request_key

REQUEST_KEY(2)            Linux Key Management Calls            REQUEST_KEY(2)

NAME
       request_key - request a key from the kernel's key management facility

SYNOPSIS
       #include <sys/types.h>
       #include <keyutils.h>

       key_serial_t request_key(const char *type, const char *description,
                                const char *callout_info,
                                key_serial_t dest_keyring);

       No glibc wrapper is provided for this system call; see NOTES.

DESCRIPTION
       request_key()  attempts to find a key of the given type with a descrip-
       tion (name) that matches the specified  description.   If  such  a  key
       could  not be found, then the key is optionally created.  If the key is
       found or created, request_key() attaches it to the keyring whose ID  is
       specified in dest_keyring and returns the key's serial number.

       request_key()  first  recursively searches for a matching key in all of
       the keyrings  attached  to  the  calling  process.   The  keyrings  are
       searched   in  the  order:  thread-specific  keyring,  process-specific
       keyring, and then session keyring.

       If request_key() is called from a program invoked by  request_key()  on
       behalf  of  some  other process to generate a key, then the keyrings of
       that other process will be searched next, using  that  other  process's
       user  ID,  group  ID,  supplementary group IDs, and security context to
       determine access.

       The search of the keyring tree  is  breadth-first:  the  keys  in  each
       keyring  searched are checked for a match before any child keyrings are
       recursed into.  Only keys for which the caller has search permission be
       found, and only keyrings for which the caller has search permission may
       be searched.

       If the key is not found and callout is NULL, then the call  fails  with
       the error ENOKEY.

       If  the  key  is  not  found  and  callout is not NULL, then the kernel
       attempts to invoke a user-space program to instantiate  the  key.   The
       details are given below.

       The dest_keyring serial number may be that of a valid keyring for which
       the caller has write permission, or it may be one of the following spe-
       cial keyring IDs:

       KEY_SPEC_THREAD_KEYRING
              This specifies the caller's thread-specific keyring (see thread-
              keyring(7)).

       KEY_SPEC_PROCESS_KEYRING
              This  specifies  the  caller's  process-specific  keyring   (see
              process-keyring(7)).

       KEY_SPEC_SESSION_KEYRING
              This  specifies  the caller's session-specific keyring (see ses-
              sion-keyring(7)).

       KEY_SPEC_USER_KEYRING
              This specifies the  caller's  UID-specific  keyring  (see  user-
              keyring(7)).

       KEY_SPEC_USER_SESSION_KEYRING
              This  specifies  the caller's UID-session keyring (see user-ses-
              sion-keyring(7)).

       When the dest_keyring is specified as 0 and  no  key  construction  has
       been performed, then no additional linking is done.

       Otherwise,  if  dest_keyring is 0 and a new key is constructed, the new
       key will be linked to the "default" keyring.  More precisely, when  the
       kernel  tries  to  determine to which keyring the newly constructed key
       should be linked, it tries the following keyrings, beginning  with  the
       keyring  set  via the keyctl(2) KEYCTL_SET_REQKEY_KEYRING operation and
       continuing in the order shown below until it finds  the  first  keyring
       that exists:

       o  The   requestor  keyring  (KEY_REQKEY_DEFL_REQUESTOR_KEYRING,  since
          Linux 2.6.29).

       o  The  thread-specific  keyring  (KEY_REQKEY_DEFL_THREAD_KEYRING;  see
          thread-keyring(7)).

       o  The  process-specific  keyring (KEY_REQKEY_DEFL_PROCESS_KEYRING; see
          process-keyring(7)).

       o  The session-specific keyring  (KEY_REQKEY_DEFL_SESSION_KEYRING;  see
          session-keyring(7)).

       o  The     session     keyring    for    the    process's    user    ID
          (KEY_REQKEY_DEFL_USER_SESSION_KEYRING; see user-session-keyring(7)).
          This keyring is expected to always exist.

       o  The  UID-specific  keyring  (KEY_REQKEY_DEFL_USER_KEYRING; see user-
          keyring(7)).  This keyring is also expected to always exist.

       If  the   keyctl(2)   KEYCTL_SET_REQKEY_KEYRING   operation   specifies
       KEY_REQKEY_DEFL_DEFAULT  (or  no KEYCTL_SET_REQKEY_KEYRING operation is
       performed), then the kernel looks  for  a  keyring  starting  from  the
       beginning of the list.

