KEYRINGS(7) Kernel key management KEYRINGS(7)
keyrings - In-kernel key management and retention facility
The keyrings facility is primarily a way for drivers to retain or cache
security data, authentication keys, encryption keys and other data in
System call interfaces are provided so that userspace programs can man-
age those objects and also use the facility for their own purposes.
A library and some userspace utilities are provided to allow access to
the facility. See keyutils(7) manual page for more information.
This document contains the following sections:
- Key types.
- Anchoring keys.
- Access rights.
- Searching for keys.
- On-demand key creation.
The facility provides the concept of a 'key', where all of the above
examples are encapsulated within objects of this type.
A 'key' contains the following elements:
This is a unique integer handle by which a key is referred to in
system call arguments.
Type This defines what sort of data can be held in the key, how the
proposed content of the key will be parsed and how the payload
will be used.
There are a number of general purpose types available, plus some
specialist types defined by specific drivers.
This is a printable string that is used as the search term for
the key (in conjunction with the type) as well as a display
name. The description may be partially matched or exactly
This is the actual content of a key. This is usually set when a
key is created, but it is possible for the kernel to upcall to
userspace to finish the instantiation of a key if that key
wasn't already known to the kernel when it was requested.
A key's payload can be read and updated if the key type supports
it and if suitable permission is granted to the caller.
Each key has an owning user ID, an owning group and a security
label - much as files do. They also have a set of permissions,
though there are more than for a normal UNIX file, and there is
an additional category beyond the usual user, group and other
Note that keys are quota controlled since they represent unswap-
pable kernel memory and the owning user ID specifies whose quota
is to be debited.
Each key can have an expiration time set. When that time is
reached, the key is marked as being expired and accesses to it
fail with EKEYEXPIRED. If not deleted, updated or replaced,
after a set amount of time, expired keys are automatically
removed along with all links to them and ENOKEY will be
Each key has a reference count. Keys are referenced by
keyrings, by current active users and by a process's creden-
tials. When the reference count reaches zero, the key is sched-
uled for garbage collection.
See the keyctl_describe(3) manual page for more information.
The facility provides several basic types of key:
Keys of this type are special. The payload consists of a set of
links to other keys, analogous to a directory holding links to
files. The main purpose of a keyring is to prevent other keys
from being garbage collected because nothing refers to them.
user This is a general purpose key type. It may be instantiated with
an arbitrary blob of data of up to about 32KB. It is kept
entirely within kernel memory. It may be read and updated by
This is similar to user but it may hold data up to 1MB in size.
The data may be stored in the swap space rather than in kernel
memory if the size exceeds the overhead of doing so (a tmpfs
file is used - which requires filesystem structures to be allo-
cated in the kernel).
logon This is similar to user but the contents may not be read by
There are more specialised key types available also, but they're not
discussed here as they're not intended for normal userspace use.
As previously mentioned, keyrings are a special type of key that con-
tain links to other keys (which may include other keyrings). Keys may
be linked to by multiple keyrings. Keyrings may be considered as anal-
ogous to UNIX directories where each directory contains a set of hard
links to files.
Several of the syscall functions available may only be applied to
Adding A key may be added to a keyring by system calls that create
keys. This prevents the new key from being immediately deleted
when the system call driver releases its last reference to the
A link may be added to a keyring pointing to a key that is
already known, provided this does not create a self-referential
A link may be removed from a keyring. When the last link to a
key is removed, that key will be scheduled for deletion by the
All the links may be removed from a keyring.
A keyring may be considered the root of a tree or subtree in
which keyrings form the branches and non-keyrings the leaves.
This tree may be searched for a leaf matching a particular type
See the keyctl_clear(3), keyctl_link(3), keyctl_search(3) and
keyctl_unlink(3) manual pages for more information.
To prevent a key from being prematurely garbage collected, it must
anchored to keep its reference count elevated when it is not in active
use by the kernel.
Keyrings are used to anchor other keys - each link is a reference on a
key - but whilst keyrings are available to link to keys, keyrings them-
selves are just keys and are also subject to the same anchoring neces-
The kernel makes available a number of anchor keyrings. Note that some
of these keyrings will only be created when first accessed.
Process credentials themselves reference keyrings with specific
semantics. These keyrings are pinned as long as the set of cre-
dentials exists - which is usually as long as the process does.
There are three keyrings with different inheritance/sharing
rules: The session keyring (inherited and shared by all child
processes), the process keyring (shared by all threads in a
process) and the thread keyring (specific to a particular
Each UID known to the kernel has a record that contains two
keyrings: The user keyring and the user session keyring. These
exist for as long as the UID record in the kernel exists. A
link to the user keyring is placed in a new session keyring by
pam_keyinit when a new login session is initiated.
There is a persistent keyring available to each UID known to the
system. It may persist beyond the life of the UID record previ-
ously mentioned, but has an expiration time set such that it is
automatically cleaned up after a set time. This, for example,
permits cron scripts to use credentials left when the user logs
Note that the expiration time is reset every time the persistent
key is requested.
There are special keyrings owned by the kernel that can anchor
keys for special purposes. An example of this is the system
keyring used for holding encryption keys for module signature
These are usually closed to direct alteration by userspace.
See the thread-keyring(7), process-keyring(7), session-keyring(7),
user-keyring(7), user-session-keyring(7), and persistent-keyring(7)
manual pages for more information.
