Re: [syzbot] [netfilter?] INFO: task hung in nfnetlink_rcv_msg (5)

syzbot has found a reproducer for the following issue on:

HEAD commit:    c21bb4193868 Merge tag 'for_linus' of git://git.kernel.org..
git tree:       upstream
console output: https://syzkaller.appspot.com/x/log.txt?x=14723bb9580000
kernel config:  https://syzkaller.appspot.com/x/.config?x=db2a459b73b0bd7c
dashboard link: https://syzkaller.appspot.com/bug?extid=c4b20b80ee6a7a2f5012
compiler:       Debian clang version 22.1.8 (++20260613092233+e80beda6e255-1~exp1~20260613092250.77), Debian LLD 22.1.8
C reproducer:   https://syzkaller.appspot.com/x/repro.c?x=13c353b9580000

IMPORTANT: if you fix the issue, please add the following tag to the commit:
Reported-by: [email protected]

INFO: task syz-executor189:6036 blocked for more than 5 seconds.
      Not tainted syzkaller #0
"echo 0 > /proc/sys/kernel/hung_task_timeout_secs" disables this message.
task:syz-executor189 state:D stack:24232 pid:6036  tgid:6036  ppid:154    task_flags:0x400100 flags:0x00080000
Call Trace:
 <TASK>
 context_switch kernel/sched/core.c:5510 [inline]
 __schedule+0x17e7/0x5630 kernel/sched/core.c:7234
 __schedule_loop kernel/sched/core.c:7311 [inline]
 schedule+0x164/0x2b0 kernel/sched/core.c:7326
 schedule_preempt_disabled+0x13/0x30 kernel/sched/core.c:7383
 __mutex_lock_common kernel/locking/mutex.c:726 [inline]
 __mutex_lock+0x7bf/0x1550 kernel/locking/mutex.c:821
 nfnl_lock net/netfilter/nfnetlink.c:96 [inline]
 nfnetlink_rcv_msg+0xa69/0x12b0 net/netfilter/nfnetlink.c:293
 netlink_rcv_skb+0x226/0x4a0 net/netlink/af_netlink.c:2556
 nfnetlink_rcv+0x2b9/0x28c0 net/netfilter/nfnetlink.c:667
 netlink_unicast_kernel net/netlink/af_netlink.c:1319 [inline]
 netlink_unicast+0x7bb/0x940 net/netlink/af_netlink.c:1345
 netlink_sendmsg+0x813/0xb40 net/netlink/af_netlink.c:1900
 sock_sendmsg_nosec+0x13a/0x180 net/socket.c:775
 __sock_sendmsg net/socket.c:790 [inline]
 __sys_sendto+0x408/0x5a0 net/socket.c:2252
 __do_sys_sendto net/socket.c:2259 [inline]
 __se_sys_sendto net/socket.c:2255 [inline]
 __x64_sys_sendto+0xde/0x100 net/socket.c:2255
 do_syscall_x64 arch/x86/entry/syscall_64.c:63 [inline]
 do_syscall_64+0x174/0x580 arch/x86/entry/syscall_64.c:94
 entry_SYSCALL_64_after_hwframe+0x77/0x7f
RIP: 0033:0x7f581fdbc037
RSP: 002b:00007fff5c0188a0 EFLAGS: 00000202 ORIG_RAX: 000000000000002c
RAX: ffffffffffffffda RBX: 000055556b6f4400 RCX: 00007f581fdbc037
RDX: 0000000000000014 RSI: 00007fff5c018900 RDI: 0000000000000000
RBP: 0000000000000004 R08: 00007fff5c0188f4 R09: 000000000000000c
R10: 0000000000000000 R11: 0000000000000202 R12: 00007fff5c019f68
R13: 0000000000000002 R14: 00007f581fe22c80 R15: 0000000000000002
 </TASK>

