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NAME | DESCRIPTION | SEE ALSO |
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keyrings(7) Miscellaneous Information Manual keyrings(7)
keyrings - in-kernel key management and retention facility
The Linux key-management facility is primarily a way for various
kernel components to retain or cache security data,
authentication keys, encryption keys, and other data in the
kernel.
System call interfaces are provided so that user-space programs
can manage those objects and also use the facility for their own
purposes; see add_key(2), request_key(2), and keyctl(2).
A library and some user-space utilities are provided to allow
access to the facility. See keyctl(1), keyctl(3), and
keyutils(7) for more information.
Keys
A key has the following attributes:
Serial number (ID)
This is a unique integer handle by which a key is referred
to in system calls. The serial number is sometimes
synonymously referred as the key ID. Programmatically,
key serial numbers are represented using the type
key_serial_t.
Type A key's type 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 kernel
components.
Description (name)
The key description is a printable string that is used as
the search term for the key (in conjunction with the key
type) as well as a display name. During searches, the
description may be partially matched or exactly matched.
Payload (data)
The payload 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 user space to finish the
instantiation of a key if that key wasn't already known to
the kernel when it was requested. For further details,
see request_key(2).
A key's payload can be read and updated if the key type
supports it and if suitable permission is granted to the
caller.
Access rights
Much as files do, each key has an owning user ID, an
owning group ID, and a security label. Each key also has
a set of permissions, though there are more than for a
normal UNIX file, and there is an additional category—
possessor—beyond the usual user, group, and other (see
Possession, below).
Note that keys are quota controlled, since they require
unswappable kernel memory. The owning user ID specifies
whose quota is to be debited.
Expiration time
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 the error EKEYEXPIRED. If not
deleted, updated, or replaced, then, after a set amount of
time, an expired key is automatically removed (garbage
collected) along with all links to it, and attempts to
access the key fail with the error ENOKEY.
Reference count
Each key has a reference count. Keys are referenced by
keyrings, by currently active users, and by a process's
credentials. When the reference count reaches zero, the
key is scheduled for garbage collection.
Key types
The kernel provides several basic types of key:
"keyring"
Keyrings are special keys which store a set of links to
other keys (including other keyrings), 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.
Keyrings with descriptions (names) that begin with a
period ('.') are reserved to the implementation.
"user" This is a general-purpose key type. The key is kept
entirely within kernel memory. The payload may be read
and updated by user-space applications.
The payload for keys of this type is a blob of arbitrary
data of up to 32,767 bytes.
The description may be any valid string, though it is
preferred that it start with a colon-delimited prefix
representing the service to which the key is of interest
(for instance "afs:mykey").
"logon" (since Linux 3.3)
This key type is essentially the same as "user", but it
does not provide reading (i.e., the keyctl(2) KEYCTL_READ
operation), meaning that the key payload is never visible
from user space. This is suitable for storing username-
password pairs that should not be readable from user
space.
The description of a "logon" key must start with a non-
empty colon-delimited prefix whose purpose is to identify
the service to which the key belongs. (Note that this
differs from keys of the "user" type, where the inclusion
of a prefix is recommended but is not enforced.)
"big_key" (since Linux 3.13)
This key type is similar to the "user" key type, but it
may hold a payload of up to 1 MiB in size. This key type
is useful for purposes such as holding Kerberos ticket
caches.
The payload data may be stored in a tmpfs filesystem,
rather than in kernel memory, if the data size exceeds the
overhead of storing the data in the filesystem. (Storing
the data in a filesystem requires filesystem structures to
be allocated in the kernel. The size of these structures
determines the size threshold above which the tmpfs
storage method is used.) Since Linux 4.8, the payload
data is encrypted when stored in tmpfs, thereby preventing
it from being written unencrypted into swap space.
There are more specialized key types available also, but they
aren't discussed here because they aren't intended for normal
user-space use.
Key type names that begin with a period ('.') are reserved to the
implementation.
Keyrings
As previously mentioned, keyrings are a special type of key that
contain links to other keys (which may include other keyrings).
