Hi Debian AppArmor team, upstream AppArmor people, people who
volunteered to review this text, a few maintainers of packages that
include AppArmor policy, and some innocent bystanders!
Please review the attached proposal. I will send it to debian-devel@
tomorrow around 6pm (Montréal time) after taking your feedback
into account.
If you're at DebCamp, I guess that the process will be nicer both for
you and me if you grab me in person whenever you want to read the
draft and comment live.
Thanks in advance :)
Cheers,
--
intrigeri
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.
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 your
PDF reader and video player from accessing your GnuPG secrets keys and
executing arbitrary code. This proactive approach helps protect the
system against both 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 and 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.
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
=====================
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
* in Tails, since 2014
* in a few other Debian derivatives (Whonix, Subgraph OS) for at
least a couple years; I suspect that Simon McVittie can add to
the list.
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, we're shipping a rather small and mature AppArmor
policy. I believe this strategy has been successful so far, 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 is AppArmor popular 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 having been coping with AppArmor enforced
by default for 9 years. I see no reason why Debian users would not.
- I wanted some data to evaluate how well we've been dealing with
this so far, so I've looked at such bugs reported in the Debian BTS
during the Stretch development cycle against our supported AppArmor
policy. I've counted 14 such bugs. 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?
----------------------------------------
Package maintainers have to deal with the aforementioned 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 for
help to the pkg-apparmor team.
I expect that initially pkg-apparmor will need to provide help is 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 improved
a lot the AppArmor documentation we have 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 at least give it a try.
Will this prevent users from using another LSM?
-----------------------------------------------
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 work in progress to fix this:
https://lwn.net/Articles/719731/
Now, every user will still be able to opt-out from AppArmor and
instead enable their preferred LSM.
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 rather 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, I see this
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 Linux namespaces?
----------------------------------------------------------
In the last few years, a number of tools appeared that use Linux
namespaces to achieve similar goals as AppArmor and SELinux. To name
a few of those that support desktop applications: bubblewrap (used by
Flatpak), Subgraph OS' oz, Firejail, Subuser and Ubuntu Snappy.
And our default init system has very interesting functionality that
does similar things for system services.
Pros:
* Very interesting long-term perspectives in terms of user
experience. E.g. the concept of "portals" embraced by Flatpak and
Ubuntu Snappy will improve security *and* user experience at the
same time.
Cons:
* Not very mature: I've evaluated these tools for Tails and deemed
them not ready for prime-time (either breaks basic functionality
such as universal access technologies, or the security part is
planned but not implemented yet).
* Insufficient: the aforementioned tools either already rely on
a LSM, or soon will, to provide adequate protection. So at the end
of the day we're left with the "which LSM do we enable by
default?" question.
In any case, it is my understanding that this is not an either/or
situation: one can perfectly well use this tools with
AppArmor enabled.
How 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.
Will we depend a lot 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.
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.
How can I help?
---------------
* Enable AppArmor on your Debian systems:
https://wiki.debian.org/AppArmor/HowToUse
* If you maintain a package that we ship AppArmor policy for:
test it with AppArmor enabled before uploading.
* Join the team:
https://wiki.debian.org/AppArmor/Contribute
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