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
35 In this context, keys represent units of cryptographic data, authentication
36 tokens, keyrings, etc.. These are represented in the kernel by struct key.
38 Each key has a number of attributes:
42 - A description (for matching a key in a search).
43 - Access control information.
49 * Each key is issued a serial number of type key_serial_t that is unique for
50 the lifetime of that key. All serial numbers are positive non-zero 32-bit
53 Userspace programs can use a key's serial numbers as a way to gain access
54 to it, subject to permission checking.
56 * Each key is of a defined "type". Types must be registered inside the
57 kernel by a kernel service (such as a filesystem) before keys of that type
58 can be added or used. Userspace programs cannot define new types directly.
60 Key types are represented in the kernel by struct key_type. This defines a
61 number of operations that can be performed on a key of that type.
63 Should a type be removed from the system, all the keys of that type will
66 * Each key has a description. This should be a printable string. The key
67 type provides an operation to perform a match between the description on a
68 key and a criterion string.
70 * Each key has an owner user ID, a group ID and a permissions mask. These
71 are used to control what a process may do to a key from userspace, and
72 whether a kernel service will be able to find the key.
74 * Each key can be set to expire at a specific time by the key type's
75 instantiation function. Keys can also be immortal.
77 * Each key can have a payload. This is a quantity of data that represent the
78 actual "key". In the case of a keyring, this is a list of keys to which
79 the keyring links; in the case of a user-defined key, it's an arbitrary
82 Having a payload is not required; and the payload can, in fact, just be a
83 value stored in the struct key itself.
85 When a key is instantiated, the key type's instantiation function is
86 called with a blob of data, and that then creates the key's payload in
89 Similarly, when userspace wants to read back the contents of the key, if
90 permitted, another key type operation will be called to convert the key's
91 attached payload back into a blob of data.
93 * Each key can be in one of a number of basic states:
95 * Uninstantiated. The key exists, but does not have any data attached.
96 Keys being requested from userspace will be in this state.
98 * Instantiated. This is the normal state. The key is fully formed, and
101 * Negative. This is a relatively short-lived state. The key acts as a
102 note saying that a previous call out to userspace failed, and acts as
103 a throttle on key lookups. A negative key can be updated to a normal
106 * Expired. Keys can have lifetimes set. If their lifetime is exceeded,
107 they traverse to this state. An expired key can be updated back to a
110 * Revoked. A key is put in this state by userspace action. It can't be
111 found or operated upon (apart from by unlinking it).
113 * Dead. The key's type was unregistered, and so the key is now useless.
115 Keys in the last three states are subject to garbage collection. See the
116 section on "Garbage collection".
122 The key service provides a number of features besides keys:
124 * The key service defines three special key types:
128 Keyrings are special keys that contain a list of other keys. Keyring
129 lists can be modified using various system calls. Keyrings should not
130 be given a payload when created.
134 A key of this type has a description and a payload that are arbitrary
135 blobs of data. These can be created, updated and read by userspace,
136 and aren't intended for use by kernel services.
140 Like a "user" key, a "logon" key has a payload that is an arbitrary
141 blob of data. It is intended as a place to store secrets which are
142 accessible to the kernel but not to userspace programs.
144 The description can be arbitrary, but must be prefixed with a non-zero
145 length string that describes the key "subclass". The subclass is
146 separated from the rest of the description by a ':'. "logon" keys can
147 be created and updated from userspace, but the payload is only
148 readable from kernel space.
150 * Each process subscribes to three keyrings: a thread-specific keyring, a
151 process-specific keyring, and a session-specific keyring.
153 The thread-specific keyring is discarded from the child when any sort of
154 clone, fork, vfork or execve occurs. A new keyring is created only when
157 The process-specific keyring is replaced with an empty one in the child on
158 clone, fork, vfork unless CLONE_THREAD is supplied, in which case it is
159 shared. execve also discards the process's process keyring and creates a
162 The session-specific keyring is persistent across clone, fork, vfork and
163 execve, even when the latter executes a set-UID or set-GID binary. A
164 process can, however, replace its current session keyring with a new one
165 by using PR_JOIN_SESSION_KEYRING. It is permitted to request an anonymous
166 new one, or to attempt to create or join one of a specific name.
168 The ownership of the thread keyring changes when the real UID and GID of
171 * Each user ID resident in the system holds two special keyrings: a user
172 specific keyring and a default user session keyring. The default session
173 keyring is initialised with a link to the user-specific keyring.
175 When a process changes its real UID, if it used to have no session key, it
176 will be subscribed to the default session key for the new UID.
178 If a process attempts to access its session key when it doesn't have one,
179 it will be subscribed to the default for its current UID.
181 * Each user has two quotas against which the keys they own are tracked. One
182 limits the total number of keys and keyrings, the other limits the total
183 amount of description and payload space that can be consumed.
185 The user can view information on this and other statistics through procfs
186 files. The root user may also alter the quota limits through sysctl files
187 (see the section "New procfs files").
189 Process-specific and thread-specific keyrings are not counted towards a
192 If a system call that modifies a key or keyring in some way would put the
193 user over quota, the operation is refused and error EDQUOT is returned.
195 * There's a system call interface by which userspace programs can create and
196 manipulate keys and keyrings.
198 * There's a kernel interface by which services can register types and search
201 * There's a way for the a search done from the kernel to call back to
202 userspace to request a key that can't be found in a process's keyrings.
204 * An optional filesystem is available through which the key database can be
205 viewed and manipulated.
208 Key Access Permissions
209 ======================
211 Keys have an owner user ID, a group access ID, and a permissions mask. The mask
212 has up to eight bits each for possessor, user, group and other access. Only
213 six of each set of eight bits are defined. These permissions granted are:
217 This permits a key or keyring's attributes to be viewed - including key
218 type and description.
222 This permits a key's payload to be viewed or a keyring's list of linked
227 This permits a key's payload to be instantiated or updated, or it allows a
228 link to be added to or removed from a keyring.
232 This permits keyrings to be searched and keys to be found. Searches can
233 only recurse into nested keyrings that have search permission set.
237 This permits a key or keyring to be linked to. To create a link from a
238 keyring to a key, a process must have Write permission on the keyring and
239 Link permission on the key.
243 This permits a key's UID, GID and permissions mask to be changed.
245 For changing the ownership, group ID or permissions mask, being the owner of
246 the key or having the sysadmin capability is sufficient.
