36 -0700957 (*) int (*update)(struct key *key, const void *data, size_t datalen);
958
David Howells76d8aea2005-06-23 22:00:49 -0700959 If this type of key can be updated, then this method should be provided.
960 It is called to update a key's payload from the blob of data provided.
Linus Torvalds1da177e2005-04-16 15:20:36 -0700961
962 key_payload_reserve() should be called if the data length might change
David Howells76d8aea2005-06-23 22:00:49 -0700963 before any changes are actually made. Note that if this succeeds, the type
964 is committed to changing the key because it's already been altered, so all
965 memory allocation must be done first.
Linus Torvalds1da177e2005-04-16 15:20:36 -0700966
David Howells76d8aea2005-06-23 22:00:49 -0700967 The key will have its semaphore write-locked before this method is called,
968 but this only deters other writers; any changes to the key's payload must
969 be made under RCU conditions, and call_rcu() must be used to dispose of
970 the old payload.
Linus Torvalds1da177e2005-04-16 15:20:36 -0700971
David Howells76d8aea2005-06-23 22:00:49 -0700972 key_payload_reserve() should be called before the changes are made, but
973 after all allocations and other potentially failing function calls are
974 made.
975
976 It is safe to sleep in this method.
Linus Torvalds1da177e2005-04-16 15:20:36 -0700977
978
979 (*) int (*match)(const struct key *key, const void *desc);
980
981 This method is called to match a key against a description. It should
982 return non-zero if the two match, zero if they don't.
983
984 This method should not need to lock the key in any way. The type and
985 description can be considered invariant, and the payload should not be
986 accessed (the key may not yet be instantiated).
987
988 It is not safe to sleep in this method; the caller may hold spinlocks.
989
990
David Howells04c567d2006-06-22 14:47:18 -0700991 (*) void (*revoke)(struct key *key);
992
993 This method is optional. It is called to discard part of the payload
994 data upon a key being revoked. The caller will have the key semaphore
995 write-locked.
996
997 It is safe to sleep in this method, though care should be taken to avoid
998 a deadlock against the key semaphore.
999
1000
Linus Torvalds1da177e2005-04-16 15:20:36 -07001001 (*) void (*destroy)(struct key *key);
1002
David Howells76d8aea2005-06-23 22:00:49 -07001003 This method is optional. It is called to discard the payload data on a key
1004 when it is being destroyed.
Linus Torvalds1da177e2005-04-16 15:20:36 -07001005
David Howells76d8aea2005-06-23 22:00:49 -07001006 This method does not need to lock the key to access the payload; it can
1007 consider the key as being inaccessible at this time. Note that the key's
1008 type may have been changed before this function is called.
Linus Torvalds1da177e2005-04-16 15:20:36 -07001009
1010 It is not safe to sleep in this method; the caller may hold spinlocks.
1011
1012
1013 (*) void (*describe)(const struct key *key, struct seq_file *p);
1014
1015 This method is optional. It is called during /proc/keys reading to
1016 summarise a key's description and payload in text form.
1017
David Howells76d8aea2005-06-23 22:00:49 -07001018 This method will be called with the RCU read lock held. rcu_dereference()
1019 should be used to read the payload pointer if the payload is to be
1020 accessed. key->datalen cannot be trusted to stay consistent with the
1021 contents of the payload.
1022
1023 The description will not change, though the key's state may.
1024
1025 It is not safe to sleep in this method; the RCU read lock is held by the
1026 caller.
Linus Torvalds1da177e2005-04-16 15:20:36 -07001027
1028
1029 (*) long (*read)(const struct key *key, char __user *buffer, size_t buflen);
1030
1031 This method is optional. It is called by KEYCTL_READ to translate the
David Howells76d8aea2005-06-23 22:00:49 -07001032 key's payload into something a blob of data for userspace to deal with.
1033 Ideally, the blob should be in the same format as that passed in to the
1034 instantiate and update methods.
Linus Torvalds1da177e2005-04-16 15:20:36 -07001035
1036 If successful, the blob size that could be produced should be returned
1037 rather than the size copied.
1038
David Howells76d8aea2005-06-23 22:00:49 -07001039 This method will be called with the key's semaphore read-locked. This will
1040 prevent the key's payload changing. It is not necessary to use RCU locking
1041 when accessing the key's payload. It is safe to sleep in this method, such
1042 as might happen when the userspace buffer is accessed.
Linus Torvalds1da177e2005-04-16 15:20:36 -07001043
1044
David Howells4e54f082006-06-29 02:24:28 -07001045 (*) int (*request_key)(struct key *key, struct key *authkey, const char *op,
1046 void *aux);
1047
1048 This method is optional. If provided, request_key() and
1049 request_key_with_auxdata() will invoke this function rather than
1050 upcalling to /sbin/request-key to operate upon a key of this type.
1051
1052 The aux parameter is as passed to request_key_with_auxdata() or is NULL
1053 otherwise. Also passed are the key to be operated upon, the
1054 authorisation key for this operation and the operation type (currently
1055 only "create").
1056
1057 This function should return only when the upcall is complete. Upon return
1058 the authorisation key will be revoked, and the target key will be
1059 negatively instantiated if it is still uninstantiated. The error will be
1060 returned to the caller of request_key*().
1061
1062
Linus Torvalds1da177e2005-04-16 15:20:36 -07001063============================
1064REQUEST-KEY CALLBACK SERVICE
1065============================
1066
1067To create a new key, the kernel will attempt to execute the following command
1068line:
1069
1070 /sbin/request-key create <key> <uid> <gid> \
1071 <threadring> <processring> <sessionring> <callout_info>
1072
1073<key> is the key being constructed, and the three keyrings are the process
1074keyrings from the process that caused the search to be issued. These are
1075included for two reasons:
1076
1077 (1) There may be an authentication token in one of the keyrings that is
1078 required to obtain the key, eg: a Kerberos Ticket-Granting Ticket.
1079
1080 (2) The new key should probably be cached in one of these rings.
1081
1082This program should set it UID and GID to those specified before attempting to
1083access any more keys. It may then look around for a user specific process to
1084hand the request off to (perhaps a path held in placed in another key by, for
1085example, the KDE desktop manager).
1086
1087The program (or whatever it calls) should finish construction of the key by
1088calling KEYCTL_INSTANTIATE, which also permits it to cache the key in one of
1089the keyrings (probably the session ring) before returning. Alternatively, the
1090key can be marked as negative with KEYCTL_NEGATE; this also permits the key to
1091be cached in one of the keyrings.
1092
1093If it returns with the key remaining in the unconstructed state, the key will
1094be marked as being negative, it will be added to the session keyring, and an
1095error will be returned to the key requestor.
1096
David Howells76d8aea2005-06-23 22:00:49 -07001097Supplementary information may be provided from whoever or whatever invoked this
1098service. This will be passed as the <callout_info> parameter. If no such
Linus Torvalds1da177e2005-04-16 15:20:36 -07001099information was made available, then "-" will be passed as this parameter
1100instead.
1101
1102
1103Similarly, the kernel may attempt to update an expired or a soon to expire key
1104by executing:
1105
1106 /sbin/request-key update <key> <uid> <gid> \
1107 <threadring> <processring> <sessionring>
1108
1109In this case, the program isn't required to actually attach the key to a ring;
1110the rings are provided for reference.