The concept of 'possession' is important to understanding the keyrings
security model. Whether a thread possesses a key is determined by the
(1) Any key or keyring that does not grant Search permission to the
caller is ignored in all the following rules.
(2) A thread possesses its session, process and thread keyrings
directly because those are pointed to by its credentials.
(3) If a keyring is possessed, then any key it links to is also pos-
(4) If any key a keyring links to is itself a keyring, then rule (3)
(5) If a process is upcalled from the kernel to instantiate a key,
then it also possess's the requester's keyrings as in rule (1)
as if it were the requester.
Note that possession is not a fundamental property of a key, but must
rather be calculated each time it is needed.
Possession is designed to allow setuid programs run from, say, a user's
shell to access the user's keys. It also allows the prevention of
access to keys just on the basis of UID and GID matches.
When it creates the session keyring, the pam_keyinit module adds a link
to the user keyring, thus making the user keyring and anything it con-
tains possessed by default.
Each key has the following security-related attributes:
- The owning user ID
- The ID of a group that is permitted to access the key
- A security label
- A permissions mask
The permissions mask is used to govern the following rights:
View If set, the attributes of a key may be read. This includes the
type, description and access rights (excluding the security
Read If set, the payload of a key may be read and a list of the
serial numbers to which a keyring has links may be read.
Write If set, the payload of a key may be updated, links may be added
to or removed from a keyring, a keyring may be cleared com-
pletely and a key may be revoked.
Search If set, keyrings and subkeyrings may be searched and keys and
keyrings may be found by that search.
Link If set, an additional link may be made to a key from a keyring.
The initial link to a key when it is created doesn't require
If set, the ownership details on a key and its security label
may be changed, its expiration time may be set and it may be
The permissions mask contains four sets of rights. The first three
sets are mutually exclusive. One and only one will be in force at any
one time. In order of descending priority:
User Used if the key's user ID matches the caller's fsuid.
Group Used if the user ID didn't match and the key's group ID matches
the caller's fsgid or one of the caller's supplementary group
Other Used if neither the key's user ID nor group ID matched.
The fourth set of rights is:
Used if a key is determined to be possessed by the caller.
The complete set of rights for a key is the set union of whichever of
the first three sets is selected plus the fourth if the key is pos-
If any right is granted to a thread for a key, then that thread will
see the key listed in /proc/keys. If no rights at all are granted,
then that thread can't even tell that the key exists.
In addition to access rights, any active Linux Security Module may pre-
vent access to a key if its policy so dictates. A key may be given a
security label or other attribute by the LSM which can be retrieved.
See the keyctl_chown(3), keyctl_describe(3), keyctl_get_security(3),
keyctl_setperm(3) and selinux(8) manual pages for more information.
SEARCHING FOR KEYS
One of the key features of this facility is the ability to find a key
that it is retaining. The request_key() system call is the primary
point of access for userspace to find a key to use (the kernel has
something similar available).
The search algorithm works as follows:
(1) The three process keyrings are searched in the following order:
the thread keyring if it exists, the process keyring if it
exists and then either the session keyring if it exists or the
user session keyring if that exists.
(2) If the caller was a process that was invoked by the
request_key() upcall mechanism then the keyrings of the original
caller of that request_key() will be searched as well.
(3) Each keyring is searched first for a match, then the keyrings
referred to by that keyring are searched.
(4) If a matching key is found that is valid, then the search termi-
nates and that key is returned.
(5) If a matching key is found that has an error state attached,
that error state is noted and the search continues.
(6) If valid matching key is found, then the first noted error state
is returned or else ENOKEY is returned.
It is also possible to search a specific keyring, in which case only
steps (3) to (6) apply.
See the request_key(2) and keyctl_search(3) manual pages for more
ON-DEMAND KEY CREATION
If a key cannot be found, the request_key() system call will, if given
a callout_info argument, create a new key and then upcall to userspace
to instantiate the key. This allows keys to be created on an as-needed
Typically, this will involve the kernel forking and exec'ing request-
key program, which will then execute the appopriate handler based on
The handler is passed a special authorisation key that allows it and
only it to instantiate the new key. This is also used to permit
searches performed by the handler program to also search the
See the keyctl_assume_authority(3), keyctl_instantiate(3),
keyctl_negate(3), keyctl_reject(3), request_key(2), request-key(8) and
request-key.conf(5) manual pages for more information.
The facility has a number of users and usages, but is not limited to
those that already exist.
In-kernel users of this facility include:
Network filesystems - DNS
The kernel uses the upcall mechanism provided by the keys to
upcall to userspace to do DNS lookups and then to cache the
AF_RXRPC and kAFS - Authentication
The AF_RXRPC network protocol and the in-kernel AFS filesystem
store the ticket needed to do secured or encrypted traffic in
keys. These are then looked up by network operations on
AF_RXRPC and filesystem operations on kAFS.
NFS - User ID mapping
The NFS filesystem uses keys to store foreign user ID to local
user ID mapping.
CIFS - Password
The CIFS filesystem uses keys to store passwords for accessing
The kernel build process can be made to cryptographically sign
modules. That signature is then checked when a module is
Userspace users of this facility include:
Kerberos key storage
The MIT Kerberos 5 facility (libkrb5) can use keys to store
authentication tokens which can be made to be automatically
cleaned up a set time after the user last uses them, but until
then permits them to hang around after the user has logged out
so that cron scripts can use them.
Linux 21 Feb 2014 KEYRINGS(7)
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