   Requesting user-space instantiation of a key
       If  the  kernel  cannot  find  a key matching type and description, and
       callout is not NULL, then the kernel attempts to  invoke  a  user-space
       program  to  instantiate a key with the given type and description.  In
       this case, the following steps are performed:

       a)  The kernel creates an uninstantiated key,  U,  with  the  requested
           type and description.

       b)  The  kernel creates an authorization key, V, that refers to the key
           U and records the facts that the caller of request_key() is:

           (1) the context in which the  key  U  should  be  instantiated  and
               secured, and

           (2) the  context  from  which associated key requests may be satis-
               fied.

           The authorization key is constructed as follows:

           *  The key type is ".request_key_auth".

           *  The key's UID and GID are the same as the corresponding filesys-
              tem IDs of the requesting process.

           *  The  key  grants  view,  read, and search permissions to the key
              possessor as well as view permission for the key user.

           *  The description (name) of the key is the hexadecimal string rep-
              resenting  the  ID  of the key that is to be instantiated in the
              requesting program.

           *  The payload of the key is taken from the data specified in call-
              out_info.

           *  Internally,  the kernel also records the PID of the process that
              called request_key().

       c)  The kernel creates a process that  executes  a  user-space  service
           such  as  request-key(8) with a new session keyring that contains a
           link to the authorization key, V.

           This program is supplied with the following command-line arguments:

           [0] The string "/sbin/request-key".

           [1] The string "create" (indicating that a key is to be created).

           [2] The ID of the key that is to be instantiated.

           [3] The filesystem UID of the caller of request_key().

           [4] The filesystem GID of the caller of request_key().

           [5] The ID of the thread keyring of the  caller  of  request_key().
               This may be zero if that keyring hasn't been created.

           [6] The  ID  of the process keyring of the caller of request_key().
               This may be zero if that keyring hasn't been created.

           [7] The ID of the session keyring of the caller of request_key().

           Note: each of the command-line  arguments  that  is  a  key  ID  is
           encoded  in  decimal (unlike the key IDs shown in /proc/keys, which
           are shown as hexadecimal values).

       d)  The program spawned in the previous step:

           *  Assumes the  authority  to  instantiate  the  key  U  using  the
              keyctl(2)  KEYCTL_ASSUME_AUTHORITY  operation (typically via the
              keyctl_assume_authority(3) function).

           *  Obtains the callout data from the payload of  the  authorization
              key  V (using the keyctl(2) KEYCTL_READ operation (or, more com-
              monly, the keyctl_read(3) function)  with  a  key  ID  value  of
              KEY_SPEC_REQKEY_AUTH_KEY).

           *  Instantiates  the  key  (or  execs another program that performs
              that task), specifying  the  payload  and  destination  keyring.
              (The destination keyring that the requestor specified when call-
              ing request_key() can be  accessed  using  the  special  key  ID
              KEY_SPEC_REQUESTOR_KEYRING.)   Instantiation  is performed using
              the keyctl(2) KEYCTL_INSTANTIATE operation (or,  more  commonly,
              the   keyctl_instantiate(3)   function).   At  this  point,  the
              request_key() call completes, and  the  requesting  program  can
              continue execution.

       If  these steps are unsuccessful, then an ENOKEY error will be returned
       to the caller of request_key() and a temporary, negatively instantiated
       key  will  be installed in the keyring specified by dest_keyring.  This
       will expire after a few seconds, but will  cause  subsequent  calls  to
       request_key()  to  fail  until it does.  The purpose of this negatively
       instantiated key is to prevent (possibly  different)  processes  making
       repeated requests (that require expensive request-key(8) upcalls) for a
       key that can't (at the moment) be positively instantiated.

       Once  the  key   has   been   instantiated,   the   authorization   key
       (KEY_SPEC_REQKEY_AUTH_KEY)  is  revoked,  and  the  destination keyring
       (KEY_SPEC_REQUESTOR_KEYRING) is no longer accessible from the  request-
       key(8) program.

       If a key is created, then--regardless of whether it is a valid key or a
       negatively instantiated key--it will displace any other  key  with  the
       same type and description from the keyring specified in dest_keyring.

