This service allows cryptographic keys, authentication tokens, cross-domain
user mappings, and similar to be cached in the kernel for the use of
-filesystems other kernel services.
+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
- Key overview
- Key service overview
- Key access permissions
+ - SELinux support
- New procfs files
- Userspace system call interface
- Kernel services
+ - Notes on accessing payload contents
- Defining a key type
- Request-key callback service
- - Key access filesystem
+ - Garbage collection
============
- 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.
+ (*) 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.
+ 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.
+ 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.
+ 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
(*) 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.
+ (*) 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.
(*) 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.
+ (*) 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.
(*) 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
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 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
amount of description and payload space that can be consumed.
The user can view information on this and other statistics through procfs
- files.
+ 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 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 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.
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 user, group and other access. Only five of
-each set of eight bits are defined. These permissions granted are:
+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
(*) 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.
+ 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
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
================
(*) /proc/keys
- This lists all the keys on the system, giving information about their
- type, description and permissions. The payload of the key is not
- available this way:
-
- SERIAL FLAGS USAGE EXPY PERM UID GID TYPE DESCRIPTION: SUMMARY
- 00000001 I----- 39 perm 1f0000 0 0 keyring _uid_ses.0: 1/4
- 00000002 I----- 2 perm 1f0000 0 0 keyring _uid.0: empty
- 00000007 I----- 1 perm 1f0000 0 0 keyring _pid.1: empty
- 0000018d I----- 1 perm 1f0000 0 0 keyring _pid.412: empty
- 000004d2 I--Q-- 1 perm 1f0000 32 -1 keyring _uid.32: 1/4
- 000004d3 I--Q-- 3 perm 1f0000 32 -1 keyring _uid_ses.32: empty
- 00000892 I--QU- 1 perm 1f0000 0 0 user metal:copper: 0
- 00000893 I--Q-N 1 35s 1f0000 0 0 user metal:silver: 0
- 00000894 I--Q-- 1 10h 1f0000 0 0 user metal:gold: 0
+ 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:
R Revoked
D Dead
Q Contributes to user's quota
- U Under contruction by callback to userspace
+ U Under construction by callback to userspace
N Negative key
This file must be enabled at kernel configuration time as it allows anyone
(*) /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:
+ 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
<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
===============================
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:
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.
+ 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.
+ 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.
+ 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
/sbin/request-key will be invoked in an attempt to obtain a key. The
callout_info string will be passed as an argument to the program.
+ See also Documentation/keys-request-key.txt.
+
The keyctl syscall functions are:
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.
+ 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.
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.
+ 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().
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.
+ 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
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.
+ 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.
+ 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 to deep or EDEADLK if the link would introduce a cycle.
+ 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:
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.
+ 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:
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.
+ 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
(*) Read the payload data from a key:
- key_serial_t keyctl(KEYCTL_READ, key_serial_t keyring, char *buffer,
- size_t buflen);
+ 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
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.
+ On a successful return, the function will always return the amount of data
+ available rather than the amount copied.
(*) Instantiate a partially constructed key.
- key_serial_t keyctl(KEYCTL_INSTANTIATE, key_serial_t key,
- const void *payload, size_t plen,
- key_serial_t keyring);
+ long keyctl(KEYCTL_INSTANTIATE, key_serial_t key,
+ const void *payload, size_t plen,
+ key_serial_t keyring);
If the kernel calls back to userspace to complete the instantiation of a
key, userspace should use this call to supply data for the key before the
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.
+ that keyring, however all the constraints applying in KEYCTL_LINK apply in
+ this case too.
The payload and plen arguments describe the payload data as for add_key().
(*) Negatively instantiate a partially constructed key.
- key_serial_t keyctl(KEYCTL_NEGATE, key_serial_t key,
- unsigned timeout, key_serial_t keyring);
+ long keyctl(KEYCTL_NEGATE, key_serial_t key,
+ unsigned timeout, key_serial_t keyring);
If the kernel calls back to userspace to complete the instantiation of a
key, userspace should use this call mark the key as negative before the
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.
+ that keyring, however all the constraints applying in KEYCTL_LINK apply in
+ this case too.
+
+
+ (*) Set the default request-key destination keyring.
+
+ long keyctl(KEYCTL_SET_REQKEY_KEYRING, int reqkey_defl);
+
+ This sets the default keyring to which implicitly requested keys will be
+ attached for this thread. reqkey_defl should be one of these constants:
+
+ CONSTANT VALUE NEW DEFAULT KEYRING
+ ====================================== ====== =======================
+ KEY_REQKEY_DEFL_NO_CHANGE -1 No change
+ KEY_REQKEY_DEFL_DEFAULT 0 Default[1]
+ KEY_REQKEY_DEFL_THREAD_KEYRING 1 Thread keyring
+ KEY_REQKEY_DEFL_PROCESS_KEYRING 2 Process keyring
+ KEY_REQKEY_DEFL_SESSION_KEYRING 3 Session keyring
+ KEY_REQKEY_DEFL_USER_KEYRING 4 User keyring
+ KEY_REQKEY_DEFL_USER_SESSION_KEYRING 5 User session keyring
+ KEY_REQKEY_DEFL_GROUP_KEYRING 6 Group keyring
+
+ The old default will be returned if successful and error EINVAL will be
+ returned if reqkey_defl is not one of the above values.
