wdiff rfc7861v1.txt rfc7861.txt

Internet Engineering Task Force (IETF) W. Adamson
Request for Comments: 7861 NetApp
Updates: 5403 N. Williams
Category: Standards Track N. Williams Cryptonector
ISSN: 2070-1721 Cryptonector
April May 2016

Remote Procedure Call (RPC) Security Version 3

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 security and type enforcement, structured
privilege assertions, and channel bindings. This document updates
RFC 5403.

Status of This Memo

This is an Internet Standards Track document.

This document is a product of the Internet Engineering Task Force
(IETF). It represents the consensus of the IETF community. It has
received public review and has been approved for publication by the
Internet Engineering Steering Group (IESG). Further information on
Internet Standards is available in Section 2 of RFC 5741.

Information about the current status of this document, any errata,
and how to provide feedback on it may be obtained at
http://www.rfc-editor.org/info/rfc7861.

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
to this document. Code Components extracted from this document must
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. Requirements Language . . . . . . . . . . . . . . . . . . 3
1.2. Added Functionality . . . . . . . . . . . . . . . . . . . 3
1.3. 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
Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . . 22
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 23

1. Introduction and Motivation

The original Remote Procedure Call (RPC) security protocol
(RPCSEC_GSS) [RFC2203] provided for authentication of RPC clients and
servers to each other using the Generic Security Service 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
this shortcoming. However, it is addressed here at the application
layer, i.e., in RPCSEC_GSS.

A major motivation for version 3 of 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 Section 9.1 of
[RFC7862]).

Labeled NFS (see Section 9 of [RFC7862]) 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 9.5.3 of
[RFC7862]. RPCSEC_GSSv3 label assertions assert client MAC process
subject labels to enable Full Mode MAC when combined with Labeled NFS
as described in Section 9.5.1 of [RFC7862].

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 [RFC7862]), 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
user's behalf and re-display it for another user from the cache
without refetching 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.

1.1. 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].

1.2. Added Functionality

RPCSEC_GSS version 3 (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 9.5.1 of [RFC7862]).

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 use of
the privilege and/or the conditions in which the privilege may be
exercised. For an example, see server-side copy as described in
[RFC7862].

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 [RFC7862].

An option for enumerating server-supported Label Format Specifiers
(LFSs) is provided. See Section 9.1 of [RFC7862].

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 implementations 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 as follows:

o For label assertions, the target indicates no support by returning
the new RPCSEC_GSS_LABEL_PROBLEM auth_stat value (see
Section 2.7.1.3).

o For structured privilege assertions, the target indicates no
support by returning the new RPCSEC_GSS_UNKNOWN_MESSAGE auth_stat
value (see Section 2.7.1.4).

o For multi-principal authentication (Section 2.7.1.1), the target
indicates no support by not including an 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 an rgss3_chan_binding value in the
rgss3_create_res.

1.3. 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 it into a form that is ready to
compile. The reader can feed this document in the following shell
script to produce the machine-readable XDR description of
RPCSEC_GSSv3:

#!/bin/sh
grep "^ *///" | sed 's?^ */// ??' | sed 's?^ *///$??'

That is, 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:

sh extract.sh < spec.txt > rpcsec_gss_v3.x

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 difference is that the new
support for assertions and channel bindings is implemented via a
different mechanism.

The entire RPCSEC_GSSv3 protocol is not presented here. Only the
differences between RPCSEC_GSSv3 and RPCSEC_GSSv2 are shown.

RPCSEC_GSSv3 is implemented as follows:

o A client uses an existing RPCSEC_GSSv3 context handle established
in the usual manner (see Section 5.2 of [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 use 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.

This issue is addressed in RPCSEC_GSS version 3 by computing the
verifier using exactly the same input as the information 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.

/// /*
/// * Copyright (c) 2016 IETF Trust and the persons
/// * identified as the authors. All rights reserved.
/// *
/// * The authors of the code are identified in RFC 2203,
/// * RFC 5403, and RFC 7861.
/// *
/// * 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
/// * 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 RFC 2203, RFC 5403,
/// * and RFC 7861. 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 */
/// };
///
/// 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;
/// };
///

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 [RFC5531].

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
};

2.7. New Control Procedures

There are two new RPCSEC_GSSv3 control procedures: RPCSEC_GSS_CREATE
and 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 of [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>
/// 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<>;
/// };
///

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 (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 Sections 2.7.1.3 and
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.

Upon a 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

///
/// struct rgss3_gss_mp_auth {
/// opaque rgmp_handle<>; /* Inner handle */
/// opaque rgmp_rpcheader_mic<>;
/// };
///

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.

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.
The inner context handle of the multi-principal authentication
assertion MUST use an RPCSEC_GSSv3 context handle that corresponds to
a GSS-API security context that authenticates the user. The reverse
(parent handle authenticates user, inner context handle authenticates
an RPC client host) MUST NOT be used. Other multi-principal parent
and inner context handle uses might eventually make sense, but they
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 handle 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 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 the rgmp_rpcheader_mic field. Note
that the rgmp_rpcheader_mic only identifies the client host GSS
context by its 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 prevents an attacker from intercepting
the RPCSEC_GSS_CREATE control procedure, reassigning 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; the target then verifies the rgmp_rpcheader_mic using the
GSS-API security context associated with the rgmp_handle field.

