man-pages/man2/request_key.2

.\" Copyright (C) 2006 Red Hat, Inc. All Rights Reserved.
.\"     Written by David Howells (dhowells@redhat.com)
.\" and Copyright (C) 2016 Michael Kerrisk <mtk.man-pages@gmail.com>
.\"
.\" %%%LICENSE_START(GPLv2+_SW_ONEPARA)
.\" This program is free software; you can redistribute it and/or
.\" modify it under the terms of the GNU General Public License
.\" as published by the Free Software Foundation; either version
.\" 2 of the License, or (at your option) any later version.
.\" %%%LICENSE_END
.\"
.TH REQUEST_KEY 2 2021-08-27 Linux "Linux Key Management Calls"
.SH NAME
request_key \- request a key from the kernel's key management facility
.SH SYNOPSIS
.nf
.B #include <keyutils.h>
.PP
.BI "key_serial_t request_key(const char *" type ", const char *" description ,
.BI "                         const char *" callout_info ,
.BI "                         key_serial_t " dest_keyring ");"
.fi
.PP
.IR Note :
There is no glibc wrapper for this system call; see NOTES.
.SH DESCRIPTION
.BR request_key ()
attempts to find a key of the given
.I type
with a description (name) that matches the specified
.IR description .
If such a key could not be found, then the key is optionally created.
If the key is found or created,
.BR request_key ()
attaches it to the keyring whose ID is specified in
.I dest_keyring
and returns the key's serial number.
.PP
.BR 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.
.PP
If
.BR request_key ()
is called from a program invoked by
.BR 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.
.\" David Howells: we can then have an arbitrarily long sequence
.\" of "recursive" request-key upcalls. There is no limit, other
.\" than number of PIDs, etc.
.PP
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
.I search
permission be found, and only keyrings for which the caller has
.I search
permission may be searched.
.PP
If the key is not found and
.I callout
is NULL, then the call fails with the error
.BR ENOKEY .
.PP
If the key is not found and
.I callout
is not NULL, then the kernel attempts to invoke a user-space
program to instantiate the key.
The details are given below.
.PP
The
.I dest_keyring
serial number may be that of a valid keyring for which the caller has
.I write
permission, or it may be one of the following special keyring IDs:
.TP
.B KEY_SPEC_THREAD_KEYRING
This specifies the caller's thread-specific keyring (see
.BR thread\-keyring (7)).
.TP
.B KEY_SPEC_PROCESS_KEYRING
This specifies the caller's process-specific keyring (see
.BR process\-keyring (7)).
.TP
.B KEY_SPEC_SESSION_KEYRING
This specifies the caller's session-specific keyring (see
.BR session\-keyring (7)).
.TP
.B KEY_SPEC_USER_KEYRING
This specifies the caller's UID-specific keyring (see
.BR user\-keyring (7)).
.TP
.B KEY_SPEC_USER_SESSION_KEYRING
This specifies the caller's UID-session keyring (see
.BR user\-session\-keyring (7)).
.PP
When the
.I dest_keyring
is specified as 0
and no key construction has been performed,
then no additional linking is done.
.PP
Otherwise, if
.I 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
.BR keyctl (2)
.BR KEYCTL_SET_REQKEY_KEYRING
operation and continuing in the order shown below
until it finds the first keyring that exists:
.IP \(bu 3
.\" 8bbf4976b59fc9fc2861e79cab7beb3f6d647640
The requestor keyring
.RB ( KEY_REQKEY_DEFL_REQUESTOR_KEYRING ,
since Linux 2.6.29).
.\" FIXME
.\" Actually, is the preceding point correct?
.\" If I understand correctly, we'll only get here if
.\" 'dest_keyring' is zero, in which case KEY_REQKEY_DEFL_REQUESTOR_KEYRING
.\" won't refer to a keyring. Have I misunderstood?
.IP \(bu
The thread-specific keyring
.RB ( KEY_REQKEY_DEFL_THREAD_KEYRING ;
see
.BR thread\-keyring (7)).
