linux-kernel - [PATCH] drivers: staging: rtl8192u: Fix deadlock in ieee80211_beacons

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Date:   Sun, 17 Apr 2022 21:54:07 +0800
From:   Duoming Zhou <duoming@....edu.cn>
To:     linux-kernel@...r.kernel.org
Cc:     gregkh@...uxfoundation.org, davem@...emloft.net,
        alexander.deucher@....com, akpm@...ux-foundation.org,
        broonie@...nel.org, linux-staging@...ts.linux.dev,
        Duoming Zhou <duoming@....edu.cn>
Subject: [PATCH] drivers: staging: rtl8192u: Fix deadlock in ieee80211_beacons_stop()

There is a deadlock in ieee80211_beacons_stop(), which is shown below:

   (Thread 1)              |      (Thread 2)
                           | ieee80211_send_beacon()
ieee80211_beacons_stop()   |  mod_timer()
 spin_lock_irqsave() //(1) |  (wait a time)
 ...                       | ieee80211_send_beacon_cb()
 del_timer_sync()          |  spin_lock_irqsave() //(2)
 (wait timer to stop)      |  ...

We hold ieee->beacon_lock in position (1) of thread 1 and use
del_timer_sync() to wait timer to stop, but timer handler
also need ieee->beacon_lock in position (2) of thread 2.
As a result, ieee80211_beacons_stop() will block forever.

This patch extracts del_timer_sync() from the protection of
spin_lock_irqsave(), which could let timer handler to obtain
the needed lock.

Signed-off-by: Duoming Zhou <duoming@....edu.cn>
---
 drivers/staging/rtl8192u/ieee80211/ieee80211_softmac.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/drivers/staging/rtl8192u/ieee80211/ieee80211_softmac.c b/drivers/staging/rtl8192u/ieee80211/ieee80211_softmac.c
index 1a43979939a..79f3fbe2555 100644
--- a/drivers/staging/rtl8192u/ieee80211/ieee80211_softmac.c
+++ b/drivers/staging/rtl8192u/ieee80211/ieee80211_softmac.c
@@ -528,9 +528,9 @@ static void ieee80211_beacons_stop(struct ieee80211_device *ieee)
 	spin_lock_irqsave(&ieee->beacon_lock, flags);
 
 	ieee->beacon_txing = 0;
-	del_timer_sync(&ieee->beacon_timer);
 
 	spin_unlock_irqrestore(&ieee->beacon_lock, flags);
+	del_timer_sync(&ieee->beacon_timer);
 }
 
 void ieee80211_stop_send_beacons(struct ieee80211_device *ieee)
-- 
2.17.1