keyrings

KEYRINGS(7)                Linux Programmer's 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,
       encryption keys, and other data in the kernel.

       System call interfaces are provided so that user-space programs can
       manage those objects and also use the facility for their own purposes;
       see add_key(2), request_key(2), and keyctl(2).

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

   Keys
       A key has the following attributes:

       Serial number (ID)
              This is a unique integer handle by which a key is referred to in
              system calls.  The serial number is sometimes synonymously
              referred as the key ID.  Programmatically, key serial numbers
              are represented using the type key_serial_t.

       Type   A key's type defines what sort of data can be held in the key,
              how the proposed content of the key will be parsed, and how the
              payload will be used.

              There are a number of general-purpose types available, plus some
              specialist types defined by specific kernel components.

       Description (name)
              The key description is a printable string that is used as the
              search term for the key (in conjunction with the key type) as
              well as a display name.  During searches, the description may be
              partially matched or exactly matched.

       Payload (data)
              The payload is the actual content of a key.  This is usually set
              when a key is created, but it is possible for the kernel to
              upcall to user space to finish the instantiation of a key if
              that key wasn't already known to the kernel when it was
              requested.  For further details, see request_key(2).

              A key's payload can be read and updated if the key type supports
              it and if suitable permission is granted to the caller.

       Access rights
              Much as files do, each key has an owning user ID, an owning
              group ID, and a security label.  Each key also has a set of
              permissions, though there are more than for a normal UNIX file,
              and there is an additional category—possessor—beyond the usual
              user, group, and other (see Possession, below).

              Note that keys are quota controlled, since they require
              unswappable kernel memory.  The owning user ID specifies whose
              quota is to be debited.

       Expiration time
              Each key can have an expiration time set.  When that time is
              reached, the key is marked as being expired and accesses to it
              fail with the error EKEYEXPIRED.  If not deleted, updated, or
              replaced, then, after a set amount of time, an expired key is
              automatically removed (garbage collected) along with all links
              to it, and attempts to access the key fail with the error
              ENOKEY.

       Reference count
              Each key has a reference count.  Keys are referenced by
              keyrings, by currently active users, and by a process's
              credentials.  When the reference count reaches zero, the key is
              scheduled for garbage collection.

   Key types
       The kernel provides several basic types of key:

       "keyring"
              Keyrings are special keys which store a set of links to other
              keys (including other keyrings), analogous to a directory
              holding links to files.  The main purpose of a keyring is to
              prevent other keys from being garbage collected because nothing
              refers to them.

              Keyrings with descriptions (names) that begin with a period
              ('.') are reserved to the implementation.

       "user" This is a general-purpose key type.  The key is kept entirely
              within kernel memory.  The payload may be read and updated by
              user-space applications.

              The payload for keys of this type is a blob of arbitrary data of
              up to 32,767 bytes.

              The description may be any valid string, though it is preferred
              that it start with a colon-delimited prefix representing the
              service to which the key is of interest (for instance
              "afs:mykey").

       "logon" (since Linux 3.3)
              This key type is essentially the same as "user", but it does not
              provide reading (i.e., the keyctl(2) KEYCTL_READ operation),
              meaning that the key payload is never visible from user space.
              This is suitable for storing username-password pairs that should
              not be readable from user space.

              The description of a "logon" key must start with a non-empty
              colon-delimited prefix whose purpose is to identify the service
              to which the key belongs.  (Note that this differs from keys of
              the "user" type, where the inclusion of a prefix is recommended
              but is not enforced.)

       "big_key" (since Linux 3.13)
              This key type is similar to the "user" key type, but it may hold
              a payload of up to 1 MiB in size.  This key type is useful for
              purposes such as holding Kerberos ticket caches.

              The payload data may be stored in a tmpfs filesystem, rather
              than in kernel memory, if the data size exceeds the overhead of
              storing the data in the filesystem.  (Storing the data in a
              filesystem requires filesystem structures to be allocated in the
              kernel.  The size of these structures determines the size
              threshold above which the tmpfs storage method is used.)  Since
              Linux 4.8, the payload data is encrypted when stored in tmpfs,
              thereby preventing it from being written unencrypted into swap
              space.

