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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 other kernel services. | |
8 | ||
9 | Keyrings are permitted; these are a special type of key that can hold links to | |
10 | other keys. Processes each have three standard keyring subscriptions that a | |
11 | kernel service can search for relevant keys. | |
12 | ||
13 | The key service can be configured on by enabling: | |
14 | ||
15 | "Security options"/"Enable access key retention support" (CONFIG_KEYS) | |
16 | ||
17 | This document has the following sections: | |
18 | ||
19 | - Key overview | |
20 | - Key service overview | |
21 | - Key access permissions | |
22 | - New procfs files | |
23 | - Userspace system call interface | |
24 | - Kernel services | |
76d8aeab | 25 | - Notes on accessing payload contents |
1da177e4 LT |
26 | - Defining a key type |
27 | - Request-key callback service | |
28 | - Key access filesystem | |
29 | ||
30 | ||
31 | ============ | |
32 | KEY OVERVIEW | |
33 | ============ | |
34 | ||
35 | In this context, keys represent units of cryptographic data, authentication | |
36 | tokens, keyrings, etc.. These are represented in the kernel by struct key. | |
37 | ||
38 | Each key has a number of attributes: | |
39 | ||
40 | - A serial number. | |
41 | - A type. | |
42 | - A description (for matching a key in a search). | |
43 | - Access control information. | |
44 | - An expiry time. | |
45 | - A payload. | |
46 | - State. | |
47 | ||
48 | ||
76d8aeab DH |
49 | (*) Each key is issued a serial number of type key_serial_t that is unique for |
50 | the lifetime of that key. All serial numbers are positive non-zero 32-bit | |
51 | integers. | |
1da177e4 LT |
52 | |
53 | Userspace programs can use a key's serial numbers as a way to gain access | |
54 | to it, subject to permission checking. | |
55 | ||
56 | (*) Each key is of a defined "type". Types must be registered inside the | |
76d8aeab DH |
57 | kernel by a kernel service (such as a filesystem) before keys of that type |
58 | can be added or used. Userspace programs cannot define new types directly. | |
1da177e4 | 59 | |
76d8aeab DH |
60 | Key types are represented in the kernel by struct key_type. This defines a |
61 | number of operations that can be performed on a key of that type. | |
1da177e4 LT |
62 | |
63 | Should a type be removed from the system, all the keys of that type will | |
64 | be invalidated. | |
65 | ||
66 | (*) Each key has a description. This should be a printable string. The key | |
76d8aeab DH |
67 | type provides an operation to perform a match between the description on a |
68 | key and a criterion string. | |
1da177e4 LT |
69 | |
70 | (*) Each key has an owner user ID, a group ID and a permissions mask. These | |
71 | are used to control what a process may do to a key from userspace, and | |
72 | whether a kernel service will be able to find the key. | |
73 | ||
74 | (*) Each key can be set to expire at a specific time by the key type's | |
75 | instantiation function. Keys can also be immortal. | |
76 | ||
76d8aeab DH |
77 | (*) Each key can have a payload. This is a quantity of data that represent the |
78 | actual "key". In the case of a keyring, this is a list of keys to which | |
79 | the keyring links; in the case of a user-defined key, it's an arbitrary | |
80 | blob of data. | |
1da177e4 LT |
81 | |
82 | Having a payload is not required; and the payload can, in fact, just be a | |
83 | value stored in the struct key itself. | |
84 | ||
85 | When a key is instantiated, the key type's instantiation function is | |
86 | called with a blob of data, and that then creates the key's payload in | |
87 | some way. | |
88 | ||
89 | Similarly, when userspace wants to read back the contents of the key, if | |
90 | permitted, another key type operation will be called to convert the key's | |
91 | attached payload back into a blob of data. | |
92 | ||
93 | (*) Each key can be in one of a number of basic states: | |
94 | ||
76d8aeab DH |
95 | (*) Uninstantiated. The key exists, but does not have any data attached. |
96 | Keys being requested from userspace will be in this state. | |
1da177e4 LT |
97 | |
98 | (*) Instantiated. This is the normal state. The key is fully formed, and | |
99 | has data attached. | |
100 | ||
101 | (*) Negative. This is a relatively short-lived state. The key acts as a | |
102 | note saying that a previous call out to userspace failed, and acts as | |
103 | a throttle on key lookups. A negative key can be updated to a normal | |
104 | state. | |
105 | ||
106 | (*) Expired. Keys can have lifetimes set. If their lifetime is exceeded, | |
107 | they traverse to this state. An expired key can be updated back to a | |
108 | normal state. | |
109 | ||
110 | (*) Revoked. A key is put in this state by userspace action. It can't be | |
111 | found or operated upon (apart from by unlinking it). | |
112 | ||
113 | (*) Dead. The key's type was unregistered, and so the key is now useless. | |
114 | ||
115 | ||
116 | ==================== | |
117 | KEY SERVICE OVERVIEW | |
118 | ==================== | |
119 | ||
120 | The key service provides a number of features besides keys: | |
121 | ||
122 | (*) The key service defines two special key types: | |
123 | ||
124 | (+) "keyring" | |
125 | ||
126 | Keyrings are special keys that contain a list of other keys. Keyring | |
127 | lists can be modified using various system calls. Keyrings should not | |
128 | be given a payload when created. | |
129 | ||
130 | (+) "user" | |
131 | ||
132 | A key of this type has a description and a payload that are arbitrary | |
133 | blobs of data. These can be created, updated and read by userspace, | |
134 | and aren't intended for use by kernel services. | |
135 | ||
136 | (*) Each process subscribes to three keyrings: a thread-specific keyring, a | |
