boinc/doc/sandbox.php

<?php
require_once("docutil.php");
page_head("Sandbox design");

echo "
This document describes a proposed modification to the Unix
(including Linux and Mac OS X) versions of BOINC.
The goal of this change is to 'sandbox' BOINC applications,
i.e. to limit the amount of damage that a malicious
or malfunctioning application can cause.
<p>
In our design, BOINC applications run under a specially-created account
having a minimal set of privileges.
Previously, the applications typically ran as the user who installed BOINC,
and had the full privileges of that account.
";

function prot($user, $group, $perm) {
    return "
    <br>
    &nbsp;&nbsp; user: $user
    <br>
    &nbsp;&nbsp; group: $group
    <br>
    &nbsp;&nbsp; protection: $perm";
}

$pp06610771 = prot('boinc_project', 'boinc_project', '0661 or 0771');
$pp06640775 = prot('boinc_project', 'boinc_project', '0664 or 0775');
$mp2500 = prot('boinc_master', 'boinc_project', '0500+setgid');
$rm4050 = prot('root', 'boinc_master', '0050+setuid');
$rm4555 = prot('root', 'boinc_master', '0555+setuid');
$mm0550 = prot('boinc_master', 'boinc_master', '0550');
$mm0440 = prot('boinc_master', 'boinc_master', '0440');
$mm0660 = prot('boinc_master', 'boinc_master', '0660');
$mm0771 = prot('boinc_master', 'boinc_master', '0771');
$mm0775 = prot('boinc_master', 'boinc_master', '0775');
$mp0775 = prot('boinc_master', 'boinc_project', '0775');
$mp06610771 = prot('boinc_master', 'boinc_project', '0661 or 0771');
$mp06640775 = prot('boinc_master', 'boinc_project', '0664 or 0775');
$mm2555 = prot('boinc_master', 'boinc_master', '0555+setgid');
$mm6555 = prot('boinc_master', 'boinc_master', '0555+setuid+setgid');
$ua0555 = prot('(installing user)', 'admin', '0555');

$colors = array('ddddff', 'ccccff', 'bbbbff');

function show_dir($level, $name, $prot, $contents) {
    global $colors;
    $color = $colors[$level];
    $x = "
        <table bgcolor=$color cellpadding=6 cellspacing=0 border=1 width=100%>
        <tr>
            <td valign=top><b>$name</b> <font size=-1>$prot</font></td><td valign=top>
    ";
    for ($i=0; $i<sizeof($contents); $i++) {
        if ($i) $x .= '<br>';
        $c = $contents[$i];
        $x .= $c;
    }
    $x .= "
        </td></tr>
        </table>
    ";
    return $x;
}

function show_file($name, $prot) {
    return "
            $name <font size=-1>$prot</font><br>
    ";
}

echo "

<p>
Our design uses two users and two groups,
both specially created for use by BOINC.
These users and groups are created by the installation process.
<ul>
<li>Group: <b>boinc_master</b>
<li>Group: <b>boinc_project</b>
<li>User: <b>boinc_master</b>
<ul>
<li>Primary group: <b>boinc_master</b>
<li>Supplementary groups: none
</ul>
<li>User: <b>boinc_project</b>
<ul>
<li>Primary group: <b>boinc_project</b>
<li>Supplementary groups: none
</ul>
</ul>
On Mac OS X, <b>boinc_project</b> and <b>boinc_master</b>
are added to the Supplementary Groups Lists of those other users
who are members of group <b>admin</b>.
This gives admin users full access to all BOINC and project files.
<p>
The following diagram shows user, group and permissions
for the BOINC file and directory tree:
<p>
";

echo
    show_dir(0, 'BOINC data', $mm0771, array(
        show_dir(1, 'projects', $mm0775, array(
            show_dir(2, 'setiathome.berkeley.edu', $mp0775, array(
                show_file('files created by BOINC Client', $mp06610771),
                show_file('files created by project apps', $pp06610771)
            ))
        )),
        show_dir(1, 'slots', $mm0775, array(
            show_dir(2, '0', $mp0775, array(
                show_file('files created by BOINC Client', $mp06610771),
                show_file('files created by project apps', $pp06610771)
            ))
        )),
        show_dir(1, 'switcher (directory)', $mm0550, array(
            show_file('switcher (executable)', $rm4050),
            show_file('setprojectgrp (executable)', $mp2500)
        )),
        show_dir(1, 'locale', $mm0550, array(
            show_dir(2, 'de', $mm0550, array(
                show_file('BOINC Manager.mo', $mm0440),
                show_file('wxstd.mo', $mm0440)
            ))
        )),
        show_file('account_*.xml', $mm0660),
        show_file('acct_mgr_login.xml', $mm0660),
        show_file('client_state.xml', $mm0660),
        show_file('gui_rpc_auth.cfg', $mm0660),
        show_file('sched_reply*', $mm0660),
        show_file('sched_request*', $mm0660)
    ));

echo "<br><br>";

echo
    show_dir(0, 'BOINC executables', $ua0555, array(
        show_file('BOINC Manager', $mm2555),
        show_file('BOINC Client', $mm6555),
        show_dir(1, 'screensaver (directory)', $ua0555, array(
            show_file('gfx_switcher (executable)', $rm4555)
        )),
    ));
    
echo "

<p>Implementation notes:

