1 ============================
2 Kernel Key Retention Service
3 ============================
4
5 This service allows cryptographic keys, authentication tokens, cross-domain
6 user mappings, and similar to be cached in the kernel for the use of
7 filesystems and other kernel services.
8
9 Keyrings are permitted; these are a special type of key that can hold links to
10 other keys. Processes each have three standard keyring subscriptions that a
11 kernel service can search for relevant keys.
12
13 The key service can be configured on by enabling:
14
15 "Security options"/"Enable access key retention support" (CONFIG_KEYS)
16
17 This document has the following sections:
18
19 .. contents:: :local:
20
21
22 Key Overview
23 ============
24
25 In this context, keys represent units of cryptographic data, authentication
26 tokens, keyrings, etc.. These are represented in the kernel by struct key.
27
28 Each key has a number of attributes:
29
30 - A serial number.
31 - A type.
32 - A description (for matching a key in a search).
33 - Access control information.
34 - An expiry time.
35 - A payload.
36 - State.
37
38
39 * Each key is issued a serial number of type key_serial_t that is unique for
40 the lifetime of that key. All serial numbers are positive non-zero 32-bit
41 integers.
42
43 Userspace programs can use a key's serial numbers as a way to gain access
44 to it, subject to permission checking.
45
46 * Each key is of a defined "type". Types must be registered inside the
47 kernel by a kernel service (such as a filesystem) before keys of that type
48 can be added or used. Userspace programs cannot define new types directly.
49
50 Key types are represented in the kernel by struct key_type. This defines a
51 number of operations that can be performed on a key of that type.
52
53 Should a type be removed from the system, all the keys of that type will
54 be invalidated.
55
56 * Each key has a description. This should be a printable string. The key
57 type provides an operation to perform a match between the description on a
58 key and a criterion string.
59
60 * Each key has an owner user ID, a group ID and a permissions mask. These
61 are used to control what a process may do to a key from userspace, and
62 whether a kernel service will be able to find the key.
63
64 * Each key can be set to expire at a specific time by the key type's
65 instantiation function. Keys can also be immortal.
66
67 * Each key can have a payload. This is a quantity of data that represent the
68 actual "key". In the case of a keyring, this is a list of keys to which
69 the keyring links; in the case of a user-defined key, it's an arbitrary
70 blob of data.
71
72 Having a payload is not required; and the payload can, in fact, just be a
73 value stored in the struct key itself.
74
75 When a key is instantiated, the key type's instantiation function is
76 called with a blob of data, and that then creates the key's payload in
77 some way.
78
79 Similarly, when userspace wants to read back the contents of the key, if
80 permitted, another key type operation will be called to convert the key's
81 attached payload back into a blob of data.
82
83 * Each key can be in one of a number of basic states:
84
85 * Uninstantiated. The key exists, but does not have any data attached.
86 Keys being requested from userspace will be in this state.
87
88 * Instantiated. This is the normal state. The key is fully formed, and
89 has data attached.
90
91 * Negative. This is a relatively short-lived state. The key acts as a
92 note saying that a previous call out to userspace failed, and acts as
93 a throttle on key lookups. A negative key can be updated to a normal
94 state.
95
96 * Expired. Keys can have lifetimes set. If their lifetime is exceeded,
97 they traverse to this state. An expired key can be updated back to a
98 normal state.
99
100 * Revoked. A key is put in this state by userspace action. It can't be
101 found or operated upon (apart from by unlinking it).
102
103 * Dead. The key's type was unregistered, and so the key is now useless.
104
105 Keys in the last three states are subject to garbage collection. See the
106 section on "Garbage collection".
107
108
109 Key Service Overview
110 ====================
111
112 The key service provides a number of features besides keys:
113
114 * The key service defines three special key types:
115
116 (+) "keyring"
117
118 Keyrings are special keys that contain a list of other keys. Keyring
119 lists can be modified using various system calls. Keyrings should not
120 be given a payload when created.
121
122 (+) "user"
123
124 A key of this type has a description and a payload that are arbitrary
125 blobs of data. These can be created, updated and read by userspace,
126 and aren't intended for use by kernel services.
127
128 (+) "logon"
129
130 Like a "user" key, a "logon" key has a payload that is an arbitrary
131 blob of data. It is intended as a place to store secrets which are
132 accessible to the kernel but not to userspace programs.
133
134 The description can be arbitrary, but must be prefixed with a non-zero
135 length string that describes the key "subclass". The subclass is
136 separated from the rest of the description by a ':'. "logon" keys can
137 be created and updated from userspace, but the payload is only
138 readable from kernel space.
139
140 * Each process subscribes to three keyrings: a thread-specific keyring, a
141 process-specific keyring, and a session-specific keyring.
142
143 The thread-specific keyring is discarded from the child when any sort of
144 clone, fork, vfork or execve occurs. A new keyring is created only when
145 required.
146
147 The process-specific keyring is replaced with an empty one in the child on
148 clone, fork, vfork unless CLONE_THREAD is supplied, in which case it is
149 shared. execve also discards the process's process keyring and creates a
150 new one.
151
152 The session-specific keyring is persistent across clone, fork, vfork and
153 execve, even when the latter executes a set-UID or set-GID binary. A
154 process can, however, replace its current session keyring with a new one
155 by using PR_JOIN_SESSION_KEYRING. It is permitted to request an anonymous
156 new one, or to attempt to create or join one of a specific name.
157
158 The ownership of the thread keyring changes when the real UID and GID of
159 the thread changes.
160
161 * Each user ID resident in the system holds two special keyrings: a user
162 specific keyring and a default user session keyring. The default session
163 keyring is initialised with a link to the user-specific keyring.
164
165 When a process changes its real UID, if it used to have no session key, it
166 will be subscribed to the default session key for the new UID.
167
168 If a process attempts to access its session key when it doesn't have one,
169 it will be subscribed to the default for its current UID.
170
171 * Each user has two quotas against which the keys they own are tracked. One
172 limits the total number of keys and keyrings, the other limits the total
173 amount of description and payload space that can be consumed.
174
175 The user can view information on this and other statistics through procfs
176 files. The root user may also alter the quota limits through sysctl files
177 (see the section "New procfs files").
178
179 Process-specific and thread-specific keyrings are not counted towards a
180 user's quota.
181
182 If a system call that modifies a key or keyring in some way would put the
183 user over quota, the operation is refused and error EDQUOT is returned.
184
185 * There's a system call interface by which userspace programs can create and
186 manipulate keys and keyrings.
187
188 * There's a kernel interface by which services can register types and search
189 for keys.
190
191 * There's a way for the a search done from the kernel to call back to
192 userspace to request a key that can't be found in a process's keyrings.
193
194 * An optional filesystem is available through which the key database can be
195 viewed and manipulated.
196
197
198 Key Access Permissions
199 ======================
200
201 Keys have an owner user ID, a group access ID, and a permissions mask. The mask
202 has up to eight bits each for possessor, user, group and other access. Only
203 six of each set of eight bits are defined. These permissions granted are:
204
205 * View
206
207 This permits a key or keyring's attributes to be viewed - including key
208 type and description.
209
210 * Read
211
212 This permits a key's payload to be viewed or a keyring's list of linked
213 keys.
214
215 * Write
216
217 This permits a key's payload to be instantiated or updated, or it allows a
218 link to be added to or removed from a keyring.
219
220 * Search
221
222 This permits keyrings to be searched and keys to be found. Searches can
223 only recurse into nested keyrings that have search permission set.
224
225 * Link
226
227 This permits a key or keyring to be linked to. To create a link from a
228 keyring to a key, a process must have Write permission on the keyring and
229 Link permission on the key.
230
231 * Set Attribute
232
233 This permits a key's UID, GID and permissions mask to be changed.
234
235 For changing the ownership, group ID or permissions mask, being the owner of
236 the key or having the sysadmin capability is sufficient.
237
238
239 SELinux Support
240 ===============
241
242 The security class "key" has been added to SELinux so that mandatory access
243 controls can be applied to keys created within various contexts. This support
244 is preliminary, and is likely to change quite significantly in the near future.
245 Currently, all of the basic permissions explained above are provided in SELinux
246 as well; SELinux is simply invoked after all basic permission checks have been
247 performed.
248
249 The value of the file /proc/self/attr/keycreate influences the labeling of
250 newly-created keys. If the contents of that file correspond to an SELinux
251 security context, then the key will be assigned that context. Otherwise, the
252 key will be assigned the current context of the task that invoked the key
253 creation request. Tasks must be granted explicit permission to assign a
254 particular context to newly-created keys, using the "create" permission in the
255 key security class.
