Re: [syzbot] [kernel?] WARNING: refcount bug in bnep_session (2)

[syzbot <syzbot+98baa83c342e86a836d7@syzkaller.appspotmail.com>]

syzbot has found a reproducer for the following issue on:

HEAD commit:    818bebeb63dd drm/xe: Don't hand out the flat CCS storage a..
git tree:       git://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git
console output: https://syzkaller.appspot.com/x/log.txt?x=13956d49580000
kernel config:  https://syzkaller.appspot.com/x/.config?x=ccca94d2c01b9e78
dashboard link: https://syzkaller.appspot.com/bug?extid=98baa83c342e86a836d7
compiler:       gcc (Debian 14.2.0-19) 14.2.0, GNU ld (GNU Binutils for Debian) 2.44
C reproducer:   https://syzkaller.appspot.com/x/repro.c?x=13435625580000

IMPORTANT: if you fix the issue, please add the following tag to the commit:
Reported-by: syzbot+98baa83c342e86a836d7@syzkaller.appspotmail.com

------------[ cut here ]------------
refcount_t: underflow; use-after-free.
WARNING: lib/refcount.c:28 at refcount_warn_saturate+0xf4/0x130 lib/refcount.c:28, CPU#3: kbnepd bnep0/6057
Modules linked in:
CPU: 3 UID: 0 PID: 6057 Comm: kbnepd bnep0 Not tainted syzkaller #0 PREEMPT(full) 
Hardware name: QEMU Standard PC (Q35 + ICH9, 2009), BIOS 1.16.3-debian-1.16.3-2 04/01/2014
RIP: 0010:refcount_warn_saturate+0xf4/0x130 lib/refcount.c:28
Code: 06 e8 70 ca e6 fc 48 8d 3d c9 dd 29 0c 67 48 0f b9 3a e8 5f ca e6 fc 5b 5d c3 cc cc cc cc e8 53 ca e6 fc 48 8d 3d bc dd 29 0c <67> 48 0f b9 3a e8 42 ca e6 fc 5b 5d e9 4b ee a5 06 e8 36 ca e6 fc
RSP: 0018:ffffc90005c6f958 EFLAGS: 00010293
RAX: 0000000000000000 RBX: ffff8880507dfc78 RCX: 0000000000000001
RDX: ffff88803142cb00 RSI: ffffffff8524377d RDI: ffffffff914e1540
RBP: 0000000000000003 R08: 0000000000000005 R09: 0000000000000004
R10: 0000000000000003 R11: 0000000000000000 R12: ffff8880134cd400
R13: ffff8880507dfc78 R14: ffffffff8661d790 R15: ffff888035896000
FS:  0000000000000000(0000) GS:ffff8880d5ea2000(0000) knlGS:0000000000000000
CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
CR2: 00007fcfb9c4bd58 CR3: 0000000036184000 CR4: 0000000000352ef0
Call Trace:
 <TASK>
 __refcount_sub_and_test include/linux/refcount.h:400 [inline]
 __refcount_dec_and_test include/linux/refcount.h:432 [inline]
 refcount_dec_and_test include/linux/refcount.h:450 [inline]
 kref_put include/linux/kref.h:64 [inline]
 klist_dec_and_del lib/klist.c:206 [inline]
 klist_put+0x18a/0x1d0 lib/klist.c:217
 device_del+0x1b8/0x9b0 drivers/base/core.c:3942
 unregister_netdevice_many_notify+0x1503/0x2150 net/core/dev.c:12551
 unregister_netdevice_many net/core/dev.c:12589 [inline]
 unregister_netdevice_queue net/core/dev.c:12390 [inline]
 unregister_netdevice_queue+0x30b/0x3c0 net/core/dev.c:12376
 unregister_netdevice include/linux/netdevice.h:3487 [inline]
 unregister_netdev+0x1f/0x60 net/core/dev.c:12697
 bnep_session+0x22e6/0x2f40 net/bluetooth/bnep/core.c:544
 kthread+0x373/0x450 kernel/kthread.c:436
 ret_from_fork+0x730/0xd60 arch/x86/kernel/process.c:158
 ret_from_fork_asm+0x1a/0x30 arch/x86/entry/entry_64.S:245
 </TASK>
----------------
Code disassembly (best guess), 1 bytes skipped:
   0:	e8 70 ca e6 fc       	call   0xfce6ca75
   5:	48 8d 3d c9 dd 29 0c 	lea    0xc29ddc9(%rip),%rdi        # 0xc29ddd5
   c:	67 48 0f b9 3a       	ud1    (%edx),%rdi
  11:	e8 5f ca e6 fc       	call   0xfce6ca75
  16:	5b                   	pop    %rbx
  17:	5d                   	pop    %rbp
  18:	c3                   	ret
  19:	cc                   	int3
  1a:	cc                   	int3
  1b:	cc                   	int3
  1c:	cc                   	int3
  1d:	e8 53 ca e6 fc       	call   0xfce6ca75
  22:	48 8d 3d bc dd 29 0c 	lea    0xc29ddbc(%rip),%rdi        # 0xc29dde5
* 29:	67 48 0f b9 3a       	ud1    (%edx),%rdi <-- trapping instruction
  2e:	e8 42 ca e6 fc       	call   0xfce6ca75
  33:	5b                   	pop    %rbx
  34:	5d                   	pop    %rbp
  35:	e9 4b ee a5 06       	jmp    0x6a5ee85
  3a:	e8 36 ca e6 fc       	call   0xfce6ca75


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