1 ============================
2 KERNEL KEY RETENTION SERVICE
3 ============================
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.
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.
13 The key service can be configured on by enabling:
15 "Security options"/"Enable access key retention support" (CONFIG_KEYS)
17 This document has the following sections:
20 - Key service overview
21 - Key access permissions
24 - Userspace system call interface
26 - Notes on accessing payload contents
28 - Request-key callback service
29 - Key access filesystem
36 In this context, keys represent units of cryptographic data, authentication
37 tokens, keyrings, etc.. These are represented in the kernel by struct key.
39 Each key has a number of attributes:
43 - A description (for matching a key in a search).
44 - Access control information.
50 (*) Each key is issued a serial number of type key_serial_t that is unique for
51 the lifetime of that key. All serial numbers are positive non-zero 32-bit
54 Userspace programs can use a key's serial numbers as a way to gain access
55 to it, subject to permission checking.
57 (*) Each key is of a defined "type". Types must be registered inside the
58 kernel by a kernel service (such as a filesystem) before keys of that type
59 can be added or used. Userspace programs cannot define new types directly.
61 Key types are represented in the kernel by struct key_type. This defines a
62 number of operations that can be performed on a key of that type.
64 Should a type be removed from the system, all the keys of that type will
67 (*) Each key has a description. This should be a printable string. The key
68 type provides an operation to perform a match between the description on a
69 key and a criterion string.
71 (*) Each key has an owner user ID, a group ID and a permissions mask. These
72 are used to control what a process may do to a key from userspace, and
73 whether a kernel service will be able to find the key.
75 (*) Each key can be set to expire at a specific time by the key type's
76 instantiation function. Keys can also be immortal.
78 (*) Each key can have a payload. This is a quantity of data that represent the
79 actual "key". In the case of a keyring, this is a list of keys to which
80 the keyring links; in the case of a user-defined key, it's an arbitrary
83 Having a payload is not required; and the payload can, in fact, just be a
84 value stored in the struct key itself.
86 When a key is instantiated, the key type's instantiation function is
87 called with a blob of data, and that then creates the key's payload in
90 Similarly, when userspace wants to read back the contents of the key, if
91 permitted, another key type operation will be called to convert the key's
92 attached payload back into a blob of data.
94 (*) Each key can be in one of a number of basic states:
96 (*) Uninstantiated. The key exists, but does not have any data attached.
97 Keys being requested from userspace will be in this state.
99 (*) Instantiated. This is the normal state. The key is fully formed, and
102 (*) Negative. This is a relatively short-lived state. The key acts as a
103 note saying that a previous call out to userspace failed, and acts as
104 a throttle on key lookups. A negative key can be updated to a normal
107 (*) Expired. Keys can have lifetimes set. If their lifetime is exceeded,
108 they traverse to this state. An expired key can be updated back to a
111 (*) Revoked. A key is put in this state by userspace action. It can't be
112 found or operated upon (apart from by unlinking it).
114 (*) Dead. The key's type was unregistered, and so the key is now useless.
121 The key service provides a number of features besides keys:
123 (*) The key service defines two special key types:
127 Keyrings are special keys that contain a list of other keys. Keyring
128 lists can be modified using various system calls. Keyrings should not
129 be given a payload when created.
133 A key of this type has a description and a payload that are arbitrary
134 blobs of data. These can be created, updated and read by userspace,
135 and aren't intended for use by kernel services.
137 (*) Each process subscribes to three keyrings: a thread-specific keyring, a
138 process-specific keyring, and a session-specific keyring.
140 The thread-specific keyring is discarded from the child when any sort of
141 clone, fork, vfork or execve occurs. A new keyring is created only when
144 The process-specific keyring is replaced with an empty one in the child on
145 clone, fork, vfork unless CLONE_THREAD is supplied, in which case it is
146 shared. execve also discards the process's process keyring and creates a
149 The session-specific keyring is persistent across clone, fork, vfork and
150 execve, even when the latter executes a set-UID or set-GID binary. A
151 process can, however, replace its current session keyring with a new one
152 by using PR_JOIN_SESSION_KEYRING. It is permitted to request an anonymous
153 new one, or to attempt to create or join one of a specific name.
155 The ownership of the thread keyring changes when the real UID and GID of
158 (*) Each user ID resident in the system holds two special keyrings: a user
159 specific keyring and a default user session keyring. The default session
160 keyring is initialised with a link to the user-specific keyring.
162 When a process changes its real UID, if it used to have no session key, it
163 will be subscribed to the default session key for the new UID.
165 If a process attempts to access its session key when it doesn't have one,
166 it will be subscribed to the default for its current UID.