+
+ The default keyring can be overridden by the keyring indicated to the
+ request_key() system call.
+
+ Note that this setting is inherited across fork/exec.
+
+ [1] The default is: the thread keyring if there is one, otherwise
+ the process keyring if there is one, otherwise the session keyring if
+ there is one, otherwise the user default session keyring.
+
+
+ (*) Set the timeout on a key.
+
+ long keyctl(KEYCTL_SET_TIMEOUT, key_serial_t key, unsigned timeout);
+
+ This sets or clears the timeout on a key. The timeout can be 0 to clear
+ the timeout or a number of seconds to set the expiry time that far into
+ the future.
+
+ The process must have attribute modification access on a key to set its
+ timeout. Timeouts may not be set with this function on negative, revoked
+ or expired keys.
+
+
+ (*) Assume the authority granted to instantiate a key
+
+ long keyctl(KEYCTL_ASSUME_AUTHORITY, key_serial_t key);
+
+ This assumes or divests the authority required to instantiate the
+ specified key. Authority can only be assumed if the thread has the
+ authorisation key associated with the specified key in its keyrings
+ somewhere.
+
+ Once authority is assumed, searches for keys will also search the
+ requester's keyrings using the requester's security label, UID, GID and
+ groups.
+
+ If the requested authority is unavailable, error EPERM will be returned,
+ likewise if the authority has been revoked because the target key is
+ already instantiated.
+
+ If the specified key is 0, then any assumed authority will be divested.
+
+ The assumed authoritative key is inherited across fork and exec.
+
+
+ (*) Get the LSM security context attached to a key.
+
+ long keyctl(KEYCTL_GET_SECURITY, key_serial_t key, char *buffer,
+ size_t buflen)
+
+ This function returns a string that represents the LSM security context
+ attached to a key in the buffer provided.
+
+ Unless there's an error, it always returns the amount of data it could
+ produce, even if that's too big for the buffer, but it won't copy more
+ than requested to userspace. If the buffer pointer is NULL then no copy
+ will take place.
+
+ A NUL character is included at the end of the string if the buffer is
+ sufficiently big. This is included in the returned count. If no LSM is
+ in force then an empty string will be returned.
+
+ A process must have view permission on the key for this function to be
+ successful.
+
+
+ (*) Install the calling process's session keyring on its parent.
+
+ long keyctl(KEYCTL_SESSION_TO_PARENT);
+
+ This functions attempts to install the calling process's session keyring
+ on to the calling process's parent, replacing the parent's current session
+ keyring.
+
+ The calling process must have the same ownership as its parent, the
+ keyring must have the same ownership as the calling process, the calling
+ process must have LINK permission on the keyring and the active LSM module
+ mustn't deny permission, otherwise error EPERM will be returned.
+
+ Error ENOMEM will be returned if there was insufficient memory to complete
+ the operation, otherwise 0 will be returned to indicate success.
+
+ The keyring will be replaced next time the parent process leaves the
+ kernel and resumes executing userspace.
===============
KERNEL SERVICES
===============
-The kernel services for key managment are fairly simple to deal with. They can
+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.
-
-When accessing a key's payload data, key->lock should be at least read locked,
-or else the data may be changed by an update being performed from userspace
-whilst the driver or filesystem is trying to access it. If no update method is
-supplied, then the key's payload may be accessed without holding a lock as
-there is no way to change it, provided it can be guaranteed that the key's
-type definition won't go away.
+filesystem or device file, a search would probably be performed during the open
+call, and the key released upon close. How to deal with conflicting keys due to
+two different users opening the same file is left to the filesystem author to
+solve.
+
+To access the key manager, the following header must be #included:
+
+ <linux/key.h>
+
+Specific key types should have a header file under include/keys/ that should be
+used to access that type. For keys of type "user", for example, that would be:
+
+ <keys/user-type.h>
+
+Note that there are two different types of pointers to keys that may be
+encountered:
+
+ (*) struct key *
+
+ This simply points to the key structure itself. Key structures will be at
+ least four-byte aligned.
