netdev - [PATCH net 2/5] net_sched: red: fix a race in __red

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Message-ID: <20250611111515.1983366-3-edumazet@google.com>
Date: Wed, 11 Jun 2025 11:15:12 +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>
Subject: [PATCH net 2/5] net_sched: red: fix a race in __red_change()

Gerrard Tai reported a race condition in RED, 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: 0c8d13ac9607 ("net: sched: red: delay destroying child qdisc on replace")
Reported-by: Gerrard Tai <gerrard.tai@...rlabs.sg>
Suggested-by: Gerrard Tai <gerrard.tai@...rlabs.sg>
Signed-off-by: Eric Dumazet <edumazet@...gle.com>
---
 net/sched/sch_red.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/net/sched/sch_red.c b/net/sched/sch_red.c
index 1ba3e0bba54f0cc8d797ab9a7ffab529481b2d4e..4696c893cf553c4c6ecd2da75b47b3d40c80e812 100644
--- a/net/sched/sch_red.c
+++ b/net/sched/sch_red.c
@@ -285,7 +285,7 @@ static int __red_change(struct Qdisc *sch, struct nlattr **tb,
 	q->userbits = userbits;
 	q->limit = ctl->limit;
 	if (child) {
-		qdisc_tree_flush_backlog(q->qdisc);
+		qdisc_purge_queue(q->qdisc);
 		old_child = q->qdisc;
 		q->qdisc = child;
 	}
-- 
2.50.0.rc0.642.g800a2b2222-goog