Internet DRAFT - draft-ietf-nfsv4-rpcsec-gssv3
draft-ietf-nfsv4-rpcsec-gssv3
NFSv4 W. Adamson
Internet-Draft NetApp
Intended status: Standards Track N. Williams
Expires: July 31, 2016 Cryptonector
January 28, 2016
Remote Procedure Call (RPC) Security Version 3
draft-ietf-nfsv4-rpcsec-gssv3-17
Abstract
This document specifies version 3 of the Remote Procedure Call (RPC)
security protocol (RPCSEC_GSS). This protocol provides support for
multi-principal authentication of client hosts and user principals to
a server (constructed by generic composition), security label
assertions for multi-level and type enforcement, structured privilege
assertions, and channel bindings.
Requirements Language
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in RFC 2119 [RFC2119].
Status of This Memo
This Internet-Draft is submitted in full conformance with the
provisions of BCP 78 and BCP 79.
Internet-Drafts are working documents of the Internet Engineering
Task Force (IETF). Note that other groups may also distribute
working documents as Internet-Drafts. The list of current Internet-
Drafts is at http://datatracker.ietf.org/drafts/current/.
Internet-Drafts are draft documents valid for a maximum of six months
and may be updated, replaced, or obsoleted by other documents at any
time. It is inappropriate to use Internet-Drafts as reference
material or to cite them other than as "work in progress."
This Internet-Draft will expire on July 31, 2016.
Copyright Notice
Copyright (c) 2016 IETF Trust and the persons identified as the
document authors. All rights reserved.
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This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents
(http://trustee.ietf.org/license-info) in effect on the date of
publication of this document. Please review these documents
carefully, as they describe your rights and restrictions with respect
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include Simplified BSD License text as described in Section 4.e of
the Trust Legal Provisions and are provided without warranty as
described in the Simplified BSD License.
Table of Contents
1. Introduction and Motivation . . . . . . . . . . . . . . . . . 2
1.1. Added Functionality . . . . . . . . . . . . . . . . . . . 4
1.2. XDR Code Extraction . . . . . . . . . . . . . . . . . . . 5
2. The RPCSEC_GSSv3 Protocol . . . . . . . . . . . . . . . . . . 5
2.1. Compatibility with RPCSEC_GSSv2 . . . . . . . . . . . . . 6
2.2. Version Negotiation . . . . . . . . . . . . . . . . . . . 6
2.3. New REPLY Verifier . . . . . . . . . . . . . . . . . . . 6
2.4. XDR Code Preliminaries . . . . . . . . . . . . . . . . . 7
2.5. RPCSEC_GSS_BIND_CHANNEL Operation . . . . . . . . . . . . 9
2.6. New auth_stat Values . . . . . . . . . . . . . . . . . . 9
2.7. New Control Procedures . . . . . . . . . . . . . . . . . 10
2.7.1. New Control Procedure - RPCSEC_GSS_CREATE . . . . . . 10
2.7.2. New Control Procedure - RPCSEC_GSS_LIST . . . . . . . 18
2.8. Extensibility . . . . . . . . . . . . . . . . . . . . . . 19
3. Operational Recommendation for Deployment . . . . . . . . . . 20
4. Security Considerations . . . . . . . . . . . . . . . . . . . 20
5. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 21
6. References . . . . . . . . . . . . . . . . . . . . . . . . . 21
6.1. Normative References . . . . . . . . . . . . . . . . . . 21
6.2. Informative References . . . . . . . . . . . . . . . . . 22
Appendix A. Acknowledgments . . . . . . . . . . . . . . . . . . 22
Appendix B. RFC Editor Notes . . . . . . . . . . . . . . . . . . 22
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 22
1. Introduction and Motivation
The original RPCSEC_GSS protocol [RFC2203] provided for
authentication of RPC clients and servers to each other using the
Generic Security Services Application Programming Interface (GSS-API)
[RFC2743]. The second version of RPCSEC_GSS [RFC5403] added support
for channel bindings [RFC5056].
Existing GSS-API mechanisms are insufficient for communicating
certain authorization and authentication information to a server.
The GSS-API and its mechanisms certainly could be extended to address
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this shortcoming. However, here it is addressed at the application
layer, i.e., in RPCSEC_GSS.
A major motivation for version 3 RPCSEC_GSS (RPCSEC_GSSv3) is to add
support for multi-level (labeled) security and server-side copy for
NFSv4.
Multi-Level Security (MLS) is a traditional model where subjects
(processes) are given a security level (Unclassified, Secret, Top
Secret, etc.) and objects (files) are given security labels that
mandate the access of the subject to the object (see [NFSv4.2]
Section 9.2).
Labeled NFS (see Section 9 of [NFSv4.2]) uses an MLS policy with
Mandatory Access Control (MAC) systems as defined in [RFC4949].
Labeled NFS stores MAC file object labels on the NFS server and
enables client Guest Mode MAC as described in Section 9.6.2 of
[NFSv4.2]. RPCSEC_GSSv3 label assertions assert client MAC process
subject labels to enable Full Mode MAC when combined with Labeled NFS
as described in Section 9.6.1 of [NFSv4.2].
