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Quchaosheng 28bc1ef699 net/packet: guard the ll header push in packet_rcv_spkt()
packet_rcv_spkt() restores the link layer header with

	skb_push(skb, skb->data - skb_mac_header(skb));

That subtraction is only meaningful when the device actually has a link
layer header.  packet_rcv() and tpacket_rcv() both wrap it in
dev_has_header(), which is also the predicate the block comment at the
top of the file states the restore in terms of; packet_rcv_spkt() does
not.  commit d549699048 ("net/packet: fix packet receive on L3
devices without visible hard header") introduced the helper and changed
the two call sites, and this one stayed behind.

A device without a visible ll header can leave skb->mac_header at the
0xFFFF sentinel that __alloc_skb() initialises it to.  A CAN skb does:
init_can_skb() sets pkt_type and ip_summed but does not reset the
headers, and commit 9f10374bb0 ("can: remove private CAN skb
headroom infrastructure") dropped the skb_reset_*_header() calls that
used to be there.  skb_mac_header() is then 0xFFFF, the length becomes
a large negative number and skb_push() reports it through
skb_under_panic() -- from softirq context, so it is a full system panic
even with panic_on_oops=0:

  skbuff: skb_under_panic: text:ffffffff8bd21bc1 len:-65455 put:-65471 head:... data:... tail:0x50 end:0x180 dev:can0
  kernel BUG at net/core/skbuff.c:214!
  RIP: 0010:skb_panic+0x50/0x60
  Call Trace:
   <IRQ>
   skb_push+0x38/0x40
   packet_rcv_spkt+0xe1/0x170
   __netif_receive_skb_core.constprop.0+0x7e8/0xd30
   ...
  Kernel panic - not syncing: Fatal exception in interrupt

The socket type is reachable: packet_create() accepts SOCK_PACKET
alongside SOCK_RAW and SOCK_DGRAM behind the same CAP_NET_RAW check,
and neither the socket length nor a capability check keeps it away
from a CAN interface.

The missing skb_reset_*_header() calls in init_can_skb() are a
regression in their own right and are being fixed separately, but a
packet socket should not turn a link layer that did not initialise its
mac header into a kernel panic.  Guard the push the way the other two
receive paths do.

Tested on v7.3-rc5 under QEMU with a slcan device on a pty, which is
the driver RX path: vcan does not reproduce it, because can_send()
resets the headers on the way out.  One SOCK_PACKET socket bound to
can0 and one frame written into the line discipline panics an
unpatched kernel with the trace above; the same image with this patch
prints no panic and powers off normally.  Both kernels are this tree,
defconfig plus CONFIG_CAN_SLCAN=y, differing only in this hunk.

Fixes: d549699048 ("net/packet: fix packet receive on L3 devices without visible hard header")
Cc: stable@vger.kernel.org
Signed-off-by: Quchaosheng <quchaosheng000406@163.com>
Reviewed-by: Willem de Bruijn <willemb@google.com>
Link: https://patch.msgid.link/20260928113108.2127215-1-quchaosheng000406@163.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>
History 2026-10-01 18:24:13 -07:00
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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
* Email Clients: Documentation/process/email-clients.rst