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+============================
+Kernel Key Retention Service
+============================
+
+This service allows cryptographic keys, authentication tokens, cross-domain
+user mappings, and similar to be cached in the kernel for the use of
+filesystems and other kernel services.
+
+Keyrings are permitted; these are a special type of key that can hold links to
+other keys. Processes each have three standard keyring subscriptions that a
+kernel service can search for relevant keys.
+
+The key service can be configured on by enabling:
+
+ "Security options"/"Enable access key retention support" (CONFIG_KEYS)
+
+This document has the following sections:
+
+.. contents:: :local:
+
+
+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 three 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.
+
+ (+) "logon"
+
+ Like a "user" key, a "logon" key has a payload that is an arbitrary
+ blob of data. It is intended as a place to store secrets which are
+ accessible to the kernel but not to userspace programs.
+
+ The description can be arbitrary, but must be prefixed with a non-zero
+ length string that describes the key "subclass". The subclass is
+ separated from the rest of the description by a ':'. "logon" keys can
+ be created and updated from userspace, but the payload is only
+ readable from kernel space.
+
+ * 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
+
+
+ * /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.
+
+ If the key type supports it, if the description is NULL or an empty
+ string, the key type will try and generate a description from the content
+ of the payload.
+
+ 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.rst.
+
+
+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).
+
+ This function can also be used to clear special kernel keyrings if they
+ are appropriately marked if the user has CAP_SYS_ADMIN capability. The
+ DNS resolver cache keyring is an example of this.
+
+
+ * 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.
+
+ If the specified buffer is too small, then the size of the buffer required
+ will be returned. Note that in this case, the contents of the buffer may
+ have been overwritten in some undefined way.
+
+ Otherwise, on success, the function will return the amount of data copied
+ into the buffer.
+
+ * 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 fulfill 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.
+
+
+ * Invalidate a key::
+
+ long keyctl(KEYCTL_INVALIDATE, key_serial_t key);
+
+ This function marks a key as being invalidated and then wakes up the
+ garbage collector. The garbage collector immediately removes invalidated
+ keys from all keyrings and deletes the key when its reference count
+ reaches zero.
+
+ Keys that are marked invalidated become invisible to normal key operations
+ immediately, though they are still visible in /proc/keys until deleted
+ (they're marked with an 'i' flag).
+
+ A process must have search permission on the key for this function to be
+ successful.
+
+ * Compute a Diffie-Hellman shared secret or public key::
+
+ long keyctl(KEYCTL_DH_COMPUTE, struct keyctl_dh_params *params,
+ char *buffer, size_t buflen, struct keyctl_kdf_params *kdf);
+
+ The params struct contains serial numbers for three keys::
+
+ - The prime, p, known to both parties
+ - The local private key
+ - The base integer, which is either a shared generator or the
+ remote public key
+
+ The value computed is::
+
+ result = base ^ private (mod prime)
+
+ If the base is the shared generator, the result is the local
+ public key. If the base is the remote public key, the result is
+ the shared secret.
+
+ If the parameter kdf is NULL, the following applies:
+
+ - The buffer length must be at least the length of the prime, or zero.
+
+ - If the buffer length is nonzero, the length of the result is
+ returned when it is successfully calculated and copied in to the
+ buffer. When the buffer length is zero, the minimum required
+ buffer length is returned.
+
+ The kdf parameter allows the caller to apply a key derivation function
+ (KDF) on the Diffie-Hellman computation where only the result
+ of the KDF is returned to the caller. The KDF is characterized with
+ struct keyctl_kdf_params as follows:
+
+ - ``char *hashname`` specifies the NUL terminated string identifying
+ the hash used from the kernel crypto API and applied for the KDF
+ operation. The KDF implemenation complies with SP800-56A as well
+ as with SP800-108 (the counter KDF).
+
+ - ``char *otherinfo`` specifies the OtherInfo data as documented in
+ SP800-56A section 5.8.1.2. The length of the buffer is given with
+ otherinfolen. The format of OtherInfo is defined by the caller.
+ The otherinfo pointer may be NULL if no OtherInfo shall be used.
+
+ This function will return error EOPNOTSUPP if the key type is not
+ supported, error ENOKEY if the key could not be found, or error
+ EACCES if the key is not readable by the caller. In addition, the
+ function will return EMSGSIZE when the parameter kdf is non-NULL
+ and either the buffer length or the OtherInfo length exceeds the
+ allowed length.
