Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S936073AbcKDPrZ (ORCPT ); Fri, 4 Nov 2016 11:47:25 -0400 Received: from mail-oi0-f68.google.com ([209.85.218.68]:33815 "EHLO mail-oi0-f68.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S933568AbcKDPrV (ORCPT ); Fri, 4 Nov 2016 11:47:21 -0400 To: David Howells Cc: lkml , Eugene Syromyatnikov , keyrings@vger.kernel.org, linux-man From: Michael Kerrisk Subject: Revised keyrings(7) man page for review Message-ID: <51643019-bb42-4066-c824-c55b9e668ac6@man7.org> Date: Fri, 4 Nov 2016 16:47:16 +0100 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:45.0) Gecko/20100101 Thunderbird/45.2.0 MIME-Version: 1.0 Content-Type: multipart/mixed; boundary="------------938A9A6E7B0A9D0B80F45E20" Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 56478 Lines: 1466 This is a multi-part message in MIME format. --------------938A9A6E7B0A9D0B80F45E20 Content-Type: text/plain; charset=utf-8 Content-Transfer-Encoding: 8bit Hi David, As part of the process of adopting the *.7 pages from keyutils into man-pages, I've made some significant extensions to the keyrings(7) page. Among the more notable changes: * Expanded the detail in "Key types" * Addition of a section on the various /proc files * Various other minor reworkings elsewhere in the text Would you would be willing to review the new page, please? (The page source file is attached, in case you want to see all the formatting.) Cheers, Michael ==== NAME keyrings - in-kernel key management and retention facility DESCRIPTION The Linux key-management facility is primarily a way for driv‐ ers to retain or cache security data, authentication keys, encryption keys, and other data in the kernel. System call interfaces are provided so that user-space programs can manage those objects and also use the facility for their own purposes. A library and some user-space utilities are provided to allow access to the facility. See keyctl(1), keyctl(3), and keyu‐ tils(7) for more information. Keys A key has the following attributes: Serial number (ID) This is a unique integer handle by which a key is referred to in system call arguments. The serial number is sometimes synonymously referred as the key ID. Pro‐ grammatically, key serial numbers are represented using the type key_serial_t. Type A key's type defines what sort of data can be held in the key, how the proposed content of the key will be parsed, and how the payload will be used. There are a number of general purpose types available, plus some specialist types defined by specific drivers. Description (name) The key description is a printable string that is used as the search term for the key (in conjunction with the key type) as well as a display name. During searches, the description may be partially matched or exactly matched. Payload (data) The payload is the actual content of a key. This is usually set when a key is created, but it is possible for the kernel to upcall to user space to finish the instantiation of a key if that key wasn't already known to the kernel when it was requested. (Details can be found in request_key(2).) A key's payload can be read and updated if the key type supports it and if suitable permission is granted to the caller. Access rights Much as files do, each key has an owning user ID, an owning group ID, and a security label. They also have a set of permissions, though there are more than for a normal UNIX file, and there is an additional category beyond the usual user, group, and other (see below). Note that keys are quota controlled since they represent unswappable kernel memory and the owning user ID speci‐ fies whose quota is to be debited. Expiration time Each key can have an expiration time set. When that time is reached, the key is marked as being expired and accesses to it fail with EKEYEXPIRED. If not deleted, updated, or replaced, after a set amount of time, expired keys are automatically removed along with all links to them, and attempts to access the key will fail with the error ENOKEY. Reference count Each key has a reference count. Keys are referenced by keyrings, by currently active users, and by a process's credentials. When the reference count reaches zero, the key is scheduled for garbage collection. Key types The facility provides several basic types of key: "user" This is a general purpose key type. The key is kept entirely within kernel memory. The payload may be read and updated by user-space applications. The payload for keys of this type is a blob of arbitrary data of up to 32,767 bytes. The description may be any valid string, though it is preferred that it start with a colon-delimited prefix representing the service to which the key is of interest (for instance "afs:mykey"). "keyring" Keyrings are special keys whose payload consists of a set of links to other keys (including other keyrings), analogous to a directory holding links to files. The main purpose of a keyring is to prevent other keys from being garbage collected because nothing refers to them. "logon" (since Linux 3.3) This key type is essentially the same as "user", but it does not provide reading (i.e., the keyctl(2) KEYCTL_READ operation), meaning that the key payload is never visible from user space. This is suitable for storing username-password pairs that you do not want to be readable from user space. "big_key" (since Linux 3.13) This key type is similar to the "user" key type, but it may hold a payload of up to 1MiB in size. The data may be stored in the swap space rather than in kernel memory if the data size exceeds the overhead of storing the data in swap space (a tmpfs file is used, which requires filesystem structures to be allocated in the kernel). This key type is useful for tasks such as holding Ker‐ beros ticket caches. There are more specialized key types available also, but they're not discussed here as they're not intended for normal user-space use. Keyrings As previously mentioned, keyrings are a special type of key that contain links to other keys (which may include other keyrings). Keys may be linked to by multiple keyrings. Keyrings may be