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Guidong Han 8b7e8245e2 eventpoll: pin files while checking reverse paths
Commit 319c151747 ("epoll: take epitem list out of struct file")
intentionally removed temporary file references from the reverse path
check list. At the time, both epitems and their files were freed after
an RCU grace period, so unlist_file() could obtain file->f_lock through
an epitem while clear_tfile_check_list() held rcu_read_lock().

Commit 0ede61d858 ("file: convert to SLAB_TYPESAFE_BY_RCU") made
struct file SLAB_TYPESAFE_BY_RCU and removed its RCU-delayed freeing.
RCU still protects the epitem, but no longer keeps the referenced file
from being freed and reused. A concurrent close can therefore make
unlist_file() lock or unlock f_lock in a recycled file object.

This violates the documented SLAB_TYPESAFE_BY_RCU rule requiring a
reference before acquiring an object's lock. The race was reproduced,
causing a wild unlock of f_lock in a recycled file and breaking its
mutual exclusion.

Add ->file to epitems_head to remember the pinned file independently of
->epitems. A concurrent EPOLL_CTL_DEL can empty ->epitems before the head
is unlisted, leaving no epi->ffd.file from which to drop the reference.

In list_file(), acquire the reference before adding the head to the
check list. The caller either owns a reference or holds the ep->mtx for
the epitem leading to the file. In the latter case, file_ref_get() can
fail after the last reference is dropped, but eventpoll_release_file()
must acquire the same mutex before the file can be freed. The dying leaf
can be skipped because removing links cannot increase the reverse path
count.

In unlist_file(), epnested_mutex excludes another list_file() or
unlist_file(), while head->next prevents a concurrent EPOLL_CTL_DEL from
freeing the head. Save head->file locally, clear it with head->next
under f_lock, and drop the reference after the RCU-protected operation.

Christian Brauner <brauner@kernel.org> quotes:

> SLAB_TYPESAFE_BY_RCU allows a slab slot to be reused while an RCU reader
> still holds its old address. Once that address contains a new live
> struct file, KASAN sees valid, unpoisoned memory and cannot distinguish
> the stale object identity. CONFIG_DEBUG_SPINLOCK exposes the failure
> instead.
>
> The failing interleaving is:
>
> CPU0: nested EPOLL_CTL_ADD             CPU1: close/open churn
> ------------------------------------   ---------------------------------
> p = hlist_first_rcu(&head->epitems)
> epi = container_of(p, ...)
>                                        close(victim)
>                                          __fput()
>                                            eventpoll_release_file()
>                                            file_free(victim)
>                                        // the slot is free; f_lock remains
> spin_lock(&epi->ffd.file->f_lock)
>                                        open() reuses the slot as new_file
>                                          spin_lock_init(&new_file->f_lock)
> spin_unlock(&epi->ffd.file->f_lock)     // wild unlock of new_file's lock
>
> CONFIG_DEBUG_SPINLOCK reports:
>
> BUG: spinlock already unlocked on CPU#0, poc_unlist/150
>  lock: 0xffff8880067fb200, .magic: dead4ead, .owner: <none>/-1, .owner_cpu: -1
> CPU: 0 UID: 1000 PID: 150 Comm: poc_unlist Not tainted 7.2.0-rc3-dirty #22 PREEMPTLAZY
> Hardware name: QEMU Ubuntu 24.04 PC v2 (i440FX + PIIX, arch_caps fix, 1996), BIOS 1.16.3-debian-1.16.3-2 04/01/2014
> Call Trace:
>  <TASK>
>  dump_stack_lvl+0x64/0x80
>  do_raw_spin_unlock+0x75/0xb0
>  _raw_spin_unlock+0xe/0x30
>  clear_tfile_check_list+0x88/0xe0
>  do_epoll_ctl_file+0x519/0xcf0
>  ? __pfx_ep_ptable_queue_proc+0x10/0x10
>  do_epoll_ctl+0x8f/0x100
>  __x64_sys_epoll_ctl+0x6f/0xa0
>  do_syscall_64+0xdc/0x520
>  ? srso_alias_return_thunk+0x5/0xfbef5
>  entry_SYSCALL_64_after_hwframe+0x76/0x7e
> RIP: 0033:0x42034e
> Code: 48 c7 c1 b8 ff ff ff f7 d8 64 89 01 48 83 c8 ff c3 66 2e 0f 1f 84 00 00 00 00 00 90 f3 0f 1e fa 49 89 ca b8 e9 00 00 00 0f 05 <48> 3d 01 f0 ff ff 73 01 c3 48 c7 c1 b8 ff ff ff f7 d8 64 89 01 48
> RSP: 002b:00007a657ff3c198 EFLAGS: 00000202 ORIG_RAX: 00000000000000e9
> RAX: ffffffffffffffda RBX: 00007a657ff3ccdc RCX: 000000000042034e
> RDX: 0000000000000003 RSI: 0000000000000001 RDI: 0000000000000004
> RBP: 00007a657ff3c2f0 R08: 0000000000000000 R09: 00007a657ff3c6c0
> R10: 00007a657ff3c1a4 R11: 0000000000000202 R12: 00007a657ff3c6c0
> R13: ffffffffffffffb8 R14: 000000000000000d R15: 00007fffb7de0210
>  </TASK>
> ------------[ cut here ]------------
>
> unlist_file() does not appear as a separate frame because it was inlined
> into clear_tfile_check_list(). This report was obtained with mdelay()
> instrumentation immediately before spin_lock() and spin_unlock() in
> unlist_file() to widen the two race windows.
>
> More importantly, this is a wild unlock. The stale unlock can target
> f_lock of a different live file and invalidate mutual exclusion for
> state protected by that lock. Turning this into a reliable exploit
> would require precise scheduling and same-slot reuse and is likely
> difficult, but the primitive is potentially exploitable.

