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 torvalds#594 Not tainted
| ------------------------------------------------------
| iptables/3303 is trying to acquire lock:
| ffff8881057e04b8 (kn->active#48){++++}-{0:0}, at: __kernfs_remove+0x20
|
| but task is already holding lock:
| ffffffffa0249068 (list_mutex){+.+.}-{3:3}, at: idletimer_tg_destroy_v]
|
| 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/xt_idletimer/timers/testme >/dev/null
| done

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

Fixes: 0902b46 ("netfilter: xtables: idletimer target implementation")
Signed-off-by: Phil Sutter <phil@nwl.cc>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>
Signed-off-by: NipaLocal <nipa@local>

Author: Phil Sutter

net/netfilter/xt_IDLETIMER.c

@@ -407,21 +407,23 @@ static void idletimer_tg_destroy(const struct xt_tgdtor_param *par)
 
 	mutex_lock(&list_mutex);
 
-	if (--info->timer->refcnt == 0) {
-		pr_debug("deleting timer %s\n", info->label);
-
-		list_del(&info->timer->entry);
-		timer_shutdown_sync(&info->timer->timer);
-		cancel_work_sync(&info->timer->work);
-		sysfs_remove_file(idletimer_tg_kobj, &info->timer->attr.attr);
-		kfree(info->timer->attr.attr.name);
-		kfree(info->timer);
-	} else {
+	if (--info->timer->refcnt > 0) {
 		pr_debug("decreased refcnt of timer %s to %u\n",
 			 info->label, info->timer->refcnt);
+		mutex_unlock(&list_mutex);
+		return;
 	}
 
+	pr_debug("deleting timer %s\n", info->label);
+
+	list_del(&info->timer->entry);
 	mutex_unlock(&list_mutex);
+
+	timer_shutdown_sync(&info->timer->timer);
+	cancel_work_sync(&info->timer->work);
+	sysfs_remove_file(idletimer_tg_kobj, &info->timer->attr.attr);
+	kfree(info->timer->attr.attr.name);
+	kfree(info->timer);
 }
 
 static void idletimer_tg_destroy_v1(const struct xt_tgdtor_param *par)
@@ -432,25 +434,27 @@ static void idletimer_tg_destroy_v1(const struct xt_tgdtor_param *par)
 
 	mutex_lock(&list_mutex);
 
-	if (--info->timer->refcnt == 0) {
-		pr_debug("deleting timer %s\n", info->label);
-
-		list_del(&info->timer->entry);
-		if (info->timer->timer_type & XT_IDLETIMER_ALARM) {
-			alarm_cancel(&info->timer->alarm);
-		} else {
-			timer_shutdown_sync(&info->timer->timer);
-		}
-		cancel_work_sync(&info->timer->work);
-		sysfs_remove_file(idletimer_tg_kobj, &info->timer->attr.attr);
-		kfree(info->timer->attr.attr.name);
-		kfree(info->timer);
-	} else {
+	if (--info->timer->refcnt > 0) {
 		pr_debug("decreased refcnt of timer %s to %u\n",
 			 info->label, info->timer->refcnt);
+		mutex_unlock(&list_mutex);
+		return;
 	}
 
+	pr_debug("deleting timer %s\n", info->label);
+
+	list_del(&info->timer->entry);
 	mutex_unlock(&list_mutex);
+
+	if (info->timer->timer_type & XT_IDLETIMER_ALARM) {
+		alarm_cancel(&info->timer->alarm);
+	} else {
+		timer_shutdown_sync(&info->timer->timer);
+	}
+	cancel_work_sync(&info->timer->work);
+	sysfs_remove_file(idletimer_tg_kobj, &info->timer->attr.attr);
+	kfree(info->timer->attr.attr.name);
+	kfree(info->timer);
 }