Currently, if you want to properly chroot a systemd service, you could
do it using BindReadOnlyPaths=/nix/store (which is not what I'd call
"properly", because the whole store is still accessible) or use a
separate derivation that gathers the runtime closure of the service you
want to chroot. The former is the easier method and there is also a
method directly offered by systemd, called ProtectSystem, which still
leaves the whole store accessible. The latter however is a bit more
involved, because you need to bind-mount each store path of the runtime
closure of the service you want to chroot.

This can be achieved using pkgs.closureInfo and a small derivation that
packs everything into a systemd unit, which later can be added to
systemd.packages. That's also what I did several times[1][2] in the
past.

However, this process got a bit tedious, so I decided that it would be
generally useful for NixOS, so this very implementation was born.

Now if you want to chroot a systemd service, all you need to do is:

  {
    systemd.services.yourservice = {
      description = "My Shiny Service";
      wantedBy = [ "multi-user.target" ];

      chroot.enable = true;
      serviceConfig.ExecStart = "${pkgs.myservice}/bin/myservice";
    };
  }

If more than the dependencies for the ExecStart* and ExecStop* (which
btw. also includes "script" and {pre,post}Start) need to be in the
chroot, it can be specified using the chroot.packages option. By
default (which uses the "full-apivfs"[3] confinement mode), a user
namespace is set up as well and /proc, /sys and /dev are mounted
appropriately.

In addition - and by default - a /bin/sh executable is provided as well,
which is useful for most programs that use the system() C library call
to execute commands via shell. The shell providing /bin/sh is dash
instead of the default in NixOS (which is bash), because it's way more
lightweight and after all we're chrooting because we want to lower the
attack surface and it should be only used for "/bin/sh -c something".

Prior to submitting this here, I did a first implementation of this
outside[4] of nixpkgs, which duplicated the "pathSafeName" functionality
from systemd-lib.nix, just because it's only a single line.

However, I decided to just re-use the one from systemd here and
subsequently made it available when importing systemd-lib.nix, so that
the systemd-chroot implementation also benefits from fixes to that
functionality (which is now a proper function).

Unfortunately, we do have a few limitations as well. The first being
that DynamicUser doesn't work in conjunction with tmpfs, because it
already sets up a tmpfs in a different path and simply ignores the one
we define. We could probably solve this by detecting it and try to
bind-mount our paths to that different path whenever DynamicUser is
enabled.

The second limitation/issue is that RootDirectoryStartOnly doesn't work
right now, because it only affects the RootDirectory option and not the
individual bind mounts or our tmpfs. It would be helpful if systemd
would have a way to disable specific bind mounts as well or at least
have some way to ignore failures for the bind mounts/tmpfs setup.

Another quirk we do have right now is that systemd tries to create a
/usr directory within the chroot, which subsequently fails. Fortunately,
this is just an ugly error and not a hard failure.

[1]: https://github.com/headcounter/shabitica/blob/3bb01728a0237ad5e7/default.nix#L43-L62
[2]: https://github.com/aszlig/avonc/blob/dedf29e092481a33dc/nextcloud.nix#L103-L124
[3]: The reason this is called "full-apivfs" instead of just "full" is
     to make room for a *real* "full" confinement mode, which is more
     restrictive even.
[4]: https://github.com/aszlig/avonc/blob/92a20bece4df54625e/systemd-chroot.nix

Signed-off-by: aszlig <aszlig@nix.build>

Quoting @edolstra from [1]:

  I don't really like the name "chroot", something like "confine[ment]"
  or "restrict" seems better. Conceptually we're not providing a
  completely different filesystem tree but a restricted view of the same
  tree.

I already used "confinement" as a sub-option and I do agree that
"chroot" sounds a bit too specific (especially because not *only* chroot
is involved).

So this changes the module name and its option to use "confinement"
instead of "chroot" and also renames the "chroot.confinement" to
"confinement.mode".

[1]: NixOS#57519 (comment)

Signed-off-by: aszlig <aszlig@nix.build>

Another thing requested by @edolstra in [1]:

  We should not provide a different /bin/sh in the chroot, that's just
  asking for confusion and random shell script breakage. It should be
  the same shell (i.e. bash) as in a regular environment.

