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Let's enable AppArmor by default (why not?)




Hi!

tl;dr: I hereby propose we enable AppArmor by default in testing/sid,
and decide one year later if we want to keep it this way in the
Buster release.

My goals when initiating this discussion are:

 - Get a rough idea of what amount of effort the Debian project is
   happy (and able) to invest into such proactive security measures.

 - Learn about any relevant social & technical concerns I am not
   aware of.

I don't expect we'll make a decision based on this very proposal:
I expect the proposal will need to be refined, or abandoned, to take
into account what we will learn during this preliminary discussion.

Why do we need AppArmor?
========================

AppArmor is a Mandatory Access Control framework implemented as
a Linux Security Module (LSM), user space utilities, and a quite
simple language to define policy.

AppArmor confines programs according to a set of rules that specify
what operations a given program can access, e.g. it can prevent
exploited server software from accessing data it does not own and
executing arbitrary code. This proactive approach helps protect the
system against some known and unknown vulnerabilities.

Various actors are actively exploiting software. Random users are
victimized every day, and specific populations are specifically
targeted, e.g. government opponents, human rights defenders, system
administrators, software developers and distributors, as revealed by
the Snowden leaks.

Every month we learn about many new attack vectors made possible by
programming errors. We fix them after the fact, which is great but
a bit too late: users may already have been exploited. Most operating
systems have adopted proactive approaches to mitigate the impact of
such problems.

In Debian, great efforts are in progress: hardening binaries makes it
harder to write successful exploits, and making our packages build
reproducibly will make it harder to introduce vulnerabilities at the
binary level.

Still, Debian is far from being best in class on this front: we have
no widespread mechanism for sandboxing desktop applications. We can
surely do better. The great news is that there is one low-hanging
fruit waiting to be picked, and it's what this proposal is about :)

A proposal
==========

1. Enable AppArmor by default in testing/sid as soon as feasible in
   the Buster cycle.

   I can think of several possible ways to do it but for now I'd
   rather focus on the "do we want to do it at all" conversation.

   If curious some options are listed on the wiki:
   https://wiki.debian.org/AppArmor/Progress#Enabling_AppArmor_by_default.3F
   Feel free to discuss them on <pkg-apparmor-team@xxxxxxxxxxxxxxxxxxxxxxx>.

   And anyway, you can already opt-in for AppArmor on your system today:
   https://wiki.debian.org/AppArmor/HowToUse :)


2. During a year, watch out for AppArmor related issues and address
   them in a prompt manner.

   Note that the best way to address them quickly enough is sometimes
   to simply disable the problematic AppArmor profile: it's cheap,
   doesn't require advanced AppArmor skills, and IMO a smaller
   AppArmor policy enabled by default is more useful than a broader
   but less robust one that only a couple thousand users benefit from.

3. A year after AppArmor was enabled by default: evaluate how it went
   and decide if Buster should be shipped with AppArmor enabled by
   default or not.

   I commit to do an analysis using BTS data to help make this
   decision. If we need formal success criteria and a clearly defined
   team who'll make the call, I'm happy to think about it. But here
   again I'd rather focus on the general idea than on implementation
   details at this point.

Questions and Answers
=====================

Table of contents:

 - What's the benefit, exactly?
 - What do other distributions do?
 - What's the history of AppArmor in Debian?
 - How popular is AppArmor in Debian?
 - What's the cost for Debian users?
 - What's the cost for package maintainers?
 - Is the Debian AppArmor team strong enough to support this change?
 - Why AppArmor and not SELinux?
 - Why AppArmor and not sandboxing based on XYZ?
 - Will this prevent users from using another Linux Security Module?
 - What does upstream look like?
 - How much will we depend on Canonical's business priorities?
 - No thanks: I've tried AppArmor and it broke stuff too often
 - Doesn't AppArmor need out-of-tree kernel patches?
 - How can I help?
 - Credits

What's the benefit, exactly?
----------------------------

Before we even bother looking at the cost of enabling AppArmor by
default, let's look closer at the expected benefit. In other words:
what kind of attacks does AppArmor really mitigate or prevent in the
real world?

tl;dr: big benefit for server software, and for desktop applications
limited to less sophisticated, non-targeted attacks (but it'll get
better).

