[syzbot] [serial?] possible deadlock in n_tty_receive_buf_common (2)

[syzbot]

Hello,

syzbot found the following issue on:

HEAD commit: 848acc8ffe1b Merge tag 'fsverity-for-linus' of git://git.k..
git tree: upstream
console output: https://syzkaller.appspot.com/x/log.txt?x=12785bb9580000
kernel config: https://syzkaller.appspot.com/x/.config?x=f221a1b5d497ed15
dashboard link: https://syzkaller.appspot.com/bug?extid=087f6a513e0d60ee6ee4
compiler: gcc (Debian 14.2.0-19) 14.2.0, GNU ld (GNU Binutils for Debian) 2.44
userspace arch: i386

Unfortunately, I don't have any reproducer for this issue yet.

Downloadable assets:
disk image (non-bootable): https://storage.googleapis.com/syzbot-assets/d900f083ada3/non_bootable_disk-848acc8f.raw.xz
vmlinux: https://storage.googleapis.com/syzbot-assets/31f94fe4d67f/vmlinux-848acc8f.xz
kernel image: https://storage.googleapis.com/syzbot-assets/391ba15cbf42/bzImage-848acc8f.xz

IMPORTANT: if you fix the issue, please add the following tag to the commit:
Reported-by: syzbot+087f6a...@syzkaller.appspotmail.com

======================================================
WARNING: possible circular locking dependency detected
syzkaller #0 Not tainted
------------------------------------------------------
kworker/u33:3/62 is trying to acquire lock:
ffff88804b3fe2c8 (&tty->termios_rwsem){++++}-{4:4}, at: class_rwsem_read_constructor include/linux/rwsem.h:259 [inline]
ffff88804b3fe2c8 (&tty->termios_rwsem){++++}-{4:4}, at: n_tty_receive_buf_common+0x85/0x1930 drivers/tty/n_tty.c:1678

but task is already holding lock:
ffff88801404a8b8 (&buf->lock){+.+.}-{4:4}, at: flush_to_ldisc+0x34/0x730 drivers/tty/tty_buffer.c:474

which lock already depends on the new lock.


the existing dependency chain (in reverse order) is:

-> #3 (&buf->lock){+.+.}-{4:4}:
lock_acquire kernel/locking/lockdep.c:5868 [inline]
lock_acquire+0x1b9/0x370 kernel/locking/lockdep.c:5825
__mutex_lock_common kernel/locking/mutex.c:646 [inline]
__mutex_lock+0x1a4/0x1bd0 kernel/locking/mutex.c:821
tty_buffer_flush+0x72/0x310 drivers/tty/tty_buffer.c:236
tty_ldisc_flush+0xd2/0xf0 drivers/tty/tty_ldisc.c:388
__do_SAK+0x368/0x480 drivers/tty/tty_io.c:3025
vc_SAK+0x7f/0x320 drivers/tty/vt/vt_ioctl.c:994
process_one_work+0xa23/0x1940 kernel/workqueue.c:3322
process_scheduled_works kernel/workqueue.c:3405 [inline]
worker_thread+0x5ef/0xe50 kernel/workqueue.c:3486
kthread+0x370/0x450 kernel/kthread.c:436
ret_from_fork+0x72b/0xd50 arch/x86/kernel/process.c:158
ret_from_fork_asm+0x1a/0x30 arch/x86/entry/entry_64.S:245

