refcount_t: underflow; use-after-free.
------------[ cut here ]------------
WARNING: CPU: 0 PID: 2623 at net/ipv4/af_inet.c:154 inet_sock_destruct+0x729/0x950 net/ipv4/af_inet.c:154
Kernel panic - not syncing: panic_on_warn set ...

CPU: 0 PID: 2623 Comm: syz-executor5 Not tainted 4.13.0-rc6+ #20
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 01/01/2011
Call Trace:
 __dump_stack lib/dump_stack.c:16 [inline]
 dump_stack+0x194/0x257 lib/dump_stack.c:52
 panic+0x1e4/0x417 kernel/panic.c:180
 __warn+0x1c4/0x1d9 kernel/panic.c:541
 report_bug+0x211/0x2d0 lib/bug.c:183
 fixup_bug+0x40/0x90 arch/x86/kernel/traps.c:190
 do_trap_no_signal arch/x86/kernel/traps.c:224 [inline]
 do_trap+0x260/0x390 arch/x86/kernel/traps.c:273
 do_error_trap+0x120/0x390 arch/x86/kernel/traps.c:310
 do_invalid_op+0x1b/0x20 arch/x86/kernel/traps.c:323
 invalid_op+0x1e/0x30 arch/x86/entry/entry_64.S:846
RIP: 0010:inet_sock_destruct+0x729/0x950 net/ipv4/af_inet.c:154
RSP: 0018:ffff8801a5707390 EFLAGS: 00010216
RAX: 0000000000010000 RBX: ffff8801c4219000 RCX: ffffc90002dc0000
RDX: 00000000000011ec RSI: ffffffff8415c1e9 RDI: ffff8801c4219238
RBP: ffff8801a57074b8 R08: 0000000000000001 R09: 0000000000000000
R10: 0000000000000000 R11: 0000000000000000 R12: 1ffff10034ae0e82
R13: 0000000000000000 R14: ffffffff847fd440 R15: ffff8801c4219138
 sctp_destruct_sock+0x43/0x50 net/sctp/socket.c:4436
 __sk_destruct+0xfd/0x910 net/core/sock.c:1570
 sk_destruct+0x47/0x80 net/core/sock.c:1605
 __sk_free+0x57/0x230 net/core/sock.c:1613
 sk_free+0x2a/0x40 net/core/sock.c:1624
 sock_put include/net/sock.h:1649 [inline]
 sctp_association_destroy net/sctp/associola.c:425 [inline]
 sctp_association_put+0x14c/0x2f0 net/sctp/associola.c:884
 sctp_wait_for_connect+0x3b4/0x550 net/sctp/socket.c:7930
 sctp_sendmsg+0x29ec/0x31f0 net/sctp/socket.c:2001
 inet_sendmsg+0x11f/0x5e0 net/ipv4/af_inet.c:762
 sock_sendmsg_nosec net/socket.c:633 [inline]
 sock_sendmsg+0xca/0x110 net/socket.c:643
 SYSC_sendto+0x352/0x5a0 net/socket.c:1750
 SyS_sendto+0x40/0x50 net/socket.c:1718
 entry_SYSCALL_64_fastpath+0x1f/0xbe
RIP: 0033:0x4512e9
RSP: 002b:00007ffb3dee2c08 EFLAGS: 00000216 ORIG_RAX: 000000000000002c
RAX: ffffffffffffffda RBX: 00000000007183f0 RCX: 00000000004512e9
RDX: 000000000000001a RSI: 0000000020001f98 RDI: 000000000000001a
RBP: 0000000000000086 R08: 0000000020944000 R09: 0000000000000010
R10: 0000000000000004 R11: 0000000000000216 R12: 00000000004ba74c
R13: 00000000ffffffff R14: 000000000000001d R15: 0000000000000000
Dumping ftrace buffer:
   (ftrace buffer empty)
Kernel Offset: disabled
Rebooting in 86400 seconds..