Keys may be linked to by multiple keyrings. Keyrings may be
considered as analogous to UNIX directories where each directory
contains a set of hard links to files.
Various operations (system calls) may be applied only to
keyrings:
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 releases its last
reference to the key.
Linking
A link may be added to a keyring pointing to a key that is
already known, provided this does not create a self-
referential cycle.
Unlinking
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 garbage collector.
Clearing
All the links may be removed from a keyring.
Searching
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 key matching a
particular type and description.
See keyctl_clear(3), keyctl_link(3), keyctl_search(3), and
keyctl_unlink(3) for more information.
Anchoring keys
To prevent a key from being garbage collected, it must be
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. Note that keyrings themselves are just keys and are
also subject to the same anchoring requirement to prevent them
being garbage collected.
The kernel makes available a number of anchor keyrings. Note
that some of these keyrings will be created only when first
accessed.
Process keyrings
Process credentials themselves reference keyrings with
specific semantics. These keyrings are pinned as long as
the set of credentials exists, which is usually as long as
the process exists.
There are three keyrings with different
inheritance/sharing rules: the session-keyring(7)
(inherited and shared by all child processes), the
process-keyring(7) (shared by all threads in a process)
and the thread-keyring(7) (specific to a particular
thread).
As an alternative to using the actual keyring IDs, in
calls to add_key(2), keyctl(2), and request_key(2), the
special keyring values KEY_SPEC_SESSION_KEYRING,
KEY_SPEC_PROCESS_KEYRING, and KEY_SPEC_THREAD_KEYRING can
be used to refer to the caller's own instances of these
keyrings.
User keyrings
Each UID known to the kernel has a record that contains
two keyrings: the user-keyring(7) and the
user-session-keyring(7). These exist for as long as the
UID record in the kernel exists.
As an alternative to using the actual keyring IDs, in
calls to add_key(2), keyctl(2), and request_key(2), the
special keyring values KEY_SPEC_USER_KEYRING and
KEY_SPEC_USER_SESSION_KEYRING can be used to refer to the
caller's own instances of these keyrings.
A link to the user keyring is placed in a new session
keyring by pam_keyinit(8) when a new login session is
initiated.
Persistent keyrings
There is a persistent-keyring(7) available to each UID
known to the system. It may persist beyond the life of
the UID record previously mentioned, but has an expiration
time set such that it is automatically cleaned up after a
set time. The persistent keyring permits, for example,
cron(8) scripts to use credentials that are left in the
persistent keyring after the user logs out.
Note that the expiration time of the persistent keyring is
reset every time the persistent key is requested.
Special keyrings
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 verification.
These special keyrings are usually closed to direct
alteration by user space.
An originally planned "group keyring", for storing keys
associated with each GID known to the kernel, is not so far
implemented, is unlikely to be implemented. Nevertheless, the
constant KEY_SPEC_GROUP_KEYRING has been defined for this
keyring.
Possession
The concept of possession is important to understanding the
keyrings security model. Whether a thread possesses a key is
determined by the following rules:
(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-keyring(7),
process-keyring(7), and thread-keyring(7) directly because
those keyrings are referred to by its credentials.
(3) If a keyring is possessed, then any key it links to is also
possessed.
(4) If any key a keyring links to is itself a keyring, then rule
(3) applies recursively.
(5) If a process is upcalled from the kernel to instantiate a
key (see request_key(2)), then it also possesses 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 the key is needed.
Possession is designed to allow set-user-ID programs run from,
say a user's shell to access the user's keys. Granting
permissions to the key possessor while denying them to the key
owner and group allows the prevention of access to keys on the
basis of UID and GID matches.
When it creates the session keyring, pam_keyinit(8) adds a link
to the user-keyring(7), thus making the user keyring and anything
it contains possessed by default.