considered as analogous to UNIX directories where each directory contains a set of hard links to files. Various operations (system calls) may be applied only to keyrings: Adding A key may be added to a keyring by system calls that create keys. This prevents the new key from being imme‐ diately deleted when the system call driver releases its last reference to the key. Linking A link may be added to a keyring pointing to a key that is already known, provided this does not create a self- referential cycle. Unlinking A link may be removed from a keyring. When the last link to a key is removed, that key will be scheduled for deletion by the garbage collector. Clearing All the links may be removed from a keyring. Searching A keyring may be considered the root of a tree or sub‐ tree in which keyrings form the branches and non- keyrings the leaves. This tree may be searched for a leaf matching a particular type and description. See keyctl_clear(3), keyctl_link(3), keyctl_search(3), and keyctl_unlink(3) for more information. Anchoring keys To prevent a key from being prematurely garbage collected, it must anchored to keep its reference count elevated when it is not in active use by the kernel. Keyrings are used to anchor other keys - each link is a refer‐ ence on a key - but whilst keyrings are available to link to keys, keyrings themselves are just keys and are also subject to the same anchoring necessity. The kernel makes available a number of anchor keyrings. Note that some of these keyrings will be created only when first accessed. Process keyrings Process credentials themselves reference keyrings with specific semantics. These keyrings are pinned as long as the set of credentials exists, which is usually as long as the process exists. There are three keyrings with different inheri‐ tance/sharing rules: The session-keyring(7) (inherited and shared by all child processes), the process- keyring(7) (shared by all threads in a process) and the thread-keyring(7) (specific to a particular thread). User keyrings Each UID known to the kernel has a record that contains two keyrings: the user-keyring(7) and the user-session- keyring(7). These exist for as long as the UID record in the kernel exists. A link to the user keyring is placed in a new session keyring by pam_keyinit(8) when a new login session is initiated. Persistent keyrings There is a persistent-keyring(7) available to each UID known to the system. It may persist beyond the life of the UID record previously mentioned, but has an expira‐ tion time set such that it is automatically cleaned up after a set time. This, for example, permits cron scripts to use credentials left when the user logs out. Note that the expiration time is reset every time the persistent key is requested. Special keyrings There are special keyrings owned by the kernel that can anchor keys for special purposes. An example of this is the system keyring used for holding encryption keys for module signature verification. These special keyrings are usually closed to direct alteration by user space. See thread-keyring(7), process-keyring(7), session-keyring(7), user-keyring(7), user-session-keyring(7), and persistent- keyring(7) for more information. Possession The concept of possession is important to understanding the keyrings security model. Whether a thread possesses a key is determined by the following rules: (1) Any key or keyring that does not grant search permission to the caller is ignored in all the following rules. (2) A thread possesses its session, process, and thread keyrings directly because those are pointed to by its cre‐ dentials. (3) If a keyring is possessed, then any key it links to is also possessed. (4) If any key a keyring links to is itself a keyring, then rule (3) applies recursively. (5) If a process is upcalled from the kernel to instantiate a key, then it also possesses the requester's keyrings as in rule (1) as if it were the requester. Note that possession is not a fundamental property of a key, but must rather be calculated each time the key is needed. Possession is designed to allow set-user-ID programs run from, say a user's shell to access the user's keys. It also allows the prevention of access to keys just on the basis of UID and GID matches. When it creates the session keyring, pam_keyinit(8) adds a link to the user-keyring(7), thus making the user keyring and any‐ thing it contains possessed by default. Access rights Each key has the following security-related attributes: * The owning user ID * The ID of a group that is permitted to access the key * A security label * A permissions mask The permissions mask contains four sets of rights. The first three sets are mutually exclusive. One and only one will be in force for a particular access check. In order of descending priority, these three sets are: user The set specifies the rights granted if the key's user ID matches the caller's filesystem user ID. group The set specifies the rights granted if the user ID didn't match and the key's group ID matches the caller's filesystem GID or one of the caller's supplementary group IDs. other The set specifies the rights granted if neither the key's user ID nor group ID matched. The fourth set of rights is: possessor The set specifies the rights granted if a key is deter‐ mined to be possessed by the caller. The complete set of rights for a key is the union of whichever of the first three sets is applicable plus the fourth set if the key is possessed. The set of rights that may be granted in each of the four masks is as follows: view The attributes of the key may be read. This includes the type, description, and access rights (excluding the security label). read For a key: the payload of the key may be read. For a keyring: the list of serial numbers (keys) to which the keyring has links may be read. write The payload of the key may be updated. For a keyring, links may be added to or removed from the keyring, the keyring may be cleared completely (all links are removed), and the key may be revoked. search For a key (or a keyring): the key may be found by a search. For a keyring: