5.4-stable review patch. If anyone has any objections, please let me know.
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From: Eric Dumazet edumazet@google.com
[ Upstream commit 85a3e0ede38450ea3053b8c45d28cf55208409b8 ]
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@starlabs.sg Suggested-by: Gerrard Tai gerrard.tai@starlabs.sg Signed-off-by: Eric Dumazet edumazet@google.com Link: https://patch.msgid.link/20250611111515.1983366-3-edumazet@google.com Signed-off-by: Jakub Kicinski kuba@kernel.org Signed-off-by: Sasha Levin sashal@kernel.org --- 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 476853ff69894..64532ee591a96 100644 --- a/net/sched/sch_red.c +++ b/net/sched/sch_red.c @@ -235,7 +235,7 @@ static int red_change(struct Qdisc *sch, struct nlattr *opt, q->flags = ctl->flags; q->limit = ctl->limit; if (child) { - qdisc_tree_flush_backlog(q->qdisc); + qdisc_purge_queue(q->qdisc); old_child = q->qdisc; q->qdisc = child; }