256
257 The default keyrings associated with users will be labeled with the default
258 context of the user if and only if the login programs have been instrumented to
259 properly initialize keycreate during the login process. Otherwise, they will
260 be labeled with the context of the login program itself.
261
262 Note, however, that the default keyrings associated with the root user are
263 labeled with the default kernel context, since they are created early in the
264 boot process, before root has a chance to log in.
265
266 The keyrings associated with new threads are each labeled with the context of
267 their associated thread, and both session and process keyrings are handled
268 similarly.
269
270
271 New ProcFS Files
272 ================
273
274 Two files have been added to procfs by which an administrator can find out
275 about the status of the key service:
276
277 * /proc/keys
278
279 This lists the keys that are currently viewable by the task reading the
280 file, giving information about their type, description and permissions.
281 It is not possible to view the payload of the key this way, though some
282 information about it may be given.
283
284 The only keys included in the list are those that grant View permission to
285 the reading process whether or not it possesses them. Note that LSM
286 security checks are still performed, and may further filter out keys that
287 the current process is not authorised to view.
288
289 The contents of the file look like this::
290
291 SERIAL FLAGS USAGE EXPY PERM UID GID TYPE DESCRIPTION: SUMMARY
292 00000001 I----- 39 perm 1f3f0000 0 0 keyring _uid_ses.0: 1/4
293 00000002 I----- 2 perm 1f3f0000 0 0 keyring _uid.0: empty
294 00000007 I----- 1 perm 1f3f0000 0 0 keyring _pid.1: empty
295 0000018d I----- 1 perm 1f3f0000 0 0 keyring _pid.412: empty
296 000004d2 I--Q-- 1 perm 1f3f0000 32 -1 keyring _uid.32: 1/4
297 000004d3 I--Q-- 3 perm 1f3f0000 32 -1 keyring _uid_ses.32: empty
298 00000892 I--QU- 1 perm 1f000000 0 0 user metal:copper: 0
299 00000893 I--Q-N 1 35s 1f3f0000 0 0 user metal:silver: 0
300 00000894 I--Q-- 1 10h 003f0000 0 0 user metal:gold: 0
301
302 The flags are::
303
304 I Instantiated
305 R Revoked
306 D Dead
307 Q Contributes to user's quota
308 U Under construction by callback to userspace
309 N Negative key
310
311
312 * /proc/key-users
313
314 This file lists the tracking data for each user that has at least one key
315 on the system. Such data includes quota information and statistics::
316
317 [root@andromeda root]# cat /proc/key-users
318 0: 46 45/45 1/100 13/10000
319 29: 2 2/2 2/100 40/10000
320 32: 2 2/2 2/100 40/10000
321 38: 2 2/2 2/100 40/10000
322
323 The format of each line is::
324
325 <UID>: User ID to which this applies
326 <usage> Structure refcount
327 <inst>/<keys> Total number of keys and number instantiated
328 <keys>/<max> Key count quota
329 <bytes>/<max> Key size quota
330
331
332 Four new sysctl files have been added also for the purpose of controlling the
333 quota limits on keys:
334
335 * /proc/sys/kernel/keys/root_maxkeys
336 /proc/sys/kernel/keys/root_maxbytes
337
338 These files hold the maximum number of keys that root may have and the
339 maximum total number of bytes of data that root may have stored in those
340 keys.
341
342 * /proc/sys/kernel/keys/maxkeys
343 /proc/sys/kernel/keys/maxbytes
344
345 These files hold the maximum number of keys that each non-root user may
346 have and the maximum total number of bytes of data that each of those
347 users may have stored in their keys.
348
349 Root may alter these by writing each new limit as a decimal number string to
350 the appropriate file.
351
352
353 Userspace System Call Interface
354 ===============================
355
356 Userspace can manipulate keys directly through three new syscalls: add_key,
357 request_key and keyctl. The latter provides a number of functions for
358 manipulating keys.
359
360 When referring to a key directly, userspace programs should use the key's
361 serial number (a positive 32-bit integer). However, there are some special
362 values available for referring to special keys and keyrings that relate to the
363 process making the call::
364
365 CONSTANT VALUE KEY REFERENCED
366 ============================== ====== ===========================
367 KEY_SPEC_THREAD_KEYRING -1 thread-specific keyring
368 KEY_SPEC_PROCESS_KEYRING -2 process-specific keyring
369 KEY_SPEC_SESSION_KEYRING -3 session-specific keyring
370 KEY_SPEC_USER_KEYRING -4 UID-specific keyring
371 KEY_SPEC_USER_SESSION_KEYRING -5 UID-session keyring
372 KEY_SPEC_GROUP_KEYRING -6 GID-specific keyring
373 KEY_SPEC_REQKEY_AUTH_KEY -7 assumed request_key()
374 authorisation key
375
376
377 The main syscalls are:
378
379 * Create a new key of given type, description and payload and add it to the
380 nominated keyring::
381
382 key_serial_t add_key(const char *type, const char *desc,
383 const void *payload, size_t plen,
384 key_serial_t keyring);
385
386 If a key of the same type and description as that proposed already exists
387 in the keyring, this will try to update it with the given payload, or it
388 will return error EEXIST if that function is not supported by the key
389 type. The process must also have permission to write to the key to be able
390 to update it. The new key will have all user permissions granted and no
391 group or third party permissions.
392
393 Otherwise, this will attempt to create a new key of the specified type and
394 description, and to instantiate it with the supplied payload and attach it
395 to the keyring. In this case, an error will be generated if the process
396 does not have permission to write to the keyring.
397
398 If the key type supports it, if the description is NULL or an empty
399 string, the key type will try and generate a description from the content
400 of the payload.
401
402 The payload is optional, and the pointer can be NULL if not required by
403 the type. The payload is plen in size, and plen can be zero for an empty
404 payload.
405
406 A new keyring can be generated by setting type "keyring", the keyring name
407 as the description (or NULL) and setting the payload to NULL.
408
409 User defined keys can be created by specifying type "user". It is
410 recommended that a user defined key's description by prefixed with a type
411 ID and a colon, such as "krb5tgt:" for a Kerberos 5 ticket granting
412 ticket.
413
414 Any other type must have been registered with the kernel in advance by a
415 kernel service such as a filesystem.
416
417 The ID of the new or updated key is returned if successful.
418
419
420 * Search the process's keyrings for a key, potentially calling out to
421 userspace to create it::
422
423 key_serial_t request_key(const char *type, const char *description,
424 const char *callout_info,
425 key_serial_t dest_keyring);
426
427 This function searches all the process's keyrings in the order thread,
428 process, session for a matching key. This works very much like
429 KEYCTL_SEARCH, including the optional attachment of the discovered key to
430 a keyring.
431
432 If a key cannot be found, and if callout_info is not NULL, then
433 /sbin/request-key will be invoked in an attempt to obtain a key. The
434 callout_info string will be passed as an argument to the program.
435
436 To link a key into the destination keyring the key must grant link
437 permission on the key to the caller and the keyring must grant write
438 permission.
439
440 See also Documentation/security/keys/request-key.rst.
441
442
443 The keyctl syscall functions are:
444
445 * Map a special key ID to a real key ID for this process::
446
447 key_serial_t keyctl(KEYCTL_GET_KEYRING_ID, key_serial_t id,
448 int create);
449
450 The special key specified by "id" is looked up (with the key being created
451 if necessary) and the ID of the key or keyring thus found is returned if
452 it exists.
453
454 If the key does not yet exist, the key will be created if "create" is
455 non-zero; and the error ENOKEY will be returned if "create" is zero.
456
457
458 * Replace the session keyring this process subscribes to with a new one::
459
460 key_serial_t keyctl(KEYCTL_JOIN_SESSION_KEYRING, const char *name);
461
462 If name is NULL, an anonymous keyring is created attached to the process
463 as its session keyring, displacing the old session keyring.
464
465 If name is not NULL, if a keyring of that name exists, the process
466 attempts to attach it as the session keyring, returning an error if that
467 is not permitted; otherwise a new keyring of that name is created and
468 attached as the session keyring.
469
470 To attach to a named keyring, the keyring must have search permission for
471 the process's ownership.
472
473 The ID of the new session keyring is returned if successful.
474
475
476 * Update the specified key::
477
478 long keyctl(KEYCTL_UPDATE, key_serial_t key, const void *payload,
479 size_t plen);
480
481 This will try to update the specified key with the given payload, or it
482 will return error EOPNOTSUPP if that function is not supported by the key
483 type. The process must also have permission to write to the key to be able
484 to update it.
485
486 The payload is of length plen, and may be absent or empty as for
487 add_key().
