[moderation] [kernel?] KCSAN: data-race in futex_hash / futex_hash_allocate (2)

[syzbot]

Hello,

syzbot found the following issue on:

HEAD commit: e9a6fb0bcdd7 Linux 6.18-rc5
git tree: upstream
console output: https://syzkaller.appspot.com/x/log.txt?x=17179412580000
kernel config: https://syzkaller.appspot.com/x/.config?x=d2e0bc0ae94545f7
dashboard link: https://syzkaller.appspot.com/bug?extid=6d10f917d32e4646f67c
compiler: Debian clang version 20.1.8 (++20250708063551+0c9f909b7976-1~exp1~20250708183702.136), Debian LLD 20.1.8
CC: [andre...@igalia.com da...@stgolabs.net dvh...@infradead.org linux-...@vger.kernel.org mi...@redhat.com pet...@infradead.org tg...@linutronix.de]

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

Downloadable assets:
disk image: https://storage.googleapis.com/syzbot-assets/a4187a0f60fe/disk-e9a6fb0b.raw.xz
vmlinux: https://storage.googleapis.com/syzbot-assets/c8702ff2e21c/vmlinux-e9a6fb0b.xz
kernel image: https://storage.googleapis.com/syzbot-assets/328a6a6a5005/bzImage-e9a6fb0b.xz

IMPORTANT: if you fix the issue, please add the following tag to the commit:
Reported-by: syzbot+6d10f9...@syzkaller.appspotmail.com

==================================================================
BUG: KCSAN: data-race in futex_hash / futex_hash_allocate

write to 0xffff88810aee3aa8 of 8 bytes by task 9537 on cpu 0:
futex_pivot_hash kernel/futex/core.c:268 [inline]
futex_hash+0x170/0x240 kernel/futex/core.c:315
class_hb_constructor kernel/futex/futex.h:240 [inline]
futex_wake+0xdd/0x360 kernel/futex/waitwake.c:172
do_futex+0x323/0x380 kernel/futex/syscalls.c:135
__do_sys_futex kernel/futex/syscalls.c:207 [inline]
__se_sys_futex+0x2ed/0x360 kernel/futex/syscalls.c:188
__x64_sys_futex+0x78/0x90 kernel/futex/syscalls.c:188
x64_sys_call+0x2e48/0x3000 arch/x86/include/generated/asm/syscalls_64.h:203
do_syscall_x64 arch/x86/entry/syscall_64.c:63 [inline]
do_syscall_64+0xd2/0x200 arch/x86/entry/syscall_64.c:94
entry_SYSCALL_64_after_hwframe+0x77/0x7f

read to 0xffff88810aee3aa8 of 8 bytes by task 9526 on cpu 1:
futex_pivot_pending kernel/futex/core.c:1748 [inline]
futex_hash_allocate+0x601/0x9d0 kernel/futex/core.c:1829
futex_hash_prctl+0xd9/0xf0 kernel/futex/core.c:1958
__do_sys_prctl kernel/sys.c:2869 [inline]
__se_sys_prctl+0xc88/0x12e0 kernel/sys.c:2518
__x64_sys_prctl+0x67/0x80 kernel/sys.c:2518
x64_sys_call+0x20cf/0x3000 arch/x86/include/generated/asm/syscalls_64.h:158
do_syscall_x64 arch/x86/entry/syscall_64.c:63 [inline]
do_syscall_64+0xd2/0x200 arch/x86/entry/syscall_64.c:94
entry_SYSCALL_64_after_hwframe+0x77/0x7f

value changed: 0xffff888133f75a80 -> 0x0000000000000000

Reported by Kernel Concurrency Sanitizer on:
CPU: 1 UID: 0 PID: 9526 Comm: syz.4.2315 Not tainted syzkaller #0 PREEMPT(voluntary)
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 10/02/2025
==================================================================


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