==================================================================
BUG: KASAN: use-after-free in l2tp_session_queue_purge+0xe8/0x100 net/l2tp/l2tp_core.c:830
Read of size 4 at addr ffff8800b59fd400 by task syzkaller016241/3368

CPU: 1 PID: 3368 Comm: syzkaller016241 Not tainted 4.4.113-gef588ef #33
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 01/01/2011
 0000000000000000 41af1e5075f99087 ffff8801d16978c8 ffffffff81d0278d
 ffffea0002d67f00 ffff8800b59fd400 0000000000000000 ffff8800b59fd400
 ffffffff82de6370 ffff8801d1697900 ffffffff814fd053 ffff8800b59fd400
Call Trace:
 [<ffffffff81d0278d>] __dump_stack lib/dump_stack.c:15 [inline]
 [<ffffffff81d0278d>] dump_stack+0xc1/0x124 lib/dump_stack.c:51
 [<ffffffff814fd053>] print_address_description+0x73/0x260 mm/kasan/report.c:252
 [<ffffffff814fd565>] kasan_report_error mm/kasan/report.c:351 [inline]
 [<ffffffff814fd565>] kasan_report+0x285/0x370 mm/kasan/report.c:408
 [<ffffffff814fd6a4>] __asan_report_load4_noabort+0x14/0x20 mm/kasan/report.c:428
 [<ffffffff83455f48>] l2tp_session_queue_purge+0xe8/0x100 net/l2tp/l2tp_core.c:830
 [<ffffffff83462fdf>] pppol2tp_release+0x1ff/0x310 net/l2tp/l2tp_ppp.c:509
 [<ffffffff82de621d>] sock_release+0x8d/0x1e0 net/socket.c:586
 [<ffffffff82de6386>] sock_close+0x16/0x20 net/socket.c:1037
 [<ffffffff81522363>] __fput+0x233/0x6d0 fs/file_table.c:208
 [<ffffffff81522885>] ____fput+0x15/0x20 fs/file_table.c:244
 [<ffffffff8118b9d4>] task_work_run+0x104/0x180 kernel/task_work.c:115
 [<ffffffff81132f7a>] exit_task_work include/linux/task_work.h:21 [inline]
 [<ffffffff81132f7a>] do_exit+0x82a/0x2a10 kernel/exit.c:755
 [<ffffffff81139418>] do_group_exit+0x108/0x320 kernel/exit.c:885
 [<ffffffff8115c872>] get_signal+0x4f2/0x1550 kernel/signal.c:2317
 [<ffffffff8100e7fb>] do_signal+0x8b/0x1d40 arch/x86/kernel/signal.c:712
 [<ffffffff810035fa>] exit_to_usermode_loop+0x11a/0x160 arch/x86/entry/common.c:247
 [<ffffffff81006363>] prepare_exit_to_usermode+0xe3/0x100 arch/x86/entry/common.c:282
 [<ffffffff83772708>] retint_user+0x8/0x3c

Allocated by task 3367:
 [<ffffffff81035df6>] save_stack_trace+0x26/0x50 arch/x86/kernel/stacktrace.c:63
 [<ffffffff814fc0c3>] save_stack+0x43/0xd0 mm/kasan/kasan.c:512
 [<ffffffff814fc38d>] set_track mm/kasan/kasan.c:524 [inline]
 [<ffffffff814fc38d>] kasan_kmalloc+0xad/0xe0 mm/kasan/kasan.c:616
 [<ffffffff814f8674>] __kmalloc+0x124/0x320 mm/slub.c:3613
 [<ffffffff8345be59>] kmalloc include/linux/slab.h:481 [inline]
 [<ffffffff8345be59>] kzalloc include/linux/slab.h:620 [inline]
 [<ffffffff8345be59>] l2tp_session_create+0x39/0x10f0 net/l2tp/l2tp_core.c:1744
 [<ffffffff834604ec>] pppol2tp_connect+0x10fc/0x1930 net/l2tp/l2tp_ppp.c:718
 [<ffffffff82deabd6>] SYSC_connect+0x1b6/0x310 net/socket.c:1557
 [<ffffffff82ded444>] SyS_connect+0x24/0x30 net/socket.c:1538
 [<ffffffff83771b5f>] entry_SYSCALL_64_fastpath+0x1c/0x98