+
+ * Restrict keyring linkage::
+
+ long keyctl(KEYCTL_RESTRICT_KEYRING, key_serial_t keyring,
+ const char *type, const char *restriction);
+
+ An existing keyring can restrict linkage of additional keys by evaluating
+ the contents of the key according to a restriction scheme.
+
+ "keyring" is the key ID for an existing keyring to apply a restriction
+ to. It may be empty or may already have keys linked. Existing linked keys
+ will remain in the keyring even if the new restriction would reject them.
+
+ "type" is a registered key type.
+
+ "restriction" is a string describing how key linkage is to be restricted.
+ The format varies depending on the key type, and the string is passed to
+ the lookup_restriction() function for the requested type. It may specify
+ a method and relevant data for the restriction such as signature
+ verification or constraints on key payload. If the requested key type is
+ later unregistered, no keys may be added to the keyring after the key type
+ is removed.
+
+ To apply a keyring restriction the process must have Set Attribute
+ permission and the keyring must not be previously restricted.
+
+ One application of restricted keyrings is to verify X.509 certificate
+ chains or individual certificate signatures using the asymmetric key type.
+ See Documentation/crypto/asymmetric-keys.txt for specific restrictions
+ applicable to the asymmetric key type.
+
+
+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, bool possession);
+
+ struct key *key_ref_to_ptr(const key_ref_t key_ref);
+
+ bool 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 true or false).
+
+ 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_preparse()
+ method. 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.rst.
+
+
+ * 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 one of the following
+ functions::
+
+ struct key *__key_get(struct key *key);
+ struct key *key_get(struct key *key);
+
+ Keys so references will need to be disposed of by calling key_put() when
+ they've been finished with. The key pointer passed in will be returned.
+
+ In the case of key_get(), 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.
+
+
+ * A keyring can be created by::
+
+ struct key *keyring_alloc(const char *description, uid_t uid, gid_t gid,
+ const struct cred *cred,
+ key_perm_t perm,
+ struct key_restriction *restrict_link,
+ unsigned long flags,
+ struct key *dest);
+
+ This creates a keyring with the given attributes and returns it. If dest
+ is not NULL, the new keyring will be linked into the keyring to which it
+ points. No permission checks are made upon the destination keyring.
+
+ Error EDQUOT can be returned if the keyring would overload the quota (pass
+ KEY_ALLOC_NOT_IN_QUOTA in flags if the keyring shouldn't be accounted
+ towards the user's quota). Error ENOMEM can also be returned.
+
+ If restrict_link is not NULL, it should point to a structure that contains
+ the function that will be called each time an attempt is made to link a
+ key into the new keyring. The structure may also contain a key pointer
+ and an associated key type. The function is called to check whether a key
+ may be added into the keyring or not. The key type is used by the garbage
+ collector to clean up function or data pointers in this structure if the
+ given key type is unregistered. Callers of key_create_or_update() within
+ the kernel can pass KEY_ALLOC_BYPASS_RESTRICTION to suppress the check.
+ An example of using this is to manage rings of cryptographic keys that are
+ set up when the kernel boots where userspace is also permitted to add keys
+ - provided they can be verified by a key the kernel already has.
+
+ When called, the restriction function will be passed the keyring being
+ added to, the key type, the payload of the key being added, and data to be
+ used in the restriction check. Note that when a new key is being created,
+ this is called between payload preparsing and actual key creation. The
+ function should return 0 to allow the link or an error to reject it.
+
+ A convenience function, restrict_link_reject, exists to always return
+ -EPERM to in this case.
+
+
+ * 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 data stored in key->payload directly. 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 pointers to them set in the
+key->payload.data[] array. 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.
+
+ Note that key->payload.data[0] has a shadow that is marked for __rcu
+ usage. This is called key->payload.rcu_data0. The following accessors
+ wrap the RCU calls to this element:
+
+ a) Set or change the first payload pointer::
+
+ rcu_assign_keypointer(struct key *key, void *data);
+
+ b) Read the first payload pointer with the key semaphore held::
+
+ [const] void *dereference_key_locked([const] struct key *key);
+
+ Note that the return value will inherit its constness from the key
+ parameter. Static analysis will give an error if it things the lock
+ isn't held.
