FreeBSD/Linux Kernel Cross Reference
sys/Documentation/keys.txt

                              ============================
                              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
            - Garbage collection
    
    
    ============
    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.
   
   Keys in the last three states are subject to garbage collection.  See the
   section on "Garbage collection".
   
   
   ====================
   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.  The root user may also alter the quota limits through sysctl files
        (see the section "New 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
           <UID>:                  User ID to which this applies
           <usage>                 Structure refcount
           <inst>/<keys>           Total number of keys and number instantiated
           <keys>/<max>            Key count quota
           <bytes>/<max>           Key size quota
   
   
   Four new sysctl files have been added also for the purpose of controlling the
   quota limits on keys:
   
    (*) /proc/sys/kernel/keys/root_maxkeys
        /proc/sys/kernel/keys/root_maxbytes
   
        These files hold the maximum number of keys that root may have and the
        maximum total number of bytes of data that root may have stored in those
        keys.
   
    (*) /proc/sys/kernel/keys/maxkeys
        /proc/sys/kernel/keys/maxbytes
   
        These files hold the maximum number of keys that each non-root user may
        have and the maximum total number of bytes of data that each of those
        users may have stored in their keys.
   
   Root may alter these by writing each new limit as a decimal number string to
   the appropriate file.
   
   
   ===============================
   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:
   
           <type>;<uid>;<gid>;<perm>;<description>
   
        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.
   
   
    (*) Get the LSM security context attached to a key.
   
           long keyctl(KEYCTL_GET_SECURITY, key_serial_t key, char *buffer,
                       size_t buflen)
   
        This function returns a string that represents the LSM security context
        attached to a key 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 NUL character is included at the end of the string if the buffer is
        sufficiently big.  This is included in the returned count.  If no LSM is
        in force then an empty string will be returned.
   
        A process must have view permission on the key for this function to be
        successful.
   
   
    (*) Install the calling process's session keyring on its parent.
   
           long keyctl(KEYCTL_SESSION_TO_PARENT);
   
        This functions attempts to install the calling process's session keyring
        on to the calling process's parent, replacing the parent's current session
        keyring.
   
        The calling process must have the same ownership as its parent, the
        keyring must have the same ownership as the calling process, the calling
        process must have LINK permission on the keyring and the active LSM module
        mustn't deny permission, otherwise error EPERM will be returned.
   
        Error ENOMEM will be returned if there was insufficient memory to complete
        the operation, otherwise 0 will be returned to indicate success.
   
        The keyring will be replaced next time the parent process leaves the
        kernel and resumes executing userspace.
   
   
   ===============
   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:
   
           <linux/key.h>
   
   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:
   
           <keys/user-type.h>
   
   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_info);
   
       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 void *callout_info,
                                                size_t callout_len,
                                                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, and the callout_info
       is a blob of length callout_len, if given (the length may be 0).
   
   
   (*) A key can be requested asynchronously by calling one of:
   
           struct key *request_key_async(const struct key_type *type,
                                         const char *description,
                                         const void *callout_info,
                                         size_t callout_len);
   
       or:
   
           struct key *request_key_async_with_auxdata(const struct key_type *type,
                                                      const char *description,
                                                      const char *callout_info,
                                                      size_t callout_len,
                                                      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 construction 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
       is held then the contents can't change under you unexpectedly as the
       semaphore must still be used to serialise modifications to the key. The
       key management code takes care of this for the key type.
  
       However, this means using:
  
          rcu_read_lock() ... rcu_dereference() ... rcu_read_unlock()
  
       to read the pointer, and:
  
          rcu_dereference() ... rcu_assign_pointer() ... call_rcu()
  
       to set the pointer and dispose of the old contents after a grace period.
       Note that only the key type should ever modify a key's payload.
  
       Furthermore, an RCU controlled payload must hold a struct rcu_head for the
       use of call_rcu() and, if the payload is of variable size, the length of
       the payload. key->datalen cannot be relied upon to be consistent with the
       payload just dereferenced if the key's semaphore is not held.
  
