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 43eb466041216d25dedaef1c383ad7bd89929cbc ]
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@starlabs.sg Suggested-by: Gerrard Tai gerrard.tai@starlabs.sg Signed-off-by: Eric Dumazet edumazet@google.com Cc: Zhengchao Shao shaozhengchao@huawei.com Link: https://patch.msgid.link/20250611111515.1983366-4-edumazet@google.com Signed-off-by: Jakub Kicinski kuba@kernel.org Signed-off-by: Sasha Levin sashal@kernel.org --- 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 259a39ca99bfb..9b11e9256336c 100644 --- a/net/sched/sch_tbf.c +++ b/net/sched/sch_tbf.c @@ -394,7 +394,7 @@ 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; }