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Refcount of a NORCU dentry must not be incremented after having dropped to zero. Otherwise we might end up with the following race: CPU1: in fast_dput(d), rcu_read_lock(); CPU1: decrements refcount of d to 0 CPU1: notice that it's unhashed CPU2: grab a reference to d CPU2: dput(d), freeing d CPU1: ... looks like we need to evict d, let's grab ->d_lock, recheck the refcount, etc. and that spin_lock(&d->d_lock) ends up a UAF, despite still being in an RCU read-side critical area started back when the refcount had been positive. If not for DCACHE_NORCU in d->d_flags freeing would've been RCU-delayed, so we'd have grabbed ->d_lock, noticed the negative value stored into refcount by __dentry_kill(), dropped the locks and that would be it. For NORCU dentries freeing is _not_ delayed, though. Most of the non-counting references are excluded for NORCU dentries - they are not allowed to be hashed, they never get placed on LRU, they never get placed into anyone's list of children and while dput_to_list() might put them into a shrink list, nobody bumps refcount of something that had been reached that way. However, inode's list of aliases can be a problem - it does not contribute to dentry refcount (for obvious reasons) and we *do* have places that grab references to something found on that list - that's precisely what d_find_alias() is. In case of d_find_alias() we are safe - it skips unhashed aliases, so all NORCU ones are ignored there. d_find_any_alias() is *not* limited to hashed ones, though, and while it's usually called for directories (which never get NORCU dentries), there are callers that use it to get something for non-directories with no hashed aliases. Having d_find_any_alias() hit a NORCU dentry is not impossible - it can be easily arranged if you have CAP_DAC_READ_SEARCH (memfd_create() + mmap() + name_to_handle_at() for /proc/self/map_files/<...> + munmap() + open_by_handle_at() will do that, and adding a second memfd_create() for mount_fd makes it possible to do that without having memfd pinned). The race window is narrow, and it's probably not feasible on bare hardware, but... It's not hard to fix, fortunately: * separate __d_find_dir_alias() (== current __d_find_any_alias()) to be used for directory inodes. * provide dget_alias_ilocked() that would return false for NORCU dentries with zero refcount and return true incrementing refcount otherwise * make __d_find_any_alias() go over the list of aliases, using dget_alias_ilocked() and returning the alias it succeeds on (normally the first one). Any NORCU alias with zero refcount is going to be evicted by the thread that had dropped the final reference; this makes __d_find_any_alias() pretend it had lost the race with eviction. Signed-off-by: Al Viro <viro@zeniv.linux.org.uk> |
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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