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
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.
116 Keys in the last three states are subject to garbage collection. See the
117 section on "Garbage collection".
124 The key service provides a number of features besides keys:
126 (*) The key service defines two special key types:
130 Keyrings are special keys that contain a list of other keys. Keyring
131 lists can be modified using various system calls. Keyrings should not
132 be given a payload when created.
136 A key of this type has a description and a payload that are arbitrary
137 blobs of data. These can be created, updated and read by userspace,
138 and aren't intended for use by kernel services.
140 (*) Each process subscribes to three keyrings: a thread-specific keyring, a
141 process-specific keyring, and a session-specific keyring.
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
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
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.
158 The ownership of the thread keyring changes when the real UID and GID of
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.
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.
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.
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.
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").
179 Process-specific and thread-specific keyrings are not counted towards a
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.
185 (*) There's a system call interface by which userspace programs can create and
186 manipulate keys and keyrings.
188 (*) There's a kernel interface by which services can register types and search
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.
194 (*) An optional filesystem is available through which the key database can be
195 viewed and manipulated.
198 ======================
199 KEY ACCESS PERMISSIONS
200 ======================
202 Keys have an owner user ID, a group access ID, and a permissions mask. The mask
203 has up to eight bits each for possessor, user, group and other access. Only
204 six of each set of eight bits are defined. These permissions granted are:
208 This permits a key or keyring's attributes to be viewed - including key
209 type and description.
213 This permits a key's payload to be viewed or a keyring's list of linked
218 This permits a key's payload to be instantiated or updated, or it allows a
219 link to be added to or removed from a keyring.
223 This permits keyrings to be searched and keys to be found. Searches can
224 only recurse into nested keyrings that have search permission set.
228 This permits a key or keyring to be linked to. To create a link from a
229 keyring to a key, a process must have Write permission on the keyring and
230 Link permission on the key.
234 This permits a key's UID, GID and permissions mask to be changed.
236 For changing the ownership, group ID or permissions mask, being the owner of
237 the key or having the sysadmin capability is sufficient.
244 The security class "key" has been added to SELinux so that mandatory access
245 controls can be applied to keys created within various contexts. This support
246 is preliminary, and is likely to change quite significantly in the near future.
247 Currently, all of the basic permissions explained above are provided in SELinux
248 as well; SELinux is simply invoked after all basic permission checks have been
251 The value of the file /proc/self/attr/keycreate influences the labeling of
252 newly-created keys. If the contents of that file correspond to an SELinux
253 security context, then the key will be assigned that context. Otherwise, the
254 key will be assigned the current context of the task that invoked the key
255 creation request. Tasks must be granted explicit permission to assign a
256 particular context to newly-created keys, using the "create" permission in the
259 The default keyrings associated with users will be labeled with the default
260 context of the user if and only if the login programs have been instrumented to
261 properly initialize keycreate during the login process. Otherwise, they will
262 be labeled with the context of the login program itself.
264 Note, however, that the default keyrings associated with the root user are
265 labeled with the default kernel context, since they are created early in the
266 boot process, before root has a chance to log in.
268 The keyrings associated with new threads are each labeled with the context of
269 their associated thread, and both session and process keyrings are handled
277 Two files have been added to procfs by which an administrator can find out
278 about the status of the key service:
282 This lists the keys that are currently viewable by the task reading the
283 file, giving information about their type, description and permissions.
284 It is not possible to view the payload of the key this way, though some
285 information about it may be given.
287 The only keys included in the list are those that grant View permission to
288 the reading process whether or not it possesses them. Note that LSM
289 security checks are still performed, and may further filter out keys that
290 the current process is not authorised to view.
292 The contents of the file look like this:
294 SERIAL FLAGS USAGE EXPY PERM UID GID TYPE DESCRIPTION: SUMMARY
295 00000001 I----- 39 perm 1f3f0000 0 0 keyring _uid_ses.0: 1/4
296 00000002 I----- 2 perm 1f3f0000 0 0 keyring _uid.0: empty
297 00000007 I----- 1 perm 1f3f0000 0 0 keyring _pid.1: empty
298 0000018d I----- 1 perm 1f3f0000 0 0 keyring _pid.412: empty
299 000004d2 I--Q-- 1 perm 1f3f0000 32 -1 keyring _uid.32: 1/4
300 000004d3 I--Q-- 3 perm 1f3f0000 32 -1 keyring _uid_ses.32: empty
301 00000892 I--QU- 1 perm 1f000000 0 0 user metal:copper: 0
302 00000893 I--Q-N 1 35s 1f3f0000 0 0 user metal:silver: 0
303 00000894 I--Q-- 1 10h 003f0000 0 0 user metal:gold: 0
310 Q Contributes to user's quota
311 U Under construction by callback to userspace
314 This file must be enabled at kernel configuration time as it allows anyone
315 to list the keys database.
