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<title>User service daemon and client specification - Applications and notes on use</title>
-<link rev=made href="mailto:ian@davenant.greenend.org.uk">
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+[<a href="ch-ipass.html">back</a>]
+ [<a href="index.html#abstract">Abstract</a>]
+ [<a href="index.html#copyright">Copyright Notice</a>]
+ [<a href="index.html#contents">Contents</a>]
+
+<hr>
+
<h1>
-User service daemon and client specification - chapter 6<br>
+User service daemon and client specification - Chapter 6<br>
Applications and notes on use
</h1>
+
+<hr>
+
+<h2>
+<a name="s-examples">6.1 Examples</a>
+</h2>
+
+<p>
+The companion package, <code>userv-utils</code>, contains a selection of
+example services, some of which are useful tools in their own right.
+See the <code>README</code> in its top-level directory for details.
+</p>
+
<hr>
-<h2><A name="s-standards">
-6.1 Standard services and directory management
-</A></h2>
+<h2>
+<a name="s-standards">6.2 Standard services and directory management</a>
+</h2>
+
+<p>
In later versions of this specification standard service names and
interfaces for common services such as mail delivery and WWW CGI
-scripts will be specified.<P>
+scripts may be specified.
+</p>
-<kbd>userv</kbd>-using applications and system services which hide
-<kbd>userv</kbd> behind wrapper scripts may need to store information in
+<p>
+<code>userv</code>-using applications and system services which hide
+<code>userv</code> behind wrapper scripts may need to store information in
the user's filespace to preserve the correct placement of the security
perimiters. Such applications should usually do so in a directory
-(created by them) <code>~/.userv/.servdata/</code><var>service</var><code></code>, where
-<var>service</var> is the service name or application in question.<P>
-
-The use of a dot-directory inside <code>~/.userv</code> will hopefully avoid
-the user becoming confused by finding parts of a semi-privileged
-application's internal state in their filespace, and or discourage
-them from fiddling with and thus corrupting it. (Note that such
-applications should of course not rely for their global integrity on
-the integrity of the data on the user's side of the security
-boundary.)
+(created by them) <samp>~/.userv/<var>service</var></samp>, where <var>service</var>
+is the service name or application in question.
+</p>
+
+<p>
+If desired, a dot-directory inside <samp>~/.userv</samp> may be used to
+avoid the user becoming confused by finding parts of a semi-privileged
+application's internal state in their filespace, and/or discourage
+them from fiddling with and thus corrupting it.
+</p>
+
+<p>
+However, <code>userv</code> applications should of course not rely for their
+global integrity and security on the integrity of the data on the
+user's side of the security boundary.
+</p>
+
<hr>
-<h2><A name="s-reducepriv">
-6.2 Reducing the number of absolutely privileged subsystems
-</A></h2>
+<h2>
+<a name="s-reducepriv">6.3 Reducing the number of absolutely privileged subsystems</a>
+</h2>
+
+<p>
Currently most Unix systems have many components which need to run as
root, even though most of their activity does not strictly require
it. This gives rise to a large and complex body of code which must be
-trusted with the security of the system.<P>
+trusted with the security of the system.
+</p>
+
+<p>
+If they were to use <code>userv</code>, many of these subsystems would no
+longer need any unusual privilege.
+</p>
-Using <kbd>userv</kbd> many of these subsystems no longer need any unusual
-privilege.<P>
+<p>
+<code>cron</code> and <code>at</code>, <code>lpr</code> and the system's mail transfer
+agent (<code>sendmail</code>, <code>smail</code>, <code>exim</code> or the like) all
+fall into this category, though <code>userv</code>-based versions of these
+programs are not currently available.
+</p>
-<kbd>cron</kbd> and <kbd>at</kbd>, <kbd>lpr</kbd> and the system's mail transfer
-agent (<kbd>sendmail</kbd>, <kbd>smail</kbd>, <kbd>exim</kbd> or the like) all
-fall into this category.
