[moderation] [serial?] KCSAN: data-race in n_tty_receive_char / n_tty_write (12)

[syzbot]

Hello,

syzbot found the following issue on:

HEAD commit: e0d4140e8043 Merge tag 'bitmap-for-6.19-rc5' of https://gi..
git tree: upstream
console output: https://syzkaller.appspot.com/x/log.txt?x=17098522580000
kernel config: https://syzkaller.appspot.com/x/.config?x=c160236e1ef1e401
dashboard link: https://syzkaller.appspot.com/bug?extid=2e5519fdbaa3a06b4dd5
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/1ac95bdb38c5/disk-e0d4140e.raw.xz
vmlinux: https://storage.googleapis.com/syzbot-assets/0ee4ab77d2c2/vmlinux-e0d4140e.xz
kernel image: https://storage.googleapis.com/syzbot-assets/25c24f59a140/bzImage-e0d4140e.xz

IMPORTANT: if you fix the issue, please add the following tag to the commit:
Reported-by: syzbot+2e5519...@syzkaller.appspotmail.com

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

write to 0xffffc900011fb028 of 8 bytes by task 37 on cpu 0:
commit_echoes drivers/tty/n_tty.c:738 [inline]
n_tty_receive_char+0x433/0x6d0 drivers/tty/n_tty.c:1413
n_tty_receive_buf_standard+0x45f/0x2fe0 drivers/tty/n_tty.c:1582
__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:3257 [inline]
process_scheduled_works+0x4ce/0x9d0 kernel/workqueue.c:3340
worker_thread+0x582/0x770 kernel/workqueue.c:3421
kthread+0x489/0x510 kernel/kthread.c:463
ret_from_fork+0x149/0x290 arch/x86/kernel/process.c:158
ret_from_fork_asm+0x1a/0x30 arch/x86/entry/entry_64.S:246

read to 0xffffc900011fb028 of 8 bytes by task 14598 on cpu 1:
process_echoes drivers/tty/n_tty.c:762 [inline]
n_tty_write+0x14f/0xbf0 drivers/tty/n_tty.c:2352
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+0x66b/0xa20 fs/splice.c:738
do_splice_from fs/splice.c:938 [inline]
direct_splice_actor+0x156/0x2a0 fs/splice.c:1161
splice_direct_to_actor+0x312/0x680 fs/splice.c:1105
do_splice_direct_actor fs/splice.c:1204 [inline]
do_splice_direct+0xda/0x150 fs/splice.c:1230
do_sendfile+0x380/0x650 fs/read_write.c:1370
__do_sys_sendfile64 fs/read_write.c:1431 [inline]
__se_sys_sendfile64 fs/read_write.c:1417 [inline]
__x64_sys_sendfile64+0x105/0x150 fs/read_write.c:1417
x64_sys_call+0x2db1/0x3000 arch/x86/include/generated/asm/syscalls_64.h:41
do_syscall_x64 arch/x86/entry/syscall_64.c:63 [inline]
do_syscall_64+0xca/0x2b0 arch/x86/entry/syscall_64.c:94
entry_SYSCALL_64_after_hwframe+0x77/0x7f

value changed: 0x0000000000000252 -> 0x0000000000000267

Reported by Kernel Concurrency Sanitizer on:
CPU: 1 UID: 0 PID: 14598 Comm: syz.4.4740 Not tainted syzkaller #0 PREEMPT(voluntary)
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 10/25/2025
==================================================================


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