       There are more specialized key types available also, but they aren't
       discussed here because they aren't intended for normal user-space use.

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

   Keyrings
       As previously mentioned, keyrings are a special type of key that
       contain links to other keys (which may include other keyrings).  Keys
       may be linked to by multiple keyrings.  Keyrings may be considered as
       analogous to UNIX directories where each directory contains a set of
       hard links to files.

       Various operations (system calls) may be applied only to keyrings:

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

       Linking
              A link may be added to a keyring pointing to a key that is
              already known, provided this does not create a self-referential
              cycle.

       Unlinking
              A link may be removed from a keyring.  When the last link to a
              key is removed, that key will be scheduled for deletion by the
              garbage collector.

       Clearing
              All the links may be removed from a keyring.

       Searching
              A keyring may be considered the root of a tree or subtree in
              which keyrings form the branches and non-keyrings the leaves.
              This tree may be searched for a key matching a particular type
              and description.

       See keyctl_clear(3), keyctl_link(3), keyctl_search(3), and
       keyctl_unlink(3) for more information.

   Anchoring keys
       To prevent a key from being garbage collected, it must be anchored to
       keep its reference count elevated when it is not in active use by the
       kernel.

       Keyrings are used to anchor other keys: each link is a reference on a
       key.  Note that keyrings themselves are just keys and are also subject
       to the same anchoring requirement to prevent them being garbage
       collected.

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

       Process keyrings
              Process credentials themselves reference keyrings with specific
              semantics.  These keyrings are pinned as long as the set of
              credentials exists, which is usually as long as the process
              exists.

              There are three keyrings with different inheritance/sharing
              rules: the session-keyring(7) (inherited and shared by all child
              processes), the process-keyring(7) (shared by all threads in a
              process) and the thread-keyring(7) (specific to a particular
              thread).

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

       User keyrings
              Each UID known to the kernel has a record that contains two
              keyrings: the user-keyring(7) and the user-session-keyring(7).
              These exist for as long as the UID record in the kernel exists.

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

              A link to the user keyring is placed in a new session keyring by
              pam_keyinit(8) when a new login session is initiated.

       Persistent keyrings
              There is a persistent-keyring(7) available to each UID known to
              the system.  It may persist beyond the life of the UID record
              previously mentioned, but has an expiration time set such that
              it is automatically cleaned up after a set time.  The persistent
              keyring permits, for example, cron(8) scripts to use credentials
              that are left in the persistent keyring after the user logs out.

              Note that the expiration time of the persistent keyring is reset
              every time the persistent key is requested.

       Special keyrings
              There are special keyrings owned by the kernel that can anchor
              keys for special purposes.  An example of this is the system
              keyring used for holding encryption keys for module signature
              verification.

              These special keyrings  are usually closed to direct alteration
              by user space.

       An originally planned "group keyring", for storing keys associated with
       each GID known to the kernel, is not so far implemented, is unlikely to
       be implemented.  Nevertheless, the constant KEY_SPEC_GROUP_KEYRING has
       been defined for this keyring.

   Possession
       The concept of possession is important to understanding the keyrings
       security model.  Whether a thread possesses a key is determined by the
       following rules:

       (1) Any key or keyring that does not grant search permission to the
           caller is ignored in all the following rules.

       (2) A thread possesses its session-keyring(7), process-keyring(7), and
           thread-keyring(7) directly because those keyrings are referred to
           by its credentials.

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

       (4) If any key a keyring links to is itself a keyring, then rule (3)
           applies recursively.

       (5) If a process is upcalled from the kernel to instantiate a key (see
           request_key(2)), then it also possesses the requester's keyrings as
           in rule (1) as if it were the requester.

       Note that possession is not a fundamental property of a key, but must
       rather be calculated each time the key is needed.

       Possession is designed to allow set-user-ID programs run from, say a
       user's shell to access the user's keys.  Granting permissions to the
       key possessor while denying them to the key owner and group allows the
       prevention of access to keys on the basis of UID and GID matches.