<hr>
-<h2><A name="s-noexcess">
-6.3 Do not give away excessive privilege to <kbd>userv</kbd>-using facilities
-</A></h2>
+<h2>
+<a name="s-noexcess">6.4 Do not give away excessive privilege to <code>userv</code>-using facilities</a>
+</h2>
+
+<p>
There is a danger that people reimplementing the facilities I mention
-above using <kbd>userv</kbd> will discard much of the security benefit by
+above using <code>userv</code> will discard much of the security benefit by
using a naive implementation technique. This will become clearer with
-an example:<P>
+an example:
+</p>
-Consider the <kbd>lpr</kbd> program. In current systems this needs to
+<p>
+Consider the <code>lpr</code> program. In current systems this needs to
have an absolutely privileged component in order to support delayed
printing without copying: when the user queues a file to be printed
the filename is stored in the print queue, rather than a copy of it,
and the printer daemon accesses the file directly when it is ready to
print the job. In order that the user can print files which are not
world-readable the daemon is given root privilege so that it can open
-the file in the context of the user, rather than its own.<P>
+the file in the context of the user, rather than its own.
+</p>
-A simple-minded approach to converting this scheme to use <kbd>userv</kbd>
-might involve giving the printer daemon (the <kbd>lp</kbd> user) the
-ability to read the file by allowing them to run <kbd>cat</kbd> (or a
-special-purpose file-reading program) as any user. The <kbd>lpr</kbd>
-program would use a <kbd>userv</kbd> service to store the filename in the
+<p>
+A simple-minded approach to converting this scheme to use <code>userv</code>
+might involve giving the printer daemon (the <code>lp</code> user) the
+ability to read the file by allowing them to run <code>cat</code> (or a
+special-purpose file-reading program) as any user. The <code>lpr</code>
+program would use a <code>userv</code> service to store the filename in the
printer daemon's queues, and the daemon would read the file later when
-it felt like it.<P>
+it felt like it.
+</p>
+<p>
However, this would allow the printer daemon to read any file on the
system, whether or not someone had asked for it to be printed. Since
many files will contain passwords and other security-critical
information this is nearly as bad as giving the daemon root access in
the first place. Any security holes in the print server which allow a
-user to execute commands as the <kbd>lp</kbd> user will give the user the
-ability to read any file on the system.<P>
+user to execute commands as the <code>lp</code> user will give the user the
+ability to read any file on the system.
+</p>
+<p>
Instead, it is necessary to keep a record of which files the daemon
has been asked to print <em>outside</em> the control of the print daemon.
This record could be kept by a new root-privileged component, but this
is not necessary: the record of which files a user has asked to be
printed can be kept under the control of the user in question. The
-submission program <kbd>lpr</kbd> will make a record in an area under the
+submission program <code>lpr</code> will make a record in an area under the
user's control before communicating with the print server, and the
print server would be given the ability to run a special file-reading
program which would only allow files to be read which were listed in
-the user's file of things they'd asked to print.<P>
+the user's file of things they'd asked to print.
+</p>
+<p>
Now security holes in most of the printing system do not critically
affect the security of the entire system: they only allow the attacker
to read and interfere with print jobs. Bugs in the programs run by the
print server to read users' files (and to remove entries from the list
of files when it has done with them) will still be serious, but this
-program can be quite simple.<P>
+program can be quite simple.
+</p>
-Similar considerations apply to many <kbd>userv</kbd>-based versions of
-facilities which currently run as root.<P>
+<p>
+Similar considerations apply to many <code>userv</code>-based versions of
+facilities which currently run as root.
+</p>
+<p>
It is debatable whether the user-controlled state should be kept in
the user's filespace (in dotfiles, say) or kept in a separate area set
aside for the purpose; however, using the user's home directory (and
-probably creating a separate subdirectory of it as a dotfile to
-contain many subsystems' state) has fewer implications for the rest of
-the system and makes it entirely clear where the security boundaries
-lie.
+possibly creating a separate subdirectory of it as a dotfile to
+contain subsystem state) has fewer implications for the rest of the
+system and makes it entirely clear where the security boundaries lie.
+</p>
+
<hr>
-<h2><A name="s-notreally">
-6.4 <kbd>userv</kbd> is not a replacement for <kbd>really</kbd> and <kbd>sudo</kbd>
-</A></h2>
-<kbd>userv</kbd> is not intended as a general-purpose system
+<h2>
+<a name="s-notreally">6.5 <code>userv</code> can often replace <code>sudo</code>, but not <code>really</code></a>
+</h2>
+
+<p>
+<code>userv</code> is not intended as a general-purpose system
administration tool with which system administrators can execute
-privileged programs when they need to. It is unsuitable for this
-purpose precisely because it enforces a strong separation between the
-calling and the called program, which is undesirable in this context.<P>
-
-Its facilities for restricting activities to running certain programs
-may at first glance seem to provide similar functionality to
-<kbd>sudo</kbd><A href="footnotes.html#2" name="fr2">[2]</A>. However, the
-separation mentioned above is a problem here too, particular for
-interaction - it can be hard for a <kbd>userv</kbd> service program to
-interact with its real caller or the user in question.
