CVE-2025-21703

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

netem: Update sch->q.qlen before qdisc_tree_reduce_backlog()

qdisc_tree_reduce_backlog() notifies parent qdisc only if child qdisc becomes empty, therefore we need to reduce the backlog of the child qdisc before calling it. Otherwise it would miss the opportunity to call cops->qlen_notify(), in the case of DRR, it resulted in UAF since DRR uses ->qlen_notify() to maintain its active list.

The bug happens only if emulate network delay, loss, and packet re-ordering qdisc scheduler being used. By default it is disabled. It is hard to trigger it and the security impact is limited.

The CVSS score(s) listed for this vulnerability do not reflect the associated product's status, and are included for informational purposes to better understand the severity of this vulnerability.

To mitigate this issue, prevent module sch_netem from being loaded. Please see https://access.redhat.com/solutions/41278 for how to blacklist a kernel module to prevent it from loading automatically.