==================================================================
BUG: KASAN: use-after-free in pppol2tp_session_destruct+0xf2/0x110 net/l2tp/l2tp_ppp.c:476
Read of size 4 at addr ffff8800b98cc000 by task syz-executor2/9541

CPU: 0 PID: 9541 Comm: syz-executor2 Not tainted 4.4.133-g3f51ea2 #51
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 01/01/2011
 0000000000000000 e5fb9a8ed0212796 ffff8800a0c9fc68 ffffffff81e0e4cd
 ffffea0002e63300 ffff8800b98cc000 0000000000000000 ffff8800b98cc000
 ffffffff82f191d0 ffff8800a0c9fca0 ffffffff815154cc ffff8800b98cc000
Call Trace:
 [<ffffffff81e0e4cd>] __dump_stack lib/dump_stack.c:15 [inline]
 [<ffffffff81e0e4cd>] dump_stack+0xc1/0x124 lib/dump_stack.c:51
 [<ffffffff815154cc>] print_address_description+0x6c/0x216 mm/kasan/report.c:252
 [<ffffffff815157eb>] kasan_report_error mm/kasan/report.c:351 [inline]
 [<ffffffff815157eb>] kasan_report.cold.7+0x175/0x2f7 mm/kasan/report.c:408
 [<ffffffff814f92c4>] __asan_report_load4_noabort+0x14/0x20 mm/kasan/report.c:428
 [<ffffffff835a53b2>] pppol2tp_session_destruct+0xf2/0x110 net/l2tp/l2tp_ppp.c:476
 [<ffffffff82f2e23c>] sk_destruct+0x4c/0x4c0 net/core/sock.c:1447
 [<ffffffff82f2e6ff>] __sk_free+0x4f/0x220 net/core/sock.c:1480
 [<ffffffff82f2e900>] sk_free+0x30/0x40 net/core/sock.c:1491
 [<ffffffff835a616a>] sock_put include/net/sock.h:1639 [inline]
 [<ffffffff835a616a>] pppol2tp_release+0x26a/0x310 net/l2tp/l2tp_ppp.c:518
 [<ffffffff82f190a6>] sock_release+0x96/0x1c0 net/socket.c:586
 [<ffffffff82f191e6>] sock_close+0x16/0x20 net/socket.c:1037
 [<ffffffff815228b5>] __fput+0x235/0x6f0 fs/file_table.c:208
 [<ffffffff81522df5>] ____fput+0x15/0x20 fs/file_table.c:244
 [<ffffffff8118bc3f>] task_work_run+0x10f/0x190 kernel/task_work.c:115
 [<ffffffff8100362d>] tracehook_notify_resume include/linux/tracehook.h:191 [inline]
 [<ffffffff8100362d>] exit_to_usermode_loop+0x13d/0x160 arch/x86/entry/common.c:252
 [<ffffffff81006535>] prepare_exit_to_usermode arch/x86/entry/common.c:283 [inline]
 [<ffffffff81006535>] syscall_return_slowpath+0x1b5/0x1f0 arch/x86/entry/common.c:348
 [<ffffffff838c10f5>] int_ret_from_sys_call+0x25/0xa3

Allocated by task 9541:
 [<ffffffff810341d6>] save_stack_trace+0x26/0x50 arch/x86/kernel/stacktrace.c:63
 [<ffffffff814f8393>] save_stack+0x43/0xd0 mm/kasan/kasan.c:512
 [<ffffffff814f8677>] set_track mm/kasan/kasan.c:524 [inline]
 [<ffffffff814f8677>] kasan_kmalloc+0xc7/0xe0 mm/kasan/kasan.c:616
 [<ffffffff814f4d94>] __kmalloc+0x124/0x310 mm/slub.c:3613
 [<ffffffff8359ea09>] kmalloc include/linux/slab.h:481 [inline]
 [<ffffffff8359ea09>] kzalloc include/linux/slab.h:620 [inline]
