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Zhan Wei a1e9718b40
eventpoll: restore EP_UNACTIVE_PTR sentinel for ctx->tfile_check_list
Commit e09c77d940 ("eventpoll: hoist CTL_ADD scratch state into
struct ep_ctl_ctx") moved tfile_check_list from a file-scope global into
the stack-allocated struct ep_ctl_ctx, and in doing so replaced the
EP_UNACTIVE_PTR sentinel with NULL on the grounds that "NULL is the
obvious 'empty' value and zero-init handles it for free", describing the
change as "No functional change". It is not.

epitems_head->next is overloaded with two roles:

  1. the "next" pointer that threads a head onto ctx->tfile_check_list;
  2. a membership flag: ep_remove_file() uses
     !smp_load_acquire(&v->next) to mean "this head is not on any
     pending ctx->tfile_check_list and is therefore safe to free".

Before that change the EP_UNACTIVE_PTR sentinel kept the two roles
disjoint: a head on the list always had a non-NULL ->next (another head,
or the sentinel at the tail), so ->next == NULL was equivalent to "never
listed". With the sentinel gone the list is NULL-terminated, so the tail
head's ->next is NULL as well. ep_remove_file()'s gate can no longer
distinguish "never listed" from "listed at the tail", and misfires on
the tail head.

The reader (reverse_path_check_proc) holds epnested_mutex +
rcu_read_lock; the freer (ep_remove_file) holds ep->mtx + file->f_lock.
The two sides share no mutex -- the sentinel was the invariant the gate
relied on to know it could skip the read side. With it gone,
ep_remove_file() frees the tail head while reverse_path_check_proc() is
still walking it, producing the slab-use-after-free read. The syzbot
reproducer hits this within seconds on a multi-CPU VM.

Restore the sentinel: initialize ctx.tfile_check_list to EP_UNACTIVE_PTR
in do_epoll_ctl_file(), and terminate the walk on "!= EP_UNACTIVE_PTR"
in reverse_path_check() and clear_tfile_check_list(). The tail head's
->next becomes the sentinel again rather than NULL, so
ep_remove_file()'s gate regains its exclusivity and stops misfiring on
the tail. ep_remove_file() itself is unchanged.

This restores the invariant the file-scope tfile_check_list relied on
before that change while preserving the ctx packaging it introduced.

Reported-by: syzbot+e70e1b6cba8714543f7c@syzkaller.appspotmail.com
Closes: https://syzkaller.appspot.com/bug?extid=e70e1b6cba8714543f7c
Fixes: e09c77d940 ("eventpoll: hoist CTL_ADD scratch state into struct ep_ctl_ctx")
Suggested-by: Christian Brauner <brauner@kernel.org>
Link: https://lore.kernel.org/all/20260528-rotwild-summt-kuhhandel-7276ef4c33b7@brauner.io/
Signed-off-by: Zhan Wei <zhanwei919@gmail.com>
Link: https://patch.msgid.link/20260529142533.23696-1-zhanwei919@gmail.com
Signed-off-by: Christian Brauner (Amutable) <brauner@kernel.org>
History 2026-06-04 13:53:50 +02:00
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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