units: switch from system call blacklist to whitelist

This is generally the safer approach, and is what container managers
(including nspawn) do, hence let's move to this too for our own
services. This is particularly useful as this this means the new
@system-service system call filter group will get serious real-life
testing quickly.

This also switches from firing SIGSYS on unexpected syscalls to
returning EPERM. This would have probably been a better default anyway,
but it's hard to change that these days. When whitelisting system calls
SIGSYS is highly problematic as system calls that are newly introduced
to Linux become minefields for services otherwise.

Note that this enables a system call filter for udev for the first time,
and will block @clock, @mount and @swap from it. Some downstream
distributions might want to revert this locally if they want to permit
unsafe operations on udev rules, but in general this shiuld be mostly
safe, as we already set MountFlags=shared for udevd, hence at least
@mount won't change anything.

1 files changed, 2 insertions, 1 deletions

diff --git a/units/systemd-importd.service.in b/units/systemd-importd.service.in
index 8e93c2d524..20704a8232 100644
--- a/units/systemd-importd.service.in
+++ b/units/systemd-importd.service.in
@@ -23,6 +23,7 @@ MemoryDenyWriteExecute=yes
 RestrictRealtime=yes
 RestrictNamespaces=net
 RestrictAddressFamilies=AF_UNIX AF_INET AF_INET6
-SystemCallFilter=~@clock @cpu-emulation @debug @keyring @module @obsolete @raw-io @reboot @swap
+SystemCallFilter=@system-service @mount
+SystemCallErrorNumber=EPERM
 SystemCallArchitectures=native
 LockPersonality=yes