 [<ffffffff8359ea09>] l2tp_session_create+0x39/0x1030 net/l2tp/l2tp_core.c:1748
 [<ffffffff835a3710>] pppol2tp_connect+0x10f0/0x1910 net/l2tp/l2tp_ppp.c:725
 [<ffffffff82f1dac8>] SYSC_connect+0x1b8/0x300 net/socket.c:1557
 [<ffffffff82f20404>] SyS_connect+0x24/0x30 net/socket.c:1538
 [<ffffffff838c0f65>] entry_SYSCALL_64_fastpath+0x22/0x9e

Freed by task 9534:
 [<ffffffff810341d6>] save_stack_trace+0x26/0x50 arch/x86/kernel/stacktrace.c:63
 [<ffffffff814f8393>] save_stack+0x43/0xd0 mm/kasan/kasan.c:512
 [<ffffffff814f8cc2>] set_track mm/kasan/kasan.c:524 [inline]
 [<ffffffff814f8cc2>] kasan_slab_free+0x72/0xc0 mm/kasan/kasan.c:589
 [<ffffffff814f61c4>] slab_free_hook mm/slub.c:1383 [inline]
 [<ffffffff814f61c4>] slab_free_freelist_hook mm/slub.c:1405 [inline]
 [<ffffffff814f61c4>] slab_free mm/slub.c:2859 [inline]
 [<ffffffff814f61c4>] kfree+0xf4/0x310 mm/slub.c:3749
 [<ffffffff8359b970>] l2tp_session_free+0x170/0x200 net/l2tp/l2tp_core.c:1676
 [<ffffffff8359dd29>] l2tp_session_dec_refcount_1 net/l2tp/l2tp_core.h:293 [inline]
 [<ffffffff8359dd29>] l2tp_tunnel_closeall+0x2b9/0x350 net/l2tp/l2tp_core.c:1279
 [<ffffffff8359e83b>] l2tp_udp_encap_destroy+0x8b/0xf0 net/l2tp/l2tp_core.c:1300
 [<ffffffff83490de1>] udpv6_destroy_sock+0xb1/0xd0 net/ipv6/udp.c:1421
 [<ffffffff82f2e97d>] sk_common_release+0x6d/0x300 net/core/sock.c:2680
 [<ffffffff8348fa95>] udp_lib_close+0x15/0x20 include/net/udp.h:190
 [<ffffffff832f75ef>] inet_release+0xff/0x1d0 net/ipv4/af_inet.c:435
 [<ffffffff8341a1c0>] inet6_release+0x50/0x70 net/ipv6/af_inet6.c:424
 [<ffffffff82f190a6>] sock_release+0x96/0x1c0 net/socket.c:586
 [<ffffffff82f191e6>] sock_close+0x16/0x20 net/socket.c:1037
 [<ffffffff815228b5>] __fput+0x235/0x6f0 fs/file_table.c:208
 [<ffffffff81522df5>] ____fput+0x15/0x20 fs/file_table.c:244
 [<ffffffff8118bc3f>] task_work_run+0x10f/0x190 kernel/task_work.c:115
 [<ffffffff8100362d>] tracehook_notify_resume include/linux/tracehook.h:191 [inline]
 [<ffffffff8100362d>] exit_to_usermode_loop+0x13d/0x160 arch/x86/entry/common.c:252
 [<ffffffff81006535>] prepare_exit_to_usermode arch/x86/entry/common.c:283 [inline]
 [<ffffffff81006535>] syscall_return_slowpath+0x1b5/0x1f0 arch/x86/entry/common.c:348
 [<ffffffff838c10f5>] int_ret_from_sys_call+0x25/0xa3

The buggy address belongs to the object at ffff8800b98cc000
 which belongs to the cache kmalloc-512 of size 512
The buggy address is located 0 bytes inside of
 512-byte region [ffff8800b98cc000, ffff8800b98cc200)
The buggy address belongs to the page:
BUG: unable to handle kernel NULL pointer dereference at 0000000000000008
IP: [<ffffffff811559c1>] set_ti_thread_flag include/linux/thread_info.h:42 [inline]
