==================================================================
BUG: KASAN: use-after-free in pppol2tp_session_destruct+0xee/0x110 net/l2tp/l2tp_ppp.c:476
Read of size 4 at addr ffff8801d295d400 by task syz-executor7/7643

CPU: 1 PID: 7643 Comm: syz-executor7 Not tainted 4.4.120-gd63fdf6 #29
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 01/01/2011
 0000000000000000 d8d7d84bd8265f8e ffff8800af7ffc20 ffffffff81d0408d
 ffffea00074a5700 ffff8801d295d400 0000000000000000 ffff8801d295d400
 ffffffff82de7d30 ffff8800af7ffc58 ffffffff814fe143 ffff8801d295d400
Call Trace:
 [<ffffffff81d0408d>] __dump_stack lib/dump_stack.c:15 [inline]
 [<ffffffff81d0408d>] dump_stack+0xc1/0x124 lib/dump_stack.c:51
 [<ffffffff814fe143>] print_address_description+0x73/0x260 mm/kasan/report.c:252
 [<ffffffff814fe655>] kasan_report_error mm/kasan/report.c:351 [inline]
 [<ffffffff814fe655>] kasan_report+0x285/0x370 mm/kasan/report.c:408
 [<ffffffff814fe794>] __asan_report_load4_noabort+0x14/0x20 mm/kasan/report.c:428
 [<ffffffff834651fe>] pppol2tp_session_destruct+0xee/0x110 net/l2tp/l2tp_ppp.c:476
 [<ffffffff82dfcc3a>] sk_destruct+0x4a/0x4c0 net/core/sock.c:1447
 [<ffffffff82dfd107>] __sk_free+0x57/0x230 net/core/sock.c:1480
 [<ffffffff82dfd310>] sk_free+0x30/0x40 net/core/sock.c:1491
 [<ffffffff834660ba>] sock_put include/net/sock.h:1639 [inline]
 [<ffffffff834660ba>] pppol2tp_release+0x27a/0x310 net/l2tp/l2tp_ppp.c:518
 [<ffffffff82de7bdd>] sock_release+0x8d/0x1e0 net/socket.c:586
 [<ffffffff82de7d46>] sock_close+0x16/0x20 net/socket.c:1037
 [<ffffffff81523603>] __fput+0x233/0x6d0 fs/file_table.c:208
 [<ffffffff81523b25>] ____fput+0x15/0x20 fs/file_table.c:244
 [<ffffffff8118b984>] task_work_run+0x104/0x180 kernel/task_work.c:115
 [<ffffffff8100361d>] tracehook_notify_resume include/linux/tracehook.h:191 [inline]
 [<ffffffff8100361d>] exit_to_usermode_loop+0x13d/0x160 arch/x86/entry/common.c:252
 [<ffffffff81007084>] prepare_exit_to_usermode arch/x86/entry/common.c:283 [inline]
 [<ffffffff81007084>] syscall_return_slowpath arch/x86/entry/common.c:348 [inline]
 [<ffffffff81007084>] do_syscall_32_irqs_on arch/x86/entry/common.c:398 [inline]
 [<ffffffff81007084>] do_fast_syscall_32+0x614/0x8a0 arch/x86/entry/common.c:459
 [<ffffffff837752ea>] sysenter_flags_fixed+0xd/0x17

Allocated by task 7643:
 [<ffffffff81035d96>] save_stack_trace+0x26/0x50 arch/x86/kernel/stacktrace.c:63
 [<ffffffff814fd1b3>] save_stack+0x43/0xd0 mm/kasan/kasan.c:512
 [<ffffffff814fd47d>] set_track mm/kasan/kasan.c:524 [inline]
 [<ffffffff814fd47d>] kasan_kmalloc+0xad/0xe0 mm/kasan/kasan.c:616
 [<ffffffff814f9764>] __kmalloc+0x124/0x320 mm/slub.c:3613
 [<ffffffff8345eeb9>] kmalloc include/linux/slab.h:481 [inline]
 [<ffffffff8345eeb9>] kzalloc include/linux/slab.h:620 [inline]
 [<ffffffff8345eeb9>] l2tp_session_create+0x39/0x10f0 net/l2tp/l2tp_core.c:1744
 [<ffffffff8346354c>] pppol2tp_connect+0x10fc/0x1930 net/l2tp/l2tp_ppp.c:718
 [<ffffffff82dec596>] SYSC_connect+0x1b6/0x310 net/socket.c:1557
 [<ffffffff82deee04>] SyS_connect+0x24/0x30 net/socket.c:1538
 [<ffffffff81006d91>] do_syscall_32_irqs_on arch/x86/entry/common.c:392 [inline]
 [<ffffffff81006d91>] do_fast_syscall_32+0x321/0x8a0 arch/x86/entry/common.c:459
 [<ffffffff837752ea>] sysenter_flags_fixed+0xd/0x17

