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c68b15ae11Christian Brauner <brauner@kernel.org> says: While working on eliding the grace period in namespace_unlock() I went through what filesystems free when they're shut down and what a pathwalk in rcu mode can still read at that point. Since 66a97c2ec953's rcu pathwalk fixes that's the filesystem's job. Whatever ->permission(), ->d_revalidate(), ->get_link(), ->d_hash() and ->d_compare() look at in rcu mode has to be freed after a grace period. A few filesystems don't do that. Most of it is hidden by umount(2). namespace_unlock() waits for a grace period before it drops the mounts, so a walker that was inside the filesystem is gone by the time the filesystem is shut down. But the last reference to a mount doesn't have to go away in umount(2). A chdir() out of a lazily unmounted directory or the close() of the last file on it drops it without a grace period. A walker can then still be in the filesystem while ->put_super() and ->kill_sb() free what it reads. With KASAN on vfs.fixes: - ext4_get_link() reads sb->s_fs_info and the extent status counters in rcu mode for a symlink that keeps its target in a data block. ext4_put_super() destroys the counters, clears s_fs_info and frees the sbi right away. That's a NULL dereference, a use-after-free of the sbi or a fault on a destroyed percpu counter, depending on where the walker sits. - The unicode map of casefolded ext4, f2fs and tmpfs is freed from ->put_super() while generic_ci_d_hash() and generic_ci_d_compare() use it. - ntfs3 with nocase frees its sbi and mount options under ntfs_d_hash() and ntfs_d_compare(). A remount frees the old mount options right away too, that one needs no unmount at all. - eventfs frees an eventfs_inode after an SRCU grace period only, but tracefs_d_revalidate() reads it under rcu_read_lock(). No unmount needed either. The ipc patch is a different kind of bug. create_ipc_ns() leaks the mqueue mount and its superblock when it fails after mq_init_ns(). I ran into it while looking at the kern_unmount() users. afs and 9p look just as broken but nothing can reach it today. Both have a private bdi, and generic_shutdown_super() calls bdi_unregister(), which waits for a grace period before ->kill_sb() frees their private data. That grace period is there for the rcu readers of bdi_list and covers afs and 9p by accident. I don't want them to depend on that, so they free their private data after a grace period of their own as well. I appended the reproducers. The race reproducers hold a pathwalk inside the filesystem's callback for two seconds with a kprobe while the other side frees what it reads. For the ntfs3 remount the window is a single load and its dereference, so that one needs an mdelay() in the kernel. ext4 with a journal needs a kernel without lockdep since jbd2_journal_destroy() calls lockdep_unregister_key(), which waits for a grace period too. Clean this all up. * patches from https://patch.msgid.link/20261009-work-fixes-rcu-v1-0-7446d2d335e6@kernel.org: ntfs3: free replaced mount options after a grace period ntfs3: free the sbi after a grace period unicode: free the map after a grace period ipc: drop the mqueue mount with kern_unmount() on setup failure eventfs: free the eventfs_inode after an rcu grace period too 9p: free the session after a grace period afs: free the super info after a grace period ext4: free the sbi after a grace period Link: https://patch.msgid.link/20261009-work-fixes-rcu-v1-0-7446d2d335e6@kernel.org Signed-off-by: Christian Brauner (Amutable) <brauner@kernel.org> |
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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. 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