Reported-by: Qi Tang <tpluszz77@gmail.com>
Reported-by: Junxi Qian <qjx1298677004@gmail.com>
Fixes: 0ede61d858 ("file: convert to SLAB_TYPESAFE_BY_RCU")
Cc: stable@vger.kernel.org
Signed-off-by: Guidong Han <2045gemini@gmail.com>
Link: https://patch.msgid.link/20260718104406.27897-1-2045gemini@gmail.com
Signed-off-by: Christian Brauner (Amutable) <brauner@kernel.org>
2026-07-23 12:48:22 +02:00
arch RISC-V updates for v7.2-rc4 2026-07-19 12:41:00 -07:00
block block: fix aligning of bounced dio read bios 2026-07-16 06:11:37 -06:00
certs treewide: fix indentation and whitespace in Kconfig files 2026-05-28 21:24:42 -07:00
crypto crypto: aes - Fix conditions for selecting MAC dependencies 2026-07-09 11:06:22 -04:00
Documentation Crypto library fixes for v7.2-rc4 2026-07-17 10:53:57 -07:00
drivers block-7.2-20260717 2026-07-19 09:29:42 -07:00
fs eventpoll: pin files while checking reverse paths 2026-07-23 12:48:22 +02:00
include fs: push nr_cached_objects memcg gating into individual filesystems 2026-07-23 11:35:02 +02:00
init mm.git review status for mm-hotfixes-stable..mm-nonmm-stable 2026-06-21 13:20:19 -07:00
io_uring io_uring-7.2-20260717 2026-07-19 09:24:32 -07:00
ipc kernel-7.2-rc1.misc 2026-06-15 03:05:50 +05:30
kernel BPF fixes: 2026-07-17 12:55:24 -07:00
lib Crypto library fixes for v7.2-rc4 2026-07-17 10:53:57 -07:00
LICENSES LICENSES: Add modern form of the LGPL-2.1 tags to the usage guide section 2025-10-22 07:58:19 +02:00
mm fs: push nr_cached_objects memcg gating into individual filesystems 2026-07-23 11:35:02 +02:00
net BPF fixes: 2026-07-17 12:55:24 -07:00
rust Rust changes for v7.2 (addendum) 2026-06-24 15:11:08 -07:00
samples tracing fixes for 7.2: 2026-07-12 09:46:37 -07:00
scripts scripts/sorttable: Handle RISC-V patchable ftrace entries 2026-06-25 13:15:03 -06:00
security selinux/stable-7.2 PR 20260717 2026-07-17 11:04:46 -07:00
sound ALSA: usb-audio: Add delay quirk for iBasso DC-Elite 2026-07-13 17:06:20 +02:00
tools selftests/pidfd: check PIDFD_THREAD survives open_by_handle_at() 2026-07-22 13:55:20 +02:00
usr kbuild: uapi: also test UAPI headers against C++ compilers 2026-03-25 13:24:42 +01:00
virt KVM/arm64 fixes for 7.2, take #2 2026-07-15 12:13:30 +02:00
.clang-format clang-format: fix formatting of guard() and scoped_guard() statements 2026-05-28 21:24:41 -07:00
.clippy.toml rust: bump Clippy's MSRV and clean incompatible_msrv allows 2026-04-07 09:51:39 +02:00
.cocciconfig
.editorconfig editorconfig: add rst extension 2026-01-26 19:07:09 -08:00
.get_maintainer.ignore .get_maintainer.ignore: add myself 2026-04-02 16:48:25 +02:00
.gitattributes .gitattributes: set diff driver for Rust source code files 2023-05-31 17:48:25 +02:00
.gitignore SPDX patches for 7.2-rc1 2026-06-22 12:06:22 -07:00
.mailmap Including fixes from Wireless, IPsec, Netfilter and Bluetooth. 2026-07-17 10:25:13 -07:00
.pylintrc docs: Move the python libraries to tools/lib/python 2025-11-18 09:22:40 -07:00
.rustfmt.toml
COPYING
CREDITS CREDITS: Add Wolfram Sang 2026-07-14 16:34:04 +02:00
Kbuild checksyscalls: move instance functionality into generic code 2026-04-05 09:21:32 +02:00
Kconfig io_uring: Rename KConfig to Kconfig 2025-02-19 14:53:27 -07:00
MAINTAINERS SoC fixes for 7.2 2026-07-17 08:52:09 -07:00
Makefile Linux 7.2-rc4 2026-07-19 13:54:41 -07:00
README docs: add AI Coding Assistants documentation 2026-01-06 14:55:06 -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
* Email Clients: Documentation/process/email-clients.rst