Showing all locks held in the system:
1 lock held by khungtaskd/31:
 #0: ffffffff8e759c20 (rcu_read_lock){....}-{1:3}, at: rcu_lock_acquire include/linux/rcupdate.h:300 [inline]
 #0: ffffffff8e759c20 (rcu_read_lock){....}-{1:3}, at: rcu_read_lock include/linux/rcupdate.h:840 [inline]
 #0: ffffffff8e759c20 (rcu_read_lock){....}-{1:3}, at: debug_show_all_locks+0x2e/0x180 kernel/locking/lockdep.c:6775
2 locks held by kworker/u8:5/154:
 #0: ffff88813fe4c140 ((wq_completion)events_unbound){+.+.}-{0:0}, at: process_one_work kernel/workqueue.c:3297 [inline]
 #0: ffff88813fe4c140 ((wq_completion)events_unbound){+.+.}-{0:0}, at: process_scheduled_works+0xa20/0x14e0 kernel/workqueue.c:3405
 #1: ffffc90002f5fc40 ((work_completion)(&sub_info->work)){+.+.}-{0:0}, at: process_one_work kernel/workqueue.c:3297 [inline]
 #1: ffffc90002f5fc40 ((work_completion)(&sub_info->work)){+.+.}-{0:0}, at: process_scheduled_works+0xa20/0x14e0 kernel/workqueue.c:3405
2 locks held by getty/5372:
 #0: ffff888035e180a0 (&tty->ldisc_sem){++++}-{0:0}, at: tty_ldisc_ref_wait+0x25/0x70 drivers/tty/tty_ldisc.c:243
 #1: ffffc900032332e8 (&ldata->atomic_read_lock){+.+.}-{4:4}, at: n_tty_read+0x45a/0x1360 drivers/tty/n_tty.c:2211
1 lock held by syz-executor189/6035:
 #0: ffffffff9a7a3cb0 (nfnl_subsys_ctnetlink){+.+.}-{4:4}, at: nfnl_lock net/netfilter/nfnetlink.c:96 [inline]
 #0: ffffffff9a7a3cb0 (nfnl_subsys_ctnetlink){+.+.}-{4:4}, at: nfnetlink_rcv_msg+0xa69/0x12b0 net/netfilter/nfnetlink.c:293
1 lock held by syz-executor189/6036:
 #0: ffffffff9a7a3cb0 (nfnl_subsys_ctnetlink){+.+.}-{4:4}, at: nfnl_lock net/netfilter/nfnetlink.c:96 [inline]
 #0: ffffffff9a7a3cb0 (nfnl_subsys_ctnetlink){+.+.}-{4:4}, at: nfnetlink_rcv_msg+0xa69/0x12b0 net/netfilter/nfnetlink.c:293

=============================================

NMI backtrace for cpu 0
CPU: 0 UID: 0 PID: 31 Comm: khungtaskd Not tainted syzkaller #0 PREEMPT(full) 
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 07/24/2026
Call Trace:
 <TASK>
 dump_stack_lvl+0xe8/0x150 lib/dump_stack.c:120
 nmi_cpu_backtrace+0x274/0x2d0 lib/nmi_backtrace.c:122
 nmi_trigger_cpumask_backtrace+0x17a/0x380 lib/nmi_backtrace.c:65
 trigger_all_cpu_backtrace include/linux/nmi.h:162 [inline]
 __sys_info lib/sys_info.c:157 [inline]
 sys_info+0x135/0x170 lib/sys_info.c:165
 check_hung_uninterruptible_tasks kernel/hung_task.c:353 [inline]
 watchdog+0xfd7/0x1030 kernel/hung_task.c:561
 kthread+0x388/0x470 kernel/kthread.c:436
 ret_from_fork+0x514/0xb70 arch/x86/kernel/process.c:158
 ret_from_fork_asm+0x1a/0x30 arch/x86/entry/entry_64.S:245
 </TASK>
Sending NMI from CPU 0 to CPUs 1:
NMI backtrace for cpu 1
CPU: 1 UID: 0 PID: 0 Comm: swapper/1 Not tainted syzkaller #0 PREEMPT(full) 
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 07/24/2026
RIP: 0010:pv_native_safe_halt+0xf/0x20 arch/x86/kernel/paravirt.c:64
Code: ec 71 02 e9 d3 02 03 00 cc cc cc 90 90 90 90 90 90 90 90 90 90 90 90 90 90 90 90 f3 0f 1e fa 66 90 0f 00 2d d3 ca 0f 00 fb f4 <c3> cc cc cc cc cc cc cc cc cc cc cc cc cc cc cc cc 90 90 90 90 90
RSP: 0018:ffffc90000197e40 EFLAGS: 00000242
RAX: 0000000000052637 RBX: ffffffff8199752c RCX: 0000000080000001
RDX: 0000000000000001 RSI: ffffffff8ddfd062 RDI: ffffffff8c0b0280
RBP: ffffc90000197f10 R08: ffff8880b87338db R09: 1ffff110170e671b
R10: dffffc0000000000 R11: ffffed10170e671c R12: 0000000000000000
R13: 1ffff11003ada000 R14: 1ffff92000032fd0 R15: dffffc0000000000
FS:  0000000000000000(0000) GS:ffff888125546000(0000) knlGS:0000000000000000
CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
CR2: 000055c1e9808200 CR3: 000000007a26e000 CR4: 00000000003526f0
Call Trace:
 <TASK>
 arch_safe_halt arch/x86/kernel/process.c:767 [inline]
 default_idle+0x9/0x20 arch/x86/kernel/process.c:768
 default_idle_call+0x72/0xb0 kernel/sched/idle.c:122
 cpuidle_idle_call kernel/sched/idle.c:199 [inline]
 do_idle+0x1cc/0x540 kernel/sched/idle.c:355
 cpu_startup_entry+0x43/0x60 kernel/sched/idle.c:454
 start_secondary+0x101/0x110 arch/x86/kernel/smpboot.c:312
 common_startup_64+0x13e/0x157
 </TASK>


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