==================================================================
BUG: KASAN: use-after-free in l2tp_session_queue_purge+0xe8/0x100 net/l2tp/l2tp_core.c:830
Read of size 4 at addr ffff8800b3264000 by task syzkaller896976/9723

CPU: 1 PID: 9723 Comm: syzkaller896976 Not tainted 4.4.115-g5e56f24 #13
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 01/01/2011
 0000000000000000 ca3c74f6192ff2f9 ffff8801d725fcc0 ffffffff81d03d8d
 ffffea0002cc9900 ffff8800b3264000 0000000000000000 ffff8800b3264000
 ffffffff82de7bf0 ffff8801d725fcf8 ffffffff814fe1d3 ffff8800b3264000
Call Trace:
 [<ffffffff81d03d8d>] __dump_stack lib/dump_stack.c:15 [inline]
 [<ffffffff81d03d8d>] 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
 [<ffffffff834588a8>] l2tp_session_queue_purge+0xe8/0x100 net/l2tp/l2tp_core.c:830
 [<ffffffff8346593f>] pppol2tp_release+0x1ff/0x310 net/l2tp/l2tp_ppp.c:509
 [<ffffffff82de7a9d>] sock_release+0x8d/0x1e0 net/socket.c:586
 [<ffffffff82de7c06>] 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
 [<ffffffff8118b9e4>] 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
 [<ffffffff81006535>] prepare_exit_to_usermode arch/x86/entry/common.c:282 [inline]
 [<ffffffff81006535>] syscall_return_slowpath+0x1b5/0x1f0 arch/x86/entry/common.c:347
 [<ffffffff83774669>] int_ret_from_sys_call+0x25/0xa3

Allocated by task 9723:
 [<ffffffff81035de6>] 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
 [<ffffffff8345e7b9>] kmalloc include/linux/slab.h:481 [inline]
 [<ffffffff8345e7b9>] kzalloc include/linux/slab.h:620 [inline]
 [<ffffffff8345e7b9>] l2tp_session_create+0x39/0x10f0 net/l2tp/l2tp_core.c:1744
 [<ffffffff83462e4c>] pppol2tp_connect+0x10fc/0x1930 net/l2tp/l2tp_ppp.c:718
 [<ffffffff82dec456>] SYSC_connect+0x1b6/0x310 net/socket.c:1557
 [<ffffffff82deecc4>] SyS_connect+0x24/0x30 net/socket.c:1538
 [<ffffffff837744df>] entry_SYSCALL_64_fastpath+0x1c/0x98

Freed by task 9713:
 [<ffffffff81035de6>] 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
 [<ffffffff8345ad40>] l2tp_session_free+0x170/0x200 net/l2tp/l2tp_core.c:1672
 [<ffffffff8345d5f1>] l2tp_session_dec_refcount_1 net/l2tp/l2tp_core.h:293 [inline]
 [<ffffffff8345d5f1>] l2tp_tunnel_closeall+0x2d1/0x3b0 net/l2tp/l2tp_core.c:1279
 [<ffffffff8345e15b>] l2tp_udp_encap_destroy+0x8b/0xf0 net/l2tp/l2tp_core.c:1300
 [<ffffffff8336c621>] udpv6_destroy_sock+0xb1/0xd0 net/ipv6/udp.c:1421
 [<ffffffff82dfeb9b>] sk_common_release+0x6b/0x300 net/core/sock.c:2680
 [<ffffffff8336b5d5>] udp_lib_close+0x15/0x20 include/net/udp.h:190
 [<ffffffff831cf28a>] inet_release+0xfa/0x1d0 net/ipv4/af_inet.c:435
 [<ffffffff832f4e90>] inet6_release+0x50/0x70 net/ipv6/af_inet6.c:424
 [<ffffffff82de7a9d>] sock_release+0x8d/0x1e0 net/socket.c:586
 [<ffffffff82de7c06>] 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
 [<ffffffff8118b9e4>] 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
 [<ffffffff81006535>] prepare_exit_to_usermode arch/x86/entry/common.c:282 [inline]
 [<ffffffff81006535>] syscall_return_slowpath+0x1b5/0x1f0 arch/x86/entry/common.c:347
 [<ffffffff83774669>] int_ret_from_sys_call+0x25/0xa3

The buggy address belongs to the object at ffff8800b3264000
 which belongs to the cache kmalloc-512 of size 512
The buggy address is located 0 bytes inside of
 512-byte region [ffff8800b3264000, ffff8800b3264200)
The buggy address belongs to the page:
------------[ cut here ]------------
WARNING: CPU: 0 PID: 0 at lib/debugobjects.c:263 debug_print_object+0x17d/0x220 lib/debugobjects.c:260()
ODEBUG: deactivate not available (active state 0) object type: hrtimer hint: hrtimer_wakeup+0x0/0x60 kernel/time/hrtimer.c:718