[syzbot] [serial?] possible deadlock in n_tty_flush_buffer

syzbot
2025-01-22T11:21:24Z

Hello,

syzbot found the following issue on:

HEAD commit: 595523945be0 Merge tag 'devicetree-fixes-for-6.13-2' of gi..
git tree: upstream
console output: https://syzkaller.appspot.com/x/log.txt?x=110a3fc4580000
kernel config: https://syzkaller.appspot.com/x/.config?x=d1cb4a1f148c0861
dashboard link: https://syzkaller.appspot.com/bug?extid=d84934b4184d7f9a1aed
compiler: gcc (Debian 12.2.0-14) 12.2.0, GNU ld (GNU Binutils for Debian) 2.40
syz repro: https://syzkaller.appspot.com/x/repro.syz?x=14a0fa18580000
C reproducer: https://syzkaller.appspot.com/x/repro.c?x=10aa62b0580000

Downloadable assets:
disk image: https://storage.googleapis.com/syzbot-assets/6f7863c08b5c/disk-59552394.raw.xz
vmlinux: https://storage.googleapis.com/syzbot-assets/06971b2686d0/vmlinux-59552394.xz
kernel image: https://storage.googleapis.com/syzbot-assets/66c751022d90/bzImage-59552394.xz

IMPORTANT: if you fix the issue, please add the following tag to the commit:
Reported-by: syzbot+d84934b4184d7f9a1aed@syzkaller.appspotmail.com

======================================================
WARNING: possible circular locking dependency detected
6.13.0-rc7-syzkaller-00189-g595523945be0 #0 Not tainted
------------------------------------------------------
kworker/0:2/1207 is trying to acquire lock:
ffff88801c7ff2e8 (&tty->termios_rwsem){++++}-{4:4}, at: n_tty_flush_buffer+0x25/0x1b0 drivers/tty/n_tty.c:358

but task is already holding lock:
ffff88801b0990b8 (&buf->lock){+.+.}-{4:4}, at: tty_buffer_flush+0x72/0x310 drivers/tty/tty_buffer.c:229

which lock already depends on the new lock.

the existing dependency chain (in reverse order) is:

-> #3 (&buf->lock){+.+.}-{4:4}:
__mutex_lock_common kernel/locking/mutex.c:585 [inline]
__mutex_lock+0x19b/0xa60 kernel/locking/mutex.c:735
tty_buffer_flush+0x72/0x310 drivers/tty/tty_buffer.c:229
tty_ldisc_flush+0x64/0xe0 drivers/tty/tty_ldisc.c:388
__do_SAK+0x6a1/0x800 drivers/tty/tty_io.c:3038
vc_SAK+0x7f/0x310 drivers/tty/vt/vt_ioctl.c:993
process_one_work+0x9c5/0x1ba0 kernel/workqueue.c:3236
process_scheduled_works kernel/workqueue.c:3317 [inline]
worker_thread+0x6c8/0xf00 kernel/workqueue.c:3398
kthread+0x2c1/0x3a0 kernel/kthread.c:389
ret_from_fork+0x45/0x80 arch/x86/kernel/process.c:147
ret_from_fork_asm+0x1a/0x30 arch/x86/entry/entry_64.S:244

-> #2 (console_lock){+.+.}-{0:0}:
console_lock+0x7a/0xa0 kernel/printk/printk.c:2833
uart_configure_port drivers/tty/serial/serial_core.c:2615 [inline]
serial_core_add_one_port drivers/tty/serial/serial_core.c:3182 [inline]
serial_core_register_port+0xe9c/0x2570 drivers/tty/serial/serial_core.c:3419
serial8250_register_8250_port+0x158f/0x2370 drivers/tty/serial/8250/8250_core.c:823
serial_pnp_probe+0x44f/0x930 drivers/tty/serial/8250/8250_pnp.c:490
pnp_device_probe+0x2a5/0x4d0 drivers/pnp/driver.c:111
call_driver_probe drivers/base/dd.c:579 [inline]
really_probe+0x23e/0xa90 drivers/base/dd.c:658
__driver_probe_device+0x1de/0x440 drivers/base/dd.c:800
driver_probe_device+0x4c/0x1b0 drivers/base/dd.c:830
__driver_attach+0x283/0x580 drivers/base/dd.c:1216
bus_for_each_dev+0x13c/0x1d0 drivers/base/bus.c:370
bus_add_driver+0x2e9/0x690 drivers/base/bus.c:675
driver_register+0x15c/0x4b0 drivers/base/driver.c:246
serial8250_init+0xc9/0x1e0 drivers/tty/serial/8250/8250_platform.c:324
do_one_initcall+0x128/0x700 init/main.c:1266
do_initcall_level init/main.c:1328 [inline]
do_initcalls init/main.c:1344 [inline]
do_basic_setup init/main.c:1363 [inline]
kernel_init_freeable+0x5c7/0x900 init/main.c:1577
kernel_init+0x1c/0x2b0 init/main.c:1466
ret_from_fork+0x45/0x80 arch/x86/kernel/process.c:147
ret_from_fork_asm+0x1a/0x30 arch/x86/entry/entry_64.S:244

