GLEP 81 significantly changes the way that user management is handled,
and reveals some subtle issues in existing packages that have remained
hidden until now. Many of these issues can be avoided (in GLEP 81, but
also in general) by exercising some discipline when choosing the data
for new users and groups:

  * User and group names
  * Default shell
  * Home directory
  * Home directory ownership and permissions

This commit adds a few new sections to the "Users and Groups" chapter
that explain the current best practices, and give suggestions for
choosing the user and group data listed above. The rationale for these
choices is explained, and some notable exceptions are mentioned.
 ebuild-writing/users-and-groups/text.xml | 178 +++++++++++++++++++++++
 1 file changed, 178 insertions(+)

diff --git a/ebuild-writing/users-and-groups/text.xml 
index 190a099..fca3959 100644
--- a/ebuild-writing/users-and-groups/text.xml
+++ b/ebuild-writing/users-and-groups/text.xml
@@ -103,6 +103,184 @@ The defaults are the user and primary group for the 
owner, and 0755 for the
+  Whenever possible, the default shell and home directory should be
+  used. The rationale for this is explained below.
+  You should embark upon a GLEP81 migration like an EAPI
+  update. Rather than simply copy your user's settings into an
+  <c>acct-user</c> package, you should take the opportunity to
+  re-evaluate your user's name, shell, home directory, and its
+  permissions. Our GLEP 81 implementation will reveal many user
+  management issues that were allowed to fester in the past.
+  <title>Choosing a shell</title>
+  <body>
+    <p>
+      In most cases, the default shell (that is, no shell) should be
+      used. Services can still be started as a user who has no shell,
+      and daemons are able to drop privileges to a user who has no
+      shell. If necessary, the administrator can override a user's
+      default shell with <c>su -s &lt;shell&gt;
+      &lt;username&gt;</c>. This is sufficient for testing, management
+      of SSH credentials, and for initial configuration in an ebuild's
+      <c>pkg_config</c> phase.
+    </p>
+    <p>
+      An obvious exception to this rule is if a human being will need
+      to log into the account interactively, as is the case with the
+      <c>root</c> user. Other exceptions certainly exist, but should
+      be evaluated case-by-case. In other words, if you haven't
+      checked, don't set your user's shell to <c>/bin/bash</c> because
+      you think he <e>might</e> need it.
+    </p>
+    <p>
+      The goal here is twofold. First, the principle of least
+      privilege says that if a user doesn't need a real shell, he
+      shouldn't have one. And along those same lines, not having a
+      shell gives the system administrator some peace of mind: he
+      doesn't have to be as concerned with whether or not that user
+      has a password (and how strong it is), or whether or not its
+      filesystem permissions are all set correctly, or whether or not
+      it can log in via SSH, et cetera.
+    </p>
+  </body>
+  <title>Choosing a home directory</title>
+  <body>
+    <p>
+      In most cases, the default home directory (that is, no home
+      directory) should be used. GLEP81 changed two aspects of user
+      management with respect to home directories:
+    </p>
+    <ol>
+      <li>
+       Creating a user can now modify the permissions on an existing
+       directory. Should the need arise, this is necessary for a new
+       version of an <c>acct-user</c> package to be able to fix the
+       ownership and permissions of its home directory.
+      </li>
+      <li>
+       All user data aside from the username became non-local to
+       ebuilds that depend on that user. This is merely a side-effect
+       of moving the user creation out of the client package, and
+       into a separate <c>acct-user</c> package.
+      </li>
+    </ol>
+    <p>
+      The first item means that you should be conservative when
+      choosing a home directory. If at all possible, avoid choosing a
+      home directory that is used by another package. In particular,
+      no two <c>acct-user</c> packages should use the same home
+      directory. At best, the ownership and permissions on a shared
+      home directory would need to be kept synchronized between all
+      packages that share it. At worst, one package goes out-of-sync
+      and introduces a security hole for the others who no longer have
+      the expected permissions.
+    </p>
+    <p>
+      The second item means that if your package requires a user, you
+      can no longer be sure of that user's home directory or its
+      ownership and permissions. If your package requires a directory
+      to be owned and writable by some user, then your package's
+      ebuild should create that directory and ensure that it is
+      writable by the user. In other words, you should not rely on the
+      directory being created "transitively" by a dependency, even if
+      that dependency is an <c>acct-user</c> package.
+    </p>
+    <p>
+      In summary,
+    </p>
+    <ul>
+      <li>
+       Avoid using an <c>ACCT_USER_HOME</c> that belongs to another
+       package.
+      </li>
+      <li>
+       No two acct-user packages should define the same
+       <c>ACCT_USER_HOME</c>.
+      </li>
+      <li>
+       If for example your package's configuration needs
+       &lt;username&gt; to be able to write to
+       <c>/var/lib/&lt;username&gt;</c>, then your package's ebuild
+       should create that directory and set its ownership and
+       permissions. Barring any other considerations, the
+       corresponding <c>acct-user</c> package should leave
+       <c>ACCT_USER_HOME</c> at its default (empty) value; setting
+       <c>ACCT_USER_HOME=/var/lib/&lt;username&gt;</c> creates
+       unnecessary duplication and risks desynchronizing the
+       permissions.
+      </li>
+    </ul>
+  </body>
+  <title>Choosing home directory ownership</title>
+  <body>
+    <p>
+      In most cases, the default home directory ownership is correct.
+      If a non-default home directory is needed at all, then it should
+      be writable by its user and giving ownership of it to someone
+      else would prevent that. Being unwritable indicates that a
+      shared and potentially sensitive location was chosen. Moreover,
+      the fact that the home directory is not writable suggests that
+      the default home directory (which is also not writable) would
+      suffice instead; the home directory guidelines explain why the
+      default is preferable in that case. For example, setting
+      <c>ACCT_USER_HOME_OWNER="root:root"</c> is suspicious because it
+      appears intended to "undo" the ownership changed by your user
+      package, and that would only be necessary if the path in
+      question is used by some other package.
+    </p>
+  </body>
+  <title>Choosing home directory permissions</title>
+  <body>
+    <p>
+      In many cases, the default home directory permissions (0755)
+      will suffice. But, if your package will work with mode 0700 or
+      0750, then those are preferable. This is the principle of least
+      privilege again. If your package works with a non-writable home
+      directory, then you should probably be using the default of
+      <e>no</e> home directory!
+    </p>
+    <warning>
+      The world-writable bit should never be set in
+      <c>ACCT_USER_HOME_PERMS</c>. This should never be necessary, and
+      is usually exploitable.
+    </warning>
+  </body>
+  These suggestions are not rules and are not written in stone, except
+  perhaps for the world-writable warning. There are packages in the
+  tree that <c>chroot()</c> to <c>$HOME</c>, and require that
+  directory to be owned by <c>root:root</c> for security reasons. In
+  cases like that, it's impossible to avoid implicitly tying the user
+  to the package that needs it via the home directory, and the best you
+  can do is attempt to ensure that the users and paths are unique so
+  that no conflicts arise.
+  Unless your package is exceptional, though, following these
+  guidelines will minimize the potential for problems down the
+  road.

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