+
+ (*) key_ref_t
+
+ This is equivalent to a struct key *, but the least significant bit is set
+ if the caller "possesses" the key. By "possession" it is meant that the
+ calling processes has a searchable link to the key from one of its
+ keyrings. There are three functions for dealing with these:
+
+ key_ref_t make_key_ref(const struct key *key,
+ unsigned long possession);
+
+ struct key *key_ref_to_ptr(const key_ref_t key_ref);
+
+ unsigned long is_key_possessed(const key_ref_t key_ref);
+
+ The first function constructs a key reference from a key pointer and
+ possession information (which must be 0 or 1 and not any other value).
+
+ The second function retrieves the key pointer from a reference and the
+ third retrieves the possession flag.
+
+When accessing a key's payload contents, certain precautions must be taken to
+prevent access vs modification races. See the section "Notes on accessing
+payload contents" for more information.
(*) To search for a key, call:
struct key *request_key(const struct key_type *type,
const char *description,
- const char *callout_string);
+ 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
Should the function fail error ENOKEY, EKEYEXPIRED or EKEYREVOKED will be
returned.
+ If successful, the key will have been attached to the default keyring for
+ implicitly obtained request-key keys, as set by KEYCTL_SET_REQKEY_KEYRING.
+
+ See also Documentation/keys-request-key.txt.
+
+
+(*) To search for a key, passing auxiliary data to the upcaller, call:
+
+ struct key *request_key_with_auxdata(const struct key_type *type,
+ const char *description,
+ const void *callout_info,
+ size_t callout_len,
+ void *aux);
+
+ This is identical to request_key(), except that the auxiliary data is
+ passed to the key_type->request_key() op if it exists, and the callout_info
+ is a blob of length callout_len, if given (the length may be 0).
+
+
+(*) A key can be requested asynchronously by calling one of:
+
+ struct key *request_key_async(const struct key_type *type,
+ const char *description,
+ const void *callout_info,
+ size_t callout_len);
+
+ or:
+
+ struct key *request_key_async_with_auxdata(const struct key_type *type,
+ const char *description,
+ const char *callout_info,
+ size_t callout_len,
+ void *aux);
+
+ which are asynchronous equivalents of request_key() and
+ request_key_with_auxdata() respectively.
+
+ These two functions return with the key potentially still under
+ construction. To wait for construction completion, the following should be
+ called:
+
+ int wait_for_key_construction(struct key *key, bool intr);
+
+ The function will wait for the key to finish being constructed and then
+ invokes key_validate() to return an appropriate value to indicate the state
+ of the key (0 indicates the key is usable).
+
+ If intr is true, then the wait can be interrupted by a signal, in which
+ case error ERESTARTSYS will be returned.
+
(*) When it is no longer required, the key should be released using:
void key_put(struct key *key);
- This can be called from interrupt context. If CONFIG_KEYS is not set then
+ 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.
(*) If a keyring was found in the search, this can be further searched by:
- struct key *keyring_search(struct key *keyring,
- const struct key_type *type,
- const char *description)
+ 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. If successful, the returned key will need to be
- released.
+ 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:
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 struct key_type and registers it with the system.
+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:
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.
+ With the revised data length. Error EDQUOT will be returned if this is not
+ viable.
(*) int (*instantiate)(struct key *key, const void *data, size_t datalen);
This method is called to attach a payload to a key during construction.
- The payload attached need not bear any relation to the data passed to
- this function.
+ 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.
The fact that KEY_FLAG_INSTANTIATED is not set in key->flags prevents
anything else from gaining access to the key.
- This method may sleep if it wishes.
-
-
- (*) int (*duplicate)(struct key *key, const struct key *source);
-
- If this type of key can be duplicated, then this method should be
- provided. It is called to copy the payload attached to the source into
- the new key. The data length on the new key will have been updated and
- the quota adjusted already.
-
- This method will be called with the source key's semaphore read-locked to
- prevent its payload from being changed. It is safe to sleep here.
+ 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.
+ 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.
+ 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 with the key->lock write locked,
- and the changes to the key's attached payload should be made before the
- key is locked.
+ key_payload_reserve() should be called before the changes are made, but
+ after all allocations and other potentially failing function calls are
+ made.
- The key will have its semaphore write-locked before this method is
- called. Any changes to the key should be made with the key's rwlock
- write-locked also. It is safe to sleep here.
+ It is safe to sleep in this method.
(*) int (*match)(const struct key *key, const void *desc);
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 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; it can consider the key as
- being inaccessible. Note that the key's type may have changed before this
- function is called.
+ 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.
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 key's rwlock read-locked. This will
- prevent the key's payload and state changing; also the description should
- not change. This also means it is not safe to sleep in this method.
+ 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.
+ 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 also
- read-lock key->lock when accessing the key's payload. It is safe to sleep
- in this method, such as might happen when the userspace buffer is
- accessed.
+ 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.
============================
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
+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.
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
git://ftp.safe.ca / safe / jmp / linux-2.6 / blobdiff