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From:   Vladislav Efanov <VEfanov@ispras.ru>
To:     Marek Lindner <mareklindner@neomailbox.ch>
Cc:     Vladislav Efanov <VEfanov@ispras.ru>,
        Simon Wunderlich <sw@simonwunderlich.de>,
        Antonio Quartulli <a@unstable.cc>,
        Sven Eckelmann <sven@narfation.org>,
        "David S. Miller" <davem@davemloft.net>,
        Eric Dumazet <edumazet@google.com>,
        Jakub Kicinski <kuba@kernel.org>,
        Paolo Abeni <pabeni@redhat.com>,
        b.a.t.m.a.n@lists.open-mesh.org, netdev@vger.kernel.org,
        linux-kernel@vger.kernel.org, lvc-project@linuxtesting.org
Subject: [PATCH] batman-adv: Broken sync while rescheduling delayed work
Date:   Fri, 26 May 2023 19:16:32 +0300
Message-Id: <20230526161632.1460753-1-VEfanov@ispras.ru>
MIME-Version: 1.0
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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: 2f1dfbe18507 ("batman-adv: Distributed ARP Table - implement local storage")
Signed-off-by: Vladislav Efanov <VEfanov@ispras.ru>
---
 net/batman-adv/distributed-arp-table.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/net/batman-adv/distributed-arp-table.c b/net/batman-adv/distributed-arp-table.c
index 6968e55eb971..28a939d56090 100644
--- 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_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 *bat_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,
-- 
2.34.1