==================================================================
BUG: KASAN: use-after-free in pppol2tp_session_destruct+0xf2/0x110 net/l2tp/l2tp_ppp.c:476
Read of size 4 at addr ffff8800b9a37900 by task syz-executor7/13386

CPU: 0 PID: 13386 Comm: syz-executor7 Not tainted 4.4.131-g1e1da81 #40
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 01/01/2011
 0000000000000000 d27db0a9e99d591f ffff8801a3e77c68 ffffffff81e0df8d
 ffffea0002e68d80 ffff8800b9a37900 0000000000000000 ffff8800b9a37900
 ffffffff82f18dc0 ffff8801a3e77ca0 ffffffff8151520c ffff8800b9a37900
Call Trace:
 [<ffffffff81e0df8d>] __dump_stack lib/dump_stack.c:15 [inline]
 [<ffffffff81e0df8d>] dump_stack+0xc1/0x124 lib/dump_stack.c:51
 [<ffffffff8151520c>] print_address_description+0x6c/0x216 mm/kasan/report.c:252
 [<ffffffff8151552b>] kasan_report_error mm/kasan/report.c:351 [inline]
 [<ffffffff8151552b>] kasan_report.cold.7+0x175/0x2f7 mm/kasan/report.c:408
 [<ffffffff814f9004>] __asan_report_load4_noabort+0x14/0x20 mm/kasan/report.c:428
 [<ffffffff835a4682>] pppol2tp_session_destruct+0xf2/0x110 net/l2tp/l2tp_ppp.c:476
 [<ffffffff82f2de2c>] sk_destruct+0x4c/0x4c0 net/core/sock.c:1447
 [<ffffffff82f2e3fe>] __sk_free+0x15e/0x220 net/core/sock.c:1480
 [<ffffffff82f2e4f0>] sk_free+0x30/0x40 net/core/sock.c:1491
 [<ffffffff835a543a>] sock_put include/net/sock.h:1639 [inline]
 [<ffffffff835a543a>] pppol2tp_release+0x26a/0x310 net/l2tp/l2tp_ppp.c:518
 [<ffffffff82f18c96>] sock_release+0x96/0x1c0 net/socket.c:586
 [<ffffffff82f18dd6>] sock_close+0x16/0x20 net/socket.c:1037
 [<ffffffff815225f5>] __fput+0x235/0x6f0 fs/file_table.c:208
 [<ffffffff81522b35>] ____fput+0x15/0x20 fs/file_table.c:244
 [<ffffffff8118bb8f>] 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
 [<ffffffff838bfff5>] int_ret_from_sys_call+0x25/0xa3

Allocated by task 13386:
 [<ffffffff810341d6>] save_stack_trace+0x26/0x50 arch/x86/kernel/stacktrace.c:63
 [<ffffffff814f80d3>] save_stack+0x43/0xd0 mm/kasan/kasan.c:512
 [<ffffffff814f83b7>] set_track mm/kasan/kasan.c:524 [inline]
 [<ffffffff814f83b7>] kasan_kmalloc+0xc7/0xe0 mm/kasan/kasan.c:616
 [<ffffffff814f4ad4>] __kmalloc+0x124/0x310 mm/slub.c:3613
 [<ffffffff8359dcd9>] kmalloc include/linux/slab.h:481 [inline]
 [<ffffffff8359dcd9>] kzalloc include/linux/slab.h:620 [inline]
 [<ffffffff8359dcd9>] l2tp_session_create+0x39/0x1030 net/l2tp/l2tp_core.c:1748
 [<ffffffff835a29e0>] pppol2tp_connect+0x10f0/0x1910 net/l2tp/l2tp_ppp.c:725
 [<ffffffff82f1d6b8>] SYSC_connect+0x1b8/0x300 net/socket.c:1557
 [<ffffffff82f1fff4>] SyS_connect+0x24/0x30 net/socket.c:1538
 [<ffffffff838bfe65>] entry_SYSCALL_64_fastpath+0x22/0x9e

Freed by task 13397:
 [<ffffffff810341d6>] save_stack_trace+0x26/0x50 arch/x86/kernel/stacktrace.c:63
 [<ffffffff814f80d3>] save_stack+0x43/0xd0 mm/kasan/kasan.c:512
 [<ffffffff814f8a02>] set_track mm/kasan/kasan.c:524 [inline]
 [<ffffffff814f8a02>] kasan_slab_free+0x72/0xc0 mm/kasan/kasan.c:589
 [<ffffffff814f5f04>] slab_free_hook mm/slub.c:1383 [inline]
 [<ffffffff814f5f04>] slab_free_freelist_hook mm/slub.c:1405 [inline]
 [<ffffffff814f5f04>] slab_free mm/slub.c:2859 [inline]
 [<ffffffff814f5f04>] kfree+0xf4/0x310 mm/slub.c:3749
 [<ffffffff8359ac40>] l2tp_session_free+0x170/0x200 net/l2tp/l2tp_core.c:1676
 [<ffffffff8359cff9>] l2tp_session_dec_refcount_1 net/l2tp/l2tp_core.h:293 [inline]
 [<ffffffff8359cff9>] l2tp_tunnel_closeall+0x2b9/0x350 net/l2tp/l2tp_core.c:1279
 [<ffffffff8359db0b>] l2tp_udp_encap_destroy+0x8b/0xf0 net/l2tp/l2tp_core.c:1300
 [<ffffffff83490101>] udpv6_destroy_sock+0xb1/0xd0 net/ipv6/udp.c:1421
 [<ffffffff82f2e56d>] sk_common_release+0x6d/0x300 net/core/sock.c:2680
 [<ffffffff8348edb5>] udp_lib_close+0x15/0x20 include/net/udp.h:190
 [<ffffffff832f6a6f>] inet_release+0xff/0x1d0 net/ipv4/af_inet.c:435
 [<ffffffff83419600>] inet6_release+0x50/0x70 net/ipv6/af_inet6.c:424
 [<ffffffff82f18c96>] sock_release+0x96/0x1c0 net/socket.c:586
 [<ffffffff82f18dd6>] sock_close+0x16/0x20 net/socket.c:1037
 [<ffffffff815225f5>] __fput+0x235/0x6f0 fs/file_table.c:208
 [<ffffffff81522b35>] ____fput+0x15/0x20 fs/file_table.c:244
 [<ffffffff8118bb8f>] 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
 [<ffffffff838bfff5>] int_ret_from_sys_call+0x25/0xa3

