==================================================================
BUG: KASAN: use-after-free in l2tp_session_queue_purge+0xf4/0x100 net/l2tp/l2tp_core.c:830
Read of size 4 at addr ffff8800bad50c80 by task syz-executor275/3820

CPU: 1 PID: 3820 Comm: syz-executor275 Not tainted 4.4.140-ged9bdc8 #68
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 01/01/2011
 0000000000000000 17971abf2d7d1762 ffff8801d8a57cc0 ffffffff81e0e08d
 ffffea0002eb5400 ffff8800bad50c80 0000000000000000 ffff8800bad50c80
 ffffffff82f19f30 ffff8801d8a57cf8 ffffffff81515a56 ffff8800bad50c80
Call Trace:
 [<ffffffff81e0e08d>] __dump_stack lib/dump_stack.c:15 [inline]
 [<ffffffff81e0e08d>] dump_stack+0xc1/0x124 lib/dump_stack.c:51
 [<ffffffff81515a56>] print_address_description+0x6c/0x216 mm/kasan/report.c:252
 [<ffffffff81515d75>] kasan_report_error mm/kasan/report.c:351 [inline]
 [<ffffffff81515d75>] kasan_report.cold.7+0x175/0x2f7 mm/kasan/report.c:408
 [<ffffffff814f9844>] __asan_report_load4_noabort+0x14/0x20 mm/kasan/report.c:428
 [<ffffffff8359adb4>] l2tp_session_queue_purge+0xf4/0x100 net/l2tp/l2tp_core.c:830
 [<ffffffff835a78ef>] pppol2tp_release+0x1ff/0x310 net/l2tp/l2tp_ppp.c:509
 [<ffffffff82f19e06>] sock_release+0x96/0x1c0 net/socket.c:586
 [<ffffffff82f19f46>] sock_close+0x16/0x20 net/socket.c:1037
 [<ffffffff81522e45>] __fput+0x235/0x6f0 fs/file_table.c:208
 [<ffffffff81523385>] ____fput+0x15/0x20 fs/file_table.c:244
 [<ffffffff8118bdbf>] 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
 [<ffffffff838c2935>] int_ret_from_sys_call+0x25/0xa3

Allocated by task 3825:
 [<ffffffff81033e46>] save_stack_trace+0x26/0x50 arch/x86/kernel/stacktrace.c:63
 [<ffffffff814f8913>] save_stack+0x43/0xd0 mm/kasan/kasan.c:512
 [<ffffffff814f8bf7>] set_track mm/kasan/kasan.c:524 [inline]
 [<ffffffff814f8bf7>] kasan_kmalloc+0xc7/0xe0 mm/kasan/kasan.c:616
 [<ffffffff814f5314>] __kmalloc+0x124/0x310 mm/slub.c:3613
 [<ffffffff835a01f9>] kmalloc include/linux/slab.h:481 [inline]
 [<ffffffff835a01f9>] kzalloc include/linux/slab.h:620 [inline]
 [<ffffffff835a01f9>] l2tp_session_create+0x39/0x1030 net/l2tp/l2tp_core.c:1748
 [<ffffffff835a4f00>] pppol2tp_connect+0x10f0/0x1910 net/l2tp/l2tp_ppp.c:725
 [<ffffffff82f1e828>] SYSC_connect+0x1b8/0x300 net/socket.c:1557
 [<ffffffff82f21164>] SyS_connect+0x24/0x30 net/socket.c:1538
 [<ffffffff838c27a5>] entry_SYSCALL_64_fastpath+0x22/0x9e

Freed by task 3787:
 [<ffffffff81033e46>] save_stack_trace+0x26/0x50 arch/x86/kernel/stacktrace.c:63
 [<ffffffff814f8913>] save_stack+0x43/0xd0 mm/kasan/kasan.c:512
 [<ffffffff814f9242>] set_track mm/kasan/kasan.c:524 [inline]
 [<ffffffff814f9242>] kasan_slab_free+0x72/0xc0 mm/kasan/kasan.c:589
 [<ffffffff814f6744>] slab_free_hook mm/slub.c:1383 [inline]
 [<ffffffff814f6744>] slab_free_freelist_hook mm/slub.c:1405 [inline]
 [<ffffffff814f6744>] slab_free mm/slub.c:2859 [inline]
 [<ffffffff814f6744>] kfree+0xf4/0x310 mm/slub.c:3749
 [<ffffffff8359d160>] l2tp_session_free+0x170/0x200 net/l2tp/l2tp_core.c:1676
 [<ffffffff8359f519>] l2tp_session_dec_refcount_1 net/l2tp/l2tp_core.h:293 [inline]
 [<ffffffff8359f519>] l2tp_tunnel_closeall+0x2b9/0x350 net/l2tp/l2tp_core.c:1279
 [<ffffffff835a002b>] l2tp_udp_encap_destroy+0x8b/0xf0 net/l2tp/l2tp_core.c:1300
 [<ffffffff83492361>] udpv6_destroy_sock+0xb1/0xd0 net/ipv6/udp.c:1421
 [<ffffffff82f2f6dd>] sk_common_release+0x6d/0x300 net/core/sock.c:2680
 [<ffffffff83491015>] udp_lib_close+0x15/0x20 include/net/udp.h:190
 [<ffffffff832f8adf>] inet_release+0xff/0x1d0 net/ipv4/af_inet.c:435
 [<ffffffff8341b740>] inet6_release+0x50/0x70 net/ipv6/af_inet6.c:424
 [<ffffffff82f19e06>] sock_release+0x96/0x1c0 net/socket.c:586
 [<ffffffff82f19f46>] sock_close+0x16/0x20 net/socket.c:1037
 [<ffffffff81522e45>] __fput+0x235/0x6f0 fs/file_table.c:208
 [<ffffffff81523385>] ____fput+0x15/0x20 fs/file_table.c:244
 [<ffffffff8118bdbf>] 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
 [<ffffffff838c2935>] int_ret_from_sys_call+0x25/0xa3

The buggy address belongs to the object at ffff8800bad50c80
 which belongs to the cache kmalloc-512 of size 512
The buggy address is located 0 bytes inside of
 512-byte region [ffff8800bad50c80, ffff8800bad50e80)
The buggy address belongs to the page:
kasan: CONFIG_KASAN_INLINE enabled
kasan: GPF could be caused by NULL-ptr deref or user memory access[  436.902452] ------------[ cut here ]------------
WARNING: CPU: 0 PID: 3778 at kernel/sched/core.c:7950 __might_sleep+0x138/0x1a0 kernel/sched/core.c:7945()
do not call blocking ops when !TASK_RUNNING; state=1 set at [<ffffffff838c0262>] do_nanosleep+0x102/0x4f0 kernel/time/hrtimer.c:1502