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                         ============================

                         KERNEL KEY RETENTION SERVICE

                         ============================

This service allows cryptographic keys, authentication tokens, cross-domain

user mappings, and similar to be cached in the kernel for the use of

filesystems and other kernel services.

Keyrings are permitted; these are a special type of key that can hold links to

other keys. Processes each have three standard keyring subscriptions that a

kernel service can search for relevant keys.

The key service can be configured on by enabling:

        "Security options"/"Enable access key retention support" (CONFIG_KEYS)

This document has the following sections:

        - Key overview

        - Key service overview

        - Key access permissions

        - SELinux support

        - New procfs files

        - Userspace system call interface

        - Kernel services

        - Notes on accessing payload contents

        - Defining a key type

        - Request-key callback service

        - Key access filesystem

============

KEY OVERVIEW

============

In this context, keys represent units of cryptographic data, authentication

tokens, keyrings, etc.. These are represented in the kernel by struct key.

Each key has a number of attributes:

        - A serial number.

        - A type.

        - A description (for matching a key in a search).

        - Access control information.

        - An expiry time.

        - A payload.

        - State.

 (*) Each key is issued a serial number of type key_serial_t that is unique for

     the lifetime of that key. All serial numbers are positive non-zero 32-bit

     integers.

     Userspace programs can use a key's serial numbers as a way to gain access

     to it, subject to permission checking.

 (*) Each key is of a defined "type". Types must be registered inside the

     kernel by a kernel service (such as a filesystem) before keys of that type

     can be added or used. Userspace programs cannot define new types directly.

     Key types are represented in the kernel by struct key_type. This defines a

     number of operations that can be performed on a key of that type.

     Should a type be removed from the system, all the keys of that type will

     be invalidated.

 (*) Each key has a description. This should be a printable string. The key

     type provides an operation to perform a match between the description on a

     key and a criterion string.

 (*) Each key has an owner user ID, a group ID and a permissions mask. These

     are used to control what a process may do to a key from userspace, and

     whether a kernel service will be able to find the key.

 (*) Each key can be set to expire at a specific time by the key type's

     instantiation function. Keys can also be immortal.

 (*) Each key can have a payload. This is a quantity of data that represent the

     actual "key". In the case of a keyring, this is a list of keys to which

     the keyring links; in the case of a user-defined key, it's an arbitrary

     blob of data.

     Having a payload is not required; and the payload can, in fact, just be a

     value stored in the struct key itself.

     When a key is instantiated, the key type's instantiation function is

     called with a blob of data, and that then creates the key's payload in

     some way.

     Similarly, when userspace wants to read back the contents of the key, if

     permitted, another key type operation will be called to convert the key's

     attached payload back into a blob of data.

 (*) Each key can be in one of a number of basic states:

     (*) Uninstantiated. The key exists, but does not have any data attached.

         Keys being requested from userspace will be in this state.

     (*) Instantiated. This is the normal state. The key is fully formed, and

         has data attached.

     (*) Negative. This is a relatively short-lived state. The key acts as a

         note saying that a previous call out to userspace failed, and acts as

         a throttle on key lookups. A negative key can be updated to a normal

         state.

     (*) Expired. Keys can have lifetimes set. If their lifetime is exceeded,

         they traverse to this state. An expired key can be updated back to a

         normal state.

     (*) Revoked. A key is put in this state by userspace action. It can't be

         found or operated upon (apart from by unlinking it).

     (*) Dead. The key's type was unregistered, and so the key is now useless.

====================

KEY SERVICE OVERVIEW

====================

The key service provides a number of features besides keys:

 (*) The key service defines two special key types:

     (+) "keyring"

         Keyrings are special keys that contain a list of other keys. Keyring

         lists can be modified using various system calls. Keyrings should not

         be given a payload when created.

     (+) "user"

         A key of this type has a description and a payload that are arbitrary

         blobs of data. These can be created, updated and read by userspace,

         and aren't intended for use by kernel services.

 (*) Each process subscribes to three keyrings: a thread-specific keyring, a

     process-specific keyring, and a session-specific keyring.

     The thread-specific keyring is discarded from the child when any sort of

     clone, fork, vfork or execve occurs. A new keyring is created only when

     required.

