[syzbot] WARNING: refcount bug in l2cap_sock_kill

[syzbot]

Hello,

syzbot found the following issue on:

HEAD commit:    5e437416 Merge branch 'dsa-mv88e6xxx-topaz-fixes'
git tree:       net-next
console output: https://syzkaller.appspot.com/x/log.txt?x=112445d8300000
kernel config:  https://syzkaller.appspot.com/x/.config?x=4cb84363d46e9fc3
dashboard link: https://syzkaller.appspot.com/bug?extid=845954aabaa368d3ef45

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

IMPORTANT: if you fix the issue, please add the following tag to the commit:
Reported-by: syzbot+845954aabaa368d3ef45@xxxxxxxxxxxxxxxxxxxxxxxxx

refcount_t: underflow; use-after-free.
WARNING: CPU: 1 PID: 8496 at lib/refcount.c:28 refcount_warn_saturate+0x1d1/0x1e0 lib/refcount.c:28
Modules linked in:
CPU: 1 PID: 8496 Comm: syz-executor.5 Tainted: G        W         5.13.0-syzkaller #0
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 01/01/2011
RIP: 0010:refcount_warn_saturate+0x1d1/0x1e0 lib/refcount.c:28
Code: e9 db fe ff ff 48 89 df e8 0c 90 e8 fd e9 8a fe ff ff e8 32 f1 a2 fd 48 c7 c7 a0 25 e3 89 c6 05 84 0c 84 09 01 e8 ec c0 0b 05 <0f> 0b e9 af fe ff ff 0f 1f 84 00 00 00 00 00 41 56 41 55 41 54 55
RSP: 0018:ffffc9000178fa88 EFLAGS: 00010282
RAX: 0000000000000000 RBX: 0000000000000000 RCX: 0000000000000000
RDX: ffff88801a591c40 RSI: ffffffff815d7235 RDI: fffff520002f1f43
RBP: 0000000000000003 R08: 0000000000000000 R09: 0000000000000000
R10: ffffffff815d109e R11: 0000000000000000 R12: ffff88806ca9b080
R13: dffffc0000000000 R14: ffff88806977c4b8 R15: 0000000000000067
FS:  0000000000000000(0000) GS:ffff8880b9d00000(0000) knlGS:0000000000000000
CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
CR2: 00007f516584b718 CR3: 000000002e987000 CR4: 00000000001506e0
DR0: 0000000000000000 DR1: 0000000000000000 DR2: 0000000000000000
DR3: 0000000000000000 DR6: 00000000fffe0ff0 DR7: 0000000000000400
Call Trace:
 __refcount_sub_and_test include/linux/refcount.h:283 [inline]
 __refcount_dec_and_test include/linux/refcount.h:315 [inline]
 refcount_dec_and_test include/linux/refcount.h:333 [inline]
 sock_put include/net/sock.h:1812 [inline]
 l2cap_sock_kill+0x153/0x240 net/bluetooth/l2cap_sock.c:1227
 l2cap_conn_del+0x3fc/0x7b0 net/bluetooth/l2cap_core.c:1900
 l2cap_disconn_cfm net/bluetooth/l2cap_core.c:8177 [inline]
 l2cap_disconn_cfm+0x95/0xd0 net/bluetooth/l2cap_core.c:8170
 hci_disconn_cfm include/net/bluetooth/hci_core.h:1500 [inline]
 hci_conn_hash_flush+0x127/0x260 net/bluetooth/hci_conn.c:1608
 hci_dev_do_close+0x528/0x1130 net/bluetooth/hci_core.c:1778
 hci_unregister_dev+0x263/0x1130 net/bluetooth/hci_core.c:4019
 vhci_release+0x70/0xe0 drivers/bluetooth/hci_vhci.c:340
 __fput+0x288/0x920 fs/file_table.c:280
 task_work_run+0xdd/0x1a0 kernel/task_work.c:164
 exit_task_work include/linux/task_work.h:32 [inline]
 do_exit+0xbd4/0x2a50 kernel/exit.c:825
 do_group_exit+0x125/0x310 kernel/exit.c:922
 __do_sys_exit_group kernel/exit.c:933 [inline]
 __se_sys_exit_group kernel/exit.c:931 [inline]
 __x64_sys_exit_group+0x3a/0x50 kernel/exit.c:931
 do_syscall_x64 arch/x86/entry/common.c:50 [inline]
 do_syscall_64+0x35/0xb0 arch/x86/entry/common.c:80
 entry_SYSCALL_64_after_hwframe+0x44/0xae
RIP: 0033:0x4665d9
Code: Unable to access opcode bytes at RIP 0x4665af.
RSP: 002b:00007fff2d0a39f8 EFLAGS: 00000246 ORIG_RAX: 00000000000000e7
RAX: ffffffffffffffda RBX: 00000000000007d0 RCX: 00000000004665d9
RDX: 0000000000000000 RSI: 0000000000000001 RDI: 0000000000000043
RBP: 0000000000000000 R08: 0000000000000014 R09: 00000000000007d0
R10: 0000000000000000 R11: 0000000000000246 R12: 00000000004bfe04
R13: 0000000000000000 R14: 0000000000000009 R15: 00007fff2d0a3bf0


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