137 | process-specific keyring, and a session-specific keyring. | |
138 | ||
139 | The thread-specific keyring is discarded from the child when any sort of | |
140 | clone, fork, vfork or execve occurs. A new keyring is created only when | |
141 | required. | |
142 | ||
76d8aeab DH |
143 | The process-specific keyring is replaced with an empty one in the child on |
144 | clone, fork, vfork unless CLONE_THREAD is supplied, in which case it is | |
145 | shared. execve also discards the process's process keyring and creates a | |
146 | new one. | |
1da177e4 LT |
147 | |
148 | The session-specific keyring is persistent across clone, fork, vfork and | |
149 | execve, even when the latter executes a set-UID or set-GID binary. A | |
150 | process can, however, replace its current session keyring with a new one | |
151 | by using PR_JOIN_SESSION_KEYRING. It is permitted to request an anonymous | |
152 | new one, or to attempt to create or join one of a specific name. | |
153 | ||
154 | The ownership of the thread keyring changes when the real UID and GID of | |
155 | the thread changes. | |
156 | ||
157 | (*) Each user ID resident in the system holds two special keyrings: a user | |
158 | specific keyring and a default user session keyring. The default session | |
159 | keyring is initialised with a link to the user-specific keyring. | |
160 | ||
161 | When a process changes its real UID, if it used to have no session key, it | |
162 | will be subscribed to the default session key for the new UID. | |
163 | ||
164 | If a process attempts to access its session key when it doesn't have one, | |
165 | it will be subscribed to the default for its current UID. | |
166 | ||
167 | (*) Each user has two quotas against which the keys they own are tracked. One | |
168 | limits the total number of keys and keyrings, the other limits the total | |
169 | amount of description and payload space that can be consumed. | |
170 | ||
171 | The user can view information on this and other statistics through procfs | |
172 | files. | |
173 | ||
174 | Process-specific and thread-specific keyrings are not counted towards a | |
175 | user's quota. | |
176 | ||
177 | If a system call that modifies a key or keyring in some way would put the | |
178 | user over quota, the operation is refused and error EDQUOT is returned. | |
179 | ||
76d8aeab DH |
180 | (*) There's a system call interface by which userspace programs can create and |
181 | manipulate keys and keyrings. | |
1da177e4 | 182 | |
76d8aeab DH |
183 | (*) There's a kernel interface by which services can register types and search |
184 | for keys. | |
1da177e4 LT |
185 | |
186 | (*) There's a way for the a search done from the kernel to call back to | |
187 | userspace to request a key that can't be found in a process's keyrings. | |
188 | ||
189 | (*) An optional filesystem is available through which the key database can be | |
190 | viewed and manipulated. | |
191 | ||
192 | ||
193 | ====================== | |
194 | KEY ACCESS PERMISSIONS | |
195 | ====================== | |
196 | ||
76d8aeab | 197 | Keys have an owner user ID, a group access ID, and a permissions mask. The mask |
664cceb0 | 198 | has up to eight bits each for possessor, user, group and other access. Only |
29db9190 | 199 | six of each set of eight bits are defined. These permissions granted are: |
1da177e4 LT |
200 | |
201 | (*) View | |
202 | ||
203 | This permits a key or keyring's attributes to be viewed - including key | |
204 | type and description. | |
205 | ||
206 | (*) Read | |
207 | ||
208 | This permits a key's payload to be viewed or a keyring's list of linked | |
209 | keys. | |
210 | ||
211 | (*) Write | |
212 | ||
76d8aeab DH |
213 | This permits a key's payload to be instantiated or updated, or it allows a |
214 | link to be added to or removed from a keyring. | |
1da177e4 LT |
215 | |
216 | (*) Search | |
217 | ||
218 | This permits keyrings to be searched and keys to be found. Searches can | |
219 | only recurse into nested keyrings that have search permission set. | |
220 | ||
221 | (*) Link | |
222 | ||
223 | This permits a key or keyring to be linked to. To create a link from a | |
224 | keyring to a key, a process must have Write permission on the keyring and | |
225 | Link permission on the key. | |
226 | ||
29db9190 DH |
227 | (*) Set Attribute |
228 | ||
229 | This permits a key's UID, GID and permissions mask to be changed. | |
230 | ||
1da177e4 LT |
231 | For changing the ownership, group ID or permissions mask, being the owner of |
232 | the key or having the sysadmin capability is sufficient. | |
233 | ||
234 | ||
235 | ================ | |
236 | NEW PROCFS FILES | |
237 | ================ | |
238 | ||
239 | Two files have been added to procfs by which an administrator can find out | |
240 | about the status of the key service: | |
241 | ||
242 | (*) /proc/keys | |
243 | ||
244 | This lists all the keys on the system, giving information about their | |
76d8aeab DH |
245 | type, description and permissions. The payload of the key is not available |
246 | this way: | |
1da177e4 | 247 | |
664cceb0 | 248 | SERIAL FLAGS USAGE EXPY PERM UID GID TYPE DESCRIPTION: SUMMARY |
29db9190 DH |
249 | 00000001 I----- 39 perm 1f3f0000 0 0 keyring _uid_ses.0: 1/4 |
250 | 00000002 I----- 2 perm 1f3f0000 0 0 keyring _uid.0: empty | |
251 | 00000007 I----- 1 perm 1f3f0000 0 0 keyring _pid.1: empty | |
252 | 0000018d I----- 1 perm 1f3f0000 0 0 keyring _pid.412: empty | |
253 | 000004d2 I--Q-- 1 perm 1f3f0000 32 -1 keyring _uid.32: 1/4 | |
254 | 000004d3 I--Q-- 3 perm 1f3f0000 32 -1 keyring _uid_ses.32: empty | |
664cceb0 | 255 | 00000892 I--QU- 1 perm 1f000000 0 0 user metal:copper: 0 |
29db9190 DH |
256 | 00000893 I--Q-N 1 35s 1f3f0000 0 0 user metal:silver: 0 |