AppArmor is well suited to protect against remote exploitation of
security issues in server software and non-GUI programs often run with
elevated privileges (think of dhclient, ping, tcpdump). I'm sure one
could identify a few serious issues that would have been mitigated or
prevented by our current AppArmor policy, by looking at a list of
DSA/CVE. Also, one gets interesting security properties when software
is tuned for AppArmor: e.g. a given libvirt/QEMU virtual machine can
only access its assigned storage volumes, and not other VMs'; this is
useful against QEMU security issues that allow guests to escape the
virtualization layer.

On the desktop, to be honest things look pretty bad *currently*:
AppArmor is not enough, and we need new concepts and technologies to
fix serious limitations on the desktop sandboxing front.
Thankfully this is being actively worked on and the future of desktop
sandboxing on GNU/Linux looks bright and shiny! Some of the future
options rely on AppArmor, some others don't. See the "Why AppArmor and
not sandboxing based on XYZ?" section below.

Still, confining desktop apps with AppArmor has benefit against
scripted or non-targeted attacks: it will mitigate some attacks and
others will get through. So while it's probably not worth starting to
write lots of new policy for GUI applications now, I think that
leveraging the existing one will already serve our users.

What do other distributions do?
-------------------------------

AppArmor has been enabled by default in several other GNU/Linux
distributions and Debian derivatives for a while:

 * in openSUSE + SLES since 2006
 * in Ubuntu since 2008, with a growing policy:
   https://wiki.ubuntu.com/SecurityTeam/KnowledgeBase/AppArmorProfiles
 * in Tails, since 2014 for a few important services (CUPS, Tor) and
   a few desktop applications (e.g. Totem, Evince, Pidgin, Tor
   Browser, Thunderbird)
 * in a few other Debian derivatives (Whonix, Subgraph OS) for at
   least a couple years.

What's the history of AppArmor in Debian?
-----------------------------------------

AppArmor has been available (opt-in) in Debian since 2011. In 2014
a Debian AppArmor packaging team was created, that has been taking
care of the AppArmor packages and policy since then.

In the last 3 years the AppArmor policy shipped in Debian was extended
substantially and its coverage is now on par with Ubuntu's. It's still
rather small due to the strategy we chose: we wanted to avoid
traumatizing early adopters and to avoid creating a culture of
"AppArmor always breaks stuff, let's get used to disabling it".
So like Ubuntu and openSUSE, we're shipping a rather small and mature
AppArmor policy. I believe this strategy has been successful so far,
and even some non-trivial pieces of software like Thunderbird now ship
an AppArmor policy; but of course it has one drawback: most software,
including web browsers, is not confined with AppArmor whatsoever.
Surely with more people contributing to our AppArmor policy we could
have it cover other important pieces of software; time will tell.

A number of maintainers accepted shipping AppArmor policy in their own
package. If you're one of them, please consider providing feedback
about how it went for you.

How popular is AppArmor in Debian?
----------------------------------

tl;dr: AppArmor has steadily become more and more popular in Debian in
the last few years. I think the user base has reached a critical mass
that proves it works OK.

Here's what popcon says ("Vote" count) for the apparmor binary
package, that's needed to use AppArmor:

 * 2015-01:  ~400
 * 2016-01:  ~700 (+75% in a year)
 * 2017-01: ~1300 (+85% in a year)
 * today:    1870 (+44% in 7 months)

But we have no way to tell whether a user who has AppArmor packages
installed actually enabled the AppArmor LSM, so the data for
apparmor-profiles-extra might be more useful here: I expect that only
users who really want to use AppArmor with an extended policy would
bother installing it. This one has 435 registered installations
("Vote" has always been 0 for some reason that I did not investigate);
it was introduced in October 2014, and since then its popcon stats
have been steadily increasing.

