[SRU][J][PATCH 1/1] net_sched: Prevent creation of classes with TC_H_ROOT

[Tim Whisonant]

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

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 at theori.io>
Signed-off-by: Cong Wang <xiyou.wangcong at gmail.com>
Reviewed-by: Simon Horman <horms at 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 at kernel.org>
(cherry picked from commit 0c3057a5a04d07120b3d0ec9c79568fceb9c921e)
CVE-2025-21971
Signed-off-by: Tim Whisonant <tim.whisonant at canonical.com>
---
 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 516874d943cd9..d9ce273ba43d8 100644
--- a/net/sched/sch_api.c
+++ b/net/sched/sch_api.c
@@ -2164,6 +2164,12 @@ 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)
-- 
2.43.0