[PATCH v2] Bluetooth: l2cap: defer conn param update to avoid conn->lock/hdev->lock inversion

[PATCH v2] Bluetooth: l2cap: defer conn param update to avoid conn->lock/hdev->lock inversion

[Mikhail Gavrilov]

When a BLE peripheral sends an L2CAP Connection Parameter Update Request
the processing path is:

  process_pending_rx()          [takes conn->lock]
    l2cap_le_sig_channel()
      l2cap_conn_param_update_req()
        hci_le_conn_update()    [takes hdev->lock]

Meanwhile other code paths take the locks in the opposite order:

  l2cap_chan_connect()          [takes hdev->lock]
    ...
      mutex_lock(&conn->lock)

  l2cap_conn_ready()            [hdev->lock via hci_cb_list_lock]
    ...
      mutex_lock(&conn->lock)

This is a classic AB/BA deadlock which lockdep reports as a circular
locking dependency when connecting a BLE MIDI keyboard (Carry-On FC-49).

Fix this by introducing l2cap_conn_param_update_sync() which is scheduled
via hci_cmd_sync_queue() to run on the hci_cmd_sync workqueue, outside
any of the involved locks.  The necessary connection parameters are
captured into a heap-allocated struct conn_param_update_data and the
sync callback performs the hci_conn_params update, sends the HCI LE
Connection Update command, and notifies mgmt of the new parameters.

To guard against a theoretical handle-reuse race (the connection could
drop and its handle be reassigned between queuing and execution), the
sync callback verifies the connection still exists and its destination
address matches the original request before proceeding.

The allocation is performed before sending the L2CAP_CONN_PARAM_ACCEPTED
response so that on OOM the peer receives a REJECTED response instead of
an inconsistent state where the peer applies new parameters but the local
controller does not.

Fixes: f044eb0524a0 ("Bluetooth: Store latency and supervision timeout in connection params")
Signed-off-by: Mikhail Gavrilov <mikhail.v.gavrilov@gmail.com>
---

Changes in v2 (Paul Menzel, Sashiko/Gemini AI review):
- Allocate before sending ACCEPTED response to avoid state mismatch on OOM
- Verify connection handle and address in sync callback against reuse race
- Expand commit message with implementation details

 net/bluetooth/l2cap_core.c | 93 ++++++++++++++++++++++++++++++++++----
 1 file changed, 85 insertions(+), 8 deletions(-)

diff --git a/net/bluetooth/l2cap_core.c b/net/bluetooth/l2cap_core.c
index 95c65fece39b..94dfef9df498 100644
--- a/net/bluetooth/l2cap_core.c
+++ b/net/bluetooth/l2cap_core.c
@@ -4670,6 +4670,70 @@ static inline int l2cap_information_rsp(struct l2cap_conn *conn,
 	return 0;
 }
 
