syzbot

unregister_netdevice: waiting for lo to become free. Usage count = 1
unregister_netdevice: waiting for lo to become free. Usage count = 1
unregister_netdevice: waiting for lo to become free. Usage count = 1
unregister_netdevice: waiting for lo to become free. Usage count = 1
unregister_netdevice: waiting for lo to become free. Usage count = 1
INFO: task syz-executor4:22408 blocked for more than 120 seconds.
      Not tainted 4.4.125-g38f41ec #63
"echo 0 > /proc/sys/kernel/hung_task_timeout_secs" disables this message.
syz-executor4   D ffff8800a8f77c38 28376 22408      1 0x00000004
 ffff8800a8f77c38 ffff8801cc2c1800 0000000000000000 0000000000000007
 ffff8801cc2c1800 ffff8801db31fdb8 ffff8801db31fde0 ffff8801db31f4d8
 ffff8801db31f4c0 ffff8801d9b49800 ffff8801cc2c1800 0000000000000000
Call Trace:
 [<ffffffff8376b82a>] schedule+0x7a/0x1b0 kernel/sched/core.c:3359
 [<ffffffff8376c023>] schedule_preempt_disabled+0x13/0x20 kernel/sched/core.c:3392
 [<ffffffff837705e6>] __mutex_lock_common kernel/locking/mutex.c:582 [inline]
 [<ffffffff837705e6>] mutex_lock_nested+0x306/0x850 kernel/locking/mutex.c:621
 [<ffffffff82e33be7>] copy_net_ns+0xc7/0x190 net/core/net_namespace.c:368
 [<ffffffff81193166>] create_new_namespaces+0x2f6/0x610 kernel/nsproxy.c:95
 [<ffffffff81193a1e>] unshare_nsproxy_namespaces+0xae/0x1e0 kernel/nsproxy.c:190
 [<ffffffff8112cc8a>] SYSC_unshare kernel/fork.c:2068 [inline]
 [<ffffffff8112cc8a>] SyS_unshare+0x3da/0x700 kernel/fork.c:2018
 [<ffffffff83779965>] entry_SYSCALL_64_fastpath+0x22/0x9e
1 lock held by syz-executor4/22408:
 #0:  (net_mutex){+.+.+.}, at: [<ffffffff82e33be7>] copy_net_ns+0xc7/0x190 net/core/net_namespace.c:368
Sending NMI to all CPUs:
NMI backtrace for cpu 0
CPU: 0 PID: 487 Comm: khungtaskd Not tainted 4.4.125-g38f41ec #63
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 01/01/2011
task: ffff8800bb38c800 task.stack: ffff8800bb9b8000
RIP: 0010:[<ffffffff810c3f87>]  [<ffffffff810c3f87>] native_apic_mem_write arch/x86/include/asm/apic.h:94 [inline]
RIP: 0010:[<ffffffff810c3f87>]  [<ffffffff810c3f87>] __default_send_IPI_dest_field arch/x86/include/asm/ipi.h:119 [inline]
RIP: 0010:[<ffffffff810c3f87>]  [<ffffffff810c3f87>] _flat_send_IPI_mask arch/x86/kernel/apic/apic_flat_64.c:61 [inline]
RIP: 0010:[<ffffffff810c3f87>]  [<ffffffff810c3f87>] flat_send_IPI_mask+0xf7/0x1a0 arch/x86/kernel/apic/apic_flat_64.c:69
RSP: 0018:ffff8800bb9bfcb8  EFLAGS: 00000046
RAX: 0000000003000000 RBX: 0000000000000c00 RCX: 0000000000000000
RDX: 0000000000000c00 RSI: 0000000000000000 RDI: ffffffffff5fb300
RBP: ffff8800bb9bfce0 R08: 0000000000000001 R09: 0000000000000000
R10: 0000000000000000 R11: 0000000000000001 R12: 0000000000000246
R13: 0000000000000003 R14: ffffffff8426f5a0 R15: 0000000000000002
FS:  0000000000000000(0000) GS:ffff8801db200000(0000) knlGS:0000000000000000
CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
CR2: 00007fbfe9a4d000 CR3: 00000001d16c8000 CR4: 0000000000160670
DR0: 0000000000000000 DR1: 0000000000000000 DR2: 0000000000000000
DR3: 0000000000000000 DR6: 00000000fffe0ff0 DR7: 0000000000000400
Stack:
 ffffffff8426f5a0 ffffffff847f0b40 fffffbfff08fdc1c dffffc0000000000
