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       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

       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

       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.

   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

              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 muststart 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.

              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

   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.

       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

       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

       The fourth set of rights is:

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

       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.

              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

       (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)

       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

       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)
              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-

                     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).

                         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(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

                         key:c9a9b19 pid:28880 ci:10

                     The three subfields are as follows:

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

                     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

                     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, 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

       /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

       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

       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


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