  
  ===================
  DEFINING A KEY TYPE
  ===================
  
  A kernel service may want to define its own key type. For instance, an AFS
  filesystem might want to define a Kerberos 5 ticket key type. To do this, it
  author fills in a key_type struct and registers it with the system.
  
  Source files that implement key types should include the following header file:
  
          <linux/key-type.h>
  
  The structure has a number of fields, some of which are mandatory:
  
   (*) const char *name
  
       The name of the key type. This is used to translate a key type name
       supplied by userspace into a pointer to the structure.
  
  
   (*) size_t def_datalen
  
       This is optional - it supplies the default payload data length as
       contributed to the quota. If the key type's payload is always or almost
       always the same size, then this is a more efficient way to do things.
  
       The data length (and quota) on a particular key can always be changed
       during instantiation or update by calling:
  
          int key_payload_reserve(struct key *key, size_t datalen);
  
       With the revised data length. Error EDQUOT will be returned if this is not
       viable.
  
  
   (*) int (*instantiate)(struct key *key, const void *data, size_t datalen);
  
       This method is called to attach a payload to a key during construction.
       The payload attached need not bear any relation to the data passed to this
       function.
  
       If the amount of data attached to the key differs from the size in
       keytype->def_datalen, then key_payload_reserve() should be called.
  
       This method does not have to lock the key in order to attach a payload.
       The fact that KEY_FLAG_INSTANTIATED is not set in key->flags prevents
       anything else from gaining access to the key.
  
       It is safe to sleep in this method.
  
  
   (*) int (*update)(struct key *key, const void *data, size_t datalen);
  
       If this type of key can be updated, then this method should be provided.
       It is called to update a key's payload from the blob of data provided.
  
       key_payload_reserve() should be called if the data length might change
       before any changes are actually made. Note that if this succeeds, the type
       is committed to changing the key because it's already been altered, so all
       memory allocation must be done first.
  
       The key will have its semaphore write-locked before this method is called,
       but this only deters other writers; any changes to the key's payload must
       be made under RCU conditions, and call_rcu() must be used to dispose of
       the old payload.
  
       key_payload_reserve() should be called before the changes are made, but
       after all allocations and other potentially failing function calls are
       made.
  
       It is safe to sleep in this method.
  
  
   (*) int (*match)(const struct key *key, const void *desc);
  
       This method is called to match a key against a description. It should
       return non-zero if the two match, zero if they don't.
  
       This method should not need to lock the key in any way. The type and
       description can be considered invariant, and the payload should not be
       accessed (the key may not yet be instantiated).
  
       It is not safe to sleep in this method; the caller may hold spinlocks.
  
  
   (*) void (*revoke)(struct key *key);
  
       This method is optional.  It is called to discard part of the payload
       data upon a key being revoked.  The caller will have the key semaphore
       write-locked.
  
       It is safe to sleep in this method, though care should be taken to avoid
       a deadlock against the key semaphore.
  
  
   (*) void (*destroy)(struct key *key);
  
       This method is optional. It is called to discard the payload data on a key
       when it is being destroyed.
  
       This method does not need to lock the key to access the payload; it can
       consider the key as being inaccessible at this time. Note that the key's
       type may have been changed before this function is called.
  
       It is not safe to sleep in this method; the caller may hold spinlocks.
  
  
   (*) void (*describe)(const struct key *key, struct seq_file *p);
  
       This method is optional. It is called during /proc/keys reading to
       summarise a key's description and payload in text form.
  
       This method will be called with the RCU read lock held. rcu_dereference()
       should be used to read the payload pointer if the payload is to be
       accessed. key->datalen cannot be trusted to stay consistent with the
       contents of the payload.
  
       The description will not change, though the key's state may.
  
       It is not safe to sleep in this method; the RCU read lock is held by the
       caller.
  