252 The security class "key" has been added to SELinux so that mandatory access
253 controls can be applied to keys created within various contexts. This support
254 is preliminary, and is likely to change quite significantly in the near future.
255 Currently, all of the basic permissions explained above are provided in SELinux
256 as well; SELinux is simply invoked after all basic permission checks have been
259 The value of the file /proc/self/attr/keycreate influences the labeling of
260 newly-created keys. If the contents of that file correspond to an SELinux
261 security context, then the key will be assigned that context. Otherwise, the
262 key will be assigned the current context of the task that invoked the key
263 creation request. Tasks must be granted explicit permission to assign a
264 particular context to newly-created keys, using the "create" permission in the
267 The default keyrings associated with users will be labeled with the default
268 context of the user if and only if the login programs have been instrumented to
269 properly initialize keycreate during the login process. Otherwise, they will
270 be labeled with the context of the login program itself.
272 Note, however, that the default keyrings associated with the root user are
273 labeled with the default kernel context, since they are created early in the
274 boot process, before root has a chance to log in.
276 The keyrings associated with new threads are each labeled with the context of
277 their associated thread, and both session and process keyrings are handled
284 Two files have been added to procfs by which an administrator can find out
285 about the status of the key service:
289 This lists the keys that are currently viewable by the task reading the
290 file, giving information about their type, description and permissions.
291 It is not possible to view the payload of the key this way, though some
292 information about it may be given.
294 The only keys included in the list are those that grant View permission to
295 the reading process whether or not it possesses them. Note that LSM
296 security checks are still performed, and may further filter out keys that
297 the current process is not authorised to view.
299 The contents of the file look like this::
301 SERIAL FLAGS USAGE EXPY PERM UID GID TYPE DESCRIPTION: SUMMARY
302 00000001 I----- 39 perm 1f3f0000 0 0 keyring _uid_ses.0: 1/4
303 00000002 I----- 2 perm 1f3f0000 0 0 keyring _uid.0: empty
304 00000007 I----- 1 perm 1f3f0000 0 0 keyring _pid.1: empty
305 0000018d I----- 1 perm 1f3f0000 0 0 keyring _pid.412: empty
306 000004d2 I--Q-- 1 perm 1f3f0000 32 -1 keyring _uid.32: 1/4
307 000004d3 I--Q-- 3 perm 1f3f0000 32 -1 keyring _uid_ses.32: empty
308 00000892 I--QU- 1 perm 1f000000 0 0 user metal:copper: 0
309 00000893 I--Q-N 1 35s 1f3f0000 0 0 user metal:silver: 0
310 00000894 I--Q-- 1 10h 003f0000 0 0 user metal:gold: 0
317 Q Contributes to user's quota
318 U Under construction by callback to userspace
324 This file lists the tracking data for each user that has at least one key
325 on the system. Such data includes quota information and statistics::
327 [root@andromeda root]# cat /proc/key-users
328 0: 46 45/45 1/100 13/10000
329 29: 2 2/2 2/100 40/10000
330 32: 2 2/2 2/100 40/10000
331 38: 2 2/2 2/100 40/10000
333 The format of each line is::
335 <UID>: User ID to which this applies
336 <usage> Structure refcount
337 <inst>/<keys> Total number of keys and number instantiated
338 <keys>/<max> Key count quota
339 <bytes>/<max> Key size quota
342 Four new sysctl files have been added also for the purpose of controlling the
343 quota limits on keys:
345 * /proc/sys/kernel/keys/root_maxkeys
346 /proc/sys/kernel/keys/root_maxbytes
348 These files hold the maximum number of keys that root may have and the
349 maximum total number of bytes of data that root may have stored in those
352 * /proc/sys/kernel/keys/maxkeys
353 /proc/sys/kernel/keys/maxbytes
355 These files hold the maximum number of keys that each non-root user may
356 have and the maximum total number of bytes of data that each of those
357 users may have stored in their keys.
359 Root may alter these by writing each new limit as a decimal number string to
360 the appropriate file.
363 Userspace System Call Interface
364 ===============================
366 Userspace can manipulate keys directly through three new syscalls: add_key,
367 request_key and keyctl. The latter provides a number of functions for
370 When referring to a key directly, userspace programs should use the key's
371 serial number (a positive 32-bit integer). However, there are some special
372 values available for referring to special keys and keyrings that relate to the
373 process making the call::
375 CONSTANT VALUE KEY REFERENCED
376 ============================== ====== ===========================
377 KEY_SPEC_THREAD_KEYRING -1 thread-specific keyring
378 KEY_SPEC_PROCESS_KEYRING -2 process-specific keyring
379 KEY_SPEC_SESSION_KEYRING -3 session-specific keyring
380 KEY_SPEC_USER_KEYRING -4 UID-specific keyring
381 KEY_SPEC_USER_SESSION_KEYRING -5 UID-session keyring
382 KEY_SPEC_GROUP_KEYRING -6 GID-specific keyring
383 KEY_SPEC_REQKEY_AUTH_KEY -7 assumed request_key()
387 The main syscalls are:
389 * Create a new key of given type, description and payload and add it to the
392 key_serial_t add_key(const char *type, const char *desc,
393 const void *payload, size_t plen,
394 key_serial_t keyring);
396 If a key of the same type and description as that proposed already exists
397 in the keyring, this will try to update it with the given payload, or it
398 will return error EEXIST if that function is not supported by the key
399 type. The process must also have permission to write to the key to be able
400 to update it. The new key will have all user permissions granted and no
401 group or third party permissions.
403 Otherwise, this will attempt to create a new key of the specified type and
404 description, and to instantiate it with the supplied payload and attach it
405 to the keyring. In this case, an error will be generated if the process
406 does not have permission to write to the keyring.
408 If the key type supports it, if the description is NULL or an empty
409 string, the key type will try and generate a description from the content
412 The payload is optional, and the pointer can be NULL if not required by
413 the type. The payload is plen in size, and plen can be zero for an empty
416 A new keyring can be generated by setting type "keyring", the keyring name
417 as the description (or NULL) and setting the payload to NULL.
419 User defined keys can be created by specifying type "user". It is
420 recommended that a user defined key's description by prefixed with a type
421 ID and a colon, such as "krb5tgt:" for a Kerberos 5 ticket granting
424 Any other type must have been registered with the kernel in advance by a
425 kernel service such as a filesystem.
