neighbour: Disregard DEAD dst in neigh_update

After a short network outage, the dst_entry is timed out and put
in DST_OBSOLETE_DEAD. We are in this code because arp reply comes
from this neighbour after network recovers. There is a potential
race condition that dst_entry is still in DST_OBSOLETE_DEAD.
With that, another neighbour lookup causes more harm than good.

In best case all packets in arp_queue are lost. This is
counterproductive to the original goal of finding a better path
for those packets.

I observed a worst case with 4.x kernel where a dst_entry in
DST_OBSOLETE_DEAD state is associated with loopback net_device.
It leads to an ethernet header with all zero addresses.
A packet with all zero source MAC address is quite deadly with
mac80211, ath9k and 802.11 block ack.  It fails
ieee80211_find_sta_by_ifaddr in ath9k (xmit.c). Ath9k flushes tx
queue (ath_tx_complete_aggr). BAW (block ack window) is not
updated. BAW logic is damaged and ath9k transmission is disabled.

Signed-off-by: Tong Zhu <zhutong@amazon.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

diff --git a/net/core/neighbour.c b/net/core/neighbour.c
index e2982b3970b88dfa671c7d7da23bd55154f8988c..8379719d1dcef1bb1ea0b673d3dcf1030380439c 100644 (file)
--- a/net/core/neighbour.c
+++ b/net/core/neighbour.c
@@ -1379,7 +1379,7 @@ static int __neigh_update(struct neighbour *neigh, const u8 *lladdr,
                         * we can reinject the packet there.
                         */
                        n2 = NULL;
-                       if (dst) {
+                       if (dst && dst->obsolete != DST_OBSOLETE_DEAD) {
                                n2 = dst_neigh_lookup_skb(dst, skb);
                                if (n2)
                                        n1 = n2;