Allowing inheritable NFSv4 ACLs to override the umask
draft-ietf-nfsv4-umask-02

Abstract

In some environments, inheritable NFSv4 ACLs can be rendered ineffective by the application of the per-process umask. This is easily worked around by transmitting the umask and create mode separately, to allow servers to make more intelligent decisions about the new mode of a file.

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Table of Contents

• 1. Conventions Used in This Document
• 2. Problem Statement
• 3. mode_umask Attribute
• 4. Security Considerations
• 5. Normative References
• Appendix A. Acknowledgments
• Authors' Addresses

1. Conventions Used in This Document

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 [RFC2119].

2. Problem Statement

On Unix-like systems, each process is associated with a file mode creation mask (umask). In the absence of inheritable permissions, the umask specifies which permissions must be turned off when creating new file system objects. With “POSIX” Access Control Lists [POSIX-1003.1e], in the presence of inheritable permissions, the umask must be ignored. Other Access Control List implementations on Unix-like systems may ignore the umask in a similar way.

The NFSv4 protocol currently does not include the umask concept; applying the umask is left to clients. Unfortunately, clients have no way of atomically checking for inheritable permissions and applying the umask only when necessary. Instead, they err on the safe side and always apply the umask. Thus the mode the server receives in an OPEN already has the umask applied.

When applying the mode, section 6.4.1.1 of [RFC7530] recommends that servers SHOULD restrict permissions granted to any user or group named in the ACL to be no more than the permissions granted by the MODE4_RGRP, MODE4_WGRP, and MODE4_XGRP bits. Servers aiming to provide clients with Unix-like chmod behavior may also be motivated by the same requirements in [SUSv4]. (See the discussion of additional and alternate access control mechanisms in section "4.4 File Permissions".)

On many existing installations, all ordinary users by default use the same effective group ID. To prevent granting all users full access to each other's files, such installations usually default to a umask with very restrictive permissions. Thus the named users and groups in an inherited ACL end up being mostly ignored.

This leads to file permissions which are more restrictive than they should be in common cases; permission inheritance over NFSv4 is broken.

To address this problem, a new attribute is proposed which allows the server to apply the umask only when there are no inheritable permissions.

3. mode_umask Attribute

      struct mode_umask4 {
        mode4  mu_mode;
        mode4  mu_umask;
      };