488
489
490 * Revoke a key::
491
492 long keyctl(KEYCTL_REVOKE, key_serial_t key);
493
494 This makes a key unavailable for further operations. Further attempts to
495 use the key will be met with error EKEYREVOKED, and the key will no longer
496 be findable.
497
498
499 * Change the ownership of a key::
500
501 long keyctl(KEYCTL_CHOWN, key_serial_t key, uid_t uid, gid_t gid);
502
503 This function permits a key's owner and group ID to be changed. Either one
504 of uid or gid can be set to -1 to suppress that change.
505
506 Only the superuser can change a key's owner to something other than the
507 key's current owner. Similarly, only the superuser can change a key's
508 group ID to something other than the calling process's group ID or one of
509 its group list members.
510
511
512 * Change the permissions mask on a key::
513
514 long keyctl(KEYCTL_SETPERM, key_serial_t key, key_perm_t perm);
515
516 This function permits the owner of a key or the superuser to change the
517 permissions mask on a key.
518
519 Only bits the available bits are permitted; if any other bits are set,
520 error EINVAL will be returned.
521
522
523 * Describe a key::
524
525 long keyctl(KEYCTL_DESCRIBE, key_serial_t key, char *buffer,
526 size_t buflen);
527
528 This function returns a summary of the key's attributes (but not its
529 payload data) as a string in the buffer provided.
530
531 Unless there's an error, it always returns the amount of data it could
532 produce, even if that's too big for the buffer, but it won't copy more
533 than requested to userspace. If the buffer pointer is NULL then no copy
534 will take place.
535
536 A process must have view permission on the key for this function to be
537 successful.
538
539 If successful, a string is placed in the buffer in the following format::
540
541 <type>;<uid>;<gid>;<perm>;<description>
542
543 Where type and description are strings, uid and gid are decimal, and perm
544 is hexadecimal. A NUL character is included at the end of the string if
545 the buffer is sufficiently big.
546
547 This can be parsed with::
548
549 sscanf(buffer, "%[^;];%d;%d;%o;%s", type, &uid, &gid, &mode, desc);
550
551
552 * Clear out a keyring::
553
554 long keyctl(KEYCTL_CLEAR, key_serial_t keyring);
555
556 This function clears the list of keys attached to a keyring. The calling
557 process must have write permission on the keyring, and it must be a
558 keyring (or else error ENOTDIR will result).
559
560 This function can also be used to clear special kernel keyrings if they
561 are appropriately marked if the user has CAP_SYS_ADMIN capability. The
562 DNS resolver cache keyring is an example of this.
563
564
565 * Link a key into a keyring::
566
567 long keyctl(KEYCTL_LINK, key_serial_t keyring, key_serial_t key);
568
569 This function creates a link from the keyring to the key. The process must
570 have write permission on the keyring and must have link permission on the
571 key.
572
573 Should the keyring not be a keyring, error ENOTDIR will result; and if the
574 keyring is full, error ENFILE will result.
575
576 The link procedure checks the nesting of the keyrings, returning ELOOP if
577 it appears too deep or EDEADLK if the link would introduce a cycle.
578
579 Any links within the keyring to keys that match the new key in terms of
580 type and description will be discarded from the keyring as the new one is
581 added.
582
583
584 * Move a key from one keyring to another::
585
586 long keyctl(KEYCTL_MOVE,
587 key_serial_t id,
588 key_serial_t from_ring_id,
589 key_serial_t to_ring_id,
590 unsigned int flags);
591
592 Move the key specified by "id" from the keyring specified by
593 "from_ring_id" to the keyring specified by "to_ring_id". If the two
594 keyrings are the same, nothing is done.
595
596 "flags" can have KEYCTL_MOVE_EXCL set in it to cause the operation to fail
597 with EEXIST if a matching key exists in the destination keyring, otherwise
598 such a key will be replaced.
599
600 A process must have link permission on the key for this function to be
601 successful and write permission on both keyrings. Any errors that can
602 occur from KEYCTL_LINK also apply on the destination keyring here.
603
604
605 * Unlink a key or keyring from another keyring::
606
607 long keyctl(KEYCTL_UNLINK, key_serial_t keyring, key_serial_t key);
608
609 This function looks through the keyring for the first link to the
610 specified key, and removes it if found. Subsequent links to that key are
611 ignored. The process must have write permission on the keyring.
612
613 If the keyring is not a keyring, error ENOTDIR will result; and if the key
614 is not present, error ENOENT will be the result.
615
616
617 * Search a keyring tree for a key::
618
619 key_serial_t keyctl(KEYCTL_SEARCH, key_serial_t keyring,
620 const char *type, const char *description,
621 key_serial_t dest_keyring);
622
623 This searches the keyring tree headed by the specified keyring until a key
624 is found that matches the type and description criteria. Each keyring is
625 checked for keys before recursion into its children occurs.
626
627 The process must have search permission on the top level keyring, or else
628 error EACCES will result. Only keyrings that the process has search
629 permission on will be recursed into, and only keys and keyrings for which
630 a process has search permission can be matched. If the specified keyring
631 is not a keyring, ENOTDIR will result.
632
633 If the search succeeds, the function will attempt to link the found key
634 into the destination keyring if one is supplied (non-zero ID). All the
635 constraints applicable to KEYCTL_LINK apply in this case too.
636
637 Error ENOKEY, EKEYREVOKED or EKEYEXPIRED will be returned if the search
638 fails. On success, the resulting key ID will be returned.
639
640
641 * Read the payload data from a key::
642
643 long keyctl(KEYCTL_READ, key_serial_t keyring, char *buffer,
644 size_t buflen);
645
646 This function attempts to read the payload data from the specified key
647 into the buffer. The process must have read permission on the key to
648 succeed.
649
650 The returned data will be processed for presentation by the key type. For
651 instance, a keyring will return an array of key_serial_t entries
652 representing the IDs of all the keys to which it is subscribed. The user
653 defined key type will return its data as is. If a key type does not
654 implement this function, error EOPNOTSUPP will result.
655
656 If the specified buffer is too small, then the size of the buffer required
657 will be returned. Note that in this case, the contents of the buffer may
658 have been overwritten in some undefined way.
659
660 Otherwise, on success, the function will return the amount of data copied
661 into the buffer.
662
663 * Instantiate a partially constructed key::
664
665 long keyctl(KEYCTL_INSTANTIATE, key_serial_t key,
666 const void *payload, size_t plen,
667 key_serial_t keyring);
668 long keyctl(KEYCTL_INSTANTIATE_IOV, key_serial_t key,
669 const struct iovec *payload_iov, unsigned ioc,
670 key_serial_t keyring);
671
672 If the kernel calls back to userspace to complete the instantiation of a
673 key, userspace should use this call to supply data for the key before the
674 invoked process returns, or else the key will be marked negative
675 automatically.
676
677 The process must have write access on the key to be able to instantiate
678 it, and the key must be uninstantiated.
679
680 If a keyring is specified (non-zero), the key will also be linked into
681 that keyring, however all the constraints applying in KEYCTL_LINK apply in
682 this case too.
683
684 The payload and plen arguments describe the payload data as for add_key().
685
686 The payload_iov and ioc arguments describe the payload data in an iovec
687 array instead of a single buffer.
688
689
690 * Negatively instantiate a partially constructed key::
691
692 long keyctl(KEYCTL_NEGATE, key_serial_t key,
693 unsigned timeout, key_serial_t keyring);
694 long keyctl(KEYCTL_REJECT, key_serial_t key,
695 unsigned timeout, unsigned error, key_serial_t keyring);
696
697 If the kernel calls back to userspace to complete the instantiation of a
698 key, userspace should use this call mark the key as negative before the
699 invoked process returns if it is unable to fulfill the request.
700
701 The process must have write access on the key to be able to instantiate
702 it, and the key must be uninstantiated.
703
704 If a keyring is specified (non-zero), the key will also be linked into
705 that keyring, however all the constraints applying in KEYCTL_LINK apply in
706 this case too.
707
708 If the key is rejected, future searches for it will return the specified
709 error code until the rejected key expires. Negating the key is the same
710 as rejecting the key with ENOKEY as the error code.