A traditional inter-server file copy entails the user gaining access
to a file on the source, reading it, and writing it to a file on the
destination. In secure NFSv4 inter-server server-side copy (see
Section 4 of [NFSv4.2]), the user first secures access to both source
and destination files, and then uses NFSv4.2 defined RPCSEC_GSSv3
structured privileges to authorize the destination to copy the file
from the source on behalf of the user.
Multi-principal assertions can be used to address shared cache
poisoning attacks (see Section 9 of [AFS-RXGK]) on the client cache
by a user. As described in Section 7 of [AFS-RXGK], multi-user
machines with a single cache manager can fetch and cache data on a
users' behalf, and re-display it for another user from the cache
without re-fetching the data from the server. The initial data
acquisition is authenticated by the first user's credentials, and if
only that user's credentials are used, it may be possible for a
malicious user or users to "poison" the cache for other users by
introducing bogus data into the cache.
Another use of the multi-principal assertion is the secure conveyance
of privilege information for processes running with more (or even
with less) privilege than the user normally would be accorded.
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1.1. Added Functionality
RPCSEC_GSS version 3 (RPCSEC_GSSv3) is therefore described.
RPCSEC_GSSv3 is the same as RPCSEC_GSSv2 [RFC5403], except that the
following assertions of authority have been added.
o Security labels for Full Mode security type enforcement, and other
labeled security models (See Section 9.6.2 in [NFSv4.2]).
o Application-specific structured privileges. These allow an RPC
application client to pass structured information to the
corresponding application code in a server to control the
applicability of the privilege and/or the conditions in which the
privilege may be exercised. For an example see server-side copy
[NFSv4.2].
o Multi-principal authentication of the client host and user to the
server done by binding two RPCSEC_GSS handles.
o Simplified channel binding.
Assertions of labels and privileges are evaluated by the server,
which may then map the asserted values to other values, all according
to server-side policy. See [NFSv4.2].
An option for enumerating server supported label format specifiers
(LFS) is provided. See Section 9.2 in [NFSv4.2].
Note that there is no RPCSEC_GSS_CREATE payload that is REQUIRED to
implement. RPCSEC_GSSv3 implementations are feature driven. Besides
implementing the RPCSEC_GSS_CREATE operation and payloads for the
desired features, all RPCSEC_GSSv3 implementation MUST implement:
o The new GSS version number (Section 2.2).
o The new reply verifier (Section 2.3).
o The new auth stat values (Section 2.6).
RPCSEC_GSSv3 targets implementing a desired feature must also
implement the RPCSEC_GSS_LIST operation, and the RPCSEC_GSS_CREATE
operation replies for unsupported features.
o For label assertions the target indicates no support by returning
the new RPCSEC_GSS_LABEL_PROBLEM auth stat (See Section 2.7.1.3).
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o For structured privilege assertions the target indicates no
support by returning the new RPCSEC_GSS_UNKNOWN_MESSAGE auth stat
(See Section 2.7.1.4).
o For multi-principal authentication (Section 2.7.1.1), the target
indicates no support by not including a rgss3_gss_mp_auth value in
the rgss3_create_res.
o For channel bindings (Section 2.7.1.2) the target indicates no
support by not including a rgss3_chan_binding value in the
rgss3_create_res.
1.2. XDR Code Extraction
This document contains the External Data Representation (XDR)
([RFC4506]) definitions for the RPCSEC_GSSv3 protocol. The XDR
description is provided in this document in a way that makes it
simple for the reader to extract into ready to compile form. The
reader can feed this document in the following shell script to
produce the machine readable XDR description of RPCSEC_GSSv3:
<CODE BEGINS>
#!/bin/sh
grep "^ *///" | sed 's?^ */// ??' | sed 's?^ *///$??'
<CODE ENDS>
I.e. if the above script is stored in a file called "extract.sh", and
this document is in a file called "spec.txt", then the reader can do:
<CODE BEGINS>
sh extract.sh < spec.txt > rpcsec_gss_v3.x
<CODE ENDS>
The effect of the script is to remove leading white space from each
line, plus a sentinel sequence of "///".
2. The RPCSEC_GSSv3 Protocol
RPCSEC_GSS version 3 (RPCSEC_GSSv3) is very similar to RPCSEC_GSS
version 2 (RPCSEC_GSSv2) [RFC5403]. The differences are the addition
of support for assertions and channel bindings are supported via a
different mechanism.
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The entire RPCSEC_GSSv3 protocol is not presented here. Only the
differences between it and RPCSEC_GSSv2 are shown.
The use of RPCSEC_GSSv3 is structured as follows:
o A client uses an existing RPCSEC_GSSv3 context handle established
in the usual manner (See Section 5.2 [RFC2203]) to protect
RPCSEC_GSSv3 exchanges, this will be termed the "parent" handle.
o The server issues a "child" RPCSEC_GSSv3 handle in the
RPCSEC_GSS_CREATE response which uses the underlying GSS-API
security context of the parent handle in all subsequent exchanges
that uses the child handle.
o An RPCSEC_GSSv3 child handle MUST NOT be used as the parent handle
in an RPCSEC_GSS3_CREATE control message.