+
+ c) Read the first payload pointer with the RCU read lock held::
+
+ const void *dereference_key_rcu(const struct key *key);
+
+
+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 (*preparse)(struct key_preparsed_payload *prep);``
+
+ This optional method permits the key type to attempt to parse payload
+ before a key is created (add key) or the key semaphore is taken (update or
+ instantiate key). The structure pointed to by prep looks like::
+
+ struct key_preparsed_payload {
+ char *description;
+ union key_payload payload;
+ const void *data;
+ size_t datalen;
+ size_t quotalen;
+ time_t expiry;
+ };
+
+ Before calling the method, the caller will fill in data and datalen with
+ the payload blob parameters; quotalen will be filled in with the default
+ quota size from the key type; expiry will be set to TIME_T_MAX and the
+ rest will be cleared.
+
+ If a description can be proposed from the payload contents, that should be
+ attached as a string to the description field. This will be used for the
+ key description if the caller of add_key() passes NULL or "".
+
+ The method can attach anything it likes to payload. This is merely passed
+ along to the instantiate() or update() operations. If set, the expiry
+ time will be applied to the key if it is instantiated from this data.
+
+ The method should return 0 if successful or a negative error code
+ otherwise.
+
+
+ * ``void (*free_preparse)(struct key_preparsed_payload *prep);``
+
+ This method is only required if the preparse() method is provided,
+ otherwise it is unused. It cleans up anything attached to the description
+ and payload fields of the key_preparsed_payload struct as filled in by the
+ preparse() method. It will always be called after preparse() returns
+ successfully, even if instantiate() or update() succeed.
+
+
+ * ``int (*instantiate)(struct key *key, struct key_preparsed_payload *prep);``
+
+ 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.
+
+ The prep->data and prep->datalen fields will define the original payload
+ blob. If preparse() was supplied then other fields may be filled in also.
+
+ 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.
+
+ generic_key_instantiate() is provided to simply copy the data from
+ prep->payload.data[] to key->payload.data[], with RCU-safe assignment on
+ the first element. It will then clear prep->payload.data[] so that the
+ free_preparse method doesn't release the data.
+
+
+ * ``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.
+
+ The prep->data and prep->datalen fields will define the original payload
+ blob. If preparse() was supplied then other fields may be filled in also.
+
+ 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_preparse)(struct key_match_data *match_data);``
+
+ This method is optional. It is called when a key search is about to be
+ performed. It is given the following structure::
+
+ struct key_match_data {
+ bool (*cmp)(const struct key *key,
+ const struct key_match_data *match_data);
+ const void *raw_data;
+ void *preparsed;
+ unsigned lookup_type;
+ };
+
+ On entry, raw_data will be pointing to the criteria to be used in matching
+ a key by the caller and should not be modified. ``(*cmp)()`` will be pointing
+ to the default matcher function (which does an exact description match
+ against raw_data) and lookup_type will be set to indicate a direct lookup.
+
+ The following lookup_type values are available:
+
+ * KEYRING_SEARCH_LOOKUP_DIRECT - A direct lookup hashes the type and
+ description to narrow down the search to a small number of keys.
+
+ * KEYRING_SEARCH_LOOKUP_ITERATE - An iterative lookup walks all the
+ keys in the keyring until one is matched. This must be used for any
+ search that's not doing a simple direct match on the key description.
+
+ The method may set cmp to point to a function of its choice that does some
+ other form of match, may set lookup_type to KEYRING_SEARCH_LOOKUP_ITERATE
+ and may attach something to the preparsed pointer for use by ``(*cmp)()``.
+ ``(*cmp)()`` should return true if a key matches and false otherwise.
+
+ If preparsed is set, it may be necessary to use the match_free() method to
+ clean it up.
+
+ The method should return 0 if successful or a negative error code
+ otherwise.
+
+ It is permitted to sleep in this method, but ``(*cmp)()`` may not sleep as
+ locks will be held over it.
+
+ If match_preparse() is not provided, keys of this type will be matched
+ exactly by their description.
+
+
+ * ``void (*match_free)(struct key_match_data *match_data);``
+
+ This method is optional. If given, it called to clean up
+ match_data->preparsed after a successful call to match_preparse().
