batman-adv: mcast: fix use-after-free in orig_node RCU release

batadv_mcast_purge_orig() removes entries from RCU-protected hlists but
does not wait for an RCU grace period before returning. Concurrent RCU
readers may still accesses references to those entries at the point of
removal. RCU-protected readers trying to operate on entries like
orig->mcast_want_all_ipv6_node will then access already freed memory.

Fix this by moving batadv_mcast_purge_orig() to batadv_orig_node_release(),
just before the call_rcu() invocation. This ensures RCU readers that were
active at purge time have drained before the orig_node memory is reclaimed.

Cc: stable@kernel.org
Fixes: ab49886e3d ("batman-adv: Add IPv4 link-local/IPv6-ll-all-nodes multicast support")
Acked-by: Linus Lüssing <linus.luessing@c0d3.blue>
Signed-off-by: Sven Eckelmann <sven@narfation.org>

net/batman-adv/originator.c

@@ -835,8 +835,6 @@ static void batadv_orig_node_free_rcu(struct rcu_head *rcu)
 
 	orig_node = container_of(rcu, struct batadv_orig_node, rcu);
 
-	batadv_mcast_purge_orig(orig_node);
-
 	batadv_frag_purge_orig(orig_node, NULL);
 
 	kfree(orig_node->tt_buff);
@@ -887,6 +885,8 @@ void batadv_orig_node_release(struct kref *ref)
 	}
 	spin_unlock_bh(&orig_node->vlan_list_lock);
 
+	batadv_mcast_purge_orig(orig_node);
+
 	call_rcu(&orig_node->rcu, batadv_orig_node_free_rcu);
 }