This document describes the permissions structure for
BOINC on the Macintosh. It has been updated for BOINC versions 6.8.20 and 6.10.30 and later.
The purpose of this scheme is to 'sandbox' BOINC applications,
i.e. to limit the amount of damage that a malicious
or malfunctioning application can cause.
In our design, BOINC applications run under a specially-created account
having a minimal set of privileges.
In early versions of BOINC, the applications typically ran as the user who installed BOINC,
and had the full privileges of that account.
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.
Group: boinc_master
Group: boinc_project
User: boinc_master
Primary group: boinc_master
Supplementary groups: none
User: boinc_project
Primary group: boinc_project
Supplementary groups: none
On Mac OS X, boinc_project and boinc_master
are added to the Supplementary Groups Lists of those other users
who are members of group admin.
This gives admin users full access to all BOINC and project files.
The following diagram shows user, group and permissions
for the BOINC file and directory tree:
BOINC data
user: boinc_master
group: boinc_master
protection: 0771
BOINC Client runs setuid and setgid to boinc_master:boinc_master.
BOINC Client uses the helper application setprojectgrp to
set project and slot files and directories to group boinc_project.
BOINC Client does not directly execute project applications.
It runs the helper application switcher,
passing the request in the argument list.
switcher runs setuid root and immediately changes its real and
effective user ID and group ID to boinc_project,
so all project applications inherit user and group boinc_project.
This blocks project applications from accessing unauthorized files.
In most cases, it is best to avoid running setuid root because
it can present a security risk. In this case, however, this is necessary to
reduce the risk because only the superuser can change the real
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 real user and
group IDs.
BOINC's use of setuid root for the switcher application is
safe because:
The switcher application is inside the switcher directory.
This directory is accessible only by user and group boinc_master,
so that project applications cannot modify the switcher
application's permissions or code. This also prevents unauthorized users
from using switcher to damage or manipulate project files.
The switcher application is readable and executable only by
group boinc_master; all other access is forbidden.
When it is run, the switcher application immediately changes
its real and effective user ID and group ID to boinc_project, disabling
its superuser privileges.
As of BOINC Version 6.10.5, BOINC Manager no longer runs setgid to group
boinc_master, because Mac OS 10.6 does not allow it. So it can be run
only by users who are members of group boinc_master. By default, the
BOINC installer automatically adds all users who are members of group
admin to group boinc_master, and optionally adds non-admin
users to group boinc_master. The Manager 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.
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 Show Graphics button. Running the graphics application
with the BOINC Manager's user and group would be a security risk, so
BOINC Manager uses the switcher application to launch them as
user and group boinc_project.
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 gfx_switcher which it uses to
launch and kill the graphics applications.
Like the switcher application, gfx_switcher runs setuid
root and immediately changes its real and effective user ID and
group ID to boinc_project.
Starting with BOINC version 6.7, a default screenaver graphics application
is provided with BOINC. The screensaver (now more properly called the
screensaver coordinator) runs the default graphics alternating with science
graphics applications according to a schedule set by the data file ss_config.xml.
The default graphics are run also when no science graphics are available, such as
when BOINC is suspended. The default graphics executable is run as user and group
boinc_project.
The BOINC screensaver's use of setuid root for the
gfx_switcher application is safe because:
When it is run, the gfx_switcher application immediately changes
its real and effective user ID and group ID to boinc_project, disabling
its superuser privileges.
The gfx_switcher application has very limited functionality. It
accepts only three commands as its first argument:
launch_gfx: the second argument is the slot number. It looks for
a soft-link named graphics_app in the specified slot directory and launches
the referenced graphics application as user and group boinc_project.
default_gfx: launches the default graphics application boincscr
in the BOINC data directory as user and group boinc_project.
kill_gfx: the second argument is the process ID. It kills the
application with the process ID; since it is running as user and group
boinc_project, it can affect only processes belonging to that user.
This is used to exit all screensaver graphics applications.
To hide account keys from unauthorized users, BOINC Client sets its umask
to 006 and (as of versions 6.8.20 and 6.10.30) makes all *.xml files at the top level
directory not world-readable (except ss_config.xml, which must be read by the
screensaver coordinator). This means that third-party add-ons cannot read BOINC
data files; they must use GUI RPCs to access BOINC Data.
BOINC sets the umask for project applications to 002; the default permissions
for all files and directories they create prevent modification outside the
boinc_project user and group.
Files written by projects are world-readable so that the BOINC Client can read
them. But, starting with BOINC versions 6.8.20 and 6.10.30, the slots directory and the projects
directory are executable (traversable) only by user boinc_master and group
boinc_projects, to prevent unauthorized users from reading account keys from the
init_data.xml files.
Unauthorized users cannot modify BOINC or project files.
Users with admin access are members of groups boinc_master
and boinc_project so that they do have
direct access to all BOINC and project files
to simplify maintenance and administration.
The RPC password file gui_rpc_auth.cfg
is accessible only by user and group boinc_master.
In other words, only BOINC Manager, BOINC Client and authorized
users can read or modify it, restricting access to those BOINC RPC functions
which modify BOINC's operation.
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 BOINC executables in the
BOINC tree diagram shown above.
Some Macintosh system administrators may wish to further limit which users
can perform BOINC Manager functions (Activity Menu, etc.).
This can be done by moving BOINC Manager out of the
/Applications directory into a directory with restricted access.
Important information for project developers: The BOINC
installer traverses the BOINC Data directory and sets the users, groups and
permissions of all files as shown in the above table. This allows it to
repair corrupted permissions. Note that the BOINC installer will
change all files and subdirectories in the projects directory, the slots directory, and
all their subdirectories to user boinc_master and
group boinc_project. This means that:
If a project file needs to be
executable by another project file, its executable-by-group permission bit
must be set. The BOINC installer will not alter the
executable-by-user and executable-by-group permission bits of files in these
directories (though it will set these bits for the directories themselves.)
It is critical that all files and subdirectories under the projects
and slots directories have both their read-by-owner and read-by-group
permission bits set, and that all subdirectories have both their
executable-by-owner and executable-by-group permission bits set.