168 (*) Each user has two quotas against which the keys they own are tracked. One
169 limits the total number of keys and keyrings, the other limits the total
170 amount of description and payload space that can be consumed.
172 The user can view information on this and other statistics through procfs
175 Process-specific and thread-specific keyrings are not counted towards a
178 If a system call that modifies a key or keyring in some way would put the
179 user over quota, the operation is refused and error EDQUOT is returned.
181 (*) There's a system call interface by which userspace programs can create and
182 manipulate keys and keyrings.
184 (*) There's a kernel interface by which services can register types and search
187 (*) There's a way for the a search done from the kernel to call back to
188 userspace to request a key that can't be found in a process's keyrings.
190 (*) An optional filesystem is available through which the key database can be
191 viewed and manipulated.
194 ======================
195 KEY ACCESS PERMISSIONS
196 ======================
198 Keys have an owner user ID, a group access ID, and a permissions mask. The mask
199 has up to eight bits each for possessor, user, group and other access. Only
200 six of each set of eight bits are defined. These permissions granted are:
204 This permits a key or keyring's attributes to be viewed - including key
205 type and description.
209 This permits a key's payload to be viewed or a keyring's list of linked
214 This permits a key's payload to be instantiated or updated, or it allows a
215 link to be added to or removed from a keyring.
219 This permits keyrings to be searched and keys to be found. Searches can
220 only recurse into nested keyrings that have search permission set.
224 This permits a key or keyring to be linked to. To create a link from a
225 keyring to a key, a process must have Write permission on the keyring and
226 Link permission on the key.
230 This permits a key's UID, GID and permissions mask to be changed.
232 For changing the ownership, group ID or permissions mask, being the owner of
233 the key or having the sysadmin capability is sufficient.
240 The security class "key" has been added to SELinux so that mandatory access
241 controls can be applied to keys created within various contexts. This support
242 is preliminary, and is likely to change quite significantly in the near future.
243 Currently, all of the basic permissions explained above are provided in SELinux
244 as well; SELinux is simply invoked after all basic permission checks have been
247 The value of the file /proc/self/attr/keycreate influences the labeling of
248 newly-created keys. If the contents of that file correspond to an SELinux
249 security context, then the key will be assigned that context. Otherwise, the
250 key will be assigned the current context of the task that invoked the key
251 creation request. Tasks must be granted explicit permission to assign a
252 particular context to newly-created keys, using the "create" permission in the
255 The default keyrings associated with users will be labeled with the default
256 context of the user if and only if the login programs have been instrumented to
257 properly initialize keycreate during the login process. Otherwise, they will
258 be labeled with the context of the login program itself.
260 Note, however, that the default keyrings associated with the root user are
261 labeled with the default kernel context, since they are created early in the
262 boot process, before root has a chance to log in.
264 The keyrings associated with new threads are each labeled with the context of
265 their associated thread, and both session and process keyrings are handled
273 Two files have been added to procfs by which an administrator can find out
274 about the status of the key service:
278 This lists the keys that are currently viewable by the task reading the
279 file, giving information about their type, description and permissions.
280 It is not possible to view the payload of the key this way, though some
281 information about it may be given.
283 The only keys included in the list are those that grant View permission to
284 the reading process whether or not it possesses them. Note that LSM
285 security checks are still performed, and may further filter out keys that
286 the current process is not authorised to view.
288 The contents of the file look like this:
290 SERIAL FLAGS USAGE EXPY PERM UID GID TYPE DESCRIPTION: SUMMARY
291 00000001 I----- 39 perm 1f3f0000 0 0 keyring _uid_ses.0: 1/4
292 00000002 I----- 2 perm 1f3f0000 0 0 keyring _uid.0: empty
293 00000007 I----- 1 perm 1f3f0000 0 0 keyring _pid.1: empty
294 0000018d I----- 1 perm 1f3f0000 0 0 keyring _pid.412: empty
295 000004d2 I--Q-- 1 perm 1f3f0000 32 -1 keyring _uid.32: 1/4
296 000004d3 I--Q-- 3 perm 1f3f0000 32 -1 keyring _uid_ses.32: empty
297 00000892 I--QU- 1 perm 1f000000 0 0 user metal:copper: 0
298 00000893 I--Q-N 1 35s 1f3f0000 0 0 user metal:silver: 0
299 00000894 I--Q-- 1 10h 003f0000 0 0 user metal:gold: 0
306 Q Contributes to user's quota
307 U Under construction by callback to userspace
310 This file must be enabled at kernel configuration time as it allows anyone
311 to list the keys database.