keys and keyrings that are linked to by the keyring may be searched. link Links may be created from keyrings to the key. The ini‐ tial link to a key that is established when the key is created doesn't require this permission. setattr The ownership details and security label of the key may be changed, the key's expiration time may be set, and the key may be revoked. If any right is granted to a thread for a key, ┌─────────────────────────────────────────────────────┐ │FIXME │ ├─────────────────────────────────────────────────────┤ │This seems to contradict the text below, which says │ │that 'view' permission is what is significant. │ │Which is correct? │ └─────────────────────────────────────────────────────┘ then that thread will see the key listed in /proc/keys. If no rights at all are granted, then that thread can't even tell that the key exists. In addition to access rights, any active Linux Security Module (LSM) may prevent access to a key if its policy so dictates. A key may be given a security label or other attribute by the LSM which can be retrieved. See keyctl_chown(3), keyctl_describe(3), keyctl_get_secu‐ rity(3), keyctl_setperm(3), and selinux(8) for more informa‐ tion. Searching for keys One of the key features of the Linux key-management facility is the ability to find a key that a process is retaining. The request_key(2) system call is the primary point of access for user-space applications to find a key. (internally, the kernel has something similar available for use by internal components that make use of keys.) The search algorithm works as follows: (1) The three process keyrings are searched in the following order: the thread thread-keyring(7) if it exists, the process-keyring(7) if it exists, and then either the ses‐ sion-keyring(7) if it exists or the user-session-keyring(7) if that exists. (2) If the caller was a process that was invoked by the request_key(2) upcall mechanism then the keyrings of the original caller of that request_key(2) will be searched as well. (3) The search of the keyring tree is in preorder: each keyring is searched first for a match, then the keyrings referred to by that keyring are searched. (4) If a matching key is found that is valid, then the search terminates and that key is returned. (5) If a matching key is found that has an error state attached, that error state is noted and the search contin‐ ues. (6) If valid matching key is found, then the first noted error state is returned; otherwise, an ENOKEY error is returned. It is also possible to search a specific keyring, in which case only steps (3) to (6) apply. See request_key(2) and keyctl_search(3) for more information. On-demand key creation If a key cannot be found, request_key(2) will, if given a call‐ out_info argument, create a new key and then upcall to user space to instantiate the key. This allows keys to be created on an as-needed basis. Typically, this will involve the kernel forking and exec'ing the request-key(8) program, which will then execute the appro‐ priate handler based on its configuration. The handler is passed a special authorization key that allows it and only it to instantiate the new key. This is also used to permit searches performed by the handler program to also search the requester's keyrings. See request_key(2), keyctl_assume_authority(3), keyctl_instan‐ tiate(3), keyctl_negate(3), keyctl_reject(3), request-key(8) and request-key.conf(5) for more information. /proc files The kernel provides various /proc files that expose information about keys or define limits on key usage. /proc/keys (since Linux 2.6.10) This file exposes a list of the keys that are viewable by the reading process, providing various information about each key. The only keys included in the list are those that grant view permission to the reading process, regardless of whether or not it possesses them. LSM security checks are still performed, and may filter out further keys that the process is not authorized to view. An example of the data that one might see in this file is the following: 009a2028 I--Q--- 1 perm 3f010000 1000 1000 user krb_ccache:primary: 12 1806c4ba I--Q--- 1 perm 3f010000 1000 1000 keyring _pid: 2 25d3a08f I--Q--- 1 perm 1f3f0000 1000 65534 keyring _uid_ses.1000: 1 28576bd8 I--Q--- 3 perm 3f010000 1000 1000 keyring _krb: 1 2c546d21 I--Q--- 190 perm 3f030000 1000 1000 keyring _ses: 2 30a4e0be I------ 4 2d 1f030000 1000 65534 keyring _persistent.1000: 1 32100fab I--Q--- 4 perm 1f3f0000 1000 65534 keyring _uid.1000: 2 32a387ea I--Q--- 1 perm 3f010000 1000 1000 keyring _pid: 2 3ce56aea I--Q--- 5 perm 3f030000 1000 1000 keyring _ses: 1 The fields shown in each line of this file are as fol‐ lows: ID The ID (serial number) of the key, expressed in hexadecimal. Flags A set of flags describing the state of the key: I The key has been instantiated. R The key has been revoked. D The key is dead (i.e., has been deleted). (A key may be briefly in this state during garbage collection.) Q The key contributes to the user's quota. U The key is under construction via a callback to user space; see request-key(2). N The key is negatively instantiated. i The key has been invalidated. Usage This is a count of the number of kernel creden‐ tial structures that are pinning the key (approx‐ imately: the number of threads and open file ref‐ erences that refer to this key). Timeout The amount of time until the key will expire, expressed in human-readable form (weeks, days, hours, minutes, and seconds). The string perm here means that the key is permanent (no time‐ out). The string expd means that the key has already expired, but has not yet been garbage collected. Permissions The key permissions, expressed as four hexadeci‐ mal bytes containing, from left to right, the possessor, user, group, and other permissions. UID The user ID of the key owner. GID The group ID of the key. The value -1 here means that the key as no group ID; this can occur in certain circumstances for keys created by the kernel. Type The key type (user, keyring, etc.) Description The key description (name). Description This field contains descriptive