What's the cost for Debian users?
---------------------------------

AppArmor unavoidably breaks functionality from time to time: e.g.
new versions of software we package (or of their dependencies)
regularly start needing access to new file locations.

And then users see broken applications from time to time, after
upgrading their testing/sid system. This is to be taken seriously, but
not a big concern IMO:

 - Apparently Ubuntu users have been coping with AppArmor enforced
   by default for 9 years. I see no reason why Debian users would not.

 - I've counted 14 bugs bugs reported in the Debian BTS during the
   Stretch development cycle against our supported AppArmor policy.
   Among those, 11 were closed (106 days after being reported on
   average); all the important ones were closed within 2 months;
   larger delays were due to users developing fixes and/or upstream
   taking some time to review merge requests. About the 3 bugs still
   open: one is waiting for input from other package maintainers since
   2 years, another one had a patch waiting to be applied, and the
   last one needs to be fixed in libvirt upstream.

 - Serious breakage is less likely to happen once AppArmor is enabled
   by default, as there are greater chances that the maintainer would
   have noticed it before uploading.

 - Workarounds are regularly suggested to the bug reporter on the BTS,
   and in many cases the bug reporter documents in the bug report the
   workaround they have *already* applied.

   Implementing a suggested workarounds requires being able to edit
   a text file and running one command as root, which should be doable
   by the vast majority of testing/sid users.

What's the cost for package maintainers?
----------------------------------------

For most of them: none at all. As said earlier, our AppArmor policy
does not cover that much software yet.

But maintainers of software confined by AppArmor will have to deal
with a new kind of bug reports, whose number is likely to grow
significantly once AppArmor is enabled by default. This means they
have to:

1. identify if a bug report can possibly be related to AppArmor;

2. either learn how to debug AppArmor issues themselves, or ask the
   pkg-apparmor team for help (thanks to usertags:
   https://wiki.debian.org/AppArmor/Reportbug#Usertags).

I expect that initially pkg-apparmor will need to provide help in many
cases, but over time the affected maintainers will slowly learn just
enough about AppArmor to handle at least the simplest cases
themselves, just like it happened in Ubuntu years ago.

Is the Debian AppArmor team strong enough to support this change?
-----------------------------------------------------------------

This is a valid concern, as I have been doing the greatest part of the
work on this team.

So far I've found my AppArmor-related workload totally sustainable:
it took me just a few hours here and there, and I would be doing this
work for Tails anyway, so better do it directly in Debian. Still,
primarily relying on one single person is concerning.

Thankfully, a number of other people have been contributing in various
ways. A few Debian users and contributors got used to reporting bugs
and contribute improvements to our AppArmor policy upstream.
Another team member uploaded src:apparmor once. Ulrike Uhlig wrote
documentation about AppArmor for Debian users and
contributors during an Outreachy project whose outcome was posted to
debian-devel-announce in March, 2015.

Also, just like any such distro-wide change, I expect the amount of
work required to support the broader project:

 - will be large initially; I'm confident that the current state of
   our team is good enough to support the project during the first
   stage of the transition;

 - will only decrease over time, as Debian people get used to it and
   learn the small bits they need to know about the new technology,
   and eventually the cases when our AppArmor team has to give a hand
   will become rare;

 - will be done by AppArmor people from other distributions as well:
   a few of them actively participate on the pkg-apparmor mailing list
   and help on issues reported in the Debian BTS.

So I think it's totally reasonable to give it a try.

Why AppArmor and not SELinux?
-----------------------------

SELinux is another LSM that tackles similar problems.

Disclaimer: I've picked AppArmor years ago and didn't look much at
SELinux recently, so some of what follows may be totally wrong or
outdated. Sorry! Debian SELinux people, if you don't mind please help
me get the basic facts right :)

Pros:

 * Allows mediating more kernel objects / interfaces than AppArmor, so
   policy can be made stricter and safer given sufficient expertise
   and available time for maintenance.

 * Enabled by default in RHEL so in theory a great number of sysadmins
   are at ease with it (see below why reality may not match this).

