[PATCH net] tun: delete the flow entries of the moved queue on detach

[Umang Pokhriyal]

When a queue is detached, __tun_detach() moves the last queue into the
freed slot and decrements tun->numqueues. The flow entries that point at
the old index of the moved queue, now tun->numqueues, are meant to be
deleted, but the call passes tun->numqueues + 1, an index that never
exists, so nothing is deleted.

Until those entries are refreshed by a write on their flow or age out,
tun_automq_select_queue() returns an index past the last queue, and
netdev_core_pick_tx() sends every such flow to queue 0 and warns:

  tapq0 selects TX queue 3, but real number of TX queues is 3

Found with an LLM while testing how a VMM detaches tap queues when a
guest lowers its virtio-net queue pairs. With four queues, 30 flows
learned on queue 3 and queue 1 detached, all 30 went to queue 0 with the
warning; with this patch they are spread by hash and no warning is
printed (3 runs each, in a guest booted on net).

Pass tun->numqueues.

Fixes: 96442e42429e ("tuntap: choose the txq based on rxq")
Signed-off-by: Umang Pokhriyal <umangpokhriyall@xxxxxxxxx>
---
 drivers/net/tun.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/drivers/net/tun.c b/drivers/net/tun.c
index dcec7311e..3b292bce4 100644
--- a/drivers/net/tun.c
+++ b/drivers/net/tun.c
@@ -615,7 +615,7 @@ static void __tun_detach(struct tun_file *tfile, bool clean)
 		}
 
 		synchronize_net();
-		tun_flow_delete_by_queue(tun, tun->numqueues + 1);
+		tun_flow_delete_by_queue(tun, tun->numqueues);
 		/* Drop read queue */
 		tun_queue_purge(tfile);
 		spin_lock_bh(&ntfile->tx_ring.consumer_lock);
-- 
2.53.0