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<section id="security">
<title>Security</title>
<para>
This section discusses security: avoiding downtime, avoiding revealing
sensitive information, avoiding unwanted modifications to the data;
either through accident or malice.
A good introduction to secure programming can be found in
<ulink url="http://www.dwheeler.com/secure-programs/Secure-Programs-HOWTO/">
<citetitle>
Secure Programming for Linux and Unix HOWTO</citetitle></ulink>.
</para>
<para>
For <application>yaird</application>, security is not very
complicated: although it runs with root privileges, the program is
not setuid, and all external input comes from files or programs
installed by the admnistrator, so our main focus is on avoiding
downtime caused by ignored error codes.
A full blown risk assessment would be overkill, so we'll just use
the HOWTO as a checklist to verify that the basic precautions are
in place.
</para>
<para>
<informaltable frame='topbot'>
<tgroup cols='3' align='left'>
<thead>
<row>
<entry>Group</entry>
<entry>Mitigation</entry>
<entry>Status</entry>
</row>
</thead>
<tbody valign='top'>
<row>
<entry>Bad input</entry>
<entry>Verify command line</entry>
<entry>
Yes.
</entry>
</row>
<row>
<entry>Bad input</entry>
<entry>Verify and clean up environment</entry>
<entry>
Complete environment is reset at start of program.
</entry>
</row>
<row>
<entry>Bad input</entry>
<entry>Avoid assumptions about file descriptors</entry>
<entry>
Handled by perl.
</entry>
</row>
<row>
<entry>Bad input</entry>
<entry>Verify file names</entry>
<entry>
Perl taint check shows filenames are verified for
absence of odd characters before passing to
subprocesses.
TODO: examine UTF-8 impact.
</entry>
</row>
<row>
<entry>Bad input</entry>
<entry>Verify file content</entry>
<entry>
File contents in sysfs verified.
Fstab entries properly quoted.
TODO: check for spaces in names of LVM volume or of
modules; could end up in generated /sbin/init.
</entry>
</row>
<row>
<entry>Bad input</entry>
<entry>Verify locale settings</entry>
<entry>
All locale related environment variables are wiped at
program startup.
</entry>
</row>
<row>
<entry>Bad input</entry>
<entry>Verify character encoding</entry>
<entry>
All IO is byte oriented.
</entry>
</row>
<row>
<entry>Bad input</entry>
<entry>Buffer overflow</entry>
<entry>
In perl?
</entry>
</row>
<row>
<entry>Program structure</entry>
<entry>Separate data and control</entry>
<entry>
Under this heading, the HOWTO discusses the dangers of
auto-executing macros in data files. The closest thing we
have to a data file are the templates that tune the image
to the distribution. We use a templating language that
does not allow code embedding, and the image generation
module does not make it possible for template output to
end up outside of the image. Conclusion: broken templates
can produce a broken image, but cannot affect the running
system.
</entry>
</row>
<row>
<entry>Program structure</entry>
<entry>Minimize privileges</entry>
<entry>
The user is supposed to bring his own root privileges to
the party, not much to be done here. A related issue
is the minimizing of privileges in the system that is
started with the generated image. This would include
starting SELinux at the earliest possible moment.
At least in Fedora, that earliest possible moment is
in <filename>rc.sysinit</filename>, well past the moment
where the initial boot image hands over control to the newly
mount root file system. No <application>yaird</application>
support needed.
</entry>
</row>
<row>
<entry>Program structure</entry>
<entry>Safe defaults</entry>
<entry>
Configuration only specifies sources of information,
like /etc/hotplug, not much can go wrong here.
</entry>
</row>
<row>
<entry>Program structure</entry>
<entry>Safe Initialisation</entry>
<entry>
The location of the main configuration file is configured
as an absolute path into the application.
</entry>
</row>
<row>
<entry>Program structure</entry>
<entry>Fail safe</entry>
<entry>
Planning and writing the image is separated;
writing only starts after planning is succesfully completed.
Todo: consider backout on write failure.
</entry>
</row>
<row>
<entry>Program structure</entry>
<entry>Avoid race conditions</entry>
<entry>
Temporary files and directories are created
with the <code>File::Temp</code> module, which is
resistant to name guessing attacks.
The completed image is installed with <code>rename</code>
rather than <code>link</code>; if an existing file is
overwritten, this guarantees there's no race where the
old image has been deleted bu the new one is not yet in
place. (Note that there is no option in place yet which
allows overwriting of existing files.)
To do: examine File::Temp safe_level=HIGH.
</entry>
</row>
<row>
<entry>Underlying resources</entry>
<entry>Handle meta characters</entry>
<entry>
Protection against terminal escape sequences in output
is not yet in place.
</entry>
</row>
<row>
<entry>Underlying resources</entry>
<entry>Check system call results</entry>
<entry>
Yes.
</entry>
</row>
<row>
<entry>Language specific</entry>
<entry>Verify perl behaviour with taint.</entry>
<entry>
Yes.
</entry>
</row>
<row>
<entry>Language specific</entry>
<entry>Avoid perl open magic with 3rd argument.</entry>
<entry>
Yes.
</entry>
</row>
</tbody>
</tgroup>
</informaltable>
</para>
</section>
|