 * A quick look at popcon suggests that SELinux might be more popular
   in Debian than AppArmor, but I'm not sure I am interpreting the
   numbers right (and I suspect that just like AppArmor, the popcon
   won't tell us if users who have installed the relevant support
   packages actually run their system with the corresponding LSM
   enabled & enforced).

Cons:

 * Writing, maintaining, auditing and debugging SELinux policy
   requires grasping a complex conceptual model; I am told this is not
   as easy as doing the same with AppArmor.

 * As far as I could understand when chatting with sysadmins of Red
   Hat systems, this has resulted in a culture where many users got
   used to disable SELinux entirely on their systems, instead of
   trying to fix the buggy policy. I've seen the opposite happen with
   AppArmor, which is good: for example, pretty often bug reporters to
   the Debian BTS document themselves how they could workaround the
   problem locally *without* turning AppArmor off. Looking at open
   bugs in the BTS against src:refpolicy, this seems to happen very
   rarely for SELinux, so I wonder if it would be realistic to ship
   Debian with SELinux enforced by default and have our community
   support it.

 * https://wiki.debian.org/SELinux/Issues says "Graphical/Desktop
   installs of Debian are not heavily tested with selinux, so you
   might run into quite some issues".

 * I'm not aware of any Debian derivative shipping with SELinux
   enabled by default. If that's correct, then it means that we would
   have to deal with quite some policy compatibility issues ourselves.

To me, the complexity of SELinux is a deal breaker: it seems that we
would need to get lots more expertise and energy to enforce SELinux by
default than doing the same with AppArmor.

Now, if for some reason the project prefers to ship with SELinux
enforced instead of AppArmor, fine by me: I would strongly prefer this
option to nothing at all.

Why AppArmor and not sandboxing based on XYZ?
---------------------------------------------

(Replace "XYZ" with Flatpak, Ubuntu's Snappy, Subgraph OS' oz,
Firejail, Subuser, or you preferred other promising option.)

We need both!

AppArmor covers server software well, and on the desktop it currently
protects against not-too-sophisticated, non-targeted attacks.

In a nutshell, the GNU/Linux desktop really wasn't designed for
applications to be siloed. To fix that we need new concepts and
technologies, such as Wayland, portals, and fine-grained D-Bus
mediation. Next generation desktop sandboxing technologies will fix
this and improve UX at the same time, and it will be amazing. But they
are not ready for prime-time yet. A Debian user cannot benefit from
them *today* much; this might change in time for Buster, but really
we're comparing a well-established, polished solution with a bunch of
other ones whose integration with Debian is being brainstormed.

Anyway, this is not an either/or situation: even though there are
currently compatibility issues, one can perfectly well develop/adapt
such tools in a way that makes them work fine with AppArmor enabled.

Let's enable AppArmor so we cover at least the server use case and the
low-hanging fruits of the desktop one, and figure out later where we
should put our efforts for securing the desktop, once the dust has
settled and next generation solutions have matured and been integrated
in Debian.

Will this prevent users from using another Linux Security Module?
-----------------------------------------------------------------

Some "minor" Linux Security Modules, such as Yama, live perfectly well
with others.

But currently it is not possible to enable several of the major
security modules. There's been (slow) work in progress to fix this for
a while, but it has picked up recently and there is a lot of interest
to have, say, AppArmor and SELinux stackable:

  https://lwn.net/Articles/719731/

Now, every user will still be able to opt-out from AppArmor and
instead enable their preferred LSM.

What does upstream look like?
-----------------------------

The upstream project is almost 20 years old, very mature and
cooperative with Debian. E.g. the upstream release schedule has been
adjusted both for Jessie and Stretch to accommodate Debian's
schedule nicely.

Regarding who does the work:

 - Canonical employees do most of the kernel work. They also maintain
   the library and other C code, e.g. the policy parser.

 - The Python utilities are primarily maintained by openSUSE's
   Christian Boltz.

 - Maintaining AppArmor policy is a cross-distro team effort, mostly
   done by Debian, Ubuntu and openSUSE people.