Access rights
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 contains four sets of rights. The first
three sets are mutually exclusive. One and only one will be in
force for a particular access check. In order of descending
priority, these three sets are:
user The set specifies the rights granted if the key's user ID
matches the caller's filesystem user ID.
group The set specifies the rights granted if the user ID didn't
match and the key's group ID matches the caller's
filesystem GID or one of the caller's supplementary group
IDs.
other The set specifies the rights granted if neither the key's
user ID nor group ID matched.
The fourth set of rights is:
possessor
The set specifies the rights granted if a key is
determined to be possessed by the caller.
The complete set of rights for a key is the union of whichever of
the first three sets is applicable plus the fourth set if the key
is possessed.
The set of rights that may be granted in each of the four masks
is as follows:
view The attributes of the key may be read. This includes the
type, description, and access rights (excluding the
security label).
read For a key: the payload of the key may be read. For a
keyring: the list of serial numbers (keys) to which the
keyring has links may be read.
write The payload of the key may be updated and the key may be
revoked. For a keyring, links may be added to or removed
from the keyring, and the keyring may be cleared
completely (all links are removed),
search For a key (or a keyring): the key may be found by a
search. For a keyring: keys and keyrings that are linked
to by the keyring may be searched.
link Links may be created from keyrings to the key. The
initial link to a key that is established when the key is
created doesn't require this permission.
setattr
The ownership details and security label of the key may be
changed, the key's expiration time may be set, and the key
may be revoked.
In addition to access rights, any active Linux Security Module
(LSM) may prevent access to a key if its policy so dictates. A
key may be given a security label or other attribute by the LSM;
this label is retrievable via keyctl_get_security(3).
See keyctl_chown(3), keyctl_describe(3), keyctl_get_security(3),
keyctl_setperm(3), and selinux(8) for more information.
Searching for keys
One of the key features of the Linux key-management facility is
the ability to find a key that a process is retaining. The
request_key(2) system call is the primary point of access for
user-space applications to find a key. (Internally, the kernel
has something similar available for use by internal components
that make use of keys.)
The search algorithm works as follows:
(1) The process keyrings are searched in the following order:
the thread-keyring(7) if it exists, the process-keyring(7)
if it exists, and then either the session-keyring(7) if it
exists or the user-session-keyring(7) if that exists.
(2) If the caller was a process that was invoked by the
request_key(2) upcall mechanism, then the keyrings of the
original caller of request_key(2) will be searched as well.
(3) The search of a keyring tree is in breadth-first order: 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
terminates 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 no valid matching key is found, then the first noted
error state is returned; otherwise, an ENOKEY error is
returned.
It is also possible to search a specific keyring, in which case
only steps (3) to (6) apply.
See request_key(2) and keyctl_search(3) for more information.
On-demand key creation
If a key cannot be found, request_key(2) will, if given a
callout_info argument, create a new key and then upcall to user
space to instantiate the key. This allows keys to be created on
an as-needed basis.
Typically, this will involve the kernel creating a new process
that executes the request-key(8) program, which will then execute
the appropriate handler based on its configuration.
The handler is passed a special authorization 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 requester's keyrings.
See request_key(2), keyctl_assume_authority(3),
keyctl_instantiate(3), keyctl_negate(3), keyctl_reject(3),
request-key(8), and request-key.conf(5) for more information.
/proc files
The kernel provides various /proc files that expose information
about keys or define limits on key usage.
/proc/keys (since Linux 2.6.10)
This file exposes a list of the keys for which the reading
thread has view permission, providing various information
about each key. The thread need not possess the key for
it to be visible in this file.
The only keys included in the list are those that grant
view permission to the reading process (regardless of
whether or not it possesses them). LSM security checks
are still performed, and may filter out further keys that
the process is not authorized to view.
