Jamal Hadi Salim 320fb29ea2 net/sched: sch_cbs: Call qdisc_reset for child qdisc
During a reset, CBS is not calling reset on its child qdisc, which
might cause qlen/backlog accounting issues. For example, if we have CBS
with a QFQ parent and a netem child with delay, we can create a scenario
where the parent's qlen underflows. QFQ, specifically, uses qlen to
check whether it should deference a pointer, so this scenario may cause
a null-ptr deref in QFQ:

[   43.875639][  T319] Oops: general protection fault, probably for non-canonical address 0xdffffc0000000009: 0000 [#1] SMP KASAN NOPTI
[   43.876124][  T319] KASAN: null-ptr-deref in range [0x0000000000000048-0x000000000000004f]
[   43.876417][  T319] CPU: 10 UID: 0 PID: 319 Comm: ping Not tainted 7.0.0-13039-ge728258debd5 #773 PREEMPT(full)
[   43.876751][  T319] Hardware name: Bochs Bochs, BIOS Bochs 01/01/2011
[   43.876949][  T319] RIP: 0010:qfq_dequeue+0x35c/0x1650
[   43.877123][  T319] Code: 00 fc ff df 80 3c 02 00 0f 85 17 0e 00 00 4c 8d 73 48 48 89 9d b8 02 00 00 48 b8 00 00 00 00 00 fc ff df 4c 89 f2 48 c1 ea 03 <80> 3c 02 00 0f 85 76 0c 00 00 48 b8 00 00 00 00 00 fc ff df 4c 8b
[   43.877648][  T319] RSP: 0018:ffff8881017ef4f0 EFLAGS: 00010216
[   43.877845][  T319] RAX: dffffc0000000000 RBX: 0000000000000000 RCX: dffffc0000000000
[   43.878073][  T319] RDX: 0000000000000009 RSI: 0000000c40000000 RDI: ffff88810eef02b0
[   43.878306][  T319] RBP: ffff88810eef0000 R08: ffff88810eef0280 R09: 1ffff1102120fd63
[   43.878523][  T319] R10: 1ffff1102120fd66 R11: 1ffff1102120fd67 R12: 0000000c40000000
[   43.878742][  T319] R13: ffff88810eef02b8 R14: 0000000000000048 R15: 0000000020000000
[   43.878959][  T319] FS:  00007f9c51c47c40(0000) GS:ffff88817a0be000(0000) knlGS:0000000000000000
[   43.879214][  T319] CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
[   43.879403][  T319] CR2: 000055e69a2230a8 CR3: 000000010c07a000 CR4: 0000000000750ef0
[   43.879621][  T319] PKRU: 55555554
[   43.879735][  T319] Call Trace:
[   43.879844][  T319]  <TASK>
[   43.879924][  T319]  __qdisc_run+0x169/0x1900
[   43.880075][  T319]  ? dev_qdisc_enqueue+0x8b/0x210
[   43.880222][  T319]  __dev_queue_xmit+0x2346/0x37a0
[   43.880376][  T319]  ? register_lock_class+0x3f/0x800
[   43.880531][  T319]  ? srso_alias_return_thunk+0x5/0xfbef5
[   43.880684][  T319]  ? __pfx___dev_queue_xmit+0x10/0x10
[   43.880834][  T319]  ? srso_alias_return_thunk+0x5/0xfbef5
[   43.880977][  T319]  ? __lock_acquire+0x819/0x1df0
[   43.881124][  T319]  ? srso_alias_return_thunk+0x5/0xfbef5
[   43.881275][  T319]  ? srso_alias_return_thunk+0x5/0xfbef5
[   43.881418][  T319]  ? __asan_memcpy+0x3c/0x60
[   43.881563][  T319]  ? srso_alias_return_thunk+0x5/0xfbef5
[   43.881708][  T319]  ? eth_header+0x165/0x1a0
[   43.881853][  T319]  ? lockdep_hardirqs_on_prepare+0xdb/0x1a0
[   43.882031][  T319]  ? srso_alias_return_thunk+0x5/0xfbef5
[   43.882174][  T319]  ? neigh_resolve_output+0x3cc/0x7e0
[   43.882325][  T319]  ? srso_alias_return_thunk+0x5/0xfbef5
[   43.882471][  T319]  ip_finish_output2+0x6b6/0x1e10

