| Network File System Version 4 | C. Lever |
| Internet-Draft | Oracle |
| Intended status: Standards Track | October 8, 2018 |
| Expires: April 11, 2019 |
File Content Provenance for Network File System version 4
draft-ietf-nfsv4-integrity-measurement-02
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.
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 April 11, 2019.
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.
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:
A more extensive mechanism is needed to guarantee that no modification of a particular file has occurred since it was created, perhaps even 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 content. The checksum and its signature are verified as the file's content is read into memory immediately before it is used. If verification fails, access to the file's content is prevented. 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 refer to this metadata using the generic term "file provenance information".
File provenance information is generated and signed by a "provenance authority", and then placed in the file system using special tools.
A security module separate from the file system stack specifies the format of the file provenance information and enforces a policy for utilizing it to determine when a protected file's content is safe to use on the local system. For the purposes of this document, we refer to this module as a "provenance assessor", and the policy it uses as the "provenance assessment policy".
NFS acts as a conduit by which file provenance information and file content move between storage on an NFS server and the provenance assessor where that content is to be accessed. NFS peers accessing a set of shared files must all agree on the at-rest file provenance information format. The format is specified by the provenance assessor and is therefore not described in this document.
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 OPTIONAL extension specified in this document.
The Linux Integrity Measurement Architecture (IMA) is an example of a mechanism that provides a full provenance assessment service [IMA-WP]. The protocol extension in this document enables the storage and use of file provenance information to protect files stored on an NFS server.
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.
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].
<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.
This section defines a new data type to encapsulate and a new OPTIONAL attribute to access and update file provenance information associated with a particular file.
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 in this section, the length of file provenance information 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 Information /// % */ /// const FATTR4_FILE_PROVENANCE = XXX; /* to be assigned */ /// /// typedef file_prov4 fattr4_file_provenance; <CODE ENDS>
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 object 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. [ cel: Does writing to a file have any effect on IMA metadata? ]
When a SETATTR is presented to an NFS version 4.2 server with a credential that is not authorized 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_INVAL.
When a SETATTR is presented to an NFS version 4.2 server and the target object resides in a file system which supports FATTR4_FILE_PROVENANCE but the object does not support this attribute, the server MUST respond with NFS4ERR_WRONGTYPE.
When a SETATTR is presented to an NFS version 4.2 server but the target object resides in a file system which does not support FATTR4_FILE_PROVENANCE, the server MUST respond with NFS4ERR_ATTRNOTSUPP.
A detailed description of the SETATTR operation can be found in Section 18.30 of [RFC5661].
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 object 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 and the target object resides in a file system which supports FATTR4_FILE_PROVENANCE but the object does not support this attribute, the server MUST respond with NFS4ERR_WRONGTYPE.
When a GETATTR is presented to an NFS version 4.2 server but the target object resides in a file system which does not support FATTR4_FILE_PROVENANCE, this does not result in an error and the FATTR4_FILE_PROVENANCE attribute bit is clear in the server's response.
Otherwise, if the target object supports FATTR4_FILE_PROVENANCE and there is no file provenance information is available for the target object, the server returns a FATTR4_FILE_PROVENANCE attribute whose length is zero.
Provenance assessors operate after file content has been delivered but immediately before that content is to be used. To enable provenance assessors on NFS clients to verify file provenance information, NFS version 4.2 servers do not prevent access to file content if they have a local provenance assessor and it indicates that provenance verification has failed.
A detailed description of the GETATTR operation can be found in Section 18.7 of [RFC5661].
To aid the discussion in this section, we define a few handy terms:
In addition, there are intermediate modes of operation on participating peers:
Once a file is written, file provenance information is generated and signed by an appropriate provenance authority. Using the OPTIONAL extension specified in this document, the information can be associated with a file on either a full-function server or client using a tool with appropriate privileges that writes the provenance information to the shared file system. When using a store-only server, only a full-function client can place file provenance information in the shared file system.
