==================================================================
BUG: KASAN: use-after-free in pppol2tp_session_destruct+0xf2/0x110 net/l2tp/l2tp_ppp.c:476
Read of size 4 at addr ffff8800a2e8d400 by task syz-executor0/18026

CPU: 1 PID: 18026 Comm: syz-executor0 Not tainted 4.4.128-gbd23e3a #20
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 01/01/2011
 0000000000000000 69cf93a7010f1369 ffff8800ad387c68 ffffffff81e0daad
 ffffea00028ba300 ffff8800a2e8d400 0000000000000000 ffff8800a2e8d400
 ffffffff82f17b20 ffff8800ad387ca0 ffffffff815150ac ffff8800a2e8d400
Call Trace:
 [<ffffffff81e0daad>] __dump_stack lib/dump_stack.c:15 [inline]
 [<ffffffff81e0daad>] dump_stack+0xc1/0x124 lib/dump_stack.c:51
 [<ffffffff815150ac>] print_address_description+0x6c/0x216 mm/kasan/report.c:252
 [<ffffffff815153cb>] kasan_report_error mm/kasan/report.c:351 [inline]
 [<ffffffff815153cb>] kasan_report.cold.7+0x175/0x2f7 mm/kasan/report.c:408
 [<ffffffff814f8ea4>] __asan_report_load4_noabort+0x14/0x20 mm/kasan/report.c:428
 [<ffffffff835a2fd2>] pppol2tp_session_destruct+0xf2/0x110 net/l2tp/l2tp_ppp.c:476
 [<ffffffff82f2cb8c>] sk_destruct+0x4c/0x4c0 net/core/sock.c:1447
 [<ffffffff82f2d15e>] __sk_free+0x15e/0x220 net/core/sock.c:1480
 [<ffffffff82f2d250>] sk_free+0x30/0x40 net/core/sock.c:1491
 [<ffffffff835a3d8a>] sock_put include/net/sock.h:1639 [inline]
 [<ffffffff835a3d8a>] pppol2tp_release+0x26a/0x310 net/l2tp/l2tp_ppp.c:518
 [<ffffffff82f179f6>] sock_release+0x96/0x1c0 net/socket.c:586
 [<ffffffff82f17b36>] sock_close+0x16/0x20 net/socket.c:1037
 [<ffffffff81522495>] __fput+0x235/0x6f0 fs/file_table.c:208
 [<ffffffff815229d5>] ____fput+0x15/0x20 fs/file_table.c:244
 [<ffffffff8118bb6f>] 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
 [<ffffffff838be8f5>] int_ret_from_sys_call+0x25/0xa3

Allocated by task 18026:
 [<ffffffff810341d6>] save_stack_trace+0x26/0x50 arch/x86/kernel/stacktrace.c:63
 [<ffffffff814f7f73>] save_stack+0x43/0xd0 mm/kasan/kasan.c:512
 [<ffffffff814f8257>] set_track mm/kasan/kasan.c:524 [inline]
 [<ffffffff814f8257>] kasan_kmalloc+0xc7/0xe0 mm/kasan/kasan.c:616
 [<ffffffff814f4974>] __kmalloc+0x124/0x310 mm/slub.c:3613
 [<ffffffff8359c699>] kmalloc include/linux/slab.h:481 [inline]
 [<ffffffff8359c699>] kzalloc include/linux/slab.h:620 [inline]
 [<ffffffff8359c699>] l2tp_session_create+0x39/0x1030 net/l2tp/l2tp_core.c:1748
 [<ffffffff835a1345>] pppol2tp_connect+0x1095/0x18a0 net/l2tp/l2tp_ppp.c:718
 [<ffffffff82f1c418>] SYSC_connect+0x1b8/0x300 net/socket.c:1557
 [<ffffffff82f1ed54>] SyS_connect+0x24/0x30 net/socket.c:1538
 [<ffffffff838be765>] entry_SYSCALL_64_fastpath+0x22/0x9e

