[PATCH net 10/11] batman-adv: bla: avoid double decrement of bla.num_requests

[Simon Wunderlich <sw@simonwunderlich.de>]

From: Sven Eckelmann <sven@narfation.org>

The bla.num_requests is increased when no request_sent was in progress. And
it is decremented in various places (announcement was received, backbone is
purged, periodic work). But the check if the request_sent is actually set
to a specific state and the atomic_dec/_inc are not safe because they are
not atomic (TOCTOU) and multiple such code portions can run concurrently.

At the same time, it is necessary to modify request_sent (state) and
bla.num_requests atomically. Otherwise batadv_bla_send_request() might set
request_sent to 1 and is interrupted.  batadv_handle_announce() can then
set request_sent back to 0 and decrement num_requests before
batadv_bla_send_request() incremented it.

The two operations must therefore be locked. And since state (request_sent)
and wait_periods are only accessed inside this lock, they can be converted
to simpler datatypes. And to avoid that the bla.num_requests is touched by
a parallel running context with a valid backbone_gw reference after
batadv_bla_purge_backbone_gw() ran, a third state "stopped" is required to
correctly signal that a backbone_gw is in the state of being cleaned up.

Cc: stable@kernel.org
Fixes: 23721387c409 ("batman-adv: add basic bridge loop avoidance code")
Signed-off-by: Sven Eckelmann <sven@narfation.org>
Signed-off-by: Simon Wunderlich <sw@simonwunderlich.de>
---
 net/batman-adv/bridge_loop_avoidance.c | 51 ++++++++++++++++++--------
 net/batman-adv/mesh-interface.c        |  1 +
 net/batman-adv/types.h                 | 39 ++++++++++++++++----
 3 files changed, 67 insertions(+), 24 deletions(-)

diff --git a/net/batman-adv/bridge_loop_avoidance.c b/net/batman-adv/bridge_loop_avoidance.c
index df1dfdf4a1a12..1bef12e659cb2 100644
--- a/net/batman-adv/bridge_loop_avoidance.c
+++ b/net/batman-adv/bridge_loop_avoidance.c
@@ -514,8 +514,8 @@ batadv_bla_get_backbone_gw(struct batadv_priv *bat_priv, const u8 *orig,
 	entry->crc = BATADV_BLA_CRC_INIT;
 	entry->bat_priv = bat_priv;
 	spin_lock_init(&entry->crc_lock);
-	atomic_set(&entry->request_sent, 0);
-	atomic_set(&entry->wait_periods, 0);
+	entry->state = BATADV_BLA_BACKBONE_GW_SYNCED;
+	entry->wait_periods = 0;
 	ether_addr_copy(entry->orig, orig);
 	INIT_WORK(&entry->report_work, batadv_bla_loopdetect_report);
 	kref_init(&entry->refcount);
@@ -544,9 +544,13 @@ batadv_bla_get_backbone_gw(struct batadv_priv *bat_priv, const u8 *orig,
 		batadv_bla_send_announce(bat_priv, entry);
 
 		/* this will be decreased in the worker thread */
-		atomic_inc(&entry->request_sent);
-		atomic_set(&entry->wait_periods, BATADV_BLA_WAIT_PERIODS);
-		atomic_inc(&bat_priv->bla.num_requests);
+		spin_lock_bh(&bat_priv->bla.num_requests_lock);
+		if (entry->state == BATADV_BLA_BACKBONE_GW_SYNCED) {
+			entry->state = BATADV_BLA_BACKBONE_GW_UNSYNCED;
+			entry->wait_periods = BATADV_BLA_WAIT_PERIODS;
+			atomic_inc(&bat_priv->bla.num_requests);
+		}
+		spin_unlock_bh(&bat_priv->bla.num_requests_lock);
 	}
 
 	return entry;
@@ -649,10 +653,12 @@ static void batadv_bla_send_request(struct batadv_bla_backbone_gw *backbone_gw)
 			      backbone_gw->vid, BATADV_CLAIM_TYPE_REQUEST);
 
 	/* no local broadcasts should be sent or received, for now. */
-	if (!atomic_read(&backbone_gw->request_sent)) {
+	spin_lock_bh(&backbone_gw->bat_priv->bla.num_requests_lock);
+	if (backbone_gw->state == BATADV_BLA_BACKBONE_GW_SYNCED) {
+		backbone_gw->state = BATADV_BLA_BACKBONE_GW_UNSYNCED;
 		atomic_inc(&backbone_gw->bat_priv->bla.num_requests);
-		atomic_set(&backbone_gw->request_sent, 1);
 	}
+	spin_unlock_bh(&backbone_gw->bat_priv->bla.num_requests_lock);
 }
 
 /**
@@ -873,10 +879,12 @@ static bool batadv_handle_announce(struct batadv_priv *bat_priv, u8 *an_addr,
 		/* if we have sent a request and the crc was OK,
 		 * we can allow traffic again.
 		 */
-		if (atomic_read(&backbone_gw->request_sent)) {
+		spin_lock_bh(&bat_priv->bla.num_requests_lock);
+		if (backbone_gw->state == BATADV_BLA_BACKBONE_GW_UNSYNCED) {
+			backbone_gw->state = BATADV_BLA_BACKBONE_GW_SYNCED;
 			atomic_dec(&backbone_gw->bat_priv->bla.num_requests);
-			atomic_set(&backbone_gw->request_sent, 0);
 		}
+		spin_unlock_bh(&bat_priv->bla.num_requests_lock);
 	}
 
 	batadv_backbone_gw_put(backbone_gw);
@@ -1255,9 +1263,13 @@ static void batadv_bla_purge_backbone_gw(struct batadv_priv *bat_priv, int now)
 				purged = true;
 