427 The ID of the new or updated key is returned if successful.
430 * Search the process's keyrings for a key, potentially calling out to
431 userspace to create it::
433 key_serial_t request_key(const char *type, const char *description,
434 const char *callout_info,
435 key_serial_t dest_keyring);
437 This function searches all the process's keyrings in the order thread,
438 process, session for a matching key. This works very much like
439 KEYCTL_SEARCH, including the optional attachment of the discovered key to
442 If a key cannot be found, and if callout_info is not NULL, then
443 /sbin/request-key will be invoked in an attempt to obtain a key. The
444 callout_info string will be passed as an argument to the program.
446 See also Documentation/security/keys-request-key.txt.
449 The keyctl syscall functions are:
451 * Map a special key ID to a real key ID for this process::
453 key_serial_t keyctl(KEYCTL_GET_KEYRING_ID, key_serial_t id,
456 The special key specified by "id" is looked up (with the key being created
457 if necessary) and the ID of the key or keyring thus found is returned if
460 If the key does not yet exist, the key will be created if "create" is
461 non-zero; and the error ENOKEY will be returned if "create" is zero.
464 * Replace the session keyring this process subscribes to with a new one::
466 key_serial_t keyctl(KEYCTL_JOIN_SESSION_KEYRING, const char *name);
468 If name is NULL, an anonymous keyring is created attached to the process
469 as its session keyring, displacing the old session keyring.
471 If name is not NULL, if a keyring of that name exists, the process
472 attempts to attach it as the session keyring, returning an error if that
473 is not permitted; otherwise a new keyring of that name is created and
474 attached as the session keyring.
476 To attach to a named keyring, the keyring must have search permission for
477 the process's ownership.
479 The ID of the new session keyring is returned if successful.
482 * Update the specified key::
484 long keyctl(KEYCTL_UPDATE, key_serial_t key, const void *payload,
487 This will try to update the specified key with the given payload, or it
488 will return error EOPNOTSUPP if that function is not supported by the key
489 type. The process must also have permission to write to the key to be able
492 The payload is of length plen, and may be absent or empty as for
498 long keyctl(KEYCTL_REVOKE, key_serial_t key);
500 This makes a key unavailable for further operations. Further attempts to
501 use the key will be met with error EKEYREVOKED, and the key will no longer
505 * Change the ownership of a key::
507 long keyctl(KEYCTL_CHOWN, key_serial_t key, uid_t uid, gid_t gid);
509 This function permits a key's owner and group ID to be changed. Either one
510 of uid or gid can be set to -1 to suppress that change.
512 Only the superuser can change a key's owner to something other than the
513 key's current owner. Similarly, only the superuser can change a key's
514 group ID to something other than the calling process's group ID or one of
515 its group list members.
518 * Change the permissions mask on a key::
520 long keyctl(KEYCTL_SETPERM, key_serial_t key, key_perm_t perm);
522 This function permits the owner of a key or the superuser to change the
523 permissions mask on a key.
525 Only bits the available bits are permitted; if any other bits are set,
526 error EINVAL will be returned.
531 long keyctl(KEYCTL_DESCRIBE, key_serial_t key, char *buffer,
534 This function returns a summary of the key's attributes (but not its
535 payload data) as a string in the buffer provided.
537 Unless there's an error, it always returns the amount of data it could
538 produce, even if that's too big for the buffer, but it won't copy more
539 than requested to userspace. If the buffer pointer is NULL then no copy
542 A process must have view permission on the key for this function to be
545 If successful, a string is placed in the buffer in the following format::
547 <type>;<uid>;<gid>;<perm>;<description>
549 Where type and description are strings, uid and gid are decimal, and perm
550 is hexadecimal. A NUL character is included at the end of the string if
551 the buffer is sufficiently big.
553 This can be parsed with::
555 sscanf(buffer, "%[^;];%d;%d;%o;%s", type, &uid, &gid, &mode, desc);
558 * Clear out a keyring::
560 long keyctl(KEYCTL_CLEAR, key_serial_t keyring);
562 This function clears the list of keys attached to a keyring. The calling
563 process must have write permission on the keyring, and it must be a
564 keyring (or else error ENOTDIR will result).
566 This function can also be used to clear special kernel keyrings if they
567 are appropriately marked if the user has CAP_SYS_ADMIN capability. The
568 DNS resolver cache keyring is an example of this.
571 * Link a key into a keyring::
573 long keyctl(KEYCTL_LINK, key_serial_t keyring, key_serial_t key);
575 This function creates a link from the keyring to the key. The process must
576 have write permission on the keyring and must have link permission on the
579 Should the keyring not be a keyring, error ENOTDIR will result; and if the
580 keyring is full, error ENFILE will result.
582 The link procedure checks the nesting of the keyrings, returning ELOOP if
583 it appears too deep or EDEADLK if the link would introduce a cycle.
585 Any links within the keyring to keys that match the new key in terms of
586 type and description will be discarded from the keyring as the new one is
590 * Unlink a key or keyring from another keyring::
592 long keyctl(KEYCTL_UNLINK, key_serial_t keyring, key_serial_t key);
594 This function looks through the keyring for the first link to the
595 specified key, and removes it if found. Subsequent links to that key are
596 ignored. The process must have write permission on the keyring.
598 If the keyring is not a keyring, error ENOTDIR will result; and if the key
599 is not present, error ENOENT will be the result.
602 * Search a keyring tree for a key::
604 key_serial_t keyctl(KEYCTL_SEARCH, key_serial_t keyring,
605 const char *type, const char *description,
606 key_serial_t dest_keyring);
608 This searches the keyring tree headed by the specified keyring until a key
609 is found that matches the type and description criteria. Each keyring is
610 checked for keys before recursion into its children occurs.
612 The process must have search permission on the top level keyring, or else
613 error EACCES will result. Only keyrings that the process has search
614 permission on will be recursed into, and only keys and keyrings for which
615 a process has search permission can be matched. If the specified keyring
616 is not a keyring, ENOTDIR will result.
618 If the search succeeds, the function will attempt to link the found key
619 into the destination keyring if one is supplied (non-zero ID). All the
620 constraints applicable to KEYCTL_LINK apply in this case too.
