[moderation] [serial?] KCSAN: data-race in __stop_tty / pty_write_room (10)

[syzbot]

Hello,

syzbot found the following issue on:

HEAD commit: 971199ad2a0f Merge tag 'arm64-fixes' of git://git.kernel.o..
git tree: upstream
console output: https://syzkaller.appspot.com/x/log.txt?x=11070542580000
kernel config: https://syzkaller.appspot.com/x/.config?x=6ffeccb21767c55b
dashboard link: https://syzkaller.appspot.com/bug?extid=68c7e260675ee6dcb49e
compiler: Debian clang version 20.1.8 (++20250708063551+0c9f909b7976-1~exp1~20250708183702.136), Debian LLD 20.1.8
CC: [gre...@linuxfoundation.org jiri...@kernel.org linux-...@vger.kernel.org linux-...@vger.kernel.org]

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

Downloadable assets:
disk image: https://storage.googleapis.com/syzbot-assets/c56fb70a6c65/disk-971199ad.raw.xz
vmlinux: https://storage.googleapis.com/syzbot-assets/38aad1e37a04/vmlinux-971199ad.xz
kernel image: https://storage.googleapis.com/syzbot-assets/6ad6c6a42ba7/bzImage-971199ad.xz

IMPORTANT: if you fix the issue, please add the following tag to the commit:
Reported-by: syzbot+68c7e2...@syzkaller.appspotmail.com

==================================================================
BUG: KCSAN: data-race in __stop_tty / pty_write_room

write to 0xffff88811acc1dbc of 1 bytes by task 3996 on cpu 1:
__stop_tty+0x36/0x90 drivers/tty/tty_io.c:744
n_tty_ioctl_helper+0x1e9/0x210 drivers/tty/tty_ioctl.c:951
n_tty_ioctl+0x101/0x200 drivers/tty/n_tty.c:2509
tty_ioctl+0x83f/0xb80 drivers/tty/tty_io.c:2801
vfs_ioctl fs/ioctl.c:51 [inline]
__do_sys_ioctl fs/ioctl.c:597 [inline]
__se_sys_ioctl+0xce/0x140 fs/ioctl.c:583
__x64_sys_ioctl+0x43/0x50 fs/ioctl.c:583
x64_sys_call+0x1816/0x3000 arch/x86/include/generated/asm/syscalls_64.h:17
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 0xffff88811acc1dbc of 1 bytes by task 37 on cpu 0:
pty_write_room+0x19/0x70 drivers/tty/pty.c:131
tty_write_room+0x3f/0x60 drivers/tty/tty_ioctl.c:69
__process_echoes+0x36/0x6b0 drivers/tty/n_tty.c:687
flush_echoes drivers/tty/n_tty.c:791 [inline]
__receive_buf drivers/tty/n_tty.c:1626 [inline]
n_tty_receive_buf_common+0x8e4/0xbe0 drivers/tty/n_tty.c:1723
n_tty_receive_buf2+0x33/0x40 drivers/tty/n_tty.c:1769
tty_ldisc_receive_buf+0x66/0xf0 drivers/tty/tty_buffer.c:387
tty_port_default_receive_buf+0x59/0x90 drivers/tty/tty_port.c:37
receive_buf drivers/tty/tty_buffer.c:445 [inline]
flush_to_ldisc+0x148/0x340 drivers/tty/tty_buffer.c:495
process_one_work kernel/workqueue.c:3263 [inline]
process_scheduled_works+0x4ce/0x9d0 kernel/workqueue.c:3346
worker_thread+0x582/0x770 kernel/workqueue.c:3427
kthread+0x489/0x510 kernel/kthread.c:463
ret_from_fork+0x122/0x1b0 arch/x86/kernel/process.c:158
ret_from_fork_asm+0x1a/0x30 arch/x86/entry/entry_64.S:245

value changed: 0x00 -> 0x01

Reported by Kernel Concurrency Sanitizer on:
CPU: 0 UID: 0 PID: 37 Comm: kworker/u8:2 Not tainted syzkaller #0 PREEMPT(voluntary)
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 08/18/2025
Workqueue: events_unbound flush_to_ldisc
==================================================================


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