     The process-specific keyring is replaced with an empty one in the child on

     clone, fork, vfork unless CLONE_THREAD is supplied, in which case it is

     shared. execve also discards the process's process keyring and creates a

     new one.

     The session-specific keyring is persistent across clone, fork, vfork and

     execve, even when the latter executes a set-UID or set-GID binary. A

     process can, however, replace its current session keyring with a new one

     by using PR_JOIN_SESSION_KEYRING. It is permitted to request an anonymous

     new one, or to attempt to create or join one of a specific name.

     The ownership of the thread keyring changes when the real UID and GID of

     the thread changes.

 (*) Each user ID resident in the system holds two special keyrings: a user

     specific keyring and a default user session keyring. The default session

     keyring is initialised with a link to the user-specific keyring.

     When a process changes its real UID, if it used to have no session key, it

     will be subscribed to the default session key for the new UID.

     If a process attempts to access its session key when it doesn't have one,

     it will be subscribed to the default for its current UID.

 (*) Each user has two quotas against which the keys they own are tracked. One

     limits the total number of keys and keyrings, the other limits the total

     amount of description and payload space that can be consumed.

     The user can view information on this and other statistics through procfs

     files.

     Process-specific and thread-specific keyrings are not counted towards a

     user's quota.

     If a system call that modifies a key or keyring in some way would put the

     user over quota, the operation is refused and error EDQUOT is returned.

 (*) There's a system call interface by which userspace programs can create and

     manipulate keys and keyrings.

 (*) There's a kernel interface by which services can register types and search

     for keys.

 (*) There's a way for the a search done from the kernel to call back to

     userspace to request a key that can't be found in a process's keyrings.

 (*) An optional filesystem is available through which the key database can be

     viewed and manipulated.

======================

KEY ACCESS PERMISSIONS

======================

Keys have an owner user ID, a group access ID, and a permissions mask. The mask

has up to eight bits each for possessor, user, group and other access. Only

six of each set of eight bits are defined. These permissions granted are:

 (*) View

     This permits a key or keyring's attributes to be viewed - including key

     type and description.

 (*) Read

     This permits a key's payload to be viewed or a keyring's list of linked

     keys.

 (*) Write

     This permits a key's payload to be instantiated or updated, or it allows a

     link to be added to or removed from a keyring.

 (*) Search

     This permits keyrings to be searched and keys to be found. Searches can

     only recurse into nested keyrings that have search permission set.

 (*) Link

     This permits a key or keyring to be linked to. To create a link from a

     keyring to a key, a process must have Write permission on the keyring and

     Link permission on the key.

 (*) Set Attribute

     This permits a key's UID, GID and permissions mask to be changed.

For changing the ownership, group ID or permissions mask, being the owner of

the key or having the sysadmin capability is sufficient.

===============

SELINUX SUPPORT

===============

The security class "key" has been added to SELinux so that mandatory access

controls can be applied to keys created within various contexts.  This support

is preliminary, and is likely to change quite significantly in the near future.

Currently, all of the basic permissions explained above are provided in SELinux

as well; SELinux is simply invoked after all basic permission checks have been

performed.

The value of the file /proc/self/attr/keycreate influences the labeling of

newly-created keys.  If the contents of that file correspond to an SELinux

security context, then the key will be assigned that context.  Otherwise, the

key will be assigned the current context of the task that invoked the key

creation request.  Tasks must be granted explicit permission to assign a

particular context to newly-created keys, using the "create" permission in the

key security class.

The default keyrings associated with users will be labeled with the default

context of the user if and only if the login programs have been instrumented to

properly initialize keycreate during the login process.  Otherwise, they will

be labeled with the context of the login program itself.

Note, however, that the default keyrings associated with the root user are

labeled with the default kernel context, since they are created early in the

boot process, before root has a chance to log in.

The keyrings associated with new threads are each labeled with the context of

their associated thread, and both session and process keyrings are handled

similarly.

================

NEW PROCFS FILES

================

Two files have been added to procfs by which an administrator can find out

about the status of the key service:

 (*) /proc/keys

     This lists the keys that are currently viewable by the task reading the

     file, giving information about their type, description and permissions.

     It is not possible to view the payload of the key this way, though some

     information about it may be given.

     The only keys included in the list are those that grant View permission to

     the reading process whether or not it possesses them.  Note that LSM

     security checks are still performed, and may further filter out keys that

     the current process is not authorised to view.

