execute: make StateDirectory= and friends compatible with DynamicUser=1 and RootDirectory=/RootImage=

Let's clean up the interaction of StateDirectory= (and friends) to
DynamicUser=1: instead of creating these directories directly below
/var/lib, place them in /var/lib/private instead if DynamicUser=1 is
set, making that directory 0700 and owned by root:root. This way, if a
dynamic UID is later reused, access to the old run's state directory is
prohibited for that user. Then, use file system namespacing inside the
service to make /var/lib/private a readable tmpfs, hiding all state
directories that are not listed in StateDirectory=, and making access to
the actual state directory possible. Mount all directories listed in
StateDirectory= to the same places inside the service (which means
they'll now be mounted into the tmpfs instance). Finally, add a symlink
from the state directory name in /var/lib/ to the one in
/var/lib/private, so that both the host and the service can access the
path under the same location.

Here's an example: let's say a service runs with StateDirectory=foo.
When DynamicUser=0 is set, it will get the following setup, and no
difference between what the unit and what the host sees:

        /var/lib/foo (created as directory)

Now, if DynamicUser=1 is set, we'll instead get this on the host:

        /var/lib/private (created as directory with mode 0700, root:root)
        /var/lib/private/foo (created as directory)
        /var/lib/foo → private/foo (created as symlink)

And from inside the unit:

        /var/lib/private (a tmpfs mount with mode 0755, root:root)
        /var/lib/private/foo (bind mounted from the host)
        /var/lib/foo → private/foo (the same symlink as above)

This takes inspiration from how container trees are protected below
/var/lib/machines: they generally reuse UIDs/GIDs of the host, but
because /var/lib/machines itself is set to 0700 host users cannot access
files in the container tree even if the UIDs/GIDs are reused. However,
for this commit we add one further trick: inside and outside of the unit
/var/lib/private is a different thing: outside it is a plain,
inaccessible directory, and inside it is a world-readable tmpfs mount
with only the whitelisted subdirs below it, bind mounte din.  This
means, from the outside the dir acts as an access barrier, but from the
inside it does not. And the symlink created in /var/lib/foo itself
points across the barrier in both cases, so that root and the unit's
user always have access to these dirs without knowing the details of
this mounting magic.

This logic resolves a major shortcoming of DynamicUser=1 units:
previously they couldn't safely store persistant data. With this change
they can have their own private state, log and data directories, which
they can write to, but which are protected from UID recycling.

With this change, if RootDirectory= or RootImage= are used it is ensured
that the specified state/log/cache directories are always mounted in
from the host. This change of semantics I think is much preferable since
this means the root directory/image logic can be used easily for
read-only resource bundling (as all writable data resides outside of the
image). Note that this is a change of behaviour, but given that we
haven't released any systemd version with StateDirectory= and friends
implemented this should be a safe change to make (in particular as
previously it wasn't clear what would actually happen when used in
combination). Moreover, by making this change we can later add a "+"
modifier to these setings too working similar to the same modifier in
ReadOnlyPaths= and friends, making specified paths relative to the
container itself.

1 files changed, 1 insertions, 0 deletions

diff --git a/src/core/namespace.h b/src/core/namespace.h
index f54954bd86..da8d85dbc5 100644
--- a/src/core/namespace.h
+++ b/src/core/namespace.h
@@ -69,6 +69,7 @@ int setup_namespace(
                 char **read_write_paths,
                 char **read_only_paths,
                 char **inaccessible_paths,
+                char **empty_directories,
                 const BindMount *bind_mounts,
                 unsigned n_bind_mounts,
                 const char *tmp_dir,