622 Error ENOKEY, EKEYREVOKED or EKEYEXPIRED will be returned if the search
623 fails. On success, the resulting key ID will be returned.
626 * Read the payload data from a key::
628 long keyctl(KEYCTL_READ, key_serial_t keyring, char *buffer,
631 This function attempts to read the payload data from the specified key
632 into the buffer. The process must have read permission on the key to
635 The returned data will be processed for presentation by the key type. For
636 instance, a keyring will return an array of key_serial_t entries
637 representing the IDs of all the keys to which it is subscribed. The user
638 defined key type will return its data as is. If a key type does not
639 implement this function, error EOPNOTSUPP will result.
641 As much of the data as can be fitted into the buffer will be copied to
642 userspace if the buffer pointer is not NULL.
644 On a successful return, the function will always return the amount of data
645 available rather than the amount copied.
648 * Instantiate a partially constructed key::
650 long keyctl(KEYCTL_INSTANTIATE, key_serial_t key,
651 const void *payload, size_t plen,
652 key_serial_t keyring);
653 long keyctl(KEYCTL_INSTANTIATE_IOV, key_serial_t key,
654 const struct iovec *payload_iov, unsigned ioc,
655 key_serial_t keyring);
657 If the kernel calls back to userspace to complete the instantiation of a
658 key, userspace should use this call to supply data for the key before the
659 invoked process returns, or else the key will be marked negative
662 The process must have write access on the key to be able to instantiate
663 it, and the key must be uninstantiated.
665 If a keyring is specified (non-zero), the key will also be linked into
666 that keyring, however all the constraints applying in KEYCTL_LINK apply in
669 The payload and plen arguments describe the payload data as for add_key().
671 The payload_iov and ioc arguments describe the payload data in an iovec
672 array instead of a single buffer.
675 * Negatively instantiate a partially constructed key::
677 long keyctl(KEYCTL_NEGATE, key_serial_t key,
678 unsigned timeout, key_serial_t keyring);
679 long keyctl(KEYCTL_REJECT, key_serial_t key,
680 unsigned timeout, unsigned error, key_serial_t keyring);
682 If the kernel calls back to userspace to complete the instantiation of a
683 key, userspace should use this call mark the key as negative before the
684 invoked process returns if it is unable to fulfill the request.
686 The process must have write access on the key to be able to instantiate
687 it, and the key must be uninstantiated.
689 If a keyring is specified (non-zero), the key will also be linked into
690 that keyring, however all the constraints applying in KEYCTL_LINK apply in
693 If the key is rejected, future searches for it will return the specified
694 error code until the rejected key expires. Negating the key is the same
695 as rejecting the key with ENOKEY as the error code.
698 * Set the default request-key destination keyring::
700 long keyctl(KEYCTL_SET_REQKEY_KEYRING, int reqkey_defl);
702 This sets the default keyring to which implicitly requested keys will be
703 attached for this thread. reqkey_defl should be one of these constants::
705 CONSTANT VALUE NEW DEFAULT KEYRING
706 ====================================== ====== =======================
707 KEY_REQKEY_DEFL_NO_CHANGE -1 No change
708 KEY_REQKEY_DEFL_DEFAULT 0 Default[1]
709 KEY_REQKEY_DEFL_THREAD_KEYRING 1 Thread keyring
710 KEY_REQKEY_DEFL_PROCESS_KEYRING 2 Process keyring
711 KEY_REQKEY_DEFL_SESSION_KEYRING 3 Session keyring
712 KEY_REQKEY_DEFL_USER_KEYRING 4 User keyring
713 KEY_REQKEY_DEFL_USER_SESSION_KEYRING 5 User session keyring
714 KEY_REQKEY_DEFL_GROUP_KEYRING 6 Group keyring
716 The old default will be returned if successful and error EINVAL will be
717 returned if reqkey_defl is not one of the above values.
719 The default keyring can be overridden by the keyring indicated to the
720 request_key() system call.
722 Note that this setting is inherited across fork/exec.
724 [1] The default is: the thread keyring if there is one, otherwise
725 the process keyring if there is one, otherwise the session keyring if
726 there is one, otherwise the user default session keyring.
729 * Set the timeout on a key::
731 long keyctl(KEYCTL_SET_TIMEOUT, key_serial_t key, unsigned timeout);
733 This sets or clears the timeout on a key. The timeout can be 0 to clear
734 the timeout or a number of seconds to set the expiry time that far into
737 The process must have attribute modification access on a key to set its
738 timeout. Timeouts may not be set with this function on negative, revoked
742 * Assume the authority granted to instantiate a key::
744 long keyctl(KEYCTL_ASSUME_AUTHORITY, key_serial_t key);
746 This assumes or divests the authority required to instantiate the
747 specified key. Authority can only be assumed if the thread has the
748 authorisation key associated with the specified key in its keyrings
751 Once authority is assumed, searches for keys will also search the
752 requester's keyrings using the requester's security label, UID, GID and
755 If the requested authority is unavailable, error EPERM will be returned,
756 likewise if the authority has been revoked because the target key is
757 already instantiated.
759 If the specified key is 0, then any assumed authority will be divested.
761 The assumed authoritative key is inherited across fork and exec.
764 * Get the LSM security context attached to a key::
766 long keyctl(KEYCTL_GET_SECURITY, key_serial_t key, char *buffer,
769 This function returns a string that represents the LSM security context
770 attached to a key in the buffer provided.
772 Unless there's an error, it always returns the amount of data it could
773 produce, even if that's too big for the buffer, but it won't copy more
774 than requested to userspace. If the buffer pointer is NULL then no copy
777 A NUL character is included at the end of the string if the buffer is
778 sufficiently big. This is included in the returned count. If no LSM is
779 in force then an empty string will be returned.
781 A process must have view permission on the key for this function to be
785 * Install the calling process's session keyring on its parent::
787 long keyctl(KEYCTL_SESSION_TO_PARENT);
789 This functions attempts to install the calling process's session keyring
790 on to the calling process's parent, replacing the parent's current session
793 The calling process must have the same ownership as its parent, the
794 keyring must have the same ownership as the calling process, the calling
795 process must have LINK permission on the keyring and the active LSM module
796 mustn't deny permission, otherwise error EPERM will be returned.
798 Error ENOMEM will be returned if there was insufficient memory to complete
799 the operation, otherwise 0 will be returned to indicate success.