257 | 00000894 I--Q-- 1 10h 003f0000 0 0 user metal:gold: 0 | |
1da177e4 LT |
258 | |
259 | The flags are: | |
260 | ||
261 | I Instantiated | |
262 | R Revoked | |
263 | D Dead | |
264 | Q Contributes to user's quota | |
265 | U Under contruction by callback to userspace | |
266 | N Negative key | |
267 | ||
268 | This file must be enabled at kernel configuration time as it allows anyone | |
269 | to list the keys database. | |
270 | ||
271 | (*) /proc/key-users | |
272 | ||
273 | This file lists the tracking data for each user that has at least one key | |
274 | on the system. Such data includes quota information and statistics: | |
275 | ||
276 | [root@andromeda root]# cat /proc/key-users | |
277 | 0: 46 45/45 1/100 13/10000 | |
278 | 29: 2 2/2 2/100 40/10000 | |
279 | 32: 2 2/2 2/100 40/10000 | |
280 | 38: 2 2/2 2/100 40/10000 | |
281 | ||
282 | The format of each line is | |
283 | <UID>: User ID to which this applies | |
284 | <usage> Structure refcount | |
285 | <inst>/<keys> Total number of keys and number instantiated | |
286 | <keys>/<max> Key count quota | |
287 | <bytes>/<max> Key size quota | |
288 | ||
289 | ||
290 | =============================== | |
291 | USERSPACE SYSTEM CALL INTERFACE | |
292 | =============================== | |
293 | ||
294 | Userspace can manipulate keys directly through three new syscalls: add_key, | |
295 | request_key and keyctl. The latter provides a number of functions for | |
296 | manipulating keys. | |
297 | ||
298 | When referring to a key directly, userspace programs should use the key's | |
299 | serial number (a positive 32-bit integer). However, there are some special | |
300 | values available for referring to special keys and keyrings that relate to the | |
301 | process making the call: | |
302 | ||
303 | CONSTANT VALUE KEY REFERENCED | |
304 | ============================== ====== =========================== | |
305 | KEY_SPEC_THREAD_KEYRING -1 thread-specific keyring | |
306 | KEY_SPEC_PROCESS_KEYRING -2 process-specific keyring | |
307 | KEY_SPEC_SESSION_KEYRING -3 session-specific keyring | |
308 | KEY_SPEC_USER_KEYRING -4 UID-specific keyring | |
309 | KEY_SPEC_USER_SESSION_KEYRING -5 UID-session keyring | |
310 | KEY_SPEC_GROUP_KEYRING -6 GID-specific keyring | |
311 | ||
312 | ||
313 | The main syscalls are: | |
314 | ||
315 | (*) Create a new key of given type, description and payload and add it to the | |
316 | nominated keyring: | |
317 | ||
318 | key_serial_t add_key(const char *type, const char *desc, | |
319 | const void *payload, size_t plen, | |
320 | key_serial_t keyring); | |
321 | ||
322 | If a key of the same type and description as that proposed already exists | |
323 | in the keyring, this will try to update it with the given payload, or it | |
324 | will return error EEXIST if that function is not supported by the key | |
76d8aeab DH |
325 | type. The process must also have permission to write to the key to be able |
326 | to update it. The new key will have all user permissions granted and no | |
327 | group or third party permissions. | |
1da177e4 | 328 | |
76d8aeab DH |
329 | Otherwise, this will attempt to create a new key of the specified type and |
330 | description, and to instantiate it with the supplied payload and attach it | |
331 | to the keyring. In this case, an error will be generated if the process | |
332 | does not have permission to write to the keyring. | |
1da177e4 LT |
333 | |
334 | The payload is optional, and the pointer can be NULL if not required by | |
335 | the type. The payload is plen in size, and plen can be zero for an empty | |
336 | payload. | |
337 | ||
76d8aeab DH |
338 | A new keyring can be generated by setting type "keyring", the keyring name |
339 | as the description (or NULL) and setting the payload to NULL. | |
1da177e4 LT |
340 | |
341 | User defined keys can be created by specifying type "user". It is | |
342 | recommended that a user defined key's description by prefixed with a type | |
343 | ID and a colon, such as "krb5tgt:" for a Kerberos 5 ticket granting | |
344 | ticket. | |
345 | ||
346 | Any other type must have been registered with the kernel in advance by a | |
347 | kernel service such as a filesystem. | |
348 | ||
349 | The ID of the new or updated key is returned if successful. | |
350 | ||
351 | ||
352 | (*) Search the process's keyrings for a key, potentially calling out to | |
353 | userspace to create it. | |
354 | ||
355 | key_serial_t request_key(const char *type, const char *description, | |
356 | const char *callout_info, | |
357 | key_serial_t dest_keyring); | |
358 | ||
359 | This function searches all the process's keyrings in the order thread, | |
360 | process, session for a matching key. This works very much like | |
361 | KEYCTL_SEARCH, including the optional attachment of the discovered key to | |
362 | a keyring. | |
363 | ||
364 | If a key cannot be found, and if callout_info is not NULL, then | |
365 | /sbin/request-key will be invoked in an attempt to obtain a key. The | |
366 | callout_info string will be passed as an argument to the program. | |
367 | ||
f1a9badc DH |
368 | See also Documentation/keys-request-key.txt. |
369 | ||
1da177e4 LT |
370 | |
371 | The keyctl syscall functions are: | |
372 | ||
373 | (*) Map a special key ID to a real key ID for this process: | |
374 | ||
375 | key_serial_t keyctl(KEYCTL_GET_KEYRING_ID, key_serial_t id, | |
376 | int create); | |
377 | ||
76d8aeab DH |
378 | The special key specified by "id" is looked up (with the key being created |
379 | if necessary) and the ID of the key or keyring thus found is returned if | |
380 | it exists. | |
1da177e4 LT |
381 | |
382 | If the key does not yet exist, the key will be created if "create" is | |
383 | non-zero; and the error ENOKEY will be returned if "create" is zero. | |
384 | ||
385 | ||