Freed by task 17989:
 [<ffffffff810341d6>] save_stack_trace+0x26/0x50 arch/x86/kernel/stacktrace.c:63
 [<ffffffff814f7f73>] save_stack+0x43/0xd0 mm/kasan/kasan.c:512
 [<ffffffff814f88a2>] set_track mm/kasan/kasan.c:524 [inline]
 [<ffffffff814f88a2>] kasan_slab_free+0x72/0xc0 mm/kasan/kasan.c:589
 [<ffffffff814f5da4>] slab_free_hook mm/slub.c:1383 [inline]
 [<ffffffff814f5da4>] slab_free_freelist_hook mm/slub.c:1405 [inline]
 [<ffffffff814f5da4>] slab_free mm/slub.c:2859 [inline]
 [<ffffffff814f5da4>] kfree+0xf4/0x310 mm/slub.c:3749
 [<ffffffff83599600>] l2tp_session_free+0x170/0x200 net/l2tp/l2tp_core.c:1676
 [<ffffffff8359b9b9>] l2tp_session_dec_refcount_1 net/l2tp/l2tp_core.h:293 [inline]
 [<ffffffff8359b9b9>] l2tp_tunnel_closeall+0x2b9/0x350 net/l2tp/l2tp_core.c:1279
 [<ffffffff8359c4cb>] l2tp_udp_encap_destroy+0x8b/0xf0 net/l2tp/l2tp_core.c:1300
 [<ffffffff8348ec51>] udpv6_destroy_sock+0xb1/0xd0 net/ipv6/udp.c:1421
 [<ffffffff82f2d2cd>] sk_common_release+0x6d/0x300 net/core/sock.c:2680
 [<ffffffff8348d905>] udp_lib_close+0x15/0x20 include/net/udp.h:190
 [<ffffffff832f55bf>] inet_release+0xff/0x1d0 net/ipv4/af_inet.c:435
 [<ffffffff83418150>] inet6_release+0x50/0x70 net/ipv6/af_inet6.c:424
 [<ffffffff82f179f6>] sock_release+0x96/0x1c0 net/socket.c:586
 [<ffffffff82f17b36>] sock_close+0x16/0x20 net/socket.c:1037
 [<ffffffff81522495>] __fput+0x235/0x6f0 fs/file_table.c:208
 [<ffffffff815229d5>] ____fput+0x15/0x20 fs/file_table.c:244
 [<ffffffff8118bb6f>] 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
 [<ffffffff838be8f5>] int_ret_from_sys_call+0x25/0xa3

The buggy address belongs to the object at ffff8800a2e8d400
 which belongs to the cache kmalloc-512 of size 512
The buggy address is located 0 bytes inside of
 512-byte region [ffff8800a2e8d400, ffff8800a2e8d600)
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: 0 PID: 17969 Comm: syz-executor4 Not tainted 4.4.128-gbd23e3a #20
Hardware name: Google Google Compute Engine/Google Co[  217.839337] kernel tried to execute NX-protected page - exploit attempt? (uid: 0)
BUG: unable to handle kernel paging request at ffff8800a2e8d400
IP: [<ffff8800a2e8d400>] 0xffff8800a2e8d400
PGD 632d067 PUD 21ffff067 PMD 80000000a2e000e3 
Thread overran stack, or stack corrupted
Oops: 0011 [#2] PREEMPT SMP KASAN
Dumping ftrace buffer:
   (ftrace buffer empty)
Modules linked in:
CPU: 0 PID: 17969 Comm: syz-executor4 Not tainted 4.4.128-gbd23e3a #20
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 01/01/2011
task: ffff88009eeee000 task.stack: ffff8800a1c70000
RIP: 0010:[<ffff8800a2e8d400>]  [<ffff8800a2e8d400>] 0xffff8800a2e8d400
RSP: 0018:ffff8800a1c771c0  EFLAGS: 00010002
RAX: 0000000000000000 RBX: ffffea00028ba300 RCX: 0000000000000000
RDX: 00000000000003fd RSI: ffffffff82082ac1 RDI: ffffffff862aa4f8
RBP: ffffffff83aa9be0 R08: 0000000000000001 R09: 0000000000000000
R10: 0000000000000002 R11: ffffffff858ed167 R12: 0000000000000000
R13: 0000000000000020 R14: fffffbfff0c554df R15: fffffbfff0c554a1
FS:  0000000000000000(0000) GS:ffff8801db200000(0000) knlGS:0000000000000000
CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
CR2: ffff8800a2e8d400 CR3: 00000001ca048000 CR4: 0000000000160670
DR0: 0000000000000000 DR1: 0000000000000000 DR2: 0000000000000000
DR3: 0000000000000000 DR6: 00000000fffe0ff0 DR7: 0000000000000400
Stack:
 ffff8800a2e8d600 ffff8800a1c77208 ffffffff8148ca77 ffffffff862aa4c0
 ffffea00028ba300 0000000000000000 ffffffff83aa9be0 ffff8800a2e8d400
 ffff8800a2e8d600 ffff8800a1c77248 ffffffff8148ca77 ffffffff862aa4c0
Call Trace:
Code: ff ff ff 00 00 00 00 00 00 00 00 00 a3 8b 02 00 ea ff ff 00 00 00 00 00 00 00 00 e0 9b aa 83 ff ff ff ff 00 d4 e8 a2 00 88 ff ff <00> d6 e8 a2 00 88 ff ff 48 d4 e8 a2 00 88 ff ff 77 ca 48 81 ff 
RIP  [<ffff8800a2e8d400>] 0xffff8800a2e8d400
 RSP <ffff8800a1c771c0>
CR2: ffff8800a2e8d400
---[ end trace c20489b9b429f599 ]---