The default, which is /tmp, has a few issues associated with it:
One being that it makes it easy for users on the system to spoof a
PostgreSQL server if it's not running, causing applications to connect
to their provided sockets instead of just failing to connect.
Another one is that it makes sandboxing of PostgreSQL and other services
unnecessarily difficult. This is already the case if only PrivateTmp is
used in a systemd service, so in order for such a service to be able to
connect to PostgreSQL, a bind mount needs to be done from /tmp to some
other path, so the service can access it. This pretty much defeats the
whole purpose of PrivateTmp.
We regularily run into issues with this in the past already (one example
would be https://github.com/NixOS/nixpkgs/pull/24317) and with the new
systemd-confinement mode upcoming in
https://github.com/NixOS/nixpkgs/pull/57519, it makes it even more
tedious to sandbox services.
I've tested this change against all the postgresql NixOS VM tests and
they still succeed and I also grepped through the source tree to replace
other occasions where we might have /tmp hardcoded. Luckily there were
very few occasions.
Signed-off-by: aszlig <aszlig@nix.build>
Cc: @ocharles, @thoughtpolice, @danbst
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>
