1 files changed, 1290 insertions, 0 deletions

diff --git a/Documentation/security/keys.txt b/Documentation/security/keys.txt
new file mode 100644
index 000000000000..4d75931d2d79
--- /dev/null
+++ b/Documentation/security/keys.txt
@@ -0,0 +1,1290 @@
+                         ============================
+                         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/security/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);
+        long keyctl(KEYCTL_INSTANTIATE_IOV, key_serial_t key,
+                    const struct iovec *payload_iov, unsigned ioc,
+                    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().
+
+     The payload_iov and ioc arguments describe the payload data in an iovec
+     array instead of a single buffer.
+
+
+ (*) Negatively instantiate a partially constructed key.
+
+        long keyctl(KEYCTL_NEGATE, key_serial_t key,
+                    unsigned timeout, key_serial_t keyring);
+        long keyctl(KEYCTL_REJECT, key_serial_t key,
+                    unsigned timeout, unsigned error, 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.
+
+     If the key is rejected, future searches for it will return the specified
+     error code until the rejected key expires.  Negating the key is the same
+     as rejecting the key with ENOKEY as the error code.
+
+
+ (*) 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/security/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 (*vet_description)(const char *description);
+
+     This optional method is called to vet a key description.  If the key type
+     doesn't approve of the key description, it may return an error, otherwise
+     it should return 0.
+
+
+ (*) 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 or KEYCTL_INSTANTIATE_IOV, 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
+or KEYCTL_REJECT; 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