possible deadlock in l2cap_chan_connect

Status: upstream: reported on 2026/09/14 22:12
Reported-by: syzbot+c765b5fd13e1030220e2@syzkaller.appspotmail.com
First crash: 17d, last: 3d18h
▶ ▼ Similar bugs (2)
Kernel Title Rank 🛈 Repro Cause bisect Fix bisect Count Last Reported Patched Status
linux-6.6 possible deadlock in l2cap_chan_connect 4 57 8h13m 17d 0/2 upstream: reported on 2026/09/14 16:49
linux-6.1 possible deadlock in l2cap_chan_connect 4 syz, C 94 now 17d 0/3 upstream: reported C repro on 2026/09/14 13:12

Sample crash report:
======================================================
WARNING: possible circular locking dependency detected
syzkaller #0 Not tainted
------------------------------------------------------
syz.3.135/4644 is trying to acquire lock:
ffff888022de8478 (&hdev->lock){+.+.}-{3:3}, at: l2cap_chan_connect+0xfb/0xe20 net/bluetooth/l2cap_core.c:8205

but task is already holding lock:
ffffffff8d5b37c8 (rfcomm_mutex){+.+.}-{3:3}, at: rfcomm_dlc_open+0x99/0x10c0 net/bluetooth/rfcomm/core.c:425

which lock already depends on the new lock.


the existing dependency chain (in reverse order) is:

-> #2 (rfcomm_mutex){+.+.}-{3:3}:
       __mutex_lock_common+0x1e5/0x2400 kernel/locking/mutex.c:596
       __mutex_lock kernel/locking/mutex.c:729 [inline]
       mutex_lock_nested+0x17/0x20 kernel/locking/mutex.c:743
       rfcomm_security_cfm+0x95/0x780 net/bluetooth/rfcomm/core.c:2210
       hci_encrypt_cfm+0x363/0x4d0 include/net/bluetooth/hci_core.h:1583
       hci_encrypt_change_evt+0x62d/0xdd0 net/bluetooth/hci_event.c:3243
       hci_event_packet+0x67e/0x1310 net/bluetooth/hci_event.c:6447
       hci_rx_work+0x255/0xa10 net/bluetooth/hci_core.c:5190
       process_one_work+0x867/0xff0 kernel/workqueue.c:2310
       worker_thread+0xad7/0x12a0 kernel/workqueue.c:2457
       kthread+0x42e/0x520 kernel/kthread.c:334
       ret_from_fork+0x1f/0x30 arch/x86/entry/entry_64.S:287

-> #1 (hci_cb_list_lock){+.+.}-{3:3}:
       __mutex_lock_common+0x1e5/0x2400 kernel/locking/mutex.c:596
       __mutex_lock kernel/locking/mutex.c:729 [inline]
       mutex_lock_nested+0x17/0x20 kernel/locking/mutex.c:743
       hci_connect_cfm include/net/bluetooth/hci_core.h:1504 [inline]
       hci_remote_features_evt+0x5d9/0xa10 net/bluetooth/hci_event.c:3367
       hci_event_packet+0x600/0x1310 net/bluetooth/hci_event.c:6455
       hci_rx_work+0x255/0xa10 net/bluetooth/hci_core.c:5190
       process_one_work+0x867/0xff0 kernel/workqueue.c:2310
       worker_thread+0xad7/0x12a0 kernel/workqueue.c:2457
       kthread+0x42e/0x520 kernel/kthread.c:334
       ret_from_fork+0x1f/0x30 arch/x86/entry/entry_64.S:287

