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- ============================
- KERNEL KEY RETENTION SERVICE
- ============================
-
-This service allows cryptographic keys, authentication tokens, cross-domain
-user mappings, and similar to be cached in the kernel for the use of
-filesystems and other kernel services.
-
-Keyrings are permitted; these are a special type of key that can hold links to
-other keys. Processes each have three standard keyring subscriptions that a
-kernel service can search for relevant keys.
-
-The key service can be configured on by enabling:
-
- "Security options"/"Enable access key retention support" (CONFIG_KEYS)
-
-This document has the following sections:
-
- - Key overview
- - Key service overview
- - Key access permissions
- - SELinux support
- - New procfs files
- - Userspace system call interface
- - Kernel services
- - Notes on accessing payload contents
- - Defining a key type
- - Request-key callback service
- - Garbage collection
-
-
-============
-KEY OVERVIEW
-============
-
-In this context, keys represent units of cryptographic data, authentication
-tokens, keyrings, etc.. These are represented in the kernel by struct key.
-
-Each key has a number of attributes:
-
- - A serial number.
- - A type.
- - A description (for matching a key in a search).
- - Access control information.
- - An expiry time.
- - A payload.
- - State.
-
-
- (*) Each key is issued a serial number of type key_serial_t that is unique for
- the lifetime of that key. All serial numbers are positive non-zero 32-bit
- integers.
-
- Userspace programs can use a key's serial numbers as a way to gain access
- to it, subject to permission checking.
-
- (*) Each key is of a defined "type". Types must be registered inside the
- kernel by a kernel service (such as a filesystem) before keys of that type
- can be added or used. Userspace programs cannot define new types directly.
-
- Key types are represented in the kernel by struct key_type. This defines a
- number of operations that can be performed on a key of that type.
-
- Should a type be removed from the system, all the keys of that type will
- be invalidated.
-
- (*) Each key has a description. This should be a printable string. The key
- type provides an operation to perform a match between the description on a
- key and a criterion string.
-
- (*) Each key has an owner user ID, a group ID and a permissions mask. These
- are used to control what a process may do to a key from userspace, and
- whether a kernel service will be able to find the key.
-
- (*) Each key can be set to expire at a specific time by the key type's
- instantiation function. Keys can also be immortal.
-
- (*) Each key can have a payload. This is a quantity of data that represent the
- actual "key". In the case of a keyring, this is a list of keys to which
- the keyring links; in the case of a user-defined key, it's an arbitrary
- blob of data.
-
- Having a payload is not required; and the payload can, in fact, just be a
- value stored in the struct key itself.
-
- When a key is instantiated, the key type's instantiation function is
- called with a blob of data, and that then creates the key's payload in
- some way.
-
- Similarly, when userspace wants to read back the contents of the key, if
- permitted, another key type operation will be called to convert the key's
- attached payload back into a blob of data.
-
- (*) Each key can be in one of a number of basic states:
-
- (*) Uninstantiated. The key exists, but does not have any data attached.
- Keys being requested from userspace will be in this state.
-
- (*) Instantiated. This is the normal state. The key is fully formed, and
- has data attached.
-
- (*) Negative. This is a relatively short-lived state. The key acts as a
- note saying that a previous call out to userspace failed, and acts as
- a throttle on key lookups. A negative key can be updated to a normal
- state.
-
- (*) Expired. Keys can have lifetimes set. If their lifetime is exceeded,
- they traverse to this state. An expired key can be updated back to a
- normal state.
-
- (*) Revoked. A key is put in this state by userspace action. It can't be
- found or operated upon (apart from by unlinking it).
-
- (*) Dead. The key's type was unregistered, and so the key is now useless.
-
-Keys in the last three states are subject to garbage collection. See the
-section on "Garbage collection".
-
-
-====================
-KEY SERVICE OVERVIEW
-====================
-
-The key service provides a number of features besides keys:
-
- (*) The key service defines 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
-
- This file must be enabled at kernel configuration time as it allows anyone
- to list the keys database.
