System hardening will proceed in roughly three phases. The first phase applies comprehensive userland isolation using the two main containment options.
Phase 1 focuses largely on putting existing services (and user logins)
in to SECURE_NOROOT+namespaced jails with as little system impact as
possible. This can be done by tweaking initialization scripts and
modifying file extended attributes. In particular, this will be done
in our custom upstart configuration files and other boot scripts.
It turns out that this is pretty easy as long as we don't focus on making all of Xorg non-root-based. For example, there is a command run by the login manager upon successful user authentication:
/sbin/capsh --secbits=0x2f --drop=[all] -- --login /etc/X11/Xsessionrc %session
in order to run the user's session in SECURE_NOROOT, if we were using the capsh tool (capabilities-aware shell).
However, since we have minijail, we can use it to do more than just run in SECURE_NOROOT:
This will dump the new process in its own namespace where minijail
acts as pid 1. The entire X session will only be able to see a /proc
that is related to its pid namespace, and it will have access only to
devices enabled for the cgroup: chromeuser. No SUID binaries or
binaries with any additional extended capability attributes set will be
executable in this session.
The biggest impact is that when it comes time to perform screen
unlocking, the xscreensaver process will not be able to do anything
privileged.
With this Big Note in mind, we can look back at our
slim.conf. If we find that we need to launch some processes with some
privileges, we can tone down how aggressive the first call to minijail
is. It can set up the namespace and lock down root, but it can leave the
bounding set a bit wider; that way, we can launch utilities that may
need capabilities like pulseaudio. This can be done inside the
Xsessionrc by calling minijail on all subsequent binaries with specific
bounding set changes, etc. They can all, thankfully, live in their new
pid namespace unless we lock them down further (chroot, etc).
Initially, we'll start with the above configuration and tweak as
problems are introduced.
Control groups (cgroups) will be used to segment the population with
respect to device access and resource utilization. To that end, we can
preconfigure a few control groups at start via a simple
/etc/init.d/cgroups script:
With that done, we can leave it to minijail invocations to add the pid
to the /cgroups/<cgroup>/task file. The biggest challenge will
be nested cgroups like chrome/sandbox, since we will not mount /cgroups
in the chrome user namespace and users will be unable to see the
/cgroups file system. In Phase 1, we'll just have to let renderers live
in the chrome cgroup and hope for the best. Segmenting chrome user
processes from the system services should be enough to guarantee that a
Chromium browser CPU DoS won't peg the system too badly, but we'll see. If it
can tie up xorg, the user experience will be the same. However, in
Phase 2, we will introduce a cgroupsd daemon. This daemon will monitor
new process creation (via a TBD mechanism with _low_ power/cpu needs)
and automatically add them to the appropriate cgroup.
Devices will be added quite simply with:
echo 'c 1:3 mr' > /cgroups/1/devices.allow
Memory per group can be determined based on system memory. Below, limit chrome to using 80 percent of available memory:
total_mem=$(free -b | grep Mem | tr -s ' ' | cut -f2 -d' ')
echo $((total_mem / 5 * 4)) > /cgroups/user/chrome/memory.limit_in_bytes
CPU usage can also be determined using the system total, which
is available in cpu.shares. Below, we give all chrome processes 80 percent of
the CPU shares:
total_cpu=$(cat /cgroups/cpu.shares)
echo $((total_cpu / 5 * 4)) > /cgroups/user/chrome/cpu.shares
Of
course, we can tweak the total number of shares to make specific
allocations. The allocations should then be used for fair scheduling.
Longer term, we may be able to use 'freezer' support to freeze all
processes prior to suspend or use cpusets to ensure that the Chromium browser, or perhaps even an
extension, is privately allocated an entire CPU core (using
cpusets).
In addition, if any of these items imply too much overhead, it is
possible to achieve similar (and even more focused) results using
RLIMITS.
