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
============
(*) 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
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.
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:
+ 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
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:
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:
Note that this setting is inherited across fork/exec.
- [1] The default default is: the thread keyring if there is one, otherwise
+ [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.
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
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 *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
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);
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
===================================
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:
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
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
============================
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