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:
25 In this context, keys represent units of cryptographic data, authentication
26 tokens, keyrings, etc.. These are represented in the kernel by struct key.
28 Each key has a number of attributes:
32 - A description (for matching a key in a search).
33 - Access control information.
39 * Each key is issued a serial number of type key_serial_t that is unique for
40 the lifetime of that key. All serial numbers are positive non-zero 32-bit
43 Userspace programs can use a key's serial numbers as a way to gain access
44 to it, subject to permission checking.
46 * Each key is of a defined "type". Types must be registered inside the
47 kernel by a kernel service (such as a filesystem) before keys of that type
48 can be added or used. Userspace programs cannot define new types directly.
50 Key types are represented in the kernel by struct key_type. This defines a
51 number of operations that can be performed on a key of that type.
53 Should a type be removed from the system, all the keys of that type will
56 * Each key has a description. This should be a printable string. The key
57 type provides an operation to perform a match between the description on a
58 key and a criterion string.
60 * Each key has an owner user ID, a group ID and a permissions mask. These
61 are used to control what a process may do to a key from userspace, and
62 whether a kernel service will be able to find the key.
64 * Each key can be set to expire at a specific time by the key type's
65 instantiation function. Keys can also be immortal.
67 * Each key can have a payload. This is a quantity of data that represent the
68 actual "key". In the case of a keyring, this is a list of keys to which
69 the keyring links; in the case of a user-defined key, it's an arbitrary
72 Having a payload is not required; and the payload can, in fact, just be a
73 value stored in the struct key itself.
75 When a key is instantiated, the key type's instantiation function is
76 called with a blob of data, and that then creates the key's payload in
79 Similarly, when userspace wants to read back the contents of the key, if
80 permitted, another key type operation will be called to convert the key's
81 attached payload back into a blob of data.
83 * Each key can be in one of a number of basic states:
85 * Uninstantiated. The key exists, but does not have any data attached.
86 Keys being requested from userspace will be in this state.
88 * Instantiated. This is the normal state. The key is fully formed, and
91 * Negative. This is a relatively short-lived state. The key acts as a
92 note saying that a previous call out to userspace failed, and acts as
93 a throttle on key lookups. A negative key can be updated to a normal
96 * Expired. Keys can have lifetimes set. If their lifetime is exceeded,
97 they traverse to this state. An expired key can be updated back to a
100 * Revoked. A key is put in this state by userspace action. It can't be
101 found or operated upon (apart from by unlinking it).
103 * Dead. The key's type was unregistered, and so the key is now useless.
105 Keys in the last three states are subject to garbage collection. See the
106 section on "Garbage collection".
112 The key service provides a number of features besides keys:
114 * The key service defines three special key types:
118 Keyrings are special keys that contain a list of other keys. Keyring
119 lists can be modified using various system calls. Keyrings should not
120 be given a payload when created.
124 A key of this type has a description and a payload that are arbitrary
125 blobs of data. These can be created, updated and read by userspace,
126 and aren't intended for use by kernel services.
130 Like a "user" key, a "logon" key has a payload that is an arbitrary
131 blob of data. It is intended as a place to store secrets which are
132 accessible to the kernel but not to userspace programs.
134 The description can be arbitrary, but must be prefixed with a non-zero
135 length string that describes the key "subclass". The subclass is
136 separated from the rest of the description by a ':'. "logon" keys can
137 be created and updated from userspace, but the payload is only
138 readable from kernel space.
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 Key Access Permissions
199 ======================
201 Keys have an owner user ID, a group access ID, and a permissions mask. The mask
202 has up to eight bits each for possessor, user, group and other access. Only
203 six of each set of eight bits are defined. These permissions granted are:
207 This permits a key or keyring's attributes to be viewed - including key
208 type and description.
212 This permits a key's payload to be viewed or a keyring's list of linked
217 This permits a key's payload to be instantiated or updated, or it allows a
218 link to be added to or removed from a keyring.
222 This permits keyrings to be searched and keys to be found. Searches can
223 only recurse into nested keyrings that have search permission set.
227 This permits a key or keyring to be linked to. To create a link from a
228 keyring to a key, a process must have Write permission on the keyring and
229 Link permission on the key.
233 This permits a key's UID, GID and permissions mask to be changed.
235 For changing the ownership, group ID or permissions mask, being the owner of
236 the key or having the sysadmin capability is sufficient.
242 The security class "key" has been added to SELinux so that mandatory access
243 controls can be applied to keys created within various contexts. This support
244 is preliminary, and is likely to change quite significantly in the near future.
245 Currently, all of the basic permissions explained above are provided in SELinux
246 as well; SELinux is simply invoked after all basic permission checks have been
249 The value of the file /proc/self/attr/keycreate influences the labeling of
250 newly-created keys. If the contents of that file correspond to an SELinux
251 security context, then the key will be assigned that context. Otherwise, the
252 key will be assigned the current context of the task that invoked the key
253 creation request. Tasks must be granted explicit permission to assign a
254 particular context to newly-created keys, using the "create" permission in the
257 The default keyrings associated with users will be labeled with the default
258 context of the user if and only if the login programs have been instrumented to
259 properly initialize keycreate during the login process. Otherwise, they will
260 be labeled with the context of the login program itself.
262 Note, however, that the default keyrings associated with the root user are
263 labeled with the default kernel context, since they are created early in the
264 boot process, before root has a chance to log in.
266 The keyrings associated with new threads are each labeled with the context of
267 their associated thread, and both session and process keyrings are handled
274 Two files have been added to procfs by which an administrator can find out
275 about the status of the key service:
279 This lists the keys that are currently viewable by the task reading the
280 file, giving information about their type, description and permissions.
281 It is not possible to view the payload of the key this way, though some
282 information about it may be given.
284 The only keys included in the list are those that grant View permission to
285 the reading process whether or not it possesses them. Note that LSM
286 security checks are still performed, and may further filter out keys that
287 the current process is not authorised to view.
289 The contents of the file look like this::
291 SERIAL FLAGS USAGE EXPY PERM UID GID TYPE DESCRIPTION: SUMMARY
292 00000001 I----- 39 perm 1f3f0000 0 0 keyring _uid_ses.0: 1/4
293 00000002 I----- 2 perm 1f3f0000 0 0 keyring _uid.0: empty
294 00000007 I----- 1 perm 1f3f0000 0 0 keyring _pid.1: empty
295 0000018d I----- 1 perm 1f3f0000 0 0 keyring _pid.412: empty
296 000004d2 I--Q-- 1 perm 1f3f0000 32 -1 keyring _uid.32: 1/4
297 000004d3 I--Q-- 3 perm 1f3f0000 32 -1 keyring _uid_ses.32: empty
298 00000892 I--QU- 1 perm 1f000000 0 0 user metal:copper: 0
299 00000893 I--Q-N 1 35s 1f3f0000 0 0 user metal:silver: 0
300 00000894 I--Q-- 1 10h 003f0000 0 0 user metal:gold: 0
307 Q Contributes to user's quota
308 U Under construction by callback to userspace
314 This file lists the tracking data for each user that has at least one key
315 on the system. Such data includes quota information and statistics::
317 [root@andromeda root]# cat /proc/key-users
318 0: 46 45/45 1/100 13/10000
319 29: 2 2/2 2/100 40/10000
320 32: 2 2/2 2/100 40/10000
321 38: 2 2/2 2/100 40/10000
323 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 Four new sysctl files have been added also for the purpose of controlling the
333 quota limits on keys:
335 * /proc/sys/kernel/keys/root_maxkeys
336 /proc/sys/kernel/keys/root_maxbytes
338 These files hold the maximum number of keys that root may have and the
339 maximum total number of bytes of data that root may have stored in those
342 * /proc/sys/kernel/keys/maxkeys
343 /proc/sys/kernel/keys/maxbytes
345 These files hold the maximum number of keys that each non-root user may
346 have and the maximum total number of bytes of data that each of those
347 users may have stored in their keys.