+struct conn_param_update_data {
+	u16	handle;
+	bdaddr_t dst;
+	u8	dst_type;
+	u16	min;
+	u16	max;
+	u16	latency;
+	u16	to_multiplier;
+};
+
+static int l2cap_conn_param_update_sync(struct hci_dev *hdev, void *data)
+{
+	struct conn_param_update_data *d = data;
+	struct hci_conn *conn;
+	struct hci_conn_params *params;
+	struct hci_cp_le_conn_update cp;
+	u8 store_hint = 0x00;
+
+	/* Verify the connection is still alive and matches the original
+	 * request, since the handle could theoretically be reused after
+	 * a disconnect/reconnect cycle.
+	 */
+	hci_dev_lock(hdev);
+
+	conn = hci_conn_hash_lookup_handle(hdev, d->handle);
+	if (!conn || bacmp(&conn->dst, &d->dst)) {
+		hci_dev_unlock(hdev);
+		return 0;
+	}
+
+	params = hci_conn_params_lookup(hdev, &d->dst, d->dst_type);
+	if (params) {
+		params->conn_min_interval = d->min;
+		params->conn_max_interval = d->max;
+		params->conn_latency = d->latency;
+		params->supervision_timeout = d->to_multiplier;
+		store_hint = 0x01;
+	}
+
+	hci_dev_unlock(hdev);
+
+	memset(&cp, 0, sizeof(cp));
+	cp.handle		= cpu_to_le16(d->handle);
+	cp.conn_interval_min	= cpu_to_le16(d->min);
+	cp.conn_interval_max	= cpu_to_le16(d->max);
+	cp.conn_latency		= cpu_to_le16(d->latency);
+	cp.supervision_timeout	= cpu_to_le16(d->to_multiplier);
+	cp.min_ce_len		= cpu_to_le16(0x0000);
+	cp.max_ce_len		= cpu_to_le16(0x0000);
+
+	hci_send_cmd(hdev, HCI_OP_LE_CONN_UPDATE, sizeof(cp), &cp);
+
+	mgmt_new_conn_param(hdev, &d->dst, d->dst_type, store_hint,
+			    d->min, d->max, d->latency, d->to_multiplier);
+
+	return 0;
+}
+
+static void l2cap_conn_param_update_destroy(struct hci_dev *hdev, void *data,
+					    int err)
+{
+	kfree(data);
+}
+
 static inline int l2cap_conn_param_update_req(struct l2cap_conn *conn,
 					      struct l2cap_cmd_hdr *cmd,
 					      u16 cmd_len, u8 *data)
@@ -4677,6 +4741,7 @@ static inline int l2cap_conn_param_update_req(struct l2cap_conn *conn,
 	struct hci_conn *hcon = conn->hcon;
 	struct l2cap_conn_param_update_req *req;
 	struct l2cap_conn_param_update_rsp rsp;
+	struct conn_param_update_data *d;
 	u16 min, max, latency, to_multiplier;
 	int err;
 
@@ -4703,18 +4768,30 @@ static inline int l2cap_conn_param_update_req(struct l2cap_conn *conn,
 	else
 		rsp.result = cpu_to_le16(L2CAP_CONN_PARAM_ACCEPTED);
 
+	if (!err) {
+		d = kmalloc(sizeof(*d), GFP_KERNEL);
+		if (!d) {
+			rsp.result = cpu_to_le16(L2CAP_CONN_PARAM_REJECTED);
+			err = -ENOMEM;
+		}
+	}
+
 	l2cap_send_cmd(conn, cmd->ident, L2CAP_CONN_PARAM_UPDATE_RSP,
 		       sizeof(rsp), &rsp);
 
 	if (!err) {
-		u8 store_hint;
-
-		store_hint = hci_le_conn_update(hcon, min, max, latency,
-						to_multiplier);
-		mgmt_new_conn_param(hcon->hdev, &hcon->dst, hcon->dst_type,
-				    store_hint, min, max, latency,
-				    to_multiplier);
-
+		d->handle = hcon->handle;
+		bacpy(&d->dst, &hcon->dst);
+		d->dst_type = hcon->dst_type;
+		d->min = min;
+		d->max = max;
+		d->latency = latency;
+		d->to_multiplier = to_multiplier;
+
+		if (hci_cmd_sync_queue(hcon->hdev,
+				       l2cap_conn_param_update_sync, d,
+				       l2cap_conn_param_update_destroy) < 0)
+			kfree(d);
 	}
 
 	return 0;
-- 
2.53.0

Re: [PATCH v2] Bluetooth: l2cap: defer conn param update to avoid conn->lock/hdev->lock inversion

[Paul Menzel]

Dear Mikhail,


Thank you for the improved version. (Just a note, that there are two 
spaces in the summary after *to avoid*. No need to resend.)

Am 10.04.26 um 08:30 schrieb Mikhail Gavrilov:
> When a BLE peripheral sends an L2CAP Connection Parameter Update Request
> the processing path is:
> 
>    process_pending_rx()          [takes conn->lock]
>      l2cap_le_sig_channel()
>        l2cap_conn_param_update_req()
>          hci_le_conn_update()    [takes hdev->lock]
> 
> Meanwhile other code paths take the locks in the opposite order:
> 
>    l2cap_chan_connect()          [takes hdev->lock]
>      ...
>        mutex_lock(&conn->lock)
> 
>    l2cap_conn_ready()            [hdev->lock via hci_cb_list_lock]
>      ...
>        mutex_lock(&conn->lock)
> 
> This is a classic AB/BA deadlock which lockdep reports as a circular
> locking dependency when connecting a BLE MIDI keyboard (Carry-On FC-49).