+
+
+ * ``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.
+
+
+ * ``struct key_restriction *(*lookup_restriction)(const char *params);``
+
+ This optional method is used to enable userspace configuration of keyring
+ restrictions. The restriction parameter string (not including the key type
+ name) is passed in, and this method returns a pointer to a key_restriction
+ structure containing the relevant functions and data to evaluate each
+ attempted key link operation. If there is no match, -EINVAL is returned.
+
+
+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
diff --git a/Documentation/security/keys/ecryptfs.rst b/Documentation/security/keys/ecryptfs.rst
new file mode 100644
index 0000000..4920f3a
--- /dev/null
+++ b/Documentation/security/keys/ecryptfs.rst
@@ -0,0 +1,73 @@
+==========================================
+Encrypted keys for the eCryptfs filesystem
+==========================================
+
+ECryptfs is a stacked filesystem which transparently encrypts and decrypts each
+file using a randomly generated File Encryption Key (FEK).
+
+Each FEK is in turn encrypted with a File Encryption Key Encryption Key (FEFEK)
+either in kernel space or in user space with a daemon called 'ecryptfsd'. In
+the former case the operation is performed directly by the kernel CryptoAPI
+using a key, the FEFEK, derived from a user prompted passphrase; in the latter
+the FEK is encrypted by 'ecryptfsd' with the help of external libraries in order
+to support other mechanisms like public key cryptography, PKCS#11 and TPM based
+operations.
+
+The data structure defined by eCryptfs to contain information required for the
+FEK decryption is called authentication token and, currently, can be stored in a
+kernel key of the 'user' type, inserted in the user's session specific keyring
+by the userspace utility 'mount.ecryptfs' shipped with the package
+'ecryptfs-utils'.
+
+The 'encrypted' key type has been extended with the introduction of the new
+format 'ecryptfs' in order to be used in conjunction with the eCryptfs
+filesystem. Encrypted keys of the newly introduced format store an
+authentication token in its payload with a FEFEK randomly generated by the
+kernel and protected by the parent master key.
+
+In order to avoid known-plaintext attacks, the datablob obtained through
+commands 'keyctl print' or 'keyctl pipe' does not contain the overall
+authentication token, which content is well known, but only the FEFEK in
+encrypted form.
+
+The eCryptfs filesystem may really benefit from using encrypted keys in that the
+required key can be securely generated by an Administrator and provided at boot
+time after the unsealing of a 'trusted' key in order to perform the mount in a
+controlled environment. Another advantage is that the key is not exposed to
+threats of malicious software, because it is available in clear form only at
+kernel level.
+
+Usage::
+
+ keyctl add encrypted name "new ecryptfs key-type:master-key-name keylen" ring
+ keyctl add encrypted name "load hex_blob" ring
+ keyctl update keyid "update key-type:master-key-name"
+
+Where::
+
+ name:= '<16 hexadecimal characters>'
+ key-type:= 'trusted' | 'user'
+ keylen:= 64
+
+
+Example of encrypted key usage with the eCryptfs filesystem:
+
+Create an encrypted key "1000100010001000" of length 64 bytes with format
+'ecryptfs' and save it using a previously loaded user key "test"::
+
+ $ keyctl add encrypted 1000100010001000 "new ecryptfs user:test 64" @u
+ 19184530
+
+ $ keyctl print 19184530
+ ecryptfs user:test 64 490045d4bfe48c99f0d465fbbbb79e7500da954178e2de0697
+ dd85091f5450a0511219e9f7cd70dcd498038181466f78ac8d4c19504fcc72402bfc41c2
+ f253a41b7507ccaa4b2b03fff19a69d1cc0b16e71746473f023a95488b6edfd86f7fdd40
+ 9d292e4bacded1258880122dd553a661
+
+ $ keyctl pipe 19184530 > ecryptfs.blob
+
+Mount an eCryptfs filesystem using the created encrypted key "1000100010001000"
+into the '/secret' directory::
+
+ $ mount -i -t ecryptfs -oecryptfs_sig=1000100010001000,\
+ ecryptfs_cipher=aes,ecryptfs_key_bytes=32 /secret /secret
diff --git a/Documentation/security/keys/index.rst b/Documentation/security/keys/index.rst
new file mode 100644
index 0000000..647d58f
--- /dev/null
+++ b/Documentation/security/keys/index.rst
@@ -0,0 +1,11 @@
+===========
+Kernel Keys
+===========
+
+.. toctree::
+ :maxdepth: 1
+
+ core
+ ecryptfs
+ request-key
+ trusted-encrypted
diff --git a/Documentation/security/keys/request-key.rst b/Documentation/security/keys/request-key.rst
new file mode 100644
index 0000000..21e2723
--- /dev/null
+++ b/Documentation/security/keys/request-key.rst
@@ -0,0 +1,199 @@
+===================
+Key Request Service
+===================
+
+The key request service is part of the key retention service (refer to
+Documentation/security/keys/core.rst). This document explains more fully how
+the requesting algorithm works.