       When it creates the session keyring, pam_keyinit(8) adds a link to the
       user-keyring(7), thus making the user keyring and anything it contains
       possessed by default.

   Access rights
       Each key has the following security-related attributes:

       *  The owning user ID

       *  The ID of a group that is permitted to access the key

       *  A security label

       *  A permissions mask

       The permissions mask contains four sets of rights.  The first three
       sets are mutually exclusive.  One and only one will be in force for a
       particular access check.  In order of descending priority, these three
       sets are:

       user   The set specifies the rights granted if the key's user ID
              matches the caller's filesystem user ID.

       group  The set specifies the rights granted if the user ID didn't match
              and the key's group ID matches the caller's filesystem GID or
              one of the caller's supplementary group IDs.

       other  The set specifies the rights granted if neither the key's user
              ID nor group ID matched.

       The fourth set of rights is:

       possessor
              The set specifies the rights granted if a key is determined to
              be possessed by the caller.

       The complete set of rights for a key is the union of whichever of the
       first three sets is applicable plus the fourth set if the key is
       possessed.

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

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

       read   For a key: the payload of the key may be read.  For a keyring:
              the list of serial numbers (keys) to which the keyring has links
              may be read.

       write  The payload of the key may be updated and the key may be
              revoked.  For a keyring, links may be added to or removed from
              the keyring, and the keyring may be cleared completely (all
              links are removed),

       search For a key (or a keyring): the key may be found by a search.  For
              a keyring: keys and keyrings that are linked to by the keyring
              may be searched.

       link   Links may be created from keyrings to the key.  The initial link
              to a key that is established when the key is created doesn't
              require this permission.

       setattr
              The ownership details and security label of the key may be
              changed, the key's expiration time may be set, and the key may
              be revoked.

       In addition to access rights, any active Linux Security Module (LSM)
       may prevent access to a key if its policy so dictates.  A key may be
       given a security label or other attribute by the LSM; this label is
       retrievable via keyctl_get_security(3).

       See keyctl_chown(3), keyctl_describe(3), keyctl_get_security(3),
       keyctl_setperm(3), and selinux(8) for more information.

   Searching for keys
       One of the key features of the Linux key-management facility is the
       ability to find a key that a process is retaining.  The request_key(2)
       system call is the primary point of access for user-space applications
       to find a key.  (Internally, the kernel has something similar available
       for use by internal components that make use of keys.)

       The search algorithm works as follows:

       (1) The process keyrings are searched in the following order: the
           thread thread-keyring(7) if it exists, the process-keyring(7) if it
           exists, and then either the session-keyring(7) if it exists or the
           user-session-keyring(7) if that exists.

       (2) If the caller was a process that was invoked by the request_key(2)
           upcall mechanism, then the keyrings of the original caller of
           request_key(2) will be searched as well.

       (3) The search of a keyring tree is in breadth-first order: each
           keyring is searched first for a match, then the keyrings referred
           to by that keyring are searched.

       (4) If a matching key is found that is valid, then the search
           terminates and that key is returned.

       (5) If a matching key is found that has an error state attached, that
           error state is noted and the search continues.

       (6) If no valid matching key is found, then the first noted error state
           is returned; otherwise, an ENOKEY error is returned.

       It is also possible to search a specific keyring, in which case only
       steps (3) to (6) apply.

       See request_key(2) and keyctl_search(3) for more information.

   On-demand key creation
       If a key cannot be found, request_key(2) will, if given a callout_info
       argument, create a new key and then upcall to user space to instantiate
       the key.  This allows keys to be created on an as-needed basis.

       Typically, this will involve the kernel creating a new process that
       executes the request-key(8) program, which will then execute the
       appropriate handler based on its configuration.

       The handler is passed a special authorization key that allows it and
       only it to instantiate the new key.  This is also used to permit
       searches performed by the handler program to also search the
       requester's keyrings.

       See request_key(2), keyctl_assume_authority(3), keyctl_instantiate(3),
       keyctl_negate(3), keyctl_reject(3), request-key(8), and request-
       key.conf(5) for more information.