  
   (*) long (*read)(const struct key *key, char __user *buffer, size_t buflen);
  
       This method is optional. It is called by KEYCTL_READ to translate the
       key's payload into something a blob of data for userspace to deal with.
       Ideally, the blob should be in the same format as that passed in to the
       instantiate and update methods.
  
       If successful, the blob size that could be produced should be returned
       rather than the size copied.
  
       This method will be called with the key's semaphore read-locked. This will
       prevent the key's payload changing. It is not necessary to use RCU locking
       when accessing the key's payload. It is safe to sleep in this method, such
       as might happen when the userspace buffer is accessed.
  
  
   (*) int (*request_key)(struct key_construction *cons, const char *op,
                          void *aux);
  
       This method is optional.  If provided, request_key() and friends will
       invoke this function rather than upcalling to /sbin/request-key to operate
       upon a key of this type.
  
       The aux parameter is as passed to request_key_async_with_auxdata() and
       similar or is NULL otherwise.  Also passed are the construction record for
       the key to be operated upon and the operation type (currently only
       "create").
  
       This method is permitted to return before the upcall is complete, but the
       following function must be called under all circumstances to complete the
       instantiation process, whether or not it succeeds, whether or not there's
       an error:
  
          void complete_request_key(struct key_construction *cons, int error);
  
       The error parameter should be 0 on success, -ve on error.  The
       construction record is destroyed by this action and the authorisation key
       will be revoked.  If an error is indicated, the key under construction
       will be negatively instantiated if it wasn't already instantiated.
  
       If this method returns an error, that error will be returned to the
       caller of request_key*().  complete_request_key() must be called prior to
       returning.
  
       The key under construction and the authorisation key can be found in the
       key_construction struct pointed to by cons:
  
       (*) struct key *key;
  
           The key under construction.
  
       (*) struct key *authkey;
  
           The authorisation key.
  
  
  ============================
  REQUEST-KEY CALLBACK SERVICE
  ============================
  
  To create a new key, the kernel will attempt to execute the following command
  line:
  
          /sbin/request-key create <key> <uid> <gid> \
                  <threadring> <processring> <sessionring> <callout_info>
  
  <key> is the key being constructed, and the three keyrings are the process
  keyrings from the process that caused the search to be issued. These are
  included for two reasons:
  
    (1) There may be an authentication token in one of the keyrings that is
        required to obtain the key, eg: a Kerberos Ticket-Granting Ticket.
  
    (2) The new key should probably be cached in one of these rings.
  
  This program should set it UID and GID to those specified before attempting to
  access any more keys. It may then look around for a user specific process to
  hand the request off to (perhaps a path held in placed in another key by, for
  example, the KDE desktop manager).
  
  The program (or whatever it calls) should finish construction of the key by
  calling KEYCTL_INSTANTIATE, which also permits it to cache the key in one of
  the keyrings (probably the session ring) before returning. Alternatively, the
  key can be marked as negative with KEYCTL_NEGATE; this also permits the key to
  be cached in one of the keyrings.
  
  If it returns with the key remaining in the unconstructed state, the key will
  be marked as being negative, it will be added to the session keyring, and an
  error will be returned to the key requestor.
  
  Supplementary information may be provided from whoever or whatever invoked this
  service. This will be passed as the <callout_info> parameter. If no such
  information was made available, then "-" will be passed as this parameter
  instead.
  
  
  Similarly, the kernel may attempt to update an expired or a soon to expire key
  by executing:
  
          /sbin/request-key update <key> <uid> <gid> \
                  <threadring> <processring> <sessionring>
  
  In this case, the program isn't required to actually attach the key to a ring;
  the rings are provided for reference.
  
  
  ==================
  GARBAGE COLLECTION
  ==================
  
  Dead keys (for which the type has been removed) will be automatically unlinked
  from those keyrings that point to them and deleted as soon as possible by a
  background garbage collector.
  
  Similarly, revoked and expired keys will be garbage collected, but only after a
  certain amount of time has passed.  This time is set as a number of seconds in:
  
          /proc/sys/kernel/keys/gc_delay

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