-
- (*) /proc/key-users
-
- This file lists the tracking data for each user that has at least one key
- on the system. Such data includes quota information and statistics:
-
- [root@andromeda root]# cat /proc/key-users
- 0: 46 45/45 1/100 13/10000
- 29: 2 2/2 2/100 40/10000
- 32: 2 2/2 2/100 40/10000
- 38: 2 2/2 2/100 40/10000
-
- The format of each line is
- <UID>: User ID to which this applies
- <usage> Structure refcount
- <inst>/<keys> Total number of keys and number instantiated
- <keys>/<max> Key count quota
- <bytes>/<max> Key size quota
-
-
-Four new sysctl files have been added also for the purpose of controlling the
-quota limits on keys:
-
- (*) /proc/sys/kernel/keys/root_maxkeys
- /proc/sys/kernel/keys/root_maxbytes
-
- These files hold the maximum number of keys that root may have and the
- maximum total number of bytes of data that root may have stored in those
- keys.
-
- (*) /proc/sys/kernel/keys/maxkeys
- /proc/sys/kernel/keys/maxbytes
-
- These files hold the maximum number of keys that each non-root user may
- have and the maximum total number of bytes of data that each of those
- users may have stored in their keys.
-
-Root may alter these by writing each new limit as a decimal number string to
-the appropriate file.
-
-
-===============================
-USERSPACE SYSTEM CALL INTERFACE
-===============================
-
-Userspace can manipulate keys directly through three new syscalls: add_key,
-request_key and keyctl. The latter provides a number of functions for
-manipulating keys.
-
-When referring to a key directly, userspace programs should use the key's
-serial number (a positive 32-bit integer). However, there are some special
-values available for referring to special keys and keyrings that relate to the
-process making the call:
-
- CONSTANT VALUE KEY REFERENCED
- ============================== ====== ===========================
- KEY_SPEC_THREAD_KEYRING -1 thread-specific keyring
- KEY_SPEC_PROCESS_KEYRING -2 process-specific keyring
- KEY_SPEC_SESSION_KEYRING -3 session-specific keyring
- KEY_SPEC_USER_KEYRING -4 UID-specific keyring
- KEY_SPEC_USER_SESSION_KEYRING -5 UID-session keyring
- KEY_SPEC_GROUP_KEYRING -6 GID-specific keyring
- KEY_SPEC_REQKEY_AUTH_KEY -7 assumed request_key()
- authorisation key
-
-
-The main syscalls are:
-
- (*) Create a new key of given type, description and payload and add it to the
- nominated keyring:
-
- key_serial_t add_key(const char *type, const char *desc,
- const void *payload, size_t plen,
- key_serial_t keyring);
-
- If a key of the same type and description as that proposed already exists
- in the keyring, this will try to update it with the given payload, or it
- will return error EEXIST if that function is not supported by the key
- type. The process must also have permission to write to the key to be able
- to update it. The new key will have all user permissions granted and no
- group or third party permissions.
-
- Otherwise, this will attempt to create a new key of the specified type and
- description, and to instantiate it with the supplied payload and attach it
- to the keyring. In this case, an error will be generated if the process
- does not have permission to write to the keyring.
-
- The payload is optional, and the pointer can be NULL if not required by
- the type. The payload is plen in size, and plen can be zero for an empty
- payload.
-
- A new keyring can be generated by setting type "keyring", the keyring name
- as the description (or NULL) and setting the payload to NULL.
-
- User defined keys can be created by specifying type "user". It is
- recommended that a user defined key's description by prefixed with a type
- ID and a colon, such as "krb5tgt:" for a Kerberos 5 ticket granting
- ticket.
-
- Any other type must have been registered with the kernel in advance by a
- kernel service such as a filesystem.
-
- The ID of the new or updated key is returned if successful.
-
-
- (*) Search the process's keyrings for a key, potentially calling out to
- userspace to create it.
-
- key_serial_t request_key(const char *type, const char *description,
- const char *callout_info,
- key_serial_t dest_keyring);
-
- This function searches all the process's keyrings in the order thread,
- process, session for a matching key. This works very much like
- KEYCTL_SEARCH, including the optional attachment of the discovered key to
- a keyring.
-
- If a key cannot be found, and if callout_info is not NULL, then
- /sbin/request-key will be invoked in an attempt to obtain a key. The
- callout_info string will be passed as an argument to the program.
-
- See also Documentation/security/keys-request-key.txt.
