==================================================================
BUG: KASAN: use-after-free in l2tp_session_queue_purge+0xf4/0x100 net/l2tp/l2tp_core.c:830
Read of size 4 at addr ffff8801d9703900 by task syz-executor2/20695

CPU: 1 PID: 20695 Comm: syz-executor2 Not tainted 4.4.153-g5e24b4e #90
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 01/01/2011
 0000000000000000 f9f4f0590b67be1d ffff8801d67efcb8 ffffffff81e162ed
 ffffea000765c080 ffff8801d9703900 0000000000000000 ffff8801d9703900
 0000000000000000 ffff8801d67efcf0 ffffffff8151b4d9 ffff8801d9703900
Call Trace:
 [<ffffffff81e162ed>] __dump_stack lib/dump_stack.c:15 [inline]
 [<ffffffff81e162ed>] dump_stack+0xc1/0x124 lib/dump_stack.c:51
 [<ffffffff8151b4d9>] print_address_description+0x6c/0x216 mm/kasan/report.c:252
 [<ffffffff8151b7f8>] kasan_report_error mm/kasan/report.c:351 [inline]
 [<ffffffff8151b7f8>] kasan_report.cold.7+0x175/0x2f7 mm/kasan/report.c:408
 [<ffffffff814fe764>] __asan_report_load4_noabort+0x14/0x20 mm/kasan/report.c:428
 [<ffffffff835a4e64>] l2tp_session_queue_purge+0xf4/0x100 net/l2tp/l2tp_core.c:830
 [<ffffffff835b196f>] pppol2tp_release+0x1ff/0x310 net/l2tp/l2tp_ppp.c:509
 [<ffffffff82f213c9>] __sock_release+0xd9/0x260 net/socket.c:592
 [<ffffffff82f21569>] sock_close+0x19/0x20 net/socket.c:1050
 [<ffffffff815288c5>] __fput+0x235/0x6f0 fs/file_table.c:208
 [<ffffffff81528e05>] ____fput+0x15/0x20 fs/file_table.c:244
 [<ffffffff8118ee9f>] 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
 [<ffffffff838cc9f5>] int_ret_from_sys_call+0x25/0xa3

Allocated by task 20702:
 [<ffffffff81034676>] save_stack_trace+0x26/0x50 arch/x86/kernel/stacktrace.c:63
 [<ffffffff814fd823>] save_stack+0x43/0xd0 mm/kasan/kasan.c:512
 [<ffffffff814fdb07>] set_track mm/kasan/kasan.c:524 [inline]
 [<ffffffff814fdb07>] kasan_kmalloc+0xc7/0xe0 mm/kasan/kasan.c:616
 [<ffffffff814fa244>] __kmalloc+0x124/0x310 mm/slub.c:3613
 [<ffffffff835aa279>] kmalloc include/linux/slab.h:481 [inline]
 [<ffffffff835aa279>] kzalloc include/linux/slab.h:620 [inline]
 [<ffffffff835aa279>] l2tp_session_create+0x39/0x1030 net/l2tp/l2tp_core.c:1748
 [<ffffffff835aef80>] pppol2tp_connect+0x10f0/0x1910 net/l2tp/l2tp_ppp.c:725
 [<ffffffff82f27ef8>] SYSC_connect+0x1b8/0x300 net/socket.c:1570
 [<ffffffff82f2a844>] SyS_connect+0x24/0x30 net/socket.c:1551
 [<ffffffff838cc865>] entry_SYSCALL_64_fastpath+0x22/0x9e

Freed by task 20700:
 [<ffffffff81034676>] save_stack_trace+0x26/0x50 arch/x86/kernel/stacktrace.c:63
 [<ffffffff814fd823>] save_stack+0x43/0xd0 mm/kasan/kasan.c:512
 [<ffffffff814fe152>] set_track mm/kasan/kasan.c:524 [inline]
 [<ffffffff814fe152>] kasan_slab_free+0x72/0xc0 mm/kasan/kasan.c:589
 [<ffffffff814fb6b4>] slab_free_hook mm/slub.c:1383 [inline]
 [<ffffffff814fb6b4>] slab_free_freelist_hook mm/slub.c:1405 [inline]
 [<ffffffff814fb6b4>] slab_free mm/slub.c:2859 [inline]
 [<ffffffff814fb6b4>] kfree+0xf4/0x310 mm/slub.c:3749
 [<ffffffff835a71e0>] l2tp_session_free+0x170/0x200 net/l2tp/l2tp_core.c:1676
 [<ffffffff835a9599>] l2tp_session_dec_refcount_1 net/l2tp/l2tp_core.h:293 [inline]
 [<ffffffff835a9599>] l2tp_tunnel_closeall+0x2b9/0x350 net/l2tp/l2tp_core.c:1279
 [<ffffffff835aa0ab>] l2tp_udp_encap_destroy+0x8b/0xf0 net/l2tp/l2tp_core.c:1300
 [<ffffffff8349bf11>] udpv6_destroy_sock+0xb1/0xd0 net/ipv6/udp.c:1421
 [<ffffffff82f38ddd>] sk_common_release+0x6d/0x300 net/core/sock.c:2680
 [<ffffffff8349abc5>] udp_lib_close+0x15/0x20 include/net/udp.h:190
 [<ffffffff8330274f>] inet_release+0xff/0x1d0 net/ipv4/af_inet.c:435
 [<ffffffff834251c0>] inet6_release+0x50/0x70 net/ipv6/af_inet6.c:424
 [<ffffffff82f213c9>] __sock_release+0xd9/0x260 net/socket.c:592
 [<ffffffff82f21569>] sock_close+0x19/0x20 net/socket.c:1050
 [<ffffffff815288c5>] __fput+0x235/0x6f0 fs/file_table.c:208
 [<ffffffff81528e05>] ____fput+0x15/0x20 fs/file_table.c:244
 [<ffffffff8118ee9f>] 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
 [<ffffffff838cc9f5>] int_ret_from_sys_call+0x25/0xa3

The buggy address belongs to the object at ffff8801d9703900
 which belongs to the cache kmalloc-512 of size 512
The buggy address is located 0 bytes inside of
 512-byte region [ffff8801d9703900, ffff8801d9703b00)
The buggy address belongs to the page:
------------[ cut here ]------------
WARNING: CPU: 0 PID: 0 at lib/debugobjects.c:263 debug_print_object+0x181/0x210 lib/debugobjects.c:260()
ODEBUG: deactivate not available (active state 0) object type: hrtimer hint: hrtimer_wakeup+0x0/0x60 kernel/time/hrtimer.c:719