[moderation] [serial?] KCSAN: data-race in n_tty_receive_buf_common / n_tty_write (8)

[syzbot]

Hello,

syzbot found the following issue on:

HEAD commit: cd5a0afbdf80 Merge tag 'mailbox-v6.18' of git://git.kernel..
git tree: upstream
console output: https://syzkaller.appspot.com/x/log.txt?x=13150b34580000
kernel config: https://syzkaller.appspot.com/x/.config?x=655a48029989f36f
dashboard link: https://syzkaller.appspot.com/bug?extid=07cd7ba34f8abdb81413
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/1d38d44fa88c/disk-cd5a0afb.raw.xz
vmlinux: https://storage.googleapis.com/syzbot-assets/6f64cc50d445/vmlinux-cd5a0afb.xz
kernel image: https://storage.googleapis.com/syzbot-assets/46764ef5440b/bzImage-cd5a0afb.xz

IMPORTANT: if you fix the issue, please add the following tag to the commit:
Reported-by: syzbot+07cd7b...@syzkaller.appspotmail.com

==================================================================
BUG: KCSAN: data-race in n_tty_receive_buf_common / n_tty_write

write to 0xffffc900112b9020 of 8 bytes by task 6707 on cpu 0:
process_echoes drivers/tty/n_tty.c:772 [inline]
n_tty_write+0x1aa/0xb40 drivers/tty/n_tty.c:2363
iterate_tty_write drivers/tty/tty_io.c:1006 [inline]
file_tty_write+0x378/0x690 drivers/tty/tty_io.c:1081
tty_write+0x25/0x30 drivers/tty/tty_io.c:1102
new_sync_write fs/read_write.c:593 [inline]
vfs_write+0x52a/0x960 fs/read_write.c:686
ksys_write+0xda/0x1a0 fs/read_write.c:738
__do_sys_write fs/read_write.c:749 [inline]
__se_sys_write fs/read_write.c:746 [inline]
__x64_sys_write+0x40/0x50 fs/read_write.c:746
x64_sys_call+0x2802/0x3000 arch/x86/include/generated/asm/syscalls_64.h:2
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 0xffffc900112b9020 of 8 bytes by task 52 on cpu 1:
flush_echoes drivers/tty/n_tty.c:786 [inline]
__receive_buf drivers/tty/n_tty.c:1626 [inline]
n_tty_receive_buf_common+0x87c/0xbe0 drivers/tty/n_tty.c:1723
n_tty_receive_buf2+0x33/0x40 drivers/tty/n_tty.c:1769
tty_ldisc_receive_buf+0x63/0xf0 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+0x4cb/0x9d0 kernel/workqueue.c:3346
worker_thread+0x582/0x770 kernel/workqueue.c:3427
kthread+0x486/0x510 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: 0x0000000000000000 -> 0x0000000000000009

Reported by Kernel Concurrency Sanitizer on:
CPU: 1 UID: 0 PID: 52 Comm: kworker/u8:3 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
==================================================================


---
This report is generated by a bot. It may contain errors.
See https://goo.gl/tpsmEJ for more information about syzbot.
syzbot engineers can be reached at syzk...@googlegroups.com.

syzbot will keep track of this issue. See:
https://goo.gl/tpsmEJ#status for how to communicate with syzbot.

If the report is already addressed, let syzbot know by replying with:
#syz fix: exact-commit-title

If you want to overwrite report's subsystems, reply with:
#syz set subsystems: new-subsystem
(See the list of subsystem names on the web dashboard)

If the report is a duplicate of another one, reply with:
#syz dup: exact-subject-of-another-report

If you want to undo deduplication, reply with:
#syz undup