[PATCH 5.4 018/154] net_sched: Prevent creation of classes with TC_H_ROOT

Greg Kroah-Hartman
2025-04-08T12:03:00Z

From: Cong Wang <xiyou.wangcong@gmail.com>

[ Upstream commit 0c3057a5a04d07120b3d0ec9c79568fceb9c921e ]

The function qdisc_tree_reduce_backlog() uses TC_H_ROOT as a termination
condition when traversing up the qdisc tree to update parent backlog
counters. However, if a class is created with classid TC_H_ROOT, the
traversal terminates prematurely at this class instead of reaching the
actual root qdisc, causing parent statistics to be incorrectly maintained.
In case of DRR, this could lead to a crash as reported by Mingi Cho.

Prevent the creation of any Qdisc class with classid TC_H_ROOT
(0xFFFFFFFF) across all qdisc types, as suggested by Jamal.

Reported-by: Mingi Cho <mincho@theori.io>
Signed-off-by: Cong Wang <xiyou.wangcong@gmail.com>
Reviewed-by: Simon Horman <horms@kernel.org>
Fixes: 066a3b5b2346 ("[NET_SCHED] sch_api: fix qdisc_tree_decrease_qlen() loop")
Link: https://patch.msgid.link/20250306232355.93864-2-xiyou.wangcong@gmail.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>
Signed-off-by: Sasha Levin <sashal@kernel.org>
---
net/sched/sch_api.c | 6 ++++++
1 file changed, 6 insertions(+)

diff --git a/net/sched/sch_api.c b/net/sched/sch_api.c
index 178044a845dfd..60c8b81a22dcd 100644
--- a/net/sched/sch_api.c
+++ b/net/sched/sch_api.c
@@ -2159,6 +2159,12 @@ static int tc_ctl_tclass(struct sk_buff *skb, struct nlmsghdr *n,
return -EOPNOTSUPP;
}

+ /* Prevent creation of traffic classes with classid TC_H_ROOT */
+ if (clid == TC_H_ROOT) {
+ NL_SET_ERR_MSG(extack, "Cannot create traffic class with classid TC_H_ROOT");
+ return -EINVAL;
+ }
+
new_cl = cl;
err = -EOPNOTSUPP;
if (cops->change)
--
2.39.5