Typically, once file provenance information is associated with a file, the file's content is essentially immutable, even if the file has write permissions. This is because changing the content without updating the associated file provenance information will make the content inaccessible, depending on the provenance assessment policy in effect. Thus updating the file content usually requires generating fresh file provenance information.
A participating server should ensure that modifications to file provenance information are done only by appropriately authorized agents.
[ cel: TBD. Regular users are probably not able to modify a local security.ima xattr. What kind of authority should be required to modify FPI remotely? ]
Because the protocol extension described herein is OPTIONAL, clients and servers that support it must necessarily interact with clients and servers that do not support it. To set the stage for a discussion of interactions that might occur, consider the following possible simple provenance assessment policies that might be adopted by a provenance assessor (actual polices are left to provenance assessors):
Given the above example policies and the definitions we provided earlier for participating and non-participating implementations, the following statements are true:
A provenance assessor on an NFS version 4.2 peer needs to be prepared to deal with file provenance information it does not recognize or cannot parse. Typically its policy treats this case as a provenance verification failure.
Note that an NFS version 4.2 server may use a provenance assessor to prevent access by local users to protected files. To enable NFS version 4.2 clients to do their own assessment, an NFS version 4.2 server should never prevent remote access to clients if local provenance assessment fails.
A provenance assessor typically checksums 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 its local provenance assessor can compute a fresh checksum to verify the file's content.
Thus provenance assessment 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). A provenance assessor 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 assessors 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.
The design of the OPTIONAL extension described in this document assumes that all file provenance information is keyed or otherwise cryptographically signed by a provenance authority to prevent unwanted alteration at rest or in transit.
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.
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.
To prevent a malicious denial-of-service attempt by altering file provenance information at rest, an NFS version 4.2 server should enforce a suitable level of privilege before authorizing a local or remote agent to alter this information. See Section 5.2.1 for more detail.
This document does not require any actions by IANA.
| [RFC2119] | Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, DOI 10.17487/RFC2119, March 1997. |
| [RFC4506] | Eisler, M., "XDR: External Data Representation Standard", STD 67, RFC 4506, DOI 10.17487/RFC4506, May 2006. |
| [RFC5661] | Shepler, S., Eisler, M. and D. Noveck, "Network File System (NFS) Version 4 Minor Version 1 Protocol", RFC 5661, DOI 10.17487/RFC5661, January 2010. |
| [RFC7862] | Haynes, T., "Network File System (NFS) Version 4 Minor Version 2 Protocol", RFC 7862, DOI 10.17487/RFC7862, November 2016. |
| [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. |
| [RFC8174] | Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC 2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174, May 2017. |
| [RFC8178] | Noveck, D., "Rules for NFSv4 Extensions and Minor Versions", RFC 8178, DOI 10.17487/RFC8178, July 2017. |
| [IMA-WP] | Safford, D., "An Overview of The Linux Integrity Subsystem" |
| [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. |
| [RFC2104] | Krawczyk, H., Bellare, M. and R. Canetti, "HMAC: Keyed-Hashing for Message Authentication", RFC 2104, DOI 10.17487/RFC2104, February 1997. |
| [RFC5662] | Shepler, S., Eisler, M. and D. Noveck, "Network File System (NFS) Version 4 Minor Version 1 External Data Representation Standard (XDR) Description", RFC 5662, DOI 10.17487/RFC5662, January 2010. |
| [RFC7861] | Adamson, A. and N. Williams, "Remote Procedure Call (RPC) Security Version 3", RFC 7861, DOI 10.17487/RFC7861, November 2016. |
| [TPM-SUM] | Trusted Computing Group, "Trusted Platform Module (TPM) Summary", April 2008. |
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 author wishes to acknowledge review comments from Dave Noveck, Craig Everhart, and Bruce Fields which helped to make this a better document.
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.