Documentation / keys.txt

Based on kernel version 2.6.25. Page generated on 2008-04-18 21:22 EST.

				 ============================
				 KERNEL KEY RETENTION SERVICE
				 ============================
	
	This service allows cryptographic keys, authentication tokens, cross-domain
	user mappings, and similar to be cached in the kernel for the use of
	filesystems and other kernel services.
	
	Keyrings are permitted; these are a special type of key that can hold links to
	other keys. Processes each have three standard keyring subscriptions that a
	kernel service can search for relevant keys.
	
	The key service can be configured on by enabling:
	
		"Security options"/"Enable access key retention support" (CONFIG_KEYS)
	
	This document has the following sections:
	
		- Key overview
		- Key service overview
		- Key access permissions
		- SELinux support
		- New procfs files
		- Userspace system call interface
		- Kernel services
		- Notes on accessing payload contents
		- Defining a key type
		- Request-key callback service
		- Key access filesystem
	
	
	============
	KEY OVERVIEW
	============
	
	In this context, keys represent units of cryptographic data, authentication
	tokens, keyrings, etc.. These are represented in the kernel by struct key.
	
	Each key has a number of attributes:
	
		- A serial number.
		- A type.
		- A description (for matching a key in a search).
		- Access control information.
		- An expiry time.
		- A payload.
		- State.
	
	
	 (*) Each key is issued a serial number of type key_serial_t that is unique for
	     the lifetime of that key. All serial numbers are positive non-zero 32-bit
	     integers.
	
	     Userspace programs can use a key's serial numbers as a way to gain access
	     to it, subject to permission checking.
	
	 (*) Each key is of a defined "type". Types must be registered inside the
	     kernel by a kernel service (such as a filesystem) before keys of that type
	     can be added or used. Userspace programs cannot define new types directly.
	
	     Key types are represented in the kernel by struct key_type. This defines a
	     number of operations that can be performed on a key of that type.
	
	     Should a type be removed from the system, all the keys of that type will
	     be invalidated.
	
	 (*) Each key has a description. This should be a printable string. The key
	     type provides an operation to perform a match between the description on a
	     key and a criterion string.
	
	 (*) Each key has an owner user ID, a group ID and a permissions mask. These
	     are used to control what a process may do to a key from userspace, and
	     whether a kernel service will be able to find the key.
	
	 (*) Each key can be set to expire at a specific time by the key type's
	     instantiation function. Keys can also be immortal.
	
	 (*) Each key can have a payload. This is a quantity of data that represent the
	     actual "key". In the case of a keyring, this is a list of keys to which
	     the keyring links; in the case of a user-defined key, it's an arbitrary
	     blob of data.
	
	     Having a payload is not required; and the payload can, in fact, just be a
	     value stored in the struct key itself.
	
	     When a key is instantiated, the key type's instantiation function is
	     called with a blob of data, and that then creates the key's payload in
	     some way.
	
	     Similarly, when userspace wants to read back the contents of the key, if
	     permitted, another key type operation will be called to convert the key's
	     attached payload back into a blob of data.
	
	 (*) Each key can be in one of a number of basic states:
	
	     (*) Uninstantiated. The key exists, but does not have any data attached.
	     	 Keys being requested from userspace will be in this state.
	
	     (*) Instantiated. This is the normal state. The key is fully formed, and
		 has data attached.
	
	     (*) Negative. This is a relatively short-lived state. The key acts as a
		 note saying that a previous call out to userspace failed, and acts as
		 a throttle on key lookups. A negative key can be updated to a normal
		 state.
	
	     (*) Expired. Keys can have lifetimes set. If their lifetime is exceeded,
		 they traverse to this state. An expired key can be updated back to a
		 normal state.
	
	     (*) Revoked. A key is put in this state by userspace action. It can't be
		 found or operated upon (apart from by unlinking it).
	
	     (*) Dead. The key's type was unregistered, and so the key is now useless.
	
	
	====================
	KEY SERVICE OVERVIEW
	====================
	
	The key service provides a number of features besides keys:
	
	 (*) The key service defines two special key types:
	
	     (+) "keyring"
	
		 Keyrings are special keys that contain a list of other keys. Keyring
		 lists can be modified using various system calls. Keyrings should not
		 be given a payload when created.
	
