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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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).