NFSv4 W. Adamson
Internet-Draft NetApp
Intended status: Standards Track N. Williams
Expires: June 11, 2015 Cryptonector
December 08, 2014
Remote Procedure Call (RPC) Security Version 3
draft-ietf-nfsv4-rpcsec-gssv3-10
Abstract
This document specifies version 3 of the Remote Procedure Call (RPC)
security protocol (RPCSEC_GSS). This protocol provides for multi-
principal authentication of client hosts and user principals to
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 [1].
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 June 11, 2015.
Copyright Notice
Copyright (c) 2014 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
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described in the Simplified BSD License.
Table of Contents
1. Introduction and Motivation . . . . . . . . . . . . . . . . . 2
2. The RPCSEC_GSSv3 Protocol . . . . . . . . . . . . . . . . . . 4
2.1. Compatibility with RPCSEC_GSSv2 . . . . . . . . . . . . . 5
2.2. Version Negotiation . . . . . . . . . . . . . . . . . . . 5
2.3. New REPLY Verifier . . . . . . . . . . . . . . . . . . . 5
2.4. New Version Number . . . . . . . . . . . . . . . . . . . 6
2.5. RPCSEC_GSS_BIND_CHANNEL Operation Deprecated . . . . . . 8
2.6. New auth_stat Values . . . . . . . . . . . . . . . . . . 8
2.7. New Control Procedures . . . . . . . . . . . . . . . . . 8
2.7.1. New Control Procedure - RPCSEC_GSS_CREATE . . . . . . 9
2.7.2. New Control Procedure - RPCSEC_GSS_LIST . . . . . . . 16
2.8. Extensibility . . . . . . . . . . . . . . . . . . . . . . 17
3. Operational Recommendation for Deployment . . . . . . . . . . 18
4. Security Considerations . . . . . . . . . . . . . . . . . . . 18
5. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 19
6. References . . . . . . . . . . . . . . . . . . . . . . . . . 19
6.1. Normative References . . . . . . . . . . . . . . . . . . 19
6.2. Informative References . . . . . . . . . . . . . . . . . 19
Appendix A. Acknowledgments . . . . . . . . . . . . . . . . . . 20
Appendix B. RFC Editor Notes . . . . . . . . . . . . . . . . . . 20
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 20
1. Introduction and Motivation
The original RPCSEC_GSS protocol [2] provided for authentication of
RPC clients and servers to each other using the Generic Security
Services Application Programming Interface (GSS-API) [3]. The second
version of RPCSEC_GSS [8] added support for channel bindings [6].
We find that GSS-API mechanisms are insufficient for communicating
certain aspects of authority to a server. The GSS-API and its
mechanisms certainly could be extended to address this shortcoming,
but it seems be far simpler to address it at the application layer,
namely, in this case, RPCSEC_GSS.
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A major motivation for RPCSEC_GSSv3 is to add support for labeled
security and server-side copy for NFSv4.
Labeled NFS (see Section 8 of [4]) uses the subject label provided by
the client via the RPCSEC_GSSv3 layer to enforce MAC access to
objects owned by the server to enable server guest mode.
RPCSEC_GSSv3 label assertions provide the means to achieve full mode
labeled NFS.
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 3.4.1 of [4]), 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 on the client cache by a user. As described in
Section 7 of [14], 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.
We therefore describe RPCSEC_GSS version 3 (RPCSEC_GSSv3).
RPCSEC_GSSv3 is the same as RPCSEC_GSSv2 [8], except that the
following assertions of authority have been added.
o Security labels for multi-level, type enforcement, and other
labeled security models. See [10], [11], [12], [4] and [9].
o Application-specific structured privileges. For an example see
server-side copy [4].
o Multi-principal authentication of the client host and user to the
server done by binding two RPCSEC_GSS handles.
o Simplified channel binding.
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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.
We add an option for enumerating server supported label format
specifiers (LFS). The LFS and Label Format Registry are described in
detail in [13].
This document contains the External Data Representation (XDR) ([7])
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_GSSv3 is the same as RPCSEC_GSSv2 [8], except that support for
assertions has been added. The entire RPCSEC_GSSv3 protocol is not
presented. Instead the differences between RPCSEC_GSSv3 and
RPCSEC_GSSv2 are shown.
RPCSEC_GSSv3 is patterned as follows:
o A client uses an existing RPCSEC_GSSv3 context handle established
in the usual manner (See Section 5.2 [2]) to protect RPCSEC_GSSv3
exchanges, this will be termed the "parent" handle.
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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 [8] is fully supported by
RPCSEC_GSSv3 with the exception of the RPCSEC_GSS_BIND_CHANNEL
operation which is deprecated (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 [2].
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 due to the following:
The RPCSEC_GSSv3 child handle uses the same GSS context as the parent
handle, so 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 MIC on just the sequence number,
this provides opportunities for man in the middle attacks.