315 This file lists the tracking data for each user that has at least one key
316 on the system. Such data includes quota information and statistics:
318 [root@andromeda root]# cat /proc/key-users
319 0: 46 45/45 1/100 13/10000
320 29: 2 2/2 2/100 40/10000
321 32: 2 2/2 2/100 40/10000
322 38: 2 2/2 2/100 40/10000
324 The format of each line is
325 <UID>: User ID to which this applies
326 <usage> Structure refcount
327 <inst>/<keys> Total number of keys and number instantiated
328 <keys>/<max> Key count quota
329 <bytes>/<max> Key size quota
332 ===============================
333 USERSPACE SYSTEM CALL INTERFACE
334 ===============================
336 Userspace can manipulate keys directly through three new syscalls: add_key,
337 request_key and keyctl. The latter provides a number of functions for
340 When referring to a key directly, userspace programs should use the key's
341 serial number (a positive 32-bit integer). However, there are some special
342 values available for referring to special keys and keyrings that relate to the
343 process making the call:
345 CONSTANT VALUE KEY REFERENCED
346 ============================== ====== ===========================
347 KEY_SPEC_THREAD_KEYRING -1 thread-specific keyring
348 KEY_SPEC_PROCESS_KEYRING -2 process-specific keyring
349 KEY_SPEC_SESSION_KEYRING -3 session-specific keyring
350 KEY_SPEC_USER_KEYRING -4 UID-specific keyring
351 KEY_SPEC_USER_SESSION_KEYRING -5 UID-session keyring
352 KEY_SPEC_GROUP_KEYRING -6 GID-specific keyring
353 KEY_SPEC_REQKEY_AUTH_KEY -7 assumed request_key()
357 The main syscalls are:
359 (*) Create a new key of given type, description and payload and add it to the
362 key_serial_t add_key(const char *type, const char *desc,
363 const void *payload, size_t plen,
364 key_serial_t keyring);
366 If a key of the same type and description as that proposed already exists
367 in the keyring, this will try to update it with the given payload, or it
368 will return error EEXIST if that function is not supported by the key
369 type. The process must also have permission to write to the key to be able
370 to update it. The new key will have all user permissions granted and no
371 group or third party permissions.
373 Otherwise, this will attempt to create a new key of the specified type and
374 description, and to instantiate it with the supplied payload and attach it
375 to the keyring. In this case, an error will be generated if the process
376 does not have permission to write to the keyring.
378 The payload is optional, and the pointer can be NULL if not required by
379 the type. The payload is plen in size, and plen can be zero for an empty
382 A new keyring can be generated by setting type "keyring", the keyring name
383 as the description (or NULL) and setting the payload to NULL.
385 User defined keys can be created by specifying type "user". It is
386 recommended that a user defined key's description by prefixed with a type
387 ID and a colon, such as "krb5tgt:" for a Kerberos 5 ticket granting
390 Any other type must have been registered with the kernel in advance by a
391 kernel service such as a filesystem.
393 The ID of the new or updated key is returned if successful.
396 (*) Search the process's keyrings for a key, potentially calling out to
397 userspace to create it.
399 key_serial_t request_key(const char *type, const char *description,
400 const char *callout_info,
401 key_serial_t dest_keyring);
403 This function searches all the process's keyrings in the order thread,
404 process, session for a matching key. This works very much like
405 KEYCTL_SEARCH, including the optional attachment of the discovered key to
408 If a key cannot be found, and if callout_info is not NULL, then
409 /sbin/request-key will be invoked in an attempt to obtain a key. The
410 callout_info string will be passed as an argument to the program.
412 See also Documentation/keys-request-key.txt.
415 The keyctl syscall functions are:
417 (*) Map a special key ID to a real key ID for this process:
419 key_serial_t keyctl(KEYCTL_GET_KEYRING_ID, key_serial_t id,
422 The special key specified by "id" is looked up (with the key being created
423 if necessary) and the ID of the key or keyring thus found is returned if
426 If the key does not yet exist, the key will be created if "create" is
427 non-zero; and the error ENOKEY will be returned if "create" is zero.
430 (*) Replace the session keyring this process subscribes to with a new one:
432 key_serial_t keyctl(KEYCTL_JOIN_SESSION_KEYRING, const char *name);
434 If name is NULL, an anonymous keyring is created attached to the process
435 as its session keyring, displacing the old session keyring.
437 If name is not NULL, if a keyring of that name exists, the process
438 attempts to attach it as the session keyring, returning an error if that
439 is not permitted; otherwise a new keyring of that name is created and
440 attached as the session keyring.
442 To attach to a named keyring, the keyring must have search permission for
443 the process's ownership.
