[PATCH net] net: neighbour: Serialize proxy queue admission

[Chengfeng Ye <nicoyip.dev@gmail.com>]

pneigh_enqueue() checks p->qlen before taking proxy_queue.lock, although
both enqueueing and the timer and purge paths update it under that lock.
The unlocked read races with those updates and allows concurrent proxy
ARP or NDP requests to bypass the queue's admission limit.

For example, CPU0 and CPU1 can both observe p->qlen == PROXY_QLEN before
either takes the lock. CPU0 then locks and enqueues, raising the count to
PROXY_QLEN + 1. CPU1 subsequently locks and enqueues using its stale
admission decision, raising the count to PROXY_QLEN + 2. The extra queued
packets consume memory until the timer or purge path removes them.

With a temporary 20 ms delay before locking and a queue-length assertion,
the kernel reported qlen=6 with PROXY_QLEN=4:

  WARNING: net/core/neighbour.c:1757 at pneigh_enqueue+0x4ee/0x650
  Call Trace:
   <IRQ>
   arp_process+0x1846/0x2060
   __netif_receive_skb_core.constprop.0+0x1524/0x2bd0
   __netif_receive_skb_one_core+0xa9/0x1b0
   process_backlog+0x1e5/0x5e0
   __napi_poll+0x9c/0x540
   net_rx_action+0x988/0xfb0
   handle_softirqs+0x18d/0x5b0
   do_softirq+0x3b/0x60
   </IRQ>

Move the existing lock acquisition before the admission check so that the
check and increment are serialized with all queue-length updates. Unlock
before freeing a rejected packet. Keep the existing > comparison so that
serial admission behavior is unchanged.

Fixes: 1da177e4c3f4 ("Linux-2.6.12-rc2")
Signed-off-by: Chengfeng Ye <nicoyip.dev@gmail.com>
---
 net/core/neighbour.c | 3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)

diff --git a/net/core/neighbour.c b/net/core/neighbour.c
index 7448320f7ad5..e35ce26b4f8a 100644
--- a/net/core/neighbour.c
+++ b/net/core/neighbour.c
@@ -1721,7 +1721,9 @@ void pneigh_enqueue(struct neigh_table *tbl, struct neigh_parms *p,
 {
 	unsigned long sched_next = neigh_proxy_delay(p);
 
+	spin_lock(&tbl->proxy_queue.lock);
 	if (p->qlen > NEIGH_VAR(p, PROXY_QLEN)) {
+		spin_unlock(&tbl->proxy_queue.lock);
 		kfree_skb(skb);
 		return;
 	}
@@ -1729,7 +1731,6 @@ void pneigh_enqueue(struct neigh_table *tbl, struct neigh_parms *p,
 	NEIGH_CB(skb)->sched_next = sched_next;
 	NEIGH_CB(skb)->flags |= LOCALLY_ENQUEUED;
 
-	spin_lock(&tbl->proxy_queue.lock);
 	if (timer_delete(&tbl->proxy_timer)) {
 		if (time_before(tbl->proxy_timer.expires, sched_next))
 			sched_next = tbl->proxy_timer.expires;
-- 
2.43.0