     The contents of the file look like this:

        SERIAL   FLAGS  USAGE EXPY PERM     UID   GID   TYPE      DESCRIPTION: SUMMARY

        00000001 I-----    39 perm 1f3f0000     0     0 keyring   _uid_ses.0: 1/4

        00000002 I-----     2 perm 1f3f0000     0     0 keyring   _uid.0: empty

        00000007 I-----     1 perm 1f3f0000     0     0 keyring   _pid.1: empty

        0000018d I-----     1 perm 1f3f0000     0     0 keyring   _pid.412: empty

        000004d2 I--Q--     1 perm 1f3f0000    32    -1 keyring   _uid.32: 1/4

        000004d3 I--Q--     3 perm 1f3f0000    32    -1 keyring   _uid_ses.32: empty

        00000892 I--QU-     1 perm 1f000000     0     0 user      metal:copper: 0

        00000893 I--Q-N     1  35s 1f3f0000     0     0 user      metal:silver: 0

        00000894 I--Q--     1  10h 003f0000     0     0 user      metal:gold: 0

     The flags are:

        I       Instantiated

        R       Revoked

        D       Dead

        Q       Contributes to user's quota

        U       Under construction by callback to userspace

        N       Negative key

     This file must be enabled at kernel configuration time as it allows anyone

     to list the keys database.

 (*) /proc/key-users

     This file lists the tracking data for each user that has at least one key

     on the system.  Such data includes quota information and statistics:

        [root@andromeda root]# cat /proc/key-users

        0:     46 45/45 1/100 13/10000

        29:     2 2/2 2/100 40/10000

        32:     2 2/2 2/100 40/10000

        38:     2 2/2 2/100 40/10000

     The format of each line is

        :                       User ID to which this applies

                                Structure refcount

        /               Total number of keys and number instantiated

        /               Key count quota

        /               Key size quota

===============================

USERSPACE SYSTEM CALL INTERFACE

===============================

Userspace can manipulate keys directly through three new syscalls: add_key,

request_key and keyctl. The latter provides a number of functions for

manipulating keys.

When referring to a key directly, userspace programs should use the key's

serial number (a positive 32-bit integer). However, there are some special

values available for referring to special keys and keyrings that relate to the

process making the call:

        CONSTANT                        VALUE   KEY REFERENCED

        ==============================  ======  ===========================

        KEY_SPEC_THREAD_KEYRING         -1      thread-specific keyring

        KEY_SPEC_PROCESS_KEYRING        -2      process-specific keyring

        KEY_SPEC_SESSION_KEYRING        -3      session-specific keyring

        KEY_SPEC_USER_KEYRING           -4      UID-specific keyring

        KEY_SPEC_USER_SESSION_KEYRING   -5      UID-session keyring

        KEY_SPEC_GROUP_KEYRING          -6      GID-specific keyring

        KEY_SPEC_REQKEY_AUTH_KEY        -7      assumed request_key()

                                                  authorisation key

The main syscalls are:

 (*) Create a new key of given type, description and payload and add it to the

     nominated keyring:

        key_serial_t add_key(const char *type, const char *desc,

                             const void *payload, size_t plen,

                             key_serial_t keyring);

     If a key of the same type and description as that proposed already exists

     in the keyring, this will try to update it with the given payload, or it

     will return error EEXIST if that function is not supported by the key

     type. The process must also have permission to write to the key to be able

     to update it. The new key will have all user permissions granted and no

     group or third party permissions.

     Otherwise, this will attempt to create a new key of the specified type and

     description, and to instantiate it with the supplied payload and attach it

     to the keyring. In this case, an error will be generated if the process

     does not have permission to write to the keyring.

     The payload is optional, and the pointer can be NULL if not required by

     the type. The payload is plen in size, and plen can be zero for an empty

     payload.

     A new keyring can be generated by setting type "keyring", the keyring name

     as the description (or NULL) and setting the payload to NULL.

     User defined keys can be created by specifying type "user". It is

     recommended that a user defined key's description by prefixed with a type

     ID and a colon, such as "krb5tgt:" for a Kerberos 5 ticket granting

     ticket.

     Any other type must have been registered with the kernel in advance by a

     kernel service such as a filesystem.

     The ID of the new or updated key is returned if successful.

