==================================================================
BUG: KASAN: use-after-free in l2tp_session_queue_purge+0xf4/0x100 net/l2tp/l2tp_core.c:830
Read of size 4 at addr ffff8801d4abdb80 by task syz-executor050/8182

CPU: 0 PID: 8182 Comm: syz-executor050 Not tainted 4.4.139-g7ba5557 #65
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 01/01/2011
 0000000000000000 f0f6a453b0623f52 ffff8801d463fcc0 ffffffff81e0d58d
 ffffea000752af00 ffff8801d4abdb80 0000000000000000 ffff8801d4abdb80
 ffffffff82f19100 ffff8801d463fcf8 ffffffff81515a16 ffff8801d4abdb80
Call Trace:
 [<ffffffff81e0d58d>] __dump_stack lib/dump_stack.c:15 [inline]
 [<ffffffff81e0d58d>] dump_stack+0xc1/0x124 lib/dump_stack.c:51
 [<ffffffff81515a16>] print_address_description+0x6c/0x216 mm/kasan/report.c:252
 [<ffffffff81515d35>] kasan_report_error mm/kasan/report.c:351 [inline]
 [<ffffffff81515d35>] kasan_report.cold.7+0x175/0x2f7 mm/kasan/report.c:408
 [<ffffffff814f9804>] __asan_report_load4_noabort+0x14/0x20 mm/kasan/report.c:428
 [<ffffffff83599e64>] l2tp_session_queue_purge+0xf4/0x100 net/l2tp/l2tp_core.c:830
 [<ffffffff835a699f>] pppol2tp_release+0x1ff/0x310 net/l2tp/l2tp_ppp.c:509
 [<ffffffff82f18fd6>] sock_release+0x96/0x1c0 net/socket.c:586
 [<ffffffff82f19116>] sock_close+0x16/0x20 net/socket.c:1037
 [<ffffffff81522e05>] __fput+0x235/0x6f0 fs/file_table.c:208
 [<ffffffff81523345>] ____fput+0x15/0x20 fs/file_table.c:244
 [<ffffffff8118bd7f>] 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
 [<ffffffff838c19f5>] int_ret_from_sys_call+0x25/0xa3

Allocated by task 8182:
 [<ffffffff81033e46>] save_stack_trace+0x26/0x50 arch/x86/kernel/stacktrace.c:63
 [<ffffffff814f88d3>] save_stack+0x43/0xd0 mm/kasan/kasan.c:512
 [<ffffffff814f8bb7>] set_track mm/kasan/kasan.c:524 [inline]
 [<ffffffff814f8bb7>] kasan_kmalloc+0xc7/0xe0 mm/kasan/kasan.c:616
 [<ffffffff814f52d4>] __kmalloc+0x124/0x310 mm/slub.c:3613
 [<ffffffff8359f2a9>] kmalloc include/linux/slab.h:481 [inline]
 [<ffffffff8359f2a9>] kzalloc include/linux/slab.h:620 [inline]
 [<ffffffff8359f2a9>] l2tp_session_create+0x39/0x1030 net/l2tp/l2tp_core.c:1748
 [<ffffffff835a3fb0>] pppol2tp_connect+0x10f0/0x1910 net/l2tp/l2tp_ppp.c:725
 [<ffffffff82f1d9f8>] SYSC_connect+0x1b8/0x300 net/socket.c:1557
 [<ffffffff82f20334>] SyS_connect+0x24/0x30 net/socket.c:1538
 [<ffffffff838c1865>] entry_SYSCALL_64_fastpath+0x22/0x9e

Freed by task 8183:
 [<ffffffff81033e46>] save_stack_trace+0x26/0x50 arch/x86/kernel/stacktrace.c:63
 [<ffffffff814f88d3>] save_stack+0x43/0xd0 mm/kasan/kasan.c:512
 [<ffffffff814f9202>] set_track mm/kasan/kasan.c:524 [inline]
 [<ffffffff814f9202>] kasan_slab_free+0x72/0xc0 mm/kasan/kasan.c:589
 [<ffffffff814f6704>] slab_free_hook mm/slub.c:1383 [inline]
 [<ffffffff814f6704>] slab_free_freelist_hook mm/slub.c:1405 [inline]
 [<ffffffff814f6704>] slab_free mm/slub.c:2859 [inline]
 [<ffffffff814f6704>] kfree+0xf4/0x310 mm/slub.c:3749
 [<ffffffff8359c210>] l2tp_session_free+0x170/0x200 net/l2tp/l2tp_core.c:1676
 [<ffffffff8359e5c9>] l2tp_session_dec_refcount_1 net/l2tp/l2tp_core.h:293 [inline]
 [<ffffffff8359e5c9>] l2tp_tunnel_closeall+0x2b9/0x350 net/l2tp/l2tp_core.c:1279
 [<ffffffff8359f0db>] l2tp_udp_encap_destroy+0x8b/0xf0 net/l2tp/l2tp_core.c:1300
 [<ffffffff83491411>] udpv6_destroy_sock+0xb1/0xd0 net/ipv6/udp.c:1421
 [<ffffffff82f2e8ad>] sk_common_release+0x6d/0x300 net/core/sock.c:2680
 [<ffffffff834900c5>] udp_lib_close+0x15/0x20 include/net/udp.h:190
 [<ffffffff832f7b8f>] inet_release+0xff/0x1d0 net/ipv4/af_inet.c:435
 [<ffffffff8341a7f0>] inet6_release+0x50/0x70 net/ipv6/af_inet6.c:424
 [<ffffffff82f18fd6>] sock_release+0x96/0x1c0 net/socket.c:586
 [<ffffffff82f19116>] sock_close+0x16/0x20 net/socket.c:1037
 [<ffffffff81522e05>] __fput+0x235/0x6f0 fs/file_table.c:208
 [<ffffffff81523345>] ____fput+0x15/0x20 fs/file_table.c:244
 [<ffffffff8118bd7f>] 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
 [<ffffffff838c19f5>] int_ret_from_sys_call+0x25/0xa3

The buggy address belongs to the object at ffff8801d4abdb80
 which belongs to the cache kmalloc-512 of size 512
The buggy address is located 0 bytes inside of
 512-byte region [ffff8801d4abdb80, ffff8801d4abdd80)
The buggy address belongs to the page:
kasan: CONFIG_KASAN_INLINE enabled
------------[ cut here ]------------
WARNING: CPU: 1 PID: 3843 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