Locking down existing daemons with extended attributes and a bit of luck
At present, there are a number of processes running as root:
- SLiM and X: both run with privilege. SLiM starts Xorg, and Xorg needs ioctl and ioperm access, which equates to root access in
most cases. We will explore a non-root Xorg in Phase 2.
-
connman will need CAP_NET_ADMIN, CAP_NET_RAWIO at most in its bounding
set so that they may be used by properly annotated files: ping,
dhclient, wpa_supplicant.
-
dhclient is used for acquiring a network address and configuring an
existing network device. It needs to broadcast UDP and change network
device parameters.
-
wpa_supplicant is used for configuring the wireless device to properly associate with wireless access points.
-
acpid handles power management and other events: lid close, low battery, etc.
-
getty handles the standalone console, which will most likely be disabled in nondeveloper installs.
-
udev handles firmware loading and hot-pluggable device.
The final goal is to move to a pure capability-based system which means that no service should need
root access. To this end, we'll need to modify the startup process for
these daemons. The easiest approach is to just wrap their
start-stop-daemon calls with calls to minijail, either in the
control panel or in /etc/init.d. Each one should get its own
namespacing with chroot'ing if possible. (If
it seems difficult to chroot a specific binary, then we should consider
doing so with the Chromium OS project's LinuxSUIDSandbox.) Capabilities required will be determined using strace | grep 'EPERM|EACCESS'
while locking down the binaries. Each binary will have the
capabilities it needs added to its extended attributes. For
example, dhclient needs
CAP_NET_BIND_SERVICE|CAP_NET_BROADCAST|CAP_NET_ADMIN:
capset cap_net_bind_service,cap_net_broadcast,cap_net_admin=ep /sbin/dhclient3
Then, dhclient will be called with minijail dropping all
capabilities except those three (and cap_setcaps if needed). If
dhclient is called from connman, then that process can already be
running with a restricted capability bounding set.
At present, our root file system supports extended attributes. If
we move to squashfs or another file system without xattr, we will have to
work around the restriction. This can be done using the patch referenced
in the Technology section or by mounting a loopback file system with the
desired binaries with the appropriate xattrs.
Supporting development mode before we finish designing it
Once we dump the user in SECURE_NOROOT land, sudo and su become
useless. To allow continued tinkering, we can use the secondary
console if enabled or a loopback ssh daemon. As long as we don't lock
these alternative entry points the same way as the primary user session, they will be perfectly sane ways to
implement a secure but useful development mode.
Adding fine-grained controls over coarse-grained capabilities with mandatory access controls
For our Mandatory Access Control (MAC) needs, we are considering the external grsecurity kernel patch as well as the currently in-kernel Tomoyo module and accompanying tomoyo-ccstools
package. In either case, a similar approach applies. Once installed, an initial policy
can be configured using learning mode. We can then enable enforcement
on process trees which shouldn't change: dhclient, wpa_supplicant,
etc. The Chromium browser itself will likely run in a permissive
mode as we explore extensions and other changes. However, as we
approach releases, we can configure a final policy using learning mode
both with active users and through automated testing that exercises
all the expected use cases for the system.
The Tomoyo tool for editing the policy is ccs-editpolicy.
Initially, there should be no need to perform any special customization
other than converting process trees from disabled to learning or
permissive. In addition, we will need to make sure that the
development mode runs in permissive or disabled mode.
Locking down the file system (discretionary access controls)
Discretionary access controls have been satisfactory at
implementing basic security in Linux for years. An audit of the root file system will ensure that no end user can read
files or directories he has no need to read, execute files he has no need to access, or chdir to directories he has no need to enter. This effort will be
further expanded by the Phase 2 efforts around changing file ownership
to limit root's discretionary access—even when its capabilities have
been revoked.