.IP \(bu
The process-specific keyring
.RB ( KEY_REQKEY_DEFL_PROCESS_KEYRING ;
see
.BR process\-keyring (7)).
.IP \(bu
The session-specific keyring
.RB ( KEY_REQKEY_DEFL_SESSION_KEYRING ;
see
.BR session\-keyring (7)).
.IP \(bu
The session keyring for the process's user ID
.RB ( KEY_REQKEY_DEFL_USER_SESSION_KEYRING ;
see
.BR user\-session\-keyring (7)).
This keyring is expected to always exist.
.IP \(bu
The UID-specific keyring
.RB ( KEY_REQKEY_DEFL_USER_KEYRING ;
see
.BR user\-keyring (7)).
This keyring is also expected to always exist.
.\" mtk: Are there circumstances where the user sessions and UID-specific
.\" keyrings do not exist?
.\"
.\" David Howells:
.\"     The uid keyrings don't exist until someone tries to access them -
.\"     at which point they're both created.  When you log in, pam_keyinit
.\"     creates a link to your user keyring in the session keyring it just
.\"     created, thereby creating the user and user-session keyrings.
.\"
.\" and David elaborated that "access" means:
.\"
.\"     It means lookup_user_key() was passed KEY_LOOKUP_CREATE.  So:
.\"
.\"         add_key() - destination keyring
.\"         request_key() - destination keyring
.\"         KEYCTL_GET_KEYRING_ID - if create arg is true
.\"         KEYCTL_CLEAR
.\"         KEYCTL_LINK - both args
.\"         KEYCTL_SEARCH - destination keyring
.\"         KEYCTL_CHOWN
.\"         KEYCTL_SETPERM
.\"         KEYCTL_SET_TIMEOUT
.\"         KEYCTL_INSTANTIATE - destination keyring
.\"         KEYCTL_INSTANTIATE_IOV - destination keyring
.\"         KEYCTL_NEGATE - destination keyring
.\"         KEYCTL_REJECT - destination keyring
.\"         KEYCTL_GET_PERSISTENT - destination keyring
.\"
.\"     will all create a keyring under some circumstances.  Whereas the rest,
.\"     such as KEYCTL_GET_SECURITY, KEYCTL_READ and KEYCTL_REVOKE, won't.
.PP
If the
.BR keyctl (2)
.BR KEYCTL_SET_REQKEY_KEYRING
operation specifies
.BR KEY_REQKEY_DEFL_DEFAULT
(or no
.BR KEYCTL_SET_REQKEY_KEYRING
operation is performed),
then the kernel looks for a keyring
starting from the beginning of the list.
.\"
.SS Requesting user-space instantiation of a key
If the kernel cannot find a key matching
.IR type
and
.IR description ,
and
.I callout
is not NULL, then the kernel attempts to invoke a user-space
program to instantiate a key with the given
.IR type
and
.IR description .
In this case, the following steps are performed:
.IP a) 4
The kernel creates an uninstantiated key, U, with the requested
.I type
and
.IR description .
.IP b)
The kernel creates an authorization key, V,
.\" struct request_key_auth, defined in security/keys/internal.h
that refers to the key U and records the facts that the caller of
.BR request_key ()
is:
.RS
.IP (1) 4
the context in which the key U should be instantiated and secured, and
.IP (2)
the context from which associated key requests may be satisfied.
.RE
.IP
The authorization key is constructed as follows:
.RS
.IP * 3
The key type is
.IR """.request_key_auth""" .
.IP *
The key's UID and GID are the same as the corresponding filesystem IDs
of the requesting process.
.IP *
The key grants
.IR view ,
.IR read ,
and
.IR search
permissions to the key possessor as well as
.IR view
permission for the key user.
.IP *
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.
.IP *
The payload of the key is taken from the data specified in
.IR callout_info .
.IP *
Internally, the kernel also records the PID of the process that called
.BR request_key ().
