==================================================================
BUG: KASAN: use-after-free in pppol2tp_session_destruct+0xf2/0x110 net/l2tp/l2tp_ppp.c:476
Read of size 4 at addr ffff8801da3c1180 by task syz-executor7/17209

CPU: 0 PID: 17209 Comm: syz-executor7 Not tainted 4.4.131-g033c952 #37
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 01/01/2011
 0000000000000000 d113cebb11e839ec ffff8801d459fc68 ffffffff81e0df8d
 ffffea000768f000 ffff8801da3c1180 0000000000000000 ffff8801da3c1180
 ffffffff82f18cb0 ffff8801d459fca0 ffffffff8151520c ffff8801da3c1180
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
 [<ffffffff835a4572>] pppol2tp_session_destruct+0xf2/0x110 net/l2tp/l2tp_ppp.c:476
 [<ffffffff82f2dd1c>] sk_destruct+0x4c/0x4c0 net/core/sock.c:1447
 [<ffffffff82f2e2ee>] __sk_free+0x15e/0x220 net/core/sock.c:1480
 [<ffffffff82f2e3e0>] sk_free+0x30/0x40 net/core/sock.c:1491
 [<ffffffff835a532a>] sock_put include/net/sock.h:1639 [inline]
 [<ffffffff835a532a>] pppol2tp_release+0x26a/0x310 net/l2tp/l2tp_ppp.c:518
 [<ffffffff82f18b86>] sock_release+0x96/0x1c0 net/socket.c:586
 [<ffffffff82f18cc6>] 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
 [<ffffffff838bfef5>] int_ret_from_sys_call+0x25/0xa3

Allocated by task 17209:
 [<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
 [<ffffffff8359dbc9>] kmalloc include/linux/slab.h:481 [inline]
 [<ffffffff8359dbc9>] kzalloc include/linux/slab.h:620 [inline]
 [<ffffffff8359dbc9>] l2tp_session_create+0x39/0x1030 net/l2tp/l2tp_core.c:1748
 [<ffffffff835a28d0>] pppol2tp_connect+0x10f0/0x1910 net/l2tp/l2tp_ppp.c:725
 [<ffffffff82f1d5a8>] SYSC_connect+0x1b8/0x300 net/socket.c:1557
 [<ffffffff82f1fee4>] SyS_connect+0x24/0x30 net/socket.c:1538
 [<ffffffff838bfd65>] entry_SYSCALL_64_fastpath+0x22/0x9e

Freed by task 17212:
 [<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
 [<ffffffff8359ab30>] l2tp_session_free+0x170/0x200 net/l2tp/l2tp_core.c:1676
 [<ffffffff8359cee9>] l2tp_session_dec_refcount_1 net/l2tp/l2tp_core.h:293 [inline]
 [<ffffffff8359cee9>] l2tp_tunnel_closeall+0x2b9/0x350 net/l2tp/l2tp_core.c:1279
 [<ffffffff8359d9fb>] l2tp_udp_encap_destroy+0x8b/0xf0 net/l2tp/l2tp_core.c:1300
 [<ffffffff8348fff1>] udpv6_destroy_sock+0xb1/0xd0 net/ipv6/udp.c:1421
 [<ffffffff82f2e45d>] sk_common_release+0x6d/0x300 net/core/sock.c:2680
 [<ffffffff8348eca5>] udp_lib_close+0x15/0x20 include/net/udp.h:190
 [<ffffffff832f695f>] inet_release+0xff/0x1d0 net/ipv4/af_inet.c:435
 [<ffffffff834194f0>] inet6_release+0x50/0x70 net/ipv6/af_inet6.c:424
 [<ffffffff82f18b86>] sock_release+0x96/0x1c0 net/socket.c:586
 [<ffffffff82f18cc6>] 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
 [<ffffffff838bfef5>] int_ret_from_sys_call+0x25/0xa3

The buggy address belongs to the object at ffff8801da3c1180
 which belongs to the cache kmalloc-512 of size 512
The buggy address is located 0 bytes inside of
 512-byte region [ffff8801da3c1180, ffff8801da3c1380)
The buggy address belongs to the page:
PANIC: double fault, error_code: 0x0
CPU: 0 PID: 17209 Comm: syz-executor7 Not tainted 4.4.131-g033c952 #37
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 01/01/2011
task: ffff8801d9ba8000 task.stack: ffff8801d4598000
RIP: 0010:[<ffffffff8148c8f6>]  [<ffffffff8148c8f6>] dump_page_badflags+0x6/0x70 mm/debug.c:84
RSP: 0018:ffff880100000000  EFLAGS: 00010046
RAX: 0000000000040000 RBX: ffffea000768f000 RCX: ffffc900048c3000
RDX: 0000000000000000 RSI: ffffffff83aa9be0 RDI: ffffea000768f000
RBP: ffff880100000008 R08: 0000000000000001 R09: 0000000000000000
R10: 0000000000000001 R11: ffffffff858ed134 R12: 0000000000000000
R13: ffffffff83aa9be0 R14: ffff8801da3c1180 R15: ffff8801da3c1380
FS:  00007fd1879a7700(0000) GS:ffff8801db200000(0000) knlGS:0000000000000000
CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
CR2: ffff8800fffffff8 CR3: 00000000b5f75000 CR4: 00000000001606f0
DR0: 0000000000000000 DR1: 0000000000000000 DR2: 0000000000000000
DR3: 0000000000000000 DR6: 00000000fffe0ff0 DR7: 0000000000000400
Stack:

Call Trace:
 <UNK> 
Code: 48 69 c0 80 06 00 00 f0 48 ff 80 68 3d 9f 84 5b 5d c3 48 89 df e8 2b c7 06 00 eb dd 66 0f 1f 84 00 00 00 00 00 55 48 89 e5 41 57 <41> 56 41 55 49 89 f5 41 54 49 89 d4 53 48 89 fb 48 83 ec 08 e8