On 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 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.

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-principal 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-principal 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

///
/// typedef opaque rgss3_chan_binding<>;
///

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 is 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.

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 the
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 an
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 an 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
or 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.

2.7.1.3. Label Assertions

/// struct rgss3_label {
/// rgss3_lfs rl_lfs;
/// opaque rl_label<>;
/// };
///
/// struct rgss3_lfs {
/// unsigned int rlf_lfs_id;
/// unsigned int rlf_pi_id;
/// };
///

The client discovers, via the RPCSEC_GSS_LIST control message, which
LFSs the server supports. 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 [RFC7862]. The 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 that
allows clients and the server to identify specific security policies.
The opaque label field (rgss3_label) is dependent on the MAC model to
interpret and enforce.

If a label itself requires privacy protection (i.e., requires that
the user can assert that the 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 LFS 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 Labeled NFS object labels.

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

///
/// typedef opaque utf8string<>; /* UTF-8 encoding */
/// typedef utf8string utf8str_cs; /* Case-sensitive UTF-8 */
///
/// struct rgss3_privs {
/// utf8str_cs rp_name<>;
/// opaque rp_privilege<>;
/// };

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.

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 4.9.1.1 of [RFC7862] ("Inter-Server Copy via ONC RPC with
RPCSEC_GSSv3") shows an example of structured privilege definition
and use.

2.7.2. New Control Procedure - RPCSEC_GSS_LIST

<CODE BEGINS>
/// 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<>;
///

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 objects 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 (Section 2.7.1). 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.

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 RPCSEC_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 SSV (Secret State Verifier) mechanisms.
Specifically, the NFSv4 callback scheme requires that the server
initiate GSS-API security contexts, which does not work well in
practice; 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 that
authenticate the client to protect any callbacks related to server
state initiated by RPCs protected by RPCSEC_GSSv3 contexts.

As described in Section 2.10.10 of [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 of [RFC5661]
are solved by 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 RPC Authentication Status Numbers have been added
to the IANA registry:

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

[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119,
DOI 10.17487/RFC2119, March 1997,
<http://www.rfc-editor.org/info/rfc2119>.

[RFC2203] Eisler, M., Chiu, A., and L. Ling, "RPCSEC_GSS Protocol
Specification", RFC 2203, DOI 10.17487/RFC2203, September
1997, <http://www.rfc-editor.org/info/rfc2203>.

[RFC2743] Linn, J., "Generic Security Service Application Program
Interface Version 2, Update 1", RFC 2743,
DOI 10.17487/RFC2743, January 2000,
<http://www.rfc-editor.org/info/rfc2743>.

[RFC4506] Eisler, M., Ed., "XDR: External Data Representation
Standard", STD 67, RFC 4506, DOI 10.17487/RFC4506, May
2006, <http://www.rfc-editor.org/info/rfc4506>.

[RFC5056] Williams, N., "On the Use of Channel Bindings to Secure
Channels", RFC 5056, DOI 10.17487/RFC5056, November 2007,
<http://www.rfc-editor.org/info/rfc5056>.

[RFC5403] Eisler, M., "RPCSEC_GSS Version 2", RFC 5403,
DOI 10.17487/RFC5403, February 2009,
<http://www.rfc-editor.org/info/rfc5403>.

[RFC5661] Shepler, S., Ed., Eisler, M., Ed., and D. Noveck, Ed.,
"Network File System (NFS) Version 4 Minor Version 1
Protocol", RFC 5661, DOI 10.17487/RFC5661, January 2010,
<http://www.rfc-editor.org/info/rfc5661>.

[RFC7530] Haynes, T., Ed. and D. Noveck, Ed., "Network File System
(NFS) Version 4 Protocol", RFC 7530, DOI 10.17487/RFC7530,
March 2015, <http://www.rfc-editor.org/info/rfc7530>.

[RFC7862] Haynes, T., "NFS "Network File System (NFS) Version 4 Minor
Version 2", 2 Protocol", RFC 7862, DOI 10.17487/RFC7862, April May
2016, <http://www.rfc-editor.org/info/rfc7862>.

6.2. Informative References

[AFS-RXGK]
Wilkinson, S. and B. Kaduk, "Integrating rxgk with AFS",
Work in Progress, draft-wilkinson-afs3-rxgk-afs-08, May
2015.

[RFC4949] Shirey, R., "Internet Security Glossary, Version 2",
FYI 36, RFC 4949, DOI 10.17487/RFC4949, August 2007,
<http://www.rfc-editor.org/info/rfc4949>.

[RFC5531] Thurlow, R., "RPC: Remote Procedure Call Protocol
Specification Version 2", RFC 5531, DOI 10.17487/RFC5531,
May 2009, <http://www.rfc-editor.org/info/rfc5531>.

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.

Authors' Addresses

William A. (Andy) Adamson
NetApp
3629 Wagner Ridge Ct.
Ann Arbor, MI 48103
United States

Phone: +1 734 665 1204
Email: andros@netapp.com

Nico Williams
cryptonector.com
13115 Tamayo Dr.
Austin, TX 78729
United States

Email: nico@cryptonector.com