Currently if you want to properly chroot a systemd service, you could do
it using BindReadOnlyPaths=/nix/store or use a separate derivation which
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.
However, this process is a bit tedious, so the changes here implement
this in a more generic way.
Now if you want to chroot a systemd service, all you need to do is:
{
systemd.services.myservice = {
description = "My Shiny Service";
wantedBy = [ "multi-user.target" ];
confinement.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 confinement.packages option. By default
(which uses the full-apivfs 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, which
is useful for most programs that use the system() C library call to
execute commands via shell.
Unfortunately, there are a few limitations at the moment. The first
being that DynamicUser doesn't work in conjunction with tmpfs, because
systemd seems to ignore the TemporaryFileSystem option if DynamicUser is
enabled. I started implementing a workaround to do this, but I decided
to not include it as part of this pull request, because it needs a lot
more testing to ensure it's consistent with the behaviour without
DynamicUser.
The second limitation/issue is that RootDirectoryStartOnly doesn't work
right now, because it only affects the RootDirectory option and doesn't
include/exclude the individual bind mounts or the tmpfs.
A 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.
The changes also come with a changelog entry for NixOS 19.03, which is
why I asked for a vote of the NixOS 19.03 stable maintainers whether to
include it (I admit it's a bit late a few days before official release,
sorry for that):
@samueldr:
Via pull request comment[1]:
+1 for backporting as this only enhances the feature set of nixos,
and does not (at a glance) change existing behaviours.
Via IRC:
new feature: -1, tests +1, we're at zero, self-contained, with no
global effects without actively using it, +1, I think it's good
@lheckemann:
Via pull request comment[2]:
I'm neutral on backporting. On the one hand, as @samueldr says,
this doesn't change any existing functionality. On the other hand,
it's a new feature and we're well past the feature freeze, which
AFAIU is intended so that new, potentially buggy features aren't
introduced in the "stabilisation period". It is a cool feature
though? :)
A few other people on IRC didn't have opposition either against late
inclusion into NixOS 19.03:
@edolstra: "I'm not against it"
@Infinisil: "+1 from me as well"
@grahamc: "IMO its up to the RMs"
So that makes +1 from @samueldr, 0 from @lheckemann, 0 from @edolstra
and +1 from @Infinisil (even though he's not a release manager) and no
opposition from anyone, which is the reason why I'm merging this right
now.
I also would like to thank @Infinisil, @edolstra and @danbst for their
reviews.
[1]: https://github.com/NixOS/nixpkgs/pull/57519#issuecomment-477322127
[2]: https://github.com/NixOS/nixpkgs/pull/57519#issuecomment-477548395
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>
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>
seems that this got broken when the config option was made to use enums. "secure" got replaced with "enum", which isn't a valid option for the failure mode.
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]: https://github.com/NixOS/nixpkgs/issues/14645
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 = ${pkgs.writeText "secrets" ''
user=admin
password=abcdefg
'';
};
}
We really do not want the *file* to end up in the chroot, but rather
just the environment variables to be exported.
Another thing is that this makes it less predictable what actually will
end up in the chroot, because we have a "globalEnvironment" option that
will get merged in as well, so users adding stuff to that option will
also make it available in confined units.
I also added a big fat warning about that in the description of the
fullUnit option.
[1]: https://github.com/NixOS/nixpkgs/pull/57519#issuecomment-472855704
Signed-off-by: aszlig <aszlig@nix.build>
Another thing requested by @edolstra in [1]:
We should not provide a different /bin/sh in the chroot, that's just
asking for confusion and random shell script breakage. It should be
the same shell (i.e. bash) as in a regular environment.
While I personally would even go as far to even have a very restricted
shell that is not even a shell and basically *only* allows "/bin/sh -c"
with only *very* minimal parsing of shell syntax, I do agree that people
expect /bin/sh to be bash (or the one configured by environment.binsh)
on NixOS.
So this should make both others and me happy in that I could just use
confinement.binSh = "${pkgs.dash}/bin/dash" for the services I confine.
[1]: https://github.com/NixOS/nixpkgs/pull/57519#issuecomment-472855704
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]: https://github.com/NixOS/nixpkgs/pull/57519#issuecomment-472855704
Signed-off-by: aszlig <aszlig@nix.build>
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>
For the hardened profile disable symmetric multi threading. There seems to be
no *proven* method of exploiting cache sharing between threads on the same CPU
core, so this may be considered quite paranoid, considering the perf cost.
SMT can be controlled at runtime, however. This is in keeping with OpenBSD
defaults.
TODO: since SMT is left to be controlled at runtime, changing the option
definition should take effect on system activation. Write to
/sys/devices/system/cpu/smt/control
For the hardened profile enable flushing whenever the hypervisor enters the
guest, but otherwise leave at kernel default (conditional flushing as of
writing).
Introduces the option security.protectKernelImage that is intended to control
various mitigations to protect the integrity of the running kernel
image (i.e., prevent replacing it without rebooting).
This makes sense as a dedicated module as it is otherwise somewhat difficult
to override for hardened profile users who want e.g., hibernation to work.
The OS Login package enables the following components:
AuthorizedKeysCommand to query valid SSH keys from the user's OS Login
profile during ssh authentication phase.
NSS Module to provide user and group information
PAM Module for the sshd service, providing authorization and
authentication support, allowing the system to use data stored in
Google Cloud IAM permissions to control both, the ability to log into
an instance, and to perform operations as root (sudo).
Having pam_unix set to "sufficient" means early-succeeding account
management group, as soon as pam_unix.so is succeeding.
This is not sufficient. For example, nixos modules might install nss
modules for user lookup, so pam_unix.so succeeds, and we end the stack
successfully, even though other pam account modules might want to do
more extensive checks.
Other distros seem to set pam_unix.so to 'required', so if there are
other pam modules in that management group, they get a chance to do some
validation too.
For SSSD, @PsyanticY already added a workaround knob in
https://github.com/NixOS/nixpkgs/pull/31969, while stating this should
be the default anyway.
I did some thinking in what could break - after this commit, we require
pam_unix to succeed, means we require `getent passwd $username` to
return something.
This is the case for all local users due to the passwd nss module, and
also the case for all modules installing their nss module to
nsswitch.conf - true for ldap (if not explicitly disabled) and sssd.
I'm not so sure about krb5, cc @eqyiel for opinions. Is there some nss
module loaded? Should the pam account module be placed before pam_unix?
We don't drop the `security.pam.services.<name?>.sssdStrictAccess`
option, as it's also used some lines below to tweak error behaviour
inside the pam sssd module itself (by changing it's 'control' field).
This is also required to get admin login for Google OS Login working
(#51566), as their pam_oslogin_admin accounts module takes care of sudo
configuration.
A module for security options that are too small to warrant their own module.
The impetus for adding this module is to make it more convenient to override
the behavior of the hardened profile wrt user namespaces.
Without a dedicated option for user namespaces, the user needs to
1) know which sysctl knob controls userns
2) know how large a value the sysctl knob needs to allow e.g.,
Nix sandbox builds to work
In the future, other mitigations currently enabled by the hardened profile may
be promoted to options in this module.
I think pam_lastlog is the only thing that writes to these files in
practice on a modern Linux system, so in a configuration that doesn't
use that module, we don't need to create these files.
I used tmpfiles.d instead of activation snippets to create the logs.
It's good enough for upstream and other distros; it's probably good
enough for us.