No need to resend, but out of curiosity, is the result, that the device 
does not work?

> Fix this by introducing l2cap_conn_param_update_sync() which is scheduled
> via hci_cmd_sync_queue() to run on the hci_cmd_sync workqueue, outside
> any of the involved locks.  The necessary connection parameters are
> captured into a heap-allocated struct conn_param_update_data and the
> sync callback performs the hci_conn_params update, sends the HCI LE
> Connection Update command, and notifies mgmt of the new parameters.
> 
> To guard against a theoretical handle-reuse race (the connection could
> drop and its handle be reassigned between queuing and execution), the
> sync callback verifies the connection still exists and its destination
> address matches the original request before proceeding.
> 
> The allocation is performed before sending the L2CAP_CONN_PARAM_ACCEPTED
> response so that on OOM the peer receives a REJECTED response instead of
> an inconsistent state where the peer applies new parameters but the local
> controller does not.
> 
> Fixes: f044eb0524a0 ("Bluetooth: Store latency and supervision timeout in connection params")
> Signed-off-by: Mikhail Gavrilov <mikhail.v.gavrilov@gmail.com>
> ---
> 
> Changes in v2 (Paul Menzel, Sashiko/Gemini AI review):
> - Allocate before sending ACCEPTED response to avoid state mismatch on OOM
> - Verify connection handle and address in sync callback against reuse race
> - Expand commit message with implementation details
> 
>   net/bluetooth/l2cap_core.c | 93 ++++++++++++++++++++++++++++++++++----
>   1 file changed, 85 insertions(+), 8 deletions(-)
> 
> diff --git a/net/bluetooth/l2cap_core.c b/net/bluetooth/l2cap_core.c
> index 95c65fece39b..94dfef9df498 100644
> --- a/net/bluetooth/l2cap_core.c
> +++ b/net/bluetooth/l2cap_core.c
> @@ -4670,6 +4670,70 @@ static inline int l2cap_information_rsp(struct l2cap_conn *conn,
>   	return 0;
>   }
>   
> +struct conn_param_update_data {
> +	u16	handle;
> +	bdaddr_t dst;
> +	u8	dst_type;
> +	u16	min;
> +	u16	max;
> +	u16	latency;
> +	u16	to_multiplier;
> +};
> +
> +static int l2cap_conn_param_update_sync(struct hci_dev *hdev, void *data)
> +{
> +	struct conn_param_update_data *d = data;
> +	struct hci_conn *conn;
> +	struct hci_conn_params *params;
> +	struct hci_cp_le_conn_update cp;
> +	u8 store_hint = 0x00;
> +
> +	/* Verify the connection is still alive and matches the original
> +	 * request, since the handle could theoretically be reused after
> +	 * a disconnect/reconnect cycle.
> +	 */
> +	hci_dev_lock(hdev);
> +
> +	conn = hci_conn_hash_lookup_handle(hdev, d->handle);
> +	if (!conn || bacmp(&conn->dst, &d->dst)) {
> +		hci_dev_unlock(hdev);
> +		return 0;
> +	}
> +
> +	params = hci_conn_params_lookup(hdev, &d->dst, d->dst_type);
> +	if (params) {
> +		params->conn_min_interval = d->min;
> +		params->conn_max_interval = d->max;
> +		params->conn_latency = d->latency;
> +		params->supervision_timeout = d->to_multiplier;
> +		store_hint = 0x01;
> +	}
> +
> +	hci_dev_unlock(hdev);
> +
> +	memset(&cp, 0, sizeof(cp));
> +	cp.handle		= cpu_to_le16(d->handle);
> +	cp.conn_interval_min	= cpu_to_le16(d->min);
> +	cp.conn_interval_max	= cpu_to_le16(d->max);
> +	cp.conn_latency		= cpu_to_le16(d->latency);
> +	cp.supervision_timeout	= cpu_to_le16(d->to_multiplier);
> +	cp.min_ce_len		= cpu_to_le16(0x0000);
> +	cp.max_ce_len		= cpu_to_le16(0x0000);
> +
> +	hci_send_cmd(hdev, HCI_OP_LE_CONN_UPDATE, sizeof(cp), &cp);
> +
> +	mgmt_new_conn_param(hdev, &d->dst, d->dst_type, store_hint,
> +			    d->min, d->max, d->latency, d->to_multiplier);
> +
> +	return 0;
> +}
> +
> +static void l2cap_conn_param_update_destroy(struct hci_dev *hdev, void *data,
> +					    int err)
> +{
> +	kfree(data);
> +}
> +
>   static inline int l2cap_conn_param_update_req(struct l2cap_conn *conn,
>   					      struct l2cap_cmd_hdr *cmd,
>   					      u16 cmd_len, u8 *data)
> @@ -4677,6 +4741,7 @@ static inline int l2cap_conn_param_update_req(struct l2cap_conn *conn,
>   	struct hci_conn *hcon = conn->hcon;
>   	struct l2cap_conn_param_update_req *req;
>   	struct l2cap_conn_param_update_rsp rsp;
> +	struct conn_param_update_data *d;
>   	u16 min, max, latency, to_multiplier;
>   	int err;
>   
> @@ -4703,18 +4768,30 @@ static inline int l2cap_conn_param_update_req(struct l2cap_conn *conn,
>   	else
>   		rsp.result = cpu_to_le16(L2CAP_CONN_PARAM_ACCEPTED);
>   
> +	if (!err) {
> +		d = kmalloc(sizeof(*d), GFP_KERNEL);
> +		if (!d) {
> +			rsp.result = cpu_to_le16(L2CAP_CONN_PARAM_REJECTED);
> +			err = -ENOMEM;
> +		}
> +	}
> +
>   	l2cap_send_cmd(conn, cmd->ident, L2CAP_CONN_PARAM_UPDATE_RSP,
>   		       sizeof(rsp), &rsp);
>   
>   	if (!err) {
> -		u8 store_hint;
> -
> -		store_hint = hci_le_conn_update(hcon, min, max, latency,
> -						to_multiplier);
> -		mgmt_new_conn_param(hcon->hdev, &hcon->dst, hcon->dst_type,
> -				    store_hint, min, max, latency,
> -				    to_multiplier);
> -
> +		d->handle = hcon->handle;
> +		bacpy(&d->dst, &hcon->dst);
> +		d->dst_type = hcon->dst_type;
> +		d->min = min;
> +		d->max = max;
> +		d->latency = latency;
> +		d->to_multiplier = to_multiplier;
> +
> +		if (hci_cmd_sync_queue(hcon->hdev,
> +				       l2cap_conn_param_update_sync, d,
> +				       l2cap_conn_param_update_destroy) < 0)
> +			kfree(d);
>   	}
>   
>   	return 0;

