==================================================================
BUG: KASAN: use-after-free in l2tp_session_create+0x160f/0x16f0 net/l2tp/l2tp_core.c:1914
Read of size 4 at addr ffff8801d06b6790 by task syz-executor304/7685

CPU: 0 PID: 7685 Comm: syz-executor304 Not tainted 4.9.107-gdb2c520 #49
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 01/01/2011
 ffff8801d03e7a20 ffffffff81eb3da9 ffffea000741ad80 ffff8801d06b6790
 0000000000000000 ffff8801d06b6790 0000000000000000 ffff8801d03e7a58
 ffffffff815679f9 ffff8801d06b6790 0000000000000004 0000000000000000
Call Trace:
 [<ffffffff81eb3da9>] __dump_stack lib/dump_stack.c:15 [inline]
 [<ffffffff81eb3da9>] dump_stack+0xc1/0x128 lib/dump_stack.c:51
 [<ffffffff815679f9>] print_address_description+0x6c/0x234 mm/kasan/report.c:256
 [<ffffffff81567e03>] kasan_report_error mm/kasan/report.c:355 [inline]
 [<ffffffff81567e03>] kasan_report.cold.6+0x242/0x2fe mm/kasan/report.c:412
 [<ffffffff8153ba34>] __asan_report_load4_noabort+0x14/0x20 mm/kasan/report.c:432
 [<ffffffff836c166f>] l2tp_session_create+0x160f/0x16f0 net/l2tp/l2tp_core.c:1914
 [<ffffffff836c5e77>] pppol2tp_connect+0x10d7/0x18f0 net/l2tp/l2tp_ppp.c:718
 [<ffffffff83017958>] SYSC_connect+0x1b8/0x300 net/socket.c:1562
 [<ffffffff8301a224>] SyS_connect+0x24/0x30 net/socket.c:1543
 [<ffffffff81006316>] do_syscall_64+0x1a6/0x490 arch/x86/entry/common.c:282
 [<ffffffff839f9b13>] entry_SYSCALL_64_after_swapgs+0x5d/0xdb

Allocated by task 7685:
 save_stack_trace+0x16/0x20 arch/x86/kernel/stacktrace.c:57
 save_stack+0x43/0xd0 mm/kasan/kasan.c:505
 set_track mm/kasan/kasan.c:517 [inline]
 kasan_kmalloc+0xc7/0xe0 mm/kasan/kasan.c:609
 __kmalloc+0x11d/0x300 mm/slub.c:3741
 kmalloc include/linux/slab.h:495 [inline]
 kzalloc include/linux/slab.h:636 [inline]
 l2tp_session_create+0x38/0x16f0 net/l2tp/l2tp_core.c:1843
 pppol2tp_connect+0x10d7/0x18f0 net/l2tp/l2tp_ppp.c:718
 SYSC_connect+0x1b8/0x300 net/socket.c:1562
 SyS_connect+0x24/0x30 net/socket.c:1543
 do_syscall_64+0x1a6/0x490 arch/x86/entry/common.c:282
 entry_SYSCALL_64_after_swapgs+0x5d/0xdb

Freed by task 7684:
 save_stack_trace+0x16/0x20 arch/x86/kernel/stacktrace.c:57
 save_stack+0x43/0xd0 mm/kasan/kasan.c:505
 set_track mm/kasan/kasan.c:517 [inline]
 kasan_slab_free+0x72/0xc0 mm/kasan/kasan.c:582
 slab_free_hook mm/slub.c:1355 [inline]
 slab_free_freelist_hook mm/slub.c:1377 [inline]
 slab_free mm/slub.c:2958 [inline]
 kfree+0xfb/0x310 mm/slub.c:3878
 l2tp_session_free+0x166/0x200 net/l2tp/l2tp_core.c:1770
 l2tp_session_dec_refcount_1 net/l2tp/l2tp_core.h:297 [inline]
 pppol2tp_connect+0xc55/0x18f0 net/l2tp/l2tp_ppp.c:790
 SYSC_connect+0x1b8/0x300 net/socket.c:1562
 SyS_connect+0x24/0x30 net/socket.c:1543
 do_syscall_64+0x1a6/0x490 arch/x86/entry/common.c:282
 entry_SYSCALL_64_after_swapgs+0x5d/0xdb

The buggy address belongs to the object at ffff8801d06b6780
 which belongs to the cache kmalloc-512 of size 512
The buggy address is located 16 bytes inside of
 512-byte region [ffff8801d06b6780, ffff8801d06b6980)
The buggy address belongs to the page:
page:ffffea000741ad80 count:1 mapcount:0 mapping:          (null) index:0x0 compound_mapcount: 0
flags: 0x8000000000004080(slab|head)
page dumped because: kasan: bad access detected