801 The keyring will be replaced next time the parent process leaves the
802 kernel and resumes executing userspace.
807 long keyctl(KEYCTL_INVALIDATE, key_serial_t key);
809 This function marks a key as being invalidated and then wakes up the
810 garbage collector. The garbage collector immediately removes invalidated
811 keys from all keyrings and deletes the key when its reference count
814 Keys that are marked invalidated become invisible to normal key operations
815 immediately, though they are still visible in /proc/keys until deleted
816 (they're marked with an 'i' flag).
818 A process must have search permission on the key for this function to be
821 * Compute a Diffie-Hellman shared secret or public key::
823 long keyctl(KEYCTL_DH_COMPUTE, struct keyctl_dh_params *params,
824 char *buffer, size_t buflen, struct keyctl_kdf_params *kdf);
826 The params struct contains serial numbers for three keys::
828 - The prime, p, known to both parties
829 - The local private key
830 - The base integer, which is either a shared generator or the
833 The value computed is::
835 result = base ^ private (mod prime)
837 If the base is the shared generator, the result is the local
838 public key. If the base is the remote public key, the result is
841 If the parameter kdf is NULL, the following applies:
843 - The buffer length must be at least the length of the prime, or zero.
845 - If the buffer length is nonzero, the length of the result is
846 returned when it is successfully calculated and copied in to the
847 buffer. When the buffer length is zero, the minimum required
848 buffer length is returned.
850 The kdf parameter allows the caller to apply a key derivation function
851 (KDF) on the Diffie-Hellman computation where only the result
852 of the KDF is returned to the caller. The KDF is characterized with
853 struct keyctl_kdf_params as follows:
855 - ``char *hashname`` specifies the NUL terminated string identifying
856 the hash used from the kernel crypto API and applied for the KDF
857 operation. The KDF implemenation complies with SP800-56A as well
858 as with SP800-108 (the counter KDF).
860 - ``char *otherinfo`` specifies the OtherInfo data as documented in
861 SP800-56A section 5.8.1.2. The length of the buffer is given with
862 otherinfolen. The format of OtherInfo is defined by the caller.
863 The otherinfo pointer may be NULL if no OtherInfo shall be used.
865 This function will return error EOPNOTSUPP if the key type is not
866 supported, error ENOKEY if the key could not be found, or error
867 EACCES if the key is not readable by the caller. In addition, the
868 function will return EMSGSIZE when the parameter kdf is non-NULL
869 and either the buffer length or the OtherInfo length exceeds the
872 * Restrict keyring linkage::
874 long keyctl(KEYCTL_RESTRICT_KEYRING, key_serial_t keyring,
875 const char *type, const char *restriction);
877 An existing keyring can restrict linkage of additional keys by evaluating
878 the contents of the key according to a restriction scheme.
880 "keyring" is the key ID for an existing keyring to apply a restriction
881 to. It may be empty or may already have keys linked. Existing linked keys
882 will remain in the keyring even if the new restriction would reject them.
884 "type" is a registered key type.
886 "restriction" is a string describing how key linkage is to be restricted.
887 The format varies depending on the key type, and the string is passed to
888 the lookup_restriction() function for the requested type. It may specify
889 a method and relevant data for the restriction such as signature
890 verification or constraints on key payload. If the requested key type is
891 later unregistered, no keys may be added to the keyring after the key type
894 To apply a keyring restriction the process must have Set Attribute
895 permission and the keyring must not be previously restricted.
897 One application of restricted keyrings is to verify X.509 certificate
898 chains or individual certificate signatures using the asymmetric key type.
899 See Documentation/crypto/asymmetric-keys.txt for specific restrictions
900 applicable to the asymmetric key type.
906 The kernel services for key management are fairly simple to deal with. They can
907 be broken down into two areas: keys and key types.
909 Dealing with keys is fairly straightforward. Firstly, the kernel service
910 registers its type, then it searches for a key of that type. It should retain
911 the key as long as it has need of it, and then it should release it. For a
912 filesystem or device file, a search would probably be performed during the open
913 call, and the key released upon close. How to deal with conflicting keys due to
914 two different users opening the same file is left to the filesystem author to
917 To access the key manager, the following header must be #included::
921 Specific key types should have a header file under include/keys/ that should be
922 used to access that type. For keys of type "user", for example, that would be::
926 Note that there are two different types of pointers to keys that may be
931 This simply points to the key structure itself. Key structures will be at
932 least four-byte aligned.
936 This is equivalent to a ``struct key *``, but the least significant bit is set
937 if the caller "possesses" the key. By "possession" it is meant that the
938 calling processes has a searchable link to the key from one of its
939 keyrings. There are three functions for dealing with these::
941 key_ref_t make_key_ref(const struct key *key, bool possession);
943 struct key *key_ref_to_ptr(const key_ref_t key_ref);
945 bool is_key_possessed(const key_ref_t key_ref);
947 The first function constructs a key reference from a key pointer and
948 possession information (which must be true or false).
950 The second function retrieves the key pointer from a reference and the
951 third retrieves the possession flag.
953 When accessing a key's payload contents, certain precautions must be taken to
954 prevent access vs modification races. See the section "Notes on accessing
955 payload contents" for more information.
957 * To search for a key, call::
959 struct key *request_key(const struct key_type *type,
960 const char *description,
961 const char *callout_info);
963 This is used to request a key or keyring with a description that matches
964 the description specified according to the key type's match_preparse()
965 method. This permits approximate matching to occur. If callout_string is
966 not NULL, then /sbin/request-key will be invoked in an attempt to obtain
967 the key from userspace. In that case, callout_string will be passed as an
968 argument to the program.
970 Should the function fail error ENOKEY, EKEYEXPIRED or EKEYREVOKED will be
973 If successful, the key will have been attached to the default keyring for
974 implicitly obtained request-key keys, as set by KEYCTL_SET_REQKEY_KEYRING.
976 See also Documentation/security/keys-request-key.txt.
979 * To search for a key, passing auxiliary data to the upcaller, call::
981 struct key *request_key_with_auxdata(const struct key_type *type,
982 const char *description,
983 const void *callout_info,
987 This is identical to request_key(), except that the auxiliary data is
988 passed to the key_type->request_key() op if it exists, and the callout_info
989 is a blob of length callout_len, if given (the length may be 0).
