fix(systemd): let the WireGuard kernel TUN write rp_filter under the sandbox
ProtectKernelTunables=yes (#6718) mounts all of /proc/sys read-only, and a
WireGuard outbound in kernel-TUN mode zeroes net.ipv4.conf.all.rp_filter (and
its ipv6 and per-device siblings) while Xray creates its handlers, so the core
aborted with "failed to disable ipv4 rp_filter for all: ... read-only file
system". The unit is reinstalled on every update, so a hand edit did not stick.
Keep ProtectKernelTunables and reopen only /proc/sys/net. Turning the
protection off (#6797) also lets the core start, but leaves kernel.core_pattern
writable to the root panel, which turns any panel-level code execution into a
way out of the whole sandbox. net.* is per network namespace and grants nothing
beyond the CAP_NET_ADMIN the panel already holds.
Verified on systemd 257 (Debian trixie) with the shipped unit and the pinned
core: the v3.10.0 unit reproduces the error, the new one starts the kernel TUN,
and kernel.core_pattern stays read-only for the service.
Closes #6796
MHSanaei