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syzbot reported:
BUG: KMSAN: uninit-value in dentry_string_cmp fs/dcache.c:291 [inline]
BUG: KMSAN: uninit-value in dentry_cmp fs/dcache.c:322 [inline]
BUG: KMSAN: uninit-value in __d_lookup_rcu+0x37d/0x5e0 fs/dcache.c:2522
dentry_string_cmp fs/dcache.c:291 [inline]
dentry_cmp fs/dcache.c:322 [inline]
__d_lookup_rcu+0x37d/0x5e0 fs/dcache.c:2522
lookup_fast+0x194/0xa40 fs/namei.c:1854
lookup_fast_for_open fs/namei.c:4545 [inline]
open_last_lookups fs/namei.c:4579 [inline]
path_openat+0x9ef/0x6540 fs/namei.c:4856
do_file_open+0x2aa/0x680 fs/namei.c:4888
do_sys_openat2+0x17c/0x390 fs/open.c:1395
do_sys_open fs/open.c:1401 [inline]
__do_sys_openat fs/open.c:1417 [inline]
__se_sys_openat fs/open.c:1412 [inline]
__x64_sys_openat+0x240/0x300 fs/open.c:1412
x64_sys_call+0x2445/0x3ea0 arch/x86/include/generated/asm/syscalls_64.h:258
do_syscall_x64 arch/x86/entry/syscall_64.c:63 [inline]
do_syscall_64+0x15d/0x3c0 arch/x86/entry/syscall_64.c:94
entry_SYSCALL_64_after_hwframe+0x77/0x7f
Uninit was stored to memory at:
copy_name fs/dcache.c:3031 [inline]
__d_move+0xd29/0x21f0 fs/dcache.c:3099
d_move+0x71/0xf0 fs/dcache.c:3147
vfs_rename+0x2619/0x2770 fs/namei.c:6085
filename_renameat2+0xa59/0x1230 fs/namei.c:6188
__do_sys_rename fs/namei.c:6232 [inline]
__se_sys_rename+0xc5/0x5c0 fs/namei.c:6228
__x64_sys_rename+0x78/0xb0 fs/namei.c:6228
x64_sys_call+0x329/0x3ea0 arch/x86/include/generated/asm/syscalls_64.h:83
do_syscall_x64 arch/x86/entry/syscall_64.c:63
do_syscall_64+0x15d/0x3c0 arch/x86/entry/syscall_64.c:94
entry_SYSCALL_64_after_hwframe+0x77/0x7f
Uninit was created at:
slab_post_alloc_hook mm/slub.c:4617 [inline]
slab_alloc_node mm/slub.c:4939 [inline]
kmem_cache_alloc_lru_noprof+0x376/0x1230 mm/s
__d_alloc+0x52/0x9f0 fs/dcache.c:1902
d_alloc+0x57/0x300 fs/dcache.c:1981
lookup_one_qstr_excl+0x19d/0x7a0 fs/namei.c:1806
__start_renaming+0x341/0x850 fs/namei.c:3888
filename_renameat2+0x625/0x1230 fs/namei.c:6163
__do_sys_rename fs/namei.c:6232 [inline]
__se_sys_rename+0xc5/0x5c0 fs/namei.c:6228
__x64_sys_rename+0x78/0xb0 fs/namei.c:6228
x64_sys_call+0x329/0x3ea0 arch/x86/include/generated/asm/syscalls_64.h:83
do_syscall_x64 arch/x86/entry/syscall_64.c:63
do_syscall_64+0x15d/0x3c0 arch/x86/entry/syscall_64.c:94
entry_SYSCALL_64_after_hwframe+0x77/0x7f
The race is between a concurrent open() and rename() of the same path.
__d_alloc() only stores the name itself and its terminating NUL, so the
rest of the inline buffer (d_shortname, DNAME_INLINE_LEN bytes) is left
uninitialized. copy_name(), called from rename(), copies that buffer as a
whole, so the uninitialized tail is propagated into the dentry that is
being moved. Meanwhile __d_lookup_rcu(), called from open(), is an
optimistic lockless lookup: it checks d_name.hash_len first and leaves the
seqcount retry to its caller, so it can end up comparing against a dentry
whose name a rename is rewriting in place, using a stale (longer) length.
The comparison then runs past the terminating NUL and reads bytes of the
uninitialized tail, which KMSAN reports.
The read is harmless by design: it stays inside the buffer, the name is
still NUL-terminated, and the result is thrown away by the seqcount retry.
It is not specific to KMSAN either - with CONFIG_DCACHE_WORD_ACCESS
enabled the very same bytes are read by read_word_at_a_time(), which is
__no_sanitize_or_inline and therefore invisible to KMSAN. KMSAN builds
only see the instrumented byte-at-a-time dentry_string_cmp() because
CONFIG_DCACHE_WORD_ACCESS is disabled when KMSAN is enabled on x86:
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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. 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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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