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Message-ID: <528b203d-ac72-e4a6-8517-e8c5c11055a4@gmail.com>
Date: Fri, 4 Nov 2016 16:45:38 +0100
From: "Michael Kerrisk (man-pages)" <mtk.manpages@...il.com>
To: David Howells <dhowells@...hat.com>
Cc: mtk.manpages@...il.com, keyrings@...r.kernel.org,
linux-man <linux-man@...r.kernel.org>,
Eugene Syromyatnikov <evgsyr@...il.com>,
lkml <linux-kernel@...r.kernel.org>
Subject: Revised request_key(2) man page for review
Hi David (and anyone else with an interest to review)
Triggered by Eugene Syromyatnikov's recent input for the keyctl(2)
man page, I've been doing a fair bit of work on improving the key
management syscall man pages, and wonder if I could ask you for
some review assistance?
First off, below, I've pasted the current draft of the request_key(2)
page, which has seen quite a a lot of changes (increasing from around
100 rendered lines to nearly 300). Could you take a look (and also take
a look at the outstanding FIXME)? (The page source file is attached,
in case you want to see all the formatting.)
Thanks,
Michael
====
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 cre‐
ated. 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.
┌─────────────────────────────────────────────────────┐
│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? │
│ │
└─────────────────────────────────────────────────────┘
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, supplemen‐
tary 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 ker‐
nel 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
(thread-keyring(7)).
KEY_SPEC_PROCESS_KEYRING
This specifies the caller's process-specific keyring
(process-keyring(7)).
KEY_SPEC_SESSION_KEYRING
This specifies the caller's session-specific keyring
(session-keyring(7)).
KEY_SPEC_USER_KEYRING
This specifies the caller's UID-specific keyring (user-
keyring(7)).
KEY_SPEC_USER_SESSION_KEYRING
This specifies the caller's UID-session keyring (user-
session-keyring(7)).
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(2) 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 a record of the PID
of the process that called request_key(2).
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 autho‐
rization key V (using the keyctl(2) KEYCTL_READ opera‐
tion (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 per‐
forms 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(2) call completes, and the requesting pro‐
gram can continue execution.
If these steps are unsuccessful, then an ENOKEY error will be
returned to the caller of request_key() and a temporary nega‐
tive 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 negative 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 creat‐
ing this key or linking it to the keyring.
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.
VERSIONS
This system call first appeared in Linux 2.6.10.
CONFORMING TO
This system call is a nonstandard Linux extension.
NOTES
No wrapper for this system call is provided in glibc. A wrap‐
per is provided in the libkeyutils package. When employing the
wrapper in that library, link with -lkeyutils.
EXAMPLE
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 tar‐
get keyring.
In order to demonstrate this program, we first create a suit‐
able 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 program
could The arguments supplied to the instantiate operation are:
the ID of the uninstantiated key (%k); the callout data sup‐
plied to the request_key() call (%c); and the session keyring
(%S) of the requestor (i.e., the caller of request)key()).
i(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 kay has been instantiated:
$ ./a.out user mtk:key1 "Payload data"
$ grep '2dddaf50' /proc/keys
2dddaf50 I--Q--- 1 perm 3f010000 1000 1000 user mtk:key1: 12
Program source
The program below
#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);
}
SEE ALSO
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.
Linux 2016-10-08 REQUEST_KEY(2)
--
Michael Kerrisk
Linux man-pages maintainer; http://www.kernel.org/doc/man-pages/
Linux/UNIX System Programming Training: http://man7.org/training/
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