+
+The process starts by either the kernel requesting a service by calling
+``request_key*()``::
+
+ struct key *request_key(const struct key_type *type,
+ const char *description,
+ const char *callout_info);
+
+or::
+
+ struct key *request_key_with_auxdata(const struct key_type *type,
+ const char *description,
+ const char *callout_info,
+ size_t callout_len,
+ void *aux);
+
+or::
+
+ struct key *request_key_async(const struct key_type *type,
+ const char *description,
+ const char *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);
+
+Or by userspace invoking the request_key system call::
+
+ key_serial_t request_key(const char *type,
+ const char *description,
+ const char *callout_info,
+ key_serial_t dest_keyring);
+
+The main difference between the access points is that the in-kernel interface
+does not need to link the key to a keyring to prevent it from being immediately
+destroyed. The kernel interface returns a pointer directly to the key, and
+it's up to the caller to destroy the key.
+
+The request_key*_with_auxdata() calls are like the in-kernel request_key*()
+calls, except that they permit auxiliary data to be passed to the upcaller (the
+default is NULL). This is only useful for those key types that define their
+own upcall mechanism rather than using /sbin/request-key.
+
+The two async in-kernel calls may return keys that are still in the process of
+being constructed. The two non-async ones will wait for construction to
+complete first.
+
+The userspace interface links the key to a keyring associated with the process
+to prevent the key from going away, and returns the serial number of the key to
+the caller.
+
+
+The following example assumes that the key types involved don't define their
+own upcall mechanisms. If they do, then those should be substituted for the
+forking and execution of /sbin/request-key.
+
+
+The Process
+===========
+
+A request proceeds in the following manner:
+
+ 1) Process A calls request_key() [the userspace syscall calls the kernel
+ interface].
+
+ 2) request_key() searches the process's subscribed keyrings to see if there's
+ a suitable key there. If there is, it returns the key. If there isn't,
+ and callout_info is not set, an error is returned. Otherwise the process
+ proceeds to the next step.
+
+ 3) request_key() sees that A doesn't have the desired key yet, so it creates
+ two things:
+
+ a) An uninstantiated key U of requested type and description.
+
+ b) An authorisation key V that refers to key U and notes that process A
+ is the context in which key U should be instantiated and secured, and
+ from which associated key requests may be satisfied.
+
+ 4) request_key() then forks and executes /sbin/request-key with a new session
+ keyring that contains a link to auth key V.
+
+ 5) /sbin/request-key assumes the authority associated with key U.
+
+ 6) /sbin/request-key execs an appropriate program to perform the actual
+ instantiation.
+
+ 7) The program may want to access another key from A's context (say a
+ Kerberos TGT key). It just requests the appropriate key, and the keyring
+ search notes that the session keyring has auth key V in its bottom level.
+
+ This will permit it to then search the keyrings of process A with the
+ UID, GID, groups and security info of process A as if it was process A,
+ and come up with key W.
+
+ 8) The program then does what it must to get the data with which to
+ instantiate key U, using key W as a reference (perhaps it contacts a
+ Kerberos server using the TGT) and then instantiates key U.
+
+ 9) Upon instantiating key U, auth key V is automatically revoked so that it
+ may not be used again.
+
+ 10) The program then exits 0 and request_key() deletes key V and returns key
+ U to the caller.
+
+This also extends further. If key W (step 7 above) didn't exist, key W would
+be created uninstantiated, another auth key (X) would be created (as per step
+3) and another copy of /sbin/request-key spawned (as per step 4); but the
+context specified by auth key X will still be process A, as it was in auth key
+V.
