bisecting fixing commit since aea8526edf59da3ff5306ca408e13d8f6ab89b34
building syzkaller on 8304907db07a4c6f7ef3164be77c1bacc94d6d51
testing commit aea8526edf59da3ff5306ca408e13d8f6ab89b34 with gcc (GCC) 8.1.0
kernel signature: 33ec35d1bdd222e61b14f29b4ebb2cae4741024f
all runs: crashed: WARNING: refcount bug in l2tp_session_create
testing current HEAD 6d0c334a400db31751c787c411e7187ab59a3f1d
testing commit 6d0c334a400db31751c787c411e7187ab59a3f1d with gcc (GCC) 8.1.0
kernel signature: ac55a0004c2f2bee8dce1d507d458382fdf51c08
all runs: crashed: WARNING: refcount bug in l2tp_session_create
revisions tested: 2, total time: 25m19.939125934s (build: 17m52.176708732s, test: 6m31.844862038s)
the crash still happens on HEAD
commit msg: Linux 4.14.164
crash: WARNING: refcount bug in l2tp_session_create
IPv6: ADDRCONF(NETDEV_CHANGE): veth1_to_hsr: link becomes ready
IPv6: ADDRCONF(NETDEV_CHANGE): hsr_slave_1: link becomes ready
device hsr_slave_0 entered promiscuous mode
refcount_t: increment on 0; use-after-free.
------------[ cut here ]------------
WARNING: CPU: 1 PID: 6903 at lib/refcount.c:153 refcount_inc.cold.12+0x13/0x1a lib/refcount.c:153
Kernel panic - not syncing: panic_on_warn set ...

CPU: 1 PID: 6903 Comm: syz-executor.5 Not tainted 4.14.164-syzkaller #0
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 01/01/2011
Call Trace:
 __dump_stack lib/dump_stack.c:17 [inline]
 dump_stack+0xf7/0x13b lib/dump_stack.c:58
 panic+0x1b0/0x358 kernel/panic.c:183
 __warn.cold.8+0x25/0x2c kernel/panic.c:547
 report_bug+0x1a4/0x1f3 lib/bug.c:186
 fixup_bug arch/x86/kernel/traps.c:177 [inline]
 do_error_trap+0x1bd/0x310 arch/x86/kernel/traps.c:295
 do_invalid_op+0x1b/0x20 arch/x86/kernel/traps.c:314
 invalid_op+0x1b/0x40 arch/x86/entry/entry_64.S:963
RIP: 0010:refcount_inc.cold.12+0x13/0x1a lib/refcount.c:153
RSP: 0018:ffff8880832dfb08 EFLAGS: 00010286
RAX: 000000000000002b RBX: ffff88808fd02d00 RCX: 0000000000000000
RDX: 000000000000002b RSI: ffffffff868cf180 RDI: ffffed101065bf58
RBP: ffff8880832dfb08 R08: ffff88809ffc0d88 R09: 0000000000000000
R10: 0000000000000000 R11: dffffc0000000000 R12: ffff8880a1718080
R13: 0000000000000000 R14: ffff8880832dfcc8 R15: ffff8880a1718130
 sock_hold include/net/sock.h:619 [inline]
 l2tp_session_add_to_tunnel net/l2tp/l2tp_core.c:374 [inline]
 l2tp_session_create+0xb31/0x1630 net/l2tp/l2tp_core.c:1846
 pppol2tp_connect+0x10c1/0x1900 net/l2tp/l2tp_ppp.c:721
 SYSC_connect+0x1e3/0x2a0 net/socket.c:1655
 SyS_connect+0x9/0x10 net/socket.c:1636
 do_syscall_64+0x1c7/0x5b0 arch/x86/entry/common.c:292
 entry_SYSCALL_64_after_hwframe+0x42/0xb7
RIP: 0033:0x459819
RSP: 002b:00007fc75a309c78 EFLAGS: 00000246 ORIG_RAX: 000000000000002a
RAX: ffffffffffffffda RBX: 0000000000000003 RCX: 0000000000459819
RDX: 0000000000000026 RSI: 0000000020000180 RDI: 0000000000000004
RBP: 000000000075bf20 R08: 0000000000000000 R09: 0000000000000000
R10: 0000000000000000 R11: 0000000000000246 R12: 00007fc75a30a6d4
R13: 00000000004bfd8b R14: 00000000004d1948 R15: 00000000ffffffff
Kernel Offset: disabled
Rebooting in 86400 seconds..