-> #0 (&hdev->lock){+.+.}-{3:3}:
       check_prev_add kernel/locking/lockdep.c:3053 [inline]
       check_prevs_add kernel/locking/lockdep.c:3172 [inline]
       validate_chain kernel/locking/lockdep.c:3788 [inline]
       __lock_acquire+0x2c66/0x7b50 kernel/locking/lockdep.c:5012
       lock_acquire+0x19e/0x400 kernel/locking/lockdep.c:5625
       __mutex_lock_common+0x1e5/0x2400 kernel/locking/mutex.c:596
       __mutex_lock kernel/locking/mutex.c:729 [inline]
       mutex_lock_nested+0x17/0x20 kernel/locking/mutex.c:743
       l2cap_chan_connect+0xfb/0xe20 net/bluetooth/l2cap_core.c:8205
       l2cap_sock_connect+0x5a3/0x770 net/bluetooth/l2cap_sock.c:255
       kernel_connect+0x11c/0x180 net/socket.c:3498
       rfcomm_session_create net/bluetooth/rfcomm/core.c:807 [inline]
       __rfcomm_dlc_open net/bluetooth/rfcomm/core.c:384 [inline]
       rfcomm_dlc_open+0x600/0x10c0 net/bluetooth/rfcomm/core.c:427
       rfcomm_sock_connect+0x2e9/0x600 net/bluetooth/rfcomm/sock.c:421
       __sys_connect_file net/socket.c:1933 [inline]
       __sys_connect+0x3cb/0x450 net/socket.c:1950
       __do_sys_connect net/socket.c:1960 [inline]
       __se_sys_connect net/socket.c:1957 [inline]
       __x64_sys_connect+0x76/0x80 net/socket.c:1957
       do_syscall_x64 arch/x86/entry/common.c:50 [inline]
       do_syscall_64+0x4c/0xa0 arch/x86/entry/common.c:80
       entry_SYSCALL_64_after_hwframe+0x66/0xd0

other info that might help us debug this:

Chain exists of:
  &hdev->lock --> hci_cb_list_lock --> rfcomm_mutex

 Possible unsafe locking scenario:

       CPU0                    CPU1
       ----                    ----
  lock(rfcomm_mutex);
                               lock(hci_cb_list_lock);
                               lock(rfcomm_mutex);
  lock(&hdev->lock);

 *** DEADLOCK ***

1 lock held by syz.3.135/4644:
 #0: ffffffff8d5b37c8 (rfcomm_mutex){+.+.}-{3:3}, at: rfcomm_dlc_open+0x99/0x10c0 net/bluetooth/rfcomm/core.c:425

stack backtrace:
CPU: 0 PID: 4644 Comm: syz.3.135 Not tainted syzkaller #0
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 07/24/2026
Call Trace:
 <TASK>
 dump_stack_lvl+0x188/0x250 lib/dump_stack.c:106
 check_noncircular+0x296/0x330 kernel/locking/lockdep.c:2133
 check_prev_add kernel/locking/lockdep.c:3053 [inline]
 check_prevs_add kernel/locking/lockdep.c:3172 [inline]
 validate_chain kernel/locking/lockdep.c:3788 [inline]
 __lock_acquire+0x2c66/0x7b50 kernel/locking/lockdep.c:5012
 lock_acquire+0x19e/0x400 kernel/locking/lockdep.c:5625
 __mutex_lock_common+0x1e5/0x2400 kernel/locking/mutex.c:596
 __mutex_lock kernel/locking/mutex.c:729 [inline]
 mutex_lock_nested+0x17/0x20 kernel/locking/mutex.c:743
 l2cap_chan_connect+0xfb/0xe20 net/bluetooth/l2cap_core.c:8205
 l2cap_sock_connect+0x5a3/0x770 net/bluetooth/l2cap_sock.c:255
 kernel_connect+0x11c/0x180 net/socket.c:3498
 rfcomm_session_create net/bluetooth/rfcomm/core.c:807 [inline]
 __rfcomm_dlc_open net/bluetooth/rfcomm/core.c:384 [inline]
 rfcomm_dlc_open+0x600/0x10c0 net/bluetooth/rfcomm/core.c:427
 rfcomm_sock_connect+0x2e9/0x600 net/bluetooth/rfcomm/sock.c:421
 __sys_connect_file net/socket.c:1933 [inline]
 __sys_connect+0x3cb/0x450 net/socket.c:1950
 __do_sys_connect net/socket.c:1960 [inline]
 __se_sys_connect net/socket.c:1957 [inline]
 __x64_sys_connect+0x76/0x80 net/socket.c:1957
 do_syscall_x64 arch/x86/entry/common.c:50 [inline]
 do_syscall_64+0x4c/0xa0 arch/x86/entry/common.c:80
 entry_SYSCALL_64_after_hwframe+0x66/0xd0
RIP: 0033:0x7f39272cd159
Code: ff c3 66 2e 0f 1f 84 00 00 00 00 00 0f 1f 44 00 00 48 89 f8 48 89 f7 48 89 d6 48 89 ca 4d 89 c2 4d 89 c8 4c 8b 4c 24 08 0f 05 <48> 3d 01 f0 ff ff 73 01 c3 48 c7 c1 e8 ff ff ff f7 d8 64 89 01 48
RSP: 002b:00007f3925525028 EFLAGS: 00000246 ORIG_RAX: 000000000000002a
RAX: ffffffffffffffda RBX: 00007f3927554fa0 RCX: 00007f39272cd159
RDX: 0000000000000014 RSI: 0000200000000200 RDI: 0000000000000004
RBP: 00007f392736406b R08: 0000000000000000 R09: 0000000000000000
R10: 0000000000000000 R11: 0000000000000246 R12: 0000000000000000
R13: 00007f3927555038 R14: 00007f3927554fa0 R15: 00007ffd89db3908
 </TASK>