711
712
713 * Set the default request-key destination keyring::
714
715 long keyctl(KEYCTL_SET_REQKEY_KEYRING, int reqkey_defl);
716
717 This sets the default keyring to which implicitly requested keys will be
718 attached for this thread. reqkey_defl should be one of these constants::
719
720 CONSTANT VALUE NEW DEFAULT KEYRING
721 ====================================== ====== =======================
722 KEY_REQKEY_DEFL_NO_CHANGE -1 No change
723 KEY_REQKEY_DEFL_DEFAULT 0 Default[1]
724 KEY_REQKEY_DEFL_THREAD_KEYRING 1 Thread keyring
725 KEY_REQKEY_DEFL_PROCESS_KEYRING 2 Process keyring
726 KEY_REQKEY_DEFL_SESSION_KEYRING 3 Session keyring
727 KEY_REQKEY_DEFL_USER_KEYRING 4 User keyring
728 KEY_REQKEY_DEFL_USER_SESSION_KEYRING 5 User session keyring
729 KEY_REQKEY_DEFL_GROUP_KEYRING 6 Group keyring
730
731 The old default will be returned if successful and error EINVAL will be
732 returned if reqkey_defl is not one of the above values.
733
734 The default keyring can be overridden by the keyring indicated to the
735 request_key() system call.
736
737 Note that this setting is inherited across fork/exec.
738
739 [1] The default is: the thread keyring if there is one, otherwise
740 the process keyring if there is one, otherwise the session keyring if
741 there is one, otherwise the user default session keyring.
742
743
744 * Set the timeout on a key::
745
746 long keyctl(KEYCTL_SET_TIMEOUT, key_serial_t key, unsigned timeout);
747
748 This sets or clears the timeout on a key. The timeout can be 0 to clear
749 the timeout or a number of seconds to set the expiry time that far into
750 the future.
751
752 The process must have attribute modification access on a key to set its
753 timeout. Timeouts may not be set with this function on negative, revoked
754 or expired keys.
755
756
757 * Assume the authority granted to instantiate a key::
758
759 long keyctl(KEYCTL_ASSUME_AUTHORITY, key_serial_t key);
760
761 This assumes or divests the authority required to instantiate the
762 specified key. Authority can only be assumed if the thread has the
763 authorisation key associated with the specified key in its keyrings
764 somewhere.
765
766 Once authority is assumed, searches for keys will also search the
767 requester's keyrings using the requester's security label, UID, GID and
768 groups.
769
770 If the requested authority is unavailable, error EPERM will be returned,
771 likewise if the authority has been revoked because the target key is
772 already instantiated.
773
774 If the specified key is 0, then any assumed authority will be divested.
775
776 The assumed authoritative key is inherited across fork and exec.
777
778
779 * Get the LSM security context attached to a key::
780
781 long keyctl(KEYCTL_GET_SECURITY, key_serial_t key, char *buffer,
782 size_t buflen)
783
784 This function returns a string that represents the LSM security context
785 attached to a key in the buffer provided.
786
787 Unless there's an error, it always returns the amount of data it could
788 produce, even if that's too big for the buffer, but it won't copy more
789 than requested to userspace. If the buffer pointer is NULL then no copy
790 will take place.
791
792 A NUL character is included at the end of the string if the buffer is
793 sufficiently big. This is included in the returned count. If no LSM is
794 in force then an empty string will be returned.
795
796 A process must have view permission on the key for this function to be
797 successful.
798
799
800 * Install the calling process's session keyring on its parent::
801
802 long keyctl(KEYCTL_SESSION_TO_PARENT);
803
804 This functions attempts to install the calling process's session keyring
805 on to the calling process's parent, replacing the parent's current session
806 keyring.
807
808 The calling process must have the same ownership as its parent, the
809 keyring must have the same ownership as the calling process, the calling
810 process must have LINK permission on the keyring and the active LSM module
811 mustn't deny permission, otherwise error EPERM will be returned.
812
813 Error ENOMEM will be returned if there was insufficient memory to complete
814 the operation, otherwise 0 will be returned to indicate success.
815
816 The keyring will be replaced next time the parent process leaves the
817 kernel and resumes executing userspace.
818
819
820 * Invalidate a key::
821
822 long keyctl(KEYCTL_INVALIDATE, key_serial_t key);
823
824 This function marks a key as being invalidated and then wakes up the
825 garbage collector. The garbage collector immediately removes invalidated
826 keys from all keyrings and deletes the key when its reference count
827 reaches zero.
828
829 Keys that are marked invalidated become invisible to normal key operations
830 immediately, though they are still visible in /proc/keys until deleted
831 (they're marked with an 'i' flag).
832
833 A process must have search permission on the key for this function to be
834 successful.
835
836 * Compute a Diffie-Hellman shared secret or public key::
837
838 long keyctl(KEYCTL_DH_COMPUTE, struct keyctl_dh_params *params,
839 char *buffer, size_t buflen, struct keyctl_kdf_params *kdf);
840
841 The params struct contains serial numbers for three keys::
842
843 - The prime, p, known to both parties
844 - The local private key
845 - The base integer, which is either a shared generator or the
846 remote public key
847
848 The value computed is::
849
850 result = base ^ private (mod prime)
851
852 If the base is the shared generator, the result is the local
853 public key. If the base is the remote public key, the result is
854 the shared secret.
855
856 If the parameter kdf is NULL, the following applies:
857
858 - The buffer length must be at least the length of the prime, or zero.
859
860 - If the buffer length is nonzero, the length of the result is
861 returned when it is successfully calculated and copied in to the
862 buffer. When the buffer length is zero, the minimum required
863 buffer length is returned.
864
865 The kdf parameter allows the caller to apply a key derivation function
866 (KDF) on the Diffie-Hellman computation where only the result
867 of the KDF is returned to the caller. The KDF is characterized with
868 struct keyctl_kdf_params as follows:
869
870 - ``char *hashname`` specifies the NUL terminated string identifying
871 the hash used from the kernel crypto API and applied for the KDF
872 operation. The KDF implementation complies with SP800-56A as well
873 as with SP800-108 (the counter KDF).
874
875 - ``char *otherinfo`` specifies the OtherInfo data as documented in
876 SP800-56A section 5.8.1.2. The length of the buffer is given with
877 otherinfolen. The format of OtherInfo is defined by the caller.
878 The otherinfo pointer may be NULL if no OtherInfo shall be used.
879
880 This function will return error EOPNOTSUPP if the key type is not
881 supported, error ENOKEY if the key could not be found, or error
882 EACCES if the key is not readable by the caller. In addition, the
883 function will return EMSGSIZE when the parameter kdf is non-NULL
884 and either the buffer length or the OtherInfo length exceeds the
885 allowed length.
886
887
888 * Restrict keyring linkage::
889
890 long keyctl(KEYCTL_RESTRICT_KEYRING, key_serial_t keyring,
891 const char *type, const char *restriction);
892
893 An existing keyring can restrict linkage of additional keys by evaluating
894 the contents of the key according to a restriction scheme.
895
896 "keyring" is the key ID for an existing keyring to apply a restriction
897 to. It may be empty or may already have keys linked. Existing linked keys
898 will remain in the keyring even if the new restriction would reject them.
899
900 "type" is a registered key type.
901
902 "restriction" is a string describing how key linkage is to be restricted.
903 The format varies depending on the key type, and the string is passed to
904 the lookup_restriction() function for the requested type. It may specify
905 a method and relevant data for the restriction such as signature
906 verification or constraints on key payload. If the requested key type is
907 later unregistered, no keys may be added to the keyring after the key type
908 is removed.
909
910 To apply a keyring restriction the process must have Set Attribute
911 permission and the keyring must not be previously restricted.
912
913 One application of restricted keyrings is to verify X.509 certificate
914 chains or individual certificate signatures using the asymmetric key type.
915 See Documentation/crypto/asymmetric-keys.rst for specific restrictions
916 applicable to the asymmetric key type.
917
918
919 * Query an asymmetric key::
920
921 long keyctl(KEYCTL_PKEY_QUERY,
922 key_serial_t key_id, unsigned long reserved,
923 const char *params,
924 struct keyctl_pkey_query *info);
925
926 Get information about an asymmetric key. Specific algorithms and
927 encodings may be queried by using the ``params`` argument. This is a
928 string containing a space- or tab-separated string of key-value pairs.
929 Currently supported keys include ``enc`` and ``hash``. The information
930 is returned in the keyctl_pkey_query struct::
931
932 __u32 supported_ops;
933 __u32 key_size;
934 __u16 max_data_size;
935 __u16 max_sig_size;
936 __u16 max_enc_size;
937 __u16 max_dec_size;
938 __u32 __spare[10];
939
940 ``supported_ops`` contains a bit mask of flags indicating which ops are
941 supported. This is constructed from a bitwise-OR of::
942
943 KEYCTL_SUPPORTS_{ENCRYPT,DECRYPT,SIGN,VERIFY}
944
945 ``key_size`` indicated the size of the key in bits.
