netdev - [PATCH net 3/5] net_sched: tbf: fix a race in tbf

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Message-ID: <20250611111515.1983366-4-edumazet@google.com>
Date: Wed, 11 Jun 2025 11:15:13 +0000
From: Eric Dumazet <edumazet@...gle.com>
To: "David S . Miller" <davem@...emloft.net>, Jakub Kicinski <kuba@...nel.org>, 
	Paolo Abeni <pabeni@...hat.com>
Cc: Simon Horman <horms@...nel.org>, Jamal Hadi Salim <jhs@...atatu.com>, 
	Cong Wang <xiyou.wangcong@...il.com>, Jiri Pirko <jiri@...nulli.us>, netdev@...r.kernel.org, 
	eric.dumazet@...il.com, Eric Dumazet <edumazet@...gle.com>, 
	Gerrard Tai <gerrard.tai@...rlabs.sg>, Zhengchao Shao <shaozhengchao@...wei.com>
Subject: [PATCH net 3/5] 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 <gerrard.tai@...rlabs.sg>
Suggested-by: Gerrard Tai <gerrard.tai@...rlabs.sg>
Signed-off-by: Eric Dumazet <edumazet@...gle.com>
Cc: Zhengchao Shao <shaozhengchao@...wei.com>
---
 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 dc26b22d53c73461c6f6d64f768136ad3f334143..4c977f049670a600eafd219c898e5f29597be2c1 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;
 	}
-- 
2.50.0.rc0.642.g800a2b2222-goog