	     (+) "user"
	
		 A key of this type has a description and a payload that are arbitrary
		 blobs of data. These can be created, updated and read by userspace,
		 and aren't intended for use by kernel services.
	
	 (*) Each process subscribes to three keyrings: a thread-specific keyring, a
	     process-specific keyring, and a session-specific keyring.
	
	     The thread-specific keyring is discarded from the child when any sort of
	     clone, fork, vfork or execve occurs. A new keyring is created only when
	     required.
	
	     The process-specific keyring is replaced with an empty one in the child on
	     clone, fork, vfork unless CLONE_THREAD is supplied, in which case it is
	     shared. execve also discards the process's process keyring and creates a
	     new one.
	
	     The session-specific keyring is persistent across clone, fork, vfork and
	     execve, even when the latter executes a set-UID or set-GID binary. A
	     process can, however, replace its current session keyring with a new one
	     by using PR_JOIN_SESSION_KEYRING. It is permitted to request an anonymous
	     new one, or to attempt to create or join one of a specific name.
	
	     The ownership of the thread keyring changes when the real UID and GID of
	     the thread changes.
	
	 (*) Each user ID resident in the system holds two special keyrings: a user
	     specific keyring and a default user session keyring. The default session
	     keyring is initialised with a link to the user-specific keyring.
	
	     When a process changes its real UID, if it used to have no session key, it
	     will be subscribed to the default session key for the new UID.
	
	     If a process attempts to access its session key when it doesn't have one,
	     it will be subscribed to the default for its current UID.
	
	 (*) Each user has two quotas against which the keys they own are tracked. One
	     limits the total number of keys and keyrings, the other limits the total
	     amount of description and payload space that can be consumed.
	
	     The user can view information on this and other statistics through procfs
	     files.
	
	     Process-specific and thread-specific keyrings are not counted towards a
	     user's quota.
	
	     If a system call that modifies a key or keyring in some way would put the
	     user over quota, the operation is refused and error EDQUOT is returned.
	
	 (*) There's a system call interface by which userspace programs can create and
	     manipulate keys and keyrings.
	
	 (*) There's a kernel interface by which services can register types and search
	     for keys.
	
	 (*) There's a way for the a search done from the kernel to call back to
	     userspace to request a key that can't be found in a process's keyrings.
	
	 (*) An optional filesystem is available through which the key database can be
	     viewed and manipulated.
	
	
	======================
	KEY ACCESS PERMISSIONS
	======================
	
	Keys have an owner user ID, a group access ID, and a permissions mask. The mask
	has up to eight bits each for possessor, user, group and other access. Only
	six of each set of eight bits are defined. These permissions granted are:
	
	 (*) View
	
	     This permits a key or keyring's attributes to be viewed - including key
	     type and description.
	
	 (*) Read
	
	     This permits a key's payload to be viewed or a keyring's list of linked
	     keys.
	
	 (*) Write
	
	     This permits a key's payload to be instantiated or updated, or it allows a
	     link to be added to or removed from a keyring.
	
	 (*) Search
	
	     This permits keyrings to be searched and keys to be found. Searches can
	     only recurse into nested keyrings that have search permission set.
	
	 (*) Link
	
	     This permits a key or keyring to be linked to. To create a link from a
	     keyring to a key, a process must have Write permission on the keyring and
	     Link permission on the key.
	
	 (*) Set Attribute
	
	     This permits a key's UID, GID and permissions mask to be changed.
	
	For changing the ownership, group ID or permissions mask, being the owner of
	the key or having the sysadmin capability is sufficient.
	
	
	===============
	SELINUX SUPPORT
	===============
	
	The security class "key" has been added to SELinux so that mandatory access
	controls can be applied to keys created within various contexts.  This support
	is preliminary, and is likely to change quite significantly in the near future.
	Currently, all of the basic permissions explained above are provided in SELinux
	as well; SELinux is simply invoked after all basic permission checks have been
	performed.
	
	The value of the file /proc/self/attr/keycreate influences the labeling of
	newly-created keys.  If the contents of that file correspond to an SELinux
	security context, then the key will be assigned that context.  Otherwise, the
	key will be assigned the current context of the task that invoked the key
	creation request.  Tasks must be granted explicit permission to assign a
	particular context to newly-created keys, using the "create" permission in the
	key security class.
	