319 This file lists the tracking data for each user that has at least one key
320 on the system. Such data includes quota information and statistics:
322 [root@andromeda root]# cat /proc/key-users
323 0: 46 45/45 1/100 13/10000
324 29: 2 2/2 2/100 40/10000
325 32: 2 2/2 2/100 40/10000
326 38: 2 2/2 2/100 40/10000
328 The format of each line is
329 <UID>: User ID to which this applies
330 <usage> Structure refcount
331 <inst>/<keys> Total number of keys and number instantiated
332 <keys>/<max> Key count quota
333 <bytes>/<max> Key size quota
336 Four new sysctl files have been added also for the purpose of controlling the
337 quota limits on keys:
339 (*) /proc/sys/kernel/keys/root_maxkeys
340 /proc/sys/kernel/keys/root_maxbytes
342 These files hold the maximum number of keys that root may have and the
343 maximum total number of bytes of data that root may have stored in those
346 (*) /proc/sys/kernel/keys/maxkeys
347 /proc/sys/kernel/keys/maxbytes
349 These files hold the maximum number of keys that each non-root user may
350 have and the maximum total number of bytes of data that each of those
351 users may have stored in their keys.
353 Root may alter these by writing each new limit as a decimal number string to
354 the appropriate file.
357 ===============================
358 USERSPACE SYSTEM CALL INTERFACE
359 ===============================
361 Userspace can manipulate keys directly through three new syscalls: add_key,
362 request_key and keyctl. The latter provides a number of functions for
365 When referring to a key directly, userspace programs should use the key's
366 serial number (a positive 32-bit integer). However, there are some special
367 values available for referring to special keys and keyrings that relate to the
368 process making the call:
370 CONSTANT VALUE KEY REFERENCED
371 ============================== ====== ===========================
372 KEY_SPEC_THREAD_KEYRING -1 thread-specific keyring
373 KEY_SPEC_PROCESS_KEYRING -2 process-specific keyring
374 KEY_SPEC_SESSION_KEYRING -3 session-specific keyring
375 KEY_SPEC_USER_KEYRING -4 UID-specific keyring
376 KEY_SPEC_USER_SESSION_KEYRING -5 UID-session keyring
377 KEY_SPEC_GROUP_KEYRING -6 GID-specific keyring
378 KEY_SPEC_REQKEY_AUTH_KEY -7 assumed request_key()
382 The main syscalls are:
384 (*) Create a new key of given type, description and payload and add it to the
387 key_serial_t add_key(const char *type, const char *desc,
388 const void *payload, size_t plen,
389 key_serial_t keyring);
391 If a key of the same type and description as that proposed already exists
392 in the keyring, this will try to update it with the given payload, or it
393 will return error EEXIST if that function is not supported by the key
394 type. The process must also have permission to write to the key to be able
395 to update it. The new key will have all user permissions granted and no
396 group or third party permissions.
398 Otherwise, this will attempt to create a new key of the specified type and
399 description, and to instantiate it with the supplied payload and attach it
400 to the keyring. In this case, an error will be generated if the process
401 does not have permission to write to the keyring.
403 The payload is optional, and the pointer can be NULL if not required by
404 the type. The payload is plen in size, and plen can be zero for an empty
407 A new keyring can be generated by setting type "keyring", the keyring name
408 as the description (or NULL) and setting the payload to NULL.
410 User defined keys can be created by specifying type "user". It is
411 recommended that a user defined key's description by prefixed with a type
412 ID and a colon, such as "krb5tgt:" for a Kerberos 5 ticket granting
415 Any other type must have been registered with the kernel in advance by a
416 kernel service such as a filesystem.
418 The ID of the new or updated key is returned if successful.
421 (*) Search the process's keyrings for a key, potentially calling out to
422 userspace to create it.
