Network File System Version 4 C. Lever
Internet-Draft Oracle
Intended status: Standards Track May 29, 2018
Expires: November 30, 2018
Integrity Measurement for Network File System version 4
draft-ietf-nfsv4-integrity-measurement-00
Abstract
This document specifies an OPTIONAL extension to NFS version 4.2 that
enables Linux Integrity Measurement Architecture metadata (IMA) to be
conveyed between NFSv4.2 servers and clients.
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 https://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 November 30, 2018.
Copyright Notice
Copyright (c) 2018 IETF Trust and the persons identified as the
document authors. All rights reserved.
This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents
(https://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.
Lever Expires November 30, 2018 [Page 1]
Internet-Draft IMA over NFS May 2018
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
2. Requirements Language . . . . . . . . . . . . . . . . . . . . 3
3. Protocol Extension Considerations . . . . . . . . . . . . . . 3
3.1. XDR Extraction . . . . . . . . . . . . . . . . . . . . . 3
4. Managing IMA Metadata on NFS Files . . . . . . . . . . . . . 4
4.1. XDR Definition . . . . . . . . . . . . . . . . . . . . . 4
4.2. Protecting File Content . . . . . . . . . . . . . . . . . 6
4.3. Protecting File Attributes . . . . . . . . . . . . . . . 7
5. Security Considerations . . . . . . . . . . . . . . . . . . . 10
6. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 11
7. References . . . . . . . . . . . . . . . . . . . . . . . . . 11
7.1. Normative References . . . . . . . . . . . . . . . . . . 11
7.2. Informative References . . . . . . . . . . . . . . . . . 11
Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . . 12
Author's Address . . . . . . . . . . . . . . . . . . . . . . . . 12
1. Introduction
The Linux Integrity Measurement Architecture (IMA) provides assurance
that the content of files is unaltered and authentic to what was
originally written to those files. In addition, an Extended
Verification Module (EVM) assures that file attribute information
remains similarly unaltered.
The primary goal is to detect when a remote attacker, a local
attacker, or unintentional software behavior has modified the content
or attributes of a file either in transit or at rest. This is done
by separately storing HMAC hashes [RFC2104] of a file's byte stream
content and attribute metadata. These hashes are updated whenever
the file is legitimately modified. The hashes themselves can be
protected by cryptographic signature.
Subsequent integrity verification is not performed by applications or
by file systems. File systems are responsible only for persistent
storage of file content and hashes. When a file is read, content,
attributes, and hashes are passed to IMA and EVM modules for
measurement and appraisal. Application access is denied if the
hashes or their signatures cannot be verified.
Some files may be immutable, in which case their integrity metadata
is signed by an RSA public key signature [RFC8017]. Such files can
be accessed in read-only mode or deleted by an appropriately
privileged agent, but cannot otherwise be modified.
Key material used to sign and verify file content and attribute
metadata must be protected. A Trusted Platform Module [TPM-SUM] can
Lever Expires November 30, 2018 [Page 2]
Internet-Draft IMA over NFS May 2018
be used to seal the key material. This use case is typical for
providing a read-only operating system image that is
cryptographically verified; for example, in a cloud environment or on
mobile devices.
On Linux, there are two parts to a file's IMA metadata:
o An HMAC hash, possibly cryptographically signed, of the file's
byte stream, stored in the file's security.ima extended attribute.
o An HMAC hash, possibly cryptographically signed, of a subset of
the file's attributes, stored in the file's security.evm extended
attribute.
The goals and use cases of the Linux Integrity Measurement
Architecture (IMA) are presented in further detail in [IMA-WP].
2. 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 BCP 14 [RFC2119]
[RFC8174] when, and only when, they appear in all capitals, as shown
here.
3. Protocol Extension Considerations
This document specifies an OPTIONAL extension to NFSv4 minor version
2 [RFC7862]. NFSv4.2 servers and clients implemented without
knowledge of this extension will continue to interoperate with
NFSv4.2 clients and servers that are aware of the extension, whether
or not they support it.
Because [RFC7862] does not define NFSv4.2 as non-extensible,
[RFC8178] treats it as an extensible minor version. Therefore this
Standards Track RFC extends NFSv4.2 but does not update [RFC7862] or
[RFC7863].