445 The ID of the new session keyring is returned if successful.
448 (*) Update the specified key:
450 long keyctl(KEYCTL_UPDATE, key_serial_t key, const void *payload,
453 This will try to update the specified key with the given payload, or it
454 will return error EOPNOTSUPP if that function is not supported by the key
455 type. The process must also have permission to write to the key to be able
458 The payload is of length plen, and may be absent or empty as for
464 long keyctl(KEYCTL_REVOKE, key_serial_t key);
466 This makes a key unavailable for further operations. Further attempts to
467 use the key will be met with error EKEYREVOKED, and the key will no longer
471 (*) Change the ownership of a key:
473 long keyctl(KEYCTL_CHOWN, key_serial_t key, uid_t uid, gid_t gid);
475 This function permits a key's owner and group ID to be changed. Either one
476 of uid or gid can be set to -1 to suppress that change.
478 Only the superuser can change a key's owner to something other than the
479 key's current owner. Similarly, only the superuser can change a key's
480 group ID to something other than the calling process's group ID or one of
481 its group list members.
484 (*) Change the permissions mask on a key:
486 long keyctl(KEYCTL_SETPERM, key_serial_t key, key_perm_t perm);
488 This function permits the owner of a key or the superuser to change the
489 permissions mask on a key.
491 Only bits the available bits are permitted; if any other bits are set,
492 error EINVAL will be returned.
497 long keyctl(KEYCTL_DESCRIBE, key_serial_t key, char *buffer,
500 This function returns a summary of the key's attributes (but not its
501 payload data) as a string in the buffer provided.
503 Unless there's an error, it always returns the amount of data it could
504 produce, even if that's too big for the buffer, but it won't copy more
505 than requested to userspace. If the buffer pointer is NULL then no copy
508 A process must have view permission on the key for this function to be
511 If successful, a string is placed in the buffer in the following format:
513 <type>;<uid>;<gid>;<perm>;<description>
515 Where type and description are strings, uid and gid are decimal, and perm
516 is hexadecimal. A NUL character is included at the end of the string if
517 the buffer is sufficiently big.
519 This can be parsed with
521 sscanf(buffer, "%[^;];%d;%d;%o;%s", type, &uid, &gid, &mode, desc);
524 (*) Clear out a keyring:
526 long keyctl(KEYCTL_CLEAR, key_serial_t keyring);
528 This function clears the list of keys attached to a keyring. The calling
529 process must have write permission on the keyring, and it must be a
530 keyring (or else error ENOTDIR will result).
533 (*) Link a key into a keyring:
535 long keyctl(KEYCTL_LINK, key_serial_t keyring, key_serial_t key);
537 This function creates a link from the keyring to the key. The process must
538 have write permission on the keyring and must have link permission on the
541 Should the keyring not be a keyring, error ENOTDIR will result; and if the
542 keyring is full, error ENFILE will result.
544 The link procedure checks the nesting of the keyrings, returning ELOOP if
545 it appears too deep or EDEADLK if the link would introduce a cycle.
547 Any links within the keyring to keys that match the new key in terms of
548 type and description will be discarded from the keyring as the new one is
552 (*) Unlink a key or keyring from another keyring:
554 long keyctl(KEYCTL_UNLINK, key_serial_t keyring, key_serial_t key);
556 This function looks through the keyring for the first link to the
557 specified key, and removes it if found. Subsequent links to that key are
558 ignored. The process must have write permission on the keyring.
560 If the keyring is not a keyring, error ENOTDIR will result; and if the key
561 is not present, error ENOENT will be the result.
564 (*) Search a keyring tree for a key:
566 key_serial_t keyctl(KEYCTL_SEARCH, key_serial_t keyring,
567 const char *type, const char *description,
568 key_serial_t dest_keyring);
570 This searches the keyring tree headed by the specified keyring until a key
571 is found that matches the type and description criteria. Each keyring is
572 checked for keys before recursion into its children occurs.
574 The process must have search permission on the top level keyring, or else
575 error EACCES will result. Only keyrings that the process has search
576 permission on will be recursed into, and only keys and keyrings for which
577 a process has search permission can be matched. If the specified keyring
578 is not a keyring, ENOTDIR will result.
580 If the search succeeds, the function will attempt to link the found key
581 into the destination keyring if one is supplied (non-zero ID). All the
582 constraints applicable to KEYCTL_LINK apply in this case too.
584 Error ENOKEY, EKEYREVOKED or EKEYEXPIRED will be returned if the search
585 fails. On success, the resulting key ID will be returned.
588 (*) Read the payload data from a key:
590 long keyctl(KEYCTL_READ, key_serial_t keyring, char *buffer,
593 This function attempts to read the payload data from the specified key
594 into the buffer. The process must have read permission on the key to
597 The returned data will be processed for presentation by the key type. For
598 instance, a keyring will return an array of key_serial_t entries
599 representing the IDs of all the keys to which it is subscribed. The user
600 defined key type will return its data as is. If a key type does not
601 implement this function, error EOPNOTSUPP will result.