   /proc files
       The kernel provides various /proc files that expose information about
       keys or define limits on key usage.

       /proc/keys (since Linux 2.6.10)
              This file exposes a list of the keys for which the reading
              thread has view permission, providing various information about
              each key.  The thread need not possess the key for it to be
              visible in this file.

              The only keys included in the list are those that grant view
              permission to the reading process (regardless of whether or not
              it possesses them).  LSM security checks are still performed,
              and may filter out further keys that the process is not
              authorized to view.

              An example of the data that one might see in this file (with the
              columns numbered for easy reference below) is the following:

  (1)     (2)     (3)(4)    (5)     (6)   (7)   (8)        (9)
009a2028 I--Q---   1 perm 3f010000  1000  1000 user     krb_ccache:primary: 12
1806c4ba I--Q---   1 perm 3f010000  1000  1000 keyring  _pid: 2
25d3a08f I--Q---   1 perm 1f3f0000  1000 65534 keyring  _uid_ses.1000: 1
28576bd8 I--Q---   3 perm 3f010000  1000  1000 keyring  _krb: 1
2c546d21 I--Q--- 190 perm 3f030000  1000  1000 keyring  _ses: 2
30a4e0be I------   4   2d 1f030000  1000 65534 keyring  _persistent.1000: 1
32100fab I--Q---   4 perm 1f3f0000  1000 65534 keyring  _uid.1000: 2
32a387ea I--Q---   1 perm 3f010000  1000  1000 keyring  _pid: 2
3ce56aea I--Q---   5 perm 3f030000  1000  1000 keyring  _ses: 1

              The fields shown in each line of this file are as follows:

              ID (1) The ID (serial number) of the key, expressed in
                     hexadecimal.

              Flags (2)
                     A set of flags describing the state of the key:

                     I   The key has been instantiated.

                     R   The key has been revoked.

                     D   The key is dead (i.e., the key type has been
                         unregistered).  (A key may be briefly in this state
                         during garbage collection.)

                     Q   The key contributes to the user's quota.

                     U   The key is under construction via a callback to user
                         space; see request-key(2).

                     N   The key is negatively instantiated.

                     i   The key has been invalidated.

              Usage (3)
                     This is a count of the number of kernel credential
                     structures that are pinning the key (approximately: the
                     number of threads and open file references that refer to
                     this key).

              Timeout (4)
                     The amount of time until the key will expire, expressed
                     in human-readable form (weeks, days, hours, minutes, and
                     seconds).  The string perm here means that the key is
                     permanent (no timeout).  The string expd means that the
                     key has already expired, but has not yet been garbage
                     collected.

              Permissions (5)
                     The key permissions, expressed as four hexadecimal bytes
                     containing, from left to right, the possessor, user,
                     group, and other permissions.  Within each byte, the
                     permission bits are as follows:

                          0x01   view
                          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
                     circumstances for keys created by the kernel.

              Type (8)
                     The key type (user, keyring, etc.)

              Description (9)
                     The key description (name).  This field contains
                     descriptive information about the key.  For most key
                     types, it has the form

                          name[: extra-info]

                     The name subfield is the key's description (name).  The
                     optional extra-info field provides some further
                     information about the key.  The information that appears
                     here depends on the key type, as follows:

                     "user" and "logon"
                            The size in bytes of the key payload (expressed in
                            decimal).

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

                     "big_key"
                            The payload size in bytes, followed either by the
                            string [file], if the key payload exceeds the
                            threshold that means that the payload is stored in
                            a (swappable) tmpfs(5) filesystem, or otherwise
                            the string [buff], indicating that the key is
                            small enough to reside in kernel memory.

                     For the ".request_key_auth" key type (authorization key;
                     see request_key(2)), the description field has the form
                     shown in the following example:

                         key:c9a9b19 pid:28880 ci:10

                     The three subfields are as follows:

                     key    The hexadecimal ID of the key being instantiated
                            in the requesting program.

                     pid    The PID of the requesting program.

                     ci     The length of the callout data with which the
                            requested key should be instantiated (i.e., the
                            length of the payload associated with the
                            authorization key).