3.1. XDR Extraction
Section 4.1 contains a description of an extension to the NFSv4.2
protocol, expressed in the External Data Representation (XDR)
language [RFC4506]. This description is provided in a way that makes
it simple to extract into ready-to-compile form. The reader can
apply the following sed script to this document to produce a machine-
readable XDR description of the extension.
Lever Expires November 30, 2018 [Page 3]
Internet-Draft IMA over NFS May 2018
<CODE BEGINS>
sed -E 's:^ */// ?::;t;d'
<CODE ENDS>
That is, if this document is in a file called "integrity-
extension.txt" then the reader can do the following to extract an XDR
description file:
<CODE BEGINS>
sed -E 's:^ */// ?::;t;d' < integrity-extension.txt > ima.x
<CODE ENDS>
Once that extraction is done, these added lines need to be inserted
into an appropriate base XDR of the generated XDR from [RFC7863]
together with XDR from any additional extensions to be recognized by
the implementation. This will result in a ready-to-compile XDR file.
4. Managing IMA Metadata on NFS Files
4.1. XDR Definition
This section defines a new data structure used to transport HMAC
data, and two new OPTIONAL GETATTR attributes used to access and
update HMAC data associated with a file.
HMAC data is a varying length opaque array of octets. To enable a
single HMAC payload to be retrieved or updated in a single RPC, an
HMAC (including signatures) MUST NOT exceed 4096 bytes in length.
Lever Expires November 30, 2018 [Page 4]
Internet-Draft IMA over NFS May 2018
<CODE BEGINS>
/// /*
/// * Copyright (c) 2018 IETF Trust and the person identified
/// * as author of the code. All rights reserved.
/// *
/// * The author of the code is: C. Lever
/// */
///
/// const IMA_HMAC_MAXSIZE = 4096;
///
/// typedef opaque ima_hmac4<IMA_HMAC_MAXSIZE>;
/// typedef opaque uuid4[16];
/// typedef opaque verified_attribute4<>;
///
/// struct evm_verflist4 {
/// uuid4 *evm_uuid;
/// verified_attribute4 evm_attrlist<>;
/// };
///
/// %/*
/// % * New For Integrity Measurement
/// % */
/// const FATTR4_IMA_HMAC_CONTENT = 85;
/// const FATTR4_IMA_HMAC_ATTR = 86;
/// const FATTR4_EVM_VERF_LIST = 87;
///
/// typedef ima_hmac4 fattr4_ima_hmac_content;
/// typedef ima_hmac4 fattr4_ima_hmac_attr;
/// typedef evm_verflist4 fattr4_evm_verf_list;
<CODE ENDS>
+------------------+-----+---------------+------+--------------+
| Name | Id | Data Type | Acc | Defined in |
+------------------+-----+---------------+------+--------------+
| IMA_HMAC_CONTENT | 85 | ima_hmac4 | W | Section 4.2 |
| IMA_HMAC_ATTR | 86 | ima_hmac4 | W | Section 4.3 |
| EVM_VERF_LIST | 87 | evm_verflist4 | W | Section 4.3 |
+------------------+-----+---------------+------+--------------+
Table 1
When an NFSv4.2 server does not recognize, or does recognize but does
not support, these new attributes, it must respond according to the
requirements defined in Section 4.3 of [RFC8178].
Lever Expires November 30, 2018 [Page 5]
Internet-Draft IMA over NFS May 2018
4.2. Protecting File Content
4.2.1. Computing an IMA HMAC on an NFS File
Integrity measurement is performed on the entirety of a file's
primary byte stream. When a file is first accessed, after file
content changes, or if any portion of a file is evicted from an
NFSv4.2 client's cache, the file's entire byte stream must be read in
order for the IMA module to verify that the stored HMAC matches the
just-computed HMAC. This requirement can incur a significant
performance impact for large files or files that change frequently.
An NFSv4.2 client may employ mechanisms, not specified here, to
reduce this performance impact. For example, instead of signing a
hash of the file's byte stream, a Merkle tree can be constructed that
allows clients to verify the integrity of smaller portions of a large
file [MERKLE]. The top hash of that tree can be signed instead of
signing the HMAC of the file content. This Merkle tree is then used
to verify subsections of the file's byte stream that are needed by
applications running on an NFSv4.2 client.