RETURN VALUE
       On success, request_key() returns the serial number of the key it found
       or caused to be created.  On error, -1 is returned and errno is set  to
       indicate the cause of the error.

ERRORS
       EACCES The keyring wasn't available for modification by the user.

       EDQUOT The  key  quota for this user would be exceeded by creating this
              key or linking it to the keyring.

       EFAULT One of type, description, or  callout_info  points  outside  the
              process's accessible address space.

       EINTR  The request was interrupted by a signal; see signal(7).

       EINVAL The  size  of  the  string (including the terminating null byte)
              specified in type or description exceeded the  limit  (32  bytes
              and 4096 bytes respectively).

       EINVAL The  size  of  the  string (including the terminating null byte)
              specified in callout_info exceeded the system page size.

       EKEYEXPIRED
              An expired key was found, but no replacement could be obtained.

       EKEYREJECTED
              The attempt to generate a new key was rejected.

       EKEYREVOKED
              A revoked key was found, but no replacement could be obtained.

       ENOKEY No matching key was found.

       ENOMEM Insufficient memory to create a key.

       EPERM  The type argument started with a period ('.').

VERSIONS
       This system call first  appeared  in  Linux  2.6.10.   The  ability  to
       instantiate keys upon request was added in Linux 2.6.13.

CONFORMING TO
       This system call is a nonstandard Linux extension.

NOTES
       No  wrapper  for  this  system call is provided in glibc.  A wrapper is
       provided in the libkeyutils package.  When  employing  the  wrapper  in
       that library, link with -lkeyutils.

EXAMPLE
       The  program  below  demonstrates  the use of request_key().  The type,
       description, and callout_info arguments for the system call  are  taken
       from the values supplied in the command-line arguments.  The call spec-
       ifies the session keyring as the target keyring.

       In order to demonstrate this program, we first create a suitable  entry
       in the file /etc/request-key.conf.

           $ sudo sh
           # echo 'create user mtk:* *   /bin/keyctl instantiate %k %c %S' \
                     > /etc/request-key.conf
           # exit

       This  entry specifies that when a new "user" key with the prefix "mtk:"
       must be instantiated, that task should be performed via  the  keyctl(1)
       command's instantiate operation.  The arguments supplied to the instan-
       tiate operation are: the ID of the uninstantiated key (%k); the callout
       data  supplied  to the request_key() call (%c); and the session keyring
       (%S) of  the  requestor  (i.e.,  the  caller  of  request_key()).   See
       request-key.conf(5) for details of these % specifiers.

       Then  we run the program and check the contents of /proc/keys to verify
       that the requested key has been instantiated:

           $ ./t_request_key user mtk:key1 "Payload data"
           $ grep '2dddaf50' /proc/keys
           2dddaf50 I--Q---  1 perm 3f010000  1000  1000 user  mtk:key1: 12

       For another example of the use of this program, see keyctl(2).

   Program source

       /* t_request_key.c */

       #include <sys/types.h>
       #include <keyutils.h>
       #include <stdio.h>
       #include <stdlib.h>
       #include <string.h>

       int
       main(int argc, char *argv[])
       {
           key_serial_t key;

           if (argc != 4) {
               fprintf(stderr, "Usage: %s type description callout-data\n",
                       argv[0]);
               exit(EXIT_FAILURE);
           }

           key = request_key(argv[1], argv[2], argv[3],
                             KEY_SPEC_SESSION_KEYRING);
           if (key == -1) {
               perror("request_key");
               exit(EXIT_FAILURE);
           }

           printf("Key ID is %lx\n", (long) key);

           exit(EXIT_SUCCESS);
       }

SEE ALSO
       keyctl(1), add_key(2), keyctl(2), keyctl(3), capabilities(7),
       keyrings(7), keyutils(7), persistent-keyring(7), process-keyring(7),
       session-keyring(7), thread-keyring(7), user-keyring(7),
       user-session-keyring(7), request-key(8)

       The kernel source files Documentation/security/keys/core.rst and
       Documentation/keys/request-key.rst (or, before Linux 4.13, in the files
       Documentation/security/keys.txt and
       Documentation/security/keys-request-key.txt).

COLOPHON
       This page is part of release 4.15 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                             2017-09-15                    REQUEST_KEY(2)
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