2.1. Compatibility with RPCSEC_GSSv2
The functionality of RPCSEC_GSSv2 [RFC5403] is fully supported by
RPCSEC_GSSv3 with the exception of the RPCSEC_GSS_BIND_CHANNEL
operation which is not supported when RPCSEC_GSSv3 is in use (see
Section 2.5).
2.2. Version Negotiation
An initiator that supports version 3 of RPCSEC_GSS simply issues an
RPCSEC_GSS request with the rgc_version field set to
RPCSEC_GSS_VERS_3. If the target does not recognize
RPCSEC_GSS_VERS_3, the target will return an RPC error per
Section 5.1 of [RFC2203].
The initiator MUST NOT attempt to use an RPCSEC_GSS handle returned
by version 3 of a target with version 1 or version 2 of the same
target. The initiator MUST NOT attempt to use an RPCSEC_GSS handle
returned by version 1 or version 2 of a target with version 3 of the
same target.
2.3. New REPLY Verifier
A new reply verifier is needed for RPCSEC_GSSv3 because of a
situation that arises from the use of the same GSS context by child
and parent handles. Because the RPCSEC_GSSv3 child handle uses the
same GSS context as the parent handle, a child and parent
RPCSEC_GSSv3 handle could have the same RPCSEC_GSS sequence numbers.
Since the reply verifier of previous versions of RPCSEC_GSS computes
a Message Integrity Code (MIC) on just the sequence number, this
provides opportunities for man in the middle attacks.
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This issue is addressed in RPCSEC_GSS version 3 by computing the
verifier using the exact same input as is used to compute the request
verifier, except that the mtype is changed from CALL to REPLY. The
new reply verifier computes a MIC over the following RPC reply header
data:
unsigned int xid;
msg_type mtype; /* set to REPLY */
unsigned int rpcvers;
unsigned int prog;
unsigned int vers;
unsigned int proc;
opaque_auth cred; /* captures the RPCSEC_GSS handle */
2.4. XDR Code Preliminaries
The following code fragment replaces the corresponding preliminary
code shown in Figure 1 of [RFC5403]. The values in the code fragment
in Section 2.6 are additions to the auth_stat enumeration.
Subsequent code fragments are additions to the code for version 2
that support the new procedures defined in version 3.
<CODE BEGINS>
/// /*
/// * Copyright (c) 2013 IETF Trust and the persons
/// * identified as the document authors. All rights
/// * reserved.
/// *
/// * The document authors are identified in [RFC2203],
/// * [RFC5403], and [RFCTBD].
/// *
/// * Redistribution and use in source and binary forms,
/// * with or without modification, are permitted
/// * provided that the following conditions are met:
/// *
/// * o Redistributions of source code must retain the above
/// * copyright notice, this list of conditions and the
/// * following disclaimer.
/// *
/// * o Redistributions in binary form must reproduce the
/// * above copyright notice, this list of
/// * conditions and the following disclaimer in
/// * the documentation and/or other materials
/// * provided with the distribution.
/// *
/// * o Neither the name of Internet Society, IETF or IETF
/// * Trust, nor the names of specific contributors, may be
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/// * used to endorse or promote products derived from this
/// * software without specific prior written permission.
/// *
/// * THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS
/// * AND CONTRIBUTORS "AS IS" AND ANY EXPRESS OR IMPLIED
/// * WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
/// * IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS
/// * FOR A PARTICULAR PURPOSE ARE DISCLAIMED. IN NO
/// * EVENT SHALL THE COPYRIGHT OWNER OR CONTRIBUTORS BE
/// * LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL,
/// * EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT
/// * NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR
/// * SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS
/// * INTERRUPTION) HOWEVER CAUSED AND ON ANY THEORY OF
/// * LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY,
/// * OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING
/// * IN ANY WAY OUT OF THE USE OF THIS SOFTWARE, EVEN IF
/// * ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
/// */
///
/// /*
/// * This code was derived from RFC2203, RFC5403, and RFCTBD.
/// * Please reproduce this note if possible.
/// */
///
/// enum rpc_gss_service_t {
/// /* Note: the enumerated value for 0 is reserved. */
/// rpc_gss_svc_none = 1,
/// rpc_gss_svc_integrity = 2,
/// rpc_gss_svc_privacy = 3,
/// rpc_gss_svc_channel_prot = 4
/// };
///
/// enum rpc_gss_proc_t {
/// RPCSEC_GSS_DATA = 0,
/// RPCSEC_GSS_INIT = 1,
/// RPCSEC_GSS_CONTINUE_INIT = 2,
/// RPCSEC_GSS_DESTROY = 3,
/// RPCSEC_GSS_BIND_CHANNEL = 4, /* not used */
/// RPCSEC_GSS_CREATE = 5, /* new */
/// RPCSEC_GSS_LIST = 6 /* new */
/// };
///
/// struct rpc_gss_cred_vers_1_t {
/// rpc_gss_proc_t gss_proc; /* control procedure */
/// unsigned int seq_num; /* sequence number */
/// rpc_gss_service_t service; /* service used */
/// opaque handle<>; /* context handle */
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/// };
///
/// const RPCSEC_GSS_VERS_1 = 1;
/// const RPCSEC_GSS_VERS_2 = 2;
/// const RPCSEC_GSS_VERS_3 = 3; /* new */
///
/// union rpc_gss_cred_t switch (unsigned int rgc_version) {
/// case RPCSEC_GSS_VERS_1:
/// case RPCSEC_GSS_VERS_2:
/// case RPCSEC_GSS_VERS_3: /* new */
/// rpc_gss_cred_vers_1_t rgc_cred_v1;
/// };
///
<CODE ENDS>
As seen above, the RPCSEC_GSSv3 credential has the same format as the
RPCSEC_GSSv1 [RFC2203] and RPCSEC_GSSv2 [RFC5403] credential.
