request_key(2) — Linux manual page

NAME | SYNOPSIS | DESCRIPTION | RETURN VALUE | ERRORS | VERSIONS | CONFORMING TO | NOTES | EXAMPLES | SEE ALSO | COLOPHON

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 <keyutils.h>

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

       Note: There is no glibc wrapper 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 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 special 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
              session-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-session-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:

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

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

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

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

       •  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.

       •  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
               satisfied.

           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
              callout_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 commonly, 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 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 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.

       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         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

       Glibc does not provide a wrapper for this system call.  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 % 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 <stdint.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 %jx\n", (uintmax_t) key);

           exit(EXIT_SUCCESS);
       }

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
       Documentation/security/keys-request-key.txt).

COLOPHON         top

       This page is part of release 5.11 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                          2021-03-22                 REQUEST_KEY(2)

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