+arbitrary programs like text editors as root (or other system users)
+when they need to. It is unsuitable for this purpose precisely
+because it enforces a strong separation between the calling and the
+called program, which is undesirable in this context.
+</p>
+
+<p>
+However, its use when restricted to running particular programs in
+particular ways is very similar to many common uses of
+<code>sudo</code><a href="footnotes.html#2" name="fr2">[2]</a>. <code>userv</code> is
+generally much better than restricted <code>sudo</code>, because it protects
+the called program much more strongly from bad environmental
+conditions set up by the caller. Most programs that one might want to
+run via restricted <code>sudo</code>, have not been designed to run in a
+partially hostile environment. <code>userv</code> allows these programs to
+be run in a safer environment and should be used instead.
+</p>
+
+<hr>
+
+<h2>
+<a name="s-stdinerr">6.6 Error handling and input streams (eg stdin)</a>
+</h2>
+
+<p>
+When the service program is reading from a file descriptor connected
+to the calling side, the fd that the service program refers to a pipe
+set up by <code>userv</code> and not to the same object as was presented by
+the caller.
+</p>
+
+<p>
+Therefore if there is some kind of error it is possible for the
+service-side fd to give premature end of file. If it is important to
+tell whether all of the intended data has been received by the service
+program, the datastream must contain an explicit end-of-file
+indication of some kind.
+</p>
+
+<p>
+For example, consider a <code>userv</code> service for submitting a mail
+message, where message is supplied on the service's stdin. However,
+if the calling process is interrupted before it has written all of the
+message, the service program will get EOF on the message data. In a
+naive arrangement this would cause a half-complete message to be
+sent. To prevent this, it is necessary to adopt some kind of explicit
+end indication; for example, the end of the message could be signalled
+by a dot on a line by itself, and dots doubled, as in SMTP. Then the
+service program would know when the entire message had been received,
+and could avoid queueing incomplete messages.
+</p>
+
<hr>
-<h2><A name="s-nogeneral">
-6.5 Don't give access to general-purpose utilities
-</A></h2>
-Do not specify general purpose programs like <kbd>mv</kbd> or <kbd>cat</kbd>
-in <kbd>execute-</kbd> directives without careful thought about their
-arguments, and certainly not if <kbd>no-suppress-args</kbd> is specified.
+<h2>
+<a name="s-nogeneral">6.7 Don't give access to general-purpose utilities</a>
+</h2>
+
+<p>
+Do not specify general purpose programs like <code>mv</code> or <code>cat</code>
+in <code>execute-</code> directives without careful thought about their
+arguments, and certainly not if <code>no-suppress-args</code> is specified.
If you do so it will give the caller much more privilige than you
-probably intend.<P>
+probably intend.
+</p>
+<p>
It is a shame that I have to say this here, but inexperienced
administrators have made similar mistakes with programs like
-<kbd>sudo</kbd>.
+<code>sudo</code>.
+</p>
+
+<hr>
+
+[<a href="ch-ipass.html">back</a>]
+ [<a href="index.html#abstract">Abstract</a>]
+ [<a href="index.html#copyright">Copyright Notice</a>]
+ [<a href="index.html#contents">Contents</a>]
+
<hr>
-User service daemon and client specification
-- <A href="index.html#copyright"><kbd>userv</kbd> is Copyright 1996-1999 Ian Jackson.</A>
-<br>
-<A href="index.html#toc">Contents</A>; <A href="index.html#abstract">abstract</A>; <A href="ch-ipass.html">back</A>.
-<br>
-<address>0.61.4<br>
-Ian Jackson <A href="mailto:ian@davenant.greenend.org.uk">ian@davenant.greenend.org.uk</A></address>
-</body></html>
+
+User service daemon and client specification<br>
+
+<address>
+1.0.1<br>
+Ian Jackson <a href="mailto:ian@davenant.greenend.org.uk">ian@davenant.greenend.org.uk</a>
+</address>
+
+</body>
+
+</html>
+
~ian / userv.git / blobdiff