992 * A key can be requested asynchronously by calling one of::
994 struct key *request_key_async(const struct key_type *type,
995 const char *description,
996 const void *callout_info,
1001 struct key *request_key_async_with_auxdata(const struct key_type *type,
1002 const char *description,
1003 const char *callout_info,
1007 which are asynchronous equivalents of request_key() and
1008 request_key_with_auxdata() respectively.
1010 These two functions return with the key potentially still under
1011 construction. To wait for construction completion, the following should be
1014 int wait_for_key_construction(struct key *key, bool intr);
1016 The function will wait for the key to finish being constructed and then
1017 invokes key_validate() to return an appropriate value to indicate the state
1018 of the key (0 indicates the key is usable).
1020 If intr is true, then the wait can be interrupted by a signal, in which
1021 case error ERESTARTSYS will be returned.
1024 * When it is no longer required, the key should be released using::
1026 void key_put(struct key *key);
1030 void key_ref_put(key_ref_t key_ref);
1032 These can be called from interrupt context. If CONFIG_KEYS is not set then
1033 the argument will not be parsed.
1036 * Extra references can be made to a key by calling one of the following
1039 struct key *__key_get(struct key *key);
1040 struct key *key_get(struct key *key);
1042 Keys so references will need to be disposed of by calling key_put() when
1043 they've been finished with. The key pointer passed in will be returned.
1045 In the case of key_get(), if the pointer is NULL or CONFIG_KEYS is not set
1046 then the key will not be dereferenced and no increment will take place.
1049 * A key's serial number can be obtained by calling::
1051 key_serial_t key_serial(struct key *key);
1053 If key is NULL or if CONFIG_KEYS is not set then 0 will be returned (in the
1054 latter case without parsing the argument).
1057 * If a keyring was found in the search, this can be further searched by::
1059 key_ref_t keyring_search(key_ref_t keyring_ref,
1060 const struct key_type *type,
1061 const char *description)
1063 This searches the keyring tree specified for a matching key. Error ENOKEY
1064 is returned upon failure (use IS_ERR/PTR_ERR to determine). If successful,
1065 the returned key will need to be released.
1067 The possession attribute from the keyring reference is used to control
1068 access through the permissions mask and is propagated to the returned key
1069 reference pointer if successful.
1072 * A keyring can be created by::
1074 struct key *keyring_alloc(const char *description, uid_t uid, gid_t gid,
1075 const struct cred *cred,
1077 struct key_restriction *restrict_link,
1078 unsigned long flags,
1081 This creates a keyring with the given attributes and returns it. If dest
1082 is not NULL, the new keyring will be linked into the keyring to which it
1083 points. No permission checks are made upon the destination keyring.
1085 Error EDQUOT can be returned if the keyring would overload the quota (pass
1086 KEY_ALLOC_NOT_IN_QUOTA in flags if the keyring shouldn't be accounted
1087 towards the user's quota). Error ENOMEM can also be returned.
1089 If restrict_link is not NULL, it should point to a structure that contains
1090 the function that will be called each time an attempt is made to link a
1091 key into the new keyring. The structure may also contain a key pointer
1092 and an associated key type. The function is called to check whether a key
1093 may be added into the keyring or not. The key type is used by the garbage
1094 collector to clean up function or data pointers in this structure if the
1095 given key type is unregistered. Callers of key_create_or_update() within
1096 the kernel can pass KEY_ALLOC_BYPASS_RESTRICTION to suppress the check.
1097 An example of using this is to manage rings of cryptographic keys that are
1098 set up when the kernel boots where userspace is also permitted to add keys
1099 - provided they can be verified by a key the kernel already has.
1101 When called, the restriction function will be passed the keyring being
1102 added to, the key type, the payload of the key being added, and data to be
1103 used in the restriction check. Note that when a new key is being created,
1104 this is called between payload preparsing and actual key creation. The
1105 function should return 0 to allow the link or an error to reject it.
1107 A convenience function, restrict_link_reject, exists to always return
1108 -EPERM to in this case.
1111 * To check the validity of a key, this function can be called::
1113 int validate_key(struct key *key);
1115 This checks that the key in question hasn't expired or and hasn't been
1116 revoked. Should the key be invalid, error EKEYEXPIRED or EKEYREVOKED will
1117 be returned. If the key is NULL or if CONFIG_KEYS is not set then 0 will be
1118 returned (in the latter case without parsing the argument).
1121 * To register a key type, the following function should be called::
1123 int register_key_type(struct key_type *type);
1125 This will return error EEXIST if a type of the same name is already
1129 * To unregister a key type, call::
1131 void unregister_key_type(struct key_type *type);
1134 Under some circumstances, it may be desirable to deal with a bundle of keys.
1135 The facility provides access to the keyring type for managing such a bundle::
1137 struct key_type key_type_keyring;
1139 This can be used with a function such as request_key() to find a specific
1140 keyring in a process's keyrings. A keyring thus found can then be searched
1141 with keyring_search(). Note that it is not possible to use request_key() to
1142 search a specific keyring, so using keyrings in this way is of limited utility.
1145 Notes On Accessing Payload Contents
1146 ===================================
1148 The simplest payload is just data stored in key->payload directly. In this
1149 case, there's no need to indulge in RCU or locking when accessing the payload.
1151 More complex payload contents must be allocated and pointers to them set in the
1152 key->payload.data[] array. One of the following ways must be selected to
1155 1) Unmodifiable key type.
1157 If the key type does not have a modify method, then the key's payload can
1158 be accessed without any form of locking, provided that it's known to be
1159 instantiated (uninstantiated keys cannot be "found").
1161 2) The key's semaphore.
1163 The semaphore could be used to govern access to the payload and to control
1164 the payload pointer. It must be write-locked for modifications and would
1165 have to be read-locked for general access. The disadvantage of doing this
1166 is that the accessor may be required to sleep.
1170 RCU must be used when the semaphore isn't already held; if the semaphore
1171 is held then the contents can't change under you unexpectedly as the
1172 semaphore must still be used to serialise modifications to the key. The
1173 key management code takes care of this for the key type.
1175 However, this means using::
1177 rcu_read_lock() ... rcu_dereference() ... rcu_read_unlock()
1179 to read the pointer, and::
1181 rcu_dereference() ... rcu_assign_pointer() ... call_rcu()
1183 to set the pointer and dispose of the old contents after a grace period.
