netdev - [PATCH AUTOSEL 4.9 04/23] neighbour: Disregard DEAD dst in neigh

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Message-Id: <20210412162704.315783-4-sashal@kernel.org>
Date:   Mon, 12 Apr 2021 12:26:45 -0400
From:   Sasha Levin <sashal@...nel.org>
To:     linux-kernel@...r.kernel.org, stable@...r.kernel.org
Cc:     Tong Zhu <zhutong@...zon.com>,
        "David S . Miller" <davem@...emloft.net>,
        Sasha Levin <sashal@...nel.org>, netdev@...r.kernel.org
Subject: [PATCH AUTOSEL 4.9 04/23] neighbour: Disregard DEAD dst in neigh_update

From: Tong Zhu <zhutong@...zon.com>

[ Upstream commit d47ec7a0a7271dda08932d6208e4ab65ab0c987c ]

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@...zon.com>
Signed-off-by: David S. Miller <davem@...emloft.net>
Signed-off-by: Sasha Levin <sashal@...nel.org>
---
 net/core/neighbour.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/net/core/neighbour.c b/net/core/neighbour.c
index d267dc04d9f7..2aa5c231560d 100644
--- a/net/core/neighbour.c
+++ b/net/core/neighbour.c
@@ -1230,7 +1230,7 @@ int neigh_update(struct neighbour *neigh, const u8 *lladdr, u8 new,
 			 * 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;
-- 
2.30.2