Re: [PATCH] Bluetooth: L2CAP: Fix UAF in l2cap_chan_timeout

From: Marco Elver

Date: Wed Jun 03 2026 - 09:29:49 EST

On Wed, 3 Jun 2026 at 14:31, Marco Elver <elver@xxxxxxxxxx> wrote:
>
> l2cap_chan_timeout() accesses chan->conn without holding a reference to
> the connection object. If l2cap_conn_del() races and tears down the
> connection while the timer is waiting for locks, it can result in a
> use-after-free when the timer wakes up and attempts to acquire
> conn->lock:
>
> | BUG: KASAN: slab-use-after-free in instrument_atomic_read_write include/linux/instrumented.h:112 [inline]
> | BUG: KASAN: slab-use-after-free in atomic_long_try_cmpxchg_acquire include/linux/atomic/atomic-instrumented.h:4456 [inline]
> | BUG: KASAN: slab-use-after-free in __mutex_trylock_fast kernel/locking/mutex.c:161 [inline]
> | BUG: KASAN: slab-use-after-free in mutex_lock+0x4f/0xa0 kernel/locking/mutex.c:318
> | Write of size 8 at addr ffff8881298d9550 by task kworker/2:1/83
> |
> | CPU: 2 UID: 0 PID: 83 Comm: kworker/2:1 Not tainted 7.1.0-rc6-next-20260601-dirty #6 PREEMPT(full)
> | Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS 1.17.0-debian-1.17.0-1 04/01/2014
> | Workqueue: events l2cap_chan_timeout
> | Call Trace:
> | <TASK>
> | instrument_atomic_read_write include/linux/instrumented.h:112 [inline]
> | atomic_long_try_cmpxchg_acquire include/linux/atomic/atomic-instrumented.h:4456 [inline]
> | __mutex_trylock_fast kernel/locking/mutex.c:161 [inline]
> | mutex_lock+0x4f/0xa0 kernel/locking/mutex.c:318
> | l2cap_chan_timeout+0x5d/0x1b0 net/bluetooth/l2cap_core.c:422
> | process_one_work kernel/workqueue.c:3326 [inline]
> | process_scheduled_works+0x7c8/0xfb0 kernel/workqueue.c:3409
> | worker_thread+0x8a9/0xcf0 kernel/workqueue.c:3490
> | kthread+0x346/0x430 kernel/kthread.c:436
> | ret_from_fork+0x1a3/0x470 arch/x86/kernel/process.c:158
> | ret_from_fork_asm+0x1a/0x30 arch/x86/entry/entry_64.S:245
> | </TASK>
> |
> | Allocated by task 320:
> | l2cap_conn_add+0xa7/0x820 net/bluetooth/l2cap_core.c:7075
> | l2cap_connect_cfm+0xdb/0xd70 net/bluetooth/l2cap_core.c:7452
> | hci_connect_cfm include/net/bluetooth/hci_core.h:2139 [inline]
> | hci_remote_features_evt+0x52f/0x9f0 net/bluetooth/hci_event.c:3760
> | hci_event_func net/bluetooth/hci_event.c:7796 [inline]
> | hci_event_packet+0x561/0xa70 net/bluetooth/hci_event.c:7847
> | hci_rx_work+0x370/0x890 net/bluetooth/hci_core.c:4040
> | process_one_work kernel/workqueue.c:3326 [inline]
> | process_scheduled_works+0x7c8/0xfb0 kernel/workqueue.c:3409
> | worker_thread+0x8a9/0xcf0 kernel/workqueue.c:3490
> | kthread+0x346/0x430 kernel/kthread.c:436
> | ret_from_fork+0x1a3/0x470 arch/x86/kernel/process.c:158
> | ret_from_fork_asm+0x1a/0x30 arch/x86/entry/entry_64.S:245
> |
> | Freed by task 322:
> | hci_disconn_cfm include/net/bluetooth/hci_core.h:2154 [inline]
> | hci_conn_hash_flush+0x101/0x1f0 net/bluetooth/hci_conn.c:2736
