                     ACL Layer Design for the Wallet

Introduction

    This is a description of the ACL layer of the wallet implementation.
    This is a specification of the expected behavior of the ACL
    implementation and includes the syntax and semantics of ACL strings
    used in the database.  The ACL entries used by the wallet are intended
    to be an extensible format to which additional ACL backends may be
    added as needed.  When new ACL backends are added, they should be
    described here.

Syntax

    An ACL entry in the wallet consists of two pieces of data, a <scheme>
    and an <identifier>. <scheme> is one or more characters in the set
    [a-z0-9-] that identifies the ACL backend to use when interpreting
    this ACL.  <identifier> is zero or more characters including all
    printable ASCII characters except whitespace.  Only the implementation
    of <scheme> knows about the meaning of <identifier>.  <identifier> may
    include zero or more users.

Semantics

    All users are authenticated to the wallet by Kerberos and are
    therefore represented by a Kerberos principal, which follows the
    normal Kerberos rules for string representation.

    Whenever there is a question about whether a user is permitted an
    action by a particular ACL, the following verification algorithm is
    used:  Iterate through each ACL entry in the ACL in question.  If the
    ACL entry is malformatted or the scheme is not recognized, skip it.
    Otherwise, dispatch the question to the check function of the ACL
    implementation, passing it the principal identifying the client, the
    <identifier> portion of the ACL entry, and the type and name of the
    object the user is attempting to access.  This function returns either
    authorized or unauthorized.  If authorized, end the search; if
    unauthorized, continue to the next ACL entry.

    There is no support in this scheme for negative ACLs.

    There is one slight complication, namely that some ACL methods need to
    maintain persistant state for performance reasons (consider, for
    example, an ACL layer implemented with LDAP queries).  Therefore, each
    ACL handler should be represented by an object, and when the ACL code
    discovers it doesn't already have an object on hand for a given ACL
    scheme, it should construct one before querying it.  If construction
    fails, it should fail that scheme and any ACL that uses that scheme,
    but still allow access if an ACL not using that scheme grants access
    to the user.

ACL Schemes

  external

    The <identifier> is arguments to an external command.  Access is
    granted if the external command returns success.  The standard remctl
    environment variables are exposed to the external command.

  krb5

    The <identifier> is a fully-qualified Kerberos principal.  Access is
    granted if the principal of the client matches <identifier>.

  ldap-attr

    <identifier> is an an attribute followed by an equal sign and a value.
    If the LDAP entry corresponding to the given principal contains the
    attribute and value specified by <identifier>, access is granted.

  ldap-attr-root

    This is almost identical to netdb except that the user must be in the
    form of a root instance (<user>/root) and the "/root" portion is
    stripped before checking the NetDB roles.

  nested

    <identifier> is the name of another ACL, and access is granted if it
    is granted by that ACL.  This can be used to organize multiple ACLs
    into a group and apply their union to an object.

  netdb

    <identifier> is the name of a system.  Access is granted if the user
    is listed as an administrator, user, or admin team member of the host
    in NetDB (Stanford's system management database).

  netdb-root

    This is almost identical to netdb except that the user must be in the
    form of a root instance (<user>/root) and the "/root" portion is
    stripped before checking the NetDB roles.

  pts

    (Not yet implemented.)  <identifier> is the name of an AFS PTS group.
    Access is granted if the principal of the user is a member of that AFS
    PTS group.

License

    Copyright 2016 Russ Allbery <eagle@eyrie.org>
    Copyright 2006-2008, 2013
        The Board of Trustees of the Leland Stanford Junior University

    Copying and distribution of this file, with or without modification,
    are permitted in any medium without royalty provided the copyright
    notice and this notice are preserved.  This file is offered as-is,
    without any warranty.

    SPDX-License-Identifier: FSFAP