information about the key. For most key tpes, it has the form desc[: extra-info] The name subfield is the the key's description (name). The optional extra-info field provides some further information about the key. The information that appears here depends on the key type, as follows: "user" and "logon" The size in bytes of the key payload (expressed in decimal). "keyring" The number of keys linked to the keyring, or the string empty if there are no keys linked to the keyring. "big_key" The payload size in bytes, followed either by the string [file], if the key payload exceeds the threshold that means that the payload is stored in a (swappable) tmpfs filesystem, or otherwise the string [buff], indicating that the key is small enough to reside in kernel memory. For the ".request_key_auth" key type (authoriza‐ tion key; see request_key(2)), the description field has the form shown in the following exam‐ ple: key:c9a9b19 pid:28880 ci:10 The three subfields are as follows: key The hexadecimal ID of the key being instan‐ tiated in the requesting program. pid The PID of the requesting program. ci The length of the callout data with which the requested key should be instantiated (i.e., the length of the payload associated with the authorization key). /proc/key-users (since Linux 2.6.10) This file lists various information for each user ID that has at least one key on the system. An example of the data that one might see in this file is the follow‐ ing: 0: 10 9/9 2/1000000 22/25000000 42: 9 9/9 8/200 106/20000 1000: 11 11/11 10/200 271/20000 The fields shown in each line are as follows: uid The user ID. usage This is a kernel-internal usage count for the kernel structure used to record key users. nkeys/nikeys The total number of keys owned by the user, and the number of those keys that have been instanti‐ ated. qnkeys/maxkeys The number of keys owned by the user, and the maximum keys that the user may own. qnbytes/maxbytes The number of bytes consumed in payloads of the keys owned by this user, and the upper limit on the number of bytes in key payloads for that user. /proc/sys/kernel/keys/gc_delay (since Linux 2.6.32) The value in this file specifies the interval, in sec‐ onds, after which revoked and expired keys will be garbage collected. The purpose of having such an inter‐ val is so that there is a window of time where user space can see an error (respectively EKEYREVOKED and EKEYEXPIRED) that indicates what happened to the key. The default value in this file is 300 (i.e., 5 minutes). /proc/sys/kernel/keys/persistent_keyring_expiry (since Linux 3.13) This file defines an interval, in seconds, to which the persistent keyring's expiration timer is reset each time the keyring is accessed (via keyctl_get_persistent(3) or the keyctl(2) KEYCTL_GET_PERSISTENT operation.) The default value in this file is 259200 (i.e., 3 days). The following files (which are writable by privileged pro‐ cesses) are used to enforce quotas on the number of keys and number of bytes of data that can be stored in key payloads: /proc/sys/kernel/keys/maxbytes (since Linux 2.6.26) This is the maximum number of bytes of data that a non‐ root user can hold in the payloads of the keys owned by the user. The default value in this file is 20,000. /proc/sys/kernel/keys/maxkeys (since Linux 2.6.26) This is the maximum number of keys that a nonroot user may own. The default value in this file is 200. /proc/sys/kernel/keys/root_maxbytes (since Linux 2.6.26) This is the maximum number of bytes of data that the root user (UID 0 in the root user namespace) can hold in the payloads of the keys owned by root. The default value in this file is 25,000,000. /proc/sys/kernel/keys/root_maxkeys (since Linux 2.6.26) This is the maximum number of keys that the root user (UID 0 in the root user namespace) may own. The default value in this file is 1,000,000. With respect to keyrings, note that each link in a keyring con‐ sumes 4 bytes of the keyring payload. Users The Linux key-management facility has a number of users and usages, but is not limited to those that already exist. In-kernel users of this facility include: Network filesystems - DNS The kernel uses the upcall mechanism provided by the keys to upcall to user space to do DNS lookups and then to cache the results. AF_RXRPC and kAFS - Authentication The AF_RXRPC network protocol and the in-kernel AFS filesystem use keys to store the ticket needed to do secured or encrypted traffic. These are then looked up by network operations on AF_RXRPC and filesystem opera‐ tions on kAFS. NFS - User ID mapping The NFS filesystem uses keys to store mappings of for‐ eign user IDs to local user IDs. CIFS - Password The CIFS filesystem uses keys to store passwords for accessing remote shares. Module verification The kernel build process can be made to cryptographi‐ cally sign modules. That signature is then checked when a module is loaded. User-space users of this facility include: Kerberos key storage The MIT Kerberos 5 facility (libkrb5) can use keys to store authentication tokens which can be made to be automatically cleaned up a set time after the user last uses them, but until then permits them to hang around after the user has logged out so that cron scripts can use them. SEE ALSO keyutils(7), persistent-keyring(7), process-keyring(7), session-keyring(7), thread-keyring(7), user-keyring(7), user-session-keyring(7), pam_keyinit(8) Linux 2016-11-01 KEYRINGS(7) -- Michael Kerrisk, man7.org Training and Consulting mtk@man7.org, http://man7.org/training/ "The Linux Programming Interface" -- http://man7.org/tlpi/ --------------938A9A6E7B0A9D0B80F45E20 Content-Type: application/x-troff-man; name="keyrings.7" Content-Transfer-Encoding: 7bit Content-Disposition: attachment; filename="keyrings.7" .\" .\" Copyright (C) 2014 Red Hat, Inc. All Rights Reserved. .\" Written by David Howells (dhowells@redhat.com) .\" and Copyright (C) 2016 Michael Kerrisk .\" .\" %%%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 Licence .\" as published by the Free Software Foundation; either version .\" 2 of the Licence, or (at your option) any later version. .