In addition to the normal file system, /dev and /proc can be dangerous
for an unprivileged root user. While we are filtering devices
per-cgroup, we should ensure that CONFIG_STRICT_DEVMEM is set to limit
/dev/mem usefulness. If we patch Xorg, we may be able to get rid of
/dev/mem entirely. In addition, a review of what is available in /proc
and /dev for each group will be crucial. Whenever we place a process
tree in a new VFS, we can mount --bind in only the files we want. This
is harder with /proc, but doable if needed. /proc may be remounted
read-only at the very least. We may be able to make use of the Linux
VServer's 'setattr' tool to hide /proc entries on namespace mount. If
so, this would be done in the minijail code but would require that we
support the vserver kernel patches. However, namespacing and
chroot'ing will hopefully cover a lot of ground.
User home directories and the /home partition should be mounted nosuid,
nodev, and ideally, noexec. We should attempt to limit user access to
included scripting engines if possible to to aid in enforcing noexec
(dash, bash, or any others).
Deploying the firewall
The netfilter/iptables infrastructure provides a number of interesting
approaches for limiting network access, both inbound and outbound.
While a basic inbound-only policy will work for general TCP and UDP
level attack protection, it would be nice to limit the OUTPUT chain as
well. We can also consider using network namespaces and VETH devices,
but for Phase 1, the added complexity and potential robustness issues
make it questionable (see Phase 2 for more detail).
Xorg without the root user
On a testing system, we have restricted Xorg to CAP_SYS_RAWIO and seen everything work
except ioctl() access to the graphics device. We believe there to be patches that deal with this, but getting them working within our codebase may require some work. In
addition, we can't just drop privilege because that will make returning
from suspend quite painful.
SLiM and pam_google with limited capabilities
Any privileged behavior needed by pam_google will need to be moved into a
standalone daemon. For instance, any encrypted volume management will
need to be shifted into a daemon that handles it for the user. A
simple daemon that checks SO_PEERCRED and the current mount state
would do the trick.
Phase 2: Diving deeper
Phase 2 is where we start making changes that are farther reaching.
cgroupsd
cgroupsd is a simple daemon that will automagically add new processes
to a control group specified in a libcg-style configuration file. The
only useful design point is that it will do so by using
CONFIG_CONNECTOR and CONFIG_PROC_EVENTS to be notified by the kernel of
new processes via netlink. It will need access to the /cgroups mountpoint, but
otherwise, will not need any additional privileges. In particular, if cgroupsd is used
only for Chromium browser sandbox processes, it can run with privileges to modify only /cgroups/user/chrome/sandbox/tasks.
cgroupsd will also be used to enforce device filtering and resource management on plugins like Flash.
Other local system management daemons (network, sound, removable storage, ?)
The further we lock down the user's session, the more work we'll
have to put in to brokering access to system resources. The control panel approach is
great for system management brokering (via a network loopback), but we
will need to make sure that udev and hal access can be handled safely.
We're probably also going to see some pain with Adobe Flash and other
binary plugins unless we give them access. Reviewing and integrating
plugins with this design will be critical to avoiding introducing a
trivial backdoor through the protections.
Put browser instances in their own namespaces
Chromium browsers can make use of the measures deployed in Phase 1. If the
existing sandbox isn't isolating rendered processes in their own pid
namespace, then it should be done here. In addition, every browser
process itself can be dropped into its own namespace when launched.
Applying net namespaces
One possible approach to isolating processes further is putting them all in their own
net namespace.
We can then expose to the system a virtual interface with a virtual, internal, IP address. We could even optionally enforce userland proxy use (for truly dodgy
inmates). However, this may introduce robustness issues if a user is
assigned a physical address that is the same or in the same netmask as
the virtual ones. Given that we can't control the eth0 address, we have
delayed pursuing this until Phase 2. When we get there, it will be worth
investigating and deploying if possible to keep any process from being
able to bind to an external port.
Re-chowning the file system, or why root shouldn't own everything.
Even in a SECURE_NOROOT environment, root still has discretionary
access control to a large number of files and devices, and that access may be
used to escalate privileges or make system changes. One possible
approach is to create multiple system accounts that are responsible
for files along some logical domain. One option would be to use a var
user and a home user and a bin user for each of the areas they may
own. A privileged user can always override such discretionary access control mechanisms, but it will stop any
accidental root access from being completely detrimental.