4.2.2. Storing an IMA HMAC
An NFSv4.2 client stores the HMAC of an NFS file's byte stream by
sending a SETATTR operation that specifies the
FATTR4_IMA_HMAC_CONTENT attribute. This HMAC completely replaces the
previous one. To remove the FATTR4_IMA_HMAC_CONTENT attribute from a
file, the client specifies an ima_hmac_data field whose length is
zero.
When a SETATTR is presented to an NFSv4.2 server with a credential
that is unauthorized to replace the FATTR4_IMA_HMAC_CONTENT
attribute, the server MUST respond with NFS4ERR_ACCESS. This
document does not specify a policy for authorizing changes to the
FATTR4_IMA_HMAC_CONTENT attribute.
When a SETATTR is presented to an NFSv4.2 server with an
ima_hmac_data field that is larger than IMA_HMAC_MAXSIZE, the server
MUST respond with NFS4ERR_NAMETOOLONG.
When a SETATTR is presented to an NFSv4.2 server that supports
FATTR4_IMA_HMAC_CONTENT, but the SETATTR targets an object which does
not support this attribute, the server MUST respond with
NFS4ERR_TYPE.
Likewise, an NFSv4.2 client retrieves the HMAC of an NFS file's byte
stream by retrieving the FATTR4_IMA_HMAC_CONTENT attribute via a
GETATTR operation. This HMAC may have been computed (and signed)
Lever Expires November 30, 2018 [Page 6]
Internet-Draft IMA over NFS May 2018
previously on this client or by some other agent. An NFSv4.2 server
MUST NOT prevent an NFSv4.2 client from accessing a file based on
HMAC verification failures on the server.
4.3. Protecting File Attributes
4.3.1. Computing an EVM HMAC on an NFS File
The EVM HMAC protects a subset of file attributes, referred to as
"verified attributes". There are several categories of verified
attributes:
o Normal (N). Such an attribute is always present on a file, and is
always verified by the file's EVM HMAC hash.
o Extended (E). Such an attribute is a Linux extended attribute
that may be present on a file. If this attribute is present on a
file, it is always verified by the file's EVM HMAC hash.
o File system (F). Such an attribute is a file system attribute
(ie., its value is the same for all files that share the same
FSID). If the local system configuration calls for it, this
attribute is verified by a file's EVM HMAC hash.
o Optional (O). Such an attribute is a Linux extended attribute
that may be present on a file. If the local system configuration
calls for it, this attribute is verified by a file's EVM HMAC
hash.
The following table enumerates all attributes that may be a verified
attribute. File attributes not listed in this table are never
included when computing an EVM HMAC hash.
Lever Expires November 30, 2018 [Page 7]
Internet-Draft IMA over NFS May 2018
+---------------------------+-----+-------------------------+-------+
| Name | Cat | NFSv4 equivalent | Notes |
+---------------------------+-----+-------------------------+-------+
| inode number | N | fattr4_fileid | 1 |
| inode generation | N | fattr4_change | 1 |
| UID | N | fattr4_owner | 2 |
| GID | N | fattr4_owner_group | 2 |
| file mode | N | fattr4_mode | 3 |
| security.selinux | E | NFSv4 SecLabel | 4 |
| security.ima | E | fattr4_ima_hmac_content | |
| security.SMACK64 | E | None | 5 |
| security.capabilities | E | None | 5 |
| File system UUID | F | fattr4_evm_verf_list | |
| security.SMACK64EXEC | O | None | 5 |
| security.SMACK64TRANSMUTE | O | None | 5 |
| security.SMACK64MMAP | O | None | 5 |
+---------------------------+-----+-------------------------+-------+
Table 2
Notes:
1. NFSv4.2 server and client implementations are responsible for
ensuring that the native representation of these values correctly
correspond anywhere that an EVM HMAC is to be computed.
2. The NFSv4 ID mapping configuration must ensure that numeric user
ID values map identically whereever an EVM HMAC is to be
computed.
3. Some implementations may alter the file mode bits depending on
the presence of ACLs or a umask.
4. The security.selinux extended attribute typically maps to NFSv4
Security Label LFS value 0.
5. This attribute is not exposed in current versions of NFSv4, but
may be exposed by future extensions to the NFSv4 protocol, or it
may be accessible by other means.
