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NAME | LIBRARY | SYNOPSIS | DESCRIPTION | RETURN VALUE | ERRORS | STANDARDS | HISTORY | EXAMPLES | SEE ALSO |
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request_key(2) System Calls Manual request_key(2)
request_key - request a key from the kernel's key management
facility
Linux Key Management Utilities (libkeyutils, -lkeyutils)
#include <keyutils.h>
key_serial_t request_key(const char *type, const char *description,
const char *_Nullable callout_info,
key_serial_t dest_keyring);
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:
(1) The kernel creates an uninstantiated key, U, with the
requested type and description.
(2) The kernel creates an authorization key, V, that refers to
the key U and records the facts that the caller of
request_key() is:
(2.1) the context in which the key U should be instantiated
and secured, and
(2.2) 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().
(3) 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).
(4) 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 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 ('.').
Linux.
Linux 2.6.10.
The ability to instantiate keys upon request was added in Linux
2.6.13.
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 <keyutils.h>
#include <stdint.h>
#include <stdio.h>
#include <stdlib.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);
}
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).
Linux man-pages 6.04 2023-03-30 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_capabilities(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_move(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), keyctl_watch_key(3), keyrings(7), keyutils(7), persistent-keyring(7), user-keyring(7), user-session-keyring(7)
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HTML rendering created 2023-06-24 by Michael Kerrisk, author of The Linux Programming Interface. For details of in-depth Linux/UNIX system programming training courses that I teach, look here. Hosting by jambit GmbH. |
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