[PATCH batadv 3/8] batman-adv: bla: only purge non-released claims

[Sven Eckelmann <sven@narfation.org>]

When batadv_bla_purge_claims() goes through the list of claims, it is only
traversing the hash list with an rcu_read_lock(). Due to a potential
parallel batadv_claim_put(), it can happen that it encounters a claim which
was actually in the process of being released+freed by
batadv_claim_release(). In this case, backbone_gw is set to NULL before the
delayed RCU kfree is started. Calling batadv_bla_claim_get_backbone_gw() is
then no longer allowed because it would cause a NULL-ptr derefence.

To avoid this, only claims with a valid reference counter must be purged.
All others are already taken care of.

Cc: stable@kernel.org
Fixes: 23721387c409 ("batman-adv: add basic bridge loop avoidance code")
Signed-off-by: Sven Eckelmann <sven@narfation.org>
---
 net/batman-adv/bridge_loop_avoidance.c | 8 ++++++++
 1 file changed, 8 insertions(+)

diff --git a/net/batman-adv/bridge_loop_avoidance.c b/net/batman-adv/bridge_loop_avoidance.c
index 8b77dd2ecfa4..9dbf945b4922 100644
--- a/net/batman-adv/bridge_loop_avoidance.c
+++ b/net/batman-adv/bridge_loop_avoidance.c
@@ -1288,6 +1288,13 @@ static void batadv_bla_purge_claims(struct batadv_priv *bat_priv,
 
 		rcu_read_lock();
 		hlist_for_each_entry_rcu(claim, head, hash_entry) {
+			/* only purge claims not currently in the process of being released.
+			 * Such claims could otherwise have a NULL-ptr* backbone_gw set because
+			 * they already went through batadv_handle_unclaim()
+			 */
+			if (!kref_get_unless_zero(&claim->refcount))
+				continue;
+
 			backbone_gw = batadv_bla_claim_get_backbone_gw(claim);
 			if (now)
 				goto purge_now;
@@ -1313,6 +1320,7 @@ static void batadv_bla_purge_claims(struct batadv_priv *bat_priv,
 					      claim->addr, claim->vid);
 skip:
 			batadv_backbone_gw_put(backbone_gw);
+			batadv_claim_put(claim);
 		}
 		rcu_read_unlock();
 	}

-- 
2.47.3