CVE-2024-54683: netfilter: IDLETIMER: Fix for possible ABBA deadlock

In the Linux kernel, the following vulnerability has been resolved:

netfilter: IDLETIMER: Fix for possible ABBA deadlock

Deletion of the last rule referencing a given idletimer may happen at the same time as a read of its file in sysfs:

| ====================================================== | WARNING: possible circular locking dependency detected | 6.12.0-rc7-01692-g5e9a28f41134-dirty #594 Not tainted | ------------------------------------------------------ | iptables/3303 is trying to acquire lock: | ffff8881057e04b8 (kn->active#48){++++}-{0:0}, at: kernfsremove+0x20 | | but task is already holding lock: | ffffffffa0249068 (listmutex){+.+.}-{3:3}, at: idletimertgdestroyv] | | which lock already depends on the new lock.

A simple reproducer is:

| #!/bin/bash | | while true; do | iptables -A INPUT -i foo -j IDLETIMER --timeout 10 --label "testme" | iptables -D INPUT -i foo -j IDLETIMER --timeout 10 --label "testme" | done & | while true; do | cat /sys/class/xtidletimer/timers/testme >/dev/null | done

Avoid this by freeing listmutex right after deleting the element from the list, then continuing with the teardown.