<ul>
<li>BOINC Client runs setuid and setgid to <b>boinc_master:boinc_master</b>.  
<li>BOINC Client uses the helper application <i>setprojectgrp</i> to 
set project and slot files and directories to group <b>boinc_project</b>.  
<li>BOINC Client does not directly execute project applications.
It runs the helper application <i>switcher</i>, 
passing the request in the argument list.
<i>switcher</i> runs setuid <b>root</b> and immediately changes its real and 
effective user ID and group ID to <b>boinc_project</b>,
so all project applications inherit user and group <b>boinc_project</b>.  
This blocks project applications from accessing unauthorized files.
<li>In most cases, it is best to avoid running setuid <b>root</b> because 
it can present a security risk.  In this case, however, this is necessary to 
<i>reduce</i> the risk because only the superuser can change the <i>real</i> 
user and group of a process.  This prevents a malicious or malfunctioning 
application from reverting to the user and group who launched BOINC, since any 
process can change its user and group back to the <i>real</i> user and 
group IDs.
<li>BOINC's use of setuid <b>root</b> for the <i>switcher</i> application is 
safe because:
<ul>
<li>The <i>switcher</i> application is inside the <i>switcher</i> directory.
This directory is accessible only by user and group <b>boinc_master</b>,
so that project applications cannot modify the <i>switcher</i> 
application's permissions or code.  This also prevents unauthorized users 
from using <i>switcher</i> to damage or manipulate project files.
<li>The <i>switcher</i> application is readable and executable only by 
group <b>boinc_master</b>; all other access is forbidden.
<li>When it is run, the <i>switcher</i> application immediately changes 
its real and effective user ID and group ID to <b>boinc_project</b>, disabling 
its superuser privileges.
</ul>
<li>BOINC Manager runs setgid to group <b>boinc_master</b>.
It can access all files in group <b>boinc_master</b>.  
It runs as the user who launched it,
which is necessary for a number of GUI features to work correctly.  
Although this means that BOINC Manager cannot modify files
created by project applications, there is no need for it to do so.  
<li>Starting with BOINC version 6.0, project science applications use a 
separate companion application to display graphics.  These graphics 
applications are launched by the BOINC Manager when the user clicks on 
the <i>Show Graphics</i> button.  Running the graphics application 
with the BOINC Manager's user and group would be a security risk, so 
BOINC Manager uses the <i>switcher</i> application to launch them as 
user and group <b>boinc_project</b>. 
<li>The screensaver also can run the graphics applications.  The Macintosh 
screensaver is launched by the operating system, so it runs as the 
currently logged in user and group.  Since running the science projects' graphics applications 
with this user and group would be a security risk, the screensaver has 
its own embedded helper application <i>gfx_switcher</i> which it uses to 
launch the graphics applications.  
Like the <i>switcher</i> application, <i>gfx_switcher</i> runs setuid 
<b>root</b> and immediately changes its real and effective user ID and 
group ID to <b>boinc_project</b>
<li>The BOINC screensaver's use of setuid <b>root</b> for the 
<i>gfx_switcher</i> application is safe because:
<ul>
<li>When it is run, the <i>gfx_switcher</i> application immediately changes 
its real and effective user ID and group ID to <b>boinc_project</b>, disabling 
its superuser privileges.
<li>The <i>gfx_switcher</i> application has very limited functionality.  It 
accepts only two commands as its first argument:.  
<ul>
<li><i>launch_gfx</i>: the second argument is the slot number.  It looks for 
a soft-link named <b>graphics_app</b> in the specified slot directory and launches 
the referenced graphics application.
<li><i>kill_gfx</i>: the second argument is the process ID.  It kills the 
application with the process ID; since it is running as user and group 
<b>boinc_project</b>, it can affect only processes belonging to that user.
</ul>
</ul>
<li>BOINC Client sets its umask to 006 to hide account keys from unauthorized 
users.  This means that third-party add-ons cannot read BOINC data files; they 
must use GUI RPCs to access BOINC Data.  
<li>BOINC sets the umask for project applications to 002; the default permissions 
for all files and directories they create prevent modification outside the 
<b>boinc_project</b> user and group.  Files written by projects are world-readable so 
that the BOINC Client can read them.
<li>Non-admin users cannot directly modify BOINC or project files.
They can modify these files only by running the BOINC Manager and Client.  
<li>Users with admin access are members of groups <b>boinc_master</b>
and <b>boinc_project</b> so that they do have 
direct access to all BOINC and project files
to simplify maintenance and administration.
<li>The RPC password file <i>gui_rpc_auth.cfg</i>
is accessible only by user and group <b>boinc_master</b>.
In other words, only BOINC Manager, BOINC Client and
authorized administrative users can read or modify it,
limiting access to most BOINC RPC functions.
<li>BOINC Manager restricts certain functions to authorized users:
Attach to Project, Detach from Project, Reset Project, Abort Task,
Abort Transfer, Update Account Manager.  
If an unauthorized user requests these functions,
the Manager requires password authentication.
<li>On Macintosh computers, the actual directory structures
of the BOINC Manager application bundle and the screensaver bundle are 
more complex than implied by the box <i>BOINC executables</i> in the 
BOINC tree diagram shown above.
<li>Some Macintosh system administrators may wish to limit which users
can perform BOINC Manager functions (Activity Menu, etc.).
This can be done by moving BOINC Manager out of the
<b>/Applications</b> directory into a directory with restricted access.
</ul>
</p>
";

page_tail();
?>