 (*) Search the process's keyrings for a key, potentially calling out to

     userspace to create it.

        key_serial_t request_key(const char *type, const char *description,

                                 const char *callout_info,

                                 key_serial_t dest_keyring);

     This function searches all the process's keyrings in the order thread,

     process, session for a matching key. This works very much like

     KEYCTL_SEARCH, including the optional attachment of the discovered key to

     a keyring.

     If a key cannot be found, and if callout_info is not NULL, then

     /sbin/request-key will be invoked in an attempt to obtain a key. The

     callout_info string will be passed as an argument to the program.

     See also Documentation/keys-request-key.txt.

The keyctl syscall functions are:

 (*) Map a special key ID to a real key ID for this process:

        key_serial_t keyctl(KEYCTL_GET_KEYRING_ID, key_serial_t id,

                            int create);

     The special key specified by "id" is looked up (with the key being created

     if necessary) and the ID of the key or keyring thus found is returned if

     it exists.

     If the key does not yet exist, the key will be created if "create" is

     non-zero; and the error ENOKEY will be returned if "create" is zero.

 (*) Replace the session keyring this process subscribes to with a new one:

        key_serial_t keyctl(KEYCTL_JOIN_SESSION_KEYRING, const char *name);

     If name is NULL, an anonymous keyring is created attached to the process

     as its session keyring, displacing the old session keyring.

     If name is not NULL, if a keyring of that name exists, the process

     attempts to attach it as the session keyring, returning an error if that

     is not permitted; otherwise a new keyring of that name is created and

     attached as the session keyring.

     To attach to a named keyring, the keyring must have search permission for

     the process's ownership.

     The ID of the new session keyring is returned if successful.

 (*) Update the specified key:

        long keyctl(KEYCTL_UPDATE, key_serial_t key, const void *payload,

                    size_t plen);

     This will try to update the specified key with the given payload, or it

     will return error EOPNOTSUPP if that function is not supported by the key

     type. The process must also have permission to write to the key to be able

     to update it.

     The payload is of length plen, and may be absent or empty as for

     add_key().

 (*) Revoke a key:

        long keyctl(KEYCTL_REVOKE, key_serial_t key);

     This makes a key unavailable for further operations. Further attempts to

     use the key will be met with error EKEYREVOKED, and the key will no longer

     be findable.

 (*) Change the ownership of a key:

        long keyctl(KEYCTL_CHOWN, key_serial_t key, uid_t uid, gid_t gid);

     This function permits a key's owner and group ID to be changed. Either one

     of uid or gid can be set to -1 to suppress that change.

     Only the superuser can change a key's owner to something other than the

     key's current owner. Similarly, only the superuser can change a key's

     group ID to something other than the calling process's group ID or one of

     its group list members.

 (*) Change the permissions mask on a key:

        long keyctl(KEYCTL_SETPERM, key_serial_t key, key_perm_t perm);

     This function permits the owner of a key or the superuser to change the

     permissions mask on a key.

     Only bits the available bits are permitted; if any other bits are set,

     error EINVAL will be returned.

 (*) Describe a key:

        long keyctl(KEYCTL_DESCRIBE, key_serial_t key, char *buffer,

                    size_t buflen);

     This function returns a summary of the key's attributes (but not its

     payload data) as a string in the buffer provided.

     Unless there's an error, it always returns the amount of data it could

     produce, even if that's too big for the buffer, but it won't copy more

     than requested to userspace. If the buffer pointer is NULL then no copy

     will take place.

     A process must have view permission on the key for this function to be

     successful.

     If successful, a string is placed in the buffer in the following format:

        ;;;;

     Where type and description are strings, uid and gid are decimal, and perm

     is hexadecimal. A NUL character is included at the end of the string if

     the buffer is sufficiently big.

     This can be parsed with

        sscanf(buffer, "%[^;];%d;%d;%o;%s", type, &uid, &gid, &mode, desc);

 (*) Clear out a keyring:

        long keyctl(KEYCTL_CLEAR, key_serial_t keyring);

     This function clears the list of keys attached to a keyring. The calling

     process must have write permission on the keyring, and it must be a

     keyring (or else error ENOTDIR will result).

 (*) Link a key into a keyring:

        long keyctl(KEYCTL_LINK, key_serial_t keyring, key_serial_t key);

     This function creates a link from the keyring to the key. The process must

     have write permission on the keyring and must have link permission on the

     key.