Reviewed-by: Paul Menzel <pmenzel@molgen.mpg.de>


Kind regards,

Paul

RE: [v2] Bluetooth: l2cap: defer conn param update to avoid conn->lock/hdev->lock inversion

[bluez.test.bot]

[-- Attachment #1: Type: text/plain, Size: 7136 bytes --]

This is automated email and please do not reply to this email!

Dear submitter,

Thank you for submitting the patches to the linux bluetooth mailing list.
This is a CI test results with your patch series:
PW Link:https://patchwork.kernel.org/project/bluetooth/list/?series=1079574

---Test result---

Test Summary:
CheckPatch                    PENDING   0.49 seconds
GitLint                       PENDING   0.25 seconds
SubjectPrefix                 PASS      0.10 seconds
BuildKernel                   PASS      25.97 seconds
CheckAllWarning               PASS      28.59 seconds
CheckSparse                   PASS      27.21 seconds
BuildKernel32                 PASS      25.14 seconds
TestRunnerSetup               PASS      565.16 seconds
TestRunner_l2cap-tester       FAIL      27.91 seconds
TestRunner_iso-tester         PASS      37.58 seconds
TestRunner_bnep-tester        PASS      6.27 seconds
TestRunner_mgmt-tester        FAIL      113.08 seconds
TestRunner_rfcomm-tester      PASS      9.35 seconds
TestRunner_sco-tester         FAIL      14.87 seconds
TestRunner_ioctl-tester       PASS      9.99 seconds
TestRunner_mesh-tester        FAIL      11.55 seconds
TestRunner_smp-tester         PASS      8.56 seconds
TestRunner_userchan-tester    PASS      6.62 seconds
TestRunner_6lowpan-tester     FAIL      8.81 seconds
IncrementalBuild              PENDING   0.27 seconds