Freed by task 7654:
 [<ffffffff81035d96>] save_stack_trace+0x26/0x50 arch/x86/kernel/stacktrace.c:63
 [<ffffffff814fd1b3>] save_stack+0x43/0xd0 mm/kasan/kasan.c:512
 [<ffffffff814fdad2>] set_track mm/kasan/kasan.c:524 [inline]
 [<ffffffff814fdad2>] kasan_slab_free+0x72/0xc0 mm/kasan/kasan.c:589
 [<ffffffff814fa56c>] slab_free_hook mm/slub.c:1383 [inline]
 [<ffffffff814fa56c>] slab_free_freelist_hook mm/slub.c:1405 [inline]
 [<ffffffff814fa56c>] slab_free mm/slub.c:2859 [inline]
 [<ffffffff814fa56c>] kfree+0xfc/0x300 mm/slub.c:3749
 [<ffffffff8345b440>] l2tp_session_free+0x170/0x200 net/l2tp/l2tp_core.c:1672
 [<ffffffff8345dcf1>] l2tp_session_dec_refcount_1 net/l2tp/l2tp_core.h:293 [inline]
 [<ffffffff8345dcf1>] l2tp_tunnel_closeall+0x2d1/0x3b0 net/l2tp/l2tp_core.c:1279
 [<ffffffff8345e85b>] l2tp_udp_encap_destroy+0x8b/0xf0 net/l2tp/l2tp_core.c:1300
 [<ffffffff8336cc51>] udpv6_destroy_sock+0xb1/0xd0 net/ipv6/udp.c:1421
 [<ffffffff82dfecdb>] sk_common_release+0x6b/0x300 net/core/sock.c:2680
 [<ffffffff8336bc05>] udp_lib_close+0x15/0x20 include/net/udp.h:190
 [<ffffffff831cfdda>] inet_release+0xfa/0x1d0 net/ipv4/af_inet.c:435
 [<ffffffff832f55a0>] inet6_release+0x50/0x70 net/ipv6/af_inet6.c:424
 [<ffffffff82de7bdd>] sock_release+0x8d/0x1e0 net/socket.c:586
 [<ffffffff82de7d46>] sock_close+0x16/0x20 net/socket.c:1037
 [<ffffffff81523603>] __fput+0x233/0x6d0 fs/file_table.c:208
 [<ffffffff81523b25>] ____fput+0x15/0x20 fs/file_table.c:244
 [<ffffffff8118b984>] task_work_run+0x104/0x180 kernel/task_work.c:115
 [<ffffffff8100361d>] tracehook_notify_resume include/linux/tracehook.h:191 [inline]
 [<ffffffff8100361d>] exit_to_usermode_loop+0x13d/0x160 arch/x86/entry/common.c:252
 [<ffffffff81007084>] prepare_exit_to_usermode arch/x86/entry/common.c:283 [inline]
 [<ffffffff81007084>] syscall_return_slowpath arch/x86/entry/common.c:348 [inline]
 [<ffffffff81007084>] do_syscall_32_irqs_on arch/x86/entry/common.c:398 [inline]
 [<ffffffff81007084>] do_fast_syscall_32+0x614/0x8a0 arch/x86/entry/common.c:459
 [<ffffffff837752ea>] sysenter_flags_fixed+0xd/0x17

The buggy address belongs to the object at ffff8801d295d400
 which belongs to the cache kmalloc-512 of size 512
The buggy address is located 0 bytes inside of
 512-byte region [ffff8801d295d400, ffff8801d295d600)
The buggy address belongs to the page:
audit: type=1400 audit(1520667437.341:19): avc:  denied  { sigchld } for  pid=3858 comm="syz-executor7" scontext=system_u:object_r:unlabeled_t:s0 tcontext=system_u:object_r:unlabeled_t:s0 tclass=process permissive=1
kasan: CONFIG_KASAN_INLINE enabled
kasan: GPF could be caused by NULL-ptr deref or user memory access
------------[ cut here ]------------
WARNING: CPU: 0 PID: 3858 at kernel/sched/core.c:7944 __might_sleep+0x138/0x1a0 kernel/sched/core.c:7939()
do not call blocking ops when !TASK_RUNNING; state=1 set at [<ffffffff81138a30>] do_wait+0x270/0xa20 kernel/exit.c:1503