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SubjectWARNING: refcount bug in chrdev_open
From
Hello,

syzbot found the following crash on:

HEAD commit: a69e9051 Merge tag 'xfs-5.3-fixes-2' of git://git.kernel.o..
git tree: upstream
console output: https://syzkaller.appspot.com/x/log.txt?x=1450b25a600000
kernel config: https://syzkaller.appspot.com/x/.config?x=3d7eaed8496da4da
dashboard link: https://syzkaller.appspot.com/bug?extid=1c85a21f1c6bc88eb388
compiler: clang version 9.0.0 (/home/glider/llvm/clang
80fee25776c2fb61e74c1ecb1a523375c2500b69)
syz repro: https://syzkaller.appspot.com/x/repro.syz?x=1088edba600000
C reproducer: https://syzkaller.appspot.com/x/repro.c?x=10477772600000

Bisection is inconclusive: the bug happens on the oldest tested release.

bisection log: https://syzkaller.appspot.com/x/bisect.txt?x=17d7389c600000
final crash: https://syzkaller.appspot.com/x/report.txt?x=1437389c600000
console output: https://syzkaller.appspot.com/x/log.txt?x=1037389c600000

IMPORTANT: if you fix the bug, please add the following tag to the commit:
Reported-by: syzbot+1c85a21f1c6bc88eb388@syzkaller.appspotmail.com

------------[ cut here ]------------
refcount_t: increment on 0; use-after-free.
WARNING: CPU: 1 PID: 12599 at lib/refcount.c:156
refcount_inc_checked+0x4b/0x50 lib/refcount.c:156
Kernel panic - not syncing: panic_on_warn set ...
CPU: 1 PID: 12599 Comm: syz-executor314 Not tainted 5.3.0-rc4+ #78
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+0x1d8/0x2f8 lib/dump_stack.c:113
panic+0x25c/0x799 kernel/panic.c:219
__warn+0x22f/0x230 kernel/panic.c:576
report_bug+0x190/0x290 lib/bug.c:186
fixup_bug arch/x86/kernel/traps.c:179 [inline]
do_error_trap+0xd7/0x440 arch/x86/kernel/traps.c:272
do_invalid_op+0x36/0x40 arch/x86/kernel/traps.c:291
invalid_op+0x23/0x30 arch/x86/entry/entry_64.S:1028
RIP: 0010:refcount_inc_checked+0x4b/0x50 lib/refcount.c:156
Code: 3d 2e 8b 68 05 01 75 08 e8 52 53 21 fe 5b 5d c3 e8 4a 53 21 fe c6 05
18 8b 68 05 01 48 c7 c7 34 4b 45 88 31 c0 e8 85 3a f4 fd <0f> 0b eb df 90
55 48 89 e5 41 57 41 56 41 55 41 54 53 48 83 ec 10
RSP: 0018:ffff88808cb579b0 EFLAGS: 00010246
RAX: cd29ce2624bf4d00 RBX: ffff8880934dcc00 RCX: ffff88809eec65c0
RDX: 0000000000000000 RSI: 0000000000000001 RDI: 0000000000000000
RBP: ffff88808cb579b8 R08: ffffffff815cf524 R09: ffffed1015d640d2
R10: ffffed1015d640d2 R11: 0000000000000000 R12: dffffc0000000000
R13: ffff8880934dcbc8 R14: ffff8880934dcc04 R15: ffff8880934dcbc8
kref_get include/linux/kref.h:45 [inline]
kobject_get+0x91/0xc0 lib/kobject.c:644
cdev_get fs/char_dev.c:355 [inline]
chrdev_open+0x17e/0x590 fs/char_dev.c:400
do_dentry_open+0x73b/0xf90 fs/open.c:797
vfs_open+0x73/0x80 fs/open.c:906
do_last fs/namei.c:3416 [inline]
path_openat+0x1397/0x4460 fs/namei.c:3533
do_filp_open+0x192/0x3d0 fs/namei.c:3563
do_sys_open+0x29f/0x560 fs/open.c:1089
__do_sys_open fs/open.c:1107 [inline]
__se_sys_open fs/open.c:1102 [inline]
__x64_sys_open+0x87/0x90 fs/open.c:1102
do_syscall_64+0xfe/0x140 arch/x86/entry/common.c:296
entry_SYSCALL_64_after_hwframe+0x49/0xbe
RIP: 0033:0x406311
Code: 75 14 b8 02 00 00 00 0f 05 48 3d 01 f0 ff ff 0f 83 a4 18 00 00 c3 48
83 ec 08 e8 0a fc ff ff 48 89 04 24 b8 02 00 00 00 0f 05 <48> 8b 3c 24 48
89 c2 e8 53 fc ff ff 48 89 d0 48 83 c4 08 48 3d 01
RSP: 002b:00007fb29f268960 EFLAGS: 00000293 ORIG_RAX: 0000000000000002
RAX: ffffffffffffffda RBX: 0000000000000000 RCX: 0000000000406311
RDX: 0000000000000000 RSI: 0000000000000000 RDI: 00007fb29f268970
RBP: 6666666666666667 R08: 000000000000000f R09: 00007fb29f269700
R10: 00007fb29f2699d0 R11: 0000000000000293 R12: 00000000006dbc3c
R13: 0000000000000000 R14: 0000000000000000 R15: 00000000317a7973
Kernel Offset: disabled
Rebooting in 86400 seconds..


---
This bug is generated by a bot. It may contain errors.
See https://goo.gl/tpsmEJ for more information about syzbot.
syzbot engineers can be reached at syzkaller@googlegroups.com.

syzbot will keep track of this bug report. See:
https://goo.gl/tpsmEJ#status for how to communicate with syzbot.
For information about bisection process see: https://goo.gl/tpsmEJ#bisection
syzbot can test patches for this bug, for details see:
https://goo.gl/tpsmEJ#testing-patches

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