Details
##############################
Test: CheckPatch - PENDING
Desc: Run checkpatch.pl script
Output:

##############################
Test: GitLint - PENDING
Desc: Run gitlint
Output:

##############################
Test: TestRunner_l2cap-tester - FAIL
Desc: Run l2cap-tester with test-runner
Output:
Total: 96, Passed: 95 (99.0%), Failed: 1, Not Run: 0

Failed Test Cases
L2CAP BR/EDR Server - Set PHY 3M                     Failed       0.115 seconds
##############################
Test: TestRunner_mgmt-tester - FAIL
Desc: Run mgmt-tester with test-runner
Output:
Total: 494, Passed: 489 (99.0%), Failed: 1, Not Run: 4

Failed Test Cases
Read Exp Feature - Success                           Failed       0.105 seconds
##############################
Test: TestRunner_sco-tester - FAIL
Desc: Run sco-tester with test-runner
Output:
WARNING: possible circular locking dependency detected
7.0.0-rc2-gc006ce61ce3a #1 Not tainted
------------------------------------------------------
kworker/u5:2/117 is trying to acquire lock:
ffff888001946240 (sk_lock-AF_BLUETOOTH-BTPROTO_SCO){+.+.}-{0:0}, at: sco_connect_cfm+0x358/0x8d0

but task is already holding lock:
ffff8880025fa220 (&conn->lock){+.+.}-{3:3}, at: sco_connect_cfm+0x22d/0x8d0

which lock already depends on the new lock.


the existing dependency chain (in reverse order) is:

-> #1 (&conn->lock){+.+.}-{3:3}:
       lock_acquire+0xf7/0x2c0
       _raw_spin_lock+0x2a/0x40
       sco_sock_connect+0x4d7/0x1280
       __sys_connect+0x1a3/0x260
       __x64_sys_connect+0x6e/0xb0
       do_syscall_64+0xa0/0x570
       entry_SYSCALL_64_after_hwframe+0x74/0x7c

-> #0 (sk_lock-AF_BLUETOOTH-BTPROTO_SCO){+.+.}-{0:0}:
       check_prev_add+0xe9/0xc70
       __lock_acquire+0x1457/0x1df0
       lock_acquire+0xf7/0x2c0
       lock_sock_nested+0x36/0xd0
       sco_connect_cfm+0x358/0x8d0
       hci_sync_conn_complete_evt+0x3d3/0x8e0
       hci_event_packet+0x74f/0xb10
       hci_rx_work+0x398/0xd00
       process_scheduled_works+0xb16/0x1ac0
       worker_thread+0x4ff/0xba0
       kthread+0x368/0x490
       ret_from_fork+0x498/0x7e0
       ret_from_fork_asm+0x19/0x30

other info that might help us debug this:

...
BUG: sleeping function called from invalid context at net/core/sock.c:3782
in_atomic(): 1, irqs_disabled(): 0, non_block: 0, pid: 117, name: kworker/u5:2
preempt_count: 1, expected: 0
RCU nest depth: 0, expected: 0
INFO: lockdep is turned off.
CPU: 0 UID: 0 PID: 117 Comm: kworker/u5:2 Not tainted 7.0.0-rc2-gc006ce61ce3a #1 PREEMPT(lazy) 
Hardware name: QEMU Standard PC (Q35 + ICH9, 2009), BIOS 1.13.0-1ubuntu1.1 04/01/2014
Workqueue: hci0 hci_rx_work
Call Trace:
 <TASK>
 dump_stack_lvl+0x49/0x60
 __might_resched+0x2ea/0x500
 lock_sock_nested+0x47/0xd0
 ? sco_connect_cfm+0x358/0x8d0
 sco_connect_cfm+0x358/0x8d0
 ? hci_debugfs_create_conn+0x190/0x210
 ? __pfx_sco_connect_cfm+0x10/0x10
 hci_sync_conn_complete_evt+0x3d3/0x8e0
 hci_event_packet+0x74f/0xb10
 ? __pfx_hci_sync_conn_complete_evt+0x10/0x10
 ? __pfx_hci_event_packet+0x10/0x10
 ? mark_held_locks+0x49/0x80
 ? lockdep_hardirqs_on_prepare+0xd4/0x180
 ? _raw_spin_unlock_irqrestore+0x2c/0x50
 hci_rx_work+0x398/0xd00
 process_scheduled_works+0xb16/0x1ac0
 ? __pfx_process_scheduled_works+0x10/0x10
 ? lock_acquire+0xf7/0x2c0
 ? lock_is_held_type+0x9b/0x110
 ? __pfx_hci_rx_work+0x10/0x10
 worker_thread+0x4ff/0xba0
 ? _raw_spin_unlock_irqrestore+0x2c/0x50
 ? __pfx_worker_thread+0x10/0x10
 kthread+0x368/0x490
 ? _raw_spin_unlock_irq+0x23/0x40
 ? __pfx_kthread+0x10/0x10
 ret_from_fork+0x498/0x7e0
 ? __pfx_ret_from_fork+0x10/0x10
 ? __switch_to+0x9e4/0xe50
 ? __switch_to_asm+0x32/0x60
...
Total: 30, Passed: 30 (100.0%), Failed: 0, Not Run: 0
##############################
Test: TestRunner_mesh-tester - FAIL
Desc: Run mesh-tester with test-runner
Output:
Total: 10, Passed: 8 (80.0%), Failed: 2, Not Run: 0