946
947 ``max_*_size`` indicate the maximum sizes in bytes of a blob of data to be
948 signed, a signature blob, a blob to be encrypted and a blob to be
949 decrypted.
950
951 ``__spare[]`` must be set to 0. This is intended for future use to hand
952 over one or more passphrases needed unlock a key.
953
954 If successful, 0 is returned. If the key is not an asymmetric key,
955 EOPNOTSUPP is returned.
956
957
958 * Encrypt, decrypt, sign or verify a blob using an asymmetric key::
959
960 long keyctl(KEYCTL_PKEY_ENCRYPT,
961 const struct keyctl_pkey_params *params,
962 const char *info,
963 const void *in,
964 void *out);
965
966 long keyctl(KEYCTL_PKEY_DECRYPT,
967 const struct keyctl_pkey_params *params,
968 const char *info,
969 const void *in,
970 void *out);
971
972 long keyctl(KEYCTL_PKEY_SIGN,
973 const struct keyctl_pkey_params *params,
974 const char *info,
975 const void *in,
976 void *out);
977
978 long keyctl(KEYCTL_PKEY_VERIFY,
979 const struct keyctl_pkey_params *params,
980 const char *info,
981 const void *in,
982 const void *in2);
983
984 Use an asymmetric key to perform a public-key cryptographic operation a
985 blob of data. For encryption and verification, the asymmetric key may
986 only need the public parts to be available, but for decryption and signing
987 the private parts are required also.
988
989 The parameter block pointed to by params contains a number of integer
990 values::
991
992 __s32 key_id;
993 __u32 in_len;
994 __u32 out_len;
995 __u32 in2_len;
996
997 ``key_id`` is the ID of the asymmetric key to be used. ``in_len`` and
998 ``in2_len`` indicate the amount of data in the in and in2 buffers and
999 ``out_len`` indicates the size of the out buffer as appropriate for the
1000 above operations.
1001
1002 For a given operation, the in and out buffers are used as follows::
1003
1004 Operation ID in,in_len out,out_len in2,in2_len
1005 ======================= =============== =============== ===============
1006 KEYCTL_PKEY_ENCRYPT Raw data Encrypted data -
1007 KEYCTL_PKEY_DECRYPT Encrypted data Raw data -
1008 KEYCTL_PKEY_SIGN Raw data Signature -
1009 KEYCTL_PKEY_VERIFY Raw data - Signature
1010
1011 ``info`` is a string of key=value pairs that supply supplementary
1012 information. These include:
1013
1014 ``enc=<encoding>`` The encoding of the encrypted/signature blob. This
1015 can be "pkcs1" for RSASSA-PKCS1-v1.5 or
1016 RSAES-PKCS1-v1.5; "pss" for "RSASSA-PSS"; "oaep" for
1017 "RSAES-OAEP". If omitted or is "raw", the raw output
1018 of the encryption function is specified.
1019
1020 ``hash=<algo>`` If the data buffer contains the output of a hash
1021 function and the encoding includes some indication of
1022 which hash function was used, the hash function can be
1023 specified with this, eg. "hash=sha256".
1024
1025 The ``__spare[]`` space in the parameter block must be set to 0. This is
1026 intended, amongst other things, to allow the passing of passphrases
1027 required to unlock a key.
1028
1029 If successful, encrypt, decrypt and sign all return the amount of data
1030 written into the output buffer. Verification returns 0 on success.
1031
1032
1033 * Watch a key or keyring for changes::
1034
1035 long keyctl(KEYCTL_WATCH_KEY, key_serial_t key, int queue_fd,
1036 const struct watch_notification_filter *filter);
1037
1038 This will set or remove a watch for changes on the specified key or
1039 keyring.
1040
1041 "key" is the ID of the key to be watched.
1042
1043 "queue_fd" is a file descriptor referring to an open pipe which
1044 manages the buffer into which notifications will be delivered.
1045
1046 "filter" is either NULL to remove a watch or a filter specification to
1047 indicate what events are required from the key.
1048
1049 See Documentation/core-api/watch_queue.rst for more information.
1050
1051 Note that only one watch may be emplaced for any particular { key,
1052 queue_fd } combination.
1053
1054 Notification records look like::
1055
1056 struct key_notification {
1057 struct watch_notification watch;
1058 __u32 key_id;
1059 __u32 aux;
1060 };
1061
1062 In this, watch::type will be "WATCH_TYPE_KEY_NOTIFY" and subtype will be
1063 one of::
1064
1065 NOTIFY_KEY_INSTANTIATED
1066 NOTIFY_KEY_UPDATED
1067 NOTIFY_KEY_LINKED
1068 NOTIFY_KEY_UNLINKED
1069 NOTIFY_KEY_CLEARED
1070 NOTIFY_KEY_REVOKED
1071 NOTIFY_KEY_INVALIDATED
1072 NOTIFY_KEY_SETATTR
1073
1074 Where these indicate a key being instantiated/rejected, updated, a link
1075 being made in a keyring, a link being removed from a keyring, a keyring
1076 being cleared, a key being revoked, a key being invalidated or a key
1077 having one of its attributes changed (user, group, perm, timeout,
1078 restriction).
1079
1080 If a watched key is deleted, a basic watch_notification will be issued
1081 with "type" set to WATCH_TYPE_META and "subtype" set to
1082 watch_meta_removal_notification. The watchpoint ID will be set in the
1083 "info" field.
1084
1085 This needs to be configured by enabling:
1086
1087 "Provide key/keyring change notifications" (KEY_NOTIFICATIONS)
1088
1089
1090 Kernel Services
1091 ===============
1092
1093 The kernel services for key management are fairly simple to deal with. They can
1094 be broken down into two areas: keys and key types.
1095
1096 Dealing with keys is fairly straightforward. Firstly, the kernel service
1097 registers its type, then it searches for a key of that type. It should retain
1098 the key as long as it has need of it, and then it should release it. For a
1099 filesystem or device file, a search would probably be performed during the open
1100 call, and the key released upon close. How to deal with conflicting keys due to
1101 two different users opening the same file is left to the filesystem author to
1102 solve.
1103
1104 To access the key manager, the following header must be #included::
1105
1106 <linux/key.h>
1107
1108 Specific key types should have a header file under include/keys/ that should be
1109 used to access that type. For keys of type "user", for example, that would be::
1110
1111 <keys/user-type.h>
1112
1113 Note that there are two different types of pointers to keys that may be
1114 encountered:
1115
1116 * struct key *
1117
1118 This simply points to the key structure itself. Key structures will be at
1119 least four-byte aligned.
1120
1121 * key_ref_t
1122
1123 This is equivalent to a ``struct key *``, but the least significant bit is set
1124 if the caller "possesses" the key. By "possession" it is meant that the
1125 calling processes has a searchable link to the key from one of its
1126 keyrings. There are three functions for dealing with these::
1127
1128 key_ref_t make_key_ref(const struct key *key, bool possession);
1129
1130 struct key *key_ref_to_ptr(const key_ref_t key_ref);
1131
1132 bool is_key_possessed(const key_ref_t key_ref);
1133
1134 The first function constructs a key reference from a key pointer and
1135 possession information (which must be true or false).
1136
1137 The second function retrieves the key pointer from a reference and the
1138 third retrieves the possession flag.
1139
1140 When accessing a key's payload contents, certain precautions must be taken to
1141 prevent access vs modification races. See the section "Notes on accessing
1142 payload contents" for more information.
1143
1144 * To search for a key, call::
1145
1146 struct key *request_key(const struct key_type *type,
1147 const char *description,
1148 const char *callout_info);
1149
1150 This is used to request a key or keyring with a description that matches
1151 the description specified according to the key type's match_preparse()
1152 method. This permits approximate matching to occur. If callout_string is
1153 not NULL, then /sbin/request-key will be invoked in an attempt to obtain
1154 the key from userspace. In that case, callout_string will be passed as an
1155 argument to the program.
1156
1157 Should the function fail error ENOKEY, EKEYEXPIRED or EKEYREVOKED will be
1158 returned.
1159
1160 If successful, the key will have been attached to the default keyring for
1161 implicitly obtained request-key keys, as set by KEYCTL_SET_REQKEY_KEYRING.
1162
1163 See also Documentation/security/keys/request-key.rst.
1164
1165
1166 * To search for a key in a specific domain, call::
1167
1168 struct key *request_key_tag(const struct key_type *type,
1169 const char *description,
1170 struct key_tag *domain_tag,
1171 const char *callout_info);
1172
1173 This is identical to request_key(), except that a domain tag may be
1174 specifies that causes search algorithm to only match keys matching that
1175 tag. The domain_tag may be NULL, specifying a global domain that is
1176 separate from any nominated domain.