\" %%%LICENSE_END .\" .TH KEYRINGS 7 2016-11-01 Linux "Linux Programmer's Manual" .SH NAME keyrings \- in-kernel key management and retention facility .SH DESCRIPTION The Linux key-management facility is primarily a way for drivers to retain or cache security data, authentication keys, encryption keys, and other data in the kernel. .P System call interfaces are provided so that user-space programs can manage those objects and also use the facility for their own purposes. .P A library and some user-space utilities are provided to allow access to the facility. See .BR keyctl (1), .BR keyctl (3), and .BR keyutils (7) for more information. .\""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""" .SS Keys A key has the following attributes: .TP Serial number (ID) This is a unique integer handle by which a key is referred to in system call arguments. The serial number is sometimes synonymously referred as the key ID. Programmatically, key serial numbers are represented using the type .IR key_serial_t . .TP Type A key's type defines what sort of data can be held in the key, how the proposed content of the key will be parsed, and how the payload will be used. There are a number of general purpose types available, plus some specialist types defined by specific drivers. .TP Description (name) The key description is a printable string that is used as the search term for the key (in conjunction with the key type) as well as a display name. During searches, the description may be partially matched or exactly matched. .TP Payload (data) The payload is the actual content of a key. This is usually set when a key is created, but it is possible for the kernel to upcall to user space to finish the instantiation of a key if that key wasn't already known to the kernel when it was requested. (Details can be found in .BR request_key (2).) A key's payload can be read and updated if the key type supports it and if suitable permission is granted to the caller. .TP Access rights Much as files do, each key has an owning user ID, an owning group ID, and a security label. They also have a set of permissions, though there are more than for a normal UNIX file, and there is an additional category beyond the usual user, group, and other (see below). Note that keys are quota controlled since they represent unswappable kernel memory and the owning user ID specifies whose quota is to be debited. .TP Expiration time Each key can have an expiration time set. When that time is reached, the key is marked as being expired and accesses to it fail with .BR EKEYEXPIRED . If not deleted, updated, or replaced, after a set amount of time, expired keys are automatically removed along with all links to them, and attempts to access the key will fail with the error .BR ENOKEY . .TP Reference count Each key has a reference count. Keys are referenced by keyrings, by currently active users, and by a process's credentials. When the reference count reaches zero, the key is scheduled for garbage collection. .\""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""" .SS Key types The facility provides several basic types of key: .TP .I """user""" This is a general purpose key type. The key is kept entirely within kernel memory. The payload may be read and updated by user-space applications. The payload for keys of this type is a blob of arbitrary data of up to 32,767 bytes. The description may be any valid string, though it is preferred that it start with a colon-delimited prefix representing the service to which the key is of interest (for instance .IR """afs:mykey""" ). .TP .I """keyring""" Keyrings are special keys whose payload consists of a set of links to other keys (including other keyrings), analogous to a directory holding links to files. The main purpose of a keyring is to prevent other keys from being garbage collected because nothing refers to them. .TP .IR """logon""" " (since Linux 3.3)" .\" commit 9f6ed2ca257fa8650b876377833e6f14e272848b This key type is essentially the same as .IR """user""" , but it does not provide reading (i.e., the .BR keyctl (2) .BR KEYCTL_READ operation), meaning that the key payload is never visible from user space. This is suitable for storing username-password pairs that you do not want to be readable from user space. .TP .IR """big_key""" " (since Linux 3.13)" .\" commit ab3c3587f8cda9083209a61dbe3a4407d3cada10 This key type is similar to the .I """user""" key type, but it may hold a payload of up to 1MiB in size. The data may be stored in the swap space rather than in kernel memory if the data size exceeds the overhead of storing the data in swap space (a tmpfs file is used, which requires filesystem structures to be allocated in the kernel). This key type is useful for tasks such as holding Kerberos ticket caches. .PP There are more specialized key types available also, but they're not discussed here as they're not intended for normal user-space use. .\""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""" .SS Keyrings As previously mentioned, keyrings are a special type of key that contain links to other keys (which may include other keyrings). Keys may be linked to by multiple keyrings. Keyrings may be considered as analogous to UNIX directories where each directory contains a set of hard links to files. .P Various operations (system calls) may be applied only to keyrings: .IP Adding A key may be added to a keyring by system calls that create keys. This prevents the new key from being immediately deleted when the system call driver releases its last reference to the key. .IP Linking A link may be added to a keyring pointing to a key that is already known, provided this does not create a self-referential cycle. .IP Unlinking A link may be removed from a keyring. When the last link to a key is removed, that key will be scheduled for deletion by the garbage collector. .IP Clearing All the links may be removed from a keyring. .IP Searching A keyring may be considered the root of a tree or subtree in which keyrings form the branches and non-keyrings the leaves. This tree may be searched for a leaf matching a particular type and description. .P See .BR keyctl_clear (3), .BR keyctl_link (3), .BR keyctl_search (3), and .BR keyctl_unlink (3) for more information. .