Memory state around the buggy address:
 ffff8801d06b6680: fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb
 ffff8801d06b6700: fc fc fc fc fc fc fc fc fc fc fc fc fc fc fc fc
>ffff8801d06b6780: fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb
                         ^
 ffff8801d06b6800: fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb
 ffff8801d06b6880: fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb
==================================================================
------------[ cut here ]------------
kernel BUG at net/l2tp/l2tp_core.c:917!
invalid opcode: 0000 [#1] PREEMPT SMP KASAN
Dumping ftrace buffer:
   (ftrace buffer empty)
Modules linked in:
CPU: 0 PID: 7674 Comm: syz-executor304 Tainted: G    B           4.9.107-gdb2c520 #49
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 01/01/2011
task: ffff8801cec28000 task.stack: ffff8801cec88000
RIP: 0010:[<ffffffff836bb760>]  [<ffffffff836bb760>] l2tp_session_queue_purge+0xe0/0x100 net/l2tp/l2tp_core.c:917
RSP: 0018:ffff8801cec8fb40  EFLAGS: 00010293
RAX: ffff8801cec28000 RBX: ffff8801d06b6780 RCX: ffffffff836bb774
RDX: 0000000000000000 RSI: ffffffff836bb760 RDI: ffff8801d06b6780
RBP: ffff8801cec8fb68 R08: 0000000000000000 R09: 0000000000000000
R10: 0000000000000000 R11: 0000000000000000 R12: dffffc0000000000
R13: ffff8801d06b6780 R14: 0000000000000000 R15: ffff8801d766e7d8
FS:  00007f13edde6700(0000) GS:ffff8801db200000(0000) knlGS:0000000000000000
CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
CR2: 00007fff995ca434 CR3: 00000001cfffc000 CR4: 00000000001606f0
DR0: 0000000000000000 DR1: 0000000000000000 DR2: 0000000000000000
DR3: 0000000000000000 DR6: 00000000fffe0ff0 DR7: 0000000000000400
Stack:
 ffff8801d06b6828 dffffc0000000000 ffff8801d06b6780 0000000000000000
 ffff8801d766e7d8 ffff8801cec8fbc0 ffffffff836bd04f ffff8801d766e858
 ffffed003aecdcfb ffff8801d766e7d8 ffff8801d766e7a0 ffff8801d766e780
Call Trace:
 [<ffffffff836bd04f>] l2tp_tunnel_closeall+0x1ff/0x350 net/l2tp/l2tp_core.c:1365
 [<ffffffff836bd902>] l2tp_tunnel_destruct+0x2f2/0x590 net/l2tp/l2tp_core.c:1324
 [<ffffffff83021095>] __sk_destruct+0x55/0x590 net/core/sock.c:1428
 [<ffffffff83028b23>] sk_destruct+0x63/0x80 net/core/sock.c:1463
 [<ffffffff83028b8f>] __sk_free+0x4f/0x220 net/core/sock.c:1471
 [<ffffffff83028d8b>] sk_free+0x2b/0x40 net/core/sock.c:1482
 [<ffffffff836bc8cc>] sock_put include/net/sock.h:1588 [inline]
 [<ffffffff836bc8cc>] l2tp_session_free+0x19c/0x200 net/l2tp/l2tp_core.c:1765
 [<ffffffff836c4132>] l2tp_session_dec_refcount_1 net/l2tp/l2tp_core.h:297 [inline]
 [<ffffffff836c4132>] pppol2tp_session_destruct+0xd2/0x110 net/l2tp/l2tp_ppp.c:460
 [<ffffffff83021095>] __sk_destruct+0x55/0x590 net/core/sock.c:1428
 [<ffffffff83028b23>] sk_destruct+0x63/0x80 net/core/sock.c:1463
 [<ffffffff83028b8f>] __sk_free+0x4f/0x220 net/core/sock.c:1471
 [<ffffffff83028d8b>] sk_free+0x2b/0x40 net/core/sock.c:1482
 [<ffffffff836c7439>] sock_put include/net/sock.h:1588 [inline]
 [<ffffffff836c7439>] pppol2tp_release+0x239/0x2e0 net/l2tp/l2tp_ppp.c:501
 [<ffffffff83013ab6>] sock_release+0x96/0x1c0 net/socket.c:599
 [<ffffffff83013bf6>] sock_close+0x16/0x20 net/socket.c:1046
 [<ffffffff81578103>] __fput+0x263/0x700 fs/file_table.c:208
 [<ffffffff81578625>] ____fput+0x15/0x20 fs/file_table.c:244
 [<ffffffff8119832c>] task_work_run+0x10c/0x180 kernel/task_work.c:116
 [<ffffffff8100559c>] tracehook_notify_resume include/linux/tracehook.h:191 [inline]
 [<ffffffff8100559c>] exit_to_usermode_loop+0xfc/0x120 arch/x86/entry/common.c:161
 [<ffffffff810064d4>] prepare_exit_to_usermode arch/x86/entry/common.c:191 [inline]
 [<ffffffff810064d4>] syscall_return_slowpath arch/x86/entry/common.c:260 [inline]
 [<ffffffff810064d4>] do_syscall_64+0x364/0x490 arch/x86/entry/common.c:287
 [<ffffffff839f9b13>] entry_SYSCALL_64_after_swapgs+0x5d/0xdb
Code: 32 ca fd 31 c0 5b 41 5c 41 5d 41 5e 41 5f 5d c3 e8 f6 31 ca fd 48 89 df e8 4e 7c 34 00 eb ca e8 e7 31 ca fd 0f 0b e8 e0 31 ca fd <0f> 0b 4c 89 ff e8 d6 02 e8 fd eb a6 48 89 df e8 ac 02 e8 fd e9 
RIP  [<ffffffff836bb760>] l2tp_session_queue_purge+0xe0/0x100 net/l2tp/l2tp_core.c:917
 RSP <ffff8801cec8fb40>