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Linux kernel source tree
28d2905d77
is_child_subreaper, has_child_subreaper, autoreap and, with
CONFIG_POSIX_TIMERS=y, timer_create_restore_ids are 1-bit fields that
share one memory location in struct signal_struct. A store to any of
them is a read-modify-write of the whole word, and three of the stores
run without a common lock:
- PR_SET_CHILD_SUBREAPER writes me->signal->is_child_subreaper with
no lock held.
- propagate_has_child_subreaper(), run by walk_process_tree() from
the same prctl in an ancestor, writes p->signal->has_child_subreaper
of every descendant p under read_lock(&tasklist_lock), which does
not exclude the other writers.
- PR_TIMER_CREATE_RESTORE_IDS writes
current->signal->timer_create_restore_ids with no lock held.
When a process calls prctl(PR_SET_CHILD_SUBREAPER, 1) while an
ancestor's PR_SET_CHILD_SUBREAPER walk visits it, or while another
thread of the same process calls prctl(PR_TIMER_CREATE_RESTORE_IDS),
the read-modify-write sequences interleave and one of the bits is
silently lost:
- If has_child_subreaper is lost, the process's orphaned descendants
are reparented straight to init when it exits: find_new_reaper()
tests father->signal->has_child_subreaper first and never looks for
the ancestor subreaper. The service manager that registered as the
subreaper never gets SIGCHLD for them and cannot wait() on them.
- If is_child_subreaper is lost, the prctl returned 0 but the process
is not a subreaper: orphans of its descendants skip it and are
reparented to the next subreaper up or to init.
PR_GET_CHILD_SUBREAPER reads back 0.
- If timer_create_restore_ids is lost, the prctl returned 0 but
timer_create() does not restore the requested timer ids (or keeps
restoring them after _OFF), and _GET reads back the old value.
Documentation/memory-barriers.txt requires all fields of one bitfield
to be protected by one lock for exactly this reason. No lock is taken
or documented for these flags, and the prctls need no privilege, so any
process can hit the race against its own ancestors or descendants, or
between two of its own threads.
Make is_child_subreaper and has_child_subreaper separate bool members.
Each store then has its own memory location and no read-modify-write
of a neighbour. The readers and their locking are unchanged.
autoreap stays a bit-field and so shares a memory location only with
timer_create_restore_ids. That pair is race-free: autoreap is stored
exactly once, in copy_process() before the new signal_struct is
reachable by any other task, and timer_create_restore_ids is only
stored to the signal_struct of an already running process.
Fixes:
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|---|---|---|
| arch | ||
| block | ||
| certs | ||
| crypto | ||
| Documentation | ||
| drivers | ||
| fs | ||
| include | ||
| init | ||
| io_uring | ||
| ipc | ||
| kernel | ||
| lib | ||
| LICENSES | ||
| mm | ||
| net | ||
| rust | ||
| samples | ||
| scripts | ||
| security | ||
| sound | ||
| tools | ||
| usr | ||
| virt | ||
| .clang-format | ||
| .clippy.toml | ||
| .cocciconfig | ||
| .editorconfig | ||
| .get_maintainer.ignore | ||
| .gitattributes | ||
| .gitignore | ||
| .mailmap | ||
| .pylintrc | ||
| .rustfmt.toml | ||
| COPYING | ||
| CREDITS | ||
| Kbuild | ||
| Kconfig | ||
| MAINTAINERS | ||
| Makefile | ||
| README | ||
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