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Writing to the netdevsim debugfs file "netdevsim/netdevsimN/fib/nexthop_bucket_activity" enters nsim_nexthop_bucket_activity_write(), which looks up a nexthop in data->nexthop_ht under rtnl_lock(). If a network namespace teardown, devlink reload or device deletion runs concurrently, nsim_fib_destroy() frees that rhashtable (and the surrounding nsim_fib_data) while the write is still in flight, leading to a slab-use-after-free: BUG: KASAN: slab-use-after-free in nsim_nexthop_bucket_activity_write+0xb9e/0xdf0 Read of size 4 at addr ff1100001a379808 by task syz.0.11967/27894 CPU: 0 UID: 0 PID: 27894 Comm: syz.0.11967 Not tainted 7.1.0-rc4-gf6f1bfc1980a #4 Call Trace: nsim_nexthop_bucket_activity_write+0xb9e/0xdf0 full_proxy_write+0x135/0x1a0 vfs_write+0x2e2/0x1040 ksys_write+0x146/0x270 __x64_sys_write+0x76/0xb0 do_syscall_64+0xb9/0x5b0 entry_SYSCALL_64_after_hwframe+0x74/0x7c Allocated by task 15957: rhashtable_init_noprof+0x3ec/0x860 nsim_fib_create+0x371/0xca0 nsim_drv_probe+0xd60/0x15c0 ... new_device_store+0x425/0x7f0 Freed by task 24: rhashtable_free_and_destroy+0x10d/0x620 nsim_fib_destroy+0xc9/0x1c0 nsim_dev_reload_destroy+0x1e7/0x530 nsim_dev_reload_down+0x6b/0xd0 devlink_reload+0x1b5/0x770 devlink_pernet_pre_exit+0x25d/0x3a0 ops_undo_list+0x1b7/0xb90 cleanup_net+0x47f/0x8a0 The buggy address belongs to the object at ff1100001a379800 which belongs to the cache kmalloc-1k of size 1024 The freed 1k object is the bucket table of data->nexthop_ht. Shortly after, the dangling table is dereferenced again and the machine also takes a GPF in __rht_bucket_nested() from the same call site. The root cause is a lifetime mismatch: the debugfs files reference nsim_fib_data (the writer dereferences data->nexthop_ht), but the interface is not bracketed around the lifetime of that data. nsim_fib_destroy() freed both rhashtables and only removed the debugfs directory afterwards, and nsim_fib_create() created the debugfs files before the rhashtables were initialized and, on the error path, freed them before removing the files. debugfs keeps the file itself alive across a ->write() via debugfs_file_get()/debugfs_file_put() (fs/debugfs/file.c), but it does not keep data->nexthop_ht alive, so the in-flight writer dereferenced freed memory. rtnl_lock() in the writer does not help, because the teardown path does not take rtnl around rhashtable_free_and_destroy(). Fix it by bracketing the debugfs interface around the data it exposes, keeping nsim_fib_create() and nsim_fib_destroy() symmetric: - In nsim_fib_destroy(), tear down the debugfs files before the data structures they reference. debugfs_remove_recursive() drops the initial active-user reference and then waits for every in-flight ->write() to drop its reference before returning, and rejects new opens (__debugfs_file_removed(), fs/debugfs/inode.c). Once it returns, no debugfs accessor can reach the FIB data, so the rhashtables and nsim_fib_data can be destroyed safely. This also covers the bool knobs in the same directory, which store pointers into the same nsim_fib_data, and the final kfree(data). - In nsim_fib_create(), create the debugfs files after the rhashtables and notifiers are set up. This closes the same race on the error-unwind path, where a concurrent writer could otherwise observe a half-constructed instance or a table that the unwind has already freed. (With only the destroy-side change, a writer racing the create window instead dereferences an uninitialized data->nexthop_ht.) This is reproducible by racing, in a loop, writes to /sys/kernel/debug/netdevsim/netdevsimN/fib/nexthop_bucket_activity against a teardown of the same netdevsim instance -- a devlink reload ("devlink dev reload netdevsim/netdevsimN"), destroying the network namespace it lives in, or "echo N > /sys/bus/netdevsim/del_device". It was found with syzkaller; a syzkaller reproducer is available. A standalone C reproducer does not trigger it reliably because the race needs the netns-teardown/reload path. Cc: <stable+noautosel@kernel.org> # netdevsim is a test harness, it's never loaded on production systems Signed-off-by: Zijing Yin <yzjaurora@gmail.com> Reviewed-by: Ido Schimmel <idosch@nvidia.com> Link: https://patch.msgid.link/20260529135718.1804031-1-yzjaurora@gmail.com Signed-off-by: Jakub Kicinski <kuba@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. 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