-> #2 (console_lock){+.+.}-{0:0}:
lock_acquire kernel/locking/lockdep.c:5868 [inline]
lock_acquire+0x1b9/0x370 kernel/locking/lockdep.c:5825
console_lock+0x7a/0xa0 kernel/printk/printk.c:2896
uart_configure_port drivers/tty/serial/serial_core.c:2530 [inline]
serial_core_add_one_port drivers/tty/serial/serial_core.c:3101 [inline]
serial_core_register_port+0xe65/0x16f0 drivers/tty/serial/serial_core.c:3322
serial8250_register_8250_port+0x15af/0x2330 drivers/tty/serial/8250/8250_core.c:828
serial_pnp_probe+0x436/0x930 drivers/tty/serial/8250/8250_pnp.c:480
pnp_device_probe+0x2b3/0x4a0 drivers/pnp/driver.c:111
call_driver_probe drivers/base/dd.c:628 [inline]
really_probe+0x241/0xa60 drivers/base/dd.c:706
__driver_probe_device+0x20e/0x450 drivers/base/dd.c:868
driver_probe_device+0x4a/0x140 drivers/base/dd.c:898
__driver_attach+0x21f/0x5b0 drivers/base/dd.c:1292
bus_for_each_dev+0x13e/0x1d0 drivers/base/bus.c:383
bus_add_driver+0x305/0x5b0 drivers/base/bus.c:763
driver_register+0x1e2/0x360 drivers/base/driver.c:174
serial8250_init+0xcc/0x1c0 drivers/tty/serial/8250/8250_platform.c:317
do_one_initcall+0x11d/0x700 init/main.c:1347
do_initcall_level init/main.c:1409 [inline]
do_initcalls init/main.c:1425 [inline]
do_basic_setup init/main.c:1445 [inline]
kernel_init_freeable+0x6ea/0x7b0 init/main.c:1658
kernel_init+0x1f/0x1e0 init/main.c:1548
ret_from_fork+0x72b/0xd50 arch/x86/kernel/process.c:158
ret_from_fork_asm+0x1a/0x30 arch/x86/entry/entry_64.S:245

-> #1 (&port->mutex){+.+.}-{4:4}:
lock_acquire kernel/locking/lockdep.c:5868 [inline]
lock_acquire+0x1b9/0x370 kernel/locking/lockdep.c:5825
__mutex_lock_common kernel/locking/mutex.c:646 [inline]
__mutex_lock+0x1a4/0x1bd0 kernel/locking/mutex.c:821
class_mutex_constructor include/linux/mutex.h:253 [inline]
uart_set_termios+0x8e/0x690 drivers/tty/serial/serial_core.c:1645
tty_set_termios+0x64d/0x980 drivers/tty/tty_ioctl.c:341
set_termios+0x5c8/0x880 drivers/tty/tty_ioctl.c:516
tty_mode_ioctl+0x17e/0xd40 drivers/tty/tty_ioctl.c:803
n_tty_ioctl_helper+0x47/0x2b0 drivers/tty/tty_ioctl.c:982
n_tty_ioctl+0x53/0x340 drivers/tty/n_tty.c:2496
tty_ioctl+0x11b4/0x1640 drivers/tty/tty_io.c:2801
vfs_ioctl fs/ioctl.c:51 [inline]
__do_sys_ioctl fs/ioctl.c:597 [inline]
__se_sys_ioctl fs/ioctl.c:583 [inline]
__x64_sys_ioctl+0x18e/0x210 fs/ioctl.c:583
do_syscall_x64 arch/x86/entry/syscall_64.c:63 [inline]
do_syscall_64+0x115/0x840 arch/x86/entry/syscall_64.c:94
entry_SYSCALL_64_after_hwframe+0x77/0x7f

-> #0 (&tty->termios_rwsem){++++}-{4:4}:
check_prev_add+0xeb/0xe60 kernel/locking/lockdep.c:3165
check_prevs_add kernel/locking/lockdep.c:3284 [inline]
validate_chain kernel/locking/lockdep.c:3908 [inline]
__lock_acquire+0x136c/0x1a40 kernel/locking/lockdep.c:5237
lock_acquire kernel/locking/lockdep.c:5868 [inline]
lock_acquire+0x1b9/0x370 kernel/locking/lockdep.c:5825
down_read+0x99/0x4c0 kernel/locking/rwsem.c:1574
class_rwsem_read_constructor include/linux/rwsem.h:259 [inline]
n_tty_receive_buf_common+0x85/0x1930 drivers/tty/n_tty.c:1678
tty_ldisc_receive_buf+0xaa/0x1a0 drivers/tty/tty_buffer.c:394
tty_port_default_receive_buf+0x70/0xb0 drivers/tty/tty_port.c:37
receive_buf drivers/tty/tty_buffer.c:452 [inline]
flush_to_ldisc+0x26b/0x730 drivers/tty/tty_buffer.c:502
process_one_work+0xa23/0x1940 kernel/workqueue.c:3322
process_scheduled_works kernel/workqueue.c:3405 [inline]
worker_thread+0x5ef/0xe50 kernel/workqueue.c:3486
kthread+0x370/0x450 kernel/kthread.c:436
ret_from_fork+0x72b/0xd50 arch/x86/kernel/process.c:158
ret_from_fork_asm+0x1a/0x30 arch/x86/entry/entry_64.S:245

