From 43eb466041216d25dedaef1c383ad7bd89929cbc Mon Sep 17 00:00:00 2001 From: Eric Dumazet Date: Wed, 11 Jun 2025 11:15:13 +0000 Subject: [PATCH] net_sched: tbf: fix a race in tbf_change() Gerrard Tai reported a race condition in TBF, 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: b05972f01e7d ("net: sched: tbf: don't call qdisc_put() while holding tree lock") Reported-by: Gerrard Tai Suggested-by: Gerrard Tai Signed-off-by: Eric Dumazet Cc: Zhengchao Shao Link: https://patch.msgid.link/20250611111515.1983366-4-edumazet@google.com Signed-off-by: Jakub Kicinski --- net/sched/sch_tbf.c | 2 +- 1 file changed, 1 insertion(+), 1 deletion(-) diff --git a/net/sched/sch_tbf.c b/net/sched/sch_tbf.c index dc26b22d53c7..4c977f049670 100644 --- a/net/sched/sch_tbf.c +++ b/net/sched/sch_tbf.c @@ -452,7 +452,7 @@ static int tbf_change(struct Qdisc *sch, struct nlattr *opt, sch_tree_lock(sch); if (child) { - qdisc_tree_flush_backlog(q->qdisc); + qdisc_purge_queue(q->qdisc); old = q->qdisc; q->qdisc = child; } -- 2.39.5