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 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
       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.

EXAMPLES
       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
       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 5.10 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                              2020-11-01                     REQUEST_KEY(2)