\""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""" .SS Anchoring keys To prevent a key from being prematurely garbage collected, it must anchored to keep its reference count elevated when it is not in active use by the kernel. .P Keyrings are used to anchor other keys - each link is a reference on a key - but whilst keyrings are available to link to keys, keyrings themselves are just keys and are also subject to the same anchoring necessity. .P The kernel makes available a number of anchor keyrings. Note that some of these keyrings will be created only when first accessed. .TP Process keyrings Process credentials themselves reference keyrings with specific semantics. These keyrings are pinned as long as the set of credentials exists, which is usually as long as the process exists. .IP There are three keyrings with different inheritance/sharing rules: The .BR session-keyring (7) (inherited and shared by all child processes), the .BR process-keyring (7) (shared by all threads in a process) and the .BR thread-keyring (7) (specific to a particular thread). .TP User keyrings Each UID known to the kernel has a record that contains two keyrings: the .BR user-keyring (7) and the .BR user-session-keyring (7). These exist for as long as the UID record in the kernel exists. A link to the user keyring is placed in a new session keyring by .BR pam_keyinit (8) when a new login session is initiated. .TP Persistent keyrings There is a .BR persistent-keyring (7) available to each UID known to the system. It may persist beyond the life of the UID record previously mentioned, but has an expiration time set such that it is automatically cleaned up after a set time. This, for example, permits cron scripts to use credentials left when the user logs out. .IP Note that the expiration time is reset every time the persistent key is requested. .TP Special keyrings There are special keyrings owned by the kernel that can anchor keys for special purposes. An example of this is the \fIsystem keyring\fR used for holding encryption keys for module signature verification. .IP These special keyrings are usually closed to direct alteration by user space. .P See .BR thread-keyring (7), .BR process-keyring (7), .BR session-keyring (7), .BR user-keyring (7), .BR user-session-keyring (7), and .BR persistent-keyring (7) for more information. .\""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""" .SS Possession The concept of possession is important to understanding the keyrings security model. Whether a thread possesses a key is determined by the following rules: .IP (1) 4 Any key or keyring that does not grant .I search permission to the caller is ignored in all the following rules. .IP (2) A thread \fIpossesses\fR its \fBsession\fR, \fBprocess\fR, and \fBthread\fR keyrings directly because those are pointed to by its credentials. .IP (3) If a keyring is possessed, then any key it links to is \fIalso\fR possessed. .IP (4) If any key a keyring links to is itself a keyring, then rule (3) applies \fIrecursively\fP. .IP (5) If a process is upcalled from the kernel to instantiate a key, then it also possesses the \fIrequester's\fP keyrings as in rule (1) as if it were the requester. .P Note that possession is not a fundamental property of a key, but must rather be calculated each time the key is needed. .P Possession is designed to allow set-user-ID programs run from, say a user's shell to access the user's keys. It also allows the prevention of access to keys just on the basis of UID and GID matches. .P When it creates the session keyring, .BR pam_keyinit (8) adds a link to the .BR user-keyring (7), thus making the user keyring and anything it contains possessed by default. .\""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""" .SS Access rights Each key has the following security-related attributes: .IP * 3 The owning user ID .IP * The ID of a group that is permitted to access the key .IP * A security label .IP * A permissions mask .P The permissions mask contains four sets of rights. The first three sets are mutually exclusive. One and only one will be in force for a particular access check. In order of descending priority, these three sets are: .IP \fIuser\fR The set specifies the rights granted if the key's user ID matches the caller's filesystem user ID. .IP \fIgroup\fR The set specifies the rights granted if the user ID didn't match and the key's group ID matches the caller's filesystem GID or one of the caller's supplementary group IDs. .IP \fIother\fR The set specifies the rights granted if neither the key's user ID nor group ID matched. .P The fourth set of rights is: .IP \fIpossessor\fR The set specifies the rights granted if a key is determined to be possessed by the caller. .P The complete set of rights for a key is the union of whichever of the first three sets is applicable plus the fourth set if the key is possessed. .P The set of rights that may be granted in each of the four masks is as follows: .TP .I view The attributes of the key may be read. This includes the type, description, and access rights (excluding the security label). .TP .I read For a key: the payload of the key may be read. For a keyring: the list of serial numbers (keys) to which the keyring has links may be read. .TP .I write The payload of the key may be updated. For a keyring, links may be added to or removed from the keyring, the keyring may be cleared completely (all links are removed), and the key may be revoked. .TP .I search For a key (or a keyring): the key may be found by a search. For a keyring: keys and keyrings that are linked to by the keyring may be searched. .TP .I link Links may be created from keyrings to the key. The initial link to a key that is established when the key is created doesn't require this permission. .TP .I setattr The ownership details and security label of the key may be changed, the key's expiration time may be set, and the key may be revoked. .P If any right is granted to a thread for a key, .