net_sched: Prevent creation of classes with TC_H_ROOT

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>

diff --git a/net/sched/sch_api.c b/net/sched/sch_api.c
index e3e91cf867eb99e41bac93ec30f6af3de777ce56..6c625dcd06519f151ff5e60a2d58e76c58d4360e 100644 (file)
--- a/net/sched/sch_api.c
+++ b/net/sched/sch_api.c
@@ -2254,6 +2254,12 @@ 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)