603 As much of the data as can be fitted into the buffer will be copied to
604 userspace if the buffer pointer is not NULL.
606 On a successful return, the function will always return the amount of data
607 available rather than the amount copied.
610 (*) Instantiate a partially constructed key.
612 long keyctl(KEYCTL_INSTANTIATE, key_serial_t key,
613 const void *payload, size_t plen,
614 key_serial_t keyring);
616 If the kernel calls back to userspace to complete the instantiation of a
617 key, userspace should use this call to supply data for the key before the
618 invoked process returns, or else the key will be marked negative
621 The process must have write access on the key to be able to instantiate
622 it, and the key must be uninstantiated.
624 If a keyring is specified (non-zero), the key will also be linked into
625 that keyring, however all the constraints applying in KEYCTL_LINK apply in
628 The payload and plen arguments describe the payload data as for add_key().
631 (*) Negatively instantiate a partially constructed key.
633 long keyctl(KEYCTL_NEGATE, key_serial_t key,
634 unsigned timeout, key_serial_t keyring);
636 If the kernel calls back to userspace to complete the instantiation of a
637 key, userspace should use this call mark the key as negative before the
638 invoked process returns if it is unable to fulfil the request.
640 The process must have write access on the key to be able to instantiate
641 it, and the key must be uninstantiated.
643 If a keyring is specified (non-zero), the key will also be linked into
644 that keyring, however all the constraints applying in KEYCTL_LINK apply in
648 (*) Set the default request-key destination keyring.
650 long keyctl(KEYCTL_SET_REQKEY_KEYRING, int reqkey_defl);
652 This sets the default keyring to which implicitly requested keys will be
653 attached for this thread. reqkey_defl should be one of these constants:
655 CONSTANT VALUE NEW DEFAULT KEYRING
656 ====================================== ====== =======================
657 KEY_REQKEY_DEFL_NO_CHANGE -1 No change
658 KEY_REQKEY_DEFL_DEFAULT 0 Default[1]
659 KEY_REQKEY_DEFL_THREAD_KEYRING 1 Thread keyring
660 KEY_REQKEY_DEFL_PROCESS_KEYRING 2 Process keyring
661 KEY_REQKEY_DEFL_SESSION_KEYRING 3 Session keyring
662 KEY_REQKEY_DEFL_USER_KEYRING 4 User keyring
663 KEY_REQKEY_DEFL_USER_SESSION_KEYRING 5 User session keyring
664 KEY_REQKEY_DEFL_GROUP_KEYRING 6 Group keyring
666 The old default will be returned if successful and error EINVAL will be
667 returned if reqkey_defl is not one of the above values.
669 The default keyring can be overridden by the keyring indicated to the
670 request_key() system call.
672 Note that this setting is inherited across fork/exec.
674 [1] The default is: the thread keyring if there is one, otherwise
675 the process keyring if there is one, otherwise the session keyring if
676 there is one, otherwise the user default session keyring.
679 (*) Set the timeout on a key.
681 long keyctl(KEYCTL_SET_TIMEOUT, key_serial_t key, unsigned timeout);
683 This sets or clears the timeout on a key. The timeout can be 0 to clear
684 the timeout or a number of seconds to set the expiry time that far into
687 The process must have attribute modification access on a key to set its
688 timeout. Timeouts may not be set with this function on negative, revoked
692 (*) Assume the authority granted to instantiate a key
694 long keyctl(KEYCTL_ASSUME_AUTHORITY, key_serial_t key);
696 This assumes or divests the authority required to instantiate the
697 specified key. Authority can only be assumed if the thread has the
698 authorisation key associated with the specified key in its keyrings
701 Once authority is assumed, searches for keys will also search the
702 requester's keyrings using the requester's security label, UID, GID and
705 If the requested authority is unavailable, error EPERM will be returned,
706 likewise if the authority has been revoked because the target key is
707 already instantiated.
709 If the specified key is 0, then any assumed authority will be divested.
711 The assumed authoritative key is inherited across fork and exec.
718 The kernel services for key management are fairly simple to deal with. They can
719 be broken down into two areas: keys and key types.
721 Dealing with keys is fairly straightforward. Firstly, the kernel service
722 registers its type, then it searches for a key of that type. It should retain
723 the key as long as it has need of it, and then it should release it. For a
724 filesystem or device file, a search would probably be performed during the open
725 call, and the key released upon close. How to deal with conflicting keys due to
726 two different users opening the same file is left to the filesystem author to
729 To access the key manager, the following header must be #included:
733 Specific key types should have a header file under include/keys/ that should be
734 used to access that type. For keys of type "user", for example, that would be:
738 Note that there are two different types of pointers to keys that may be
743 This simply points to the key structure itself. Key structures will be at
744 least four-byte aligned.
