Whenever delegating UID ranges to a user namespace, it can also be
useful to map the foreign UID range, so that the container running in
the user namespace with delegated UID ranges can download container
images and unpack them to the foreign UID range.
Let's add an option mapForeign to make this possible. Note that this option
gives unprivileged users full access to the any foreign UID range owned directory
that they can access. Hence it is recommended (and already was recommended) to
store foreign UID range owned directories in a 0700 directory owned by the
owner of the tree to avoid access and modifications by other users.
This is already the case for the main users of the foreign UID range,
namely /var/lib/machines, /var/lib/portables and /home/<user> which all
use 0700 as their mode.
Users will also be able to create foreign UID range owned inodes in any
directories their own user can write to (on most systems this means /tmp,
/var/tmp and /home/<user>).
mkosi does all of its environment setup in an unprivileged user
namespace with an identity mapping. When it invokes nspawn and nspawn
tries to get a transient userns from nsresourced, this fails as no
transient ranges are mapped into mkosi's unprivileged userns (as doing
so would require privileges).
To fix this problem, let's allow allocating unprivileged self user
namespaces in nsresourced, similar to what the kernel allows, except that
we also support delegations for these. This means that mkosi can get its
unprivileged userns as before from nsresourced, but it can also request a
delegated 64K range inside that userns as well, which nsresourced can then
allocate to nspawn later when it asks for one.
Similar to the kernel, we disallow setgroups for self mappings. However,
instead of doing this via /proc/self/setgroups, which applies to the current
user namespace and all its child user namespaces, we use the BPF LSM to deny
setgroups instead, so that it can still be allowed for child user namespaces.
We need this because as soon as a container launches in a child user namespace
using one of the delegated mappings, it has to be able to do setgroups() to be
able to function properly.
To allow mapping the root user, we need to add the CAP_SETFCAP capability to
nsresourced.
The purpose of the userns-restrict BPF-LSM program is to prevent the
transient ranges leaking to disk, so let's allow operations outside the
transient UID ranges, even if the mount is not allowlisted.
This is preparation for the next commits where we'll add support for mapping
the current user and the foreign UID range into the user namespaces provisioned
by nsresourced. Operations creating files/directories as these UIDs/GIDs should
not need the corresponding mount to be allowlisted with nsresourced.
Let's not leak details from src/shared and src/libsystemd into
src/basic, even though you can't actually do anything useful with
just forward declarations from src/shared.
The sd-forward.h header is put in src/libsystemd/sd-common as we
don't have a directory for shared internal headers for libsystemd
yet.
Let's also rename forward.h to basic-forward.h to keep things
self-explanatory.
In multi-seat scenarios, a display manager might need to start multiple
greeter sessions. But systemd allows at most one graphical session per
user. So, display managers now have a range of UIDs to dynamically
allocate users for their greeter sessions.
This makes the UID range configurable via build time options, but of
course it really shouldn't be changed. The default range I picked is
outside even of IPAs current (ridiculously large) allocation ranges,
hence hopefully minimizes conflicts.
A bit confusingly CONTAINER_UID_BASE_MAX is just the maximum *base* UID
for a container. Thus, with the usual 64K UID assignments, the last
actual container UID is CONTAINER_UID_BASE_MAX+0xFFFF.
To make this less confusing define CONTAINER_UID_MIN/MAX that add the
missing extra space.
Also adjust two uses where this was mishandled so far, due to this
confusion.
With this change the UID ranges we default to should properly match what
is documented on https://systemd.io/UIDS-GIDS/.
We had both uid-range.h and uid-alloc-range.h. The latter now contains helpers
like {uid,gid}_is_{system,dynamic,container}(), uid_for_system_journal(), so
the existing name is outdated. I think the uid-range.[ch] should stay separate
because it has a bunch of helpers for parsing and printing of uid ranges. So
let's rename as in $subject to better reflect the contents of the file and make
the two sets of files harder to confuse.
Daan De Meyer
Adrian Vovk
Lennart Poettering
Zbigniew Jędrzejewski-Szmek