 				/* don't wait for the pending request anymore */
-				if (atomic_read(&backbone_gw->request_sent))
+				spin_lock_bh(&bat_priv->bla.num_requests_lock);
+				if (backbone_gw->state == BATADV_BLA_BACKBONE_GW_UNSYNCED)
 					atomic_dec(&bat_priv->bla.num_requests);
 
+				backbone_gw->state = BATADV_BLA_BACKBONE_GW_STOPPED;
+				spin_unlock_bh(&bat_priv->bla.num_requests_lock);
+
 				batadv_bla_del_backbone_claims(backbone_gw);
 
 				hlist_del_rcu(&backbone_gw->hash_entry);
@@ -1508,7 +1520,7 @@ static void batadv_bla_periodic_work(struct work_struct *work)
 				batadv_bla_send_loopdetect(bat_priv,
 							   backbone_gw);
 
-			/* request_sent is only set after creation to avoid
+			/* state is only set to unsynced after creation to avoid
 			 * problems when we are not yet known as backbone gw
 			 * in the backbone.
 			 *
@@ -1517,14 +1529,21 @@ static void batadv_bla_periodic_work(struct work_struct *work)
 			 * some grace time.
 			 */
 
-			if (atomic_read(&backbone_gw->request_sent) == 0)
-				continue;
+			spin_lock_bh(&bat_priv->bla.num_requests_lock);
+			if (backbone_gw->state != BATADV_BLA_BACKBONE_GW_UNSYNCED)
+				goto unlock_next;
 
-			if (!atomic_dec_and_test(&backbone_gw->wait_periods))
-				continue;
+			if (backbone_gw->wait_periods > 0)
+				backbone_gw->wait_periods--;
+
+			if (backbone_gw->wait_periods > 0)
+				goto unlock_next;
 
+			backbone_gw->state = BATADV_BLA_BACKBONE_GW_SYNCED;
 			atomic_dec(&backbone_gw->bat_priv->bla.num_requests);
-			atomic_set(&backbone_gw->request_sent, 0);
+
+unlock_next:
+			spin_unlock_bh(&bat_priv->bla.num_requests_lock);
 		}
 		rcu_read_unlock();
 	}
diff --git a/net/batman-adv/mesh-interface.c b/net/batman-adv/mesh-interface.c
index 56ca1c1b83f27..e7aa45bc6b7ad 100644
--- a/net/batman-adv/mesh-interface.c
+++ b/net/batman-adv/mesh-interface.c
@@ -787,6 +787,7 @@ static int batadv_meshif_init_late(struct net_device *dev)
 	atomic_set(&bat_priv->tt.ogm_append_cnt, 0);
 #ifdef CONFIG_BATMAN_ADV_BLA
 	atomic_set(&bat_priv->bla.num_requests, 0);
+	spin_lock_init(&bat_priv->bla.num_requests_lock);
 #endif
 	atomic_set(&bat_priv->tp_num, 0);
 
diff --git a/net/batman-adv/types.h b/net/batman-adv/types.h
index 821ada05d86a7..a01ee46d97f34 100644
--- a/net/batman-adv/types.h
+++ b/net/batman-adv/types.h
@@ -1026,6 +1026,12 @@ struct batadv_priv_bla {
 	/** @num_requests: number of bla requests in flight */
 	atomic_t num_requests;
 
+	/**
+	 * @num_requests_lock: locks update num_requests +
+	 * batadv_backbone_gw::state + batadv_backbone_gw::wait_periods update
+	 */
+	spinlock_t num_requests_lock;
+
 	/**
 	 * @claim_hash: hash table containing mesh nodes this host has claimed
 	 */
@@ -1672,6 +1678,27 @@ struct batadv_priv {
 
 #ifdef CONFIG_BATMAN_ADV_BLA
 
+enum batadv_bla_backbone_gw_state {
+	/**
+	 * @BATADV_BLA_BACKBONE_GW_STOPPED: backbone gw is being removed
+	 * and it must not longer work on requests
+	 */
+	BATADV_BLA_BACKBONE_GW_STOPPED,
+
+	/**
+	 * @BATADV_BLA_BACKBONE_GW_UNSYNCED: backbone was detected out
+	 * of sync and a request was send. No traffic is forwarded until the
+	 * situation is resolved
+	 */
+	BATADV_BLA_BACKBONE_GW_UNSYNCED,
+
+	/**
+	 * @BATADV_BLA_BACKBONE_GW_SYNCED: backbone is consider to be in
+	 * sync. traffic can be forwarded
+	 */
+	BATADV_BLA_BACKBONE_GW_SYNCED,
+};
+
 /**
  * struct batadv_bla_backbone_gw - batman-adv gateway bridged into the LAN
  */
@@ -1697,16 +1724,12 @@ struct batadv_bla_backbone_gw {
 	/**
 	 * @wait_periods: grace time for bridge forward delays and bla group
 	 *  forming at bootup phase - no bcast traffic is formwared until it has
-	 *  elapsed
+	 *  elapsed. Must only be access with num_requests_lock.
 	 */
-	atomic_t wait_periods;
+	u8 wait_periods;
 
-	/**
-	 * @request_sent: if this bool is set to true we are out of sync with
-	 *  this backbone gateway - no bcast traffic is formwared until the
-	 *  situation was resolved
-	 */
-	atomic_t request_sent;
+	/** @state: sync state. Must only be access with num_requests_lock. */
+	enum batadv_bla_backbone_gw_state state;
 
 	/** @crc: crc16 checksum over all claims */
 	u16 crc;
-- 
2.47.3