-
-
-The keyctl syscall functions are:
-
- (*) Map a special key ID to a real key ID for this process:
-
- key_serial_t keyctl(KEYCTL_GET_KEYRING_ID, key_serial_t id,
- int create);
-
- The special key specified by "id" is looked up (with the key being created
- if necessary) and the ID of the key or keyring thus found is returned if
- it exists.
-
- If the key does not yet exist, the key will be created if "create" is
- non-zero; and the error ENOKEY will be returned if "create" is zero.
-
-
- (*) Replace the session keyring this process subscribes to with a new one:
-
- key_serial_t keyctl(KEYCTL_JOIN_SESSION_KEYRING, const char *name);
-
- If name is NULL, an anonymous keyring is created attached to the process
- as its session keyring, displacing the old session keyring.
-
- If name is not NULL, if a keyring of that name exists, the process
- attempts to attach it as the session keyring, returning an error if that
- is not permitted; otherwise a new keyring of that name is created and
- attached as the session keyring.
-
- To attach to a named keyring, the keyring must have search permission for
- the process's ownership.
-
- The ID of the new session keyring is returned if successful.
-
-
- (*) Update the specified key:
-
- long keyctl(KEYCTL_UPDATE, key_serial_t key, const void *payload,
- size_t plen);
-
- This will try to update the specified key with the given payload, or it
- will return error EOPNOTSUPP if that function is not supported by the key
- type. The process must also have permission to write to the key to be able
- to update it.
-
- The payload is of length plen, and may be absent or empty as for
- add_key().
-
-
- (*) Revoke a key:
-
- long keyctl(KEYCTL_REVOKE, key_serial_t key);
-
- This makes a key unavailable for further operations. Further attempts to
- use the key will be met with error EKEYREVOKED, and the key will no longer
- be findable.
-
-
- (*) Change the ownership of a key:
-
- long keyctl(KEYCTL_CHOWN, key_serial_t key, uid_t uid, gid_t gid);
-
- This function permits a key's owner and group ID to be changed. Either one
- of uid or gid can be set to -1 to suppress that change.
-
- Only the superuser can change a key's owner to something other than the
- key's current owner. Similarly, only the superuser can change a key's
- group ID to something other than the calling process's group ID or one of
- its group list members.
-
-
- (*) Change the permissions mask on a key:
-
- long keyctl(KEYCTL_SETPERM, key_serial_t key, key_perm_t perm);
-
- This function permits the owner of a key or the superuser to change the
- permissions mask on a key.
-
- Only bits the available bits are permitted; if any other bits are set,
- error EINVAL will be returned.
-
-
- (*) Describe a key:
-
- long keyctl(KEYCTL_DESCRIBE, key_serial_t key, char *buffer,
- size_t buflen);
-
- This function returns a summary of the key's attributes (but not its
- payload data) as a string in the buffer provided.
-
- Unless there's an error, it always returns the amount of data it could
- produce, even if that's too big for the buffer, but it won't copy more
- than requested to userspace. If the buffer pointer is NULL then no copy
- will take place.
-
- A process must have view permission on the key for this function to be
- successful.
-
- If successful, a string is placed in the buffer in the following format:
-
- <type>;<uid>;<gid>;<perm>;<description>
-
- Where type and description are strings, uid and gid are decimal, and perm
- is hexadecimal. A NUL character is included at the end of the string if
- the buffer is sufficiently big.
-
- This can be parsed with
-
- sscanf(buffer, "%[^;];%d;%d;%o;%s", type, &uid, &gid, &mode, desc);
-
-
- (*) Clear out a keyring:
-
- long keyctl(KEYCTL_CLEAR, key_serial_t keyring);
-
- This function clears the list of keys attached to a keyring. The calling
- process must have write permission on the keyring, and it must be a
- keyring (or else error ENOTDIR will result).
-
- 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.
-
- As much of the data as can be fitted into the buffer will be copied to
- userspace if the buffer pointer is not NULL.
-
- On a successful return, the function will always return the amount of data
- available rather than the amount copied.
-
-
- (*) Instantiate a partially constructed key.
-
- long keyctl(KEYCTL_INSTANTIATE, key_serial_t key,
- const void *payload, size_t plen,
- key_serial_t keyring);
- long keyctl(KEYCTL_INSTANTIATE_IOV, key_serial_t key,
- const struct iovec *payload_iov, unsigned ioc,
- key_serial_t keyring);
-
- If the kernel calls back to userspace to complete the instantiation of a
- key, userspace should use this call to supply data for the key before the
- invoked process returns, or else the key will be marked negative
- automatically.
