==================================================================
BUG: KASAN: use-after-free in pppol2tp_session_destruct+0xee/0x110 net/l2tp/l2tp_ppp.c:476
Read of size 4 at addr ffff8801c7858c80 by task syz-executor0/11159

CPU: 0 PID: 11159 Comm: syz-executor0 Not tainted 4.4.114-g4e74e98 #5
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 01/01/2011
 0000000000000000 f054d701d4441e1b ffff8801c0ac7c20 ffffffff81d03d2d
 ffffea00071e1600 ffff8801c7858c80 0000000000000000 ffff8801c7858c80
 ffffffff82de7a60 ffff8801c0ac7c58 ffffffff814fe1d3 ffff8801c7858c80
Call Trace:
 [<ffffffff81d03d2d>] __dump_stack lib/dump_stack.c:15 [inline]
 [<ffffffff81d03d2d>] dump_stack+0xc1/0x124 lib/dump_stack.c:51
 [<ffffffff814fe1d3>] print_address_description+0x73/0x260 mm/kasan/report.c:252
 [<ffffffff814fe6e5>] kasan_report_error mm/kasan/report.c:351 [inline]
 [<ffffffff814fe6e5>] kasan_report+0x285/0x370 mm/kasan/report.c:408
 [<ffffffff814fe824>] __asan_report_load4_noabort+0x14/0x20 mm/kasan/report.c:428
 [<ffffffff834648ee>] pppol2tp_session_destruct+0xee/0x110 net/l2tp/l2tp_ppp.c:476
 [<ffffffff82dfc96a>] sk_destruct+0x4a/0x4c0 net/core/sock.c:1447
 [<ffffffff82dfce37>] __sk_free+0x57/0x230 net/core/sock.c:1480
 [<ffffffff82dfd040>] sk_free+0x30/0x40 net/core/sock.c:1491
 [<ffffffff834657aa>] sock_put include/net/sock.h:1639 [inline]
 [<ffffffff834657aa>] pppol2tp_release+0x27a/0x310 net/l2tp/l2tp_ppp.c:518
 [<ffffffff82de790d>] sock_release+0x8d/0x1e0 net/socket.c:586
 [<ffffffff82de7a76>] sock_close+0x16/0x20 net/socket.c:1037
 [<ffffffff81523693>] __fput+0x233/0x6d0 fs/file_table.c:208
 [<ffffffff81523bb5>] ____fput+0x15/0x20 fs/file_table.c:244
 [<ffffffff8118b9d4>] task_work_run+0x104/0x180 kernel/task_work.c:115
 [<ffffffff8100361d>] tracehook_notify_resume include/linux/tracehook.h:191 [inline]
 [<ffffffff8100361d>] exit_to_usermode_loop+0x13d/0x160 arch/x86/entry/common.c:251
 [<ffffffff81007067>] prepare_exit_to_usermode arch/x86/entry/common.c:282 [inline]
 [<ffffffff81007067>] syscall_return_slowpath arch/x86/entry/common.c:347 [inline]
 [<ffffffff81007067>] do_syscall_32_irqs_on arch/x86/entry/common.c:396 [inline]
 [<ffffffff81007067>] do_fast_syscall_32+0x607/0x890 arch/x86/entry/common.c:457
 [<ffffffff83775c6a>] sysenter_flags_fixed+0xd/0x17

Allocated by task 11159:
 [<ffffffff81035df6>] save_stack_trace+0x26/0x50 arch/x86/kernel/stacktrace.c:63
 [<ffffffff814fd243>] save_stack+0x43/0xd0 mm/kasan/kasan.c:512
 [<ffffffff814fd50d>] set_track mm/kasan/kasan.c:524 [inline]
 [<ffffffff814fd50d>] kasan_kmalloc+0xad/0xe0 mm/kasan/kasan.c:616
 [<ffffffff814f97f4>] __kmalloc+0x124/0x320 mm/slub.c:3613