349 Root may alter these by writing each new limit as a decimal number string to
350 the appropriate file.
353 Userspace System Call Interface
354 ===============================
356 Userspace can manipulate keys directly through three new syscalls: add_key,
357 request_key and keyctl. The latter provides a number of functions for
360 When referring to a key directly, userspace programs should use the key's
361 serial number (a positive 32-bit integer). However, there are some special
362 values available for referring to special keys and keyrings that relate to the
363 process making the call::
365 CONSTANT VALUE KEY REFERENCED
366 ============================== ====== ===========================
367 KEY_SPEC_THREAD_KEYRING -1 thread-specific keyring
368 KEY_SPEC_PROCESS_KEYRING -2 process-specific keyring
369 KEY_SPEC_SESSION_KEYRING -3 session-specific keyring
370 KEY_SPEC_USER_KEYRING -4 UID-specific keyring
371 KEY_SPEC_USER_SESSION_KEYRING -5 UID-session keyring
372 KEY_SPEC_GROUP_KEYRING -6 GID-specific keyring
373 KEY_SPEC_REQKEY_AUTH_KEY -7 assumed request_key()
377 The main syscalls are:
379 * Create a new key of given type, description and payload and add it to the
382 key_serial_t add_key(const char *type, const char *desc,
383 const void *payload, size_t plen,
384 key_serial_t keyring);
386 If a key of the same type and description as that proposed already exists
387 in the keyring, this will try to update it with the given payload, or it
388 will return error EEXIST if that function is not supported by the key
389 type. The process must also have permission to write to the key to be able
390 to update it. The new key will have all user permissions granted and no
391 group or third party permissions.
393 Otherwise, this will attempt to create a new key of the specified type and
394 description, and to instantiate it with the supplied payload and attach it
395 to the keyring. In this case, an error will be generated if the process
396 does not have permission to write to the keyring.
398 If the key type supports it, if the description is NULL or an empty
399 string, the key type will try and generate a description from the content
402 The payload is optional, and the pointer can be NULL if not required by
403 the type. The payload is plen in size, and plen can be zero for an empty
406 A new keyring can be generated by setting type "keyring", the keyring name
407 as the description (or NULL) and setting the payload to NULL.
409 User defined keys can be created by specifying type "user". It is
410 recommended that a user defined key's description by prefixed with a type
411 ID and a colon, such as "krb5tgt:" for a Kerberos 5 ticket granting
414 Any other type must have been registered with the kernel in advance by a
415 kernel service such as a filesystem.
417 The ID of the new or updated key is returned if successful.
420 * Search the process's keyrings for a key, potentially calling out to
421 userspace to create it::
423 key_serial_t request_key(const char *type, const char *description,
424 const char *callout_info,
425 key_serial_t dest_keyring);
427 This function searches all the process's keyrings in the order thread,
428 process, session for a matching key. This works very much like
429 KEYCTL_SEARCH, including the optional attachment of the discovered key to
432 If a key cannot be found, and if callout_info is not NULL, then
433 /sbin/request-key will be invoked in an attempt to obtain a key. The
434 callout_info string will be passed as an argument to the program.
436 To link a key into the destination keyring the key must grant link
437 permission on the key to the caller and the keyring must grant write
440 See also Documentation/security/keys/request-key.rst.
443 The keyctl syscall functions are:
445 * Map a special key ID to a real key ID for this process::
447 key_serial_t keyctl(KEYCTL_GET_KEYRING_ID, key_serial_t id,
450 The special key specified by "id" is looked up (with the key being created
451 if necessary) and the ID of the key or keyring thus found is returned if
454 If the key does not yet exist, the key will be created if "create" is
455 non-zero; and the error ENOKEY will be returned if "create" is zero.
458 * Replace the session keyring this process subscribes to with a new one::
460 key_serial_t keyctl(KEYCTL_JOIN_SESSION_KEYRING, const char *name);
462 If name is NULL, an anonymous keyring is created attached to the process
463 as its session keyring, displacing the old session keyring.
465 If name is not NULL, if a keyring of that name exists, the process
466 attempts to attach it as the session keyring, returning an error if that
467 is not permitted; otherwise a new keyring of that name is created and
468 attached as the session keyring.
470 To attach to a named keyring, the keyring must have search permission for
471 the process's ownership.
473 The ID of the new session keyring is returned if successful.
476 * Update the specified key::
478 long keyctl(KEYCTL_UPDATE, key_serial_t key, const void *payload,
481 This will try to update the specified key with the given payload, or it
482 will return error EOPNOTSUPP if that function is not supported by the key
483 type. The process must also have permission to write to the key to be able
486 The payload is of length plen, and may be absent or empty as for
492 long keyctl(KEYCTL_REVOKE, key_serial_t key);
494 This makes a key unavailable for further operations. Further attempts to
495 use the key will be met with error EKEYREVOKED, and the key will no longer
499 * Change the ownership of a key::
501 long keyctl(KEYCTL_CHOWN, key_serial_t key, uid_t uid, gid_t gid);
503 This function permits a key's owner and group ID to be changed. Either one
504 of uid or gid can be set to -1 to suppress that change.
506 Only the superuser can change a key's owner to something other than the
507 key's current owner. Similarly, only the superuser can change a key's
508 group ID to something other than the calling process's group ID or one of
509 its group list members.
512 * Change the permissions mask on a key::
514 long keyctl(KEYCTL_SETPERM, key_serial_t key, key_perm_t perm);
516 This function permits the owner of a key or the superuser to change the
517 permissions mask on a key.
519 Only bits the available bits are permitted; if any other bits are set,
520 error EINVAL will be returned.
525 long keyctl(KEYCTL_DESCRIBE, key_serial_t key, char *buffer,
528 This function returns a summary of the key's attributes (but not its
529 payload data) as a string in the buffer provided.
531 Unless there's an error, it always returns the amount of data it could
532 produce, even if that's too big for the buffer, but it won't copy more
533 than requested to userspace. If the buffer pointer is NULL then no copy
536 A process must have view permission on the key for this function to be
539 If successful, a string is placed in the buffer in the following format::
541 <type>;<uid>;<gid>;<perm>;<description>
543 Where type and description are strings, uid and gid are decimal, and perm
544 is hexadecimal. A NUL character is included at the end of the string if
545 the buffer is sufficiently big.
