CVE-2025-21971

In the Linux kernel, the following vulnerability has been resolved:

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.