[syzbot] [i2c?] WARNING: refcount bug in i2c_get_adapter (2)

[syzbot <syzbot+c0291c8c9aaa473c7721@syzkaller.appspotmail.com>]

Hello,

syzbot found the following issue on:

HEAD commit:    e753c16cb3dd Merge tag 'spi-fix-v7.0-rc7' of git://git.ker..
git tree:       upstream
console output: https://syzkaller.appspot.com/x/log.txt?x=127f9106580000
kernel config:  https://syzkaller.appspot.com/x/.config?x=45cb3c58fd963c27
dashboard link: https://syzkaller.appspot.com/bug?extid=c0291c8c9aaa473c7721
compiler:       Debian clang version 21.1.8 (++20251221033036+2078da43e25a-1~exp1~20251221153213.50), Debian LLD 21.1.8

Unfortunately, I don't have any reproducer for this issue yet.

Downloadable assets:
disk image: https://storage.googleapis.com/syzbot-assets/5ae80002fa8a/disk-e753c16c.raw.xz
vmlinux: https://storage.googleapis.com/syzbot-assets/6ef912fb95e2/vmlinux-e753c16c.xz
kernel image: https://storage.googleapis.com/syzbot-assets/b5b121be2ebf/bzImage-e753c16c.xz

IMPORTANT: if you fix the issue, please add the following tag to the commit:
Reported-by: syzbot+c0291c8c9aaa473c7721@syzkaller.appspotmail.com

------------[ cut here ]------------
refcount_t: addition on 0; use-after-free.
WARNING: lib/refcount.c:25 at refcount_warn_saturate+0x9f/0x110 lib/refcount.c:25, CPU#0: syz.1.506/7352
Modules linked in:
CPU: 0 UID: 0 PID: 7352 Comm: syz.1.506 Tainted: G             L      syzkaller #0 PREEMPT_{RT,(full)} 
Tainted: [L]=SOFTLOCKUP
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 03/18/2026
RIP: 0010:refcount_warn_saturate+0x9f/0x110 lib/refcount.c:25
Code: eb 66 85 db 74 3e 83 fb 01 75 4c e8 bb d6 25 fd 48 8d 3d 84 01 d6 0a 67 48 0f b9 3a eb 4a e8 a8 d6 25 fd 48 8d 3d 81 01 d6 0a <67> 48 0f b9 3a eb 37 e8 95 d6 25 fd 48 8d 3d 7e 01 d6 0a 67 48 0f
RSP: 0018:ffffc9001ca9f6d8 EFLAGS: 00010283
RAX: ffffffff849eaa68 RBX: 0000000000000002 RCX: 0000000000080000
RDX: ffffc90006421000 RSI: 00000000000006c7 RDI: ffffffff8f74abf0
RBP: 0000000000000000 R08: ffff888020331e80 R09: 0000000000000005
R10: 0000000000000100 R11: 0000000000000004 R12: ffffffff8c04a688
R13: dffffc0000000000 R14: ffff88803b531188 R15: dffffc0000000000
FS:  00007f9769ca66c0(0000) GS:ffff888126332000(0000) knlGS:0000000000000000
CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
CR2: 00007f6df4ab1d80 CR3: 00000000348d0000 CR4: 00000000003526f0
Call Trace:
 <TASK>
 __refcount_add include/linux/refcount.h:-1 [inline]
 __refcount_inc include/linux/refcount.h:366 [inline]
 refcount_inc include/linux/refcount.h:383 [inline]
 kref_get include/linux/kref.h:45 [inline]
 kobject_get+0xfa/0x120 lib/kobject.c:643
 i2c_get_adapter+0x6d/0xa0 drivers/i2c/i2c-core-base.c:2612
 i2cdev_open+0x48/0x190 drivers/i2c/i2c-dev.c:603
 chrdev_open+0x4d0/0x5f0 fs/char_dev.c:411
 do_dentry_open+0x83d/0x13e0 fs/open.c:949
 vfs_open+0x3b/0x350 fs/open.c:1081
 do_open fs/namei.c:4677 [inline]
 path_openat+0x2e43/0x38a0 fs/namei.c:4836
 do_file_open+0x23e/0x4a0 fs/namei.c:4865
 do_sys_openat2+0x113/0x200 fs/open.c:1366
 do_sys_open fs/open.c:1372 [inline]
 __do_sys_openat fs/open.c:1388 [inline]
 __se_sys_openat fs/open.c:1383 [inline]
 __x64_sys_openat+0x138/0x170 fs/open.c:1383
 do_syscall_x64 arch/x86/entry/syscall_64.c:63 [inline]
 do_syscall_64+0x14d/0xf80 arch/x86/entry/syscall_64.c:94
 entry_SYSCALL_64_after_hwframe+0x77/0x7f
RIP: 0033:0x7f976ba0d04e
Code: 08 0f 85 a5 a8 ff ff 49 89 fb 48 89 f0 48 89 d7 48 89 ce 4c 89 c2 4d 89 ca 4c 8b 44 24 08 4c 8b 4c 24 10 4c 89 5c 24 08 0f 05 <c3> 66 2e 0f 1f 84 00 00 00 00 00 0f 1f 80 00 00 00 00 48 83 ec 08
RSP: 002b:00007f9769ca5b28 EFLAGS: 00000246 ORIG_RAX: 0000000000000101
RAX: ffffffffffffffda RBX: 00007f9769ca66c0 RCX: 00007f976ba0d04e
RDX: 0000000000000402 RSI: 00007f9769ca5c00 RDI: ffffffffffffff9c
RBP: 00007f9769ca5c00 R08: 0000000000000000 R09: 0000000000000000
R10: 0000000000000000 R11: 0000000000000246 R12: cccccccccccccccd
R13: 00007f976bcc6038 R14: 00007f976bcc5fa0 R15: 00007fffc400e638
 </TASK>
----------------
Code disassembly (best guess):
   0:	eb 66                	jmp    0x68
   2:	85 db                	test   %ebx,%ebx
   4:	74 3e                	je     0x44
   6:	83 fb 01             	cmp    $0x1,%ebx
   9:	75 4c                	jne    0x57
   b:	e8 bb d6 25 fd       	call   0xfd25d6cb
  10:	48 8d 3d 84 01 d6 0a 	lea    0xad60184(%rip),%rdi        # 0xad6019b
  17:	67 48 0f b9 3a       	ud1    (%edx),%rdi
  1c:	eb 4a                	jmp    0x68
  1e:	e8 a8 d6 25 fd       	call   0xfd25d6cb
  23:	48 8d 3d 81 01 d6 0a 	lea    0xad60181(%rip),%rdi        # 0xad601ab
* 2a:	67 48 0f b9 3a       	ud1    (%edx),%rdi <-- trapping instruction
  2f:	eb 37                	jmp    0x68
  31:	e8 95 d6 25 fd       	call   0xfd25d6cb
  36:	48 8d 3d 7e 01 d6 0a 	lea    0xad6017e(%rip),%rdi        # 0xad601bb
  3d:	67                   	addr32
  3e:	48                   	rex.W
  3f:	0f                   	.byte 0xf


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