Failed Test Cases
Mesh - Send cancel - 1                               Timed out    1.909 seconds
Mesh - Send cancel - 2                               Timed out    1.996 seconds
##############################
Test: TestRunner_6lowpan-tester - FAIL
Desc: Run 6lowpan-tester with test-runner
Output:
WARNING: possible circular locking dependency detected
7.0.0-rc2-gc006ce61ce3a #1 Not tainted
------------------------------------------------------
kworker/0:0/9 is trying to acquire lock:
ffff888002777140 ((wq_completion)hci0#2){+.+.}-{0:0}, at: touch_wq_lockdep_map+0x75/0x180

but task is already holding lock:
ffffffff9804d720 (rtnl_mutex){+.+.}-{4:4}, at: lowpan_unregister_netdev+0xd/0x30

which lock already depends on the new lock.


the existing dependency chain (in reverse order) is:

-> #4 (rtnl_mutex){+.+.}-{4:4}:
       lock_acquire+0xf7/0x2c0
       __mutex_lock+0x16b/0x1fc0
       lowpan_register_netdev+0x11/0x30
       chan_ready_cb+0x836/0xd00
       l2cap_recv_frame+0x6ca1/0x8b20
       l2cap_recv_acldata+0x790/0xdf0
       hci_rx_work+0x500/0xd00
       process_scheduled_works+0xb16/0x1ac0
       worker_thread+0x4ff/0xba0
       kthread+0x368/0x490
       ret_from_fork+0x498/0x7e0
       ret_from_fork_asm+0x19/0x30

-> #3 (&chan->lock#3/1){+.+.}-{4:4}:
       lock_acquire+0xf7/0x2c0
       __mutex_lock+0x16b/0x1fc0
       l2cap_chan_connect+0x74e/0x1980
       lowpan_control_write+0x523/0x660
       full_proxy_write+0x10b/0x190
       vfs_write+0x1c0/0xf60
       ksys_write+0xf1/0x1d0
       do_syscall_64+0xa0/0x570
       entry_SYSCALL_64_after_hwframe+0x74/0x7c

-> #2 (&conn->lock){+.+.}-{4:4}:
...
Total: 8, Passed: 8 (100.0%), Failed: 0, Not Run: 0
##############################
Test: IncrementalBuild - PENDING
Desc: Incremental build with the patches in the series
Output:



https://github.com/bluez/bluetooth-next/pull/59

---
Regards,
Linux Bluetooth

Re: [PATCH v2] Bluetooth: l2cap: defer conn param update to avoid conn->lock/hdev->lock inversion

[Mikhail Gavrilov]

On Fri, Apr 10, 2026 at 12:29 PM Paul Menzel <pmenzel@molgen.mpg.de> wrote:
>
> No need to resend, but out of curiosity, is the result, that the device
> does not work?

Hi Paul,

Thanks for the review!