> | hci_dev_close_sync+0x889/0xde0 net/bluetooth/hci_sync.c:5405
> | hci_dev_do_close net/bluetooth/hci_core.c:502 [inline]
> | hci_unregister_dev+0x1f7/0x370 net/bluetooth/hci_core.c:2679
> | vhci_release+0x12a/0x180 drivers/bluetooth/hci_vhci.c:690
> | __fput+0x369/0x890 fs/file_table.c:510
> | task_work_run+0x160/0x1d0 kernel/task_work.c:233
> | get_signal+0xf5b/0x1120 kernel/signal.c:2810
> | arch_do_signal_or_restart+0x4d/0x600 arch/x86/kernel/signal.c:337
> | __exit_to_user_mode_loop kernel/entry/common.c:64 [inline]
> | exit_to_user_mode_loop+0x85/0x510 kernel/entry/common.c:98
> | __exit_to_user_mode_prepare include/linux/irq-entry-common.h:207 [inline]
> | syscall_exit_to_user_mode_prepare include/linux/irq-entry-common.h:230 [inline]
> | syscall_exit_to_user_mode include/linux/entry-common.h:318 [inline]
> | do_syscall_64+0x263/0x3d0 arch/x86/entry/syscall_64.c:100
> | entry_SYSCALL_64_after_hwframe+0x77/0x7f
> |
> | Last potentially related work creation:
> | hci_connect_cfm include/net/bluetooth/hci_core.h:2139 [inline]
> | hci_remote_features_evt+0x52f/0x9f0 net/bluetooth/hci_event.c:3760
> | hci_event_func net/bluetooth/hci_event.c:7796 [inline]
> | hci_event_packet+0x561/0xa70 net/bluetooth/hci_event.c:7847
> | hci_rx_work+0x370/0x890 net/bluetooth/hci_core.c:4040
> | process_one_work kernel/workqueue.c:3326 [inline]
> | process_scheduled_works+0x7c8/0xfb0 kernel/workqueue.c:3409
> | worker_thread+0x8a9/0xcf0 kernel/workqueue.c:3490
> | kthread+0x346/0x430 kernel/kthread.c:436
> | ret_from_fork+0x1a3/0x470 arch/x86/kernel/process.c:158
> | ret_from_fork_asm+0x1a/0x30 arch/x86/entry/entry_64.S:245
> |
> | The buggy address belongs to the object at ffff8881298d9400
> | which belongs to the cache kmalloc-512 of size 512
> | The buggy address is located 336 bytes inside of
> | freed 512-byte region [ffff8881298d9400, ffff8881298d9600)
>
> Fix it by holding a reference to the connection when the channel timer
> is scheduled, and releasing it when the timer is either canceled or
> executes to completion.
>
> Since l2cap_chan_del() nullifies chan->conn to disassociate the channel
> during teardown, the timer handler might read NULL from chan->conn even
> if it held a reference. To address this, introduce a `timer_conn` field
> to `struct l2cap_chan` to store the connection pointer associated with
> the active timer. The timer handler uses this field to acquire locks and
> release the connection reference, and skips channel closing operations
> if chan->conn has already been nullified by teardown.
>
> Fixes: 75780ca4c6a8 ("Bluetooth: L2CAP: use chan timer to close channels in cleanup_listen()")
> Cc: <stable@xxxxxxxxxxxxxxx>
> Cc: Siwei Zhang <oss@xxxxxxxxxxx>
> Cc: Luiz Augusto von Dentz <luiz.von.dentz@xxxxxxxxx>
> Assisted-by: Gemini:gemini-3.1-pro-preview
> Reported-by: https://sashiko.dev/#/patchset/20260521021249.3258069-1-oss%40fourdim.xyz
> Signed-off-by: Marco Elver <elver@xxxxxxxxxx>

