Revised keyrings(7) man page for review

From: Michael Kerrisk
Date: Fri Nov 04 2016 - 11:47:30 EST

Hi David,

As part of the process of adopting the *.7 pages from
keyutils into man-pages, I've made some significant
extensions to the keyrings(7) page. Among the more notable

* Expanded the detail in "Key types"
* Addition of a section on the various /proc files
* Various other minor reworkings elsewhere in the text

Would you would be willing to review the new page, please?
(The page source file is attached, in case you want to
see all the formatting.)




keyrings - in-kernel key management and retention facility

The Linux key-management facility is primarily a way for drivâ
ers 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.

A library and some user-space utilities are provided to allow
access to the facility. See keyctl(1), keyctl(3), and keyuâ
tils(7) for more information.

A key has the following attributes:

Serial number (ID)
This is a unique integer handle by which a key is
referred to in system call arguments. The serial number
is sometimes synonymously referred as the key ID. Proâ
grammatically, 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 drivers.

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

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. (Details can be
found in 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

Access rights
Much as files do, each key has an owning user ID, an
owning group ID, and a security label. 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 (see below).

Note that keys are quota controlled since they represent
unswappable kernel memory and the owning user ID speciâ
fies 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 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 attempts to access the key will 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 facility provides several basic types of key:

"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").

Keyrings are special keys whose payload consists of 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.

"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 you do not want to
be readable from user space.

"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 1MiB in size. The data may
be stored in the swap space rather than in kernel memory
if the data size exceeds the overhead of storing the
data in swap space (a tmpfs file is used, which requires
filesystem structures to be allocated in the kernel).
This key type is useful for tasks such as holding Kerâ
beros ticket caches.

There are more specialized key types available also, but
they're not discussed here as they're not intended for normal
user-space use.

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

Adding A key may be added to a keyring by system calls that
create keys. This prevents the new key from being immeâ
diately deleted when the system call driver releases its
last reference to the key.

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.

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.

All the links may be removed from a keyring.

A keyring may be considered the root of a tree or subâ
tree in which keyrings form the branches and non-
keyrings the leaves. This tree may be searched for a
leaf 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 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 referâ
ence on a key - but whilst keyrings are available to link to
keys, keyrings themselves are just keys and are also subject to
the same anchoring necessity.

The kernel makes available a number of anchor keyrings. Note
that some of these keyrings will be created only when first

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 inheriâ
tance/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).

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. 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 expiraâ
tion 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 out.

Note that the expiration time 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.

See thread-keyring(7), process-keyring(7), session-keyring(7),
user-keyring(7), user-session-keyring(7), and persistent-
keyring(7) 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 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, process, and thread
keyrings directly because those are pointed to by its creâ

(3) If a keyring is possessed, then any key it links to is also

(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, 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. It also allows
the prevention of access to keys just 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 anyâ
thing 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:

The set specifies the rights granted if a key is deterâ
mined 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. For a keyring,
links may be added to or removed from the keyring, the
keyring may be cleared completely (all links are
removed), and the key may be revoked.

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 iniâ
tial link to a key that is established when the key is
created doesn't require this permission.

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.

If any right is granted to a thread for a key,

âThis seems to contradict the text below, which says â
âthat 'view' permission is what is significant. â
âWhich is correct? â
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
(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
which can be retrieved.

See keyctl_chown(3), keyctl_describe(3), keyctl_get_secuâ
rity(3), keyctl_setperm(3), and selinux(8) for more informaâ

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 three process keyrings are searched in the following
order: the thread thread-keyring(7) if it exists, the
process-keyring(7) if it exists, and then either the sesâ
sion-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 that request_key(2) will be searched as

(3) The search of the keyring tree is in preorder: 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 continâ

(6) If 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 callâ
out_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 forking and exec'ing
the request-key(8) program, which will then execute the approâ
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 requester's keyrings.

See request_key(2), keyctl_assume_authority(3), keyctl_instanâ
tiate(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 that are viewable
by the reading process, providing various information
about each key.

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
is the following:

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 folâ

ID The ID (serial number) of the key, expressed in

Flags 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., has been deleted). (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 This is a count of the number of kernel credenâ
tial structures that are pinning the key (approxâ
imately: the number of threads and open file refâ
erences that refer to this key).

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 timeâ
out). The string expd means that the key has
already expired, but has not yet been garbage

The key permissions, expressed as four hexadeciâ
mal bytes containing, from left to right, the
possessor, user, group, and other permissions.

UID The user ID of the key owner.

GID The group ID of the key. The value -1 here means
that the key as no group ID; this can occur in
certain circumstances for keys created by the

Type The key type (user, keyring, etc.)

The key description (name).

This field contains descriptive information about
the key. For most key tpes, it has the form

desc[: extra-info]

The name subfield is the 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).

The number of keys linked to the keyring, or
the string empty if there are no keys linked
to the keyring.

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 filesystem, or
otherwise the string [buff], indicating that
the key is small enough to reside in kernel

For the ".request_key_auth" key type (authorizaâ
tion key; see request_key(2)), the description
field has the form shown in the following examâ

key:c9a9b19 pid:28880 ci:10

The three subfields are as follows:

key The hexadecimal ID of the key being instanâ
tiated 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 followâ

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.

The total number of keys owned by the user, and
the number of those keys that have been instantiâ

The number of keys owned by the user, and the
maximum keys that the user may own.

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

/proc/sys/kernel/keys/gc_delay (since Linux 2.6.32)
The value in this file specifies the interval, in secâ
onds, after which revoked and expired keys will be
garbage collected. The purpose of having such an interâ
val 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
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 proâ
cesses) 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 nonâ
root 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.

/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.

With respect to keyrings, note that each link in a keyring conâ
sumes 4 bytes of the keyring payload.

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 operaâ
tions on kAFS.

NFS - User ID mapping
The NFS filesystem uses keys to store mappings of forâ
eign 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 cryptographiâ
cally 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 scripts can
use them.

keyutils(7), persistent-keyring(7), process-keyring(7),
session-keyring(7), thread-keyring(7), user-keyring(7),
user-session-keyring(7), pam_keyinit(8)

Linux 2016-11-01 KEYRINGS(7)

Michael Kerrisk, Training and Consulting
"The Linux Programming Interface" --

Attachment: keyrings.7
Description: Unix manual page