	The default keyrings associated with users will be labeled with the default
	context of the user if and only if the login programs have been instrumented to
	properly initialize keycreate during the login process.  Otherwise, they will
	be labeled with the context of the login program itself.
	
	Note, however, that the default keyrings associated with the root user are
	labeled with the default kernel context, since they are created early in the
	boot process, before root has a chance to log in.
	
	The keyrings associated with new threads are each labeled with the context of
	their associated thread, and both session and process keyrings are handled
	similarly.
	
	
	================
	NEW PROCFS FILES
	================
	
	Two files have been added to procfs by which an administrator can find out
	about the status of the key service:
	
	 (*) /proc/keys
	
	     This lists the keys that are currently viewable by the task reading the
	     file, giving information about their type, description and permissions.
	     It is not possible to view the payload of the key this way, though some
	     information about it may be given.
	
	     The only keys included in the list are those that grant View permission to
	     the reading process whether or not it possesses them.  Note that LSM
	     security checks are still performed, and may further filter out keys that
	     the current process is not authorised to view.
	
	     The contents of the file look like this:
	
		SERIAL   FLAGS  USAGE EXPY PERM     UID   GID   TYPE      DESCRIPTION: SUMMARY
		00000001 I-----    39 perm 1f3f0000     0     0 keyring   _uid_ses.0: 1/4
		00000002 I-----     2 perm 1f3f0000     0     0 keyring   _uid.0: empty
		00000007 I-----     1 perm 1f3f0000     0     0 keyring   _pid.1: empty
		0000018d I-----     1 perm 1f3f0000     0     0 keyring   _pid.412: empty
		000004d2 I--Q--     1 perm 1f3f0000    32    -1 keyring   _uid.32: 1/4
		000004d3 I--Q--     3 perm 1f3f0000    32    -1 keyring   _uid_ses.32: empty
		00000892 I--QU-     1 perm 1f000000     0     0 user      metal:copper: 0
		00000893 I--Q-N     1  35s 1f3f0000     0     0 user      metal:silver: 0
		00000894 I--Q--     1  10h 003f0000     0     0 user      metal:gold: 0
	
	     The flags are:
	
		I	Instantiated
		R	Revoked
		D	Dead
		Q	Contributes to user's quota
		U	Under construction by callback to userspace
		N	Negative key
	
	     This file must be enabled at kernel configuration time as it allows anyone
	     to list the keys database.
	
	 (*) /proc/key-users
	
	     This file lists the tracking data for each user that has at least one key
	     on the system.  Such data includes quota information and statistics:
	
		[root@andromeda root]# cat /proc/key-users
		0:     46 45/45 1/100 13/10000
		29:     2 2/2 2/100 40/10000
		32:     2 2/2 2/100 40/10000
		38:     2 2/2 2/100 40/10000
	
	     The format of each line is
		<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
	
	
	===============================
	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.
	
	
	===============
	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_string);
	
	    This is used to request a key or keyring with a description that matches
	    the description specified according to the key type's match function. This
	    permits approximate matching to occur. If callout_string is not NULL, then
	    /sbin/request-key will be invoked in an attempt to obtain the key from
	    userspace. In that case, callout_string will be passed as an argument to
	    the program.
	
	    Should the function fail error ENOKEY, EKEYEXPIRED or EKEYREVOKED will be
	    returned.
	
	    If successful, the key will have been attached to the default keyring for
	    implicitly obtained request-key keys, as set by KEYCTL_SET_REQKEY_KEYRING.
	
	    See also Documentation/keys-request-key.txt.
	
	
	(*) To search for a key, passing auxiliary data to the upcaller, call:
	
		struct key *request_key_with_auxdata(const struct key_type *type,
						     const char *description,
						     const char *callout_string,
						     void *aux);
	
	    This is identical to request_key(), except that the auxiliary data is
	    passed to the key_type->request_key() op if it exists.
	