Setting the rgc_version field to 3 indicates that the initiator and
target support the new RPCSEC_GSSv3 control procedures.
2.5. RPCSEC_GSS_BIND_CHANNEL Operation
RPCSEC_GSSv3 provides a channel binding assertion that replaces the
RPCSEC_GSSv2 RPCSEC_GSS_BIND_CHANNEL operation.
The RPCSEC_GSS_BIND_CHANNEL operation is not supported on RPCSEC_GSS
version 3 handles. If a server receives an RPCSEC_GSS_BIND_CHANNEL
operation on an RPCSEC_GSSv3 handle, it MUST return a reply status of
MSG_ACCEPTED with an accept stat of PROC_UNAVAIL.
2.6. New auth_stat Values
RPCSEC_GSSv3 requires the addition of several values to the auth_stat
enumerated type definition. The use of these new auth_stat values is
explained throughout this document.
enum auth_stat {
...
/*
* RPCSEC_GSSv3 errors
*/
RPCSEC_GSS_INNER_CREDPROBLEM = 15,
RPCSEC_GSS_LABEL_PROBLEM = 16,
RPCSEC_GSS_PRIVILEGE_PROBLEM = 17,
RPCSEC_GSS_UNKNOWN_MESSAGE = 18
};
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2.7. New Control Procedures
There are two new RPCSEC_GSSv3 control procedures: RPCSEC_GSS_CREATE,
RPCSEC_GSS_LIST.
The RPCSEC_GSS_CREATE procedure binds any combination of assertions:
multi-principal authentication, labels, structured privileges, or
channel bindings to a new RPCSEC_GSSv3 context returned in the
rgss3_create_res rcr_handle field.
The RPCSEC_GSS_LIST procedure queries the target for supported
assertions.
RPCSEC_GSS version 3 control messages are similar to the RPCSEC_GSS
version 1 and version 2 RPCSEC_GSS_DESTROY control message (see
section 5.4 [RFC2203]) in that the sequence number in the request
must be valid, and the header checksum in the verifier must be valid.
As in RPCSEC_GSS version 1 and version 2, the RPCSEC_GSS version 3
control messages may contain call data following the verifier in the
body of the NULLPROC procedure. In other words, they look a lot like
an RPCSEC_GSS data message with the header procedure set to NULLPROC.
The client MUST use one of the following security services to protect
the RPCSEC_GSS_CREATE or RPCSEC_GSS_LIST control message:
o rpc_gss_svc_integrity
o rpc_gss_svc_privacy
Specifically the client MUST NOT use rpc_gss_svc_none.
RPCSEC_GSS_LIST can also use rpc_gss_svc_channel_prot (see
RPCSEC_GSSv2 [RFC5403]) if the request is sent using an RPCSEC_GSSv3
child handle with channel bindings enabled as described in
Section 2.7.1.2.
2.7.1. New Control Procedure - RPCSEC_GSS_CREATE
<CODE BEGINS>
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/// struct rgss3_create_args {
/// rgss3_gss_mp_auth *rca_mp_auth;
/// rgss3_chan_binding *rca_chan_bind_mic;
/// rgss3_assertion_u rca_assertions<>;
/// };
///
/// struct rgss3_create_res {
/// opaque rcr_handle<>;
/// rgss3_gss_mp_auth *rcr_mp_auth;
/// rgss3_chan_binding *rcr_chan_bind_mic;
/// rgss3_assertion_u rcr_assertions<>;
/// };
///
/// enum rgss3_assertion_type {
/// LABEL = 0,
/// PRIVS = 1
/// };
///
/// union rgss3_assertion_u
/// switch (rgss3_assertion_type atype) {
/// case LABEL:
/// rgss3_label rau_label;
/// case PRIVS:
/// rgss3_privs rau_privs;
/// default:
/// opaque rau_ext<>;
/// };
///
<CODE ENDS>
The call data for an RPCSEC_GSS_CREATE request consists of an
rgss3_create_args which binds one or more items of several kinds to
the returned rcr_handle RPCSEC_GSSv3 context handle called the
"child" handle:
o Multi-principal authentication: another RPCSEC_GSS context handle
o A channel binding
o Authorization assertions: labels and or privileges
The reply to this message consists of either an error or an
rgss3_create_res structure. As noted in Section 2.7.1.3 and
Section 2.7.1.4 successful rgss3_assertions are enumerated in
rcr_assertions, and are REQUIRED to be enumerated in the same order
as they appeared in the rca_assertions argument.
