Subject: batman-adv: Broken sync while rescheduling delayed work
From: Vladislav Efanov <[email-masked]>
Date: Fri, 26 May 2023 19:16:32 +0300

Syzkaller got a lot of crashes like:
KASAN: use-after-free Write in *_timers*

All of these crashes point to the same memory area:

The buggy address belongs to the object at ffff88801f870000
 which belongs to the cache kmalloc-8k of size 8192
The buggy address is located 5320 bytes inside of
 8192-byte region [ffff88801f870000, ffff88801f872000)

This area belongs to :
        batadv_priv->batadv_priv_dat->delayed_work->timer_list

The reason for these issues is the lack of synchronization. Delayed
work (batadv_dat_purge) schedules new timer/work while the device
is being deleted. As the result new timer/delayed work is set after
cancel_delayed_work_sync() was called. So after the device is freed
the timer list contains pointer to already freed memory.

Found by Linux Verification Center (linuxtesting.org) with syzkaller.

Fixes: f6badf9eb582 ("batman-adv: Distributed ARP Table - implement local storage")
Signed-off-by: Vladislav Efanov <[email-masked]>
Signed-off-by: Sven Eckelmann <[email-masked]>
Origin: upstream, https://git.open-mesh.org/batman-adv.git/commit/177ba85fb2d4ca2ecd30ca16803560e80e916fac

--- a/net/batman-adv/distributed-arp-table.c
+++ b/net/batman-adv/distributed-arp-table.c
@@ -101,7 +101,6 @@ static void batadv_dat_purge(struct work
  */
 static void batadv_dat_start_timer(struct batadv_priv *bat_priv)
 {
-	INIT_DELAYED_WORK(&bat_priv->dat.work, batadv_dat_purge);
 	queue_delayed_work(batadv_event_workqueue, &bat_priv->dat.work,
 			   msecs_to_jiffies(10000));
 }
@@ -819,6 +818,7 @@ int batadv_dat_init(struct batadv_priv *
 	if (!bat_priv->dat.hash)
 		return -ENOMEM;
 
+	INIT_DELAYED_WORK(&bat_priv->dat.work, batadv_dat_purge);
 	batadv_dat_start_timer(bat_priv);
 
 	batadv_tvlv_handler_register(bat_priv, batadv_dat_tvlv_ogm_handler_v1,