-> #1 (&port->mutex){+.+.}-{4:4}:
__mutex_lock_common kernel/locking/mutex.c:585 [inline]
__mutex_lock+0x19b/0xa60 kernel/locking/mutex.c:735
class_mutex_constructor include/linux/mutex.h:201 [inline]
uart_set_termios+0x8e/0x6a0 drivers/tty/serial/serial_core.c:1704
tty_set_termios+0x667/0x9b0 drivers/tty/tty_ioctl.c:345
set_termios+0x5ba/0x7f0 drivers/tty/tty_ioctl.c:520
tty_mode_ioctl+0x57a/0xd20 drivers/tty/tty_ioctl.c:807
n_tty_ioctl_helper+0x4b/0x2b0 drivers/tty/tty_ioctl.c:986
n_tty_ioctl+0x7f/0x370 drivers/tty/n_tty.c:2521
tty_ioctl+0x6ee/0x15d0 drivers/tty/tty_io.c:2812
vfs_ioctl fs/ioctl.c:51 [inline]
__do_sys_ioctl fs/ioctl.c:906 [inline]
__se_sys_ioctl fs/ioctl.c:892 [inline]
__x64_sys_ioctl+0x190/0x200 fs/ioctl.c:892
do_syscall_x64 arch/x86/entry/common.c:52 [inline]
do_syscall_64+0xcd/0x250 arch/x86/entry/common.c:83
entry_SYSCALL_64_after_hwframe+0x77/0x7f

-> #0 (&tty->termios_rwsem){++++}-{4:4}:
check_prev_add kernel/locking/lockdep.c:3161 [inline]
check_prevs_add kernel/locking/lockdep.c:3280 [inline]
validate_chain kernel/locking/lockdep.c:3904 [inline]
__lock_acquire+0x249e/0x3c40 kernel/locking/lockdep.c:5226
lock_acquire.part.0+0x11b/0x380 kernel/locking/lockdep.c:5849
down_write+0x93/0x200 kernel/locking/rwsem.c:1577
n_tty_flush_buffer+0x25/0x1b0 drivers/tty/n_tty.c:358
tty_buffer_flush+0x236/0x310 drivers/tty/tty_buffer.c:241
tty_ldisc_flush+0x64/0xe0 drivers/tty/tty_ldisc.c:388
__do_SAK+0x6a1/0x800 drivers/tty/tty_io.c:3038
vc_SAK+0x7f/0x310 drivers/tty/vt/vt_ioctl.c:993
process_one_work+0x9c5/0x1ba0 kernel/workqueue.c:3236
process_scheduled_works kernel/workqueue.c:3317 [inline]
worker_thread+0x6c8/0xf00 kernel/workqueue.c:3398
kthread+0x2c1/0x3a0 kernel/kthread.c:389
ret_from_fork+0x45/0x80 arch/x86/kernel/process.c:147
ret_from_fork_asm+0x1a/0x30 arch/x86/entry/entry_64.S:244

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);
lock(&tty->termios_rwsem);

*** DEADLOCK ***

5 locks held by kworker/0:2/1207:
#0: ffff88801b078948 ((wq_completion)events){+.+.}-{0:0}, at: process_one_work+0x1293/0x1ba0 kernel/workqueue.c:3211
#1: ffffc900045efd80 ((work_completion)(&vc_cons[currcons].SAK_work)){+.+.}-{0:0}, at: process_one_work+0x921/0x1ba0 kernel/workqueue.c:3212
#2: ffffffff8e1a9040 (console_lock){+.+.}-{0:0}, at: vc_SAK+0x13/0x310 drivers/tty/vt/vt_ioctl.c:983
#3: ffff88801c7ff0a0 (&tty->ldisc_sem){++++}-{0:0}, at: tty_ldisc_ref drivers/tty/tty_ldisc.c:263 [inline]
#3: ffff88801c7ff0a0 (&tty->ldisc_sem){++++}-{0:0}, at: tty_ldisc_flush+0x1c/0xe0 drivers/tty/tty_ldisc.c:386
#4: ffff88801b0990b8 (&buf->lock){+.+.}-{4:4}, at: tty_buffer_flush+0x72/0x310 drivers/tty/tty_buffer.c:229