An example of the data that one might see in this file
(with the columns numbered for easy reference below) is
the following:
(1) (2) (3)(4) (5) (6) (7) (8) (9)
009a2028 I--Q--- 1 perm 3f010000 1000 1000 user krb_ccache:primary: 12
1806c4ba I--Q--- 1 perm 3f010000 1000 1000 keyring _pid: 2
25d3a08f I--Q--- 1 perm 1f3f0000 1000 65534 keyring _uid_ses.1000: 1
28576bd8 I--Q--- 3 perm 3f010000 1000 1000 keyring _krb: 1
2c546d21 I--Q--- 190 perm 3f030000 1000 1000 keyring _ses: 2
30a4e0be I------ 4 2d 1f030000 1000 65534 keyring _persistent.1000: 1
32100fab I--Q--- 4 perm 1f3f0000 1000 65534 keyring _uid.1000: 2
32a387ea I--Q--- 1 perm 3f010000 1000 1000 keyring _pid: 2
3ce56aea I--Q--- 5 perm 3f030000 1000 1000 keyring _ses: 1
The fields shown in each line of this file are as follows:
ID (1) The ID (serial number) of the key, expressed in
hexadecimal.
Flags (2)
A set of flags describing the state of the key:
I The key has been instantiated.
R The key has been revoked.
D The key is dead (i.e., the key type has been
unregistered). (A key may be briefly in
this state during garbage collection.)
Q The key contributes to the user's quota.
U The key is under construction via a callback
to user space; see request-key(2).
N The key is negatively instantiated.
i The key has been invalidated.
Usage (3)
This is a count of the number of kernel credential
structures that are pinning the key (approximately:
the number of threads and open file references that
refer to this key).
Timeout (4)
The amount of time until the key will expire,
expressed in human-readable form (weeks, days,
hours, minutes, and seconds). The string perm here
means that the key is permanent (no timeout). The
string expd means that the key has already expired,
but has not yet been garbage collected.
Permissions (5)
The key permissions, expressed as four hexadecimal
bytes containing, from left to right, the
possessor, user, group, and other permissions.
Within each byte, the permission bits are as
follows:
0x01 view
0x02 read
0x04 write
0x08 search
0x10 link
0x20 setattr
UID (6)
The user ID of the key owner.
GID (7)
The group ID of the key. The value -1 here means
that the key has no group ID; this can occur in
certain circumstances for keys created by the
kernel.
Type (8)
The key type (user, keyring, etc.)
Description (9)
The key description (name). This field contains
descriptive information about the key. For most
key types, it has the form
name[: extra-info]
The name subfield is the key's description (name).
The optional extra-info field provides some further
information about the key. The information that
appears here depends on the key type, as follows:
"user" and "logon"
The size in bytes of the key payload
(expressed in decimal).
"keyring"
The number of keys linked to the keyring, or
the string empty if there are no keys linked
to the keyring.
"big_key"
The payload size in bytes, followed either
by the string [file], if the key payload
exceeds the threshold that means that the
payload is stored in a (swappable) tmpfs(5)
filesystem, or otherwise the string [buff],
indicating that the key is small enough to
reside in kernel memory.
For the ".request_key_auth" key type (authorization
key; see request_key(2)), the description field has
the form shown in the following example:
key:c9a9b19 pid:28880 ci:10
The three subfields are as follows:
key The hexadecimal ID of the key being
instantiated in the requesting program.
pid The PID of the requesting program.
ci The length of the callout data with which
the requested key should be instantiated
(i.e., the length of the payload associated
with the authorization key).
/proc/key-users (since Linux 2.6.10)
This file lists various information for each user ID that
has at least one key on the system. An example of the
data that one might see in this file is the following:
0: 10 9/9 2/1000000 22/25000000
42: 9 9/9 8/200 106/20000
1000: 11 11/11 10/200 271/20000
The fields shown in each line are as follows:
uid The user ID.
usage This is a kernel-internal usage count for the
kernel structure used to record key users.
nkeys/nikeys
The total number of keys owned by the user, and the
number of those keys that have been instantiated.
qnkeys/maxkeys
The number of keys owned by the user, and the
maximum number of keys that the user may own.
qnbytes/maxbytes
The number of bytes consumed in payloads of the
keys owned by this user, and the upper limit on the
number of bytes in key payloads for that user.