+
+This is because process A's keyrings can't simply be attached to
+/sbin/request-key at the appropriate places because (a) execve will discard two
+of them, and (b) it requires the same UID/GID/Groups all the way through.
+
+
+Negative Instantiation And Rejection
+====================================
+
+Rather than instantiating a key, it is possible for the possessor of an
+authorisation key to negatively instantiate a key that's under construction.
+This is a short duration placeholder that causes any attempt at re-requesting
+the key whilst it exists to fail with error ENOKEY if negated or the specified
+error if rejected.
+
+This is provided to prevent excessive repeated spawning of /sbin/request-key
+processes for a key that will never be obtainable.
+
+Should the /sbin/request-key process exit anything other than 0 or die on a
+signal, the key under construction will be automatically negatively
+instantiated for a short amount of time.
+
+
+The Search Algorithm
+====================
+
+A search of any particular keyring proceeds in the following fashion:
+
+ 1) When the key management code searches for a key (keyring_search_aux) it
+ firstly calls key_permission(SEARCH) on the keyring it's starting with,
+ if this denies permission, it doesn't search further.
+
+ 2) It considers all the non-keyring keys within that keyring and, if any key
+ matches the criteria specified, calls key_permission(SEARCH) on it to see
+ if the key is allowed to be found. If it is, that key is returned; if
+ not, the search continues, and the error code is retained if of higher
+ priority than the one currently set.
+
+ 3) It then considers all the keyring-type keys in the keyring it's currently
+ searching. It calls key_permission(SEARCH) on each keyring, and if this
+ grants permission, it recurses, executing steps (2) and (3) on that
+ keyring.
+
+The process stops immediately a valid key is found with permission granted to
+use it. Any error from a previous match attempt is discarded and the key is
+returned.
+
+When search_process_keyrings() is invoked, it performs the following searches
+until one succeeds:
+
+ 1) If extant, the process's thread keyring is searched.
+
+ 2) If extant, the process's process keyring is searched.
+
+ 3) The process's session keyring is searched.
+
+ 4) If the process has assumed the authority associated with a request_key()
+ authorisation key then:
+
+ a) If extant, the calling process's thread keyring is searched.
+
+ b) If extant, the calling process's process keyring is searched.
+
+ c) The calling process's session keyring is searched.
+
+The moment one succeeds, all pending errors are discarded and the found key is
+returned.
+
+Only if all these fail does the whole thing fail with the highest priority
+error. Note that several errors may have come from LSM.
+
+The error priority is::
+
+ EKEYREVOKED > EKEYEXPIRED > ENOKEY
+
+EACCES/EPERM are only returned on a direct search of a specific keyring where
+the basal keyring does not grant Search permission.
diff --git a/Documentation/security/keys/trusted-encrypted.rst b/Documentation/security/keys/trusted-encrypted.rst
new file mode 100644
index 0000000..3bb24e0
--- /dev/null
+++ b/Documentation/security/keys/trusted-encrypted.rst
@@ -0,0 +1,175 @@
+==========================
+Trusted and Encrypted Keys
+==========================
+
+Trusted and Encrypted Keys are two new key types added to the existing kernel
+key ring service. Both of these new types are variable length symmetric keys,
+and in both cases all keys are created in the kernel, and user space sees,
+stores, and loads only encrypted blobs. Trusted Keys require the availability
+of a Trusted Platform Module (TPM) chip for greater security, while Encrypted
+Keys can be used on any system. All user level blobs, are displayed and loaded
+in hex ascii for convenience, and are integrity verified.
+
+Trusted Keys use a TPM both to generate and to seal the keys. Keys are sealed
+under a 2048 bit RSA key in the TPM, and optionally sealed to specified PCR
+(integrity measurement) values, and only unsealed by the TPM, if PCRs and blob
+integrity verifications match. A loaded Trusted Key can be updated with new
+(future) PCR values, so keys are easily migrated to new pcr values, such as
+when the kernel and initramfs are updated. The same key can have many saved
+blobs under different PCR values, so multiple boots are easily supported.
+
+By default, trusted keys are sealed under the SRK, which has the default
+authorization value (20 zeros). This can be set at takeownership time with the
+trouser's utility: "tpm_takeownership -u -z".