\" FIXME This seems to contradict the text below, which says that 'view' .\" permission is what is significant. then that thread will see the key listed in .IR /proc/keys . If no rights at all are granted, then that thread can't even tell that the key exists. .P In addition to access rights, any active Linux Security Module (LSM) may prevent access to a key if its policy so dictates. A key may be given a security label or other attribute by the LSM which can be retrieved. .P See .BR keyctl_chown (3), .BR keyctl_describe (3), .BR keyctl_get_security (3), .BR keyctl_setperm (3), and .BR selinux (8) for more information. .\""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""" .SS Searching for keys One of the key features of the Linux key-management facility is the ability to find a key that a process is retaining. The .BR request_key (2) system call is the primary point of access for user-space applications to find a key. (internally, the kernel has something similar available for use by internal components that make use of keys.) .P The search algorithm works as follows: .IP (1) 4 The three process keyrings are searched in the following order: the thread .BR thread-keyring (7) if it exists, the .BR process-keyring (7) if it exists, and then either the .BR session-keyring (7) if it exists or the .BR user-session-keyring (7) if that exists. .IP (2) If the caller was a process that was invoked by the .BR request_key (2) upcall mechanism then the keyrings of the original caller of that .BR request_key (2) will be searched as well. .IP (3) The search of the keyring tree is in preorder: each keyring is searched first for a match, then the keyrings referred to by that keyring are searched. .IP (4) If a matching key is found that is valid, then the search terminates and that key is returned. .IP (5) If a matching key is found that has an error state attached, that error state is noted and the search continues. .IP (6) If valid matching key is found, then the first noted error state is returned; otherwise, an .B ENOKEY error is returned. .P It is also possible to search a specific keyring, in which case only steps (3) to (6) apply. .P See .BR request_key (2) and .BR keyctl_search (3) for more information. .\""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""" .SS On-demand key creation If a key cannot be found, .BR request_key (2) will, if given a .I callout_info argument, create a new key and then upcall to user space to instantiate the key. This allows keys to be created on an as-needed basis. .P Typically, this will involve the kernel forking and exec'ing the .BR request-key (8) program, which will then execute the appropriate handler based on its configuration. .P The handler is passed a special authorization key that allows it and only it to instantiate the new key. This is also used to permit searches performed by the handler program to also search the requester's keyrings. .P See .BR request_key (2), .BR keyctl_assume_authority (3), .BR keyctl_instantiate (3), .BR keyctl_negate (3), .BR keyctl_reject (3), .BR request-key (8) and .BR request-key.conf (5) for more information. .\""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""" .SS /proc files The kernel provides various .I /proc files that expose information about keys or define limits on key usage. .TP .IR /proc/keys " (since Linux 2.6.10)" This file exposes a list of the keys that are viewable by the reading process, providing various information about each key. The only keys included in the list are those that grant .I view permission to the reading process, regardless of whether or not it possesses them. LSM security checks are still performed, and may filter out further keys that the process is not authorized to view. An example of the data that one might see in this file is the following: .nf .in 0n 009a2028 I--Q--- 1 perm 3f010000 1000 1000 user krb_ccache:primary: 12 1806c4ba I--Q--- 1 perm 3f010000 1000 1000 keyring _pid: 2 25d3a08f I--Q--- 1 perm 1f3f0000 1000 65534 keyring _uid_ses.1000: 1 28576bd8 I--Q--- 3 perm 3f010000 1000 1000 keyring _krb: 1 2c546d21 I--Q--- 190 perm 3f030000 1000 1000 keyring _ses: 2 30a4e0be I------ 4 2d 1f030000 1000 65534 keyring _persistent.1000: 1 32100fab I--Q--- 4 perm 1f3f0000 1000 65534 keyring _uid.1000: 2 32a387ea I--Q--- 1 perm 3f010000 1000 1000 keyring _pid: 2 3ce56aea I--Q--- 5 perm 3f030000 1000 1000 keyring _ses: 1 .in .fi The fields shown in each line of this file are as follows: .RS .TP ID The ID (serial number) of the key, expressed in hexadecimal. .TP Flags A set of flags describing the state of the key: .RS .IP I 4 The key has been instantiated. .IP R The key has been revoked. .IP D The key is dead (i.e., has been deleted). (A key may be briefly in this state during garbage collection.) .IP Q The key contributes to the user's quota. .IP U The key is under construction via a callback to user space; see .BR request-key (2). .IP N The key is negatively instantiated. .IP i The key has been invalidated. .RE .TP Usage This is a count of the number of kernel credential structures that are pinning the key (approximately: the number of threads and open file references that refer to this key). .TP Timeout The amount of time until the key will expire, expressed in human-readable form (weeks, days, hours, minutes, and seconds). The string .I perm here means that the key is permanent (no timeout). The string .I expd means that the key has already expired, but has not yet been garbage collected. .TP Permissions The key permissions, expressed as four hexadecimal bytes containing, from left to right, the possessor, user, group, and other permissions. .TP UID The user ID of the key owner. .TP GID The group ID of the key. The value \-1 here means that the key as no group ID; this can occur in certain circumstances for keys created by the kernel. .TP Type The key type (user, keyring, etc.) .TP Description The key description (name). .TP Description This field contains descriptive information about the key. For most key tpes, it has the form desc[: extra\-info] The .I name