386 | (*) Replace the session keyring this process subscribes to with a new one: | |
387 | ||
388 | key_serial_t keyctl(KEYCTL_JOIN_SESSION_KEYRING, const char *name); | |
389 | ||
390 | If name is NULL, an anonymous keyring is created attached to the process | |
391 | as its session keyring, displacing the old session keyring. | |
392 | ||
393 | If name is not NULL, if a keyring of that name exists, the process | |
394 | attempts to attach it as the session keyring, returning an error if that | |
395 | is not permitted; otherwise a new keyring of that name is created and | |
396 | attached as the session keyring. | |
397 | ||
398 | To attach to a named keyring, the keyring must have search permission for | |
399 | the process's ownership. | |
400 | ||
401 | The ID of the new session keyring is returned if successful. | |
402 | ||
403 | ||
404 | (*) Update the specified key: | |
405 | ||
406 | long keyctl(KEYCTL_UPDATE, key_serial_t key, const void *payload, | |
407 | size_t plen); | |
408 | ||
409 | This will try to update the specified key with the given payload, or it | |
410 | will return error EOPNOTSUPP if that function is not supported by the key | |
76d8aeab DH |
411 | type. The process must also have permission to write to the key to be able |
412 | to update it. | |
1da177e4 LT |
413 | |
414 | The payload is of length plen, and may be absent or empty as for | |
415 | add_key(). | |
416 | ||
417 | ||
418 | (*) Revoke a key: | |
419 | ||
420 | long keyctl(KEYCTL_REVOKE, key_serial_t key); | |
421 | ||
422 | This makes a key unavailable for further operations. Further attempts to | |
423 | use the key will be met with error EKEYREVOKED, and the key will no longer | |
424 | be findable. | |
425 | ||
426 | ||
427 | (*) Change the ownership of a key: | |
428 | ||
429 | long keyctl(KEYCTL_CHOWN, key_serial_t key, uid_t uid, gid_t gid); | |
430 | ||
76d8aeab DH |
431 | This function permits a key's owner and group ID to be changed. Either one |
432 | of uid or gid can be set to -1 to suppress that change. | |
1da177e4 LT |
433 | |
434 | Only the superuser can change a key's owner to something other than the | |
435 | key's current owner. Similarly, only the superuser can change a key's | |
436 | group ID to something other than the calling process's group ID or one of | |
437 | its group list members. | |
438 | ||
439 | ||
440 | (*) Change the permissions mask on a key: | |
441 | ||
442 | long keyctl(KEYCTL_SETPERM, key_serial_t key, key_perm_t perm); | |
443 | ||
444 | This function permits the owner of a key or the superuser to change the | |
445 | permissions mask on a key. | |
446 | ||
447 | Only bits the available bits are permitted; if any other bits are set, | |
448 | error EINVAL will be returned. | |
449 | ||
450 | ||
451 | (*) Describe a key: | |
452 | ||
453 | long keyctl(KEYCTL_DESCRIBE, key_serial_t key, char *buffer, | |
454 | size_t buflen); | |
455 | ||
456 | This function returns a summary of the key's attributes (but not its | |
457 | payload data) as a string in the buffer provided. | |
458 | ||
459 | Unless there's an error, it always returns the amount of data it could | |
460 | produce, even if that's too big for the buffer, but it won't copy more | |
461 | than requested to userspace. If the buffer pointer is NULL then no copy | |
462 | will take place. | |
463 | ||
464 | A process must have view permission on the key for this function to be | |
465 | successful. | |
466 | ||
467 | If successful, a string is placed in the buffer in the following format: | |
468 | ||
469 | <type>;<uid>;<gid>;<perm>;<description> | |
470 | ||
471 | Where type and description are strings, uid and gid are decimal, and perm | |
472 | is hexadecimal. A NUL character is included at the end of the string if | |
473 | the buffer is sufficiently big. | |
474 | ||
475 | This can be parsed with | |
476 | ||
477 | sscanf(buffer, "%[^;];%d;%d;%o;%s", type, &uid, &gid, &mode, desc); | |
478 | ||
479 | ||
480 | (*) Clear out a keyring: | |
481 | ||
482 | long keyctl(KEYCTL_CLEAR, key_serial_t keyring); | |
483 | ||
484 | This function clears the list of keys attached to a keyring. The calling | |
485 | process must have write permission on the keyring, and it must be a | |
486 | keyring (or else error ENOTDIR will result). | |
487 | ||
488 | ||
489 | (*) Link a key into a keyring: | |
490 | ||
491 | long keyctl(KEYCTL_LINK, key_serial_t keyring, key_serial_t key); | |
492 | ||
76d8aeab DH |
493 | This function creates a link from the keyring to the key. The process must |
494 | have write permission on the keyring and must have link permission on the | |
495 | key. | |
1da177e4 | 496 | |
76d8aeab DH |
497 | Should the keyring not be a keyring, error ENOTDIR will result; and if the |
498 | keyring is full, error ENFILE will result. | |
1da177e4 LT |
499 | |
500 | The link procedure checks the nesting of the keyrings, returning ELOOP if | |
017679c4 | 501 | it appears too deep or EDEADLK if the link would introduce a cycle. |
1da177e4 | 502 | |
cab8eb59 DH |
503 | Any links within the keyring to keys that match the new key in terms of |
504 | type and description will be discarded from the keyring as the new one is | |
505 | added. | |
506 | ||
1da177e4 LT |
507 | |
508 | (*) Unlink a key or keyring from another keyring: | |
509 | ||
510 | long keyctl(KEYCTL_UNLINK, key_serial_t keyring, key_serial_t key); | |
511 | ||
512 | This function looks through the keyring for the first link to the | |
513 | specified key, and removes it if found. Subsequent links to that key are | |
514 | ignored. The process must have write permission on the keyring. | |
515 | ||
76d8aeab DH |
516 | If the keyring is not a keyring, error ENOTDIR will result; and if the key |
517 | is not present, error ENOENT will be the result. | |
1da177e4 LT |
