# UNDF: UNDF-2026-000000147
# CWE-407: Algorithmic Complexity — O(P) → O(1) in net/core/neighbour.c lookup_neigh_parms()
#
# Defect: lookup_neigh_parms() walks tbl->parms_list — one entry per registered network
# device — via list_for_each_entry until the matching ifindex is found.  In VxLAN/bridge
# environments with P=hundreds of devices joined to ARP/NDP tables, every netlink neigh
# parameter command pays O(P) cost.  Called on every 'ip neigh change', ARP table config,
# and NDP parameter update.
#
# Fix: maintain tbl->parms_xa (struct xarray) keyed by ifindex alongside the existing
# parms_list.  neigh_parms_alloc() stores into it; neigh_parms_release() erases from it.
# lookup_neigh_parms() becomes a single xa_load() — O(1).  parms_list is retained for
# GC iteration and module compatibility.
#
# Complexity gate (unit/LinuxTest.java linux-0003 benchmark):
#   P=500 devices: slow O(500) avg=250 comparisons, fast O(1).
#   At P=500: >250× speedup.  20× is a conservative lower bound even at P=20.
#   Gate: ratio slow/fast must be ≥20× at P=500 sequential lookups.
#
--- a/net/core/neighbour.c
+++ b/net/core/neighbour.c
@@ -1752,11 +1752,37 @@ static void pneigh_queue_purge(struct sk_buff_head *list, struct net *net,
 	spin_unlock_irqrestore(&list->lock, flags);
 }

+/*
+ * CWE-407 fix: replace O(P) linear parms_list scan with O(1) xarray lookup.
+ *
+ * The original lookup_neigh_parms walks tbl->parms_list which holds one entry
+ * per network device registered with this neighbour table.  In VxLAN/bridge
+ * environments with hundreds of devices, this is O(D) per netlink command.
+ *
+ * Fix: maintain tbl->parms_xa (struct xarray) keyed by ifindex.
+ * neigh_parms_alloc() stores into it; neigh_parms_release() erases from it.
+ * lookup_neigh_parms() becomes a single xa_load() call.
+ *
+ * parms_list is retained for GC iteration.  ifindex==0 means global parms;
+ * stored at xarray key 0.
+ *
+ * NOTE: struct neigh_table in include/net/neighbour.h must gain:
+ *   struct xarray parms_xa;
+ * Initialised in neigh_table_init() via xa_init(&tbl->parms_xa).
+ * Destroyed in neigh_table_unregister() via xa_destroy(&tbl->parms_xa).
+ */
 static inline struct neigh_parms *lookup_neigh_parms(struct neigh_table *tbl,
 						      struct net *net, int ifindex)
 {
-	struct neigh_parms *p;
+	struct neigh_parms *p;  /* retained for slow-path fallback */

-	list_for_each_entry(p, &tbl->parms_list, list) {
-		if ((p->dev && p->dev->ifindex == ifindex && net_eq(neigh_parms_net(p), net)) ||
-		    (!p->dev && !ifindex && net_eq(net, &init_net)))
+	/* CWE-407 fast path: O(1) xarray lookup by ifindex. */
+	p = xa_load(&tbl->parms_xa, (unsigned long)ifindex);
+	if (p && net_eq(neigh_parms_net(p), net))
+		return p;
+
+	/*
+	 * Slow fallback: handles the case where parms_xa is not yet populated
+	 * (boot time, module load) or net namespace mismatch.  O(P).
+	 */
+	list_for_each_entry(p, &tbl->parms_list, list) {
+		if ((p->dev && p->dev->ifindex == ifindex &&
+		     net_eq(neigh_parms_net(p), net)) ||
+		    (!p->dev && !ifindex && net_eq(net, &init_net)))
 			return p;
 	}

 	return NULL;
 }

@@ -1790,6 +1816,10 @@ struct neigh_parms *neigh_parms_alloc(struct net_device *dev,
 		p->dev = dev;
 		p->dev_tracker = dev_tracker;
 		list_add(&p->list, &tbl->parms_list);
+		/* CWE-407 fix: populate xarray so lookup_neigh_parms() is O(1). */
+		xa_store(&tbl->parms_xa,
+			 (unsigned long)(dev ? dev->ifindex : 0),
+			 p, GFP_KERNEL);
 		write_pnet(&p->net, net);
 	}
 	return p;
@@ -1822,6 +1852,9 @@ void neigh_parms_release(struct neigh_table *tbl, struct neigh_parms *parms)
 	if (parms == &tbl->parms)
 		return;
 	list_del(&parms->list);
+	/* CWE-407 fix: evict from xarray on release. */
+	xa_erase(&tbl->parms_xa,
+		 (unsigned long)(parms->dev ? parms->dev->ifindex : 0));
 	kfree_rcu(parms, rcu_head);
 }
 EXPORT_SYMBOL(neigh_parms_release);