Crashes (17):
Time Kernel Commit Syzkaller Config Log Report Syz repro C repro VM info Assets (help?) Manager Title
2026/09/28 07:46 linux-5.15.y 0248c33e835e a3c7249d .config console log report info [disk image] [vmlinux] [kernel image] ci2-linux-5-15-kasan possible deadlock in l2cap_chan_connect
2026/09/27 22:34 linux-5.15.y 0248c33e835e a3c7249d .config console log report info [disk image] [vmlinux] [kernel image] ci2-linux-5-15-kasan possible deadlock in l2cap_chan_connect
2026/09/26 11:19 linux-5.15.y 0248c33e835e a3c7249d .config console log report info [disk image] [vmlinux] [kernel image] ci2-linux-5-15-kasan possible deadlock in l2cap_chan_connect
2026/09/25 16:29 linux-5.15.y 0248c33e835e f281d9d2 .config console log report info [disk image] [vmlinux] [kernel image] ci2-linux-5-15-kasan possible deadlock in l2cap_chan_connect
2026/09/25 15:07 linux-5.15.y 0248c33e835e f281d9d2 .config console log report info [disk image] [vmlinux] [kernel image] ci2-linux-5-15-kasan possible deadlock in l2cap_chan_connect
2026/09/24 19:11 linux-5.15.y 0248c33e835e 3c3cdf19 .config console log report info [disk image] [vmlinux] [kernel image] ci2-linux-5-15-kasan possible deadlock in l2cap_chan_connect
2026/09/24 12:40 linux-5.15.y 0248c33e835e 0281b448 .config console log report info [disk image] [vmlinux] [kernel image] ci2-linux-5-15-kasan possible deadlock in l2cap_chan_connect
2026/09/24 11:32 linux-5.15.y 0248c33e835e 0281b448 .config console log report info [disk image] [vmlinux] [kernel image] ci2-linux-5-15-kasan possible deadlock in l2cap_chan_connect
2026/09/23 19:19 linux-5.15.y 0248c33e835e abce1216 .config console log report info [disk image] [vmlinux] [kernel image] ci2-linux-5-15-kasan possible deadlock in l2cap_chan_connect
2026/09/23 19:19 linux-5.15.y 0248c33e835e abce1216 .config console log report info [disk image] [vmlinux] [kernel image] ci2-linux-5-15-kasan possible deadlock in l2cap_chan_connect
2026/09/23 19:17 linux-5.15.y 0248c33e835e abce1216 .config console log report info [disk image] [vmlinux] [kernel image] ci2-linux-5-15-kasan possible deadlock in l2cap_chan_connect
2026/09/23 19:17 linux-5.15.y 0248c33e835e abce1216 .config console log report info [disk image] [vmlinux] [kernel image] ci2-linux-5-15-kasan possible deadlock in l2cap_chan_connect
2026/09/23 04:17 linux-5.15.y 0248c33e835e 5207b90e .config console log report info [disk image] [vmlinux] [kernel image] ci2-linux-5-15-kasan possible deadlock in l2cap_chan_connect
2026/09/23 03:06 linux-5.15.y 0248c33e835e 5207b90e .config console log report info [disk image] [vmlinux] [kernel image] ci2-linux-5-15-kasan possible deadlock in l2cap_chan_connect
2026/09/20 02:45 linux-5.15.y 0248c33e835e 853b104b .config console log report info [disk image] [vmlinux] [kernel image] ci2-linux-5-15-kasan possible deadlock in l2cap_chan_connect
2026/09/18 03:11 linux-5.15.y 0248c33e835e 8d7d05f1 .config console log report info [disk image] [vmlinux] [kernel image] ci2-linux-5-15-kasan possible deadlock in l2cap_chan_connect
2026/09/14 22:12 linux-5.15.y 0248c33e835e 81a27d1a .config console log report info [disk image] [vmlinux] [kernel image] ci2-linux-5-15-kasan possible deadlock in l2cap_chan_connect
* Struck through repros no longer work on HEAD.