IP: [<ffffffff811559c1>] set_tsk_thread_flag include/linux/sched.h:3103 [inline]
IP: [<ffffffff811559c1>] signal_wake_up_state+0x31/0x70 kernel/signal.c:651
PGD 1d336c067 PUD 1d98e3067 PMD 0 
Oops: 0002 [#1] PREEMPT SMP KASAN
Dumping ftrace buffer:
   (ftrace buffer empty)
Modules linked in:
CPU: 1 PID: 9538 Comm: syz-executor6 Not tainted 4.4.133-g3f51ea2 #51
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 01/01/2011
task: ffff8801cf72c800 task.stack: ffff8801cf4b8000
RIP: 0010:[<ffffffff811559c1>]  [<ffffffff811559c1>] set_ti_thread_flag include/linux/thread_info.h:42 [inline]
RIP: 0010:[<ffffffff811559c1>]  [<ffffffff811559c1>] set_tsk_thread_flag include/linux/sched.h:3103 [inline]
RIP: 0010:[<ffffffff811559c1>]  [<ffffffff811559c1>] signal_wake_up_state+0x31/0x70 kernel/signal.c:651
RSP: 0018:ffff8801cf4bfea8  EFLAGS: 00010082
RAX: 0000000000000000 RBX: ffffffff83aa95f0 RCX: ffffffff811559fe
RDX: 0000000000000000 RSI: 0000000000000008 RDI: ffffffff83aa95f8
RBP: ffff8801cf4bfeb8 R08: ffffffff85328540 R09: 0000000000000001
R10: 0000000000000001 R11: ffff8801cf72c800 R12: 0000000000000080
R13: dffffc0000000000 R14: 0000000000000002 R15: ffff8801cf72c800
FS:  0000000001d58940(0063) GS:ffff8801db300000(0000) knlGS:0000000000000000
CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
CR2: 0000000000000008 CR3: 00000001cd20c000 CR4: 00000000001606f0
DR0: 0000000000000000 DR1: 0000000000000000 DR2: 0000000000000000
DR3: 0000000000000000 DR6: 00000000fffe0ff0 DR7: 0000000000000400
Stack:
 ffffffff83aa95f0 ffffffff83aa9be0 ffff8801cf4bfef0 ffffffff8115a463
 0000000000000b00 ffff8801b91c4500 ffff8801b91c459c ffff8801b91c4590
 ffff8801cc1448c8 ffff8801cf4bff30 ffffffff8113b21c ffffffff81003017
Call Trace:
 [<ffffffff8115a463>] signal_wake_up include/linux/sched.h:3323 [inline]
 [<ffffffff8115a463>] zap_other_threads+0x123/0x170 kernel/signal.c:1213
 [<ffffffff8113b21c>] do_group_exit+0x26c/0x330 kernel/exit.c:884
 [<ffffffff8113b2fd>] SYSC_exit_group kernel/exit.c:900 [inline]
 [<ffffffff8113b2fd>] SyS_exit_group+0x1d/0x20 kernel/exit.c:898
 [<ffffffff838c0f65>] entry_SYSCALL_64_fastpath+0x22/0x9e
Code: 41 89 f4 53 48 89 fb e8 8e b9 1f 00 48 8d 7b 08 48 b8 00 00 00 00 00 fc ff df 48 89 fa 48 c1 ea 03 80 3c 02 00 75 3c 48 8b 43 08 <f0> 80 48 08 04 44 89 e6 48 89 df 83 ce 01 e8 8c cb 06 00 85 c0 
RIP  [<ffffffff811559c1>] set_ti_thread_flag include/linux/thread_info.h:42 [inline]
RIP  [<ffffffff811559c1>] set_tsk_thread_flag include/linux/sched.h:3103 [inline]
RIP  [<ffffffff811559c1>] signal_wake_up_state+0x31/0x70 kernel/signal.c:651
 RSP <ffff8801cf4bfea8>
CR2: 0000000000000008
---[ end trace 7fb8b3c9381e17ab ]---