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 88c88067 P4D 88c88067 PUD a53ac067 PMD 0 
Oops: 0002 [#1] PREEMPT SMP KASAN
Modules linked in:
CPU: 1 PID: 10146 Comm: syz-executor.3 Not tainted 4.14.174-syzkaller #0
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 01/01/2011
task: ffff8880526a6680 task.stack: ffff8880557b8000
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: 0000:ffff8880557bfcd0 EFLAGS: 00010246
RAX: dffffc0000000000 RBX: ffff88809958b500 RCX: 0000000000000000
RDX: 1ffff110132b16d1 RSI: ffff8880526a6f00 RDI: ffff88809958b688
RBP: ffff88807fcbba00 R08: 0000000000000001 R09: 0000000000000000
R10: 0000000000000000 R11: 0000000000000000 R12: ffff88807fcbba10
R13: 0000000000000000 R14: 0000000000000000 R15: ffff8880a78eea10
FS:  0000000001869940(0000) GS:ffff8880aeb00000(0000) knlGS:0000000000000000
CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
CR2: 0000000000000080 CR3: 000000009007b000 CR4: 00000000001406e0
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+0x247/0x2f0 net/l2tp/l2tp_ppp.c:501
 __sock_release+0xcd/0x2b0 net/socket.c:602
 sock_close+0x15/0x20 net/socket.c:1139
 __fput+0x25f/0x790 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+0x1d6/0x220 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+0x42/0xb7
RIP: 0033:0x4163e1
RSP: 002b:00007ffd15408ec0 EFLAGS: 00000293 ORIG_RAX: 0000000000000003
RAX: 0000000000000000 RBX: 0000000000000008 RCX: 00000000004163e1
RDX: 0000000000000000 RSI: 0000000000000081 RDI: 0000000000000007
RBP: 0000000000000000 R08: 0000000000770d30 R09: 01ffffffffffffff
R10: 00007ffd15408f90 R11: 0000000000000293 R12: 000000000076bf00
R13: 0000000000770d38 R14: 0000000000000000 R15: 000000000076bf0c
Code: 97 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 24 80 7b 00 74 70 e8 b8 d9 97 fb 
RIP: refcount_dec_and_test arch/x86/include/asm/refcount.h:75 [inline] RSP: ffff8880557bfcd0
RIP: sock_put include/net/sock.h:1657 [inline] RSP: ffff8880557bfcd0
RIP: l2tp_session_free+0x113/0x200 net/l2tp/l2tp_core.c:1714 RSP: ffff8880557bfcd0
CR2: 0000000000000080
---[ end trace 6daa424ad049eb1f ]---