     Should the keyring not be a keyring, error ENOTDIR will result; and if the

     keyring is full, error ENFILE will result.

     The link procedure checks the nesting of the keyrings, returning ELOOP if

     it appears too deep or EDEADLK if the link would introduce a cycle.

     Any links within the keyring to keys that match the new key in terms of

     type and description will be discarded from the keyring as the new one is

     added.

 (*) Unlink a key or keyring from another keyring:

        long keyctl(KEYCTL_UNLINK, key_serial_t keyring, key_serial_t key);

     This function looks through the keyring for the first link to the

     specified key, and removes it if found. Subsequent links to that key are

     ignored. The process must have write permission on the keyring.

     If the keyring is not a keyring, error ENOTDIR will result; and if the key

     is not present, error ENOENT will be the result.

 (*) Search a keyring tree for a key:

        key_serial_t keyctl(KEYCTL_SEARCH, key_serial_t keyring,

                            const char *type, const char *description,

                            key_serial_t dest_keyring);

     This searches the keyring tree headed by the specified keyring until a key

     is found that matches the type and description criteria. Each keyring is

     checked for keys before recursion into its children occurs.

     The process must have search permission on the top level keyring, or else

     error EACCES will result. Only keyrings that the process has search

     permission on will be recursed into, and only keys and keyrings for which

     a process has search permission can be matched. If the specified keyring

     is not a keyring, ENOTDIR will result.

     If the search succeeds, the function will attempt to link the found key

     into the destination keyring if one is supplied (non-zero ID). All the

     constraints applicable to KEYCTL_LINK apply in this case too.

     Error ENOKEY, EKEYREVOKED or EKEYEXPIRED will be returned if the search

     fails. On success, the resulting key ID will be returned.

 (*) Read the payload data from a key:

        long keyctl(KEYCTL_READ, key_serial_t keyring, char *buffer,

                    size_t buflen);

     This function attempts to read the payload data from the specified key

     into the buffer. The process must have read permission on the key to

     succeed.

     The returned data will be processed for presentation by the key type. For

     instance, a keyring will return an array of key_serial_t entries

     representing the IDs of all the keys to which it is subscribed. The user

     defined key type will return its data as is. If a key type does not

     implement this function, error EOPNOTSUPP will result.

     As much of the data as can be fitted into the buffer will be copied to

     userspace if the buffer pointer is not NULL.

     On a successful return, the function will always return the amount of data

     available rather than the amount copied.

 (*) Instantiate a partially constructed key.

        long keyctl(KEYCTL_INSTANTIATE, key_serial_t key,

                    const void *payload, size_t plen,

                    key_serial_t keyring);

     If the kernel calls back to userspace to complete the instantiation of a

     key, userspace should use this call to supply data for the key before the

     invoked process returns, or else the key will be marked negative

     automatically.

     The process must have write access on the key to be able to instantiate

     it, and the key must be uninstantiated.

     If a keyring is specified (non-zero), the key will also be linked into

     that keyring, however all the constraints applying in KEYCTL_LINK apply in

     this case too.

     The payload and plen arguments describe the payload data as for add_key().

 (*) Negatively instantiate a partially constructed key.

        long keyctl(KEYCTL_NEGATE, key_serial_t key,

                    unsigned timeout, key_serial_t keyring);

     If the kernel calls back to userspace to complete the instantiation of a

     key, userspace should use this call mark the key as negative before the

     invoked process returns if it is unable to fulfil the request.

     The process must have write access on the key to be able to instantiate

     it, and the key must be uninstantiated.

     If a keyring is specified (non-zero), the key will also be linked into

     that keyring, however all the constraints applying in KEYCTL_LINK apply in

     this case too.

