bisecting fixing commit since db2d0b7c1dde59b93045a6d011f392fb04b276af
building syzkaller on 51ca0454d75437183e240f7e23165fe5458908e5
testing commit db2d0b7c1dde59b93045a6d011f392fb04b276af with gcc (GCC) 8.1.0
kernel signature: fc72741c77077ae1ea363b81278ca65c8955d323
run #0: crashed: WARNING: refcount bug in cdev_get
run #1: crashed: WARNING: refcount bug in cdev_get
run #2: crashed: WARNING: refcount bug in cdev_get
run #3: crashed: WARNING: refcount bug in cdev_get
run #4: crashed: WARNING: refcount bug in cdev_get
run #5: crashed: WARNING: refcount bug in cdev_get
run #6: crashed: WARNING: refcount bug in cdev_get
run #7: crashed: WARNING: refcount bug in cdev_get
run #8: crashed: WARNING: refcount bug in cdev_get
run #9: OK
testing current HEAD 7d120bf21c05cbe30a679f0feeca884eeaceb069
testing commit 7d120bf21c05cbe30a679f0feeca884eeaceb069 with gcc (GCC) 8.1.0
kernel signature: 379d9e85078a38cd9cb8243641ccf659dee4a035
run #0: crashed: WARNING: refcount bug in cdev_get
run #1: crashed: WARNING: refcount bug in cdev_get
run #2: crashed: WARNING: refcount bug in cdev_get
run #3: crashed: WARNING: refcount bug in cdev_get
run #4: crashed: WARNING: refcount bug in cdev_get
run #5: crashed: WARNING: refcount bug in cdev_get
run #6: crashed: WARNING: refcount bug in cdev_get
run #7: crashed: WARNING: refcount bug in cdev_get
run #8: crashed: WARNING: refcount bug in cdev_get
run #9: OK
revisions tested: 2, total time: 39m45.444467059s (build: 17m18.187763814s, test: 21m42.717617738s)
the crash still happens on HEAD
commit msg: Linux 4.19.90
crash: WARNING: refcount bug in cdev_get
input: syz1 as /devices/virtual/input/input11125
input: syz1 as /devices/virtual/input/input11126
input: syz1 as /devices/virtual/input/input11127
------------[ cut here ]------------
refcount_t: increment on 0; use-after-free.
WARNING: CPU: 1 PID: 26427 at lib/refcount.c:153 refcount_inc_checked+0x2b/0x30 lib/refcount.c:153
Kernel panic - not syncing: panic_on_warn set ...

CPU: 1 PID: 26427 Comm: syz-executor.2 Not tainted 4.19.90-syzkaller #0
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 01/01/2011
Call Trace:
 __dump_stack lib/dump_stack.c:77 [inline]
 dump_stack+0x123/0x177 lib/dump_stack.c:118
 panic+0x1cd/0x375 kernel/panic.c:186
 __warn.cold.8+0x1b/0x3e kernel/panic.c:541
 report_bug+0x1a4/0x200 lib/bug.c:186
 fixup_bug arch/x86/kernel/traps.c:178 [inline]
 do_error_trap+0x200/0x350 arch/x86/kernel/traps.c:296
 do_invalid_op+0x1b/0x20 arch/x86/kernel/traps.c:316
 invalid_op+0x14/0x20 arch/x86/entry/entry_64.S:1037
RIP: 0010:refcount_inc_checked+0x2b/0x30 lib/refcount.c:153
Code: 48 89 e5 e8 97 fe ff ff 84 c0 74 02 5d c3 80 3d 43 6c bc 05 00 75 f5 48 c7 c7 60 06 ff 86 c6 05 33 6c bc 05 01 e8 48 01 48 fe <0f> 0b 5d c3 90 55 48 89 e5 41 57 49 89 f7 be 04 00 00 00 41 56 4c
RSP: 0018:ffff888073b7f900 EFLAGS: 00010282
RAX: 0000000000000000 RBX: ffff88808f00d2b0 RCX: 0000000000000000
RDX: 0000000000000004 RSI: ffff8880a867ea80 RDI: ffffffff89a866a0
RBP: ffff888073b7f900 R08: ffff8880a867eaa0 R09: 0000000000000000
R10: 0000000000000000 R11: 0000000000000000 R12: 0000000000000000
R13: ffff888091aefb50 R14: ffff88807a4e9840 R15: ffff888091aefed0
 kref_get include/linux/kref.h:47 [inline]
 kobject_get+0x43/0x90 lib/kobject.c:613
 cdev_get+0x46/0x80 fs/char_dev.c:364
 chrdev_open+0xa3/0x5c0 fs/char_dev.c:409
 do_dentry_open+0x3f1/0x1010 fs/open.c:796
 vfs_open+0x9a/0xc0 fs/open.c:905
 do_last fs/namei.c:3418 [inline]
 path_openat+0x691/0x3c00 fs/namei.c:3534
 do_filp_open+0x177/0x250 fs/namei.c:3564
 do_sys_open+0x1dd/0x350 fs/open.c:1088
 __do_sys_open fs/open.c:1106 [inline]
 __se_sys_open fs/open.c:1101 [inline]
 __x64_sys_open+0x79/0xb0 fs/open.c:1101
 do_syscall_64+0xd0/0x4e0 arch/x86/entry/common.c:293
 entry_SYSCALL_64_after_hwframe+0x49/0xbe
RIP: 0033:0x4137d1
Code: 75 14 b8 02 00 00 00 0f 05 48 3d 01 f0 ff ff 0f 83 04 19 00 00 c3 48 83 ec 08 e8 0a fa ff ff 48 89 04 24 b8 02 00 00 00 0f 05 <48> 8b 3c 24 48 89 c2 e8 53 fa ff ff 48 89 d0 48 83 c4 08 48 3d 01
RSP: 002b:00007fe710d167a0 EFLAGS: 00000293 ORIG_RAX: 0000000000000002
RAX: ffffffffffffffda RBX: 6666666666666667 RCX: 00000000004137d1
RDX: 0000000000000000 RSI: 0000000000024440 RDI: 00007fe710d16850
RBP: 000000000075bf20 R08: 000000000000000f R09: 0000000000000000
R10: 0000000000000000 R11: 0000000000000293 R12: 00007fe710d176d4
R13: 00000000004c8c84 R14: 00000000004dfc48 R15: 00000000ffffffff
Kernel Offset: disabled
Rebooting in 86400 seconds..