BUG: unable to handle kernel NULL pointer dereference at 0000000000000080
IP: refcount_dec_and_test arch/x86/include/asm/refcount.h:75 [inline]
IP: sock_put include/net/sock.h:1657 [inline]
IP: l2tp_session_free+0x113/0x200 net/l2tp/l2tp_core.c:1714
PGD a887c067 P4D a887c067 PUD 872bb067 PMD 0 
Oops: 0002 [#1] PREEMPT SMP KASAN
Modules linked in:
CPU: 0 PID: 9956 Comm: syz-executor.4 Not tainted 4.14.181-syzkaller #0
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 01/01/2011
task: ffff88802748e3c0 task.stack: ffff88820c460000
RIP: 0010:refcount_dec_and_test arch/x86/include/asm/refcount.h:75 [inline]
RIP: 0010:sock_put include/net/sock.h:1657 [inline]
RIP: 0010:l2tp_session_free+0x113/0x200 net/l2tp/l2tp_core.c:1714
RSP: 0018:ffff88820c467cd0 EFLAGS: 00010246
RAX: dffffc0000000000 RBX: ffff8880928f7d00 RCX: 0000000000000000
RDX: 1ffff1101251efd1 RSI: ffff88802748ec48 RDI: ffff8880928f7e88
RBP: ffff88808e56ea40 R08: 0000000000000001 R09: 0000000000000000
R10: 0000000000000000 R11: 0000000000000000 R12: ffff88808e56ea50
R13: 0000000000000000 R14: 0000000000000000 R15: ffff8880a8471850
FS:  00000000024a1940(0000) GS:ffff8880aec00000(0000) knlGS:0000000000000000
CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
CR2: 0000000000000080 CR3: 000000005f145000 CR4: 00000000001426f0
DR0: 0000000000000000 DR1: 0000000000000000 DR2: 0000000000000000
DR3: 0000000000000000 DR6: 00000000fffe0ff0 DR7: 0000000000000400
Call Trace:
 l2tp_session_dec_refcount_1 net/l2tp/l2tp_core.h:300 [inline]
 pppol2tp_session_destruct+0xcd/0x110 net/l2tp/l2tp_ppp.c:460
 __sk_destruct+0x49/0x640 net/core/sock.c:1556
 sk_destruct+0x97/0xc0 net/core/sock.c:1596
 __sk_free+0x4c/0x220 net/core/sock.c:1604
 sk_free+0x2b/0x40 net/core/sock.c:1615
 sock_put include/net/sock.h:1658 [inline]
 pppol2tp_release+0x248/0x2f0 net/l2tp/l2tp_ppp.c:501
 __sock_release+0xcd/0x2b0 net/socket.c:602
 sock_close+0x15/0x20 net/socket.c:1139
 __fput+0x25f/0x7a0 fs/file_table.c:210
 task_work_run+0x113/0x190 kernel/task_work.c:113
 tracehook_notify_resume include/linux/tracehook.h:191 [inline]
 exit_to_usermode_loop+0x1ad/0x200 arch/x86/entry/common.c:164
 prepare_exit_to_usermode arch/x86/entry/common.c:199 [inline]
 syscall_return_slowpath arch/x86/entry/common.c:270 [inline]
 do_syscall_64+0x4a3/0x640 arch/x86/entry/common.c:297
 entry_SYSCALL_64_after_hwframe+0x46/0xbb
RIP: 0033:0x416621
RSP: 002b:00007ffea22be030 EFLAGS: 00000293 ORIG_RAX: 0000000000000003
RAX: 0000000000000000 RBX: 0000000000000007 RCX: 0000000000416621
RDX: 0000000000000000 RSI: 0000000000000081 RDI: 0000000000000006
RBP: 0000000000000000 R08: 0000000000790940 R09: 0000000000000000
R10: 00007ffea22be110 R11: 0000000000000293 R12: 0000000000790948
R13: 0000000000000000 R14: ffffffffffffffff R15: 000000000078bf0c
Code: 9a fb 48 8d bb 88 01 00 00 48 b8 00 00 00 00 00 fc ff df 48 89 fa 48 c1 ea 03 80 3c 02 00 0f 85 d2 00 00 00 4c 8b ab 88 01 00 00 <f0> 41 ff 8d 80 00 00 00 0f 88 ee 6d 7b 00 74 70 e8 48 3a 9a fb 
Option '                                                                                ' to dns_resolver key: bad/missing value
RIP: refcount_dec_and_test arch/x86/include/asm/refcount.h:75 [inline] RSP: ffff88820c467cd0
RIP: sock_put include/net/sock.h:1657 [inline] RSP: ffff88820c467cd0
RIP: l2tp_session_free+0x113/0x200 net/l2tp/l2tp_core.c:1714 RSP: ffff88820c467cd0
CR2: 0000000000000080
---[ end trace 515cf1e791fbf6db ]---
Option '                                                                                ' to dns_resolver key: bad/missing value