net/sched: sch_qfq: Fix race condition on qfq_aggregate
CVE: CVE-2025-38477
Component: kernel
CWE: CWE-416
Severity: Moderate
CVSS v3: 7.5/AV:L/AC:L/PR:L/UI:N/S:U/C:H/I:L/A:H
Release date: 2025-07-28T00:00:00+00:00

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

net/sched: sch_qfq: Fix race condition on qfq_aggregate

A race condition can occur when 'agg' is modified in qfq_change_agg
(called during qfq_enqueue) while other threads access it
concurrently. For example, qfq_dump_class may trigger a NULL
dereference, and qfq_delete_class may cause a use-after-free.

This patch addresses the issue by:

1. Moved qfq_destroy_class into the critical section.

2. Added sch_tree_lock protection to qfq_dump_class and
qfq_dump_class_stats.

Statement: This patch fixes a race condition in the QFQ packet scheduler, where concurrent modification and access to cl->agg may result in a use-after-free or NULL pointer dereference.
The bug occurs when qfq_change_class() reassigns the agg pointer while other threads read it in qfq_dump_class() or qfq_delete_class().
The fix synchronizes access using sch_tree_lock() and reorders destruction logic to avoid concurrency issues.
This issue can be triggered locally by users with CAP_NET_ADMIN privileges and may lead to kernel crashes or denial-of-service.
Although the vulnerability involves a use-after-free, it only results in stale reads of deallocated memory, used in netlink reporting functions.
There is no write primitive, function pointer corruption, or kernel state modification, making privilege escalation infeasible in this case.

Remediation:
For details on how to apply this update, which includes the changes described in this advisory, refer to:

https://access.redhat.com/articles/11258

The system must be rebooted for this update to take effect.
For details on how to apply this update, which includes the changes described in this advisory, refer to:

https://access.redhat.com/articles/11258

The system must be rebooted for this update to take effect.
For details on how to apply this update, which includes the changes described in this advisory, refer to:

https://access.redhat.com/articles/11258

The system must be rebooted for this update to take effect.
For details on how to apply this update, which includes the changes described in this advisory, refer to:

https://access.redhat.com/articles/11258

The system must be rebooted for this update to take effect.
For details on how to apply this update, which includes the changes described in this advisory, refer to:

https://access.redhat.com/articles/11258

The system must be rebooted for this update to take effect.
For details on how to apply this update, which includes the changes described in this advisory, refer to:

https://access.redhat.com/articles/11258

The system must be rebooted for this update to take effect.
For details on how to apply this update, which includes the changes described in this advisory, refer to:

https://access.redhat.com/articles/11258

The system must be rebooted for this update to take effect.
To mitigate this issue, prevent module sch_qfq from being loaded. Please see https://access.redhat.com/solutions/41278 for how to blacklist a kernel module to prevent it from loading automatically.