Sigh, Sashiko points out more problems here:
https://sashiko.dev/#/patchset/20260603123111.2334409-1-elver%40google.com

> Can this lockless read of chan->timer_conn cause a use-after-free or double
> free if another thread re-arms the timer concurrently?

I haven't analyzed this further yet, so consider this patch a
bug-report-only. If anyone finds a better fix sooner, please go ahead.

> ---
> include/net/bluetooth/l2cap.h | 18 ++++++++++++++++--
> net/bluetooth/l2cap_core.c | 26 +++++++++++++++-----------
> 2 files changed, 31 insertions(+), 13 deletions(-)
>
> diff --git a/include/net/bluetooth/l2cap.h b/include/net/bluetooth/l2cap.h
> index e0a1f2293679..83719777512e 100644
> --- a/include/net/bluetooth/l2cap.h
> +++ b/include/net/bluetooth/l2cap.h
> @@ -514,6 +514,7 @@ struct l2cap_seq_list {
>
> struct l2cap_chan {
> struct l2cap_conn *conn;
> + struct l2cap_conn *timer_conn; /* for chan_timer */
> struct kref kref;
> atomic_t nesting;
>
> @@ -835,6 +836,9 @@ static inline void l2cap_chan_unlock(struct l2cap_chan *chan)
> mutex_unlock(&chan->lock);
> }
>
> +struct l2cap_conn *l2cap_conn_get(struct l2cap_conn *conn);
> +void l2cap_conn_put(struct l2cap_conn *conn);
> +
> static inline void l2cap_set_timer(struct l2cap_chan *chan,
> struct delayed_work *work, long timeout)
> {
> @@ -843,8 +847,13 @@ static inline void l2cap_set_timer(struct l2cap_chan *chan,
>
> /* If delayed work cancelled do not hold(chan)
> since it is already done with previous set_timer */
> - if (!cancel_delayed_work(work))
> + if (!cancel_delayed_work(work)) {
> l2cap_chan_hold(chan);
> + if (work == &chan->chan_timer && chan->conn) {
> + l2cap_conn_get(chan->conn);
> + chan->timer_conn = chan->conn;
> + }
> + }
>
> schedule_delayed_work(work, timeout);
> }
> @@ -857,8 +866,13 @@ static inline bool l2cap_clear_timer(struct l2cap_chan *chan,
> /* put(chan) if delayed work cancelled otherwise it
> is done in delayed work function */
> ret = cancel_delayed_work(work);
> - if (ret)
> + if (ret) {
> + if (work == &chan->chan_timer && chan->timer_conn) {
> + l2cap_conn_put(chan->timer_conn);
> + chan->timer_conn = NULL;
> + }
> l2cap_chan_put(chan);
> + }
>
> return ret;
> }
> diff --git a/net/bluetooth/l2cap_core.c b/net/bluetooth/l2cap_core.c
> index c4ccfbda9d78..491b03bf6903 100644
> --- a/net/bluetooth/l2cap_core.c
> +++ b/net/bluetooth/l2cap_core.c
> @@ -406,7 +406,7 @@ static void l2cap_chan_timeout(struct work_struct *work)
> {
> struct l2cap_chan *chan = container_of(work, struct l2cap_chan,
> chan_timer.work);
> - struct l2cap_conn *conn = chan->conn;
> + struct l2cap_conn *conn = chan->timer_conn;
> int reason;
>
> BT_DBG("chan %p state %s", chan, state_to_string(chan->state));
> @@ -421,23 +421,27 @@ static void l2cap_chan_timeout(struct work_struct *work)
> * this work. No need to call l2cap_chan_hold(chan) here again.
> */
> l2cap_chan_lock(chan);
> + chan->timer_conn = NULL;
> +
> + if (chan->conn) {
> + if (chan->state == BT_CONNECTED || chan->state == BT_CONFIG)
> + reason = ECONNREFUSED;
> + else if (chan->state == BT_CONNECT &&
> + chan->sec_level != BT_SECURITY_SDP)
> + reason = ECONNREFUSED;
> + else
> + reason = ETIMEDOUT;
>
> - if (chan->state == BT_CONNECTED || chan->state == BT_CONFIG)
> - reason = ECONNREFUSED;
> - else if (chan->state == BT_CONNECT &&
> - chan->sec_level != BT_SECURITY_SDP)
> - reason = ECONNREFUSED;
> - else
> - reason = ETIMEDOUT;
> -
> - l2cap_chan_close(chan, reason);
> + l2cap_chan_close(chan, reason);
>
> - chan->ops->close(chan);
> + chan->ops->close(chan);
> + }
>
> l2cap_chan_unlock(chan);
> l2cap_chan_put(chan);
>
> mutex_unlock(&conn->lock);
> + l2cap_conn_put(conn);
> }
>
> struct l2cap_chan *l2cap_chan_create(void)
> --
> 2.54.0.1013.g208068f2d8-goog
>