-
- The process must have write access on the key to be able to instantiate
- it, and the key must be uninstantiated.
-
- If a keyring is specified (non-zero), the key will also be linked into
- that keyring, however all the constraints applying in KEYCTL_LINK apply in
- this case too.
-
- The payload and plen arguments describe the payload data as for add_key().
-
- The payload_iov and ioc arguments describe the payload data in an iovec
- array instead of a single buffer.
-
-
- (*) Negatively instantiate a partially constructed key.
-
- long keyctl(KEYCTL_NEGATE, key_serial_t key,
- unsigned timeout, key_serial_t keyring);
- long keyctl(KEYCTL_REJECT, key_serial_t key,
- unsigned timeout, unsigned error, key_serial_t keyring);
-
- If the kernel calls back to userspace to complete the instantiation of a
- key, userspace should use this call mark the key as negative before the
- invoked process returns if it is unable to 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.
-
-
-===============
-KERNEL SERVICES
-===============
-
-The kernel services for key management are fairly simple to deal with. They can
-be broken down into two areas: keys and key types.
-
-Dealing with keys is fairly straightforward. Firstly, the kernel service
-registers its type, then it searches for a key of that type. It should retain
-the key as long as it has need of it, and then it should release it. For a
-filesystem or device file, a search would probably be performed during the open
-call, and the key released upon close. How to deal with conflicting keys due to
-two different users opening the same file is left to the filesystem author to
-solve.
-
-To access the key manager, the following header must be #included:
-
- <linux/key.h>
-
-Specific key types should have a header file under include/keys/ that should be
-used to access that type. For keys of type "user", for example, that would be:
-
- <keys/user-type.h>
-
-Note that there are two different types of pointers to keys that may be
-encountered:
-
- (*) struct key *
-
- This simply points to the key structure itself. Key structures will be at
- least four-byte aligned.
-
- (*) key_ref_t
-
- This is equivalent to a struct key *, but the least significant bit is set
- if the caller "possesses" the key. By "possession" it is meant that the
- calling processes has a searchable link to the key from one of its
- keyrings. There are three functions for dealing with these:
-
- key_ref_t make_key_ref(const struct key *key,
- unsigned long possession);
-
- struct key *key_ref_to_ptr(const key_ref_t key_ref);
-
- unsigned long is_key_possessed(const key_ref_t key_ref);
-
- The first function constructs a key reference from a key pointer and
- possession information (which must be 0 or 1 and not any other value).
-
- The second function retrieves the key pointer from a reference and the
- third retrieves the possession flag.
-
-When accessing a key's payload contents, certain precautions must be taken to
-prevent access vs modification races. See the section "Notes on accessing
-payload contents" for more information.
-
-(*) To search for a key, call:
-
- struct key *request_key(const struct key_type *type,
- const char *description,
- const char *callout_info);
-
- This is used to request a key or keyring with a description that matches
- the description specified according to the key type's match function. This
- permits approximate matching to occur. If callout_string is not NULL, then
- /sbin/request-key will be invoked in an attempt to obtain the key from
- userspace. In that case, callout_string will be passed as an argument to
- the program.
-
- Should the function fail error ENOKEY, EKEYEXPIRED or EKEYREVOKED will be
- returned.
-
- If successful, the key will have been attached to the default keyring for
- implicitly obtained request-key keys, as set by KEYCTL_SET_REQKEY_KEYRING.
-
- See also Documentation/security/keys-request-key.txt.
-
-
-(*) To search for a key, passing auxiliary data to the upcaller, call:
-
- struct key *request_key_with_auxdata(const struct key_type *type,
- const char *description,
- const void *callout_info,
- size_t callout_len,
- void *aux);
-
- This is identical to request_key(), except that the auxiliary data is
- passed to the key_type->request_key() op if it exists, and the callout_info
- is a blob of length callout_len, if given (the length may be 0).
-
-
-(*) A key can be requested asynchronously by calling one of:
-
- struct key *request_key_async(const struct key_type *type,
- const char *description,
- const void *callout_info,
- size_t callout_len);
-
- or:
-
- struct key *request_key_async_with_auxdata(const struct key_type *type,
- const char *description,
- const char *callout_info,
- size_t callout_len,
- void *aux);
-
- which are asynchronous equivalents of request_key() and
- request_key_with_auxdata() respectively.