+
+Usage::
+
+ keyctl add trusted name "new keylen [options]" ring
+ keyctl add trusted name "load hex_blob [pcrlock=pcrnum]" ring
+ keyctl update key "update [options]"
+ keyctl print keyid
+
+ options:
+ keyhandle= ascii hex value of sealing key default 0x40000000 (SRK)
+ keyauth= ascii hex auth for sealing key default 0x00...i
+ (40 ascii zeros)
+ blobauth= ascii hex auth for sealed data default 0x00...
+ (40 ascii zeros)
+ pcrinfo= ascii hex of PCR_INFO or PCR_INFO_LONG (no default)
+ pcrlock= pcr number to be extended to "lock" blob
+ migratable= 0|1 indicating permission to reseal to new PCR values,
+ default 1 (resealing allowed)
+ hash= hash algorithm name as a string. For TPM 1.x the only
+ allowed value is sha1. For TPM 2.x the allowed values
+ are sha1, sha256, sha384, sha512 and sm3-256.
+ policydigest= digest for the authorization policy. must be calculated
+ with the same hash algorithm as specified by the 'hash='
+ option.
+ policyhandle= handle to an authorization policy session that defines the
+ same policy and with the same hash algorithm as was used to
+ seal the key.
+
+"keyctl print" returns an ascii hex copy of the sealed key, which is in standard
+TPM_STORED_DATA format. The key length for new keys are always in bytes.
+Trusted Keys can be 32 - 128 bytes (256 - 1024 bits), the upper limit is to fit
+within the 2048 bit SRK (RSA) keylength, with all necessary structure/padding.
+
+Encrypted keys do not depend on a TPM, and are faster, as they use AES for
+encryption/decryption. New keys are created from kernel generated random
+numbers, and are encrypted/decrypted using a specified 'master' key. The
+'master' key can either be a trusted-key or user-key type. The main
+disadvantage of encrypted keys is that if they are not rooted in a trusted key,
+they are only as secure as the user key encrypting them. The master user key
+should therefore be loaded in as secure a way as possible, preferably early in
+boot.
+
+The decrypted portion of encrypted keys can contain either a simple symmetric
+key or a more complex structure. The format of the more complex structure is
+application specific, which is identified by 'format'.
+
+Usage::
+
+ keyctl add encrypted name "new [format] key-type:master-key-name keylen"
+ ring
+ keyctl add encrypted name "load hex_blob" ring
+ keyctl update keyid "update key-type:master-key-name"
+
+Where::
+
+ format:= 'default | ecryptfs'
+ key-type:= 'trusted' | 'user'
+
+
+Examples of trusted and encrypted key usage:
+
+Create and save a trusted key named "kmk" of length 32 bytes::
+
+ $ keyctl add trusted kmk "new 32" @u
+ 440502848
+
+ $ keyctl show
+ Session Keyring
+ -3 --alswrv 500 500 keyring: _ses
+ 97833714 --alswrv 500 -1 \_ keyring: _uid.500
+ 440502848 --alswrv 500 500 \_ trusted: kmk
+
+ $ keyctl print 440502848
+ 0101000000000000000001005d01b7e3f4a6be5709930f3b70a743cbb42e0cc95e18e915
+ 3f60da455bbf1144ad12e4f92b452f966929f6105fd29ca28e4d4d5a031d068478bacb0b
+ 27351119f822911b0a11ba3d3498ba6a32e50dac7f32894dd890eb9ad578e4e292c83722
+ a52e56a097e6a68b3f56f7a52ece0cdccba1eb62cad7d817f6dc58898b3ac15f36026fec
+ d568bd4a706cb60bb37be6d8f1240661199d640b66fb0fe3b079f97f450b9ef9c22c6d5d
+ dd379f0facd1cd020281dfa3c70ba21a3fa6fc2471dc6d13ecf8298b946f65345faa5ef0