518 | |
519 | ||
520 | (*) Search a keyring tree for a key: | |
521 | ||
522 | key_serial_t keyctl(KEYCTL_SEARCH, key_serial_t keyring, | |
523 | const char *type, const char *description, | |
524 | key_serial_t dest_keyring); | |
525 | ||
76d8aeab DH |
526 | This searches the keyring tree headed by the specified keyring until a key |
527 | is found that matches the type and description criteria. Each keyring is | |
528 | checked for keys before recursion into its children occurs. | |
1da177e4 LT |
529 | |
530 | The process must have search permission on the top level keyring, or else | |
531 | error EACCES will result. Only keyrings that the process has search | |
532 | permission on will be recursed into, and only keys and keyrings for which | |
533 | a process has search permission can be matched. If the specified keyring | |
534 | is not a keyring, ENOTDIR will result. | |
535 | ||
536 | If the search succeeds, the function will attempt to link the found key | |
537 | into the destination keyring if one is supplied (non-zero ID). All the | |
538 | constraints applicable to KEYCTL_LINK apply in this case too. | |
539 | ||
540 | Error ENOKEY, EKEYREVOKED or EKEYEXPIRED will be returned if the search | |
541 | fails. On success, the resulting key ID will be returned. | |
542 | ||
543 | ||
544 | (*) Read the payload data from a key: | |
545 | ||
f1a9badc DH |
546 | long keyctl(KEYCTL_READ, key_serial_t keyring, char *buffer, |
547 | size_t buflen); | |
1da177e4 LT |
548 | |
549 | This function attempts to read the payload data from the specified key | |
550 | into the buffer. The process must have read permission on the key to | |
551 | succeed. | |
552 | ||
553 | The returned data will be processed for presentation by the key type. For | |
554 | instance, a keyring will return an array of key_serial_t entries | |
555 | representing the IDs of all the keys to which it is subscribed. The user | |
556 | defined key type will return its data as is. If a key type does not | |
557 | implement this function, error EOPNOTSUPP will result. | |
558 | ||
559 | As much of the data as can be fitted into the buffer will be copied to | |
560 | userspace if the buffer pointer is not NULL. | |
561 | ||
76d8aeab DH |
562 | On a successful return, the function will always return the amount of data |
563 | available rather than the amount copied. | |
1da177e4 LT |
564 | |
565 | ||
566 | (*) Instantiate a partially constructed key. | |
567 | ||
f1a9badc DH |
568 | long keyctl(KEYCTL_INSTANTIATE, key_serial_t key, |
569 | const void *payload, size_t plen, | |
570 | key_serial_t keyring); | |
1da177e4 LT |
571 | |
572 | If the kernel calls back to userspace to complete the instantiation of a | |
573 | key, userspace should use this call to supply data for the key before the | |
574 | invoked process returns, or else the key will be marked negative | |
575 | automatically. | |
576 | ||
577 | The process must have write access on the key to be able to instantiate | |
578 | it, and the key must be uninstantiated. | |
579 | ||
580 | If a keyring is specified (non-zero), the key will also be linked into | |
76d8aeab DH |
581 | that keyring, however all the constraints applying in KEYCTL_LINK apply in |
582 | this case too. | |
1da177e4 LT |
583 | |
584 | The payload and plen arguments describe the payload data as for add_key(). | |
585 | ||
586 | ||
587 | (*) Negatively instantiate a partially constructed key. | |
588 | ||
f1a9badc DH |
589 | long keyctl(KEYCTL_NEGATE, key_serial_t key, |
590 | unsigned timeout, key_serial_t keyring); | |
1da177e4 LT |
591 | |
592 | If the kernel calls back to userspace to complete the instantiation of a | |
593 | key, userspace should use this call mark the key as negative before the | |
594 | invoked process returns if it is unable to fulfil the request. | |
595 | ||
596 | The process must have write access on the key to be able to instantiate | |
597 | it, and the key must be uninstantiated. | |
598 | ||
599 | If a keyring is specified (non-zero), the key will also be linked into | |
76d8aeab DH |
600 | that keyring, however all the constraints applying in KEYCTL_LINK apply in |
601 | this case too. | |
1da177e4 LT |
602 | |
603 | ||
3e30148c DH |
604 | (*) Set the default request-key destination keyring. |
605 | ||
606 | long keyctl(KEYCTL_SET_REQKEY_KEYRING, int reqkey_defl); | |
607 | ||
608 | This sets the default keyring to which implicitly requested keys will be | |
609 | attached for this thread. reqkey_defl should be one of these constants: | |
610 | ||
611 | CONSTANT VALUE NEW DEFAULT KEYRING | |
612 | ====================================== ====== ======================= | |
613 | KEY_REQKEY_DEFL_NO_CHANGE -1 No change | |
614 | KEY_REQKEY_DEFL_DEFAULT 0 Default[1] | |
615 | KEY_REQKEY_DEFL_THREAD_KEYRING 1 Thread keyring | |
616 | KEY_REQKEY_DEFL_PROCESS_KEYRING 2 Process keyring | |
617 | KEY_REQKEY_DEFL_SESSION_KEYRING 3 Session keyring | |
618 | KEY_REQKEY_DEFL_USER_KEYRING 4 User keyring | |
619 | KEY_REQKEY_DEFL_USER_SESSION_KEYRING 5 User session keyring | |
620 | KEY_REQKEY_DEFL_GROUP_KEYRING 6 Group keyring | |
621 | ||
622 | The old default will be returned if successful and error EINVAL will be | |
623 | returned if reqkey_defl is not one of the above values. | |
624 | ||
625 | The default keyring can be overridden by the keyring indicated to the | |
626 | request_key() system call. | |
627 | ||
628 | Note that this setting is inherited across fork/exec. | |
629 | ||
630 | [1] The default default is: the thread keyring if there is one, otherwise | |
631 | the process keyring if there is one, otherwise the session keyring if | |
632 | there is one, otherwise the user default session keyring. | |
633 | ||
634 | ||
017679c4 DH |
635 | (*) Set the timeout on a key. |
636 | ||