547 This can be parsed with::
549 sscanf(buffer, "%[^;];%d;%d;%o;%s", type, &uid, &gid, &mode, desc);
552 * Clear out a keyring::
554 long keyctl(KEYCTL_CLEAR, key_serial_t keyring);
556 This function clears the list of keys attached to a keyring. The calling
557 process must have write permission on the keyring, and it must be a
558 keyring (or else error ENOTDIR will result).
560 This function can also be used to clear special kernel keyrings if they
561 are appropriately marked if the user has CAP_SYS_ADMIN capability. The
562 DNS resolver cache keyring is an example of this.
565 * Link a key into a keyring::
567 long keyctl(KEYCTL_LINK, key_serial_t keyring, key_serial_t key);
569 This function creates a link from the keyring to the key. The process must
570 have write permission on the keyring and must have link permission on the
573 Should the keyring not be a keyring, error ENOTDIR will result; and if the
574 keyring is full, error ENFILE will result.
576 The link procedure checks the nesting of the keyrings, returning ELOOP if
577 it appears too deep or EDEADLK if the link would introduce a cycle.
579 Any links within the keyring to keys that match the new key in terms of
580 type and description will be discarded from the keyring as the new one is
584 * Move a key from one keyring to another::
586 long keyctl(KEYCTL_MOVE,
588 key_serial_t from_ring_id,
589 key_serial_t to_ring_id,
592 Move the key specified by "id" from the keyring specified by
593 "from_ring_id" to the keyring specified by "to_ring_id". If the two
594 keyrings are the same, nothing is done.
596 "flags" can have KEYCTL_MOVE_EXCL set in it to cause the operation to fail
597 with EEXIST if a matching key exists in the destination keyring, otherwise
598 such a key will be replaced.
600 A process must have link permission on the key for this function to be
601 successful and write permission on both keyrings. Any errors that can
602 occur from KEYCTL_LINK also apply on the destination keyring here.
605 * Unlink a key or keyring from another keyring::
607 long keyctl(KEYCTL_UNLINK, key_serial_t keyring, key_serial_t key);
609 This function looks through the keyring for the first link to the
610 specified key, and removes it if found. Subsequent links to that key are
611 ignored. The process must have write permission on the keyring.
613 If the keyring is not a keyring, error ENOTDIR will result; and if the key
614 is not present, error ENOENT will be the result.
617 * Search a keyring tree for a key::
619 key_serial_t keyctl(KEYCTL_SEARCH, key_serial_t keyring,
620 const char *type, const char *description,
621 key_serial_t dest_keyring);
623 This searches the keyring tree headed by the specified keyring until a key
624 is found that matches the type and description criteria. Each keyring is
625 checked for keys before recursion into its children occurs.
627 The process must have search permission on the top level keyring, or else
628 error EACCES will result. Only keyrings that the process has search
629 permission on will be recursed into, and only keys and keyrings for which
630 a process has search permission can be matched. If the specified keyring
631 is not a keyring, ENOTDIR will result.
633 If the search succeeds, the function will attempt to link the found key
634 into the destination keyring if one is supplied (non-zero ID). All the
635 constraints applicable to KEYCTL_LINK apply in this case too.
637 Error ENOKEY, EKEYREVOKED or EKEYEXPIRED will be returned if the search
638 fails. On success, the resulting key ID will be returned.
641 * Read the payload data from a key::
643 long keyctl(KEYCTL_READ, key_serial_t keyring, char *buffer,
646 This function attempts to read the payload data from the specified key
647 into the buffer. The process must have read permission on the key to
650 The returned data will be processed for presentation by the key type. For
651 instance, a keyring will return an array of key_serial_t entries
652 representing the IDs of all the keys to which it is subscribed. The user
653 defined key type will return its data as is. If a key type does not
654 implement this function, error EOPNOTSUPP will result.
656 If the specified buffer is too small, then the size of the buffer required
657 will be returned. Note that in this case, the contents of the buffer may
658 have been overwritten in some undefined way.
660 Otherwise, on success, the function will return the amount of data copied
663 * Instantiate a partially constructed key::
665 long keyctl(KEYCTL_INSTANTIATE, key_serial_t key,
666 const void *payload, size_t plen,
667 key_serial_t keyring);
668 long keyctl(KEYCTL_INSTANTIATE_IOV, key_serial_t key,
669 const struct iovec *payload_iov, unsigned ioc,
670 key_serial_t keyring);
672 If the kernel calls back to userspace to complete the instantiation of a
673 key, userspace should use this call to supply data for the key before the
674 invoked process returns, or else the key will be marked negative
677 The process must have write access on the key to be able to instantiate
678 it, and the key must be uninstantiated.
680 If a keyring is specified (non-zero), the key will also be linked into
681 that keyring, however all the constraints applying in KEYCTL_LINK apply in
684 The payload and plen arguments describe the payload data as for add_key().
686 The payload_iov and ioc arguments describe the payload data in an iovec
687 array instead of a single buffer.
690 * Negatively instantiate a partially constructed key::
692 long keyctl(KEYCTL_NEGATE, key_serial_t key,
693 unsigned timeout, key_serial_t keyring);
694 long keyctl(KEYCTL_REJECT, key_serial_t key,
695 unsigned timeout, unsigned error, key_serial_t keyring);
697 If the kernel calls back to userspace to complete the instantiation of a
698 key, userspace should use this call mark the key as negative before the
699 invoked process returns if it is unable to fulfill the request.
701 The process must have write access on the key to be able to instantiate
702 it, and the key must be uninstantiated.
704 If a keyring is specified (non-zero), the key will also be linked into
705 that keyring, however all the constraints applying in KEYCTL_LINK apply in
708 If the key is rejected, future searches for it will return the specified
709 error code until the rejected key expires. Negating the key is the same
710 as rejecting the key with ENOKEY as the error code.
713 * Set the default request-key destination keyring::
715 long keyctl(KEYCTL_SET_REQKEY_KEYRING, int reqkey_defl);
717 This sets the default keyring to which implicitly requested keys will be
718 attached for this thread. reqkey_defl should be one of these constants::
720 CONSTANT VALUE NEW DEFAULT KEYRING
721 ====================================== ====== =======================
722 KEY_REQKEY_DEFL_NO_CHANGE -1 No change
723 KEY_REQKEY_DEFL_DEFAULT 0 Default[1]
724 KEY_REQKEY_DEFL_THREAD_KEYRING 1 Thread keyring
725 KEY_REQKEY_DEFL_PROCESS_KEYRING 2 Process keyring
726 KEY_REQKEY_DEFL_SESSION_KEYRING 3 Session keyring
727 KEY_REQKEY_DEFL_USER_KEYRING 4 User keyring
728 KEY_REQKEY_DEFL_USER_SESSION_KEYRING 5 User session keyring
729 KEY_REQKEY_DEFL_GROUP_KEYRING 6 Group keyring
731 The old default will be returned if successful and error EINVAL will be
732 returned if reqkey_defl is not one of the above values.