748 This is equivalent to a struct key *, but the least significant bit is set
749 if the caller "possesses" the key. By "possession" it is meant that the
750 calling processes has a searchable link to the key from one of its
751 keyrings. There are three functions for dealing with these:
753 key_ref_t make_key_ref(const struct key *key,
754 unsigned long possession);
756 struct key *key_ref_to_ptr(const key_ref_t key_ref);
758 unsigned long is_key_possessed(const key_ref_t key_ref);
760 The first function constructs a key reference from a key pointer and
761 possession information (which must be 0 or 1 and not any other value).
763 The second function retrieves the key pointer from a reference and the
764 third retrieves the possession flag.
766 When accessing a key's payload contents, certain precautions must be taken to
767 prevent access vs modification races. See the section "Notes on accessing
768 payload contents" for more information.
770 (*) To search for a key, call:
772 struct key *request_key(const struct key_type *type,
773 const char *description,
774 const char *callout_string);
776 This is used to request a key or keyring with a description that matches
777 the description specified according to the key type's match function. This
778 permits approximate matching to occur. If callout_string is not NULL, then
779 /sbin/request-key will be invoked in an attempt to obtain the key from
780 userspace. In that case, callout_string will be passed as an argument to
783 Should the function fail error ENOKEY, EKEYEXPIRED or EKEYREVOKED will be
786 If successful, the key will have been attached to the default keyring for
787 implicitly obtained request-key keys, as set by KEYCTL_SET_REQKEY_KEYRING.
789 See also Documentation/keys-request-key.txt.
792 (*) To search for a key, passing auxiliary data to the upcaller, call:
794 struct key *request_key_with_auxdata(const struct key_type *type,
795 const char *description,
796 const char *callout_string,
799 This is identical to request_key(), except that the auxiliary data is
800 passed to the key_type->request_key() op if it exists.
803 (*) A key can be requested asynchronously by calling one of:
805 struct key *request_key_async(const struct key_type *type,
806 const char *description,
807 const char *callout_string);
811 struct key *request_key_async_with_auxdata(const struct key_type *type,
812 const char *description,
813 const char *callout_string,
816 which are asynchronous equivalents of request_key() and
817 request_key_with_auxdata() respectively.
819 These two functions return with the key potentially still under
820 construction. To wait for contruction completion, the following should be
823 int wait_for_key_construction(struct key *key, bool intr);
825 The function will wait for the key to finish being constructed and then
826 invokes key_validate() to return an appropriate value to indicate the state
827 of the key (0 indicates the key is usable).
829 If intr is true, then the wait can be interrupted by a signal, in which
830 case error ERESTARTSYS will be returned.
833 (*) When it is no longer required, the key should be released using:
835 void key_put(struct key *key);
839 void key_ref_put(key_ref_t key_ref);
841 These can be called from interrupt context. If CONFIG_KEYS is not set then
842 the argument will not be parsed.
845 (*) Extra references can be made to a key by calling the following function:
847 struct key *key_get(struct key *key);
849 These need to be disposed of by calling key_put() when they've been
850 finished with. The key pointer passed in will be returned. If the pointer
851 is NULL or CONFIG_KEYS is not set then the key will not be dereferenced and
852 no increment will take place.
855 (*) A key's serial number can be obtained by calling:
857 key_serial_t key_serial(struct key *key);
859 If key is NULL or if CONFIG_KEYS is not set then 0 will be returned (in the
860 latter case without parsing the argument).
863 (*) If a keyring was found in the search, this can be further searched by:
865 key_ref_t keyring_search(key_ref_t keyring_ref,
866 const struct key_type *type,
867 const char *description)
869 This searches the keyring tree specified for a matching key. Error ENOKEY
870 is returned upon failure (use IS_ERR/PTR_ERR to determine). If successful,
871 the returned key will need to be released.
873 The possession attribute from the keyring reference is used to control
874 access through the permissions mask and is propagated to the returned key
875 reference pointer if successful.
878 (*) To check the validity of a key, this function can be called:
880 int validate_key(struct key *key);
882 This checks that the key in question hasn't expired or and hasn't been
883 revoked. Should the key be invalid, error EKEYEXPIRED or EKEYREVOKED will
884 be returned. If the key is NULL or if CONFIG_KEYS is not set then 0 will be
885 returned (in the latter case without parsing the argument).