424 key_serial_t request_key(const char *type, const char *description,
425 const char *callout_info,
426 key_serial_t dest_keyring);
428 This function searches all the process's keyrings in the order thread,
429 process, session for a matching key. This works very much like
430 KEYCTL_SEARCH, including the optional attachment of the discovered key to
433 If a key cannot be found, and if callout_info is not NULL, then
434 /sbin/request-key will be invoked in an attempt to obtain a key. The
435 callout_info string will be passed as an argument to the program.
437 See also Documentation/keys-request-key.txt.
440 The keyctl syscall functions are:
442 (*) Map a special key ID to a real key ID for this process:
444 key_serial_t keyctl(KEYCTL_GET_KEYRING_ID, key_serial_t id,
447 The special key specified by "id" is looked up (with the key being created
448 if necessary) and the ID of the key or keyring thus found is returned if
451 If the key does not yet exist, the key will be created if "create" is
452 non-zero; and the error ENOKEY will be returned if "create" is zero.
455 (*) Replace the session keyring this process subscribes to with a new one:
457 key_serial_t keyctl(KEYCTL_JOIN_SESSION_KEYRING, const char *name);
459 If name is NULL, an anonymous keyring is created attached to the process
460 as its session keyring, displacing the old session keyring.
462 If name is not NULL, if a keyring of that name exists, the process
463 attempts to attach it as the session keyring, returning an error if that
464 is not permitted; otherwise a new keyring of that name is created and
465 attached as the session keyring.
467 To attach to a named keyring, the keyring must have search permission for
468 the process's ownership.
470 The ID of the new session keyring is returned if successful.
473 (*) Update the specified key:
475 long keyctl(KEYCTL_UPDATE, key_serial_t key, const void *payload,
478 This will try to update the specified key with the given payload, or it
479 will return error EOPNOTSUPP if that function is not supported by the key
480 type. The process must also have permission to write to the key to be able
483 The payload is of length plen, and may be absent or empty as for
489 long keyctl(KEYCTL_REVOKE, key_serial_t key);
491 This makes a key unavailable for further operations. Further attempts to
492 use the key will be met with error EKEYREVOKED, and the key will no longer
496 (*) Change the ownership of a key:
498 long keyctl(KEYCTL_CHOWN, key_serial_t key, uid_t uid, gid_t gid);
500 This function permits a key's owner and group ID to be changed. Either one
501 of uid or gid can be set to -1 to suppress that change.
503 Only the superuser can change a key's owner to something other than the
504 key's current owner. Similarly, only the superuser can change a key's
505 group ID to something other than the calling process's group ID or one of
506 its group list members.
509 (*) Change the permissions mask on a key:
511 long keyctl(KEYCTL_SETPERM, key_serial_t key, key_perm_t perm);
513 This function permits the owner of a key or the superuser to change the
514 permissions mask on a key.
516 Only bits the available bits are permitted; if any other bits are set,
517 error EINVAL will be returned.
522 long keyctl(KEYCTL_DESCRIBE, key_serial_t key, char *buffer,
525 This function returns a summary of the key's attributes (but not its
526 payload data) as a string in the buffer provided.
528 Unless there's an error, it always returns the amount of data it could
529 produce, even if that's too big for the buffer, but it won't copy more
530 than requested to userspace. If the buffer pointer is NULL then no copy
533 A process must have view permission on the key for this function to be
536 If successful, a string is placed in the buffer in the following format:
538 <type>;<uid>;<gid>;<perm>;<description>
540 Where type and description are strings, uid and gid are decimal, and perm
541 is hexadecimal. A NUL character is included at the end of the string if
542 the buffer is sufficiently big.
544 This can be parsed with
546 sscanf(buffer, "%[^;];%d;%d;%o;%s", type, &uid, &gid, &mode, desc);
549 (*) Clear out a keyring:
551 long keyctl(KEYCTL_CLEAR, key_serial_t keyring);
553 This function clears the list of keys attached to a keyring. The calling
554 process must have write permission on the keyring, and it must be a
555 keyring (or else error ENOTDIR will result).
558 (*) Link a key into a keyring:
560 long keyctl(KEYCTL_LINK, key_serial_t keyring, key_serial_t key);
562 This function creates a link from the keyring to the key. The process must
563 have write permission on the keyring and must have link permission on the
566 Should the keyring not be a keyring, error ENOTDIR will result; and if the
567 keyring is full, error ENFILE will result.
569 The link procedure checks the nesting of the keyrings, returning ELOOP if
570 it appears too deep or EDEADLK if the link would introduce a cycle.