637 | long keyctl(KEYCTL_SET_TIMEOUT, key_serial_t key, unsigned timeout); | |
638 | ||
639 | This sets or clears the timeout on a key. The timeout can be 0 to clear | |
640 | the timeout or a number of seconds to set the expiry time that far into | |
641 | the future. | |
642 | ||
643 | The process must have attribute modification access on a key to set its | |
644 | timeout. Timeouts may not be set with this function on negative, revoked | |
645 | or expired keys. | |
646 | ||
647 | ||
1da177e4 LT |
648 | =============== |
649 | KERNEL SERVICES | |
650 | =============== | |
651 | ||
652 | The kernel services for key managment are fairly simple to deal with. They can | |
653 | be broken down into two areas: keys and key types. | |
654 | ||
655 | Dealing with keys is fairly straightforward. Firstly, the kernel service | |
656 | registers its type, then it searches for a key of that type. It should retain | |
657 | the key as long as it has need of it, and then it should release it. For a | |
76d8aeab DH |
658 | filesystem or device file, a search would probably be performed during the open |
659 | call, and the key released upon close. How to deal with conflicting keys due to | |
660 | two different users opening the same file is left to the filesystem author to | |
661 | solve. | |
662 | ||
664cceb0 DH |
663 | Note that there are two different types of pointers to keys that may be |
664 | encountered: | |
665 | ||
666 | (*) struct key * | |
667 | ||
668 | This simply points to the key structure itself. Key structures will be at | |
669 | least four-byte aligned. | |
670 | ||
671 | (*) key_ref_t | |
672 | ||
673 | This is equivalent to a struct key *, but the least significant bit is set | |
674 | if the caller "possesses" the key. By "possession" it is meant that the | |
675 | calling processes has a searchable link to the key from one of its | |
676 | keyrings. There are three functions for dealing with these: | |
677 | ||
678 | key_ref_t make_key_ref(const struct key *key, | |
679 | unsigned long possession); | |
680 | ||
681 | struct key *key_ref_to_ptr(const key_ref_t key_ref); | |
682 | ||
683 | unsigned long is_key_possessed(const key_ref_t key_ref); | |
684 | ||
685 | The first function constructs a key reference from a key pointer and | |
686 | possession information (which must be 0 or 1 and not any other value). | |
687 | ||
688 | The second function retrieves the key pointer from a reference and the | |
689 | third retrieves the possession flag. | |
690 | ||
76d8aeab DH |
691 | When accessing a key's payload contents, certain precautions must be taken to |
692 | prevent access vs modification races. See the section "Notes on accessing | |
693 | payload contents" for more information. | |
1da177e4 LT |
694 | |
695 | (*) To search for a key, call: | |
696 | ||
697 | struct key *request_key(const struct key_type *type, | |
698 | const char *description, | |
699 | const char *callout_string); | |
700 | ||
701 | This is used to request a key or keyring with a description that matches | |
702 | the description specified according to the key type's match function. This | |
703 | permits approximate matching to occur. If callout_string is not NULL, then | |
704 | /sbin/request-key will be invoked in an attempt to obtain the key from | |
705 | userspace. In that case, callout_string will be passed as an argument to | |
706 | the program. | |
707 | ||
708 | Should the function fail error ENOKEY, EKEYEXPIRED or EKEYREVOKED will be | |
709 | returned. | |
710 | ||
3e30148c DH |
711 | If successful, the key will have been attached to the default keyring for |
712 | implicitly obtained request-key keys, as set by KEYCTL_SET_REQKEY_KEYRING. | |
713 | ||
f1a9badc DH |
714 | See also Documentation/keys-request-key.txt. |
715 | ||
1da177e4 LT |
716 | |
717 | (*) When it is no longer required, the key should be released using: | |
718 | ||
719 | void key_put(struct key *key); | |
720 | ||
664cceb0 DH |
721 | Or: |
722 | ||
723 | void key_ref_put(key_ref_t key_ref); | |
724 | ||
725 | These can be called from interrupt context. If CONFIG_KEYS is not set then | |
1da177e4 LT |
726 | the argument will not be parsed. |
727 | ||
728 | ||
729 | (*) Extra references can be made to a key by calling the following function: | |
730 | ||
731 | struct key *key_get(struct key *key); | |
732 | ||
733 | These need to be disposed of by calling key_put() when they've been | |
734 | finished with. The key pointer passed in will be returned. If the pointer | |
735 | is NULL or CONFIG_KEYS is not set then the key will not be dereferenced and | |
736 | no increment will take place. | |
737 | ||
738 | ||
739 | (*) A key's serial number can be obtained by calling: | |
740 | ||
741 | key_serial_t key_serial(struct key *key); | |
742 | ||
743 | If key is NULL or if CONFIG_KEYS is not set then 0 will be returned (in the | |
744 | latter case without parsing the argument). | |
745 | ||
746 | ||
747 | (*) If a keyring was found in the search, this can be further searched by: | |
748 | ||
664cceb0 DH |
749 | key_ref_t keyring_search(key_ref_t keyring_ref, |
750 | const struct key_type *type, | |
751 | const char *description) | |
1da177e4 LT |
752 | |
753 | This searches the keyring tree specified for a matching key. Error ENOKEY | |
664cceb0 DH |
754 | is returned upon failure (use IS_ERR/PTR_ERR to determine). If successful, |
755 | the returned key will need to be released. | |
756 | ||
757 | The possession attribute from the keyring reference is used to control | |
758 | access through the permissions mask and is propagated to the returned key | |
759 | reference pointer if successful. | |
1da177e4 LT |
760 | |
761 | ||
762 | (*) To check the validity of a key, this function can be called: | |
763 | ||
764 | int validate_key(struct key *key); | |