/proc/sys/kernel/keys/gc_delay (since Linux 2.6.32)
The value in this file specifies the interval, in seconds,
after which revoked and expired keys will be garbage
collected. The purpose of having such an interval is so
that there is a window of time where user space can see an
error (respectively EKEYREVOKED and EKEYEXPIRED) that
indicates what happened to the key.
The default value in this file is 300 (i.e., 5 minutes).
/proc/sys/kernel/keys/persistent_keyring_expiry (since Linux
3.13)
This file defines an interval, in seconds, to which the
persistent keyring's expiration timer is reset each time
the keyring is accessed (via keyctl_get_persistent(3) or
the keyctl(2) KEYCTL_GET_PERSISTENT operation.)
The default value in this file is 259200 (i.e., 3 days).
The following files (which are writable by privileged processes)
are used to enforce quotas on the number of keys and number of
bytes of data that can be stored in key payloads:
/proc/sys/kernel/keys/maxbytes (since Linux 2.6.26)
This is the maximum number of bytes of data that a nonroot
user can hold in the payloads of the keys owned by the
user.
The default value in this file is 20,000.
/proc/sys/kernel/keys/maxkeys (since Linux 2.6.26)
This is the maximum number of keys that a nonroot user may
own.
The default value in this file is 200.
/proc/sys/kernel/keys/root_maxbytes (since Linux 2.6.26)
This is the maximum number of bytes of data that the root
user (UID 0 in the root user namespace) can hold in the
payloads of the keys owned by root.
The default value in this file is 25,000,000 (20,000
before Linux 3.17).
/proc/sys/kernel/keys/root_maxkeys (since Linux 2.6.26)
This is the maximum number of keys that the root user (UID
0 in the root user namespace) may own.
The default value in this file is 1,000,000 (200 before
Linux 3.17).
With respect to keyrings, note that each link in a keyring
consumes 4 bytes of the keyring payload.
Users
The Linux key-management 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 user space to do DNS lookups and then to
cache the results.
AF_RXRPC and kAFS - Authentication
The AF_RXRPC network protocol and the in-kernel AFS
filesystem use keys to store the ticket needed to do
secured or encrypted traffic. 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 mappings of foreign
user IDs to local user IDs.
CIFS - Password
The CIFS filesystem uses keys to store passwords for
accessing remote shares.
Module verification
The kernel build process can be made to cryptographically
sign modules. That signature is then checked when a
module is loaded.
User-space 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(8) scripts can
use them.
keyctl(1), add_key(2), keyctl(2), request_key(2), keyctl(3),
keyutils(7), persistent-keyring(7), process-keyring(7),
session-keyring(7), thread-keyring(7), user-keyring(7),
user-session-keyring(7), pam_keyinit(8), request-key(8)
The kernel source files Documentation/crypto/asymmetric-keys.txt
and under Documentation/security/keys (or, before Linux 4.13, in
the file Documentation/security/keys.txt).
Linux man-pages (unreleased) (date) keyrings(7)
Pages that refer to this page: keyctl(1), add_key(2), keyctl(2), request_key(2), find_key_by_type_and_name(3), keyctl(3), keyctl_capabilities(3), keyctl_chown(3), keyctl_clear(3), keyctl_describe(3), keyctl_get_keyring_ID(3), keyctl_get_persistent(3), keyctl_get_security(3), keyctl_instantiate(3), keyctl_invalidate(3), keyctl_join_session_keyring(3), keyctl_link(3), keyctl_move(3), keyctl_pkey_encrypt(3), keyctl_pkey_query(3), keyctl_pkey_sign(3), keyctl_read(3), keyctl_revoke(3), keyctl_search(3), keyctl_session_to_parent(3), keyctl_setperm(3), keyctl_set_reqkey_keyring(3), keyctl_set_timeout(3), keyctl_update(3), keyctl_watch_key(3), recursive_key_scan(3), proc(5), asymmetric-key(7), keyutils(7), persistent-keyring(7), process-keyring(7), session-keyring(7), thread-keyring(7), user-keyring(7), user_namespaces(7), user-session-keyring(7), key.dns_resolver(8), request-key(8)
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