 (*) Set the default request-key destination keyring.

        long keyctl(KEYCTL_SET_REQKEY_KEYRING, int reqkey_defl);

     This sets the default keyring to which implicitly requested keys will be

     attached for this thread. reqkey_defl should be one of these constants:

        CONSTANT                                VALUE   NEW DEFAULT KEYRING

        ======================================  ======  =======================

        KEY_REQKEY_DEFL_NO_CHANGE               -1      No change

        KEY_REQKEY_DEFL_DEFAULT                 0        Default[1]

        KEY_REQKEY_DEFL_THREAD_KEYRING          1       Thread keyring

        KEY_REQKEY_DEFL_PROCESS_KEYRING         2       Process keyring

        KEY_REQKEY_DEFL_SESSION_KEYRING         3       Session keyring

        KEY_REQKEY_DEFL_USER_KEYRING            4       User keyring

        KEY_REQKEY_DEFL_USER_SESSION_KEYRING    5       User session keyring

        KEY_REQKEY_DEFL_GROUP_KEYRING           6       Group keyring

     The old default will be returned if successful and error EINVAL will be

     returned if reqkey_defl is not one of the above values.

     The default keyring can be overridden by the keyring indicated to the

     request_key() system call.

     Note that this setting is inherited across fork/exec.

     [1] The default is: the thread keyring if there is one, otherwise

     the process keyring if there is one, otherwise the session keyring if

     there is one, otherwise the user default session keyring.

 (*) Set the timeout on a key.

        long keyctl(KEYCTL_SET_TIMEOUT, key_serial_t key, unsigned timeout);

     This sets or clears the timeout on a key. The timeout can be 0 to clear

     the timeout or a number of seconds to set the expiry time that far into

     the future.

     The process must have attribute modification access on a key to set its

     timeout. Timeouts may not be set with this function on negative, revoked

     or expired keys.

 (*) Assume the authority granted to instantiate a key

        long keyctl(KEYCTL_ASSUME_AUTHORITY, key_serial_t key);

     This assumes or divests the authority required to instantiate the

     specified key. Authority can only be assumed if the thread has the

     authorisation key associated with the specified key in its keyrings

     somewhere.

     Once authority is assumed, searches for keys will also search the

     requester's keyrings using the requester's security label, UID, GID and

     groups.

     If the requested authority is unavailable, error EPERM will be returned,

     likewise if the authority has been revoked because the target key is

     already instantiated.

     If the specified key is 0, then any assumed authority will be divested.

     The assumed authoritative key is inherited across fork and exec.

===============

KERNEL SERVICES

===============

The kernel services for key management are fairly simple to deal with. They can

be broken down into two areas: keys and key types.

Dealing with keys is fairly straightforward. Firstly, the kernel service

registers its type, then it searches for a key of that type. It should retain

the key as long as it has need of it, and then it should release it. For a

filesystem or device file, a search would probably be performed during the open

call, and the key released upon close. How to deal with conflicting keys due to

two different users opening the same file is left to the filesystem author to

solve.

To access the key manager, the following header must be #included:

Specific key types should have a header file under include/keys/ that should be

used to access that type.  For keys of type "user", for example, that would be:

Note that there are two different types of pointers to keys that may be

encountered:

 (*) struct key *

     This simply points to the key structure itself. Key structures will be at

     least four-byte aligned.

 (*) key_ref_t

     This is equivalent to a struct key *, but the least significant bit is set

     if the caller "possesses" the key. By "possession" it is meant that the

     calling processes has a searchable link to the key from one of its

     keyrings. There are three functions for dealing with these:

        key_ref_t make_key_ref(const struct key *key,

                               unsigned long possession);

        struct key *key_ref_to_ptr(const key_ref_t key_ref);

        unsigned long is_key_possessed(const key_ref_t key_ref);

     The first function constructs a key reference from a key pointer and

     possession information (which must be 0 or 1 and not any other value).

     The second function retrieves the key pointer from a reference and the

     third retrieves the possession flag.

When accessing a key's payload contents, certain precautions must be taken to

prevent access vs modification races. See the section "Notes on accessing

payload contents" for more information.

(*) To search for a key, call:

        struct key *request_key(const struct key_type *type,

                                const char *description,

                                const char *callout_string);

    This is used to request a key or keyring with a description that matches

    the description specified according to the key type's match function. This

    permits approximate matching to occur. If callout_string is not NULL, then

    /sbin/request-key will be invoked in an attempt to obtain the key from

    userspace. In that case, callout_string will be passed as an argument to

    the program.

    Should the function fail error ENOKEY, EKEYEXPIRED or EKEYREVOKED will be

    returned.

    If successful, the key will have been attached to the default keyring for

    implicitly obtained request-key keys, as set by KEYCTL_SET_REQKEY_KEYRING.