other info that might help us debug this:

Chain exists of:
&tty->termios_rwsem --> console_lock --> &buf->lock

Possible unsafe locking scenario:

CPU0 CPU1
---- ----
lock(&buf->lock);
lock(console_lock);
lock(&buf->lock);
rlock(&tty->termios_rwsem);

*** DEADLOCK ***

4 locks held by kworker/u33:3/62:
#0: ffff88801bce4140 ((wq_completion)events_unbound){+.+.}-{0:0}, at: process_one_work+0x12b1/0x1940 kernel/workqueue.c:3297
#1: ffffc900009efd08 ((work_completion)(&buf->work)){+.+.}-{0:0}, at: process_one_work+0x988/0x1940 kernel/workqueue.c:3298
#2: ffff88801404a8b8 (&buf->lock){+.+.}-{4:4}, at: flush_to_ldisc+0x34/0x730 drivers/tty/tty_buffer.c:474
#3: ffff88804b3fe0a0 (&tty->ldisc_sem){++++}-{0:0}, at: tty_ldisc_ref+0x21/0x90 drivers/tty/tty_ldisc.c:263

stack backtrace:
CPU: 0 UID: 0 PID: 62 Comm: kworker/u33:3 Not tainted syzkaller #0 PREEMPT(full)
Hardware name: QEMU Standard PC (Q35 + ICH9, 2009), BIOS 1.16.3-debian-1.16.3-2 04/01/2014
Workqueue: events_unbound flush_to_ldisc
Call Trace:
<TASK>
__dump_stack lib/dump_stack.c:94 [inline]
dump_stack_lvl+0x100/0x190 lib/dump_stack.c:120
print_circular_bug.cold+0x178/0x1c7 kernel/locking/lockdep.c:2043
check_noncircular+0x146/0x160 kernel/locking/lockdep.c:2175
check_prev_add+0xeb/0xe60 kernel/locking/lockdep.c:3165
check_prevs_add kernel/locking/lockdep.c:3284 [inline]
validate_chain kernel/locking/lockdep.c:3908 [inline]
__lock_acquire+0x136c/0x1a40 kernel/locking/lockdep.c:5237
lock_acquire kernel/locking/lockdep.c:5868 [inline]
lock_acquire+0x1b9/0x370 kernel/locking/lockdep.c:5825
down_read+0x99/0x4c0 kernel/locking/rwsem.c:1574
class_rwsem_read_constructor include/linux/rwsem.h:259 [inline]
n_tty_receive_buf_common+0x85/0x1930 drivers/tty/n_tty.c:1678
tty_ldisc_receive_buf+0xaa/0x1a0 drivers/tty/tty_buffer.c:394
tty_port_default_receive_buf+0x70/0xb0 drivers/tty/tty_port.c:37
receive_buf drivers/tty/tty_buffer.c:452 [inline]
flush_to_ldisc+0x26b/0x730 drivers/tty/tty_buffer.c:502
process_one_work+0xa23/0x1940 kernel/workqueue.c:3322
process_scheduled_works kernel/workqueue.c:3405 [inline]
worker_thread+0x5ef/0xe50 kernel/workqueue.c:3486
kthread+0x370/0x450 kernel/kthread.c:436
ret_from_fork+0x72b/0xd50 arch/x86/kernel/process.c:158
ret_from_fork_asm+0x1a/0x30 arch/x86/entry/entry_64.S:245
</TASK>


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