-
- These two functions return with the key potentially still under
- construction. To wait for construction completion, the following should be
- called:
-
- int wait_for_key_construction(struct key *key, bool intr);
-
- The function will wait for the key to finish being constructed and then
- invokes key_validate() to return an appropriate value to indicate the state
- of the key (0 indicates the key is usable).
-
- If intr is true, then the wait can be interrupted by a signal, in which
- case error ERESTARTSYS will be returned.
-
-
-(*) When it is no longer required, the key should be released using:
-
- void key_put(struct key *key);
-
- Or:
-
- void key_ref_put(key_ref_t key_ref);
-
- These can be called from interrupt context. If CONFIG_KEYS is not set then
- the argument will not be parsed.
-
-
-(*) Extra references can be made to a key by calling the following function:
-
- struct key *key_get(struct key *key);
-
- These need to be disposed of by calling key_put() when they've been
- finished with. The key pointer passed in will be returned. If the pointer
- is NULL or CONFIG_KEYS is not set then the key will not be dereferenced and
- no increment will take place.
-
-
-(*) A key's serial number can be obtained by calling:
-
- key_serial_t key_serial(struct key *key);
-
- If key is NULL or if CONFIG_KEYS is not set then 0 will be returned (in the
- latter case without parsing the argument).
-
-
-(*) If a keyring was found in the search, this can be further searched by:
-
- key_ref_t keyring_search(key_ref_t keyring_ref,
- const struct key_type *type,
- const char *description)
-
- This searches the keyring tree specified for a matching key. Error ENOKEY
- is returned upon failure (use IS_ERR/PTR_ERR to determine). If successful,
- the returned key will need to be released.
-
- The possession attribute from the keyring reference is used to control
- access through the permissions mask and is propagated to the returned key
- reference pointer if successful.
-
-
-(*) To check the validity of a key, this function can be called:
-
- int validate_key(struct key *key);
-
- This checks that the key in question hasn't expired or and hasn't been
- revoked. Should the key be invalid, error EKEYEXPIRED or EKEYREVOKED will
- be returned. If the key is NULL or if CONFIG_KEYS is not set then 0 will be
- returned (in the latter case without parsing the argument).
-
-
-(*) To register a key type, the following function should be called:
-
- int register_key_type(struct key_type *type);
-
- This will return error EEXIST if a type of the same name is already
- present.
-
-
-(*) To unregister a key type, call:
-
- void unregister_key_type(struct key_type *type);
-
-
-Under some circumstances, it may be desirable to deal with a bundle of keys.
-The facility provides access to the keyring type for managing such a bundle:
-
- struct key_type key_type_keyring;
-
-This can be used with a function such as request_key() to find a specific
-keyring in a process's keyrings. A keyring thus found can then be searched
-with keyring_search(). Note that it is not possible to use request_key() to
-search a specific keyring, so using keyrings in this way is of limited utility.
-
-
-===================================
-NOTES ON ACCESSING PAYLOAD CONTENTS
-===================================
-
-The simplest payload is just a number in key->payload.value. In this case,
-there's no need to indulge in RCU or locking when accessing the payload.
-
-More complex payload contents must be allocated and a pointer to them set in
-key->payload.data. One of the following ways must be selected to access the
-data:
-
- (1) Unmodifiable key type.
-
- If the key type does not have a modify method, then the key's payload can
- be accessed without any form of locking, provided that it's known to be
- instantiated (uninstantiated keys cannot be "found").
-
- (2) The key's semaphore.
-
- The semaphore could be used to govern access to the payload and to control
- the payload pointer. It must be write-locked for modifications and would
- have to be read-locked for general access. The disadvantage of doing this
- is that the accessor may be required to sleep.
-
- (3) RCU.
-
- RCU must be used when the semaphore isn't already held; if the semaphore
- is held then the contents can't change under you unexpectedly as the
- semaphore must still be used to serialise modifications to the key. The
- key management code takes care of this for the key type.
-
- However, this means using:
-
- rcu_read_lock() ... rcu_dereference() ... rcu_read_unlock()
-
- to read the pointer, and:
-
- rcu_dereference() ... rcu_assign_pointer() ... call_rcu()
-
- to set the pointer and dispose of the old contents after a grace period.