The EVM HMAC for a file must be updated whenever one or more verified
attributes changes. Whenever a verified attribute is present on a
stored file, it MUST be accessible to the NFSv4.2 client computing
that HMAC. Otherwise, if that attribute is present and is not
accessible when the HMAC is verified, the client's EVM module denies
access to that file.
Lever Expires November 30, 2018 [Page 8]
Internet-Draft IMA over NFS May 2018
To verify an EVM HMAC, the verifier must know which optional
attributes were included when the hash was originally computed. An
NFSv4.2 server advertises this via the per file system
FATTR4_EVM_VERF_LIST attribute (Section 5.4 of [RFC5661] defines the
meaning of the term "per file system attribute"). If EVM HMACs are
supported on a file system, the server MUST also support and expose
FATTR4_EVM_VERF_LIST.
If an FS UUID is included in the EVM HMACs on this file system, the
server MUST fill in the evm_uuid field with its value. The server
MUST leave this field empty if the FS UUID is not included in EVM
HMACs. The server MUST append the names of all optional extended
attributes included in the EVM HMACs on this file system.
EVM HMAC hashes may be computed either on the NFSv4.2 server that
contains the files, or by NFSv4.2 clients. If an NFSv4.2 server or a
shared file system can store these hashes, but the hashes are always
computed elsewhere, the server still has to advertise the attribute
input set that is used to compute EVM HMACs on this file system.
An NFSv4.2 server MAY allow modifications of FATTR4_EVM_VERF_LIST by
NFSv4.2 clients. Changes to the value of this attribute render
existing EVM HMACs on this file system invalid. The format of the
FATTR4_EVM_VERF_LIST attribute a client submits via SETATTR is the
same as the the format the server uses when returning this attribute
via GETATTR.
If an NFSv4.2 client attempts to update this attribute and provides
one or more unrecognized optional extended attributes, the server
MUST respond with NFS4ERR_INVAL. If an NFSv4.2 client attempts to
update this attribute and the server does not support changes to it
by clients, the server MUST respond with NFS4ERR_INVAL, as required
by Section 5.5 of [RFC5661]. This document does not specify a policy
for authorizing changes to the FATTR4_EVM_VERF_LIST attribute.
4.3.2. Storing an EVM HMAC
An NFSv4.2 client stores the HMAC of an NFS file's attributes by
sending a SETATTR operation that specifies the FATTR4_IMA_HMAC_ATTR
attribute. This HMAC completely replaces the previous one. To
remove the FATTR4_IMA_HMAC_ATTR attribute from a file, a client
specifies an ima_hmac_data field whose length is zero.
When a SETATTR is presented to an NFSv4.2 server with a credential
that is unauthorized to replace the FATTR4_IMA_HMAC_ATTR attribute,
the server MUST respond with NFS4ERR_ACCESS. This document does not
specify a policy for authorizing changes to the FATTR4_IMA_HMAC_ATTR
attribute.
Lever Expires November 30, 2018 [Page 9]
Internet-Draft IMA over NFS May 2018
When a SETATTR is presented to an NFSv4.2 server with an
ima_hmac_data field that is larger than IMA_HMAC_MAXSIZE, the server
MUST respond with NFS4ERR_NAMETOOLONG.
When a SETATTR is presented to an NFSv4.2 server that supports
FATTR4_IMA_HMAC_ATTR, but the SETATTR targets an object which does
not support this attribute, the server MUST respond with
NFS4ERR_TYPE.
Likewise, an NFSv4.2 client retrieves the HMAC of an NFS file's
attributes by retrieving the FATTR4_IMA_HMAC_ATTR attribute via a
GETATTR operation. This HMAC may have been computed (and signed)
previously on this client or by some other agent. An NFSv4.2 server
MUST NOT prevent an NFSv4.2 client from accessing a file based on
HMAC verification failures on the server.
5. Security Considerations
An NFSv4.2 server is required to enforce a suitable level of
privilege before permiting a local or remote agent to alter IMA or
EVM HMAC hashes. This document does not specify a policy for
authorizing replacement of IMA or EVM HMAC hashes.
When protected by both IMA and EVM HMAC hashes, the content of a file
and its attributes are protected from end-to-end. Receivers can
reliably detect unintentional or malicious alteration of file content
or attributes by verifying the HMAC hashes that cover them.
Additional protection of such content or attributes while in transit
on an untrusted network is not required.