1177
1178
1179 * To search for a key, passing auxiliary data to the upcaller, call::
1180
1181 struct key *request_key_with_auxdata(const struct key_type *type,
1182 const char *description,
1183 struct key_tag *domain_tag,
1184 const void *callout_info,
1185 size_t callout_len,
1186 void *aux);
1187
1188 This is identical to request_key_tag(), except that the auxiliary data is
1189 passed to the key_type->request_key() op if it exists, and the
1190 callout_info is a blob of length callout_len, if given (the length may be
1191 0).
1192
1193
1194 * To search for a key under RCU conditions, call::
1195
1196 struct key *request_key_rcu(const struct key_type *type,
1197 const char *description,
1198 struct key_tag *domain_tag);
1199
1200 which is similar to request_key_tag() except that it does not check for
1201 keys that are under construction and it will not call out to userspace to
1202 construct a key if it can't find a match.
1203
1204
1205 * When it is no longer required, the key should be released using::
1206
1207 void key_put(struct key *key);
1208
1209 Or::
1210
1211 void key_ref_put(key_ref_t key_ref);
1212
1213 These can be called from interrupt context. If CONFIG_KEYS is not set then
1214 the argument will not be parsed.
1215
1216
1217 * Extra references can be made to a key by calling one of the following
1218 functions::
1219
1220 struct key *__key_get(struct key *key);
1221 struct key *key_get(struct key *key);
1222
1223 Keys so references will need to be disposed of by calling key_put() when
1224 they've been finished with. The key pointer passed in will be returned.
1225
1226 In the case of key_get(), if the pointer is NULL or CONFIG_KEYS is not set
1227 then the key will not be dereferenced and no increment will take place.
1228
1229
1230 * A key's serial number can be obtained by calling::
1231
1232 key_serial_t key_serial(struct key *key);
1233
1234 If key is NULL or if CONFIG_KEYS is not set then 0 will be returned (in the
1235 latter case without parsing the argument).
1236
1237
1238 * If a keyring was found in the search, this can be further searched by::
1239
1240 key_ref_t keyring_search(key_ref_t keyring_ref,
1241 const struct key_type *type,
1242 const char *description,
1243 bool recurse)
1244
1245 This searches the specified keyring only (recurse == false) or keyring tree
1246 (recurse == true) specified for a matching key. Error ENOKEY is returned
1247 upon failure (use IS_ERR/PTR_ERR to determine). If successful, the returned
1248 key will need to be released.
1249
1250 The possession attribute from the keyring reference is used to control
1251 access through the permissions mask and is propagated to the returned key
1252 reference pointer if successful.
1253
1254
1255 * A keyring can be created by::
1256
1257 struct key *keyring_alloc(const char *description, uid_t uid, gid_t gid,
1258 const struct cred *cred,
1259 key_perm_t perm,
1260 struct key_restriction *restrict_link,
1261 unsigned long flags,
1262 struct key *dest);
1263
1264 This creates a keyring with the given attributes and returns it. If dest
1265 is not NULL, the new keyring will be linked into the keyring to which it
1266 points. No permission checks are made upon the destination keyring.
1267
1268 Error EDQUOT can be returned if the keyring would overload the quota (pass
1269 KEY_ALLOC_NOT_IN_QUOTA in flags if the keyring shouldn't be accounted
1270 towards the user's quota). Error ENOMEM can also be returned.
1271
1272 If restrict_link is not NULL, it should point to a structure that contains
1273 the function that will be called each time an attempt is made to link a
1274 key into the new keyring. The structure may also contain a key pointer
1275 and an associated key type. The function is called to check whether a key
1276 may be added into the keyring or not. The key type is used by the garbage
1277 collector to clean up function or data pointers in this structure if the
1278 given key type is unregistered. Callers of key_create_or_update() within
1279 the kernel can pass KEY_ALLOC_BYPASS_RESTRICTION to suppress the check.
1280 An example of using this is to manage rings of cryptographic keys that are
1281 set up when the kernel boots where userspace is also permitted to add keys
1282 - provided they can be verified by a key the kernel already has.
1283
1284 When called, the restriction function will be passed the keyring being
1285 added to, the key type, the payload of the key being added, and data to be
1286 used in the restriction check. Note that when a new key is being created,
1287 this is called between payload preparsing and actual key creation. The
1288 function should return 0 to allow the link or an error to reject it.
1289
1290 A convenience function, restrict_link_reject, exists to always return
1291 -EPERM to in this case.
1292
1293
1294 * To check the validity of a key, this function can be called::
1295
1296 int validate_key(struct key *key);
1297
1298 This checks that the key in question hasn't expired or and hasn't been
1299 revoked. Should the key be invalid, error EKEYEXPIRED or EKEYREVOKED will
1300 be returned. If the key is NULL or if CONFIG_KEYS is not set then 0 will be
1301 returned (in the latter case without parsing the argument).
1302
1303
1304 * To register a key type, the following function should be called::
1305
1306 int register_key_type(struct key_type *type);
1307
1308 This will return error EEXIST if a type of the same name is already
1309 present.
1310
1311
1312 * To unregister a key type, call::
1313
1314 void unregister_key_type(struct key_type *type);
1315
1316
1317 Under some circumstances, it may be desirable to deal with a bundle of keys.
1318 The facility provides access to the keyring type for managing such a bundle::
1319
1320 struct key_type key_type_keyring;
1321
1322 This can be used with a function such as request_key() to find a specific
1323 keyring in a process's keyrings. A keyring thus found can then be searched
1324 with keyring_search(). Note that it is not possible to use request_key() to
1325 search a specific keyring, so using keyrings in this way is of limited utility.
1326
1327
1328 Notes On Accessing Payload Contents
1329 ===================================
1330
1331 The simplest payload is just data stored in key->payload directly. In this
1332 case, there's no need to indulge in RCU or locking when accessing the payload.
1333
1334 More complex payload contents must be allocated and pointers to them set in the
1335 key->payload.data[] array. One of the following ways must be selected to
1336 access the data:
1337
1338 1) Unmodifiable key type.
1339
1340 If the key type does not have a modify method, then the key's payload can
1341 be accessed without any form of locking, provided that it's known to be
1342 instantiated (uninstantiated keys cannot be "found").
1343
1344 2) The key's semaphore.
1345
1346 The semaphore could be used to govern access to the payload and to control
1347 the payload pointer. It must be write-locked for modifications and would
1348 have to be read-locked for general access. The disadvantage of doing this
1349 is that the accessor may be required to sleep.
1350
1351 3) RCU.
1352
1353 RCU must be used when the semaphore isn't already held; if the semaphore
1354 is held then the contents can't change under you unexpectedly as the
1355 semaphore must still be used to serialise modifications to the key. The
1356 key management code takes care of this for the key type.
1357
1358 However, this means using::
1359
1360 rcu_read_lock() ... rcu_dereference() ... rcu_read_unlock()
1361
1362 to read the pointer, and::
1363
1364 rcu_dereference() ... rcu_assign_pointer() ... call_rcu()
1365
1366 to set the pointer and dispose of the old contents after a grace period.
1367 Note that only the key type should ever modify a key's payload.
1368
1369 Furthermore, an RCU controlled payload must hold a struct rcu_head for the
1370 use of call_rcu() and, if the payload is of variable size, the length of
1371 the payload. key->datalen cannot be relied upon to be consistent with the
1372 payload just dereferenced if the key's semaphore is not held.
1373
1374 Note that key->payload.data[0] has a shadow that is marked for __rcu
1375 usage. This is called key->payload.rcu_data0. The following accessors
1376 wrap the RCU calls to this element:
1377
1378 a) Set or change the first payload pointer::
1379
1380 rcu_assign_keypointer(struct key *key, void *data);
1381
1382 b) Read the first payload pointer with the key semaphore held::
1383
1384 [const] void *dereference_key_locked([const] struct key *key);
1385
1386 Note that the return value will inherit its constness from the key
1387 parameter. Static analysis will give an error if it things the lock
1388 isn't held.
1389
1390 c) Read the first payload pointer with the RCU read lock held::
1391
1392 const void *dereference_key_rcu(const struct key *key);
1393
1394
1395 Defining a Key Type
1396 ===================
1397
1398 A kernel service may want to define its own key type. For instance, an AFS
1399 filesystem might want to define a Kerberos 5 ticket key type. To do this, it
1400 author fills in a key_type struct and registers it with the system.