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Upon successful RPCSEC_GSS_CREATE, both the client and the server
need to associate the resultant child rcr_handle context handle with
the parent context handle in their GSS context caches so as to be
able to reference the parent context given the child context handle.
RPCSEC_GSSv3 child handles MUST be destroyed upon the destruction of
the associated parent handle.
Server implementation and policy MAY result in labels, privileges,
and identities being mapped to concepts and values that are local to
the server. Server policies should take into account the identity of
the client and/or user as authenticated via the GSS-API.
2.7.1.1. Multi-principal Authentication
<CODE BEGINS>
///
/// struct rgss3_gss_mp_auth {
/// opaque rgmp_handle<>; /* inner handle */
/// opaque rgmp_rpcheader_mic<>;
/// };
///
<CODE ENDS>
RPCSEC_GSSv3 clients MAY assert a multi-principal authentication of
the RPC client host principal and a user principal. This feature is
needed, for example, when an RPC client host wishes to use authority
assertions that the server may only grant if a user and an RPC client
host are authenticated together to the server. Thus a server may
refuse to grant requested authority to a user acting alone (e.g., via
an unprivileged user-space program), or to an RPC client host acting
alone (e.g., when an RPC client host is acting on behalf of a user)
but may grant requested authority to an RPC client host acting on
behalf of a user if the server identifies the user and trusts the RPC
client host.
It is assumed that an unprivileged user-space program would not have
access to RPC client host credentials needed to establish a GSS-API
security context authenticating the RPC client host to the server,
therefore an unprivileged user-space program could not create an
RPCSEC_GSSv3 RPCSEC_GSS_CREATE message that successfully binds an RPC
client host and a user security context.
In addition to the parent handle (Section 2), the multi-principal
authentication call data has an RPCSEC_GSS version 3 handle
referenced via the rgmp_handle field termed the "inner" handle.
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Clients using RPCSEC_GSSv3 multi-principal authentication MUST use an
RPCSEC_GSSv3 context handle that corresponds to a GSS-API security
context that authenticates the RPC client host for the parent handle.
For the inner context handle with RPCSEC_GSSv3 it MUST use a context
handle to authenticate a user. The reverse (parent handle
authenticates user, inner authenticates an RPC client host) MUST NOT
be used. Other multi-principal parent and inner context handle uses
might eventually make sense, but would need to be introduced in a new
revision of the RPCSEC_GSS protocol.
The child context handle returned by a successful multi-principal
assertion binds the inner RPCSEC_GSSv3 context handle to the parent
RPCSEC_GSS context and MUST be treated by servers as authenticating
the GSS-API initiator principal authenticated by the inner context
handle's GSS-API security context. This principal may be mapped to a
server-side notion of user or principal.
Multi-principal binding is done by including an assertion of type
rgss3_gss_mp_auth in the RPCSEC_GSS_CREATE rgss3_create_args call
data. The inner context handle is placed in the rgmp_handle field.
A MIC of the RPC call header up to and including the credential is
computed using the GSS-API security context associated with the inner
context handle and is placed in rgmp_rpcheader_mic field. Note that
the rgmp_rpcheader_mic only identifies the client host GSS context by
it's context handle (the parent context handle) in the rpc header.
An RPCSEC_GSS_CREATE control procedure with a multi-principal
authentication payload MUST use the rpc_gss_svc_privacy security
service for protection. This is to prevent an attacker from
intercepting the RPCSEC_GSS_CREATE control procedure, re-assigning
the (parent) context handle, and stealing the user's identity.
The target verifies the multi-principal authentication by first
confirming that the parent context used is an RPC client host
context, and then verifies the rgmp_rpcheader_mic using the GSS-API
security context associated with the rgmp_handle field.
On a successful verification, the rgss3_gss_mp_auth field in the
rgss3_create_res reply MUST be filled in with the inner RPCSEC_GSSv3
context handle as the rgmp_handle, and a MIC computed over the RPC
reply header (see section Section 2.3) using the GSS-API security
context associated with the inner handle.
On failure, the rgss3_gss_mp_auth field is not sent
(rgss3_gss_mp_auth is an optional field). A MSG_DENIED reply to the
RPCSEC_GSS_CREATE call is formulated as usual.
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As described in Section 5.3.3.3 of [RFC2203] the server maintains a
list of contexts for the clients that are currently in session with
it. When a client request comes in, there may not be a context
corresponding to its handle. When this occurs on an
RPCSEC_GSS3_CREATE request processing of the parent handle, the
server rejects the request with a reply status of MSG_DENIED with the
reject_stat of AUTH_ERROR and with an auth_stat value of
RPCSEC_GSS_CREDPROBLEM.
A new value, RPCSEC_GSS_INNER_CREDPROBLEM, has been added to the
auth_stat type. With a multi-pricipal authorization request, the
server must also have a context corresponding to the inner context
handle. When the server does not have a context handle corresponding
to the inner context handle of a multi-pricipal authorization
request, the server sends a reply status of MSG_DENIED with the
reject_stat of AUTH_ERROR and with an auth_stat value of
RPCSEC_GSS_INNER_CREDPROBLEM.