572 Any links within the keyring to keys that match the new key in terms of
573 type and description will be discarded from the keyring as the new one is
577 (*) Unlink a key or keyring from another keyring:
579 long keyctl(KEYCTL_UNLINK, key_serial_t keyring, key_serial_t key);
581 This function looks through the keyring for the first link to the
582 specified key, and removes it if found. Subsequent links to that key are
583 ignored. The process must have write permission on the keyring.
585 If the keyring is not a keyring, error ENOTDIR will result; and if the key
586 is not present, error ENOENT will be the result.
589 (*) Search a keyring tree for a key:
591 key_serial_t keyctl(KEYCTL_SEARCH, key_serial_t keyring,
592 const char *type, const char *description,
593 key_serial_t dest_keyring);
595 This searches the keyring tree headed by the specified keyring until a key
596 is found that matches the type and description criteria. Each keyring is
597 checked for keys before recursion into its children occurs.
599 The process must have search permission on the top level keyring, or else
600 error EACCES will result. Only keyrings that the process has search
601 permission on will be recursed into, and only keys and keyrings for which
602 a process has search permission can be matched. If the specified keyring
603 is not a keyring, ENOTDIR will result.
605 If the search succeeds, the function will attempt to link the found key
606 into the destination keyring if one is supplied (non-zero ID). All the
607 constraints applicable to KEYCTL_LINK apply in this case too.
609 Error ENOKEY, EKEYREVOKED or EKEYEXPIRED will be returned if the search
610 fails. On success, the resulting key ID will be returned.
613 (*) Read the payload data from a key:
615 long keyctl(KEYCTL_READ, key_serial_t keyring, char *buffer,
618 This function attempts to read the payload data from the specified key
619 into the buffer. The process must have read permission on the key to
622 The returned data will be processed for presentation by the key type. For
623 instance, a keyring will return an array of key_serial_t entries
624 representing the IDs of all the keys to which it is subscribed. The user
625 defined key type will return its data as is. If a key type does not
626 implement this function, error EOPNOTSUPP will result.
628 As much of the data as can be fitted into the buffer will be copied to
629 userspace if the buffer pointer is not NULL.
631 On a successful return, the function will always return the amount of data
632 available rather than the amount copied.
635 (*) Instantiate a partially constructed key.
637 long keyctl(KEYCTL_INSTANTIATE, key_serial_t key,
638 const void *payload, size_t plen,
639 key_serial_t keyring);
641 If the kernel calls back to userspace to complete the instantiation of a
642 key, userspace should use this call to supply data for the key before the
643 invoked process returns, or else the key will be marked negative
646 The process must have write access on the key to be able to instantiate
647 it, and the key must be uninstantiated.
649 If a keyring is specified (non-zero), the key will also be linked into
650 that keyring, however all the constraints applying in KEYCTL_LINK apply in
653 The payload and plen arguments describe the payload data as for add_key().
656 (*) Negatively instantiate a partially constructed key.
658 long keyctl(KEYCTL_NEGATE, key_serial_t key,
659 unsigned timeout, key_serial_t keyring);
661 If the kernel calls back to userspace to complete the instantiation of a
662 key, userspace should use this call mark the key as negative before the
663 invoked process returns if it is unable to fulfil the request.
665 The process must have write access on the key to be able to instantiate
666 it, and the key must be uninstantiated.
668 If a keyring is specified (non-zero), the key will also be linked into
669 that keyring, however all the constraints applying in KEYCTL_LINK apply in
673 (*) Set the default request-key destination keyring.
675 long keyctl(KEYCTL_SET_REQKEY_KEYRING, int reqkey_defl);
677 This sets the default keyring to which implicitly requested keys will be
678 attached for this thread. reqkey_defl should be one of these constants:
680 CONSTANT VALUE NEW DEFAULT KEYRING
681 ====================================== ====== =======================
682 KEY_REQKEY_DEFL_NO_CHANGE -1 No change
683 KEY_REQKEY_DEFL_DEFAULT 0 Default[1]
684 KEY_REQKEY_DEFL_THREAD_KEYRING 1 Thread keyring
685 KEY_REQKEY_DEFL_PROCESS_KEYRING 2 Process keyring
686 KEY_REQKEY_DEFL_SESSION_KEYRING 3 Session keyring
687 KEY_REQKEY_DEFL_USER_KEYRING 4 User keyring
688 KEY_REQKEY_DEFL_USER_SESSION_KEYRING 5 User session keyring
689 KEY_REQKEY_DEFL_GROUP_KEYRING 6 Group keyring
691 The old default will be returned if successful and error EINVAL will be
692 returned if reqkey_defl is not one of the above values.
