[moderation] [serial?] KCSAN: data-race in __process_echoes / n_tty_write (14)

[syzbot]

Hello,

syzbot found the following issue on:

HEAD commit: 4c06e63b9203 Merge tag 'for-6.16-rc4-tag' of git://git.ker..
git tree: upstream
console output: https://syzkaller.appspot.com/x/log.txt?x=104f8f70580000
kernel config: https://syzkaller.appspot.com/x/.config?x=eea96b85b0259840
dashboard link: https://syzkaller.appspot.com/bug?extid=cab57903ecc2a256de4f
compiler: Debian clang version 20.1.7 (++20250616065708+6146a88f6049-1~exp1~20250616065826.132), Debian LLD 20.1.7
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/a29ad39ab029/disk-4c06e63b.raw.xz
vmlinux: https://storage.googleapis.com/syzbot-assets/ccd884b8dad2/vmlinux-4c06e63b.xz
kernel image: https://storage.googleapis.com/syzbot-assets/96b61fc8e436/bzImage-4c06e63b.xz

IMPORTANT: if you fix the issue, please add the following tag to the commit:
Reported-by: syzbot+cab579...@syzkaller.appspotmail.com

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

write to 0xffffc900011bd280 of 8 bytes by task 51 on cpu 0:
__process_echoes+0x63e/0x6b0 drivers/tty/n_tty.c:730
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+0x144/0x360 drivers/tty/tty_buffer.c:495
process_one_work kernel/workqueue.c:3238 [inline]
process_scheduled_works+0x4cb/0x9d0 kernel/workqueue.c:3321
worker_thread+0x582/0x770 kernel/workqueue.c:3402
kthread+0x489/0x510 kernel/kthread.c:464
ret_from_fork+0xda/0x150 arch/x86/kernel/process.c:148
ret_from_fork_asm+0x1a/0x30 arch/x86/entry/entry_64.S:245

read to 0xffffc900011bd280 of 8 bytes by task 31622 on cpu 1:
process_echoes drivers/tty/n_tty.c:768 [inline]
n_tty_write+0x161/0xb10 drivers/tty/n_tty.c:2363
iterate_tty_write drivers/tty/tty_io.c:1006 [inline]
file_tty_write+0x389/0x670 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+0x4a0/0x8e0 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+0x2cdd/0x2fb0 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: 0x0000000000000008 -> 0x000000000000000e

Reported by Kernel Concurrency Sanitizer on:
CPU: 1 UID: 0 PID: 31622 Comm: syz.8.9036 Tainted: G W 6.16.0-rc4-syzkaller-00123-g4c06e63b9203 #0 PREEMPT(voluntary)
Tainted: [W]=WARN
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 05/07/2025
==================================================================


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[syzbot]

Auto-closing this bug as obsolete.
Crashes did not happen for a while, no reproducer and no activity.