734 The default keyring can be overridden by the keyring indicated to the
735 request_key() system call.
737 Note that this setting is inherited across fork/exec.
739 [1] The default is: the thread keyring if there is one, otherwise
740 the process keyring if there is one, otherwise the session keyring if
741 there is one, otherwise the user default session keyring.
744 * Set the timeout on a key::
746 long keyctl(KEYCTL_SET_TIMEOUT, key_serial_t key, unsigned timeout);
748 This sets or clears the timeout on a key. The timeout can be 0 to clear
749 the timeout or a number of seconds to set the expiry time that far into
752 The process must have attribute modification access on a key to set its
753 timeout. Timeouts may not be set with this function on negative, revoked
757 * Assume the authority granted to instantiate a key::
759 long keyctl(KEYCTL_ASSUME_AUTHORITY, key_serial_t key);
761 This assumes or divests the authority required to instantiate the
762 specified key. Authority can only be assumed if the thread has the
763 authorisation key associated with the specified key in its keyrings
766 Once authority is assumed, searches for keys will also search the
767 requester's keyrings using the requester's security label, UID, GID and
770 If the requested authority is unavailable, error EPERM will be returned,
771 likewise if the authority has been revoked because the target key is
772 already instantiated.
774 If the specified key is 0, then any assumed authority will be divested.
776 The assumed authoritative key is inherited across fork and exec.
779 * Get the LSM security context attached to a key::
781 long keyctl(KEYCTL_GET_SECURITY, key_serial_t key, char *buffer,
784 This function returns a string that represents the LSM security context
785 attached to a key in the buffer provided.
787 Unless there's an error, it always returns the amount of data it could
788 produce, even if that's too big for the buffer, but it won't copy more
789 than requested to userspace. If the buffer pointer is NULL then no copy
792 A NUL character is included at the end of the string if the buffer is
793 sufficiently big. This is included in the returned count. If no LSM is
794 in force then an empty string will be returned.
796 A process must have view permission on the key for this function to be
800 * Install the calling process's session keyring on its parent::
802 long keyctl(KEYCTL_SESSION_TO_PARENT);
804 This functions attempts to install the calling process's session keyring
805 on to the calling process's parent, replacing the parent's current session
808 The calling process must have the same ownership as its parent, the
809 keyring must have the same ownership as the calling process, the calling
810 process must have LINK permission on the keyring and the active LSM module
811 mustn't deny permission, otherwise error EPERM will be returned.
813 Error ENOMEM will be returned if there was insufficient memory to complete
814 the operation, otherwise 0 will be returned to indicate success.
816 The keyring will be replaced next time the parent process leaves the
817 kernel and resumes executing userspace.
822 long keyctl(KEYCTL_INVALIDATE, key_serial_t key);
824 This function marks a key as being invalidated and then wakes up the
825 garbage collector. The garbage collector immediately removes invalidated
826 keys from all keyrings and deletes the key when its reference count
829 Keys that are marked invalidated become invisible to normal key operations
830 immediately, though they are still visible in /proc/keys until deleted
831 (they're marked with an 'i' flag).
833 A process must have search permission on the key for this function to be
836 * Compute a Diffie-Hellman shared secret or public key::
838 long keyctl(KEYCTL_DH_COMPUTE, struct keyctl_dh_params *params,
839 char *buffer, size_t buflen, struct keyctl_kdf_params *kdf);
841 The params struct contains serial numbers for three keys::
843 - The prime, p, known to both parties
844 - The local private key
845 - The base integer, which is either a shared generator or the
848 The value computed is::
850 result = base ^ private (mod prime)
852 If the base is the shared generator, the result is the local
853 public key. If the base is the remote public key, the result is
856 If the parameter kdf is NULL, the following applies:
858 - The buffer length must be at least the length of the prime, or zero.
860 - If the buffer length is nonzero, the length of the result is
861 returned when it is successfully calculated and copied in to the
862 buffer. When the buffer length is zero, the minimum required
863 buffer length is returned.
865 The kdf parameter allows the caller to apply a key derivation function
866 (KDF) on the Diffie-Hellman computation where only the result
867 of the KDF is returned to the caller. The KDF is characterized with
868 struct keyctl_kdf_params as follows:
870 - ``char *hashname`` specifies the NUL terminated string identifying
871 the hash used from the kernel crypto API and applied for the KDF
872 operation. The KDF implemenation complies with SP800-56A as well
873 as with SP800-108 (the counter KDF).
875 - ``char *otherinfo`` specifies the OtherInfo data as documented in
876 SP800-56A section 5.8.1.2. The length of the buffer is given with
877 otherinfolen. The format of OtherInfo is defined by the caller.
878 The otherinfo pointer may be NULL if no OtherInfo shall be used.
880 This function will return error EOPNOTSUPP if the key type is not
881 supported, error ENOKEY if the key could not be found, or error
882 EACCES if the key is not readable by the caller. In addition, the
883 function will return EMSGSIZE when the parameter kdf is non-NULL
884 and either the buffer length or the OtherInfo length exceeds the
888 * Restrict keyring linkage::
890 long keyctl(KEYCTL_RESTRICT_KEYRING, key_serial_t keyring,
891 const char *type, const char *restriction);
893 An existing keyring can restrict linkage of additional keys by evaluating
894 the contents of the key according to a restriction scheme.
896 "keyring" is the key ID for an existing keyring to apply a restriction
897 to. It may be empty or may already have keys linked. Existing linked keys
898 will remain in the keyring even if the new restriction would reject them.
900 "type" is a registered key type.
902 "restriction" is a string describing how key linkage is to be restricted.
903 The format varies depending on the key type, and the string is passed to
904 the lookup_restriction() function for the requested type. It may specify
905 a method and relevant data for the restriction such as signature
906 verification or constraints on key payload. If the requested key type is
907 later unregistered, no keys may be added to the keyring after the key type
910 To apply a keyring restriction the process must have Set Attribute
911 permission and the keyring must not be previously restricted.
913 One application of restricted keyrings is to verify X.509 certificate
914 chains or individual certificate signatures using the asymmetric key type.
915 See Documentation/crypto/asymmetric-keys.txt for specific restrictions
916 applicable to the asymmetric key type.
919 * Query an asymmetric key::
921 long keyctl(KEYCTL_PKEY_QUERY,
922 key_serial_t key_id, unsigned long reserved,
923 struct keyctl_pkey_query *info);
925 Get information about an asymmetric key. The information is returned in
926 the keyctl_pkey_query struct::
936 ``supported_ops`` contains a bit mask of flags indicating which ops are
937 supported. This is constructed from a bitwise-OR of::
939 KEYCTL_SUPPORTS_{ENCRYPT,DECRYPT,SIGN,VERIFY}
941 ``key_size`` indicated the size of the key in bits.
943 ``max_*_size`` indicate the maximum sizes in bytes of a blob of data to be
944 signed, a signature blob, a blob to be encrypted and a blob to be
947 ``__spare[]`` must be set to 0. This is intended for future use to hand
948 over one or more passphrases needed unlock a key.
