bisecting fixing commit since 1ec8f1f0bffe34ebdf95dbe0fd4a6635a84612a8
building syzkaller on 505ab413c77ce8c6bd4658ea5e68ea2534d47b39
testing commit 1ec8f1f0bffe34ebdf95dbe0fd4a6635a84612a8 with gcc (GCC) 8.1.0
kernel signature: 352b64083510cab7dec16ad98a3b683e280b0ccf
all runs: crashed: WARNING: refcount bug in cdev_get
testing current HEAD fbc5fe7a54d02e11972e3b2a5ddb6ffc88162c8f
testing commit fbc5fe7a54d02e11972e3b2a5ddb6ffc88162c8f with gcc (GCC) 8.1.0
kernel signature: 630ff0e89f15e04e48a0bf24e27bd90fec3c4fbe
all runs: crashed: WARNING: refcount bug in cdev_get
revisions tested: 2, total time: 28m14.133191741s (build: 16m6.621400553s, test: 11m8.678495651s)
the crash still happens on HEAD
commit msg: Linux 4.14.157
crash: WARNING: refcount bug in cdev_get
input: syz0 as /devices/virtual/input/input6194
input: syz0 as /devices/virtual/input/input6196
input: syz0 as /devices/virtual/input/input6198
refcount_t: increment on 0; use-after-free.
------------[ cut here ]------------
WARNING: CPU: 0 PID: 25253 at lib/refcount.c:153 refcount_inc.cold.12+0x13/0x1a lib/refcount.c:153
Kernel panic - not syncing: panic_on_warn set ...

CPU: 0 PID: 25253 Comm: syz-executor.1 Not tainted 4.14.157-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+0x1b0/0x36a kernel/panic.c:183
 __warn.cold.8+0x25/0x2a 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:ffff8880890f7930 EFLAGS: 00010286
RAX: 000000000000002b RBX: ffff8880a4ec6e68 RCX: 0000000000000000
RDX: 000000000000002b RSI: ffff8880a0168ac0 RDI: ffffed101121ef1d
RBP: ffff8880890f7930 R08: ffff8880a0168ae0 R09: 0000000000000000
R10: 0000000000000000 R11: dffffc0000000000 R12: 0000000000000000
R13: ffff8880804beb60 R14: ffff8880915d9340 R15: ffff8880804beee8
 kref_get include/linux/kref.h:47 [inline]
 kobject_get+0x47/0x60 lib/kobject.c:596
 cdev_get+0x46/0x80 fs/char_dev.c:364
 chrdev_open+0x9c/0x5b0 fs/char_dev.c:409
 do_dentry_open+0x623/0xdb0 fs/open.c:777
 vfs_open+0xfc/0x240 fs/open.c:891
 do_last fs/namei.c:3425 [inline]
 path_openat+0xe6d/0x3b50 fs/namei.c:3566
 do_filp_open+0x16b/0x220 fs/namei.c:3600
 do_sys_open+0x1c2/0x340 fs/open.c:1084
 SYSC_open fs/open.c:1102 [inline]
 SyS_open+0x19/0x20 fs/open.c:1097
 do_syscall_64+0x1c9/0x5b0 arch/x86/entry/common.c:292
 entry_SYSCALL_64_after_hwframe+0x42/0xb7
RIP: 0033:0x412bc1
RSP: 002b:00007fac7c77a7a0 EFLAGS: 00000293 ORIG_RAX: 0000000000000002
RAX: ffffffffffffffda RBX: 6666666666666667 RCX: 0000000000412bc1
RDX: 0000000000000000 RSI: 0000000000000000 RDI: 00007fac7c77a850
RBP: 000000000073bfa0 R08: 000000000000000f R09: 0000000000000000
R10: 00007fac7c77b9d0 R11: 0000000000000293 R12: 00007fac7c77b6d4
R13: 00000000004c7975 R14: 00000000004dda00 R15: 00000000ffffffff
Kernel Offset: disabled
Rebooting in 86400 seconds..