 [<ffffffff8345e5a9>] kmalloc include/linux/slab.h:481 [inline]
 [<ffffffff8345e5a9>] kzalloc include/linux/slab.h:620 [inline]
 [<ffffffff8345e5a9>] l2tp_session_create+0x39/0x10f0 net/l2tp/l2tp_core.c:1744
 [<ffffffff83462c3c>] pppol2tp_connect+0x10fc/0x1930 net/l2tp/l2tp_ppp.c:718
 [<ffffffff82dec2c6>] SYSC_connect+0x1b6/0x310 net/socket.c:1557
 [<ffffffff82deeb34>] SyS_connect+0x24/0x30 net/socket.c:1538
 [<ffffffff81006d74>] do_syscall_32_irqs_on arch/x86/entry/common.c:390 [inline]
 [<ffffffff81006d74>] do_fast_syscall_32+0x314/0x890 arch/x86/entry/common.c:457
 [<ffffffff83775c6a>] sysenter_flags_fixed+0xd/0x17

Freed by task 11162:
 [<ffffffff81035df6>] save_stack_trace+0x26/0x50 arch/x86/kernel/stacktrace.c:63
 [<ffffffff814fd243>] save_stack+0x43/0xd0 mm/kasan/kasan.c:512
 [<ffffffff814fdb62>] set_track mm/kasan/kasan.c:524 [inline]
 [<ffffffff814fdb62>] kasan_slab_free+0x72/0xc0 mm/kasan/kasan.c:589
 [<ffffffff814fa5fc>] slab_free_hook mm/slub.c:1383 [inline]
 [<ffffffff814fa5fc>] slab_free_freelist_hook mm/slub.c:1405 [inline]
 [<ffffffff814fa5fc>] slab_free mm/slub.c:2859 [inline]
 [<ffffffff814fa5fc>] kfree+0xfc/0x300 mm/slub.c:3749
 [<ffffffff8345ab30>] l2tp_session_free+0x170/0x200 net/l2tp/l2tp_core.c:1672
 [<ffffffff8345d3e1>] l2tp_session_dec_refcount_1 net/l2tp/l2tp_core.h:293 [inline]
 [<ffffffff8345d3e1>] l2tp_tunnel_closeall+0x2d1/0x3b0 net/l2tp/l2tp_core.c:1279
 [<ffffffff8345df4b>] l2tp_udp_encap_destroy+0x8b/0xf0 net/l2tp/l2tp_core.c:1300
 [<ffffffff8336c411>] udpv6_destroy_sock+0xb1/0xd0 net/ipv6/udp.c:1421
 [<ffffffff82dfea0b>] sk_common_release+0x6b/0x300 net/core/sock.c:2680
 [<ffffffff8336b3c5>] udp_lib_close+0x15/0x20 include/net/udp.h:190
 [<ffffffff831cf07a>] inet_release+0xfa/0x1d0 net/ipv4/af_inet.c:435
 [<ffffffff832f4c80>] inet6_release+0x50/0x70 net/ipv6/af_inet6.c:424
 [<ffffffff82de790d>] sock_release+0x8d/0x1e0 net/socket.c:586
 [<ffffffff82de7a76>] sock_close+0x16/0x20 net/socket.c:1037
 [<ffffffff81523693>] __fput+0x233/0x6d0 fs/file_table.c:208
 [<ffffffff81523bb5>] ____fput+0x15/0x20 fs/file_table.c:244
 [<ffffffff8118b9d4>] task_work_run+0x104/0x180 kernel/task_work.c:115
 [<ffffffff8100361d>] tracehook_notify_resume include/linux/tracehook.h:191 [inline]
 [<ffffffff8100361d>] exit_to_usermode_loop+0x13d/0x160 arch/x86/entry/common.c:251
 [<ffffffff81007067>] prepare_exit_to_usermode arch/x86/entry/common.c:282 [inline]
 [<ffffffff81007067>] syscall_return_slowpath arch/x86/entry/common.c:347 [inline]
 [<ffffffff81007067>] do_syscall_32_irqs_on arch/x86/entry/common.c:396 [inline]
 [<ffffffff81007067>] do_fast_syscall_32+0x607/0x890 arch/x86/entry/common.c:457
