==================================================================
BUG: KASAN: use-after-free in l2tp_session_queue_purge+0xf4/0x100 net/l2tp/l2tp_core.c:830
Read of size 4 at addr ffff8801ca96d900 by task syz-executor7/7354

CPU: 0 PID: 7354 Comm: syz-executor7 Not tainted 4.4.151-g727cfac #87
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 01/01/2011
 0000000000000000 8e67b59945d0f55f ffff8801bd26fa78 ffffffff81e15eed
 ffffea00072a5b00 ffff8801ca96d900 0000000000000000 ffff8801ca96d900
 0000000000000000 ffff8801bd26fab0 ffffffff8151b390 ffff8801ca96d900
Call Trace:
 [<ffffffff81e15eed>] __dump_stack lib/dump_stack.c:15 [inline]
 [<ffffffff81e15eed>] dump_stack+0xc1/0x124 lib/dump_stack.c:51
 [<ffffffff8151b390>] print_address_description+0x6c/0x216 mm/kasan/report.c:252
 [<ffffffff8151b6af>] kasan_report_error mm/kasan/report.c:351 [inline]
 [<ffffffff8151b6af>] kasan_report.cold.7+0x175/0x2f7 mm/kasan/report.c:408
 [<ffffffff814fe624>] __asan_report_load4_noabort+0x14/0x20 mm/kasan/report.c:428
 [<ffffffff835a4804>] l2tp_session_queue_purge+0xf4/0x100 net/l2tp/l2tp_core.c:830
 [<ffffffff835b130f>] pppol2tp_release+0x1ff/0x310 net/l2tp/l2tp_ppp.c:509
 [<ffffffff82f20d09>] __sock_release+0xd9/0x260 net/socket.c:592
 [<ffffffff82f20ea9>] sock_close+0x19/0x20 net/socket.c:1050
 [<ffffffff81528775>] __fput+0x235/0x6f0 fs/file_table.c:208
 [<ffffffff81528cb5>] ____fput+0x15/0x20 fs/file_table.c:244
 [<ffffffff8118ee4f>] task_work_run+0x10f/0x190 kernel/task_work.c:115
 [<ffffffff81162290>] get_signal+0x1190/0x14b0 kernel/signal.c:2151
 [<ffffffff8100df4b>] do_signal+0x8b/0x1d30 arch/x86/kernel/signal.c:712
 [<ffffffff8100360a>] exit_to_usermode_loop+0x11a/0x160 arch/x86/entry/common.c:249
 [<ffffffff81006535>] prepare_exit_to_usermode arch/x86/entry/common.c:284 [inline]
 [<ffffffff81006535>] syscall_return_slowpath+0x1b5/0x1f0 arch/x86/entry/common.c:349
 [<ffffffff838cc3b5>] int_ret_from_sys_call+0x25/0xa3

Allocated by task 7336:
 [<ffffffff81034676>] save_stack_trace+0x26/0x50 arch/x86/kernel/stacktrace.c:63
 [<ffffffff814fd6f3>] save_stack+0x43/0xd0 mm/kasan/kasan.c:512
 [<ffffffff814fd9d7>] set_track mm/kasan/kasan.c:524 [inline]
 [<ffffffff814fd9d7>] kasan_kmalloc+0xc7/0xe0 mm/kasan/kasan.c:616
 [<ffffffff814fa114>] __kmalloc+0x124/0x310 mm/slub.c:3613
 [<ffffffff835a9c19>] kmalloc include/linux/slab.h:481 [inline]
 [<ffffffff835a9c19>] kzalloc include/linux/slab.h:620 [inline]
 [<ffffffff835a9c19>] l2tp_session_create+0x39/0x1030 net/l2tp/l2tp_core.c:1748
 [<ffffffff835ae920>] pppol2tp_connect+0x10f0/0x1910 net/l2tp/l2tp_ppp.c:725
 [<ffffffff82f27838>] SYSC_connect+0x1b8/0x300 net/socket.c:1570
 [<ffffffff82f2a184>] SyS_connect+0x24/0x30 net/socket.c:1551
 [<ffffffff838cc225>] entry_SYSCALL_64_fastpath+0x22/0x9e

Freed by task 7336:
 [<ffffffff81034676>] save_stack_trace+0x26/0x50 arch/x86/kernel/stacktrace.c:63
 [<ffffffff814fd6f3>] save_stack+0x43/0xd0 mm/kasan/kasan.c:512
 [<ffffffff814fe022>] set_track mm/kasan/kasan.c:524 [inline]
 [<ffffffff814fe022>] kasan_slab_free+0x72/0xc0 mm/kasan/kasan.c:589
 [<ffffffff814fb584>] slab_free_hook mm/slub.c:1383 [inline]
 [<ffffffff814fb584>] slab_free_freelist_hook mm/slub.c:1405 [inline]
 [<ffffffff814fb584>] slab_free mm/slub.c:2859 [inline]
 [<ffffffff814fb584>] kfree+0xf4/0x310 mm/slub.c:3749
 [<ffffffff835a6b80>] l2tp_session_free+0x170/0x200 net/l2tp/l2tp_core.c:1676
 [<ffffffff835a8f39>] l2tp_session_dec_refcount_1 net/l2tp/l2tp_core.h:293 [inline]
 [<ffffffff835a8f39>] l2tp_tunnel_closeall+0x2b9/0x350 net/l2tp/l2tp_core.c:1279
 [<ffffffff835a9a4b>] l2tp_udp_encap_destroy+0x8b/0xf0 net/l2tp/l2tp_core.c:1300
 [<ffffffff8349ba81>] udpv6_destroy_sock+0xb1/0xd0 net/ipv6/udp.c:1421
 [<ffffffff82f3871d>] sk_common_release+0x6d/0x300 net/core/sock.c:2680
 [<ffffffff8349a735>] udp_lib_close+0x15/0x20 include/net/udp.h:190
 [<ffffffff8330209f>] inet_release+0xff/0x1d0 net/ipv4/af_inet.c:435
 [<ffffffff83424d30>] inet6_release+0x50/0x70 net/ipv6/af_inet6.c:424
 [<ffffffff82f20d09>] __sock_release+0xd9/0x260 net/socket.c:592
 [<ffffffff82f20ea9>] sock_close+0x19/0x20 net/socket.c:1050
 [<ffffffff81528775>] __fput+0x235/0x6f0 fs/file_table.c:208
 [<ffffffff81528cb5>] ____fput+0x15/0x20 fs/file_table.c:244
 [<ffffffff8118ee4f>] 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:253
 [<ffffffff81006535>] prepare_exit_to_usermode arch/x86/entry/common.c:284 [inline]
 [<ffffffff81006535>] syscall_return_slowpath+0x1b5/0x1f0 arch/x86/entry/common.c:349
 [<ffffffff838cc3b5>] int_ret_from_sys_call+0x25/0xa3

The buggy address belongs to the object at ffff8801ca96d900
 which belongs to the cache kmalloc-512 of size 512
The buggy address is located 0 bytes inside of
 512-byte region [ffff8801ca96d900, ffff8801ca96db00)
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[  115.945045] ------------[ cut here ]------------
WARNING: CPU: 1 PID: 3893 at kernel/sched/core.c:7946 __might_sleep+0x138/0x1a0 kernel/sched/core.c:7941()
do not call blocking ops when !TASK_RUNNING; state=1 set at [<ffffffff8113d81e>] do_wait+0x26e/0xa30 kernel/exit.c:1503