[moderation] [fs?] KCSAN: data-race in __d_instantiate / lookup_fast (13)

[syzbot]

Hello,

syzbot found the following issue on:

HEAD commit: 8fde5d1d47f6 Merge tag 'acpi-7.1-rc6' of git://git.kernel...
git tree: upstream
console output: https://syzkaller.appspot.com/x/log.txt?x=17c84e73980000
kernel config: https://syzkaller.appspot.com/x/.config?x=f571f22917457cd8
dashboard link: https://syzkaller.appspot.com/bug?extid=54b3bbfb896275f5fb03
compiler: Debian clang version 21.1.8 (++20251221033036+2078da43e25a-1~exp1~20251221153213.50), Debian LLD 21.1.8
CC: [bra...@kernel.org ja...@suse.cz linux-...@vger.kernel.org linux-...@vger.kernel.org vi...@zeniv.linux.org.uk]

Unfortunately, I don't have any reproducer for this issue yet.

Downloadable assets:
disk image: https://storage.googleapis.com/syzbot-assets/6fd7078e6720/disk-8fde5d1d.raw.xz
vmlinux: https://storage.googleapis.com/syzbot-assets/8a314316afc1/vmlinux-8fde5d1d.xz
kernel image: https://storage.googleapis.com/syzbot-assets/c0f4649947fb/bzImage-8fde5d1d.xz

IMPORTANT: if you fix the issue, please add the following tag to the commit:
Reported-by: syzbot+54b3bb...@syzkaller.appspotmail.com

capability: warning: `syz.0.159' uses deprecated v2 capabilities in a way that may be insecure
==================================================================
BUG: KCSAN: data-race in __d_instantiate / lookup_fast

read-write to 0xffff888106c62480 of 4 bytes by task 4160 on cpu 0:
fsnotify_update_flags include/linux/fsnotify_backend.h:-1 [inline]
__d_instantiate+0x2a8/0x390 fs/dcache.c:2054
d_make_persistent+0x72/0x110 fs/dcache.c:2870
shmem_mknod+0x15f/0x170 mm/shmem.c:3881
shmem_create+0x34/0x50 mm/shmem.c:3933
lookup_open fs/namei.c:4511 [inline]
open_last_lookups fs/namei.c:4611 [inline]
path_openat+0xd75/0x2050 fs/namei.c:4855
do_file_open+0x16c/0x290 fs/namei.c:4887
do_sys_openat2+0x94/0x130 fs/open.c:1364
do_sys_open fs/open.c:1370 [inline]
__do_sys_openat fs/open.c:1386 [inline]
__se_sys_openat fs/open.c:1381 [inline]
__x64_sys_openat+0xf2/0x120 fs/open.c:1381
x64_sys_call+0x1e39/0x3020 arch/x86/include/generated/asm/syscalls_64.h:258
do_syscall_x64 arch/x86/entry/syscall_64.c:63 [inline]
do_syscall_64+0x12c/0x3b0 arch/x86/entry/syscall_64.c:94
entry_SYSCALL_64_after_hwframe+0x77/0x7f

read to 0xffff888106c62480 of 4 bytes by task 4159 on cpu 1:
d_revalidate fs/namei.c:1029 [inline]
lookup_fast+0xf0/0x320 fs/namei.c:1863
lookup_fast_for_open fs/namei.c:4548 [inline]
open_last_lookups fs/namei.c:4582 [inline]
path_openat+0x436/0x2050 fs/namei.c:4855
do_file_open+0x16c/0x290 fs/namei.c:4887
do_sys_openat2+0x94/0x130 fs/open.c:1364
do_sys_open fs/open.c:1370 [inline]
__do_sys_openat fs/open.c:1386 [inline]
__se_sys_openat fs/open.c:1381 [inline]
__x64_sys_openat+0xf2/0x120 fs/open.c:1381
x64_sys_call+0x1e39/0x3020 arch/x86/include/generated/asm/syscalls_64.h:258
do_syscall_x64 arch/x86/entry/syscall_64.c:63 [inline]
do_syscall_64+0x12c/0x3b0 arch/x86/entry/syscall_64.c:94
entry_SYSCALL_64_after_hwframe+0x77/0x7f

value changed: 0x00000080 -> 0x08200080

Reported by Kernel Concurrency Sanitizer on:
CPU: 1 UID: 0 PID: 4159 Comm: syz.0.159 Not tainted syzkaller #0 PREEMPT(lazy)
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 04/18/2026
==================================================================


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