The lockdep warning did not noticeably affect the FC-49 functionality —
MIDI events were received fine via `aseqdump -p 128` both before and
after the patch.  The warning is about a potential deadlock that lockdep
detected through lock ordering analysis, not one that was actually hit
during my testing.  That said, under the right timing conditions the
deadlock is real and could hang the workqueue.

-- 
Thanks,
Mike Gavrilov.

Re: [PATCH v2] Bluetooth: l2cap: defer conn param update to avoid conn->lock/hdev->lock inversion

[Luiz Augusto von Dentz]

Hi Mikhail,

On Fri, Apr 10, 2026 at 2:30 AM Mikhail Gavrilov
<mikhail.v.gavrilov@gmail.com> wrote:
>
> When a BLE peripheral sends an L2CAP Connection Parameter Update Request
> the processing path is:
>
>   process_pending_rx()          [takes conn->lock]
>     l2cap_le_sig_channel()
>       l2cap_conn_param_update_req()
>         hci_le_conn_update()    [takes hdev->lock]
>
> Meanwhile other code paths take the locks in the opposite order:
>
>   l2cap_chan_connect()          [takes hdev->lock]
>     ...
>       mutex_lock(&conn->lock)
>
>   l2cap_conn_ready()            [hdev->lock via hci_cb_list_lock]
>     ...
>       mutex_lock(&conn->lock)
>
> This is a classic AB/BA deadlock which lockdep reports as a circular
> locking dependency when connecting a BLE MIDI keyboard (Carry-On FC-49).
>
> Fix this by introducing l2cap_conn_param_update_sync() which is scheduled
> via hci_cmd_sync_queue() to run on the hci_cmd_sync workqueue, outside
> any of the involved locks.  The necessary connection parameters are
> captured into a heap-allocated struct conn_param_update_data and the
> sync callback performs the hci_conn_params update, sends the HCI LE
> Connection Update command, and notifies mgmt of the new parameters.
>
> To guard against a theoretical handle-reuse race (the connection could
> drop and its handle be reassigned between queuing and execution), the
> sync callback verifies the connection still exists and its destination
> address matches the original request before proceeding.
>
> The allocation is performed before sending the L2CAP_CONN_PARAM_ACCEPTED
> response so that on OOM the peer receives a REJECTED response instead of
> an inconsistent state where the peer applies new parameters but the local
> controller does not.
>
> Fixes: f044eb0524a0 ("Bluetooth: Store latency and supervision timeout in connection params")
> Signed-off-by: Mikhail Gavrilov <mikhail.v.gavrilov@gmail.com>
> ---
>
> Changes in v2 (Paul Menzel, Sashiko/Gemini AI review):
> - Allocate before sending ACCEPTED response to avoid state mismatch on OOM
> - Verify connection handle and address in sync callback against reuse race
> - Expand commit message with implementation details
>
>  net/bluetooth/l2cap_core.c | 93 ++++++++++++++++++++++++++++++++++----
>  1 file changed, 85 insertions(+), 8 deletions(-)
>
> diff --git a/net/bluetooth/l2cap_core.c b/net/bluetooth/l2cap_core.c
> index 95c65fece39b..94dfef9df498 100644
> --- a/net/bluetooth/l2cap_core.c
> +++ b/net/bluetooth/l2cap_core.c
> @@ -4670,6 +4670,70 @@ static inline int l2cap_information_rsp(struct l2cap_conn *conn,
>         return 0;
>  }
>
> +struct conn_param_update_data {
> +       u16     handle;
> +       bdaddr_t dst;
> +       u8      dst_type;
> +       u16     min;
> +       u16     max;
> +       u16     latency;
> +       u16     to_multiplier;
> +};
> +
> +static int l2cap_conn_param_update_sync(struct hci_dev *hdev, void *data)
> +{
> +       struct conn_param_update_data *d = data;
> +       struct hci_conn *conn;
> +       struct hci_conn_params *params;
> +       struct hci_cp_le_conn_update cp;
> +       u8 store_hint = 0x00;
> +
> +       /* Verify the connection is still alive and matches the original
> +        * request, since the handle could theoretically be reused after
> +        * a disconnect/reconnect cycle.
> +        */
> +       hci_dev_lock(hdev);
> +
> +       conn = hci_conn_hash_lookup_handle(hdev, d->handle);
> +       if (!conn || bacmp(&conn->dst, &d->dst)) {
> +               hci_dev_unlock(hdev);
> +               return 0;
> +       }
> +
> +       params = hci_conn_params_lookup(hdev, &d->dst, d->dst_type);
> +       if (params) {
> +               params->conn_min_interval = d->min;
> +               params->conn_max_interval = d->max;
> +               params->conn_latency = d->latency;
> +               params->supervision_timeout = d->to_multiplier;
> +               store_hint = 0x01;
> +       }
> +
> +       hci_dev_unlock(hdev);
> +
> +       memset(&cp, 0, sizeof(cp));
> +       cp.handle               = cpu_to_le16(d->handle);
> +       cp.conn_interval_min    = cpu_to_le16(d->min);
> +       cp.conn_interval_max    = cpu_to_le16(d->max);
> +       cp.conn_latency         = cpu_to_le16(d->latency);
> +       cp.supervision_timeout  = cpu_to_le16(d->to_multiplier);
> +       cp.min_ce_len           = cpu_to_le16(0x0000);
> +       cp.max_ce_len           = cpu_to_le16(0x0000);
> +
> +       hci_send_cmd(hdev, HCI_OP_LE_CONN_UPDATE, sizeof(cp), &cp);
> +
> +       mgmt_new_conn_param(hdev, &d->dst, d->dst_type, store_hint,
> +                           d->min, d->max, d->latency, d->to_multiplier);

