[moderation] [serial?] KCSAN: data-race in n_tty_receive_buf_standard / n_tty_write (2)

[syzbot]

Hello,

syzbot found the following issue on:

HEAD commit: 7bb4d6512545 Merge tag 'v6.18rc4-SMB-client-fixes' of git:..
git tree: upstream
console output: https://syzkaller.appspot.com/x/log.txt?x=115a1412580000
kernel config: https://syzkaller.appspot.com/x/.config?x=d2e0bc0ae94545f7
dashboard link: https://syzkaller.appspot.com/bug?extid=c87b97ab5952dcfd64b8
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/b24797e9de04/disk-7bb4d651.raw.xz
vmlinux: https://storage.googleapis.com/syzbot-assets/955e7ce8739f/vmlinux-7bb4d651.xz
kernel image: https://storage.googleapis.com/syzbot-assets/eb6ce0cd8783/bzImage-7bb4d651.xz

IMPORTANT: if you fix the issue, please add the following tag to the commit:
Reported-by: syzbot+c87b97...@syzkaller.appspotmail.com

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

write to 0xffffc90006b35028 of 8 bytes by task 10380 on cpu 1:
commit_echoes drivers/tty/n_tty.c:742 [inline]
n_tty_receive_char_canon drivers/tty/n_tty.c:1289 [inline]
n_tty_receive_char_special drivers/tty/n_tty.c:1372 [inline]
n_tty_receive_buf_standard+0x2bea/0x2f10 drivers/tty/n_tty.c:1588
__receive_buf drivers/tty/n_tty.c:1624 [inline]
n_tty_receive_buf_common+0x805/0xbe0 drivers/tty/n_tty.c:1723
n_tty_receive_buf+0x30/0x40 drivers/tty/n_tty.c:1763
tiocsti+0x170/0x1d0 drivers/tty/tty_io.c:2290
tty_ioctl+0x4a4/0xb80 drivers/tty/tty_io.c:2706
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 0xffffc90006b35028 of 8 bytes by task 10379 on cpu 0:
process_echoes drivers/tty/n_tty.c:768 [inline]
n_tty_write+0x14f/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

value changed: 0x0000000000000000 -> 0x0000000000000003

Reported by Kernel Concurrency Sanitizer on:
CPU: 0 UID: 0 PID: 10379 Comm: syz.3.2505 Tainted: G W syzkaller #0 PREEMPT(voluntary)
Tainted: [W]=WARN
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 10/02/2025
==================================================================


---
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