That
change may be overkill or add more complexity than the gain. Another
option would be to add a new secure bit along the lines of
SECURE_UNSAFE. If that secure bit is set on a tree, then no process in
the tree can change to UID/GID 0.
Device interposition
Given
that Chromium browsers and plugins need access to the webcam and audio (in
and out), it's important to isolate the kernel device drivers from
software that may be attacker controlled. To this end, access to
/dev/video0 will be brokered via a userland daemon. The work may be
based on
GSTFakeVideo.
Not only will this avoid direct attacks on random webcam drivers, it
will also mean we can later offer an interface for doing real-time
video stream filtering: custom effects, etc.
In addition to
/dev/video, we'll want to position userland code for audio interception
(e.g, /dev/dsp, etc). This can be done using something like esound or
pulseaudio. If we go with one of those daemons anyway, we can get this for free.
After
the audio/video experience, we're left with one major exposed surface
for plugins which require video card device access. Since we will want
to support accelerated 3D and other fast rendering, we'll be exposing
(possibly binary-only) video card drivers via X/DRI. This is a larger
problem that will be addressed in a more detailed design document on
the issue.
Monitoring
If we can monitor our system for any clear signs of compromise without
seriously affecting battery life or performance, then we should! This
area should be branched out into a full standalone design document.
But within the scope of this document, it is worth considering a very
simple system.
A single daemon can monitor process creation and uid changes
via the proc events kernel interface. If it sees any process become
uid/euid==0, then we have someone running a privilege escalation
exploit. If we determine an exceptional-event user interface or a
reboot path that will notify the user to put-a-paperclip-in to reset the device, then we
can trigger it immediately. While an exploit can target this behavior,
it is just one more layer of defense.
The
Linux Auditing Framework may be very useful for doing detection, but
its cost may outweigh the benefit. Since we are not expecting a huge
number of process creation events, we can monitor system calls ranging
from ptrace to clone(2) to fork. If we avoid high traffic system
calls, we should be able to enforce some basic system call detection
without sandboxing explicitly. In addition, if we don't use auditd,
but instead a
custom listener,
we can immediately react to an event—such as terminating the calling
process or triggering a reboot into the recovery system.
Phase 3: Don't forget your snorkel!
Phase 3 is where we get to explore additional innovations in the security space that will require the most long-term investment. We're excited about the possibilities around integrating new kernel and user space hardening techniques and figuring out how to properly isolate drivers in kernel-space.
Here are some of the ideas we have, but there is a lot of area to research:
- Retrofit /sbin/init and remove root everywhere.
- Isolate all running Xorg windows in Xephyr transparently to mitigate keystroke theft, etc. (is the benefit worth the extra code?)
- Custom Linux Security Module for doing nested runtime sandboxes.
- Device driver security: We need to analyze device drivers and determine a plan for isolation and or robustness.
- Using KLEE or other automated dynamic analysis suite
- Consider vt-d/trustzone approaches for driver isolation (l4linux, 64-bit friendly KERNSEAL/KERNEXEC?, Nooks)
- Harden the kernel heap management.
- Chromium-browser-based isolation of user data and processes by site domain.
- Chromium browser http and https network stack isolation (as processes) to protect secure cookies from evil sites accessed over HTTP.
- Further harden userland/kernel interfaces.
Designing and developing for security
Future
software written for and included in Chromium OS-based devices must not work around or
otherwise undermine any of the features implemented to ensure system
security. In addition, it is important that software used in Chromium
OS receive sufficient peer code reviews, manual and automated security
testing, as well as security code audits. Security testing and code
review processes should be discussed in more detail in another document. Obviously, we feel that all Chromium OS devices should only run software that follows these guidelines, but we can only ensure that this is so for our official builds.