950 If successful, 0 is returned. If the key is not an asymmetric key,
951 EOPNOTSUPP is returned.
954 * Encrypt, decrypt, sign or verify a blob using an asymmetric key::
956 long keyctl(KEYCTL_PKEY_ENCRYPT,
957 const struct keyctl_pkey_params *params,
962 long keyctl(KEYCTL_PKEY_DECRYPT,
963 const struct keyctl_pkey_params *params,
968 long keyctl(KEYCTL_PKEY_SIGN,
969 const struct keyctl_pkey_params *params,
974 long keyctl(KEYCTL_PKEY_VERIFY,
975 const struct keyctl_pkey_params *params,
980 Use an asymmetric key to perform a public-key cryptographic operation a
981 blob of data. For encryption and verification, the asymmetric key may
982 only need the public parts to be available, but for decryption and signing
983 the private parts are required also.
985 The parameter block pointed to by params contains a number of integer
993 ``key_id`` is the ID of the asymmetric key to be used. ``in_len`` and
994 ``in2_len`` indicate the amount of data in the in and in2 buffers and
995 ``out_len`` indicates the size of the out buffer as appropriate for the
998 For a given operation, the in and out buffers are used as follows::
1000 Operation ID in,in_len out,out_len in2,in2_len
1001 ======================= =============== =============== ===============
1002 KEYCTL_PKEY_ENCRYPT Raw data Encrypted data -
1003 KEYCTL_PKEY_DECRYPT Encrypted data Raw data -
1004 KEYCTL_PKEY_SIGN Raw data Signature -
1005 KEYCTL_PKEY_VERIFY Raw data - Signature
1007 ``info`` is a string of key=value pairs that supply supplementary
1008 information. These include:
1010 ``enc=<encoding>`` The encoding of the encrypted/signature blob. This
1011 can be "pkcs1" for RSASSA-PKCS1-v1.5 or
1012 RSAES-PKCS1-v1.5; "pss" for "RSASSA-PSS"; "oaep" for
1013 "RSAES-OAEP". If omitted or is "raw", the raw output
1014 of the encryption function is specified.
1016 ``hash=<algo>`` If the data buffer contains the output of a hash
1017 function and the encoding includes some indication of
1018 which hash function was used, the hash function can be
1019 specified with this, eg. "hash=sha256".
1021 The ``__spare[]`` space in the parameter block must be set to 0. This is
1022 intended, amongst other things, to allow the passing of passphrases
1023 required to unlock a key.
1025 If successful, encrypt, decrypt and sign all return the amount of data
1026 written into the output buffer. Verification returns 0 on success.
1032 The kernel services for key management are fairly simple to deal with. They can
1033 be broken down into two areas: keys and key types.
1035 Dealing with keys is fairly straightforward. Firstly, the kernel service
1036 registers its type, then it searches for a key of that type. It should retain
1037 the key as long as it has need of it, and then it should release it. For a
1038 filesystem or device file, a search would probably be performed during the open
1039 call, and the key released upon close. How to deal with conflicting keys due to
1040 two different users opening the same file is left to the filesystem author to
1043 To access the key manager, the following header must be #included::
1047 Specific key types should have a header file under include/keys/ that should be
1048 used to access that type. For keys of type "user", for example, that would be::
1052 Note that there are two different types of pointers to keys that may be
1057 This simply points to the key structure itself. Key structures will be at
1058 least four-byte aligned.
1062 This is equivalent to a ``struct key *``, but the least significant bit is set
1063 if the caller "possesses" the key. By "possession" it is meant that the
1064 calling processes has a searchable link to the key from one of its
1065 keyrings. There are three functions for dealing with these::
1067 key_ref_t make_key_ref(const struct key *key, bool possession);
1069 struct key *key_ref_to_ptr(const key_ref_t key_ref);
1071 bool is_key_possessed(const key_ref_t key_ref);
1073 The first function constructs a key reference from a key pointer and
1074 possession information (which must be true or false).
1076 The second function retrieves the key pointer from a reference and the
1077 third retrieves the possession flag.
1079 When accessing a key's payload contents, certain precautions must be taken to
1080 prevent access vs modification races. See the section "Notes on accessing
1081 payload contents" for more information.
1083 * To search for a key, call::
1085 struct key *request_key(const struct key_type *type,
1086 const char *description,
1087 const char *callout_info);
1089 This is used to request a key or keyring with a description that matches
1090 the description specified according to the key type's match_preparse()
1091 method. This permits approximate matching to occur. If callout_string is
1092 not NULL, then /sbin/request-key will be invoked in an attempt to obtain
1093 the key from userspace. In that case, callout_string will be passed as an
1094 argument to the program.
1096 Should the function fail error ENOKEY, EKEYEXPIRED or EKEYREVOKED will be
1099 If successful, the key will have been attached to the default keyring for
1100 implicitly obtained request-key keys, as set by KEYCTL_SET_REQKEY_KEYRING.
1102 See also Documentation/security/keys/request-key.rst.
1105 * To search for a key in a specific domain, call:
1107 struct key *request_key_tag(const struct key_type *type,
1108 const char *description,
1109 struct key_tag *domain_tag,
1110 const char *callout_info);
1112 This is identical to request_key(), except that a domain tag may be
1113 specifies that causes search algorithm to only match keys matching that
1114 tag. The domain_tag may be NULL, specifying a global domain that is
1115 separate from any nominated domain.
1118 * To search for a key, passing auxiliary data to the upcaller, call::
1120 struct key *request_key_with_auxdata(const struct key_type *type,
1121 const char *description,
1122 struct key_tag *domain_tag,
1123 const void *callout_info,
1127 This is identical to request_key_tag(), except that the auxiliary data is
1128 passed to the key_type->request_key() op if it exists, and the
1129 callout_info is a blob of length callout_len, if given (the length may be
1133 * To search for a key under RCU conditions, call::
1135 struct key *request_key_rcu(const struct key_type *type,
1136 const char *description,
1137 struct key_tag *domain_tag);
1139 which is similar to request_key_tag() except that it does not check for
1140 keys that are under construction and it will not call out to userspace to
1141 construct a key if it can't find a match.
1144 * When it is no longer required, the key should be released using::
1146 void key_put(struct key *key);
1150 void key_ref_put(key_ref_t key_ref);
1152 These can be called from interrupt context. If CONFIG_KEYS is not set then
1153 the argument will not be parsed.
1156 * Extra references can be made to a key by calling one of the following
1159 struct key *__key_get(struct key *key);
1160 struct key *key_get(struct key *key);
1162 Keys so references will need to be disposed of by calling key_put() when
1163 they've been finished with. The key pointer passed in will be returned.
1165 In the case of key_get(), if the pointer is NULL or CONFIG_KEYS is not set
1166 then the key will not be dereferenced and no increment will take place.
1169 * A key's serial number can be obtained by calling::
1171 key_serial_t key_serial(struct key *key);
1173 If key is NULL or if CONFIG_KEYS is not set then 0 will be returned (in the
1174 latter case without parsing the argument).