.RE
.IP c)
The kernel creates a process that executes a user-space service such as
.BR request\-key (8)
with a new session keyring that contains a link to the authorization key, V.
.\" The request\-key(8) program can be invoked in circumstances *other* than
.\" when triggered by request_key(2). For example, upcalls from places such
.\" as the DNS resolver.
.IP
This program is supplied with the following command-line arguments:
.RS
.IP [0] 4
The string
.IR """/sbin/request\-key""" .
.IP [1]
The string
.I """create"""
(indicating that a key is to be created).
.IP [2]
The ID of the key that is to be instantiated.
.IP [3]
The filesystem UID of the caller of
.BR request_key ().
.IP [4]
The filesystem GID of the caller of
.BR request_key ().
.IP [5]
The ID of the thread keyring of the caller of
.BR request_key ().
This may be zero if that keyring hasn't been created.
.IP [6]
The ID of the process keyring of the caller of
.BR request_key ().
This may be zero if that keyring hasn't been created.
.IP [7]
The ID of the session keyring of the caller of
.BR request_key ().
.RE
.IP
.IR Note :
each of the command-line arguments that is a key ID is encoded in
.IR decimal
(unlike the key IDs shown in
.IR /proc/keys ,
which are shown as hexadecimal values).
.IP d)
The program spawned in the previous step:
.RS
.IP * 3
Assumes the authority to instantiate the key U using the
.BR keyctl (2)
.BR KEYCTL_ASSUME_AUTHORITY
operation (typically via the
.BR keyctl_assume_authority (3)
function).
.IP *
Obtains the callout data from the payload of the authorization key V
(using the
.BR keyctl (2)
.BR KEYCTL_READ
operation (or, more commonly, the
.BR keyctl_read (3)
function) with a key ID value of
.BR KEY_SPEC_REQKEY_AUTH_KEY ).
.IP *
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
.BR request_key ()
can be accessed using the special key ID
.BR KEY_SPEC_REQUESTOR_KEYRING .)
.\" Should an instantiating program be using KEY_SPEC_REQUESTOR_KEYRING?
.\" I couldn't find a use in the keyutils git repo.
.\" According to David Howells:
.\" * This feature is provided, but not used at the moment.
.\" * A key added to that ring is then owned by the requester
Instantiation is performed using the
.BR keyctl (2)
.BR KEYCTL_INSTANTIATE
operation (or, more commonly, the
.BR keyctl_instantiate (3)
function).
At this point, the
.BR request_key ()
call completes, and the requesting program can continue execution.
.RE
.PP
If these steps are unsuccessful, then an
.BR ENOKEY
error will be returned to the caller of
.BR request_key ()
and a temporary, negatively instantiated key will be installed
in the keyring specified by
.IR dest_keyring .
This will expire after a few seconds, but will cause subsequent calls to
.BR 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
.BR request\-key (8)
upcalls) for a key that can't (at the moment) be positively instantiated.
.PP
Once the key has been instantiated, the authorization key
.RB ( KEY_SPEC_REQKEY_AUTH_KEY )
is revoked, and the destination keyring
.RB ( KEY_SPEC_REQUESTOR_KEYRING )
is no longer accessible from the
.BR request\-key (8)
program.
.PP
If a key is created, then\(emregardless of whether it is a valid key or
a negatively instantiated key\(emit will displace any other key with
the same type and description from the keyring specified in
.IR dest_keyring .
.SH RETURN VALUE
On success,
.BR request_key ()
returns the serial number of the key it found or caused to be created.
On error, \-1 is returned and
.I errno
is set to indicate the error.
.SH ERRORS
.TP
.B EACCES
The keyring wasn't available for modification by the user.
.TP
.B EDQUOT
The key quota for this user would be exceeded by creating this key or linking
it to the keyring.
.TP
.B EFAULT
One of
.IR type ,
.IR description ,
or
.IR callout_info
points outside the process's accessible address space.
.TP
.B EINTR
The request was interrupted by a signal; see
.BR signal (7).
