From d35acc1be3480505b5931f17e4ea9b7617fea4d3 Mon Sep 17 00:00:00 2001 From: Eric Dumazet Date: Wed, 11 Jun 2025 11:15:11 +0000 Subject: [PATCH] net_sched: prio: fix a race in prio_tune() Gerrard Tai reported a race condition in PRIO, whenever SFQ perturb timer fires at the wrong time. The race is as follows: CPU 0 CPU 1 [1]: lock root [2]: qdisc_tree_flush_backlog() [3]: unlock root | | [5]: lock root | [6]: rehash | [7]: qdisc_tree_reduce_backlog() | [4]: qdisc_put() This can be abused to underflow a parent's qlen. Calling qdisc_purge_queue() instead of qdisc_tree_flush_backlog() should fix the race, because all packets will be purged from the qdisc before releasing the lock. Fixes: 7b8e0b6e6599 ("net: sched: prio: delay destroying child qdiscs on change") Reported-by: Gerrard Tai Suggested-by: Gerrard Tai Signed-off-by: Eric Dumazet Link: https://patch.msgid.link/20250611111515.1983366-2-edumazet@google.com Signed-off-by: Jakub Kicinski --- net/sched/sch_prio.c | 2 +- 1 file changed, 1 insertion(+), 1 deletion(-) diff --git a/net/sched/sch_prio.c b/net/sched/sch_prio.c index cc30f7a32f1a..9e2b9a490db2 100644 --- a/net/sched/sch_prio.c +++ b/net/sched/sch_prio.c @@ -211,7 +211,7 @@ static int prio_tune(struct Qdisc *sch, struct nlattr *opt, memcpy(q->prio2band, qopt->priomap, TC_PRIO_MAX+1); for (i = q->bands; i < oldbands; i++) - qdisc_tree_flush_backlog(q->queues[i]); + qdisc_purge_queue(q->queues[i]); for (i = oldbands; i < q->bands; i++) { q->queues[i] = queues[i]; -- 2.39.5