694 The default keyring can be overridden by the keyring indicated to the
695 request_key() system call.
697 Note that this setting is inherited across fork/exec.
699 [1] The default is: the thread keyring if there is one, otherwise
700 the process keyring if there is one, otherwise the session keyring if
701 there is one, otherwise the user default session keyring.
704 (*) Set the timeout on a key.
706 long keyctl(KEYCTL_SET_TIMEOUT, key_serial_t key, unsigned timeout);
708 This sets or clears the timeout on a key. The timeout can be 0 to clear
709 the timeout or a number of seconds to set the expiry time that far into
712 The process must have attribute modification access on a key to set its
713 timeout. Timeouts may not be set with this function on negative, revoked
717 (*) Assume the authority granted to instantiate a key
719 long keyctl(KEYCTL_ASSUME_AUTHORITY, key_serial_t key);
721 This assumes or divests the authority required to instantiate the
722 specified key. Authority can only be assumed if the thread has the
723 authorisation key associated with the specified key in its keyrings
726 Once authority is assumed, searches for keys will also search the
727 requester's keyrings using the requester's security label, UID, GID and
730 If the requested authority is unavailable, error EPERM will be returned,
731 likewise if the authority has been revoked because the target key is
732 already instantiated.
734 If the specified key is 0, then any assumed authority will be divested.
736 The assumed authoritative key is inherited across fork and exec.
739 (*) Get the LSM security context attached to a key.
741 long keyctl(KEYCTL_GET_SECURITY, key_serial_t key, char *buffer,
744 This function returns a string that represents the LSM security context
745 attached to a key in the buffer provided.
747 Unless there's an error, it always returns the amount of data it could
748 produce, even if that's too big for the buffer, but it won't copy more
749 than requested to userspace. If the buffer pointer is NULL then no copy
752 A NUL character is included at the end of the string if the buffer is
753 sufficiently big. This is included in the returned count. If no LSM is
754 in force then an empty string will be returned.
756 A process must have view permission on the key for this function to be
764 The kernel services for key management are fairly simple to deal with. They can
765 be broken down into two areas: keys and key types.
767 Dealing with keys is fairly straightforward. Firstly, the kernel service
768 registers its type, then it searches for a key of that type. It should retain
769 the key as long as it has need of it, and then it should release it. For a
770 filesystem or device file, a search would probably be performed during the open
771 call, and the key released upon close. How to deal with conflicting keys due to
772 two different users opening the same file is left to the filesystem author to
775 To access the key manager, the following header must be #included:
779 Specific key types should have a header file under include/keys/ that should be
780 used to access that type. For keys of type "user", for example, that would be:
784 Note that there are two different types of pointers to keys that may be
789 This simply points to the key structure itself. Key structures will be at
790 least four-byte aligned.
794 This is equivalent to a struct key *, but the least significant bit is set
795 if the caller "possesses" the key. By "possession" it is meant that the
796 calling processes has a searchable link to the key from one of its
797 keyrings. There are three functions for dealing with these:
799 key_ref_t make_key_ref(const struct key *key,
800 unsigned long possession);
802 struct key *key_ref_to_ptr(const key_ref_t key_ref);
804 unsigned long is_key_possessed(const key_ref_t key_ref);
806 The first function constructs a key reference from a key pointer and
807 possession information (which must be 0 or 1 and not any other value).
809 The second function retrieves the key pointer from a reference and the
810 third retrieves the possession flag.
812 When accessing a key's payload contents, certain precautions must be taken to
813 prevent access vs modification races. See the section "Notes on accessing
814 payload contents" for more information.
816 (*) To search for a key, call:
818 struct key *request_key(const struct key_type *type,
819 const char *description,
820 const char *callout_info);
822 This is used to request a key or keyring with a description that matches
823 the description specified according to the key type's match function. This
824 permits approximate matching to occur. If callout_string is not NULL, then
825 /sbin/request-key will be invoked in an attempt to obtain the key from
826 userspace. In that case, callout_string will be passed as an argument to
829 Should the function fail error ENOKEY, EKEYEXPIRED or EKEYREVOKED will be
832 If successful, the key will have been attached to the default keyring for
833 implicitly obtained request-key keys, as set by KEYCTL_SET_REQKEY_KEYRING.