1177 * If a keyring was found in the search, this can be further searched by::
1179 key_ref_t keyring_search(key_ref_t keyring_ref,
1180 const struct key_type *type,
1181 const char *description,
1184 This searches the specified keyring only (recurse == false) or keyring tree
1185 (recurse == true) specified for a matching key. Error ENOKEY is returned
1186 upon failure (use IS_ERR/PTR_ERR to determine). If successful, the returned
1187 key will need to be released.
1189 The possession attribute from the keyring reference is used to control
1190 access through the permissions mask and is propagated to the returned key
1191 reference pointer if successful.
1194 * A keyring can be created by::
1196 struct key *keyring_alloc(const char *description, uid_t uid, gid_t gid,
1197 const struct cred *cred,
1199 struct key_restriction *restrict_link,
1200 unsigned long flags,
1203 This creates a keyring with the given attributes and returns it. If dest
1204 is not NULL, the new keyring will be linked into the keyring to which it
1205 points. No permission checks are made upon the destination keyring.
1207 Error EDQUOT can be returned if the keyring would overload the quota (pass
1208 KEY_ALLOC_NOT_IN_QUOTA in flags if the keyring shouldn't be accounted
1209 towards the user's quota). Error ENOMEM can also be returned.
1211 If restrict_link is not NULL, it should point to a structure that contains
1212 the function that will be called each time an attempt is made to link a
1213 key into the new keyring. The structure may also contain a key pointer
1214 and an associated key type. The function is called to check whether a key
1215 may be added into the keyring or not. The key type is used by the garbage
1216 collector to clean up function or data pointers in this structure if the
1217 given key type is unregistered. Callers of key_create_or_update() within
1218 the kernel can pass KEY_ALLOC_BYPASS_RESTRICTION to suppress the check.
1219 An example of using this is to manage rings of cryptographic keys that are
1220 set up when the kernel boots where userspace is also permitted to add keys
1221 - provided they can be verified by a key the kernel already has.
1223 When called, the restriction function will be passed the keyring being
1224 added to, the key type, the payload of the key being added, and data to be
1225 used in the restriction check. Note that when a new key is being created,
1226 this is called between payload preparsing and actual key creation. The
1227 function should return 0 to allow the link or an error to reject it.
1229 A convenience function, restrict_link_reject, exists to always return
1230 -EPERM to in this case.
1233 * To check the validity of a key, this function can be called::
1235 int validate_key(struct key *key);
1237 This checks that the key in question hasn't expired or and hasn't been
1238 revoked. Should the key be invalid, error EKEYEXPIRED or EKEYREVOKED will
1239 be returned. If the key is NULL or if CONFIG_KEYS is not set then 0 will be
1240 returned (in the latter case without parsing the argument).
1243 * To register a key type, the following function should be called::
1245 int register_key_type(struct key_type *type);
1247 This will return error EEXIST if a type of the same name is already
1251 * To unregister a key type, call::
1253 void unregister_key_type(struct key_type *type);
1256 Under some circumstances, it may be desirable to deal with a bundle of keys.
1257 The facility provides access to the keyring type for managing such a bundle::
1259 struct key_type key_type_keyring;
1261 This can be used with a function such as request_key() to find a specific
1262 keyring in a process's keyrings. A keyring thus found can then be searched
1263 with keyring_search(). Note that it is not possible to use request_key() to
1264 search a specific keyring, so using keyrings in this way is of limited utility.
1267 Notes On Accessing Payload Contents
1268 ===================================
1270 The simplest payload is just data stored in key->payload directly. In this
1271 case, there's no need to indulge in RCU or locking when accessing the payload.
1273 More complex payload contents must be allocated and pointers to them set in the
1274 key->payload.data[] array. One of the following ways must be selected to
1277 1) Unmodifiable key type.
1279 If the key type does not have a modify method, then the key's payload can
1280 be accessed without any form of locking, provided that it's known to be
1281 instantiated (uninstantiated keys cannot be "found").
1283 2) The key's semaphore.
1285 The semaphore could be used to govern access to the payload and to control
1286 the payload pointer. It must be write-locked for modifications and would
1287 have to be read-locked for general access. The disadvantage of doing this
1288 is that the accessor may be required to sleep.
1292 RCU must be used when the semaphore isn't already held; if the semaphore
1293 is held then the contents can't change under you unexpectedly as the
1294 semaphore must still be used to serialise modifications to the key. The
1295 key management code takes care of this for the key type.
1297 However, this means using::
1299 rcu_read_lock() ... rcu_dereference() ... rcu_read_unlock()
1301 to read the pointer, and::
1303 rcu_dereference() ... rcu_assign_pointer() ... call_rcu()
1305 to set the pointer and dispose of the old contents after a grace period.
1306 Note that only the key type should ever modify a key's payload.
1308 Furthermore, an RCU controlled payload must hold a struct rcu_head for the
1309 use of call_rcu() and, if the payload is of variable size, the length of
1310 the payload. key->datalen cannot be relied upon to be consistent with the
1311 payload just dereferenced if the key's semaphore is not held.
1313 Note that key->payload.data[0] has a shadow that is marked for __rcu
1314 usage. This is called key->payload.rcu_data0. The following accessors
1315 wrap the RCU calls to this element:
1317 a) Set or change the first payload pointer::
1319 rcu_assign_keypointer(struct key *key, void *data);
1321 b) Read the first payload pointer with the key semaphore held::
1323 [const] void *dereference_key_locked([const] struct key *key);
1325 Note that the return value will inherit its constness from the key
1326 parameter. Static analysis will give an error if it things the lock
1329 c) Read the first payload pointer with the RCU read lock held::
1331 const void *dereference_key_rcu(const struct key *key);
1337 A kernel service may want to define its own key type. For instance, an AFS
1338 filesystem might want to define a Kerberos 5 ticket key type. To do this, it
1339 author fills in a key_type struct and registers it with the system.
1341 Source files that implement key types should include the following header file::
1345 The structure has a number of fields, some of which are mandatory:
1347 * ``const char *name``
1349 The name of the key type. This is used to translate a key type name
1350 supplied by userspace into a pointer to the structure.
1353 * ``size_t def_datalen``
1355 This is optional - it supplies the default payload data length as
1356 contributed to the quota. If the key type's payload is always or almost
1357 always the same size, then this is a more efficient way to do things.
1359 The data length (and quota) on a particular key can always be changed
1360 during instantiation or update by calling::
1362 int key_payload_reserve(struct key *key, size_t datalen);
1364 With the revised data length. Error EDQUOT will be returned if this is not
1368 * ``int (*vet_description)(const char *description);``
1370 This optional method is called to vet a key description. If the key type
1371 doesn't approve of the key description, it may return an error, otherwise
1375 * ``int (*preparse)(struct key_preparsed_payload *prep);``
1377 This optional method permits the key type to attempt to parse payload
1378 before a key is created (add key) or the key semaphore is taken (update or
1379 instantiate key). The structure pointed to by prep looks like::
1381 struct key_preparsed_payload {
1383 union key_payload payload;
1390 Before calling the method, the caller will fill in data and datalen with
1391 the payload blob parameters; quotalen will be filled in with the default
1392 quota size from the key type; expiry will be set to TIME_T_MAX and the
1393 rest will be cleared.