 [<ffffffff83775c6a>] sysenter_flags_fixed+0xd/0x17

The buggy address belongs to the object at ffff8801c7858c80
 which belongs to the cache kmalloc-512 of size 512
The buggy address is located 0 bytes inside of
 512-byte region [ffff8801c7858c80, ffff8801c7858e80)
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: 4124 Comm: syz-executor0 Not tainted 4.4.114-g4e74e98 #5
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 01/01/2011
task: ffff8800b7b7c800 task.stack: ffff8801cb210000
RIP: 0010:[<ffffffff81d63d31>]  [<ffffffff81d63d31>] __list_add+0x51/0x1c0 lib/list_debug.c:30
RSP: 0018:ffff8801cb217b88  EFLAGS: 00010046
RAX: dffffc0000000000 RBX: ffff8800af78be00 RCX: ffffffff8115556e
RDX: 0000000000000000 RSI: 0000000000000000 RDI: ffff8800af78be00
RBP: ffff8801cb217ba8 R08: 0000000000000000 R09: 0000000000000000
R10: ffffffff81152ca1 R11: 1ffff10039642f3a R12: 0000000000000000
R13: ffff8801c0a9dc70 R14: ffff8801c0a9dc78 R15: 0000000000000009
FS:  0000000000000000(0000) GS:ffff8801db300000(0063) knlGS:000000000a2f5900
CS:  0010 DS: 002b ES: 002b CR0: 0000000080050033
CR2: 00000000207d8000 CR3: 00000001cb20e000 CR4: 0000000000160670
DR0: 0000000000000000 DR1: 0000000000000000 DR2: 0000000000000000
DR3: 0000000000000000 DR6: 00000000fffe0ff0 DR7: 0000000000000400
Stack:
 ffff8800b92d4800 ffff8801c0a9dc70 0000000000000000 0000000000000000
 ffff8801cb217c20 ffffffff8115559f ffff880100000003 0000000000000001
 ffff880100000008 ffff8801c0a9dc80 ffff8800af78be00 ffff8800b92d4f68
Call Trace:
 [<ffffffff8115559f>] list_add_tail include/linux/list.h:77 [inline]
 [<ffffffff8115559f>] __send_signal+0x33f/0x1330 kernel/signal.c:1032
 [<ffffffff811565da>] send_signal+0x4a/0xc0 kernel/signal.c:1096
 [<ffffffff81158d64>] do_send_sig_info+0xa4/0x130 kernel/signal.c:1151
 [<ffffffff81159bed>] group_send_sig_info+0xdd/0x170 kernel/signal.c:1273
 [<ffffffff81159d10>] __kill_pgrp_info+0x90/0x120 kernel/signal.c:1291
 [<ffffffff8115a316>] kill_something_info kernel/signal.c:1397 [inline]
 [<ffffffff8115a316>] SYSC_kill+0x366/0x5b0 kernel/signal.c:2867
 [<ffffffff811604ec>] SyS_kill+0x1c/0x30 kernel/signal.c:2857
 [<ffffffff81006d74>] do_syscall_32_irqs_on arch/x86/entry/common.c:390 [inline]
 [<ffffffff81006d74>] do_fast_syscall_32+0x314/0x890 arch/x86/entry/common.c:457
 [<ffffffff83775c6a>] sysenter_flags_fixed+0xd/0x17
Code: 48 89 fb 80 3c 02 00 0f 85 48 01 00 00 4d 8b 45 08 4d 39 e0 0f 85 c2 00 00 00 48 b8 00 00 00 00 00 fc ff df 4c 89 e2 48 c1 ea 03 <80> 3c 02 00 0f 85 13 01 00 00 4d 8b 04 24 4d 39 e8 0f 85 bd 00 
RIP  [<ffffffff81d63d31>] __list_add+0x51/0x1c0 lib/list_debug.c:30
 RSP <ffff8801cb217b88>
---[ end trace a8fbd86de23353b8 ]---