File Content Provenance for Network File System version 4
draft-ietf-nfsv4-integrity-measurement-01

Abstract

This document specifies an OPTIONAL extension to NFS version 4 minor version 2 that enables file provenance information to be conveyed between NFS version 4.2 servers and clients. File provenance information authenticates the creator of a file's content and helps guarantee the content's integrity from creation to use.

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 March 28, 2019.

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.

Table of Contents

• 1. Introduction
• 2. Requirements Language
• 3. Protocol Extension Considerations
• 4. Managing File Provenance Metadata on NFS Files
• 5. Security Considerations
• 6. IANA Considerations
• 7. References
• Acknowledgments
• Author's Address

1. Introduction

The security of software distribution systems is complex and challenging, especially as software distribution has become increasingly decentralized. An end administrator needs to trust that she is running executables just as they are supplied by a software vendor; in other words, that they have not been modified by malicious actors, contracted system administration services, or broken hardware or software. Software vendors want a guarantee that customer-installed executables that fall under support contracts have similarly not been modified.

There already exist mechanisms that protect file data during certain portions of a file's life cycle:

• Whole file system checksumming can verify so-called Golden Master installation media before it is used to install the software it contains.
• File or block integrity mechanisms can protect data at rest on storage servers.
• For a distributed file system such as NFS, transport layer security or a GSS integrity service (as described in [RFC7861]) can protect data while it traverses a network between a storage server and a client.

A more extensive mechanism is needed to guarantee that no modification of a particular file has occurred since it was created, even perhaps after several generations of copies have been made of the file's content.

This guarantee can be accomplished by separately preserving a keyed hash, such as an HMAC [RFC2104], of a file's byte stream. This hash and its signature are verified as the file's content is read into memory just before it is used. If verification fails, access to the file's content is denied. The hash is updated and re-signed only when the file is legitimately modified.

A keyed hash authenticates the identity of the last modifier of a file's content and serves as a strong check of the content's integrity. For the purposes of this document, we will refer to this as file provenance information. The Linux Integrity Measurement Architecture (IMA) is an example of a mechanism for assessing file content provenance [IMA-WP].

A Trusted Platform Module [TPM-SUM] can seal the key material used to sign and verify file content. Distributing and protecting such key material is outside the scope of the NFS protocol extension specified in this document.

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 NFS version 4 minor version 2 [RFC7862], hereafter referred to as NFS version 4.2. NFS version 4.2 servers and clients implemented without knowledge of this extension will continue to interoperate with NFS version 4.2 clients and servers that are aware of the extension, whether or not they support it.

Because [RFC7862] does not define NFS version 4.2 as non-extensible, [RFC8178] treats it as an extensible minor version. Therefore this Standards Track RFC extends NFS version 4.2 but does not update [RFC7862] or [RFC7863].

3.1. XDR Extraction

<CODE BEGINS>

sed -n -e 's:^ */// ::p' -e 's:^ *///$::p'

<CODE ENDS>

Section 4.1 contains a description of an extension to the NFS version 4.2 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.

<CODE BEGINS>

sed -n -e 's:^ */// ::p' -e 's:^ *///$::p'
     < provenance-extension.txt > ima.x

<CODE ENDS>

That is, if this document is in a file called "provenance-extension.txt" then the reader can do the following to extract an XDR description file:

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 File Provenance Metadata on NFS Files

4.1. XDR Definition

This section defines a new data type to encapsulate and a new OPTIONAL GETATTR attribute to access and update file provenance information associated with a particular file.

File provenance information is opaque to the NFS protocol. To ensure interoperability among accessors of this information when it is stored on NFS version 4.2 servers, this information MUST be self-describing.

To enable a single file provenance information payload to be retrieved or updated via a single RPC, and to constrain the transport resources required for the operations defined here, the payload MUST NOT exceed 4096 bytes in length.

When an NFS version 4.2 server does not recognize, or does recognize but does not support, this new attribute, the server responds in accordance with the requirements specified in Section 4.3 of [RFC8178].

