[moderation] [serial?] KCSAN: data-race in __uart_start / stop_tty

[syzbot]

Hello,

syzbot found the following issue on:

HEAD commit: 11439c4635ed Linux 7.0-rc2
git tree: upstream
console output: https://syzkaller.appspot.com/x/log.txt?x=13c5a64a580000
kernel config: https://syzkaller.appspot.com/x/.config?x=6b32b22d8934c136
dashboard link: https://syzkaller.appspot.com/bug?extid=19879f025d7d926ac30f
compiler: Debian clang version 21.1.8 (++20251221033036+2078da43e25a-1~exp1~20251221153213.50), Debian LLD 21.1.8
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/7b78191ef5d7/disk-11439c46.raw.xz
vmlinux: https://storage.googleapis.com/syzbot-assets/fa1a3b97d273/vmlinux-11439c46.xz
kernel image: https://storage.googleapis.com/syzbot-assets/cc2539425ef0/bzImage-11439c46.xz

IMPORTANT: if you fix the issue, please add the following tag to the commit:
Reported-by: syzbot+19879f...@syzkaller.appspotmail.com

==================================================================
BUG: KCSAN: data-race in __uart_start / stop_tty

write to 0xffff88810b29d5bc of 1 bytes by task 40 on cpu 0:
__stop_tty drivers/tty/tty_io.c:744 [inline]
stop_tty+0x4d/0xe0 drivers/tty/tty_io.c:766
n_tty_receive_char_flow_ctrl drivers/tty/n_tty.c:1230 [inline]
n_tty_receive_char_special drivers/tty/n_tty.c:1335 [inline]
n_tty_receive_buf_standard+0x691/0x3f60 drivers/tty/n_tty.c:1580
__receive_buf drivers/tty/n_tty.c:1616 [inline]
n_tty_receive_buf_common+0x7f3/0xc10 drivers/tty/n_tty.c:1715
n_tty_receive_buf2+0x33/0x40 drivers/tty/n_tty.c:1759
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:3275 [inline]
process_scheduled_works+0x4de/0x9e0 kernel/workqueue.c:3358
worker_thread+0x581/0x770 kernel/workqueue.c:3439
kthread+0x22a/0x280 kernel/kthread.c:467
ret_from_fork+0x150/0x360 arch/x86/kernel/process.c:158
ret_from_fork_asm+0x1a/0x30 arch/x86/entry/entry_64.S:245

read to 0xffff88810b29d5bc of 1 bytes by task 12008 on cpu 1:
uart_tx_stopped include/linux/serial_core.h:1149 [inline]
__uart_start+0x8a/0x2d0 drivers/tty/serial/serial_core.c:143
uart_start+0x58/0x1e0 drivers/tty/serial/serial_core.c:173
uart_flush_chars+0x15/0x20 drivers/tty/serial/serial_core.c:607
n_tty_write+0x9e6/0xc20 drivers/tty/n_tty.c:2382
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
iter_file_splice_write+0x6c4/0xa80 fs/splice.c:736
do_splice_from fs/splice.c:936 [inline]
direct_splice_actor+0x156/0x2a0 fs/splice.c:1159
splice_direct_to_actor+0x311/0x670 fs/splice.c:1103
do_splice_direct_actor fs/splice.c:1202 [inline]
do_splice_direct+0x119/0x1a0 fs/splice.c:1228
do_sendfile+0x382/0x650 fs/read_write.c:1372
__do_sys_sendfile64 fs/read_write.c:1433 [inline]
__se_sys_sendfile64 fs/read_write.c:1419 [inline]
__x64_sys_sendfile64+0x105/0x150 fs/read_write.c:1419
x64_sys_call+0x2dc4/0x3020 arch/x86/include/generated/asm/syscalls_64.h:41
do_syscall_x64 arch/x86/entry/syscall_64.c:63 [inline]
do_syscall_64+0x12c/0x370 arch/x86/entry/syscall_64.c:94
entry_SYSCALL_64_after_hwframe+0x77/0x7f

value changed: 0x00 -> 0x01

Reported by Kernel Concurrency Sanitizer on:
CPU: 1 UID: 0 PID: 12008 Comm: syz.0.2495 Tainted: G W syzkaller #0 PREEMPT(full)
Tainted: [W]=WARN
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 02/12/2026
==================================================================


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