835 See also Documentation/keys-request-key.txt.
838 (*) To search for a key, passing auxiliary data to the upcaller, call:
840 struct key *request_key_with_auxdata(const struct key_type *type,
841 const char *description,
842 const void *callout_info,
846 This is identical to request_key(), except that the auxiliary data is
847 passed to the key_type->request_key() op if it exists, and the callout_info
848 is a blob of length callout_len, if given (the length may be 0).
851 (*) A key can be requested asynchronously by calling one of:
853 struct key *request_key_async(const struct key_type *type,
854 const char *description,
855 const void *callout_info,
860 struct key *request_key_async_with_auxdata(const struct key_type *type,
861 const char *description,
862 const char *callout_info,
866 which are asynchronous equivalents of request_key() and
867 request_key_with_auxdata() respectively.
869 These two functions return with the key potentially still under
870 construction. To wait for construction completion, the following should be
873 int wait_for_key_construction(struct key *key, bool intr);
875 The function will wait for the key to finish being constructed and then
876 invokes key_validate() to return an appropriate value to indicate the state
877 of the key (0 indicates the key is usable).
879 If intr is true, then the wait can be interrupted by a signal, in which
880 case error ERESTARTSYS will be returned.
883 (*) When it is no longer required, the key should be released using:
885 void key_put(struct key *key);
889 void key_ref_put(key_ref_t key_ref);
891 These can be called from interrupt context. If CONFIG_KEYS is not set then
892 the argument will not be parsed.
895 (*) Extra references can be made to a key by calling the following function:
897 struct key *key_get(struct key *key);
899 These need to be disposed of by calling key_put() when they've been
900 finished with. The key pointer passed in will be returned. If the pointer
901 is NULL or CONFIG_KEYS is not set then the key will not be dereferenced and
902 no increment will take place.
905 (*) A key's serial number can be obtained by calling:
907 key_serial_t key_serial(struct key *key);
909 If key is NULL or if CONFIG_KEYS is not set then 0 will be returned (in the
910 latter case without parsing the argument).
913 (*) If a keyring was found in the search, this can be further searched by:
915 key_ref_t keyring_search(key_ref_t keyring_ref,
916 const struct key_type *type,
917 const char *description)
919 This searches the keyring tree specified for a matching key. Error ENOKEY
920 is returned upon failure (use IS_ERR/PTR_ERR to determine). If successful,
921 the returned key will need to be released.
923 The possession attribute from the keyring reference is used to control
924 access through the permissions mask and is propagated to the returned key
925 reference pointer if successful.
928 (*) To check the validity of a key, this function can be called:
930 int validate_key(struct key *key);
932 This checks that the key in question hasn't expired or and hasn't been
933 revoked. Should the key be invalid, error EKEYEXPIRED or EKEYREVOKED will
934 be returned. If the key is NULL or if CONFIG_KEYS is not set then 0 will be
935 returned (in the latter case without parsing the argument).
938 (*) To register a key type, the following function should be called:
940 int register_key_type(struct key_type *type);
942 This will return error EEXIST if a type of the same name is already
946 (*) To unregister a key type, call:
948 void unregister_key_type(struct key_type *type);
951 Under some circumstances, it may be desirable to deal with a bundle of keys.
952 The facility provides access to the keyring type for managing such a bundle:
954 struct key_type key_type_keyring;
956 This can be used with a function such as request_key() to find a specific
957 keyring in a process's keyrings. A keyring thus found can then be searched
958 with keyring_search(). Note that it is not possible to use request_key() to
959 search a specific keyring, so using keyrings in this way is of limited utility.
962 ===================================
963 NOTES ON ACCESSING PAYLOAD CONTENTS
964 ===================================
966 The simplest payload is just a number in key->payload.value. In this case,
967 there's no need to indulge in RCU or locking when accessing the payload.
969 More complex payload contents must be allocated and a pointer to them set in
970 key->payload.data. One of the following ways must be selected to access the
973 (1) Unmodifiable key type.
975 If the key type does not have a modify method, then the key's payload can
976 be accessed without any form of locking, provided that it's known to be
977 instantiated (uninstantiated keys cannot be "found").
979 (2) The key's semaphore.