subfield is the the key's description (name). The optional .I extra\-info field provides some further information about the key. The information that appears here depends on the key type, as follows: .RS .TP 4 .IR """user""" " and " """logon""" The size in bytes of the key payload (expressed in decimal). .TP .IR """keyring""" The number of keys linked to the keyring, or the string .IR empty if there are no keys linked to the keyring. .TP .IR """big_key""" The payload size in bytes, followed either by the string .IR [file] , if the key payload exceeds the threshold that means that the payload is stored in a (swappable) tmpfs filesystem, or otherwise the string .IR [buff] , indicating that the key is small enough to reside in kernel memory. .RE .IP For the .IR """.request_key_auth""" key type (authorization key; see .BR request_key (2)), the description field has the form shown in the following example: key:c9a9b19 pid:28880 ci:10 The three subfields are as follows: .RS .TP 5 .I key The hexadecimal ID of the key being instantiated in the requesting program. .TP .I pid The PID of the requesting program. .TP .I ci The length of the callout data with which the requested key should be instantiated (i.e., the length of the payload associated with the authorization key). .RE .RE .TP .IR /proc/key-users " (since Linux 2.6.10)" This file lists various information for each user ID that has at least one key on the system. An example of the data that one might see in this file is the following: .nf .in +4n 0: 10 9/9 2/1000000 22/25000000 42: 9 9/9 8/200 106/20000 1000: 11 11/11 10/200 271/20000 .in .fi The fields shown in each line are as follows: .RS .TP .I uid The user ID. .TP .I usage This is a kernel-internal usage count for the kernel structure used to record key users. .TP .IR nkeys / nikeys The total number of keys owned by the user, and the number of those keys that have been instantiated. .TP .IR qnkeys / maxkeys The number of keys owned by the user, and the maximum keys that the user may own. .TP .IR qnbytes / maxbytes The number of bytes consumed in payloads of the keys owned by this user, and the upper limit on the number of bytes in key payloads for that user. .RE .TP .IR /proc/sys/kernel/keys/gc_delay " (since Linux 2.6.32)" .\" commit 5d135440faf7db8d566de0c6fab36b16cf9cfc3b The value in this file specifies the interval, in seconds, after which revoked and expired keys will be garbage collected. The purpose of having such an interval is so that there is a window of time where user space can see an error (respectively .BR EKEYREVOKED and .BR EKEYEXPIRED ) that indicates what happened to the key. The default value in this file is 300 (i.e., 5 minutes). .TP .IR /proc/sys/kernel/keys/persistent_keyring_expiry " (since Linux 3.13)" .\" commit f36f8c75ae2e7d4da34f4c908cebdb4aa42c977e This file defines an interval, in seconds, to which the persistent keyring's expiration timer is reset each time the keyring is accessed (via .BR keyctl_get_persistent (3) or the .BR keyctl (2) .B KEYCTL_GET_PERSISTENT operation.) The default value in this file is 259200 (i.e., 3 days). .PP The following files (which are writable by privileged processes) are used to enforce quotas on the number of keys and number of bytes of data that can be stored in key payloads: .TP .IR /proc/sys/kernel/keys/maxbytes " (since Linux 2.6.26)" .\" commit 0b77f5bfb45c13e1e5142374f9d6ca75292252a4 .\" Previously: KEYQUOTA_MAX_BYTES 10000 This is the maximum number of bytes of data that a nonroot user can hold in the payloads of the keys owned by the user. The default value in this file is 20,000. .TP .IR /proc/sys/kernel/keys/maxkeys " (since Linux 2.6.26)" .\" commit 0b77f5bfb45c13e1e5142374f9d6ca75292252a4 .\" Previously: KEYQUOTA_MAX_KEYS 100 This is the maximum number of keys that a nonroot user may own. The default value in this file is 200. .TP .IR /proc/sys/kernel/keys/root_maxbytes " (since Linux 2.6.26)" This is the maximum number of bytes of data that the root user (UID 0 in the root user namespace) can hold in the payloads of the keys owned by root. The default value in this file is 25,000,000. .\" commit 0b77f5bfb45c13e1e5142374f9d6ca75292252a4 .TP .IR /proc/sys/kernel/keys/root_maxkeys " (since Linux 2.6.26)" .\" commit 0b77f5bfb45c13e1e5142374f9d6ca75292252a4 This is the maximum number of keys that the root user (UID 0 in the root user namespace) may own. The default value in this file is 1,000,000. .PP With respect to keyrings, note that each link in a keyring consumes 4 bytes of the keyring payload. .\""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""" .SS Users The Linux key-management facility has a number of users and usages, but is not limited to those that already exist. .P In-kernel users of this facility include: .TP Network filesystems - DNS The kernel uses the upcall mechanism provided by the keys to upcall to user space to do DNS lookups and then to cache the results. .TP AF_RXRPC and kAFS - Authentication The AF_RXRPC network protocol and the in-kernel AFS filesystem use keys to store the ticket needed to do secured or encrypted traffic. These are then looked up by network operations on AF_RXRPC and filesystem operations on kAFS. .TP NFS - User ID mapping The NFS filesystem uses keys to store mappings of foreign user IDs to local user IDs. .TP CIFS - Password The CIFS filesystem uses keys to store passwords for accessing remote shares. .TP Module verification The kernel build process can be made to cryptographically sign modules. That signature is then checked when a module is loaded. .P User-space users of this facility include: .TP Kerberos key storage The MIT Kerberos 5 facility (libkrb5) can use keys to store authentication tokens which can be made to be automatically cleaned up a set time after the user last uses them, but until then permits them to hang around after the user has logged out so that cron scripts can use them. .\""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""" .SH SEE ALSO .ad l .nh .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 pam_keyinit (8) --------------938A9A6E7B0A9D0B80F45E20--