keyrings(7) — Linux manual page


KEYRINGS(7)               Linux Programmer's Manual              KEYRINGS(7)

NAME         top

       keyrings - in-kernel key management and retention facility

DESCRIPTION         top

       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

       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

       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

       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:

              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

       Key type names that begin with a period ('.') are reserved to the

       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.

              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 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

       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
              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

              As an alternative to using the actual keyring IDs, in calls to
              add_key(2), keyctl(2), and request_key(2), the special keyring
              can be used to refer to the caller's own instances of these

              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

              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

       (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:

              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

       The set of rights that may be granted in each of the four masks is as

       view   The attributes of the key may be read.  This includes the
              type, description, and access rights (excluding the security

       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.

              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 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

                (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 hexa‐

              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 unreg‐
                         istered).  (A key may be briefly in this state dur‐
                         ing 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
                          Ox02   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 de‐
                     scriptive 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 infor‐
                     mation 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

                            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 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.

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

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

       /proc/sys/kernel/keys/gc_delay (since Linux 2.6.32)
              The value in this file specifies the interval, in seconds, af‐
              ter 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 per‐
              sistent 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

              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

       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

       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

       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.

SEE ALSO         top

       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

COLOPHON         top

       This page is part of release 5.09 of the Linux man-pages project.  A
       description of the project, information about reporting bugs, and the
       latest version of this page, can be found at

Linux                            2020-08-13                      KEYRINGS(7)

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