981 The semaphore could be used to govern access to the payload and to control
982 the payload pointer. It must be write-locked for modifications and would
983 have to be read-locked for general access. The disadvantage of doing this
984 is that the accessor may be required to sleep.
988 RCU must be used when the semaphore isn't already held; if the semaphore
989 is held then the contents can't change under you unexpectedly as the
990 semaphore must still be used to serialise modifications to the key. The
991 key management code takes care of this for the key type.
993 However, this means using:
995 rcu_read_lock() ... rcu_dereference() ... rcu_read_unlock()
997 to read the pointer, and:
999 rcu_dereference() ... rcu_assign_pointer() ... call_rcu()
1001 to set the pointer and dispose of the old contents after a grace period.
1002 Note that only the key type should ever modify a key's payload.
1004 Furthermore, an RCU controlled payload must hold a struct rcu_head for the
1005 use of call_rcu() and, if the payload is of variable size, the length of
1006 the payload. key->datalen cannot be relied upon to be consistent with the
1007 payload just dereferenced if the key's semaphore is not held.
1014 A kernel service may want to define its own key type. For instance, an AFS
1015 filesystem might want to define a Kerberos 5 ticket key type. To do this, it
1016 author fills in a key_type struct and registers it with the system.
1018 Source files that implement key types should include the following header file:
1022 The structure has a number of fields, some of which are mandatory:
1024 (*) const char *name
1026 The name of the key type. This is used to translate a key type name
1027 supplied by userspace into a pointer to the structure.
1030 (*) size_t def_datalen
1032 This is optional - it supplies the default payload data length as
1033 contributed to the quota. If the key type's payload is always or almost
1034 always the same size, then this is a more efficient way to do things.
1036 The data length (and quota) on a particular key can always be changed
1037 during instantiation or update by calling:
1039 int key_payload_reserve(struct key *key, size_t datalen);
1041 With the revised data length. Error EDQUOT will be returned if this is not
1045 (*) int (*instantiate)(struct key *key, const void *data, size_t datalen);
1047 This method is called to attach a payload to a key during construction.
1048 The payload attached need not bear any relation to the data passed to this
1051 If the amount of data attached to the key differs from the size in
1052 keytype->def_datalen, then key_payload_reserve() should be called.
1054 This method does not have to lock the key in order to attach a payload.
1055 The fact that KEY_FLAG_INSTANTIATED is not set in key->flags prevents
1056 anything else from gaining access to the key.
1058 It is safe to sleep in this method.
1061 (*) int (*update)(struct key *key, const void *data, size_t datalen);
1063 If this type of key can be updated, then this method should be provided.
1064 It is called to update a key's payload from the blob of data provided.
1066 key_payload_reserve() should be called if the data length might change
1067 before any changes are actually made. Note that if this succeeds, the type
1068 is committed to changing the key because it's already been altered, so all
1069 memory allocation must be done first.
1071 The key will have its semaphore write-locked before this method is called,
1072 but this only deters other writers; any changes to the key's payload must
1073 be made under RCU conditions, and call_rcu() must be used to dispose of
1076 key_payload_reserve() should be called before the changes are made, but
1077 after all allocations and other potentially failing function calls are
1080 It is safe to sleep in this method.
1083 (*) int (*match)(const struct key *key, const void *desc);
1085 This method is called to match a key against a description. It should
1086 return non-zero if the two match, zero if they don't.
1088 This method should not need to lock the key in any way. The type and
1089 description can be considered invariant, and the payload should not be
1090 accessed (the key may not yet be instantiated).
1092 It is not safe to sleep in this method; the caller may hold spinlocks.
1095 (*) void (*revoke)(struct key *key);
1097 This method is optional. It is called to discard part of the payload
1098 data upon a key being revoked. The caller will have the key semaphore
1101 It is safe to sleep in this method, though care should be taken to avoid
1102 a deadlock against the key semaphore.
1105 (*) void (*destroy)(struct key *key);
1107 This method is optional. It is called to discard the payload data on a key
1108 when it is being destroyed.
1110 This method does not need to lock the key to access the payload; it can
1111 consider the key as being inaccessible at this time. Note that the key's
1112 type may have been changed before this function is called.
1114 It is not safe to sleep in this method; the caller may hold spinlocks.
1117 (*) void (*describe)(const struct key *key, struct seq_file *p);
1119 This method is optional. It is called during /proc/keys reading to
1120 summarise a key's description and payload in text form.