       /proc/key-users (since Linux 2.6.10)
              This file lists various information for each user ID that has at
              least one key on the system.  An example of the data that one
              might see in this file is the following:

                     0:    10 9/9 2/1000000 22/25000000
                    42:     9 9/9 8/200 106/20000
                  1000:    11 11/11 10/200 271/20000

              The fields shown in each line are as follows:

              uid    The user ID.

              usage  This is a kernel-internal usage count for the kernel
                     structure used to record key users.

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

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

              qnbytes/maxbytes
                     The number of bytes consumed in payloads of the keys
                     owned by this user, and the upper limit on the number of
                     bytes in key payloads for that user.

       /proc/sys/kernel/keys/gc_delay (since Linux 2.6.32)
              The value in this file specifies the interval, in seconds, after
              which revoked and expired keys will be garbage collected.  The
              purpose of having such an interval is so that there is a window
              of time where user space can see an error (respectively
              EKEYREVOKED and EKEYEXPIRED) that indicates what happened to the
              key.

              The default value in this file is 300 (i.e., 5 minutes).

       /proc/sys/kernel/keys/persistent_keyring_expiry (since Linux 3.13)
              This file defines an interval, in seconds, to which the
              persistent keyring's expiration timer is reset each time the
              keyring is accessed (via keyctl_get_persistent(3) or the
              keyctl(2) KEYCTL_GET_PERSISTENT operation.)

              The default value in this file is 259200 (i.e., 3 days).

       The following files (which are writable by privileged processes) are
       used to enforce quotas on the number of keys and number of bytes of
       data that can be stored in key payloads:

       /proc/sys/kernel/keys/maxbytes (since Linux 2.6.26)
              This is the maximum number of bytes of data that a nonroot user
              can hold in the payloads of the keys owned by the user.

              The default value in this file is 20,000.

       /proc/sys/kernel/keys/maxkeys (since Linux 2.6.26)
              This is the maximum number of keys that a nonroot user may own.

              The default value in this file is 200.

       /proc/sys/kernel/keys/root_maxbytes (since Linux 2.6.26)
              This is the maximum number of bytes of data that the root user
              (UID 0 in the root user namespace) can hold in the payloads of
              the keys owned by root.

              The default value in this file is 25,000,000 (20,000 before
              Linux 3.17).

       /proc/sys/kernel/keys/root_maxkeys (since Linux 2.6.26)
              This is the maximum number of keys that the root user (UID 0 in
              the root user namespace) may own.

              The default value in this file is 1,000,000 (200 before Linux
              3.17).

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

   Users
       The Linux key-management facility has a number of users and usages, but
       is not limited to those that already exist.

       In-kernel users of this facility include:

       Network filesystems - DNS
              The kernel uses the upcall mechanism provided by the keys to
              upcall to user space to do DNS lookups and then to cache the
              results.

       AF_RXRPC and kAFS - Authentication
              The AF_RXRPC network protocol and the in-kernel AFS filesystem
              use keys to store the ticket needed to do secured or encrypted
              traffic.  These are then looked up by network operations on
              AF_RXRPC and filesystem operations on kAFS.

       NFS - User ID mapping
              The NFS filesystem uses keys to store mappings of foreign user
              IDs to local user IDs.

       CIFS - Password
              The CIFS filesystem uses keys to store passwords for accessing
              remote shares.

       Module verification
              The kernel build process can be made to cryptographically sign
              modules.  That signature is then checked when a module is
              loaded.

       User-space users of this facility include:

       Kerberos key storage
              The MIT Kerberos 5 facility (libkrb5) can use keys to store
              authentication tokens which can be made to be automatically
              cleaned up a set time after the user last uses them, but until
              then permits them to hang around after the user has logged out
              so that cron(8) scripts can use them.

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).

COLOPHON
       This page is part of release 5.07 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
       https://www.kernel.org/doc/man-pages/.



Linux                             2020-04-11                       KEYRINGS(7)