Actually most of this can be defered directly to
hci_event.c:hci_le_conn_update_complete_evt, then use
hci_sent_cmd_data(hdev, HCI_OP_LE_CONN_UPDATE) to fetch the command
parameters.

> +
> +       return 0;
> +}
> +
> +static void l2cap_conn_param_update_destroy(struct hci_dev *hdev, void *data,
> +                                           int err)
> +{
> +       kfree(data);
> +}
> +
>  static inline int l2cap_conn_param_update_req(struct l2cap_conn *conn,
>                                               struct l2cap_cmd_hdr *cmd,
>                                               u16 cmd_len, u8 *data)
> @@ -4677,6 +4741,7 @@ static inline int l2cap_conn_param_update_req(struct l2cap_conn *conn,
>         struct hci_conn *hcon = conn->hcon;
>         struct l2cap_conn_param_update_req *req;
>         struct l2cap_conn_param_update_rsp rsp;
> +       struct conn_param_update_data *d;
>         u16 min, max, latency, to_multiplier;
>         int err;
>
> @@ -4703,18 +4768,30 @@ static inline int l2cap_conn_param_update_req(struct l2cap_conn *conn,
>         else
>                 rsp.result = cpu_to_le16(L2CAP_CONN_PARAM_ACCEPTED);
>
> +       if (!err) {
> +               d = kmalloc(sizeof(*d), GFP_KERNEL);
> +               if (!d) {
> +                       rsp.result = cpu_to_le16(L2CAP_CONN_PARAM_REJECTED);
> +                       err = -ENOMEM;
> +               }
> +       }
> +
>         l2cap_send_cmd(conn, cmd->ident, L2CAP_CONN_PARAM_UPDATE_RSP,
>                        sizeof(rsp), &rsp);
>
>         if (!err) {
> -               u8 store_hint;
> -
> -               store_hint = hci_le_conn_update(hcon, min, max, latency,
> -                                               to_multiplier);
> -               mgmt_new_conn_param(hcon->hdev, &hcon->dst, hcon->dst_type,
> -                                   store_hint, min, max, latency,
> -                                   to_multiplier);

Just make hci_le_conn_update internally use hci_cmd_sync_queue and
then handle the update values at hci_le_conn_update_complete_evt.

> -
> +               d->handle = hcon->handle;
> +               bacpy(&d->dst, &hcon->dst);
> +               d->dst_type = hcon->dst_type;
> +               d->min = min;
> +               d->max = max;
> +               d->latency = latency;
> +               d->to_multiplier = to_multiplier;
> +
> +               if (hci_cmd_sync_queue(hcon->hdev,
> +                                      l2cap_conn_param_update_sync, d,
> +                                      l2cap_conn_param_update_destroy) < 0)
> +                       kfree(d);
>         }
>
>         return 0;
> --
> 2.53.0
>


-- 
Luiz Augusto von Dentz