1395 If a description can be proposed from the payload contents, that should be
1396 attached as a string to the description field. This will be used for the
1397 key description if the caller of add_key() passes NULL or "".
1399 The method can attach anything it likes to payload. This is merely passed
1400 along to the instantiate() or update() operations. If set, the expiry
1401 time will be applied to the key if it is instantiated from this data.
1403 The method should return 0 if successful or a negative error code
1407 * ``void (*free_preparse)(struct key_preparsed_payload *prep);``
1409 This method is only required if the preparse() method is provided,
1410 otherwise it is unused. It cleans up anything attached to the description
1411 and payload fields of the key_preparsed_payload struct as filled in by the
1412 preparse() method. It will always be called after preparse() returns
1413 successfully, even if instantiate() or update() succeed.
1416 * ``int (*instantiate)(struct key *key, struct key_preparsed_payload *prep);``
1418 This method is called to attach a payload to a key during construction.
1419 The payload attached need not bear any relation to the data passed to this
1422 The prep->data and prep->datalen fields will define the original payload
1423 blob. If preparse() was supplied then other fields may be filled in also.
1425 If the amount of data attached to the key differs from the size in
1426 keytype->def_datalen, then key_payload_reserve() should be called.
1428 This method does not have to lock the key in order to attach a payload.
1429 The fact that KEY_FLAG_INSTANTIATED is not set in key->flags prevents
1430 anything else from gaining access to the key.
1432 It is safe to sleep in this method.
1434 generic_key_instantiate() is provided to simply copy the data from
1435 prep->payload.data[] to key->payload.data[], with RCU-safe assignment on
1436 the first element. It will then clear prep->payload.data[] so that the
1437 free_preparse method doesn't release the data.
1440 * ``int (*update)(struct key *key, const void *data, size_t datalen);``
1442 If this type of key can be updated, then this method should be provided.
1443 It is called to update a key's payload from the blob of data provided.
1445 The prep->data and prep->datalen fields will define the original payload
1446 blob. If preparse() was supplied then other fields may be filled in also.
1448 key_payload_reserve() should be called if the data length might change
1449 before any changes are actually made. Note that if this succeeds, the type
1450 is committed to changing the key because it's already been altered, so all
1451 memory allocation must be done first.
1453 The key will have its semaphore write-locked before this method is called,
1454 but this only deters other writers; any changes to the key's payload must
1455 be made under RCU conditions, and call_rcu() must be used to dispose of
1458 key_payload_reserve() should be called before the changes are made, but
1459 after all allocations and other potentially failing function calls are
1462 It is safe to sleep in this method.
1465 * ``int (*match_preparse)(struct key_match_data *match_data);``
1467 This method is optional. It is called when a key search is about to be
1468 performed. It is given the following structure::
1470 struct key_match_data {
1471 bool (*cmp)(const struct key *key,
1472 const struct key_match_data *match_data);
1473 const void *raw_data;
1475 unsigned lookup_type;
1478 On entry, raw_data will be pointing to the criteria to be used in matching
1479 a key by the caller and should not be modified. ``(*cmp)()`` will be pointing
1480 to the default matcher function (which does an exact description match
1481 against raw_data) and lookup_type will be set to indicate a direct lookup.
1483 The following lookup_type values are available:
1485 * KEYRING_SEARCH_LOOKUP_DIRECT - A direct lookup hashes the type and
1486 description to narrow down the search to a small number of keys.
1488 * KEYRING_SEARCH_LOOKUP_ITERATE - An iterative lookup walks all the
1489 keys in the keyring until one is matched. This must be used for any
1490 search that's not doing a simple direct match on the key description.
1492 The method may set cmp to point to a function of its choice that does some
1493 other form of match, may set lookup_type to KEYRING_SEARCH_LOOKUP_ITERATE
1494 and may attach something to the preparsed pointer for use by ``(*cmp)()``.
1495 ``(*cmp)()`` should return true if a key matches and false otherwise.
1497 If preparsed is set, it may be necessary to use the match_free() method to
1500 The method should return 0 if successful or a negative error code
1503 It is permitted to sleep in this method, but ``(*cmp)()`` may not sleep as
1504 locks will be held over it.
1506 If match_preparse() is not provided, keys of this type will be matched
1507 exactly by their description.
1510 * ``void (*match_free)(struct key_match_data *match_data);``
1512 This method is optional. If given, it called to clean up
1513 match_data->preparsed after a successful call to match_preparse().
1516 * ``void (*revoke)(struct key *key);``
1518 This method is optional. It is called to discard part of the payload
1519 data upon a key being revoked. The caller will have the key semaphore
1522 It is safe to sleep in this method, though care should be taken to avoid
1523 a deadlock against the key semaphore.
1526 * ``void (*destroy)(struct key *key);``
1528 This method is optional. It is called to discard the payload data on a key
1529 when it is being destroyed.
1531 This method does not need to lock the key to access the payload; it can
1532 consider the key as being inaccessible at this time. Note that the key's
1533 type may have been changed before this function is called.
1535 It is not safe to sleep in this method; the caller may hold spinlocks.
1538 * ``void (*describe)(const struct key *key, struct seq_file *p);``
1540 This method is optional. It is called during /proc/keys reading to
1541 summarise a key's description and payload in text form.
1543 This method will be called with the RCU read lock held. rcu_dereference()
1544 should be used to read the payload pointer if the payload is to be
1545 accessed. key->datalen cannot be trusted to stay consistent with the
1546 contents of the payload.
1548 The description will not change, though the key's state may.
1550 It is not safe to sleep in this method; the RCU read lock is held by the
1554 * ``long (*read)(const struct key *key, char __user *buffer, size_t buflen);``
1556 This method is optional. It is called by KEYCTL_READ to translate the
1557 key's payload into something a blob of data for userspace to deal with.
1558 Ideally, the blob should be in the same format as that passed in to the
1559 instantiate and update methods.
1561 If successful, the blob size that could be produced should be returned
1562 rather than the size copied.
1564 This method will be called with the key's semaphore read-locked. This will
1565 prevent the key's payload changing. It is not necessary to use RCU locking
1566 when accessing the key's payload. It is safe to sleep in this method, such
1567 as might happen when the userspace buffer is accessed.
1570 * ``int (*request_key)(struct key_construction *cons, const char *op, void *aux);``
1572 This method is optional. If provided, request_key() and friends will
1573 invoke this function rather than upcalling to /sbin/request-key to operate
1574 upon a key of this type.
1576 The aux parameter is as passed to request_key_async_with_auxdata() and
1577 similar or is NULL otherwise. Also passed are the construction record for
1578 the key to be operated upon and the operation type (currently only
1581 This method is permitted to return before the upcall is complete, but the
1582 following function must be called under all circumstances to complete the
1583 instantiation process, whether or not it succeeds, whether or not there's
1586 void complete_request_key(struct key_construction *cons, int error);
1588 The error parameter should be 0 on success, -ve on error. The
1589 construction record is destroyed by this action and the authorisation key
1590 will be revoked. If an error is indicated, the key under construction
1591 will be negatively instantiated if it wasn't already instantiated.