When processing the multi-principal authentication request, if the
GSS_VerifyMIC() call on the rgmp_rpcheader_mic fails to return
GSS_S_COMPLETE, the server sends a reply status of MSG_DENIED with
the reject_stat of AUTH_ERROR and with an auth_stat value of
RPCSEC_GSS_INNER_CREDPROBLEM.
2.7.1.2. Channel Binding
<CODE BEGINS>
///
/// typedef opaque rgss3_chan_binding<>;
///
<CODE ENDS>
RPCSEC_GSSv3 provides a different way to do channel binding than
RPCSEC_GSSv2 [RFC5403]. Specifically:
a. RPCSEC_GSSv3 builds on RPCSEC_GSSv1 by reusing existing,
established context handles rather than providing a different RPC
security flavor for establishing context handles,
b. channel bindings data are not hashed because there is now general
agreement that it is the secure channel's responsibility to
produce channel bindings data of manageable size.
(a) is useful in keeping RPCSEC_GSSv3 simple in general, not just for
channel binding. (b) is useful in keeping RPCSEC_GSSv3 simple
specifically for channel binding.
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Channel binding is accomplished as follows. The client prefixes the
channel bindings data octet string with the channel type as described
in [RFC5056], then the client calls GSS_GetMIC() to get a MIC of
resulting octet string, using the parent RPCSEC_GSSv3 context
handle's GSS-API security context. The MIC is then placed in the
rca_chan_bind_mic field of RPCSEC_GSS_CREATE arguments
(rgss3_create_args).
If the rca_chan_bind_mic field of the arguments of a
RPCSEC_GSS_CREATE control message is set, then the server MUST verify
the client's channel binding MIC if the server supports this feature.
If channel binding verification succeeds then the server MUST
generate a new MIC of the same channel bindings and place it in the
rcr_chan_bind_mic field of the RPCSEC_GSS_CREATE rgss3_create_res
results. If channel binding verification fails or the server doesn't
support channel binding then the server MUST indicate this in its
reply by not including a rgss3_chan_binding value in rgss3_create_res
(rgss3_chan_binding is an optional field).
The client MUST verify the result's rcr_chan_bind_mic value by
calling GSS_VerifyMIC() with the given MIC and the channel bindings
data (including the channel type prefix). If client-side channel
binding verification fails then the client MUST call
RPCSEC_GSS_DESTROY. If the client requested channel binding but the
server did not include an rcr_chan_binding_mic field in the results,
then the client MAY continue to use the resulting context handle as
though channel binding had never been requested. If the client
considers channel binding critical, it MUST call RPCSEC_GSS_DESTROY.
As per-RPCSEC_GSSv2 [RFC5403]:
"Once a successful [channel binding] procedure has been performed
on an [RPCSEC_GSSv3] context handle, the initiator's
implementation may map application requests for rpc_gss_svc_none
and rpc_gss_svc_integrity to rpc_gss_svc_channel_prot credentials.
And if the secure channel has privacy enabled, requests for
rpc_gss_svc_privacy can also be mapped to
rpc_gss_svc_channel_prot."
Any RPCSEC_GSSv3 child context handle that has been bound to a secure
channel in this way SHOULD be used only with the
rpc_gss_svc_channel_prot, and SHOULD NOT be used with
rpc_gss_svc_none nor rpc_gss_svc_integrity -- if the secure channel
does not provide privacy protection then the client MAY use
rpc_gss_svc_privacy where privacy protection is needed or desired.
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2.7.1.3. Label Assertions
<CODE BEGINS>
/// struct rgss3_label {
/// rgss3_lfs rl_lfs;
/// opaque rl_label<>;
/// };
///
/// struct rgss3_lfs {
/// unsigned int rlf_lfs_id;
/// unsigned int rlf_pi_id;
/// };
///
<CODE ENDS>
The client discovers which label format specifiers (LFS) the server
supports via the RPCSEC_GSS_LIST control message. Full mode MAC is
enabled when an RPCSEC_GSS version 3 process subject label assertion
is combined with a file object label provided by the NFSv4.2
sec_label attribute.
Label encoding is specified to mirror the NFSv4.2 sec_label attribute
described in Section 12.2.4 of [NFSv4.2]. The label format specifier
(LFS) is an identifier used by the client to establish the syntactic
format of the security label and the semantic meaning of its
components. The policy identifier (PI) is an optional part of the
definition of an LFS which allows for clients and server to identify
specific security policies. The opaque label field of rgss3_label is
dependent on the MAC model to interpret and enforce.
If a label itself requires privacy protection (i.e., that the user
can assert that label is a secret) then the client MUST use the
rpc_gss_svc_privacy protection service for the RPCSEC_GSS_CREATE
request.
RPCSEC_GSSv3 clients MAY assert a set of subject security labels in
some LSF by binding a label assertion to the RPCSEC_GSSv3 child
context handle. This is done by including an assertion of type
rgss3_label in the RPCSEC_GSS_CREATE rgss3_create_args rca_assertions
call data. The label assertion payload is the set of subject labels
asserted by the calling NFS client process. The resultant child
context is used for NFS requests asserting the client process subject
labels. The NFS server process that handles such requests then
asserts the (client) process subject label(s) as it attempts to
access a file that has associated LNFS object labels.