.TP
.B EINVAL
The size of the string (including the terminating null byte) specified in
.I type
or
.I description
exceeded the limit (32 bytes and 4096 bytes respectively).
.TP
.B EINVAL
The size of the string (including the terminating null byte) specified in
.I callout_info
exceeded the system page size.
.TP
.B EKEYEXPIRED
An expired key was found, but no replacement could be obtained.
.TP
.B EKEYREJECTED
The attempt to generate a new key was rejected.
.TP
.B EKEYREVOKED
A revoked key was found, but no replacement could be obtained.
.TP
.B ENOKEY
No matching key was found.
.TP
.B ENOMEM
Insufficient memory to create a key.
.TP
.B EPERM
The
.I type
argument started with a period (\(aq.\(aq).
.SH VERSIONS
This system call first appeared in Linux 2.6.10.
The ability to instantiate keys upon request was added
.\" commit 3e30148c3d524a9c1c63ca28261bc24c457eb07a
in Linux 2.6.13.
.SH CONFORMING TO
This system call is a nonstandard Linux extension.
.SH NOTES
Glibc does not provide a wrapper for this system call.
A wrapper is provided in the
.IR libkeyutils
library.
(The accompanying package provides the
.I <keyutils.h>
header file.)
When employing the wrapper in that library, link with
.IR \-lkeyutils .
.SH EXAMPLES
The program below demonstrates the use of
.BR request_key ().
The
.IR type ,
.IR description ,
and
.IR 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.
.PP
In order to demonstrate this program,
we first create a suitable entry in the file
.IR /etc/request\-key.conf .
.PP
.in +4n
.EX
$ sudo sh
# \fBecho \(aqcreate user mtk:* *   /bin/keyctl instantiate %k %c %S\(aq \e\fP
          \fB> /etc/request\-key.conf\fP
# \fBexit\fP
.EE
.in
.PP
This entry specifies that when a new "user" key with the prefix
"mtk:" must be instantiated, that task should be performed via the
.BR keyctl (1)
command's
.B instantiate
operation.
The arguments supplied to the
.B instantiate
operation are:
the ID of the uninstantiated key
.RI ( %k );
the callout data supplied to the
.BR request_key ()
call
.RI ( %c );
and the session keyring
.RI ( %S )
of the requestor (i.e., the caller of
.BR request_key ()).
See
.BR request\-key.conf (5)
for details of these
.I %
specifiers.
.PP
Then we run the program and check the contents of
.IR /proc/keys
to verify that the requested key has been instantiated:
.PP
.in +4n
.EX
$ \fB./t_request_key user mtk:key1 "Payload data"\fP
$ \fBgrep \(aq2dddaf50\(aq /proc/keys\fP
2dddaf50 I\-\-Q\-\-\-  1 perm 3f010000  1000  1000 user  mtk:key1: 12
.EE
.in
.PP
For another example of the use of this program, see
.BR keyctl (2).
.SS Program source
\&
.EX
/* 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\en",
                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\en", (uintmax_t) key);

    exit(EXIT_SUCCESS);
}
.EE
.SH SEE ALSO
.ad l
.nh
.BR keyctl (1),
.BR add_key (2),
.BR keyctl (2),
.BR keyctl (3),
.BR capabilities (7),
.BR keyrings (7),
.BR keyutils (7),
.BR persistent\-keyring (7),
.BR process\-keyring (7),
.BR session\-keyring (7),
.BR thread\-keyring (7),
.BR user\-keyring (7),
.BR user\-session\-keyring (7),
.BR request\-key (8)
.PP
The kernel source files
.IR Documentation/security/keys/core.rst
and
.IR Documentation/keys/request\-key.rst
(or, before Linux 4.13, in the files
.\" commit b68101a1e8f0263dbc7b8375d2a7c57c6216fb76
.IR Documentation/security/keys.txt
and
.\" commit 3db38ed76890565772fcca3279cc8d454ea6176b
.IR Documentation/security/keys\-request\-key.txt ).