How much will we depend on Canonical's business priorities?
-----------------------------------------------------------

Given Canonical employees do the greatest part of the work upstream:
indeed, we will. I see two main concerns about this:

Long-term reliability: this funding could run out some day.
I personally am not overly concerned, as Canonical has been investing
a lot into products (Snappy, LXC/LXD) that strongly depend on AppArmor
in the last few years.

Power imbalance: the company that does so much of the work has great
power over the priorities of the upstream project. This is the case
for a large amount of critical software we ship, so like it or not, it
is something we are living with already. AppArmor developers employed
by Canonical have shown great willingness in cooperating with Debian
in the last few years, so I'm confident that our contributions will be
welcome for the foreseeable future, whenever we need to adapt the
software to our needs. But of course management/business decisions can
change this at any time.

No thanks: I've tried AppArmor and it broke stuff too often
-----------------------------------------------------------

Sorry about that. I think this has had three main causes so far, that
all share one single root cause i.e. "AppArmor is not enabled by
default" (chicken'n'egg!):

1. Most package maintainers don't test packages with AppArmor before
   uploading, so users notice breakage that could easily be avoided.

2. The huge majority of our users are not affected by breakage caused
   by AppArmor, so we handle such breakage in a way that saves
   maintainers' time: e.g. in many cases I've personally preferred to
   wait for my fixes to AppArmor profiles to be approved and merged
   upstream before applying them in Debian.

   Once AppArmor is enabled by default, as far as I'm concerned
   I don't plan to wait for upstream review before fixing regressions
   in testing/sid.

3. The huge majority of our users are not affected by breakage caused
   by AppArmor, so such breakage was kinda acceptable and thus we
   *sometimes* preferred to give a specific AppArmor profile more
   exposure to testers, even if it had a couple known issues, in order
   to identify problems and help stabilize it (e.g. Tor, libvirt).

   I think we will need to be more conservative once AppArmor is
   enabled by default, i.e. profiles that break functionality too much
   or too often should not be enabled by default.

Doesn't AppArmor need out-of-tree kernel patches?
-------------------------------------------------

Yes and no.

Historically, the mainline Linux kernel has supported a rather small
subset of the AppArmor mediation made possible by the out-of-tree
kernel patch. This made the value of enabling AppArmor smaller than it
could be (e.g. LXC is not well confined in Debian: #750106), and
smaller than it is in distros that apply the out-of-tree kernel patch
(such as Ubuntu).

Still, even with the set of features available in mainline Linux
*today*, IMO enabling AppArmor already has a good cost/benefit ratio
for Debian and our users. I'm not proposing we apply out-of-tree
AppArmor kernel patches.

Thankfully, the AppArmor kernel developers recently changed how they
proceed: new features are now added to Linux mainline before they
reach Ubuntu, so I'm confident that this situation will get better and
better in the future, and Buster's kernel will support tons of new
AppArmor mediation types compared to Stretch.

How can I help?
---------------

 * Enable AppArmor on your Debian systems:
   https://wiki.debian.org/AppArmor/HowToUse

 * If you maintain a package for which we ship AppArmor policy in
   Debian: test it with AppArmor enabled before uploading.

   Ask for help if needed:
   https://wiki.debian.org/AppArmor/Reportbug#Usertags

 * Join the team:
   https://wiki.debian.org/AppArmor/Contribute

 * Talk to us:
   <pkg-apparmor-team@xxxxxxxxxxxxxxxxxxxxxxx>

Credits
-------

A huge thank you to the people who reviewed this text, provided
valuable input that I took into account & integrated, and helped me
change my mind here and there: Christian Boltz, gregoa, Guido Günther,
Jamie Strandboge, John Johansen, Sebastien Delafond, Simon McVittie
and Solveig! Sorry to those I forgot. I shamelessly stole bits of text
they wrote. I absolutely do *not* imply they endorse this proposal.

Thanks a lot to my pkg-apparmor team-mates, to Kees Cook who
introduced AppArmor in Debian in the first place, and to all AppArmor
contributors upstream and in other distros :)

Cheers,
-- 
intrigeri