The buggy address belongs to the object at ffff8800b9a37900
 which belongs to the cache kmalloc-512 of size 512
The buggy address is located 0 bytes inside of
 512-byte region [ffff8800b9a37900, ffff8800b9a37b00)
The buggy address belongs to the page:
kasan: CONFIG_KASAN_INLINE enabled
kasan: GPF could be caused by NULL-ptr deref or user memory accessgeneral protection fault: 0000 [#1] PREEMPT SMP KASAN
Dumping ftrace buffer:
   (ftrace buffer empty)
Modules linked in:
CPU: 1 PID: 3914 Comm: syz-executor7 Not tainted 4.4.131-g1e1da81 #40
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 01/01/2011
task: ffff8800a3e7b000 task.stack: ffff8800a2448000
RIP: 0010:[<ffffffff811a5f0d>]  [<ffffffff811a5f0d>] task_curr kernel/sched/core.c:1025 [inline]
RIP: 0010:[<ffffffff811a5f0d>]  [<ffffffff811a5f0d>] kick_process+0xdd/0x1c0 kernel/sched/core.c:1567
RSP: 0018:ffff8800a244fb88  EFLAGS: 00010002
RAX: dffffc0000000000 RBX: 000000000001f4c0 RCX: 0000000000000000
RDX: 0000000071dcf257 RSI: ffffffff81e6db1b RDI: 000000038ee792b8
RBP: ffff8800a244fba8 R08: ffffffff85349080 R09: 0000000000000001
R10: 0000000000000000 R11: ffff8800a3e7b000 R12: ffff8800a0c6b000
R13: 000000008148c947 R14: 000000008148c947 R15: ffff8800b69f0000
FS:  00000000015ad940(0063) GS:ffff8801db300000(0000) knlGS:0000000000000000
CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
CR2: 00007f3f233f4000 CR3: 00000000a2bef000 CR4: 00000000001606f0
DR0: 0000000000000000 DR1: 0000000000000000 DR2: 0000000000000000
DR3: 0000000000000000 DR6: 00000000fffe0ff0 DR7: 0000000000000400
Stack:
 ffff8800a0c6b000 0000000000000080 ffff8801cff5c800 0000000000000100
 ffff8800a244fbc8 ffffffff811559af ffff8800a0c6b000 0000000000000080
 ffff8800a244fc20 ffffffff81155d89 ffff8800b69f0070 ffff8800b69f0068
Call Trace:
 [<ffffffff811559af>] signal_wake_up_state+0x5f/0x70 kernel/signal.c:660
 [<ffffffff81155d89>] signal_wake_up include/linux/sched.h:3323 [inline]
 [<ffffffff81155d89>] complete_signal+0x389/0x790 kernel/signal.c:952
 [<ffffffff81157575>] __send_signal+0x4d5/0x11b0 kernel/signal.c:1080
 [<ffffffff8115829a>] send_signal+0x4a/0xc0 kernel/signal.c:1096
 [<ffffffff8115a984>] do_send_sig_info+0xa4/0x130 kernel/signal.c:1151
 [<ffffffff8115b836>] group_send_sig_info+0xf6/0x170 kernel/signal.c:1273
 [<ffffffff8115b940>] __kill_pgrp_info+0x90/0x120 kernel/signal.c:1291
 [<ffffffff8115be05>] kill_something_info kernel/signal.c:1397 [inline]
 [<ffffffff8115be05>] SYSC_kill+0x225/0x5a0 kernel/signal.c:2867
 [<ffffffff81161dcc>] SyS_kill+0x1c/0x30 kernel/signal.c:2857
 [<ffffffff838bfe65>] entry_SYSCALL_64_fastpath+0x22/0x9e
Code: 04 02 84 c0 74 08 3c 03 0f 8e b4 00 00 00 48 b8 00 00 00 00 00 fc ff df 45 8b 6d 10 4a 8d 3c ed 80 48 a1 84 48 89 fa 48 c1 ea 03 <80> 3c 02 00 0f 85 ae 00 00 00 4a 03 1c ed 80 48 a1 84 48 b8 00 
RIP  [<ffffffff811a5f0d>] task_curr kernel/sched/core.c:1025 [inline]
RIP  [<ffffffff811a5f0d>] kick_process+0xdd/0x1c0 kernel/sched/core.c:1567
 RSP <ffff8800a244fb88>
---[ end trace 44d0f4cc369bfb54 ]---