1184 Note that only the key type should ever modify a key's payload.
1186 Furthermore, an RCU controlled payload must hold a struct rcu_head for the
1187 use of call_rcu() and, if the payload is of variable size, the length of
1188 the payload. key->datalen cannot be relied upon to be consistent with the
1189 payload just dereferenced if the key's semaphore is not held.
1191 Note that key->payload.data[0] has a shadow that is marked for __rcu
1192 usage. This is called key->payload.rcu_data0. The following accessors
1193 wrap the RCU calls to this element:
1195 a) Set or change the first payload pointer::
1197 rcu_assign_keypointer(struct key *key, void *data);
1199 b) Read the first payload pointer with the key semaphore held::
1201 [const] void *dereference_key_locked([const] struct key *key);
1203 Note that the return value will inherit its constness from the key
1204 parameter. Static analysis will give an error if it things the lock
1207 c) Read the first payload pointer with the RCU read lock held::
1209 const void *dereference_key_rcu(const struct key *key);
1215 A kernel service may want to define its own key type. For instance, an AFS
1216 filesystem might want to define a Kerberos 5 ticket key type. To do this, it
1217 author fills in a key_type struct and registers it with the system.
1219 Source files that implement key types should include the following header file::
1223 The structure has a number of fields, some of which are mandatory:
1225 * ``const char *name``
1227 The name of the key type. This is used to translate a key type name
1228 supplied by userspace into a pointer to the structure.
1231 * ``size_t def_datalen``
1233 This is optional - it supplies the default payload data length as
1234 contributed to the quota. If the key type's payload is always or almost
1235 always the same size, then this is a more efficient way to do things.
1237 The data length (and quota) on a particular key can always be changed
1238 during instantiation or update by calling::
1240 int key_payload_reserve(struct key *key, size_t datalen);
1242 With the revised data length. Error EDQUOT will be returned if this is not
1246 * ``int (*vet_description)(const char *description);``
1248 This optional method is called to vet a key description. If the key type
1249 doesn't approve of the key description, it may return an error, otherwise
1253 * ``int (*preparse)(struct key_preparsed_payload *prep);``
1255 This optional method permits the key type to attempt to parse payload
1256 before a key is created (add key) or the key semaphore is taken (update or
1257 instantiate key). The structure pointed to by prep looks like::
1259 struct key_preparsed_payload {
1261 union key_payload payload;
1268 Before calling the method, the caller will fill in data and datalen with
1269 the payload blob parameters; quotalen will be filled in with the default
1270 quota size from the key type; expiry will be set to TIME_T_MAX and the
1271 rest will be cleared.
1273 If a description can be proposed from the payload contents, that should be
1274 attached as a string to the description field. This will be used for the
1275 key description if the caller of add_key() passes NULL or "".
1277 The method can attach anything it likes to payload. This is merely passed
1278 along to the instantiate() or update() operations. If set, the expiry
1279 time will be applied to the key if it is instantiated from this data.
1281 The method should return 0 if successful or a negative error code
1285 * ``void (*free_preparse)(struct key_preparsed_payload *prep);``
1287 This method is only required if the preparse() method is provided,
1288 otherwise it is unused. It cleans up anything attached to the description
1289 and payload fields of the key_preparsed_payload struct as filled in by the
1290 preparse() method. It will always be called after preparse() returns
1291 successfully, even if instantiate() or update() succeed.
1294 * ``int (*instantiate)(struct key *key, struct key_preparsed_payload *prep);``
1296 This method is called to attach a payload to a key during construction.
1297 The payload attached need not bear any relation to the data passed to this
1300 The prep->data and prep->datalen fields will define the original payload
1301 blob. If preparse() was supplied then other fields may be filled in also.
1303 If the amount of data attached to the key differs from the size in
1304 keytype->def_datalen, then key_payload_reserve() should be called.
1306 This method does not have to lock the key in order to attach a payload.
1307 The fact that KEY_FLAG_INSTANTIATED is not set in key->flags prevents
1308 anything else from gaining access to the key.
1310 It is safe to sleep in this method.
1312 generic_key_instantiate() is provided to simply copy the data from
1313 prep->payload.data[] to key->payload.data[], with RCU-safe assignment on
1314 the first element. It will then clear prep->payload.data[] so that the
1315 free_preparse method doesn't release the data.
1318 * ``int (*update)(struct key *key, const void *data, size_t datalen);``
1320 If this type of key can be updated, then this method should be provided.
1321 It is called to update a key's payload from the blob of data provided.
1323 The prep->data and prep->datalen fields will define the original payload
1324 blob. If preparse() was supplied then other fields may be filled in also.
1326 key_payload_reserve() should be called if the data length might change
1327 before any changes are actually made. Note that if this succeeds, the type
1328 is committed to changing the key because it's already been altered, so all
1329 memory allocation must be done first.
1331 The key will have its semaphore write-locked before this method is called,
1332 but this only deters other writers; any changes to the key's payload must
1333 be made under RCU conditions, and call_rcu() must be used to dispose of
1336 key_payload_reserve() should be called before the changes are made, but
1337 after all allocations and other potentially failing function calls are
1340 It is safe to sleep in this method.
1343 * ``int (*match_preparse)(struct key_match_data *match_data);``
1345 This method is optional. It is called when a key search is about to be
1346 performed. It is given the following structure::
1348 struct key_match_data {
1349 bool (*cmp)(const struct key *key,
1350 const struct key_match_data *match_data);
1351 const void *raw_data;
1353 unsigned lookup_type;
1356 On entry, raw_data will be pointing to the criteria to be used in matching
1357 a key by the caller and should not be modified. ``(*cmp)()`` will be pointing
1358 to the default matcher function (which does an exact description match
1359 against raw_data) and lookup_type will be set to indicate a direct lookup.
1361 The following lookup_type values are available:
1363 * KEYRING_SEARCH_LOOKUP_DIRECT - A direct lookup hashes the type and
1364 description to narrow down the search to a small number of keys.
1366 * KEYRING_SEARCH_LOOKUP_ITERATE - An iterative lookup walks all the
1367 keys in the keyring until one is matched. This must be used for any
1368 search that's not doing a simple direct match on the key description.
1370 The method may set cmp to point to a function of its choice that does some
1371 other form of match, may set lookup_type to KEYRING_SEARCH_LOOKUP_ITERATE
1372 and may attach something to the preparsed pointer for use by ``(*cmp)()``.