Fix this by calling qdisc_reset for CBS' child qdisc.
Sashiko caught an issue which could result in a null ptr deref if
qdisc_create_dflt() is invoked on an unitialised cbs qdisc which is exposed
by this patch. We add an early return if the qdisc is null to address this.
This is a similar approach used by two other fixes[1][2].

The proper fix for this specific issue elucidated by sashiko is to remove
the call to qdisc_reset when qdisc_create_dflt fails. Since the dflt qdisc
isn't attached anywhere yet at that point, calling the reset callback doesn't
make much sense (and as stated has been a source of two other bugs).
We plan on  submitting this fix in a later patch.
[1] https://lore.kernel.org/netdev/20221018063201.306474-2-shaozhengchao@huawei.com/
[2] https://lore.kernel.org/netdev/20221018063201.306474-4-shaozhengchao@huawei.com/

Fixes: 585d763af0 ("net/sched: Introduce Credit Based Shaper (CBS) qdisc")
Reported-by: Junyoung Jang <graypanda.inzag@gmail.com>
Tested-by: Junyoung Jang <graypanda.inzag@gmail.com>
Tested-by: Victor Nogueira <victor@mojatatu.com>
Acked-by: Vinicius Costa Gomes <vinicius.gomes@intel.com>
Signed-off-by: Jamal Hadi Salim <jhs@mojatatu.com>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>
2026-05-13 17:53:39 -07:00
2026-04-28 18:13:58 -04:00
2026-01-26 19:07:09 -08:00
2022-09-28 09:02:20 +02:00
2025-02-19 14:53:27 -07:00
2026-05-03 14:21:25 -07:00

Linux kernel
============

The Linux kernel is the core of any Linux operating system. It manages hardware,
system resources, and provides the fundamental services for all other software.

Quick Start
-----------

* Report a bug: See Documentation/admin-guide/reporting-issues.rst
* Get the latest kernel: https://kernel.org
* Build the kernel: See Documentation/admin-guide/quickly-build-trimmed-linux.rst
* Join the community: https://lore.kernel.org/

Essential Documentation
-----------------------

All users should be familiar with:

* Building requirements: Documentation/process/changes.rst
* Code of Conduct: Documentation/process/code-of-conduct.rst
* License: See COPYING

Documentation can be built with make htmldocs or viewed online at:
https://www.kernel.org/doc/html/latest/


Who Are You?
============

Find your role below:

* New Kernel Developer - Getting started with kernel development
* Academic Researcher - Studying kernel internals and architecture
* Security Expert - Hardening and vulnerability analysis
* Backport/Maintenance Engineer - Maintaining stable kernels
* System Administrator - Configuring and troubleshooting
* Maintainer - Leading subsystems and reviewing patches
* Hardware Vendor - Writing drivers for new hardware
* Distribution Maintainer - Packaging kernels for distros
* AI Coding Assistant - LLMs and AI-powered development tools


For Specific Users
==================

New Kernel Developer
--------------------

Welcome! Start your kernel development journey here:

* Getting Started: Documentation/process/development-process.rst
* Your First Patch: Documentation/process/submitting-patches.rst
* Coding Style: Documentation/process/coding-style.rst
* Build System: Documentation/kbuild/index.rst
* Development Tools: Documentation/dev-tools/index.rst
* Kernel Hacking Guide: Documentation/kernel-hacking/hacking.rst
* Core APIs: Documentation/core-api/index.rst

Academic Researcher
-------------------

Explore the kernel's architecture and internals:

* Researcher Guidelines: Documentation/process/researcher-guidelines.rst
* Memory Management: Documentation/mm/index.rst
* Scheduler: Documentation/scheduler/index.rst
* Networking Stack: Documentation/networking/index.rst
* Filesystems: Documentation/filesystems/index.rst
* RCU (Read-Copy Update): Documentation/RCU/index.rst
* Locking Primitives: Documentation/locking/index.rst
* Power Management: Documentation/power/index.rst

Security Expert
---------------

Security documentation and hardening guides:

* Security Documentation: Documentation/security/index.rst
* LSM Development: Documentation/security/lsm-development.rst
* Self Protection: Documentation/security/self-protection.rst
* Reporting Vulnerabilities: Documentation/process/security-bugs.rst
* CVE Procedures: Documentation/process/cve.rst
* Embargoed Hardware Issues: Documentation/process/embargoed-hardware-issues.rst
* Security Features: Documentation/userspace-api/seccomp_filter.rst

Backport/Maintenance Engineer
-----------------------------

Maintain and stabilize kernel versions:

* Stable Kernel Rules: Documentation/process/stable-kernel-rules.rst
* Backporting Guide: Documentation/process/backporting.rst
* Applying Patches: Documentation/process/applying-patches.rst
* Subsystem Profile: Documentation/maintainer/maintainer-entry-profile.rst
* Git for Maintainers: Documentation/maintainer/configure-git.rst

System Administrator
--------------------

Configure, tune, and troubleshoot Linux systems:

* Admin Guide: Documentation/admin-guide/index.rst
* Kernel Parameters: Documentation/admin-guide/kernel-parameters.rst
* Sysctl Tuning: Documentation/admin-guide/sysctl/index.rst
* Tracing/Debugging: Documentation/trace/index.rst
* Performance Security: Documentation/admin-guide/perf-security.rst
* Hardware Monitoring: Documentation/hwmon/index.rst

Maintainer
----------

Lead kernel subsystems and manage contributions:

* Maintainer Handbook: Documentation/maintainer/index.rst
* Pull Requests: Documentation/maintainer/pull-requests.rst
* Managing Patches: Documentation/maintainer/modifying-patches.rst
* Rebasing and Merging: Documentation/maintainer/rebasing-and-merging.rst
* Development Process: Documentation/process/maintainer-handbooks.rst
* Maintainer Entry Profile: Documentation/maintainer/maintainer-entry-profile.rst
* Git Configuration: Documentation/maintainer/configure-git.rst

Hardware Vendor
---------------

Write drivers and support new hardware:

* Driver API Guide: Documentation/driver-api/index.rst
* Driver Model: Documentation/driver-api/driver-model/driver.rst
* Device Drivers: Documentation/driver-api/infrastructure.rst
* Bus Types: Documentation/driver-api/driver-model/bus.rst
* Device Tree Bindings: Documentation/devicetree/bindings/
* Power Management: Documentation/driver-api/pm/index.rst
* DMA API: Documentation/core-api/dma-api.rst

Distribution Maintainer
-----------------------

Package and distribute the kernel:

* Stable Kernel Rules: Documentation/process/stable-kernel-rules.rst
* ABI Documentation: Documentation/ABI/README
* Kernel Configuration: Documentation/kbuild/kconfig.rst
* Module Signing: Documentation/admin-guide/module-signing.rst
* Kernel Parameters: Documentation/admin-guide/kernel-parameters.rst
* Tainted Kernels: Documentation/admin-guide/tainted-kernels.rst

AI Coding Assistant
-------------------

CRITICAL: If you are an LLM or AI-powered coding assistant, you MUST read and
follow the AI coding assistants documentation before contributing to the Linux
kernel:

* Documentation/process/coding-assistants.rst

This documentation contains essential requirements about licensing, attribution,
and the Developer Certificate of Origin that all AI tools must comply with.


Communication and Support
=========================

* Mailing Lists: https://lore.kernel.org/
* IRC: #kernelnewbies on irc.oftc.net
* Bugzilla: https://bugzilla.kernel.org/
* MAINTAINERS file: Lists subsystem maintainers and mailing lists
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