	
	(*) A key can be requested asynchronously by calling one of:
	
		struct key *request_key_async(const struct key_type *type,
					      const char *description,
					      const char *callout_string);
	
	    or:
	
		struct key *request_key_async_with_auxdata(const struct key_type *type,
							   const char *description,
							   const char *callout_string,
						     	   void *aux);
	
	    which are asynchronous equivalents of request_key() and
	    request_key_with_auxdata() respectively.
	
	    These two functions return with the key potentially still under
	    construction.  To wait for contruction completion, the following should be
	    called:
	
		int wait_for_key_construction(struct key *key, bool intr);
	
	    The function will wait for the key to finish being constructed and then
	    invokes key_validate() to return an appropriate value to indicate the state
	    of the key (0 indicates the key is usable).
	
	    If intr is true, then the wait can be interrupted by a signal, in which
	    case error ERESTARTSYS will be returned.
	
	
	(*) When it is no longer required, the key should be released using:
	
		void key_put(struct key *key);
	
	    Or:
	
		void key_ref_put(key_ref_t key_ref);
	
	    These can be called from interrupt context. If CONFIG_KEYS is not set then
	    the argument will not be parsed.
	
	
	(*) Extra references can be made to a key by calling the following function:
	
		struct key *key_get(struct key *key);
	
	    These need to be disposed of by calling key_put() when they've been
	    finished with. The key pointer passed in will be returned. If the pointer
	    is NULL or CONFIG_KEYS is not set then the key will not be dereferenced and
	    no increment will take place.
	
	
	(*) A key's serial number can be obtained by calling:
	
		key_serial_t key_serial(struct key *key);
	
	    If key is NULL or if CONFIG_KEYS is not set then 0 will be returned (in the
	    latter case without parsing the argument).
	
	
	(*) If a keyring was found in the search, this can be further searched by:
	
		key_ref_t keyring_search(key_ref_t keyring_ref,
					 const struct key_type *type,
					 const char *description)
	
	    This searches the keyring tree specified for a matching key. Error ENOKEY
	    is returned upon failure (use IS_ERR/PTR_ERR to determine). If successful,
	    the returned key will need to be released.
	
	    The possession attribute from the keyring reference is used to control
	    access through the permissions mask and is propagated to the returned key
	    reference pointer if successful.
	
	
	(*) To check the validity of a key, this function can be called:
	
		int validate_key(struct key *key);
	
	    This checks that the key in question hasn't expired or and hasn't been
	    revoked. Should the key be invalid, error EKEYEXPIRED or EKEYREVOKED will
	    be returned. If the key is NULL or if CONFIG_KEYS is not set then 0 will be
	    returned (in the latter case without parsing the argument).
	
	
	(*) To register a key type, the following function should be called:
	
		int register_key_type(struct key_type *type);
	
	    This will return error EEXIST if a type of the same name is already
	    present.
	
	
	(*) To unregister a key type, call:
	
		void unregister_key_type(struct key_type *type);
	
	
	Under some circumstances, it may be desirable to deal with a bundle of keys.
	The facility provides access to the keyring type for managing such a bundle:
	
		struct key_type key_type_keyring;
	
	This can be used with a function such as request_key() to find a specific
	keyring in a process's keyrings.  A keyring thus found can then be searched
	with keyring_search().  Note that it is not possible to use request_key() to
	search a specific keyring, so using keyrings in this way is of limited utility.
	
	
	===================================
	NOTES ON ACCESSING PAYLOAD CONTENTS
	===================================
	
	The simplest payload is just a number in key->payload.value. In this case,
	there's no need to indulge in RCU or locking when accessing the payload.
	
	More complex payload contents must be allocated and a pointer to them set in
	key->payload.data. One of the following ways must be selected to access the
	data:
	
	 (1) Unmodifiable key type.
	
	     If the key type does not have a modify method, then the key's payload can
	     be accessed without any form of locking, provided that it's known to be
	     instantiated (uninstantiated keys cannot be "found").
	
	 (2) The key's semaphore.
	
	     The semaphore could be used to govern access to the payload and to control
	     the payload pointer. It must be write-locked for modifications and would
	     have to be read-locked for general access. The disadvantage of doing this
	     is that the accessor may be required to sleep.
	
	 (3) RCU.
	
	     RCU must be used when the semaphore isn't already held; if the semaphore
	     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.

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