1593 If this method returns an error, that error will be returned to the
1594 caller of request_key*(). complete_request_key() must be called prior to
1597 The key under construction and the authorisation key can be found in the
1598 key_construction struct pointed to by cons:
1600 * ``struct key *key;``
1602 The key under construction.
1604 * ``struct key *authkey;``
1606 The authorisation key.
1609 * ``struct key_restriction *(*lookup_restriction)(const char *params);``
1611 This optional method is used to enable userspace configuration of keyring
1612 restrictions. The restriction parameter string (not including the key type
1613 name) is passed in, and this method returns a pointer to a key_restriction
1614 structure containing the relevant functions and data to evaluate each
1615 attempted key link operation. If there is no match, -EINVAL is returned.
1618 * ``asym_eds_op`` and ``asym_verify_signature``::
1620 int (*asym_eds_op)(struct kernel_pkey_params *params,
1621 const void *in, void *out);
1622 int (*asym_verify_signature)(struct kernel_pkey_params *params,
1623 const void *in, const void *in2);
1625 These methods are optional. If provided the first allows a key to be
1626 used to encrypt, decrypt or sign a blob of data, and the second allows a
1627 key to verify a signature.
1629 In all cases, the following information is provided in the params block::
1631 struct kernel_pkey_params {
1633 const char *encoding;
1634 const char *hash_algo;
1641 enum kernel_pkey_operation op : 8;
1644 This includes the key to be used; a string indicating the encoding to use
1645 (for instance, "pkcs1" may be used with an RSA key to indicate
1646 RSASSA-PKCS1-v1.5 or RSAES-PKCS1-v1.5 encoding or "raw" if no encoding);
1647 the name of the hash algorithm used to generate the data for a signature
1648 (if appropriate); the sizes of the input and output (or second input)
1649 buffers; and the ID of the operation to be performed.
1651 For a given operation ID, the input and output buffers are used as
1654 Operation ID in,in_len out,out_len in2,in2_len
1655 ======================= =============== =============== ===============
1656 kernel_pkey_encrypt Raw data Encrypted data -
1657 kernel_pkey_decrypt Encrypted data Raw data -
1658 kernel_pkey_sign Raw data Signature -
1659 kernel_pkey_verify Raw data - Signature
1661 asym_eds_op() deals with encryption, decryption and signature creation as
1662 specified by params->op. Note that params->op is also set for
1663 asym_verify_signature().
1665 Encrypting and signature creation both take raw data in the input buffer
1666 and return the encrypted result in the output buffer. Padding may have
1667 been added if an encoding was set. In the case of signature creation,
1668 depending on the encoding, the padding created may need to indicate the
1669 digest algorithm - the name of which should be supplied in hash_algo.
1671 Decryption takes encrypted data in the input buffer and returns the raw
1672 data in the output buffer. Padding will get checked and stripped off if
1673 an encoding was set.
1675 Verification takes raw data in the input buffer and the signature in the
1676 second input buffer and checks that the one matches the other. Padding
1677 will be validated. Depending on the encoding, the digest algorithm used
1678 to generate the raw data may need to be indicated in hash_algo.
1680 If successful, asym_eds_op() should return the number of bytes written
1681 into the output buffer. asym_verify_signature() should return 0.
1683 A variety of errors may be returned, including EOPNOTSUPP if the operation
1684 is not supported; EKEYREJECTED if verification fails; ENOPKG if the
1685 required crypto isn't available.
1690 int (*asym_query)(const struct kernel_pkey_params *params,
1691 struct kernel_pkey_query *info);
1693 This method is optional. If provided it allows information about the
1694 public or asymmetric key held in the key to be determined.
1696 The parameter block is as for asym_eds_op() and co. but in_len and out_len
1697 are unused. The encoding and hash_algo fields should be used to reduce
1698 the returned buffer/data sizes as appropriate.
1700 If successful, the following information is filled in::
1702 struct kernel_pkey_query {
1703 __u32 supported_ops;
1705 __u16 max_data_size;
1711 The supported_ops field will contain a bitmask indicating what operations
1712 are supported by the key, including encryption of a blob, decryption of a
1713 blob, signing a blob and verifying the signature on a blob. The following
1714 constants are defined for this::
1716 KEYCTL_SUPPORTS_{ENCRYPT,DECRYPT,SIGN,VERIFY}
1718 The key_size field is the size of the key in bits. max_data_size and
1719 max_sig_size are the maximum raw data and signature sizes for creation and
1720 verification of a signature; max_enc_size and max_dec_size are the maximum
1721 raw data and signature sizes for encryption and decryption. The
1722 max_*_size fields are measured in bytes.
1724 If successful, 0 will be returned. If the key doesn't support this,
1725 EOPNOTSUPP will be returned.
1728 Request-Key Callback Service
1729 ============================
1731 To create a new key, the kernel will attempt to execute the following command
1734 /sbin/request-key create <key> <uid> <gid> \
1735 <threadring> <processring> <sessionring> <callout_info>
1737 <key> is the key being constructed, and the three keyrings are the process
1738 keyrings from the process that caused the search to be issued. These are
1739 included for two reasons:
1741 1 There may be an authentication token in one of the keyrings that is
1742 required to obtain the key, eg: a Kerberos Ticket-Granting Ticket.
1744 2 The new key should probably be cached in one of these rings.
1746 This program should set it UID and GID to those specified before attempting to
1747 access any more keys. It may then look around for a user specific process to
1748 hand the request off to (perhaps a path held in placed in another key by, for
1749 example, the KDE desktop manager).
1751 The program (or whatever it calls) should finish construction of the key by
1752 calling KEYCTL_INSTANTIATE or KEYCTL_INSTANTIATE_IOV, which also permits it to
1753 cache the key in one of the keyrings (probably the session ring) before
1754 returning. Alternatively, the key can be marked as negative with KEYCTL_NEGATE
1755 or KEYCTL_REJECT; this also permits the key to be cached in one of the
1758 If it returns with the key remaining in the unconstructed state, the key will
1759 be marked as being negative, it will be added to the session keyring, and an
1760 error will be returned to the key requestor.
1762 Supplementary information may be provided from whoever or whatever invoked this
1763 service. This will be passed as the <callout_info> parameter. If no such
1764 information was made available, then "-" will be passed as this parameter
1768 Similarly, the kernel may attempt to update an expired or a soon to expire key
1771 /sbin/request-key update <key> <uid> <gid> \
1772 <threadring> <processring> <sessionring>
1774 In this case, the program isn't required to actually attach the key to a ring;
1775 the rings are provided for reference.
1781 Dead keys (for which the type has been removed) will be automatically unlinked
1782 from those keyrings that point to them and deleted as soon as possible by a
1783 background garbage collector.
1785 Similarly, revoked and expired keys will be garbage collected, but only after a
1786 certain amount of time has passed. This time is set as a number of seconds in::
1788 /proc/sys/kernel/keys/gc_delay