- Note that only the key type should ever modify a key's payload.
-
- Furthermore, an RCU controlled payload must hold a struct rcu_head for the
- use of call_rcu() and, if the payload is of variable size, the length of
- the payload. key->datalen cannot be relied upon to be consistent with the
- payload just dereferenced if the key's semaphore is not held.
-
-
-===================
-DEFINING A KEY TYPE
-===================
-
-A kernel service may want to define its own key type. For instance, an AFS
-filesystem might want to define a Kerberos 5 ticket key type. To do this, it
-author fills in a key_type struct and registers it with the system.
-
-Source files that implement key types should include the following header file:
-
- <linux/key-type.h>
-
-The structure has a number of fields, some of which are mandatory:
-
- (*) const char *name
-
- The name of the key type. This is used to translate a key type name
- supplied by userspace into a pointer to the structure.
-
-
- (*) size_t def_datalen
-
- This is optional - it supplies the default payload data length as
- contributed to the quota. If the key type's payload is always or almost
- always the same size, then this is a more efficient way to do things.
-
- The data length (and quota) on a particular key can always be changed
- during instantiation or update by calling:
-
- int key_payload_reserve(struct key *key, size_t datalen);
-
- With the revised data length. Error EDQUOT will be returned if this is not
- viable.
-
-
- (*) int (*vet_description)(const char *description);
-
- This optional method is called to vet a key description. If the key type
- doesn't approve of the key description, it may return an error, otherwise
- it should return 0.
-
-
- (*) int (*instantiate)(struct key *key, const void *data, size_t datalen);
-
- This method is called to attach a payload to a key during construction.
- The payload attached need not bear any relation to the data passed to this
- function.
-
- If the amount of data attached to the key differs from the size in
- keytype->def_datalen, then key_payload_reserve() should be called.
-
- This method does not have to lock the key in order to attach a payload.
- The fact that KEY_FLAG_INSTANTIATED is not set in key->flags prevents
- anything else from gaining access to the key.
-
- It is safe to sleep in this method.
-
-
- (*) int (*update)(struct key *key, const void *data, size_t datalen);
-
- If this type of key can be updated, then this method should be provided.
- It is called to update a key's payload from the blob of data provided.
-
- key_payload_reserve() should be called if the data length might change
- before any changes are actually made. Note that if this succeeds, the type
- is committed to changing the key because it's already been altered, so all
- memory allocation must be done first.
-
- The key will have its semaphore write-locked before this method is called,
- but this only deters other writers; any changes to the key's payload must
- be made under RCU conditions, and call_rcu() must be used to dispose of
- the old payload.
-
- key_payload_reserve() should be called before the changes are made, but
- after all allocations and other potentially failing function calls are
- made.
-
- It is safe to sleep in this method.
-
-
- (*) int (*match)(const struct key *key, const void *desc);
-
- This method is called to match a key against a description. It should
- return non-zero if the two match, zero if they don't.
-
- This method should not need to lock the key in any way. The type and
- description can be considered invariant, and the payload should not be
- accessed (the key may not yet be instantiated).
-
- It is not safe to sleep in this method; the caller may hold spinlocks.
-
-
- (*) void (*revoke)(struct key *key);
-
- This method is optional. It is called to discard part of the payload
- data upon a key being revoked. The caller will have the key semaphore
- write-locked.
-
- It is safe to sleep in this method, though care should be taken to avoid
- a deadlock against the key semaphore.
-
-
- (*) void (*destroy)(struct key *key);
-
- This method is optional. It is called to discard the payload data on a key
- when it is being destroyed.
-
- This method does not need to lock the key to access the payload; it can
- consider the key as being inaccessible at this time. Note that the key's
- type may have been changed before this function is called.
-
- It is not safe to sleep in this method; the caller may hold spinlocks.
-
-
- (*) void (*describe)(const struct key *key, struct seq_file *p);
-
- This method is optional. It is called during /proc/keys reading to
- summarise a key's description and payload in text form.
-
- This method will be called with the RCU read lock held. rcu_dereference()
- should be used to read the payload pointer if the payload is to be
- accessed. key->datalen cannot be trusted to stay consistent with the
- contents of the payload.