1373 ``(*cmp)()`` should return true if a key matches and false otherwise.
1375 If preparsed is set, it may be necessary to use the match_free() method to
1378 The method should return 0 if successful or a negative error code
1381 It is permitted to sleep in this method, but ``(*cmp)()`` may not sleep as
1382 locks will be held over it.
1384 If match_preparse() is not provided, keys of this type will be matched
1385 exactly by their description.
1388 * ``void (*match_free)(struct key_match_data *match_data);``
1390 This method is optional. If given, it called to clean up
1391 match_data->preparsed after a successful call to match_preparse().
1394 * ``void (*revoke)(struct key *key);``
1396 This method is optional. It is called to discard part of the payload
1397 data upon a key being revoked. The caller will have the key semaphore
1400 It is safe to sleep in this method, though care should be taken to avoid
1401 a deadlock against the key semaphore.
1404 * ``void (*destroy)(struct key *key);``
1406 This method is optional. It is called to discard the payload data on a key
1407 when it is being destroyed.
1409 This method does not need to lock the key to access the payload; it can
1410 consider the key as being inaccessible at this time. Note that the key's
1411 type may have been changed before this function is called.
1413 It is not safe to sleep in this method; the caller may hold spinlocks.
1416 * ``void (*describe)(const struct key *key, struct seq_file *p);``
1418 This method is optional. It is called during /proc/keys reading to
1419 summarise a key's description and payload in text form.
1421 This method will be called with the RCU read lock held. rcu_dereference()
1422 should be used to read the payload pointer if the payload is to be
1423 accessed. key->datalen cannot be trusted to stay consistent with the
1424 contents of the payload.
1426 The description will not change, though the key's state may.
1428 It is not safe to sleep in this method; the RCU read lock is held by the
1432 * ``long (*read)(const struct key *key, char __user *buffer, size_t buflen);``
1434 This method is optional. It is called by KEYCTL_READ to translate the
1435 key's payload into something a blob of data for userspace to deal with.
1436 Ideally, the blob should be in the same format as that passed in to the
1437 instantiate and update methods.
1439 If successful, the blob size that could be produced should be returned
1440 rather than the size copied.
1442 This method will be called with the key's semaphore read-locked. This will
1443 prevent the key's payload changing. It is not necessary to use RCU locking
1444 when accessing the key's payload. It is safe to sleep in this method, such
1445 as might happen when the userspace buffer is accessed.
1448 * ``int (*request_key)(struct key_construction *cons, const char *op, void *aux);``
1450 This method is optional. If provided, request_key() and friends will
1451 invoke this function rather than upcalling to /sbin/request-key to operate
1452 upon a key of this type.
1454 The aux parameter is as passed to request_key_async_with_auxdata() and
1455 similar or is NULL otherwise. Also passed are the construction record for
1456 the key to be operated upon and the operation type (currently only
1459 This method is permitted to return before the upcall is complete, but the
1460 following function must be called under all circumstances to complete the
1461 instantiation process, whether or not it succeeds, whether or not there's
1464 void complete_request_key(struct key_construction *cons, int error);
1466 The error parameter should be 0 on success, -ve on error. The
1467 construction record is destroyed by this action and the authorisation key
1468 will be revoked. If an error is indicated, the key under construction
1469 will be negatively instantiated if it wasn't already instantiated.
1471 If this method returns an error, that error will be returned to the
1472 caller of request_key*(). complete_request_key() must be called prior to
1475 The key under construction and the authorisation key can be found in the
1476 key_construction struct pointed to by cons:
1478 * ``struct key *key;``
1480 The key under construction.
1482 * ``struct key *authkey;``
1484 The authorisation key.
1487 * ``struct key_restriction *(*lookup_restriction)(const char *params);``
1489 This optional method is used to enable userspace configuration of keyring
1490 restrictions. The restriction parameter string (not including the key type
1491 name) is passed in, and this method returns a pointer to a key_restriction
1492 structure containing the relevant functions and data to evaluate each
1493 attempted key link operation. If there is no match, -EINVAL is returned.
1496 Request-Key Callback Service
1497 ============================
1499 To create a new key, the kernel will attempt to execute the following command
1502 /sbin/request-key create <key> <uid> <gid> \
1503 <threadring> <processring> <sessionring> <callout_info>
1505 <key> is the key being constructed, and the three keyrings are the process
1506 keyrings from the process that caused the search to be issued. These are
1507 included for two reasons:
1509 1 There may be an authentication token in one of the keyrings that is
1510 required to obtain the key, eg: a Kerberos Ticket-Granting Ticket.
1512 2 The new key should probably be cached in one of these rings.
1514 This program should set it UID and GID to those specified before attempting to
1515 access any more keys. It may then look around for a user specific process to
1516 hand the request off to (perhaps a path held in placed in another key by, for
1517 example, the KDE desktop manager).
1519 The program (or whatever it calls) should finish construction of the key by
1520 calling KEYCTL_INSTANTIATE or KEYCTL_INSTANTIATE_IOV, which also permits it to
1521 cache the key in one of the keyrings (probably the session ring) before
1522 returning. Alternatively, the key can be marked as negative with KEYCTL_NEGATE
1523 or KEYCTL_REJECT; this also permits the key to be cached in one of the
1526 If it returns with the key remaining in the unconstructed state, the key will
1527 be marked as being negative, it will be added to the session keyring, and an
1528 error will be returned to the key requestor.
1530 Supplementary information may be provided from whoever or whatever invoked this
1531 service. This will be passed as the <callout_info> parameter. If no such
1532 information was made available, then "-" will be passed as this parameter
1536 Similarly, the kernel may attempt to update an expired or a soon to expire key
1539 /sbin/request-key update <key> <uid> <gid> \
1540 <threadring> <processring> <sessionring>
1542 In this case, the program isn't required to actually attach the key to a ring;
1543 the rings are provided for reference.
1549 Dead keys (for which the type has been removed) will be automatically unlinked
1550 from those keyrings that point to them and deleted as soon as possible by a
1551 background garbage collector.
1553 Similarly, revoked and expired keys will be garbage collected, but only after a
1554 certain amount of time has passed. This time is set as a number of seconds in::
1556 /proc/sys/kernel/keys/gc_delay