1401
1402 Source files that implement key types should include the following header file::
1403
1404 <linux/key-type.h>
1405
1406 The structure has a number of fields, some of which are mandatory:
1407
1408 * ``const char *name``
1409
1410 The name of the key type. This is used to translate a key type name
1411 supplied by userspace into a pointer to the structure.
1412
1413
1414 * ``size_t def_datalen``
1415
1416 This is optional - it supplies the default payload data length as
1417 contributed to the quota. If the key type's payload is always or almost
1418 always the same size, then this is a more efficient way to do things.
1419
1420 The data length (and quota) on a particular key can always be changed
1421 during instantiation or update by calling::
1422
1423 int key_payload_reserve(struct key *key, size_t datalen);
1424
1425 With the revised data length. Error EDQUOT will be returned if this is not
1426 viable.
1427
1428
1429 * ``int (*vet_description)(const char *description);``
1430
1431 This optional method is called to vet a key description. If the key type
1432 doesn't approve of the key description, it may return an error, otherwise
1433 it should return 0.
1434
1435
1436 * ``int (*preparse)(struct key_preparsed_payload *prep);``
1437
1438 This optional method permits the key type to attempt to parse payload
1439 before a key is created (add key) or the key semaphore is taken (update or
1440 instantiate key). The structure pointed to by prep looks like::
1441
1442 struct key_preparsed_payload {
1443 char *description;
1444 union key_payload payload;
1445 const void *data;
1446 size_t datalen;
1447 size_t quotalen;
1448 time_t expiry;
1449 };
1450
1451 Before calling the method, the caller will fill in data and datalen with
1452 the payload blob parameters; quotalen will be filled in with the default
1453 quota size from the key type; expiry will be set to TIME_T_MAX and the
1454 rest will be cleared.
1455
1456 If a description can be proposed from the payload contents, that should be
1457 attached as a string to the description field. This will be used for the
1458 key description if the caller of add_key() passes NULL or "".
1459
1460 The method can attach anything it likes to payload. This is merely passed
1461 along to the instantiate() or update() operations. If set, the expiry
1462 time will be applied to the key if it is instantiated from this data.
1463
1464 The method should return 0 if successful or a negative error code
1465 otherwise.
1466
1467
1468 * ``void (*free_preparse)(struct key_preparsed_payload *prep);``
1469
1470 This method is only required if the preparse() method is provided,
1471 otherwise it is unused. It cleans up anything attached to the description
1472 and payload fields of the key_preparsed_payload struct as filled in by the
1473 preparse() method. It will always be called after preparse() returns
1474 successfully, even if instantiate() or update() succeed.
1475
1476
1477 * ``int (*instantiate)(struct key *key, struct key_preparsed_payload *prep);``
1478
1479 This method is called to attach a payload to a key during construction.
1480 The payload attached need not bear any relation to the data passed to this
1481 function.
1482
1483 The prep->data and prep->datalen fields will define the original payload
1484 blob. If preparse() was supplied then other fields may be filled in also.
1485
1486 If the amount of data attached to the key differs from the size in
1487 keytype->def_datalen, then key_payload_reserve() should be called.
1488
1489 This method does not have to lock the key in order to attach a payload.
1490 The fact that KEY_FLAG_INSTANTIATED is not set in key->flags prevents
1491 anything else from gaining access to the key.
1492
1493 It is safe to sleep in this method.
1494
1495 generic_key_instantiate() is provided to simply copy the data from
1496 prep->payload.data[] to key->payload.data[], with RCU-safe assignment on
1497 the first element. It will then clear prep->payload.data[] so that the
1498 free_preparse method doesn't release the data.
1499
1500
1501 * ``int (*update)(struct key *key, const void *data, size_t datalen);``
1502
1503 If this type of key can be updated, then this method should be provided.
1504 It is called to update a key's payload from the blob of data provided.
1505
1506 The prep->data and prep->datalen fields will define the original payload
1507 blob. If preparse() was supplied then other fields may be filled in also.
1508
1509 key_payload_reserve() should be called if the data length might change
1510 before any changes are actually made. Note that if this succeeds, the type
1511 is committed to changing the key because it's already been altered, so all
1512 memory allocation must be done first.
1513
1514 The key will have its semaphore write-locked before this method is called,
1515 but this only deters other writers; any changes to the key's payload must
1516 be made under RCU conditions, and call_rcu() must be used to dispose of
1517 the old payload.
1518
1519 key_payload_reserve() should be called before the changes are made, but
1520 after all allocations and other potentially failing function calls are
1521 made.
1522
1523 It is safe to sleep in this method.
1524
1525
1526 * ``int (*match_preparse)(struct key_match_data *match_data);``
1527
1528 This method is optional. It is called when a key search is about to be
1529 performed. It is given the following structure::
1530
1531 struct key_match_data {
1532 bool (*cmp)(const struct key *key,
1533 const struct key_match_data *match_data);
1534 const void *raw_data;
1535 void *preparsed;
1536 unsigned lookup_type;
1537 };
1538
1539 On entry, raw_data will be pointing to the criteria to be used in matching
1540 a key by the caller and should not be modified. ``(*cmp)()`` will be pointing
1541 to the default matcher function (which does an exact description match
1542 against raw_data) and lookup_type will be set to indicate a direct lookup.
1543
1544 The following lookup_type values are available:
1545
1546 * KEYRING_SEARCH_LOOKUP_DIRECT - A direct lookup hashes the type and
1547 description to narrow down the search to a small number of keys.
1548
1549 * KEYRING_SEARCH_LOOKUP_ITERATE - An iterative lookup walks all the
1550 keys in the keyring until one is matched. This must be used for any
1551 search that's not doing a simple direct match on the key description.
1552
1553 The method may set cmp to point to a function of its choice that does some
1554 other form of match, may set lookup_type to KEYRING_SEARCH_LOOKUP_ITERATE
1555 and may attach something to the preparsed pointer for use by ``(*cmp)()``.
1556 ``(*cmp)()`` should return true if a key matches and false otherwise.
1557
1558 If preparsed is set, it may be necessary to use the match_free() method to
1559 clean it up.
1560
1561 The method should return 0 if successful or a negative error code
1562 otherwise.
1563
1564 It is permitted to sleep in this method, but ``(*cmp)()`` may not sleep as
1565 locks will be held over it.
1566
1567 If match_preparse() is not provided, keys of this type will be matched
1568 exactly by their description.
1569
1570
1571 * ``void (*match_free)(struct key_match_data *match_data);``
1572
1573 This method is optional. If given, it called to clean up
1574 match_data->preparsed after a successful call to match_preparse().
1575
1576
1577 * ``void (*revoke)(struct key *key);``
1578
1579 This method is optional. It is called to discard part of the payload
1580 data upon a key being revoked. The caller will have the key semaphore
1581 write-locked.
1582
1583 It is safe to sleep in this method, though care should be taken to avoid
1584 a deadlock against the key semaphore.
1585
1586
1587 * ``void (*destroy)(struct key *key);``
1588
1589 This method is optional. It is called to discard the payload data on a key
1590 when it is being destroyed.
1591
1592 This method does not need to lock the key to access the payload; it can
1593 consider the key as being inaccessible at this time. Note that the key's
1594 type may have been changed before this function is called.
1595
1596 It is not safe to sleep in this method; the caller may hold spinlocks.
1597
1598
1599 * ``void (*describe)(const struct key *key, struct seq_file *p);``
1600
1601 This method is optional. It is called during /proc/keys reading to
1602 summarise a key's description and payload in text form.
1603
1604 This method will be called with the RCU read lock held. rcu_dereference()
1605 should be used to read the payload pointer if the payload is to be
1606 accessed. key->datalen cannot be trusted to stay consistent with the
1607 contents of the payload.
1608
1609 The description will not change, though the key's state may.
1610
1611 It is not safe to sleep in this method; the RCU read lock is held by the
1612 caller.
1613
1614
1615 * ``long (*read)(const struct key *key, char __user *buffer, size_t buflen);``
1616
1617 This method is optional. It is called by KEYCTL_READ to translate the
1618 key's payload into something a blob of data for userspace to deal with.
1619 Ideally, the blob should be in the same format as that passed in to the
1620 instantiate and update methods.
1621
1622 If successful, the blob size that could be produced should be returned
1623 rather than the size copied.
1624
1625 This method will be called with the key's semaphore read-locked. This will
1626 prevent the key's payload changing. It is not necessary to use RCU locking
1627 when accessing the key's payload. It is safe to sleep in this method, such
1628 as might happen when the userspace buffer is accessed.
1629
1630
1631 * ``int (*request_key)(struct key_construction *cons, const char *op, void *aux);``
1632
1633 This method is optional. If provided, request_key() and friends will
1634 invoke this function rather than upcalling to /sbin/request-key to operate
1635 upon a key of this type.