888 (*) To register a key type, the following function should be called:
890 int register_key_type(struct key_type *type);
892 This will return error EEXIST if a type of the same name is already
896 (*) To unregister a key type, call:
898 void unregister_key_type(struct key_type *type);
901 Under some circumstances, it may be desirable to desirable to deal with a
902 bundle of keys. The facility provides access to the keyring type for managing
905 struct key_type key_type_keyring;
907 This can be used with a function such as request_key() to find a specific
908 keyring in a process's keyrings. A keyring thus found can then be searched
909 with keyring_search(). Note that it is not possible to use request_key() to
910 search a specific keyring, so using keyrings in this way is of limited utility.
913 ===================================
914 NOTES ON ACCESSING PAYLOAD CONTENTS
915 ===================================
917 The simplest payload is just a number in key->payload.value. In this case,
918 there's no need to indulge in RCU or locking when accessing the payload.
920 More complex payload contents must be allocated and a pointer to them set in
921 key->payload.data. One of the following ways must be selected to access the
924 (1) Unmodifiable key type.
926 If the key type does not have a modify method, then the key's payload can
927 be accessed without any form of locking, provided that it's known to be
928 instantiated (uninstantiated keys cannot be "found").
930 (2) The key's semaphore.
932 The semaphore could be used to govern access to the payload and to control
933 the payload pointer. It must be write-locked for modifications and would
934 have to be read-locked for general access. The disadvantage of doing this
935 is that the accessor may be required to sleep.
939 RCU must be used when the semaphore isn't already held; if the semaphore
940 is held then the contents can't change under you unexpectedly as the
941 semaphore must still be used to serialise modifications to the key. The
942 key management code takes care of this for the key type.
944 However, this means using:
946 rcu_read_lock() ... rcu_dereference() ... rcu_read_unlock()
948 to read the pointer, and:
950 rcu_dereference() ... rcu_assign_pointer() ... call_rcu()
952 to set the pointer and dispose of the old contents after a grace period.
953 Note that only the key type should ever modify a key's payload.
955 Furthermore, an RCU controlled payload must hold a struct rcu_head for the
956 use of call_rcu() and, if the payload is of variable size, the length of
957 the payload. key->datalen cannot be relied upon to be consistent with the
958 payload just dereferenced if the key's semaphore is not held.
965 A kernel service may want to define its own key type. For instance, an AFS
966 filesystem might want to define a Kerberos 5 ticket key type. To do this, it
967 author fills in a key_type struct and registers it with the system.
969 Source files that implement key types should include the following header file:
973 The structure has a number of fields, some of which are mandatory:
977 The name of the key type. This is used to translate a key type name
978 supplied by userspace into a pointer to the structure.
981 (*) size_t def_datalen
983 This is optional - it supplies the default payload data length as
984 contributed to the quota. If the key type's payload is always or almost
985 always the same size, then this is a more efficient way to do things.
987 The data length (and quota) on a particular key can always be changed
988 during instantiation or update by calling:
990 int key_payload_reserve(struct key *key, size_t datalen);
992 With the revised data length. Error EDQUOT will be returned if this is not
996 (*) int (*instantiate)(struct key *key, const void *data, size_t datalen);
998 This method is called to attach a payload to a key during construction.
999 The payload attached need not bear any relation to the data passed to this
1002 If the amount of data attached to the key differs from the size in
1003 keytype->def_datalen, then key_payload_reserve() should be called.
1005 This method does not have to lock the key in order to attach a payload.
1006 The fact that KEY_FLAG_INSTANTIATED is not set in key->flags prevents
1007 anything else from gaining access to the key.
1009 It is safe to sleep in this method.
1012 (*) int (*update)(struct key *key, const void *data, size_t datalen);
1014 If this type of key can be updated, then this method should be provided.
1015 It is called to update a key's payload from the blob of data provided.
1017 key_payload_reserve() should be called if the data length might change
1018 before any changes are actually made. Note that if this succeeds, the type
1019 is committed to changing the key because it's already been altered, so all
1020 memory allocation must be done first.
1022 The key will have its semaphore write-locked before this method is called,
1023 but this only deters other writers; any changes to the key's payload must
1024 be made under RCU conditions, and call_rcu() must be used to dispose of
1027 key_payload_reserve() should be called before the changes are made, but
1028 after all allocations and other potentially failing function calls are
1031 It is safe to sleep in this method.
1034 (*) int (*match)(const struct key *key, const void *desc);
1036 This method is called to match a key against a description. It should
1037 return non-zero if the two match, zero if they don't.
1039 This method should not need to lock the key in any way. The type and
1040 description can be considered invariant, and the payload should not be
1041 accessed (the key may not yet be instantiated).
1043 It is not safe to sleep in this method; the caller may hold spinlocks.
1046 (*) void (*revoke)(struct key *key);
1048 This method is optional. It is called to discard part of the payload
1049 data upon a key being revoked. The caller will have the key semaphore
1052 It is safe to sleep in this method, though care should be taken to avoid
1053 a deadlock against the key semaphore.