-
- The description will not change, though the key's state may.
-
- It is not safe to sleep in this method; the RCU read lock is held by the
- caller.
-
-
- (*) long (*read)(const struct key *key, char __user *buffer, size_t buflen);
-
- This method is optional. It is called by KEYCTL_READ to translate the
- key's payload into something a blob of data for userspace to deal with.
- Ideally, the blob should be in the same format as that passed in to the
- instantiate and update methods.
-
- If successful, the blob size that could be produced should be returned
- rather than the size copied.
-
- This method will be called with the key's semaphore read-locked. This will
- prevent the key's payload changing. It is not necessary to use RCU locking
- when accessing the key's payload. It is safe to sleep in this method, such
- as might happen when the userspace buffer is accessed.
-
-
- (*) int (*request_key)(struct key_construction *cons, const char *op,
- void *aux);
-
- This method is optional. If provided, request_key() and friends will
- invoke this function rather than upcalling to /sbin/request-key to operate
- upon a key of this type.
-
- The aux parameter is as passed to request_key_async_with_auxdata() and
- similar or is NULL otherwise. Also passed are the construction record for
- the key to be operated upon and the operation type (currently only
- "create").
-
- This method is permitted to return before the upcall is complete, but the
- following function must be called under all circumstances to complete the
- instantiation process, whether or not it succeeds, whether or not there's
- an error:
-
- void complete_request_key(struct key_construction *cons, int error);
-
- The error parameter should be 0 on success, -ve on error. The
- construction record is destroyed by this action and the authorisation key
- will be revoked. If an error is indicated, the key under construction
- will be negatively instantiated if it wasn't already instantiated.
-
- If this method returns an error, that error will be returned to the
- caller of request_key*(). complete_request_key() must be called prior to
- returning.
-
- The key under construction and the authorisation key can be found in the
- key_construction struct pointed to by cons:
-
- (*) struct key *key;
-
- The key under construction.
-
- (*) struct key *authkey;
-
- The authorisation key.
-
-
-============================
-REQUEST-KEY CALLBACK SERVICE
-============================
-
-To create a new key, the kernel will attempt to execute the following command
-line:
-
- /sbin/request-key create <key> <uid> <gid> \
- <threadring> <processring> <sessionring> <callout_info>
-
-<key> is the key being constructed, and the three keyrings are the process
-keyrings from the process that caused the search to be issued. These are
-included for two reasons:
-
- (1) There may be an authentication token in one of the keyrings that is
- required to obtain the key, eg: a Kerberos Ticket-Granting Ticket.
-
- (2) The new key should probably be cached in one of these rings.
-
-This program should set it UID and GID to those specified before attempting to
-access any more keys. It may then look around for a user specific process to
-hand the request off to (perhaps a path held in placed in another key by, for
-example, the KDE desktop manager).
-
-The program (or whatever it calls) should finish construction of the key by
-calling KEYCTL_INSTANTIATE or KEYCTL_INSTANTIATE_IOV, which also permits it to
-cache the key in one of the keyrings (probably the session ring) before
-returning. Alternatively, the key can be marked as negative with KEYCTL_NEGATE
-or KEYCTL_REJECT; this also permits the key to be cached in one of the
-keyrings.
-
-If it returns with the key remaining in the unconstructed state, the key will
-be marked as being negative, it will be added to the session keyring, and an
-error will be returned to the key requestor.
-
-Supplementary information may be provided from whoever or whatever invoked this
-service. This will be passed as the <callout_info> parameter. If no such
-information was made available, then "-" will be passed as this parameter
-instead.
-
-
-Similarly, the kernel may attempt to update an expired or a soon to expire key
-by executing:
-
- /sbin/request-key update <key> <uid> <gid> \
- <threadring> <processring> <sessionring>
-
-In this case, the program isn't required to actually attach the key to a ring;
-the rings are provided for reference.
-
-
-==================
-GARBAGE COLLECTION
-==================
-
-Dead keys (for which the type has been removed) will be automatically unlinked
-from those keyrings that point to them and deleted as soon as possible by a
-background garbage collector.
-
-Similarly, revoked and expired keys will be garbage collected, but only after a
-certain amount of time has passed. This time is set as a number of seconds in:
-
- /proc/sys/kernel/keys/gc_delay