+ f1f8fff03ad0acb083725535636addb08d73dedb9832da198081e5deae84bfaf0409c22b
+ e4a8aea2b607ec96931e6f4d4fe563ba
+
+ $ keyctl pipe 440502848 > kmk.blob
+
+Load a trusted key from the saved blob::
+
+ $ keyctl add trusted kmk "load `cat kmk.blob`" @u
+ 268728824
+
+ $ keyctl print 268728824
+ 0101000000000000000001005d01b7e3f4a6be5709930f3b70a743cbb42e0cc95e18e915
+ 3f60da455bbf1144ad12e4f92b452f966929f6105fd29ca28e4d4d5a031d068478bacb0b
+ 27351119f822911b0a11ba3d3498ba6a32e50dac7f32894dd890eb9ad578e4e292c83722
+ a52e56a097e6a68b3f56f7a52ece0cdccba1eb62cad7d817f6dc58898b3ac15f36026fec
+ d568bd4a706cb60bb37be6d8f1240661199d640b66fb0fe3b079f97f450b9ef9c22c6d5d
+ dd379f0facd1cd020281dfa3c70ba21a3fa6fc2471dc6d13ecf8298b946f65345faa5ef0
+ f1f8fff03ad0acb083725535636addb08d73dedb9832da198081e5deae84bfaf0409c22b
+ e4a8aea2b607ec96931e6f4d4fe563ba
+
+Reseal a trusted key under new pcr values::
+
+ $ keyctl update 268728824 "update pcrinfo=`cat pcr.blob`"
+ $ keyctl print 268728824
+ 010100000000002c0002800093c35a09b70fff26e7a98ae786c641e678ec6ffb6b46d805
+ 77c8a6377aed9d3219c6dfec4b23ffe3000001005d37d472ac8a44023fbb3d18583a4f73
+ d3a076c0858f6f1dcaa39ea0f119911ff03f5406df4f7f27f41da8d7194f45c9f4e00f2e
+ df449f266253aa3f52e55c53de147773e00f0f9aca86c64d94c95382265968c354c5eab4
+ 9638c5ae99c89de1e0997242edfb0b501744e11ff9762dfd951cffd93227cc513384e7e6
+ e782c29435c7ec2edafaa2f4c1fe6e7a781b59549ff5296371b42133777dcc5b8b971610
+ 94bc67ede19e43ddb9dc2baacad374a36feaf0314d700af0a65c164b7082401740e489c9
+ 7ef6a24defe4846104209bf0c3eced7fa1a672ed5b125fc9d8cd88b476a658a4434644ef
+ df8ae9a178e9f83ba9f08d10fa47e4226b98b0702f06b3b8
+
+The initial consumer of trusted keys is EVM, which at boot time needs a high
+quality symmetric key for HMAC protection of file metadata. The use of a
+trusted key provides strong guarantees that the EVM key has not been
+compromised by a user level problem, and when sealed to specific boot PCR
+values, protects against boot and offline attacks. Create and save an
+encrypted key "evm" using the above trusted key "kmk":
+
+option 1: omitting 'format'::
+
+ $ keyctl add encrypted evm "new trusted:kmk 32" @u
+ 159771175
+
+option 2: explicitly defining 'format' as 'default'::
+
+ $ keyctl add encrypted evm "new default trusted:kmk 32" @u
+ 159771175
+
+ $ keyctl print 159771175
+ default trusted:kmk 32 2375725ad57798846a9bbd240de8906f006e66c03af53b1b3
+ 82dbbc55be2a44616e4959430436dc4f2a7a9659aa60bb4652aeb2120f149ed197c564e0
+ 24717c64 5972dcb82ab2dde83376d82b2e3c09ffc
+
+ $ keyctl pipe 159771175 > evm.blob
+
+Load an encrypted key "evm" from saved blob::
+
+ $ keyctl add encrypted evm "load `cat evm.blob`" @u
+ 831684262
+
+ $ keyctl print 831684262
+ default trusted:kmk 32 2375725ad57798846a9bbd240de8906f006e66c03af53b1b3
+ 82dbbc55be2a44616e4959430436dc4f2a7a9659aa60bb4652aeb2120f149ed197c564e0
+ 24717c64 5972dcb82ab2dde83376d82b2e3c09ffc
+
+Other uses for trusted and encrypted keys, such as for disk and file encryption
+are anticipated. In particular the new format 'ecryptfs' has been defined in
+in order to use encrypted keys to mount an eCryptfs filesystem. More details
+about the usage can be found in the file
+``Documentation/security/keys/ecryptfs.rst``.