When an HMAC is cryptographically signed, receivers can reliably
detect unintentional or malicious alteration simply by verifying its
signature. Additional protection of a signed HMAC while in transit
on an untrusted network is not required.
In cases when IMA and EVM HMAC hashes are not otherwise
cryptographically protected, these hashes MUST be protected while in
transit on an untrusted network using a cryptographically strong
transport layer security service that can detect tampering, such as
RPCSEC with an integrity-protecting service [RFC7861].
Like other mechanisms that protect integrity during transit, it is
possible for a malicious agent or a network malfunction to create a
denial-of-service condition by repeatedly triggering integrity
verification failures on clients.
Lever Expires November 30, 2018 [Page 10]
Internet-Draft IMA over NFS May 2018
6. IANA Considerations
This document does not require any actions by IANA.
7. References
7.1. 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,
<https://www.rfc-editor.org/info/rfc2119>.
[RFC4506] Eisler, M., Ed., "XDR: External Data Representation
Standard", STD 67, RFC 4506, DOI 10.17487/RFC4506, May
2006, <https://www.rfc-editor.org/info/rfc4506>.
[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,
<https://www.rfc-editor.org/info/rfc5661>.
[RFC7862] Haynes, T., "Network File System (NFS) Version 4 Minor
Version 2 Protocol", RFC 7862, DOI 10.17487/RFC7862,
November 2016, <https://www.rfc-editor.org/info/rfc7862>.
[RFC7863] Haynes, T., "Network File System (NFS) Version 4 Minor
Version 2 External Data Representation Standard (XDR)
Description", RFC 7863, DOI 10.17487/RFC7863, November
2016, <https://www.rfc-editor.org/info/rfc7863>.
[RFC8174] Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
May 2017, <https://www.rfc-editor.org/info/rfc8174>.
[RFC8178] Noveck, D., "Rules for NFSv4 Extensions and Minor
Versions", RFC 8178, DOI 10.17487/RFC8178, July 2017,
<https://www.rfc-editor.org/info/rfc8178>.
7.2. Informative References
[IMA-WP] Safford, D., "An Overview of The Linux Integrity
Subsystem", <http://downloads.sf.net/project/linux-ima/
linux-ima/Integrity_overview.pdf>.
[MERKLE] Merkle, R., ""A Digital Signature Based on a Conventional
Encryption Function" Advances in Cryptology - CRYPTO '87",
DOI 10.1007/3-540-48184-2_32, 1988.
Lever Expires November 30, 2018 [Page 11]
Internet-Draft IMA over NFS May 2018
[RFC2104] Krawczyk, H., Bellare, M., and R. Canetti, "HMAC: Keyed-
Hashing for Message Authentication", RFC 2104,
DOI 10.17487/RFC2104, February 1997,
<https://www.rfc-editor.org/info/rfc2104>.
[RFC7861] Adamson, A. and N. Williams, "Remote Procedure Call (RPC)
Security Version 3", RFC 7861, DOI 10.17487/RFC7861,
November 2016, <https://www.rfc-editor.org/info/rfc7861>.
[RFC8017] Moriarty, K., Ed., Kaliski, B., Jonsson, J., and A. Rusch,
"PKCS #1: RSA Cryptography Specifications Version 2.2",
RFC 8017, DOI 10.17487/RFC8017, November 2016,
<https://www.rfc-editor.org/info/rfc8017>.
[TPM-SUM] Trusted Computing Group, "Trusted Platform Module (TPM)
Summary", April 2008, <https://trustedcomputinggroup.org/
wp-content/uploads/
Trusted-Platform-Module-Summary_04292008.pdf>.
Acknowledgments
The author wishes to thank Mimi Zohar and James Morris for their
early review of the concepts in this document, and Wim Coekaerts for
his encouragement of this work. Great thanks to Calum Mackay for his
improved XDR extraction script.
Special thanks go to Transport Area Director Spencer Dawkins, NFSV4
Working Group Chair Spencer Shepler, and NFSV4 Working Group
Secretary Thomas Haynes for their support.
Author's Address
Charles Lever
Oracle Corporation
1015 Granger Avenue
Ann Arbor, MI 48104
United States of America
Phone: +1 248 816 6463
Email: chuck.lever@oracle.com
Lever Expires November 30, 2018 [Page 12]
