units: Split modprobing out into a separate service unit

Devices referred to by `DeviceAllow=` sandboxing are resolved into their
corresponding major numbers when the unit is loaded by looking at
`/proc/devices`. If a reference is made to a device which is not yet
available, the `DeviceAllow` is ignored and the unit's processes cannot
access that device.

In both logind and nspawn, we have `DeviceAllow=` lines, and `modprobe`
in `ExecStartPre=` to load some kernel modules. Those kernel modules
cause device nodes to become available when they are loaded: the device
nodes may not exist when the unit itself is loaded. This means that the
unit's processes will not be able to access the device since the
`DeviceAllow=` will have been resolved earlier and denied it.

One way to fix this would be to re-evaluate the available devices and
re-apply the policy to the cgroup, but this cannot work atomically on
cgroupsv1. So we fall back to a second approach: instead of running
`modprobe` via `ExecStartPre`, we move this out to a separate unit and
order it before the units which want the module.

Closes #14322.
Fixes: #13943.

1 files changed, 16 insertions, 0 deletions

diff --git a/units/modprobe@.service b/units/modprobe@.service
new file mode 100644
index 0000000000..4c5bd637e3
--- /dev/null
+++ b/units/modprobe@.service
@@ -0,0 +1,16 @@
+#  SPDX-License-Identifier: LGPL-2.1+
+#
+#  This file is part of systemd.
+#
+#  systemd is free software; you can redistribute it and/or modify it
+#  under the terms of the GNU Lesser General Public License as published by
+#  the Free Software Foundation; either version 2.1 of the License, or
+#  (at your option) any later version.
+
+[Unit]
+Description=Load kernel module %i
+Documentation=man:modprobe(8)
+
+[Service]
+Type=oneshot
+ExecStart=-/sbin/modprobe -abq %I