765 | ||
766 | This checks that the key in question hasn't expired or and hasn't been | |
767 | revoked. Should the key be invalid, error EKEYEXPIRED or EKEYREVOKED will | |
768 | be returned. If the key is NULL or if CONFIG_KEYS is not set then 0 will be | |
769 | returned (in the latter case without parsing the argument). | |
770 | ||
771 | ||
772 | (*) To register a key type, the following function should be called: | |
773 | ||
774 | int register_key_type(struct key_type *type); | |
775 | ||
776 | This will return error EEXIST if a type of the same name is already | |
777 | present. | |
778 | ||
779 | ||
780 | (*) To unregister a key type, call: | |
781 | ||
782 | void unregister_key_type(struct key_type *type); | |
783 | ||
784 | ||
76d8aeab DH |
785 | =================================== |
786 | NOTES ON ACCESSING PAYLOAD CONTENTS | |
787 | =================================== | |
788 | ||
789 | The simplest payload is just a number in key->payload.value. In this case, | |
790 | there's no need to indulge in RCU or locking when accessing the payload. | |
791 | ||
792 | More complex payload contents must be allocated and a pointer to them set in | |
793 | key->payload.data. One of the following ways must be selected to access the | |
794 | data: | |
795 | ||
664cceb0 | 796 | (1) Unmodifiable key type. |
76d8aeab DH |
797 | |
798 | If the key type does not have a modify method, then the key's payload can | |
799 | be accessed without any form of locking, provided that it's known to be | |
800 | instantiated (uninstantiated keys cannot be "found"). | |
801 | ||
802 | (2) The key's semaphore. | |
803 | ||
804 | The semaphore could be used to govern access to the payload and to control | |
805 | the payload pointer. It must be write-locked for modifications and would | |
806 | have to be read-locked for general access. The disadvantage of doing this | |
807 | is that the accessor may be required to sleep. | |
808 | ||
809 | (3) RCU. | |
810 | ||
811 | RCU must be used when the semaphore isn't already held; if the semaphore | |
812 | is held then the contents can't change under you unexpectedly as the | |
813 | semaphore must still be used to serialise modifications to the key. The | |
814 | key management code takes care of this for the key type. | |
815 | ||
816 | However, this means using: | |
817 | ||
818 | rcu_read_lock() ... rcu_dereference() ... rcu_read_unlock() | |
819 | ||
820 | to read the pointer, and: | |
821 | ||
822 | rcu_dereference() ... rcu_assign_pointer() ... call_rcu() | |
823 | ||
824 | to set the pointer and dispose of the old contents after a grace period. | |
825 | Note that only the key type should ever modify a key's payload. | |
826 | ||
827 | Furthermore, an RCU controlled payload must hold a struct rcu_head for the | |
828 | use of call_rcu() and, if the payload is of variable size, the length of | |
829 | the payload. key->datalen cannot be relied upon to be consistent with the | |
830 | payload just dereferenced if the key's semaphore is not held. | |
831 | ||
832 | ||
1da177e4 LT |
833 | =================== |
834 | DEFINING A KEY TYPE | |
835 | =================== | |
836 | ||
837 | A kernel service may want to define its own key type. For instance, an AFS | |
838 | filesystem might want to define a Kerberos 5 ticket key type. To do this, it | |
839 | author fills in a struct key_type and registers it with the system. | |
840 | ||
841 | The structure has a number of fields, some of which are mandatory: | |
842 | ||
843 | (*) const char *name | |
844 | ||
845 | The name of the key type. This is used to translate a key type name | |
846 | supplied by userspace into a pointer to the structure. | |
847 | ||
848 | ||
849 | (*) size_t def_datalen | |
850 | ||
851 | This is optional - it supplies the default payload data length as | |
852 | contributed to the quota. If the key type's payload is always or almost | |
853 | always the same size, then this is a more efficient way to do things. | |
854 | ||
855 | The data length (and quota) on a particular key can always be changed | |
856 | during instantiation or update by calling: | |
857 | ||
858 | int key_payload_reserve(struct key *key, size_t datalen); | |
859 | ||
76d8aeab DH |
860 | With the revised data length. Error EDQUOT will be returned if this is not |
861 | viable. | |
1da177e4 LT |
862 | |
863 | ||
864 | (*) int (*instantiate)(struct key *key, const void *data, size_t datalen); | |
865 | ||
866 | This method is called to attach a payload to a key during construction. | |
76d8aeab DH |
867 | The payload attached need not bear any relation to the data passed to this |
868 | function. | |
1da177e4 LT |
869 | |
870 | If the amount of data attached to the key differs from the size in | |
871 | keytype->def_datalen, then key_payload_reserve() should be called. | |
872 | ||
873 | This method does not have to lock the key in order to attach a payload. | |
874 | The fact that KEY_FLAG_INSTANTIATED is not set in key->flags prevents | |
875 | anything else from gaining access to the key. | |
876 | ||
76d8aeab | 877 | It is safe to sleep in this method. |
1da177e4 LT |
878 | |
879 | ||
1da177e4 LT |
880 | (*) int (*update)(struct key *key, const void *data, size_t datalen); |
881 | ||
76d8aeab DH |
882 | If this type of key can be updated, then this method should be provided. |
883 | It is called to update a key's payload from the blob of data provided. | |
1da177e4 LT |
884 | |
885 | key_payload_reserve() should be called if the data length might change | |
76d8aeab DH |
886 | before any changes are actually made. Note that if this succeeds, the type |
887 | is committed to changing the key because it's already been altered, so all | |
888 | memory allocation must be done first. | |
889 | ||
890 | The key will have its semaphore write-locked before this method is called, | |
