request_key(2) — Linux manual page


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

NAME         top

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

SYNOPSIS         top

       #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         top

       request_key() attempts to find a key of the given type with a
       description (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

       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 special keyring IDs:

              This specifies the caller's thread-specific keyring (see

              This specifies the caller's process-specific keyring (see

              This specifies the caller's session-specific keyring (see

              This specifies the caller's UID-specific keyring (see

              This specifies the caller's UID-session keyring (see

       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:

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

       ·  The thread-specific keyring (KEY_REQKEY_DEFL_THREAD_KEYRING; see

       ·  The process-specific keyring (KEY_REQKEY_DEFL_PROCESS_KEYRING; see

       ·  The session-specific keyring (KEY_REQKEY_DEFL_SESSION_KEYRING; see

       ·  The session keyring for the process's user ID
          user-session-keyring(7)).  This keyring is expected to always

       ·  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
       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

           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
              filesystem 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
              representing 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

           *  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

           [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
              commonly, the keyctl_read(3) function) with a key ID value of

           *  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
              calling 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         top

       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         top

       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.

              An expired key was found, but no replacement could be

              The attempt to generate a new key was rejected.

              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         top

       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         top

       This system call is a nonstandard Linux extension.

NOTES         top

       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.

EXAMPLES         top

       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
       specifies 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 instantiate 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 % spec‐

       Then we run the program and check the contents of /proc/keys to ver‐
       ify 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>

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

           if (argc != 4) {
               fprintf(stderr, "Usage: %s type description callout-data\n",

           key = request_key(argv[1], argv[2], argv[3],
           if (key == -1) {

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


SEE ALSO         top

       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

COLOPHON         top

       This page is part of release 5.08 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                            2020-06-09                   REQUEST_KEY(2)

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