1122 This method will be called with the RCU read lock held. rcu_dereference()
1123 should be used to read the payload pointer if the payload is to be
1124 accessed. key->datalen cannot be trusted to stay consistent with the
1125 contents of the payload.
1127 The description will not change, though the key's state may.
1129 It is not safe to sleep in this method; the RCU read lock is held by the
1133 (*) long (*read)(const struct key *key, char __user *buffer, size_t buflen);
1135 This method is optional. It is called by KEYCTL_READ to translate the
1136 key's payload into something a blob of data for userspace to deal with.
1137 Ideally, the blob should be in the same format as that passed in to the
1138 instantiate and update methods.
1140 If successful, the blob size that could be produced should be returned
1141 rather than the size copied.
1143 This method will be called with the key's semaphore read-locked. This will
1144 prevent the key's payload changing. It is not necessary to use RCU locking
1145 when accessing the key's payload. It is safe to sleep in this method, such
1146 as might happen when the userspace buffer is accessed.
1149 (*) int (*request_key)(struct key_construction *cons, const char *op,
1152 This method is optional. If provided, request_key() and friends will
1153 invoke this function rather than upcalling to /sbin/request-key to operate
1154 upon a key of this type.
1156 The aux parameter is as passed to request_key_async_with_auxdata() and
1157 similar or is NULL otherwise. Also passed are the construction record for
1158 the key to be operated upon and the operation type (currently only
1161 This method is permitted to return before the upcall is complete, but the
1162 following function must be called under all circumstances to complete the
1163 instantiation process, whether or not it succeeds, whether or not there's
1166 void complete_request_key(struct key_construction *cons, int error);
1168 The error parameter should be 0 on success, -ve on error. The
1169 construction record is destroyed by this action and the authorisation key
1170 will be revoked. If an error is indicated, the key under construction
1171 will be negatively instantiated if it wasn't already instantiated.
1173 If this method returns an error, that error will be returned to the
1174 caller of request_key*(). complete_request_key() must be called prior to
1177 The key under construction and the authorisation key can be found in the
1178 key_construction struct pointed to by cons:
1180 (*) struct key *key;
1182 The key under construction.
1184 (*) struct key *authkey;
1186 The authorisation key.
1189 ============================
1190 REQUEST-KEY CALLBACK SERVICE
1191 ============================
1193 To create a new key, the kernel will attempt to execute the following command
1196 /sbin/request-key create <key> <uid> <gid> \
1197 <threadring> <processring> <sessionring> <callout_info>
1199 <key> is the key being constructed, and the three keyrings are the process
1200 keyrings from the process that caused the search to be issued. These are
1201 included for two reasons:
1203 (1) There may be an authentication token in one of the keyrings that is
1204 required to obtain the key, eg: a Kerberos Ticket-Granting Ticket.
1206 (2) The new key should probably be cached in one of these rings.
1208 This program should set it UID and GID to those specified before attempting to
1209 access any more keys. It may then look around for a user specific process to
1210 hand the request off to (perhaps a path held in placed in another key by, for
1211 example, the KDE desktop manager).
1213 The program (or whatever it calls) should finish construction of the key by
1214 calling KEYCTL_INSTANTIATE, which also permits it to cache the key in one of
1215 the keyrings (probably the session ring) before returning. Alternatively, the
1216 key can be marked as negative with KEYCTL_NEGATE; this also permits the key to
1217 be cached in one of the keyrings.
1219 If it returns with the key remaining in the unconstructed state, the key will
1220 be marked as being negative, it will be added to the session keyring, and an
1221 error will be returned to the key requestor.
1223 Supplementary information may be provided from whoever or whatever invoked this
1224 service. This will be passed as the <callout_info> parameter. If no such
1225 information was made available, then "-" will be passed as this parameter
1229 Similarly, the kernel may attempt to update an expired or a soon to expire key
1232 /sbin/request-key update <key> <uid> <gid> \
1233 <threadring> <processring> <sessionring>
1235 In this case, the program isn't required to actually attach the key to a ring;
1236 the rings are provided for reference.
1243 Dead keys (for which the type has been removed) will be automatically unlinked
1244 from those keyrings that point to them and deleted as soon as possible by a
1245 background garbage collector.
1247 Similarly, revoked and expired keys will be garbage collected, but only after a
1248 certain amount of time has passed. This time is set as a number of seconds in:
1250 /proc/sys/kernel/keys/gc_delay