This is easily addressed: RPCSEC_GSS version 3 changes the verifier
of the reply to compute the verifier using the exact same input as
that is used for verifier of the request, except for the mtype change
from CALL to REPLY. The new reply verifier computes a MIC over the
following RPC reply header data:
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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. New Version Number
<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 [RFCxxxx].
/// *
/// * 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
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/// * 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]. 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;
/// };
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///
<CODE ENDS>
As seen above, the RPCSEC_GSSv3 credential has the same format as the
RPCSEC_GSSv1 [2] and RPCSEC_GSSv2 [8] 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 Deprecated
RPCSEC_GSSv3 provides a channel binding assertion that replaces the
RPCSEC_GSSv2 RPCSEC_GSS_BIND_CHANNEL operation.
RPCSEC_GSS_BIND_CHANNEL MUST NOT be used on RPCSEC_GSS version 3
handles.
2.6. New auth_stat Values
RPCSEC_GSSv3 requires the addition of several values to the auth_stat
enumerated type definition. The use of each 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,
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
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section 5.4 [2]) 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_GSSv 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 [8]) 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 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
SHOULD 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.
The inner context handle it SHOULD 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 is placed in rgmp_rpcheader_mic field.
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.
As described in Section 5.3.3.3 of [2] 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.
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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 [8]. 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 the community now
agrees 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 [6], 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).
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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 [8]:
"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.
2.7.1.3. Label Assertions
<CODE BEGINS>
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/// 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 labels the server supports via the
RPCSEC_GSS_LIST control message. Asserting a server supported label
via RPCSEC_GSS_CREATE enables server guest mode labels. Full mode is
enabled when an RPCSEC_GSS_CREATE label assertion is combined with
asserting the same label with the NFSv4.2 sec_label attribute.
Label encoding is specified to mirror the NFSv4.2 sec_label attribute
described in Section 12.2.2 of [4]. 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 server security label in some LSF
by binding a label assertion to the RPCSEC_GSSv3 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.
Servers that support labeling in the requested LFS MAY map the
requested label to different 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
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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>
///
/// struct rgss3_privs {
/// string rp_name<>; /* human readable */
/// opaque rp_privilege<>;
/// };
<CODE ENDS>
A structured privilege is an RPC application defined privilege.
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. Encoding, server
verification and any policies for structured privileges are described
by the RPC application definition.
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.
If a server receives a structured privilege assertion that it fails
to verify according to the requirements of the RPC application
defined behavior, the assertion 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 3.4.1.2. "Inter-Server Copy with RPCSEC_GSSv3" of [4] 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 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. Other assertion types are described
elsewhere and 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 [8] which in turn is a
superset of RPCSEC_GSSv1 [2], and so can be used in all situations
where RPCSEC_GSSv1 or RPCSEC_GSSv2 is used. 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 [5] 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 [5] 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 [5] are solved by the new
reply verifier (Section 2.3). Using an RPCSEC_GSSv3 handle backed by
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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
There are no IANA considerations in this document.
6. References
6.1. Normative References
[1] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", RFC 2119, March 1997.
[2] Eisler, M., Chiu, A., and L. Ling, "RPCSEC_GSS Protocol
Specification", RFC 2203, September 1997.
[3] Linn, J., "Generic Security Service Application Program
Interface Version 2, Update 1", RFC 2743, January 2000.
[4] Haynes, T., "NFS Version 4 Minor Version 2", draft-ietf-
nfsv4-minorversion2-29 (Work In Progress), December 2014.
[5] Shepler, S., Eisler, M., and D. Noveck, "Network File
System (NFS) Version 4 Minor Version 1 Protocol", RFC
5661, January 2010.
[6] Williams, N., "On the Use of Channel Bindings to Secure
Channels", RFC 5056, November 2007.
[7] Eisler, M., "XDR: External Data Representation Standard",
RFC 4506, May 2006.
[8] Eisler, M., "RPCSEC_GSS Version 2", RFC 5403, February
2009.
[9] Haynes, T., "Requirements for Labeled NFS", RFC 7204,
April 2014.
6.2. Informative References
[10] "Section 46.6. Multi-Level Security (MLS) of Deployment
Guide: Deployment, configuration and administration of Red
Hat Enterprise Linux 5, Edition 6", 2011.
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[11] Smalley, S., "The Distributed Trusted Operating System
(DTOS) Home Page", 2000,
<http://www.cs.utah.edu/flux/fluke/html/dtos/HTML/
dtos.html>.
[12] Carter, J., "Implementing SELinux Support for NFS", 2005,
<http://www.nsa.gov/research/_files/selinux/papers/
nfsv3.pdf>.
[13] Quigley, D. and J. Lu, "Registry Specification for MAC
Security Label Formats", draft-quigley-label-format-
registry (work in progress), 2011.
[14] Wilkinson, S. and B. Kaduk, "Integrating rxgk with AFS",
draft-wilkinson-afs3-rxgk-afs (work in progress), April
2014.
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, and Bruce Fields 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 RFCTBD10 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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