1636
1637 The aux parameter is as passed to request_key_async_with_auxdata() and
1638 similar or is NULL otherwise. Also passed are the construction record for
1639 the key to be operated upon and the operation type (currently only
1640 "create").
1641
1642 This method is permitted to return before the upcall is complete, but the
1643 following function must be called under all circumstances to complete the
1644 instantiation process, whether or not it succeeds, whether or not there's
1645 an error::
1646
1647 void complete_request_key(struct key_construction *cons, int error);
1648
1649 The error parameter should be 0 on success, -ve on error. The
1650 construction record is destroyed by this action and the authorisation key
1651 will be revoked. If an error is indicated, the key under construction
1652 will be negatively instantiated if it wasn't already instantiated.
1653
1654 If this method returns an error, that error will be returned to the
1655 caller of request_key*(). complete_request_key() must be called prior to
1656 returning.
1657
1658 The key under construction and the authorisation key can be found in the
1659 key_construction struct pointed to by cons:
1660
1661 * ``struct key *key;``
1662
1663 The key under construction.
1664
1665 * ``struct key *authkey;``
1666
1667 The authorisation key.
1668
1669
1670 * ``struct key_restriction *(*lookup_restriction)(const char *params);``
1671
1672 This optional method is used to enable userspace configuration of keyring
1673 restrictions. The restriction parameter string (not including the key type
1674 name) is passed in, and this method returns a pointer to a key_restriction
1675 structure containing the relevant functions and data to evaluate each
1676 attempted key link operation. If there is no match, -EINVAL is returned.
1677
1678
1679 * ``asym_eds_op`` and ``asym_verify_signature``::
1680
1681 int (*asym_eds_op)(struct kernel_pkey_params *params,
1682 const void *in, void *out);
1683 int (*asym_verify_signature)(struct kernel_pkey_params *params,
1684 const void *in, const void *in2);
1685
1686 These methods are optional. If provided the first allows a key to be
1687 used to encrypt, decrypt or sign a blob of data, and the second allows a
1688 key to verify a signature.
1689
1690 In all cases, the following information is provided in the params block::
1691
1692 struct kernel_pkey_params {
1693 struct key *key;
1694 const char *encoding;
1695 const char *hash_algo;
1696 char *info;
1697 __u32 in_len;
1698 union {
1699 __u32 out_len;
1700 __u32 in2_len;
1701 };
1702 enum kernel_pkey_operation op : 8;
1703 };
1704
1705 This includes the key to be used; a string indicating the encoding to use
1706 (for instance, "pkcs1" may be used with an RSA key to indicate
1707 RSASSA-PKCS1-v1.5 or RSAES-PKCS1-v1.5 encoding or "raw" if no encoding);
1708 the name of the hash algorithm used to generate the data for a signature
1709 (if appropriate); the sizes of the input and output (or second input)
1710 buffers; and the ID of the operation to be performed.
1711
1712 For a given operation ID, the input and output buffers are used as
1713 follows::
1714
1715 Operation ID in,in_len out,out_len in2,in2_len
1716 ======================= =============== =============== ===============
1717 kernel_pkey_encrypt Raw data Encrypted data -
1718 kernel_pkey_decrypt Encrypted data Raw data -
1719 kernel_pkey_sign Raw data Signature -
1720 kernel_pkey_verify Raw data - Signature
1721
1722 asym_eds_op() deals with encryption, decryption and signature creation as
1723 specified by params->op. Note that params->op is also set for
1724 asym_verify_signature().
1725
1726 Encrypting and signature creation both take raw data in the input buffer
1727 and return the encrypted result in the output buffer. Padding may have
1728 been added if an encoding was set. In the case of signature creation,
1729 depending on the encoding, the padding created may need to indicate the
1730 digest algorithm - the name of which should be supplied in hash_algo.
1731
1732 Decryption takes encrypted data in the input buffer and returns the raw
1733 data in the output buffer. Padding will get checked and stripped off if
1734 an encoding was set.
1735
1736 Verification takes raw data in the input buffer and the signature in the
1737 second input buffer and checks that the one matches the other. Padding
1738 will be validated. Depending on the encoding, the digest algorithm used
1739 to generate the raw data may need to be indicated in hash_algo.
1740
1741 If successful, asym_eds_op() should return the number of bytes written
1742 into the output buffer. asym_verify_signature() should return 0.
1743
1744 A variety of errors may be returned, including EOPNOTSUPP if the operation
1745 is not supported; EKEYREJECTED if verification fails; ENOPKG if the
1746 required crypto isn't available.
1747
1748
1749 * ``asym_query``::
1750
1751 int (*asym_query)(const struct kernel_pkey_params *params,
1752 struct kernel_pkey_query *info);
1753
1754 This method is optional. If provided it allows information about the
1755 public or asymmetric key held in the key to be determined.
1756
1757 The parameter block is as for asym_eds_op() and co. but in_len and out_len
1758 are unused. The encoding and hash_algo fields should be used to reduce
1759 the returned buffer/data sizes as appropriate.
1760
1761 If successful, the following information is filled in::
1762
1763 struct kernel_pkey_query {
1764 __u32 supported_ops;
1765 __u32 key_size;
1766 __u16 max_data_size;
1767 __u16 max_sig_size;
1768 __u16 max_enc_size;
1769 __u16 max_dec_size;
1770 };
1771
1772 The supported_ops field will contain a bitmask indicating what operations
1773 are supported by the key, including encryption of a blob, decryption of a
1774 blob, signing a blob and verifying the signature on a blob. The following
1775 constants are defined for this::
1776
1777 KEYCTL_SUPPORTS_{ENCRYPT,DECRYPT,SIGN,VERIFY}
1778
1779 The key_size field is the size of the key in bits. max_data_size and
1780 max_sig_size are the maximum raw data and signature sizes for creation and
1781 verification of a signature; max_enc_size and max_dec_size are the maximum
1782 raw data and signature sizes for encryption and decryption. The
1783 max_*_size fields are measured in bytes.
1784
1785 If successful, 0 will be returned. If the key doesn't support this,
1786 EOPNOTSUPP will be returned.
1787
1788
1789 Request-Key Callback Service
1790 ============================
1791
1792 To create a new key, the kernel will attempt to execute the following command
1793 line::
1794
1795 /sbin/request-key create <key> <uid> <gid> \
1796 <threadring> <processring> <sessionring> <callout_info>
1797
1798 <key> is the key being constructed, and the three keyrings are the process
1799 keyrings from the process that caused the search to be issued. These are
1800 included for two reasons:
1801
1802 1 There may be an authentication token in one of the keyrings that is
1803 required to obtain the key, eg: a Kerberos Ticket-Granting Ticket.
1804
1805 2 The new key should probably be cached in one of these rings.
1806
1807 This program should set it UID and GID to those specified before attempting to
1808 access any more keys. It may then look around for a user specific process to
1809 hand the request off to (perhaps a path held in placed in another key by, for
1810 example, the KDE desktop manager).
1811
1812 The program (or whatever it calls) should finish construction of the key by
1813 calling KEYCTL_INSTANTIATE or KEYCTL_INSTANTIATE_IOV, which also permits it to
1814 cache the key in one of the keyrings (probably the session ring) before
1815 returning. Alternatively, the key can be marked as negative with KEYCTL_NEGATE
1816 or KEYCTL_REJECT; this also permits the key to be cached in one of the
1817 keyrings.
1818
1819 If it returns with the key remaining in the unconstructed state, the key will
1820 be marked as being negative, it will be added to the session keyring, and an
1821 error will be returned to the key requestor.
1822
1823 Supplementary information may be provided from whoever or whatever invoked this
1824 service. This will be passed as the <callout_info> parameter. If no such
1825 information was made available, then "-" will be passed as this parameter
1826 instead.
1827
1828
1829 Similarly, the kernel may attempt to update an expired or a soon to expire key
1830 by executing::
1831
1832 /sbin/request-key update <key> <uid> <gid> \
1833 <threadring> <processring> <sessionring>
1834
1835 In this case, the program isn't required to actually attach the key to a ring;
1836 the rings are provided for reference.
1837
1838
1839 Garbage Collection
1840 ==================
1841
1842 Dead keys (for which the type has been removed) will be automatically unlinked
1843 from those keyrings that point to them and deleted as soon as possible by a
1844 background garbage collector.
1845
1846 Similarly, revoked and expired keys will be garbage collected, but only after a
1847 certain amount of time has passed. This time is set as a number of seconds in::
1848
1849 /proc/sys/kernel/keys/gc_delay
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