1056 (*) void (*destroy)(struct key *key);
1058 This method is optional. It is called to discard the payload data on a key
1059 when it is being destroyed.
1061 This method does not need to lock the key to access the payload; it can
1062 consider the key as being inaccessible at this time. Note that the key's
1063 type may have been changed before this function is called.
1065 It is not safe to sleep in this method; the caller may hold spinlocks.
1068 (*) void (*describe)(const struct key *key, struct seq_file *p);
1070 This method is optional. It is called during /proc/keys reading to
1071 summarise a key's description and payload in text form.
1073 This method will be called with the RCU read lock held. rcu_dereference()
1074 should be used to read the payload pointer if the payload is to be
1075 accessed. key->datalen cannot be trusted to stay consistent with the
1076 contents of the payload.
1078 The description will not change, though the key's state may.
1080 It is not safe to sleep in this method; the RCU read lock is held by the
1084 (*) long (*read)(const struct key *key, char __user *buffer, size_t buflen);
1086 This method is optional. It is called by KEYCTL_READ to translate the
1087 key's payload into something a blob of data for userspace to deal with.
1088 Ideally, the blob should be in the same format as that passed in to the
1089 instantiate and update methods.
1091 If successful, the blob size that could be produced should be returned
1092 rather than the size copied.
1094 This method will be called with the key's semaphore read-locked. This will
1095 prevent the key's payload changing. It is not necessary to use RCU locking
1096 when accessing the key's payload. It is safe to sleep in this method, such
1097 as might happen when the userspace buffer is accessed.
1100 (*) int (*request_key)(struct key_construction *cons, const char *op,
1103 This method is optional. If provided, request_key() and friends will
1104 invoke this function rather than upcalling to /sbin/request-key to operate
1105 upon a key of this type.
1107 The aux parameter is as passed to request_key_async_with_auxdata() and
1108 similar or is NULL otherwise. Also passed are the construction record for
1109 the key to be operated upon and the operation type (currently only
1112 This method is permitted to return before the upcall is complete, but the
1113 following function must be called under all circumstances to complete the
1114 instantiation process, whether or not it succeeds, whether or not there's
1117 void complete_request_key(struct key_construction *cons, int error);
1119 The error parameter should be 0 on success, -ve on error. The
1120 construction record is destroyed by this action and the authorisation key
1121 will be revoked. If an error is indicated, the key under construction
1122 will be negatively instantiated if it wasn't already instantiated.
1124 If this method returns an error, that error will be returned to the
1125 caller of request_key*(). complete_request_key() must be called prior to
1128 The key under construction and the authorisation key can be found in the
1129 key_construction struct pointed to by cons:
1131 (*) struct key *key;
1133 The key under construction.
1135 (*) struct key *authkey;
1137 The authorisation key.
1140 ============================
1141 REQUEST-KEY CALLBACK SERVICE
1142 ============================
1144 To create a new key, the kernel will attempt to execute the following command
1147 /sbin/request-key create <key> <uid> <gid> \
1148 <threadring> <processring> <sessionring> <callout_info>
1150 <key> is the key being constructed, and the three keyrings are the process
1151 keyrings from the process that caused the search to be issued. These are
1152 included for two reasons:
1154 (1) There may be an authentication token in one of the keyrings that is
1155 required to obtain the key, eg: a Kerberos Ticket-Granting Ticket.
1157 (2) The new key should probably be cached in one of these rings.
1159 This program should set it UID and GID to those specified before attempting to
1160 access any more keys. It may then look around for a user specific process to
1161 hand the request off to (perhaps a path held in placed in another key by, for
1162 example, the KDE desktop manager).
1164 The program (or whatever it calls) should finish construction of the key by
1165 calling KEYCTL_INSTANTIATE, which also permits it to cache the key in one of
1166 the keyrings (probably the session ring) before returning. Alternatively, the
1167 key can be marked as negative with KEYCTL_NEGATE; this also permits the key to
1168 be cached in one of the keyrings.
1170 If it returns with the key remaining in the unconstructed state, the key will
1171 be marked as being negative, it will be added to the session keyring, and an
1172 error will be returned to the key requestor.
1174 Supplementary information may be provided from whoever or whatever invoked this
1175 service. This will be passed as the <callout_info> parameter. If no such
1176 information was made available, then "-" will be passed as this parameter
1180 Similarly, the kernel may attempt to update an expired or a soon to expire key
1183 /sbin/request-key update <key> <uid> <gid> \
1184 <threadring> <processring> <sessionring>
1186 In this case, the program isn't required to actually attach the key to a ring;
1187 the rings are provided for reference.