While I personally would even go as far to even have a very restricted
shell that is not even a shell and basically *only* allows "/bin/sh -c"
with only *very* minimal parsing of shell syntax, I do agree that people
expect /bin/sh to be bash (or the one configured by environment.binsh)
on NixOS.

So this should make both others and me happy in that I could just use
confinement.binSh = "${pkgs.dash}/bin/dash" for the services I confine.

[1]: NixOS#57519 (comment)

Signed-off-by: aszlig <aszlig@nix.build>

From @edolstra at [1]:

  BTW we probably should take the closure of the whole unit rather than
  just the exec commands, to handle things like Environment variables.

With this commit, there is now a "fullUnit" option, which can be enabled
to include the full closure of the service unit into the chroot.

However, I did not enable this by default, because I do disagree here
and *especially* things like environment variables or environment files
shouldn't be in the closure of the chroot.

For example if you have something like:

  { pkgs, ... }:

  {
    systemd.services.foobar = {
      serviceConfig.EnvironmentFile = ${pkgs.writeText "secrets" ''
        user=admin
        password=abcdefg
      '';
    };
  }

We really do not want the *file* to end up in the chroot, but rather
just the environment variables to be exported.

Another thing is that this makes it less predictable what actually will
end up in the chroot, because we have a "globalEnvironment" option that
will get merged in as well, so users adding stuff to that option will
also make it available in confined units.

I also added a big fat warning about that in the description of the
fullUnit option.

[1]: NixOS#57519 (comment)

Signed-off-by: aszlig <aszlig@nix.build>

My implementation was relying on PrivateDevices, PrivateTmp,
PrivateUsers and others to be false by default if chroot-only mode is
used.

However there is an ongoing effort[1] to change these defaults, which
then will actually increase the attack surface in chroot-only mode,
because it is expected that there is no /dev, /sys or /proc.

If for example PrivateDevices is enabled by default, there suddenly will
be a mounted /dev in the chroot and we wouldn't detect it.

Fortunately, our tests cover that, but I'm preparing for this anyway so
that we have a smoother transition without the need to fix our
implementation again.

Thanks to @infinisil for the heads-up.

[1]: NixOS#14645

Signed-off-by: aszlig <aszlig@nix.build>

Noted by @infinisil on IRC:

   infinisil: Question regarding the confinement PR
   infinisil: On line 136 you do different things depending on
              RootDirectoryStartOnly
   infinisil: But on line 157 you have an assertion that disallows that
              option being true
   infinisil: Is there a reason behind this or am I missing something

I originally left this in so that once systemd supports that, we can
just flip a switch and remove the assertion and thus support
RootDirectoryStartOnly for our confinement module.

However, this doesn't seem to be on the roadmap for systemd in the
foreseeable future, so I'll just remove this, especially because it's
very easy to add it again, once it is supported.

Signed-off-by: aszlig <aszlig@nix.build>

So far we had MountFlags = "private", but as @infinisil has correctly
noticed, there is a dedicated PrivateMounts option, which does exactly
that and is better integrated than providing raw mount flags.

When checking for the reason why I used MountFlags instead of
PrivateMounts, I found that at the time I wrote the initial version of
this module (Mar 12 06:15:58 2018 +0100) the PrivateMounts option didn't
exist yet and has been added to systemd in Jun 13 08:20:18 2018 +0200.

Signed-off-by: aszlig <aszlig@nix.build>

First of all, the reason I added this to the "highlights" section is
that we want users to be aware of these options, because in the end we
really want to decrease the attack surface of NixOS services and this is
a step towards improving that situation.

The reason why I'm adding this to the changelog of the NixOS 19.03
release instead of 19.09 is that it makes backporting services that use
these options easier. Doing the backport of the confinement module after
the official release would mean that it's not part of the release
announcement and potentially could fall under the radar of most users.

These options and the whole module also do not change anything in
existing services or affect other modules, so they're purely optional.

Adding this "last minute" to the 19.03 release doesn't hurt and is
probably a good preparation for the next months where we hopefully
confine as much services as we can :-)

I also have asked @samueldr and @lheckemann, whether they're okay with
the inclusion in 19.03. While so far only @samueldr has accepted the
change, we can still move the changelog entry to the NixOS 19.09 release
notes in case @lheckemann rejects it.

Signed-off-by: aszlig <aszlig@nix.build>