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Servers that support labeling in the requested LFS MAY map the
requested subject label to a different subject label as a result of
server-side policy evaluation.
The labels that are accepted by the target and bound to the
RPCSEC_GSSv3 context MUST be enumerated in the rcr_assertions field
of the rgss3_create_res RPCSEC_GSS_CREATE reply.
Servers that do not support labeling or that do not support the
requested LFS reject the label assertion with a reply status of
MSG_DENIED, a reject_status of AUTH_ERROR, and an auth_stat of
RPCSEC_GSS_LABEL_PROBLEM.
2.7.1.4. Structured Privilege Assertions
<CODE BEGINS>
///
/// typedef opaque utf8string<>; /* UTF-8 encoding */
/// typedef utf8string utf8str_cs; /* Case-sensitive UTF-8 */
///
/// struct rgss3_privs {
/// utf8str_cs rp_name<>;
/// opaque rp_privilege<>;
/// };
<CODE ENDS>
A structured privilege is a capability defined by a specific RPC
application. To support the assertion of this privilege, by a client
using the application, in a server that also supports the
application, the application may define a private data structure that
is understood by clients and servers implementing the RPC
application.
RPCSEC_GSSv3 clients MAY assert a structured privilege by binding the
privilege to the RPCSEC_GSSv3 context handle. This is done by
including an assertion of type rgss3_privs in the RPCSEC_GSS_CREATE
rgss3_create_args rca_assertions call data.
The privilege is identified by the description string that is used by
RPCSEC_GSSv3 to identify the privilege and communicate the private
data between the relevant RPC application-specific code without
needing to be aware of the details of the structure used. Thus, as
far as RPCSEC_GSSv3 is concerned, the defined structure is passed
between client and server as opaque data encoded in the
rpc_gss3_privs rp_privilege field.
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Encoding, server verification and any server policies for structured
privileges are described by the RPC application definition. The
rp_name field of rpc_gss3_privs carries the description string used
to identify and list the privilege. The utf8str_cs definition is
from [RFC7530].
A successful structured privilege assertion MUST be enumerated in the
rcr_assertions field of the rgss3_create_res RPCSEC_GSS_CREATE reply.
If a server receives a structured privilege assertion that it does
not recognize, the assertion is rejected with a reply status of
MSG_DENIED, a reject_status of AUTH_ERROR, and an auth_stat of
RPCSEC_GSS_UNKNOWN_MESSAGE.
It is assumed that a client asserting more than one structured
privilege to be bound to a context handle would require all the
privilege assertions to succeed.
If a server receives an RPCSEC_GSS_CREATE request containing one or
more structured privilege assertions, any of which it fails to verify
according to the requirements of the RPC application defined
behavior, the request is rejected with a reply status of MSG_DENIED,
a reject_status of AUTH_ERROR, and an auth_stat of
RPCSEC_GSS_PRIVILEGE_PROBLEM.
Section 4.10.1.1. "Inter-Server Copy via ONC RPC with RPCSEC_GSSv3"
of [NFSv4.2] shows an example of structured privilege definition and
use.
2.7.2. New Control Procedure - RPCSEC_GSS_LIST
<CODE BEGINS>
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/// enum rgss3_list_item {
/// LABEL = 0,
/// PRIVS = 1
/// };
///
/// struct rgss3_list_args {
/// rgss3_list_item rla_list_what<>;
/// };
///
/// union rgss3_list_item_u
/// switch (rgss3_list_item itype) {
/// case LABEL:
/// rgss3_label rli_labels<>;
/// case PRIVS:
/// rgss3_privs rli_privs<>;
/// };
///
/// typedef rgss3_list_item_u rgss3_list_res<>;
///
<CODE ENDS>
The call data for an RPCSEC_GSS_LIST request consists of a list of
integers (rla_list_what) indicating what assertions are to be listed,
and the reply consists of an error or the requested list.
The result of requesting a list of rgss3_list_item LABEL is a list of
LFSs supported by the server. The client can then use the LFS list
to assert labels via the RPCSEC_GSS_CREATE label assertions. See
Section 2.7.1.3.
2.8. Extensibility
Assertion types may be added in the future by adding arms to the
'rgss3_assertion_u' union. Examples of other potential assertion
types include:
o Client-side assertions of identity:
* Primary client/user identity
* Supplementary group memberships of the client/user, including
support for specifying deltas to the membership list as seen on
the server.
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3. Operational Recommendation for Deployment
RPCSEC_GSSv3 is a superset of RPCSEC_GSSv2 [RFC5403] which in turn is
a superset of RPCSEC_GSSv1 [RFC2203], and so can be used in all
situations where RPCSEC_GSSv2 is used, or where RPCSEC_GSSv1 is used
and channel bindings functionality is not needed. RPCSEC_GSSv3
should be used when the new functionality is needed.
4. Security Considerations
This entire document deals with security issues.
