keyrings(7) Miscellaneous Information Manual keyrings(7)
NAME
keyrings - in-kernel key management and retention facility
DESCRIPTION
The Linux key-management facility is primarily a way for various kernel
components to retain or cache security data, authentication keys, en-
cryption 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 in-
formation.
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 re-
ferred 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 up-
call 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 per-
missions, 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 unswap-
pable 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 creden-
tials. When the reference count reaches zero, the key is sched-
uled 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 hold-
ing 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 im-
plementation.
Keyrings
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.
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 al-
ready 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_un-
link(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 col-
lected.
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 cre-
dentials 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 pos-
sessed.
(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 pos-
sessed.
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 re-
voked. 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 re-
quire 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 re-
trievable 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 ex-
ists, 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 re-
quest_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 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 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 ex-
ecutes the request-key(8) program, which will then execute the appro-
priate 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 re-
quester's keyrings.
See request_key(2), keyctl_assume_authority(3), keyctl_instantiate(3),
keyctl_negate(3), keyctl_reject(3), request-key(8), and re-
quest-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 vis-
ible 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 autho-
rized 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 hexadeci-
mal.
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 un-
registered). (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 struc-
tures 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 per-
mission 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 cir-
cumstances for keys created by the kernel.
Type (8)
The key type (user, keyring, etc.)
Description (9)
The key description (name). This field contains descrip-
tive 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 informa-
tion 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 re-
quested key should be instantiated (i.e., the
length of the payload associated with the autho-
rization 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 num-
ber 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 EKEYRE-
VOKED 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 persis-
tent 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 up-
call to user space to do DNS lookups and then to cache the re-
sults.
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 au-
thentication 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.
SEE ALSO
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 6.7 2024-02-25 keyrings(7)
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