 ffff8801db31bca0 ffff8800bb9bfd00 ffffffff810b9abb ffffffff839f6dc0
 0000000000000003 ffff8800bb9bfd60 ffffffff81d11764 ffffffff8141b993
Call Trace:
 [<ffffffff810b9abb>] nmi_raise_cpu_backtrace+0x5b/0x70 arch/x86/kernel/apic/hw_nmi.c:33
 [<ffffffff81d11764>] nmi_trigger_all_cpu_backtrace+0x4a4/0x550 lib/nmi_backtrace.c:85
 [<ffffffff810b9b54>] arch_trigger_all_cpu_backtrace+0x14/0x20 arch/x86/kernel/apic/hw_nmi.c:38
 [<ffffffff813695ca>] trigger_all_cpu_backtrace include/linux/nmi.h:44 [inline]
 [<ffffffff813695ca>] check_hung_task kernel/hung_task.c:125 [inline]
 [<ffffffff813695ca>] check_hung_uninterruptible_tasks kernel/hung_task.c:182 [inline]
 [<ffffffff813695ca>] watchdog+0x6fa/0xae0 kernel/hung_task.c:238
 [<ffffffff81190b48>] kthread+0x268/0x300 kernel/kthread.c:211
 [<ffffffff83779d95>] ret_from_fork+0x55/0x80 arch/x86/entry/entry_64.S:510
Code: b3 5f ff f6 c4 10 75 e1 44 89 e8 c1 e0 18 89 04 25 10 b3 5f ff 44 89 fa 09 da 80 cf 04 41 83 ff 02 0f 44 d3 89 14 25 00 b3 5f ff <41> f7 c4 00 02 00 00 74 1a e8 2b 33 17 00 4c 89 e7 57 9d 0f 1f 
NMI backtrace for cpu 1
CPU: 1 PID: 0 Comm: swapper/1 Not tainted 4.4.125-g38f41ec #63
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 01/01/2011
task: ffff8801d9b49800 task.stack: ffff8801d9b58000
RIP: 0010:[<ffffffff810d0666>]  [<ffffffff810d0666>] native_safe_halt+0x6/0x10 arch/x86/include/asm/irqflags.h:49
RSP: 0018:ffff8801d9b5fd98  EFLAGS: 00000246
RAX: 0000000000000007 RBX: ffffffff847dec48 RCX: 0000000000000000
RDX: 0000000000000000 RSI: 0000000000000001 RDI: ffff8801d9b4a0dc
RBP: ffff8801d9b5fd98 R08: 0000000000000000 R09: 0000000000000000
R10: 0000000000000000 R11: 0000000000000000 R12: 0000000000000001
R13: fffffbfff07088c0 R14: ffffffff847ecfb8 R15: 0000000000000000
FS:  0000000000000000(0000) GS:ffff8801db300000(0000) knlGS:0000000000000000
CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
CR2: 00000000020e2b10 CR3: 00000001ce7dc000 CR4: 0000000000160670
DR0: 0000000000000000 DR1: 0000000000000000 DR2: 0000000000000000
DR3: 0000000000000000 DR6: 00000000fffe0ff0 DR7: 0000000000000400
Stack:
 ffff8801d9b5fdd0 ffffffff81027e65 ffff8801d9b60000 ffffffff847dec48
 fffffbfff07088c0 ffffffff847ecfb8 0000000000000000 ffff8801d9b5fde0
 ffffffff810293da ffff8801d9b5fdf8 ffffffff81221948 dffffc0000000000
Call Trace:
 [<ffffffff81027e65>] arch_safe_halt arch/x86/include/asm/paravirt.h:117 [inline]
 [<ffffffff81027e65>] default_idle+0x55/0x3c0 arch/x86/kernel/process.c:290
 [<ffffffff810293da>] arch_cpu_idle+0xa/0x10 arch/x86/kernel/process.c:281
 [<ffffffff81221948>] default_idle_call+0x48/0x70 kernel/sched/idle.c:93
 [<ffffffff8122204d>] cpuidle_idle_call kernel/sched/idle.c:157 [inline]
 [<ffffffff8122204d>] cpu_idle_loop kernel/sched/idle.c:253 [inline]
 [<ffffffff8122204d>] cpu_startup_entry+0x5fd/0x8f0 kernel/sched/idle.c:301
 [<ffffffff810adcd4>] start_secondary+0x304/0x3e0 arch/x86/kernel/smpboot.c:242
Code: 00 00 00 00 00 55 48 89 e5 fa 5d c3 66 0f 1f 84 00 00 00 00 00 55 48 89 e5 fb 5d c3 66 0f 1f 84 00 00 00 00 00 55 48 89 e5 fb f4 <5d> c3 0f 1f 84 00 00 00 00 00 55 48 89 e5 f4 5d c3 66 0f 1f 84