891 | but this only deters other writers; any changes to the key's payload must | |
892 | be made under RCU conditions, and call_rcu() must be used to dispose of | |
893 | the old payload. | |
1da177e4 | 894 | |
76d8aeab DH |
895 | key_payload_reserve() should be called before the changes are made, but |
896 | after all allocations and other potentially failing function calls are | |
897 | made. | |
1da177e4 | 898 | |
76d8aeab | 899 | It is safe to sleep in this method. |
1da177e4 LT |
900 | |
901 | ||
902 | (*) int (*match)(const struct key *key, const void *desc); | |
903 | ||
904 | This method is called to match a key against a description. It should | |
905 | return non-zero if the two match, zero if they don't. | |
906 | ||
907 | This method should not need to lock the key in any way. The type and | |
908 | description can be considered invariant, and the payload should not be | |
909 | accessed (the key may not yet be instantiated). | |
910 | ||
911 | It is not safe to sleep in this method; the caller may hold spinlocks. | |
912 | ||
913 | ||
914 | (*) void (*destroy)(struct key *key); | |
915 | ||
76d8aeab DH |
916 | This method is optional. It is called to discard the payload data on a key |
917 | when it is being destroyed. | |
1da177e4 | 918 | |
76d8aeab DH |
919 | This method does not need to lock the key to access the payload; it can |
920 | consider the key as being inaccessible at this time. Note that the key's | |
921 | type may have been changed before this function is called. | |
1da177e4 LT |
922 | |
923 | It is not safe to sleep in this method; the caller may hold spinlocks. | |
924 | ||
925 | ||
926 | (*) void (*describe)(const struct key *key, struct seq_file *p); | |
927 | ||
928 | This method is optional. It is called during /proc/keys reading to | |
929 | summarise a key's description and payload in text form. | |
930 | ||
76d8aeab DH |
931 | This method will be called with the RCU read lock held. rcu_dereference() |
932 | should be used to read the payload pointer if the payload is to be | |
933 | accessed. key->datalen cannot be trusted to stay consistent with the | |
934 | contents of the payload. | |
935 | ||
936 | The description will not change, though the key's state may. | |
937 | ||
938 | It is not safe to sleep in this method; the RCU read lock is held by the | |
939 | caller. | |
1da177e4 LT |
940 | |
941 | ||
942 | (*) long (*read)(const struct key *key, char __user *buffer, size_t buflen); | |
943 | ||
944 | This method is optional. It is called by KEYCTL_READ to translate the | |
76d8aeab DH |
945 | key's payload into something a blob of data for userspace to deal with. |
946 | Ideally, the blob should be in the same format as that passed in to the | |
947 | instantiate and update methods. | |
1da177e4 LT |
948 | |
949 | If successful, the blob size that could be produced should be returned | |
950 | rather than the size copied. | |
951 | ||
76d8aeab DH |
952 | This method will be called with the key's semaphore read-locked. This will |
953 | prevent the key's payload changing. It is not necessary to use RCU locking | |
954 | when accessing the key's payload. It is safe to sleep in this method, such | |
955 | as might happen when the userspace buffer is accessed. | |
1da177e4 LT |
956 | |
957 | ||
958 | ============================ | |
959 | REQUEST-KEY CALLBACK SERVICE | |
960 | ============================ | |
961 | ||
962 | To create a new key, the kernel will attempt to execute the following command | |
963 | line: | |
964 | ||
965 | /sbin/request-key create <key> <uid> <gid> \ | |
966 | <threadring> <processring> <sessionring> <callout_info> | |
967 | ||
968 | <key> is the key being constructed, and the three keyrings are the process | |
969 | keyrings from the process that caused the search to be issued. These are | |
970 | included for two reasons: | |
971 | ||
972 | (1) There may be an authentication token in one of the keyrings that is | |
973 | required to obtain the key, eg: a Kerberos Ticket-Granting Ticket. | |
974 | ||
975 | (2) The new key should probably be cached in one of these rings. | |
976 | ||
977 | This program should set it UID and GID to those specified before attempting to | |
978 | access any more keys. It may then look around for a user specific process to | |
979 | hand the request off to (perhaps a path held in placed in another key by, for | |
980 | example, the KDE desktop manager). | |
981 | ||
982 | The program (or whatever it calls) should finish construction of the key by | |
983 | calling KEYCTL_INSTANTIATE, which also permits it to cache the key in one of | |
984 | the keyrings (probably the session ring) before returning. Alternatively, the | |
985 | key can be marked as negative with KEYCTL_NEGATE; this also permits the key to | |
986 | be cached in one of the keyrings. | |
987 | ||
988 | If it returns with the key remaining in the unconstructed state, the key will | |
989 | be marked as being negative, it will be added to the session keyring, and an | |
990 | error will be returned to the key requestor. | |
991 | ||
76d8aeab DH |
992 | Supplementary information may be provided from whoever or whatever invoked this |
993 | service. This will be passed as the <callout_info> parameter. If no such | |
1da177e4 LT |
994 | information was made available, then "-" will be passed as this parameter |
995 | instead. | |
996 | ||
997 | ||
998 | Similarly, the kernel may attempt to update an expired or a soon to expire key | |
999 | by executing: | |
1000 | ||
1001 | /sbin/request-key update <key> <uid> <gid> \ | |
1002 | <threadring> <processring> <sessionring> | |
1003 | ||
1004 | In this case, the program isn't required to actually attach the key to a ring; | |
1005 | the rings are provided for reference. |