CAP_SYS_PTRACE for making sure we can open mount namespaces of
peers via /proc/<pid>/ns and CAP_SYS_CHROOT for making sure we can
join those mount namespaces.
Since mountfsd was added in 702a52f4b5 the
caps bounding set line was commented. That's an accident. Fix that. (We
need to add a bunch of caps to the list).
The current arrangement of service and socket units is
sort of all over the place. Let's clean it up a little,
roughly following the principles below:
- socket units have implicit ordering deps (not to be confused
with default ones which are subject to DefaultDependencies=)
before associated service, so drop any explicit After=
- If socket can be enabled, remember to link to it in service
via Also= and Sockets= (the latter replaces Wants=).
If the service Requires= socket however, Sockets= is omitted.
- If socket is statically enabled, no need for service
to pull it in - machined
systemd-mountfsd so far provided a MountImage() API call for mounting a
disk image and returning a set of mount fds. This complements the API
with a new MountDirectory() API call, that operates on a directory
instead of an image file. Now, what makes this interesting is that it
applies an idmapping from the foreign UID range to the provided target
userns – and in which case unpriveleged operation is allowed (well,
under some conditions: in particular the client must own a parent dir of
the provided path).
This allows container managers to run fully unprivileged from
directories – as long as those directories are owned by the foreign UID
range. Basic operation is like this:
1. acquire a transient userns from systemd-nsresourced with 64K users
2. ask systemd-mountfsd for an idmapped mount of the container dir
matching that userns
3. join the userns and bind the mount fd as root.
Note that we have to drop various sandboxing knobs from the mountfsd
service file for this to work, since the kernel's security checks that
try to ensure than an obstructed /proc/ cannot be circumvented via
mounting a new procfs will otherwise prohibit mountfsd to duplicate the
mounts properly.
mountfsd is supposed to be available during early boot aleady, before
systemd-tmpfiles-setup-dev-early.service completes, hence make sure
loopback devices and DM already work before that.
As suggested by yuwata here:
https://github.com/systemd/systemd/pull/35685#issuecomment-2608157569
Daan De Meyer
Lennart Poettering
Mike Yuan