    See also Documentation/keys-request-key.txt.

(*) To search for a key, passing auxiliary data to the upcaller, call:

        struct key *request_key_with_auxdata(const struct key_type *type,

                                             const char *description,

                                             const char *callout_string,

                                             void *aux);

    This is identical to request_key(), except that the auxiliary data is

    passed to the key_type->request_key() op if it exists.

(*) A key can be requested asynchronously by calling one of:

        struct key *request_key_async(const struct key_type *type,

                                      const char *description,

                                      const char *callout_string);

    or:

        struct key *request_key_async_with_auxdata(const struct key_type *type,

                                                   const char *description,

                                                   const char *callout_string,

                                                   void *aux);

    which are asynchronous equivalents of request_key() and

    request_key_with_auxdata() respectively.

    These two functions return with the key potentially still under

    construction.  To wait for contruction completion, the following should be

    called:

        int wait_for_key_construction(struct key *key, bool intr);

    The function will wait for the key to finish being constructed and then

    invokes key_validate() to return an appropriate value to indicate the state

    of the key (0 indicates the key is usable).

    If intr is true, then the wait can be interrupted by a signal, in which

    case error ERESTARTSYS will be returned.

(*) When it is no longer required, the key should be released using:

        void key_put(struct key *key);

    Or:

        void key_ref_put(key_ref_t key_ref);

    These can be called from interrupt context. If CONFIG_KEYS is not set then

    the argument will not be parsed.

(*) Extra references can be made to a key by calling the following function:

        struct key *key_get(struct key *key);

    These need to be disposed of by calling key_put() when they've been

    finished with. The key pointer passed in will be returned. If the pointer

    is NULL or CONFIG_KEYS is not set then the key will not be dereferenced and

    no increment will take place.

(*) A key's serial number can be obtained by calling:

        key_serial_t key_serial(struct key *key);

    If key is NULL or if CONFIG_KEYS is not set then 0 will be returned (in the

    latter case without parsing the argument).

(*) If a keyring was found in the search, this can be further searched by:

        key_ref_t keyring_search(key_ref_t keyring_ref,

                                 const struct key_type *type,

                                 const char *description)

    This searches the keyring tree specified for a matching key. Error ENOKEY

    is returned upon failure (use IS_ERR/PTR_ERR to determine). If successful,

    the returned key will need to be released.

    The possession attribute from the keyring reference is used to control

    access through the permissions mask and is propagated to the returned key

    reference pointer if successful.

(*) To check the validity of a key, this function can be called:

        int validate_key(struct key *key);

    This checks that the key in question hasn't expired or and hasn't been

    revoked. Should the key be invalid, error EKEYEXPIRED or EKEYREVOKED will

    be returned. If the key is NULL or if CONFIG_KEYS is not set then 0 will be

    returned (in the latter case without parsing the argument).

(*) To register a key type, the following function should be called:

        int register_key_type(struct key_type *type);

    This will return error EEXIST if a type of the same name is already

    present.

(*) To unregister a key type, call:

        void unregister_key_type(struct key_type *type);

Under some circumstances, it may be desirable to deal with a bundle of keys.

The facility provides access to the keyring type for managing such a bundle:

        struct key_type key_type_keyring;

This can be used with a function such as request_key() to find a specific

keyring in a process's keyrings.  A keyring thus found can then be searched

with keyring_search().  Note that it is not possible to use request_key() to

search a specific keyring, so using keyrings in this way is of limited utility.

===================================

NOTES ON ACCESSING PAYLOAD CONTENTS

===================================

The simplest payload is just a number in key->payload.value. In this case,

there's no need to indulge in RCU or locking when accessing the payload.

More complex payload contents must be allocated and a pointer to them set in

key->payload.data. One of the following ways must be selected to access the

data:

 (1) Unmodifiable key type.

     If the key type does not have a modify method, then the key's payload can

     be accessed without any form of locking, provided that it's known to be

     instantiated (uninstantiated keys cannot be "found").

 (2) The key's semaphore.

     The semaphore could be used to govern access to the payload and to control

     the payload pointer. It must be write-locked for modifications and would

     have to be read-locked for general access. The disadvantage of doing this

     is that the accessor may be required to sleep.

 (3) RCU.

     RCU must be used when the semaphore isn't already held; if the semaphore