The RPCSEC_GSSv3 protocol allows for client-side assertions of data
that is relevant to server-side authorization decisions. These
assertions must be evaluated by the server in the context of whether
the client and/or user are authenticated, whether multi-principal
authentication was used, whether the client is trusted, what ranges
of assertions are allowed for the client and the user (separately or
together), and any relevant server-side policy.
The security semantics of assertions carried by RPCSEC_GSSv3 are
application protocol-specific.
Note that RPSEC_GSSv3 is not a complete solution for labeling: it
conveys the labels of actors, but not the labels of objects. RPC
application protocols may require extending in order to carry object
label information.
There may be interactions with NFSv4's callback security scheme and
NFSv4.1's [RFC5661] GSS-API "SSV" mechanisms. Specifically, the
NFSv4 callback scheme requires that the server initiate GSS-API
security contexts, which does not work well in practice, and in the
context of client-side processes running as the same user but with
different privileges and security labels the NFSv4 callback security
scheme seems particularly unlikely to work well. NFSv4.1 has the
server use an existing, client-initiated RPCSEC_GSS context handle to
protect server-initiated callback RPCs. The NFSv4.1 callback
security scheme lacks all the problems of the NFSv4 scheme, however,
it is important that the server pick an appropriate RPCSEC_GSS
context handle to protect any callbacks. Specifically, it is
important that the server use RPCSEC_GSS context handles which
authenticate the client to protect any callbacks relating to server
state initiated by RPCs protected by RPCSEC_GSSv3 contexts.
As described in Section 2.10.10 [RFC5661] the client is permitted to
associate multiple RPCSEC_GSS handles with a single SSV GSS context.
RPCSEC_GSSv3 handles will work well with SSV in that the man-in-the-
middle attacks described in Section 2.10.10 [RFC5661] are solved by
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the new reply verifier (Section 2.3). Using an RPCSEC_GSSv3 handle
backed by a GSS-SSV mechanism context as a parent handle in an
RPCSEC_GSS_CREATE call while permitted is complicated by the lifetime
rules of SSV contexts and their associated RPCSEC_GSS handles.
5. IANA Considerations
The following new IANA RPC Authentication Status Numbers have been
added:
o RPCSEC_GSS_INNER_CREDPROBLEM (15) "No credentials for multi-
principal assertion inner context user". See Section 2.7.1.1.
o RPCSEC_GSS_LABEL_PROBLEM (16) "Problem with label assertion". See
Section 2.7.1.3.
o RPCSEC_GSS_PRIVILEGE_PROBLEM (17) "Problem with structured
privilege assertion". See Section 2.7.1.4.
o RPCSEC_GSS_UNKNOWN_MESSAGE (18) "Unknown structured privilege
assertion". See Section 2.7.1.4.
6. References
6.1. Normative References
[NFSv4.2] Haynes, T., "NFS Version 4 Minor Version 2", draft-ietf-
nfsv4-minorversion2-29 (Work In Progress), December 2014.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", RFC 2119, March 1997.
[RFC2203] Eisler, M., Chiu, A., and L. Ling, "RPCSEC_GSS Protocol
Specification", RFC 2203, September 1997.
[RFC2743] Linn, J., "Generic Security Service Application Program
Interface Version 2, Update 1", RFC 2743, January 2000.
[RFC4506] Eisler, M., "XDR: External Data Representation Standard",
RFC 4506, May 2006.
[RFC5056] Williams, N., "On the Use of Channel Bindings to Secure
Channels", RFC 5056, November 2007.
[RFC5403] Eisler, M., "RPCSEC_GSS Version 2", RFC 5403, February
2009.
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[RFC5661] Shepler, S., Eisler, M., and D. Noveck, "Network File
System (NFS) Version 4 Minor Version 1 Protocol", RFC
5661, January 2010.
[RFC7530] Haynes, T. and D. Noveck, "Network File System (NFS)
Version 4 Protocol", RFC 7530, March 2015.
6.2. Informative References
[AFS-RXGK]
Wilkinson, S. and B. Kaduk, "Integrating rxgk with AFS",
draft-wilkinson-afs3-rxgk-afs (work in progress), April
2014.
[RFC4949] Shirley, R., "Internet Security Glossary, Version 2", RFC
4949, August 2007.
Appendix A. Acknowledgments
Andy Adamson would like to thank NetApp, Inc. for its funding of his
time on this project.
We thank Lars Eggert, Mike Eisler, Ben Kaduk, Bruce Fields, Tom
Haynes, and Dave Noveck for their most helpful reviews.
Appendix B. RFC Editor Notes
[RFC Editor: please remove this section prior to publishing this
document as an RFC]
[RFC Editor: prior to publishing this document as an RFC, please
replace all occurrences of RFCTBD with RFCxxxx where xxxx is the RFC
number of this document]
Authors' Addresses
William A. (Andy) Adamson
NetApp
3629 Wagner Ridge Ct
Ann Arbor, MI 48103
USA
Phone: +1 734 665 1204
Email: andros@netapp.com
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Nico Williams
cryptonector.com
13115 Tamayo Dr
Austin, TX 78729
USA
Email: nico@cryptonector.com
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