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NAME | SYNOPSIS | DESCRIPTION | RETURN VALUE | ERRORS | VERSIONS | CONFORMING TO | NOTES | EXAMPLE | SEE ALSO | COLOPHON |
REQUEST_KEY(2) Linux Key Management Calls REQUEST_KEY(2)
request_key - request a key from the kernel's key management facility
#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.
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.
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.
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 ('.').
This system call first appeared in Linux 2.6.10. The ability to
instantiate keys upon request was added in Linux 2.6.13.
This system call is a nonstandard Linux extension.
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.
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-keys.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 <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);
}
keyctl(1), add_key(2), keyctl(2), keyctl(3), keyrings(7),
keyutils(7), capabilities(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.txt and
Documentation/security/keys-request-key.txt.
This page is part of release 4.12 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 2017-03-13 REQUEST_KEY(2)
Pages that refer to this page: keyctl(1), add_key(2), keyctl(2), syscalls(2), find_key_by_type_and_name(3), keyctl(3), keyctl_chown(3), keyctl_clear(3), keyctl_describe(3), keyctl_get_keyring_ID(3), keyctl_get_persistent(3), keyctl_get_security(3), keyctl_instantiate(3), keyctl_invalidate(3), keyctl_join_session_keyring(3), keyctl_link(3), keyctl_read(3), keyctl_revoke(3), keyctl_search(3), keyctl_session_to_parent(3), keyctl_setperm(3), keyctl_set_reqkey_keyring(3), keyctl_set_timeout(3), keyctl_update(3), keyrings(7), keyutils(7), persistent-keyring(7), user-keyring(7), user-session-keyring(7)