<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 FILEPROV4_MAXSIZE = 4096;
   /// typedef opaque file_prov4<FILEPROV4_MAXSIZE>;
   ///
   /// %/*
   /// % * New For File Provenance Metadata
   /// % */
   /// const FATTR4_FILE_PROVENANCE = XXX;   /* to be assigned */
   ///
   /// typedef file_prov4 fattr4_file_provenance;

<CODE ENDS>

4.2. Storing File Provenance Metadata

An NFS version 4.2 client stores file provenance information by sending a SETATTR operation that specifies the FATTR4_FILE_PROVENANCE attribute, targeting the file associated with the file provenance information to be stored. This attribute completely replaces any previous one. To remove this attribute from a file, the client sends a FATTR4_FILE_PROVENANCE attribute whose length is zero.

When a SETATTR is presented to an NFS version 4.2 server with a credential that is unauthorized to replace the FATTR4_FILE_PROVENANCE attribute, the server MUST respond with NFS4ERR_ACCESS.

When a SETATTR is presented to an NFS version 4.2 server with a fattr4_file_provenance field whose length is larger than FILEPROV4_MAXSIZE, the server MUST respond with NFS4ERR_NAMETOOLONG.

When a SETATTR is presented to an NFS version 4.2 server that supports FATTR4_FILE_PROVENANCE, but the SETATTR targets an object which does not support this attribute, the server MUST respond with NFS4ERR_TYPE.

4.3. Retrieving File Provenance Metadata

An NFS version 4.2 client retrieves file provenance information by retrieving the FATTR4_FILE_PROVENANCE attribute via a GETATTR operation, specifying the file handle of the file associated with the information to be retrieved. This information may have been computed and signed previously on this client or by some other agent.

When a GETATTR is presented to an NFS version 4.2 server that supports the FATTR4_FILE_PROVENANCE attribute, but the GETATTR targets an object which does not support this attribute, the server MUST respond with NFS4ERR_TYPE. Otherwise, if no file provenance information is available for the targeted file handle, the server returns a FATTR4_FILE_PROVENANCE attribute whose length is zero.

An NFS version 4.2 server MUST NOT prevent an NFS version 4.2 client from accessing a file based on provenance verification failures on the server.

4.4. Performance Cost of Using File Provenance Metadata

Computing a file checksum is typically performed on the entirety of a file's content. When a file's content is first accessed, after it changes, or if any portion of a file is evicted from an NFS version 4.2 client's cache, the client must retrieve any missing content before a fresh checksum can be computed to verify the file's content. This can incur a significant performance impact for large files, files that change frequently, or files where only a portion of the content is used on that client (e.g., software libraries).

An NFS version 4.2 client can employ mechanisms not specified here to reduce this 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 root hash of that tree, being of sufficiently limited size, can be signed and stored as file provenance information. The Merkle tree, which is stored elsewhere, can be used to verify portions of the file's content without the need to read the whole file.

5. Security Considerations

An NFS version 4.2 server is REQUIRED to enforce a suitable level of privilege before permitting a local or remote agent to alter file provenance information. This document does not specify a policy for authorizing modification of this information.

When file provenance information for a file exists, the content of a file is protected from creation to use. Receivers can reliably detect unintentional or malicious alteration of file content by verifying its content using file provenance information. Additional protection of file content while at rest or in transit on an untrusted network is unnecessary.

Likewise, receivers can also reliably detect unintentional or malicious alteration of file provenance information that is cryptographically signed, simply by verifying its signature. Additional protection of signed file provenance information while at rest or in transit on an untrusted network is unnecessary.

In the rare cases when file provenance information is not cryptographically self-protected, the information 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 data integrity during transit, A malicious agent or a network malfunction can create a denial-of-service condition by repeatedly triggering integrity verification failures on NFS version 4.2 clients.

6. IANA Considerations

This document does not require any actions by IANA.

7. References

7.1. Normative References

7.2. Informative References

Acknowledgments

The author wishes to thank Mimi Zohar and James Morris for their early review of the concepts in this document, Wim Coekaerts for his encouragement of this work, and Dave Noveck for his work on NFS version 4 extensibility.

The XDR extraction conventions were first described by the authors of the NFS version 4.1 XDR specification [RFC5662]. Herbert van den Bergh suggested the replacement sed script used in this document.

Special thanks go to Transport Area Director Spencer Dawkins, NFSV4 Working Group Chairs Spencer Shepler and Brian Pawlowski, and NFSV4 Working Group Secretary Thomas Haynes for their support.

Author's Address