stack backtrace:
CPU: 0 UID: 0 PID: 1207 Comm: kworker/0:2 Not tainted 6.13.0-rc7-syzkaller-00189-g595523945be0 #0
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 12/27/2024
Workqueue: events vc_SAK
Call Trace:
<TASK>
__dump_stack lib/dump_stack.c:94 [inline]
dump_stack_lvl+0x116/0x1f0 lib/dump_stack.c:120
print_circular_bug+0x419/0x5d0 kernel/locking/lockdep.c:2074
check_noncircular+0x31a/0x400 kernel/locking/lockdep.c:2206
check_prev_add kernel/locking/lockdep.c:3161 [inline]
check_prevs_add kernel/locking/lockdep.c:3280 [inline]
validate_chain kernel/locking/lockdep.c:3904 [inline]
__lock_acquire+0x249e/0x3c40 kernel/locking/lockdep.c:5226
lock_acquire.part.0+0x11b/0x380 kernel/locking/lockdep.c:5849
down_write+0x93/0x200 kernel/locking/rwsem.c:1577
n_tty_flush_buffer+0x25/0x1b0 drivers/tty/n_tty.c:358
tty_buffer_flush+0x236/0x310 drivers/tty/tty_buffer.c:241
tty_ldisc_flush+0x64/0xe0 drivers/tty/tty_ldisc.c:388
__do_SAK+0x6a1/0x800 drivers/tty/tty_io.c:3038
vc_SAK+0x7f/0x310 drivers/tty/vt/vt_ioctl.c:993
process_one_work+0x9c5/0x1ba0 kernel/workqueue.c:3236
process_scheduled_works kernel/workqueue.c:3317 [inline]
worker_thread+0x6c8/0xf00 kernel/workqueue.c:3398
kthread+0x2c1/0x3a0 kernel/kthread.c:389
ret_from_fork+0x45/0x80 arch/x86/kernel/process.c:147
ret_from_fork_asm+0x1a/0x30 arch/x86/entry/entry_64.S:244
</TASK>
tty tty1: SAK: killed process 5963 (syz-executor367): by fd#4
usb 5-1: USB disconnect, device number 25
usb 5-1: new high-speed USB device number 26 using dummy_hcd
usb 5-1: config 0 interface 0 altsetting 0 endpoint 0x81 has an invalid bInterval 0, changing to 7
usb 5-1: New USB device found, idVendor=0926, idProduct=3333, bcdDevice= 0.40
usb 5-1: New USB device strings: Mfr=0, Product=0, SerialNumber=0
usb 5-1: config 0 descriptor??
keytouch 0003:0926:3333.0079: fixing up Keytouch IEC report descriptor
input: HID 0926:3333 as /devices/platform/dummy_hcd.4/usb5/5-1/5-1:0.0/0003:0926:3333.0079/input/input125
keytouch 0003:0926:3333.0079: input,hidraw0: USB HID v0.00 Keyboard [HID 0926:3333] on usb-dummy_hcd.4-1/input0
usb 4-1: USB disconnect, device number 26
usb 4-1: new high-speed USB device number 27 using dummy_hcd
usb 4-1: config 0 interface 0 altsetting 0 endpoint 0x81 has an invalid bInterval 0, changing to 7
usb 4-1: New USB device found, idVendor=0926, idProduct=3333, bcdDevice= 0.40
usb 4-1: New USB device strings: Mfr=0, Product=0, SerialNumber=0
usb 4-1: config 0 descriptor??
keytouch 0003:0926:3333.0080: fixing up Keytouch IEC report descriptor
input: HID 0926:3333 as /devices/platform/dummy_hcd.3/usb4/4-1/4-1:0.0/0003:0926:3333.0080/input/input132
keytouch 0003:0926:3333.0080: input,hidraw2: USB HID v0.00 Keyboard [HID 0926:3333] on usb-dummy_hcd.3-1/input0
usb 5-1: USB disconnect, device number 27
usb 5-1: new high-speed USB device number 28 using dummy_hcd
usb 5-1: config 0 interface 0 altsetting 0 endpoint 0x81 has an invalid bInterval 0, changing to 7
usb 5-1: New USB device found, idVendor=0926, idProduct=3333, bcdDevice= 0.40
usb 5-1: New USB device strings: Mfr=0, Product=0, SerialNumber=0
usb 5-1: config 0 descriptor??
keytouch 0003:0926:3333.0084: fixing up Keytouch IEC report descriptor
input: HID 0926:3333 as /devices/platform/dummy_hcd.4/usb5/5-1/5-1:0.0/0003:0926:3333.0084/input/input136
keytouch 0003:0926:3333.0084: input,hidraw1: USB HID v0.00 Keyboard [HID 0926:3333] on usb-dummy_hcd.4-1/input0

---
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