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 2016-10-08 Linux "Linux Key Management Calls"
.SH NAME
request_key \- request a key from the kernel's key management facility
.SH SYNOPSIS
.nf
.B #include <sys/types.h>
.B #include <keyutils.h>
.sp
.BI "key_serial_t request_key(const char *" type ", const char *" description ,
.BI "                         const char *" callout_info ,
.BI "                         key_serial_t " dest_keyring ");"
.fi

No glibc wrapper is provided 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.
.\" FIXME Is 'keyring' allowed to be 0? Reading the source, it appears so.
.\" In this case, by default, the key is assigned to the session keyring.
.\" But, the KEYCTL_SET_REQKEY_KEYRING also seems to have an influence here.
.\" What are the details here?
.\"

.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.
.P
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.
.\" FIXME Above, can we then have an arbitrarily long sequence
.\" of "recursive" request-key upcalls?
.P
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.
.P
If the key is not found and
.I callout
is NULL, then the call fails with the error
.BR ENOKEY .

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.

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
.RB ( thread-keyring (7)).
.TP
.B KEY_SPEC_PROCESS_KEYRING
This specifies the caller's process-specific keyring
.RB ( process-keyring (7)).
.TP
.B KEY_SPEC_SESSION_KEYRING
This specifies the caller's session-specific keyring
.RB ( session-keyring (7)).
.TP
.B KEY_SPEC_USER_KEYRING
This specifies the caller's UID-specific keyring
.RB ( user-keyring (7)).
.TP
.B KEY_SPEC_USER_SESSION_KEYRING
This specifies the caller's UID-session keyring
.RB ( user-session-keyring (7)).
.\"
.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, that refers to the key U
and records the facts that the caller of
.BR request_key (2)
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.
.IP *
The payload of the key is taken from the data specified in
.IR callout_info .
.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.

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 (2)
call completes, and the requesting program can continue execution.
.RE
.P
If these steps are unsuccessful, then an
.BR ENOKEY
error will be returned to the caller of
.BR request_key ()
and a temporary negative 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.

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.

If a key is created, then\(emregardless of whether it is a valid key or
a negative 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 cause of 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 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.
.SH VERSIONS
This system call first appeared in Linux 2.6.10.
.SH CONFORMING TO
This system call is a nonstandard Linux extension.
.SH NOTES
No wrapper for this system call is provided in glibc.
A wrapper is provided in the
.IR libkeyutils
package.
When employing the wrapper in that library, link with
.IR \-lkeyutils .
.SH EXAMPLE
The program below demonstrates the use of
.BR request_key ().
The
.IR type ,
.IR description ,
and
.BR 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
.IR /etc/request-key.conf .

.in +4n
.nf
$ sudo sh
# \fBecho 'create user mtk:* *   /bin/keyctl instantiate %k %c %S' \\\fP
          \fB> /etc/request-keys.conf\fP
# \fBexit\fP
.fi
.in

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 program could 
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 ()).
i(See
.BR request-key.conf (5)
for details of these
.I %
specifiers.)

Then we run the program and check the contents of
.IR /proc/keys
to verify that the requested kay has been instantiated:

.in +4n
.nf
$ \fB./a.out user mtk:key1 "Payload data"\fP
$ \fBgrep \(aq2dddaf50\(aq /proc/keys\fP
2dddaf50 I--Q---   1 perm 3f010000  1000  1000 user   mtk:key1: 12
.fi
.in
.SS Program source
\&
.nf
#include <sys/types.h>
#include <keyutils.h>
#include <stdio.h>
#include <stdlib.h>
#include <string.h>

#define errExit(msg)    do { perror(msg); exit(EXIT_FAILURE); \\
                        } while (0)

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)
        errExit("request_key");

    printf("Key ID is %lx\\n", (long) key);

    exit(EXIT_SUCCESS);
}
.fi
.SH SEE ALSO
.ad l
.nh
.BR keyctl (1),
.BR add_key (2),
.BR keyctl (2),
.BR keyctl (3),
.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)

The kernel source files
.IR Documentation/security/keys.txt
and
.IR Documentation/security/keys\-request\-key.txt .