Freed by task 3318:
 [<ffffffff81035df6>] save_stack_trace+0x26/0x50 arch/x86/kernel/stacktrace.c:63
 [<ffffffff814fc0c3>] save_stack+0x43/0xd0 mm/kasan/kasan.c:512
 [<ffffffff814fc9e2>] set_track mm/kasan/kasan.c:524 [inline]
 [<ffffffff814fc9e2>] kasan_slab_free+0x72/0xc0 mm/kasan/kasan.c:589
 [<ffffffff814f947c>] slab_free_hook mm/slub.c:1383 [inline]
 [<ffffffff814f947c>] slab_free_freelist_hook mm/slub.c:1405 [inline]
 [<ffffffff814f947c>] slab_free mm/slub.c:2859 [inline]
 [<ffffffff814f947c>] kfree+0xfc/0x300 mm/slub.c:3749
 [<ffffffff834583e0>] l2tp_session_free+0x170/0x200 net/l2tp/l2tp_core.c:1672
 [<ffffffff8345ac91>] l2tp_session_dec_refcount_1 net/l2tp/l2tp_core.h:293 [inline]
 [<ffffffff8345ac91>] l2tp_tunnel_closeall+0x2d1/0x3b0 net/l2tp/l2tp_core.c:1279
 [<ffffffff8345b7fb>] l2tp_udp_encap_destroy+0x8b/0xf0 net/l2tp/l2tp_core.c:1300
 [<ffffffff8336a031>] udpv6_destroy_sock+0xb1/0xd0 net/ipv6/udp.c:1421
 [<ffffffff82dfd31b>] sk_common_release+0x6b/0x300 net/core/sock.c:2680
 [<ffffffff83368fe5>] udp_lib_close+0x15/0x20 include/net/udp.h:190
 [<ffffffff831cd12a>] inet_release+0xfa/0x1d0 net/ipv4/af_inet.c:435
 [<ffffffff832f2920>] inet6_release+0x50/0x70 net/ipv6/af_inet6.c:424
 [<ffffffff82de621d>] sock_release+0x8d/0x1e0 net/socket.c:586
 [<ffffffff82de6386>] sock_close+0x16/0x20 net/socket.c:1037
 [<ffffffff81522363>] __fput+0x233/0x6d0 fs/file_table.c:208
 [<ffffffff81522885>] ____fput+0x15/0x20 fs/file_table.c:244
 [<ffffffff8118b9d4>] task_work_run+0x104/0x180 kernel/task_work.c:115
 [<ffffffff81132f7a>] exit_task_work include/linux/task_work.h:21 [inline]
 [<ffffffff81132f7a>] do_exit+0x82a/0x2a10 kernel/exit.c:755
 [<ffffffff81139418>] do_group_exit+0x108/0x320 kernel/exit.c:885
 [<ffffffff8115c872>] get_signal+0x4f2/0x1550 kernel/signal.c:2317
 [<ffffffff8100e7fb>] do_signal+0x8b/0x1d40 arch/x86/kernel/signal.c:712
 [<ffffffff810035fa>] exit_to_usermode_loop+0x11a/0x160 arch/x86/entry/common.c:247
 [<ffffffff81006363>] prepare_exit_to_usermode+0xe3/0x100 arch/x86/entry/common.c:282
 [<ffffffff83772708>] retint_user+0x8/0x3c

The buggy address belongs to the object at ffff8800b59fd400
 which belongs to the cache kmalloc-512 of size 512
The buggy address is located 0 bytes inside of
 512-byte region [ffff8800b59fd400, ffff8800b59fd600)
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[   27.034717] ------------[ cut here ]------------
WARNING: CPU: 0 PID: 0 at kernel/rcu/update.c:211 __rcu_read_unlock+0x140/0x1a0 kernel/rcu/update.c:211()